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Edmonds Walgreens Geo-tech Report.pdfCEO'TECH CONSULTANTS, INC. Seven Hills Properties, LLC 88 Perry Street #800 San Francisco, California 94107 Attention: Jonathan Hill 13256 Northeast 20th Street, Suite 16 Bellevue, Washington 98005 (425) 747-5618 rAX (425) 747-8561 February 28, 2012 JN 12034 via email: Jhill@sevenhillsprop.com Subject: Transmittal Letter — Geotechnical Engineering Study Proposed Walgreens Development 9801 Edmonds Way Edmonds, Washington Dear Mr. Hill: We are pleased to present this geotechnical engineering report for the Walgreens Development project to be constructed at 9801 Edmonds Way in Edmonds, Washington. The scope of our services consisted of exploring site surface and subsurface conditions, and then developing this report to provide recommendations for general earthwork and design criteria for foundations, retaining walls, and shoring. This work was authorized by your acceptance of our proposal, P- 8308, dated January 30, 2012. The attached report contains a discussion of the study and our recommendations. Please contact us if there are any questions regarding this report, or for further assistance during the design and construction phases of this project. Respectfully submitted, GEOTECH CONSULTANTS, INC. D. Robert Ward, P.E. Principal cc: Baysinger Partners Architecture — William M. Ruecker via email; billy@baysingerpartners.com JLH/DRW: jyb GF_OTECH CONSULTANTS, INC. GEOTECHNICAL ENGINEERING STUDY Proposed Walgreens Development 9801 Edmonds Way Edmonds, Washington This report presents the findings and recommendations of our geotechnical engineering study for the site of the proposed Walgreens Development project to be located at 9801 Edmonds Way in Edmonds. We were provided with preliminary plans and a topographic map. Baysinger Partners Architecture developed the plans, the latest of which is dated December 14, 2011. The topographic survey was created by Foster & Maddux Surveying, Inc., and is dated October 31, 2011. Based on the provided information, we understand that the project will consist of removing the existing bowling alley building on the site, and constructing a new, 14,490 square foot Walgreens drugstore building on the northern portion of the site. An access drive is proposed around the east and north sides of the new building, which will require a significant excavation and retaining wall of up to about 35 feet tall along the northern edge of the site and a smaller wall needed on the eastern side. South of the new drugstore, near State Route No. 104 (Edmonds Way) a new, 4,500 square -foot bank building is proposed. Some stormwater infiltration is also proposed in some new rain gardens in an existing parking lot area west of the proposed buildings. Also, there is a possibility of various additional infiltration facilities near the proposed Walgreens and bank building. If the scope of the project changes from what we have described above, we should be provided with revised plans in order to determine if modifications to the recommendations and conclusions of this report are warranted. SITE CONDITIONS SURFACE The Vicinity Map, Plate 1, illustrates the general location of the site in Edmonds. The project area includes two parcels located along the northern side of Edmonds Way. The eastern portion of the subject site is mostly developed with a bowling alley and parking lot, while the western side contains a parking lot. The site is bordered to the east and west by commercial properties currently developed with a bank, a grocery store and a pet store, respectively. The parking lot on the eastern side of the property is used by the grocery and the pet store patrons. A residential development is located upslope and north of the property. Most Of the site, which includes the bowling alley building and parking lots, is relatively level with just a slight rise to the east/northeast. This flat area has grades ranging from approximately elevation 319 feet to 325 feet. The main (and lowest) level of the bowling alley appears to have a finish floor of approximately 325 feet. The areas directly north and east of the bowling alley building are undeveloped. The northern area is mostly covered with a few large trees and blackberry bushes. The eastern area is mostly covered with grass. A steep slope that rises up to the north at approximately 80 to 90 percent is located north of the building. The height of the slope is approximately 40 to 55 feet. We did not notice any indications of slope instability or seepage along the face of this steep slope at the time of our investigation. The top of the slope appears to be relatively flat; the residential development is located in this flatter area. A stormwater pipe that apparently conveys water from the development and/or upper streets is located on the steep slope GEOTECH CONSULTANTS, INC. Seven Hills Properties JN 12034 February 28, 2012 Page 2 near the northeastern corner of the site; it discharges water onto the property at approximately elevation 345 feet. It appears that the water makes its way onto the existing bank property adjacent to the east. The undeveloped area east of the bowling alley is somewhat of a "ridge" that runs north -south between the bowling alley and the adjacent eastern bank parking lot. The ridge declines to the south from approximately elevation 345 feet down to approximately elevation 325 feet. Existing utility lines are located in the southern portion of this undeveloped area. As noted earlier, a new concrete wall is proposed north of the Walgreens building. There is currently already a concrete retaining wall located along the northern edge of the existing western parking lot. This wall is up to approximately 20 feet tall. The new wall will connect to and extend east of this existing wall. SUBSURFACE The subsurface conditions were explored by drilling five test borings at the approximate locations shown on the Site Exploration Plan, Plate 2. Our exploration program was based on the proposed construction, anticipated subsurface conditions and those encountered during exploration, and the scope of work outlined in our proposal. The test borings were drilled on February 14 and 15, 2012 using a small, track -mounted, hollow - stem auger drill. Samples were taken at 5 -foot intervals with a standard penetration sampler. This split -spoon sampler, which has a 2 -inch outside diameter, is driven into the soil with a 140 -pound hammer falling 30 inches. The number of blows required to advance the sampler a given distance is an indication of the soil density or consistency. A geotechnical engineer from our staff observed the drilling process, logged the test borings, and obtained representative samples of the soil encountered. The Test Boring Logs are attached as Plates 3 through 8. Soil Conditions The test borings generally revealed similar soil conditions beneath the surface of the site; native sand with some gravel was encountered at the surface in the test borings with the exception of the Test Boring 4, drilled in the area of the proposed bank. Up to approximately 7 feet of loose sand, apparently fill soil, was encountered over the sand in this test boring. The test borings revealed that the sand is generally dense near the existing ground surface, and becomes very dense with depth. This sand was glacially consolidated. The deepest boring, Test Boring 1, was drilled to a maximum explored depth approximately 45 feet. Test Boring 1, conducted along the steep northern slope, indicates that dense to very dense native sand comprise the core of the steep slope. No obstructions were revealed by our explorations. However, debris, buried utilities, and old foundation and slab elements are commonly encountered on sites that have had previous development. Groundwater Conditions No groundwater seepage was observed in our test borings. The test borings were left open for only a short time period. Therefore, the seepage levels on the logs represent the location of transient water seepage and may not indicate the static groundwater level. Groundwater levels encountered during drilling can be deceptive, because seepage into the boring can be blocked or slowed by the auger itself. GEOTECH CONSULTANTS, INC. Seven Hills Properties February 28, 2012 JN 12034 Page 3 It should be noted that groundwater levels vary seasonally with rainfall and other factors. It is possible that groundwater could be found in more permeable soil layers, coarser sand and gravel lenses, and between the near -surface, more weathered soil and the underlying denser soil. The stratification lines on the logs represent the approximate boundaries between soil types at the exploration locations. The actual transition between soil types may be gradual, and subsurface conditions can vary between exploration locations. The logs provide specific subsurface information only at the locations tested. The relative densities and moisture descriptions indicated on the test boring logs are interpretive descriptions based on the conditions observed during drilling. CONCLUSIONS AND RECOMMENDATIONS GENERAL THIS SECTION CONTAINS A SUMMARY OF OUR STUDY AND FINDINGS FOR THE PURPOSES OF A GENERAL OVERVIEW ONLY. MORE SPECIFIC RECOMMENDATIONS AND CONCLUSIONS ARE CONTAINED IN THE REMAINDER OF THIS REPORT. ANY PARTY RELYING ON THIS REPORT SHOULD READ THE ENTIRE DOCUMENT. The test borings conducted for this study generally encountered dense native sand beneath the surface of the site in most locations. The exception being Test Boring 4, conducted in the area of the proposed bank building, which encountered approximately 7 feet of loose soil overlying the dense native sand. It appears in the location of the new Walgreens building that dense native sands will likely be exposed at or near the depth of the planned foundation excavations. The possible exception where some overexcavation may be needed to reach the dense sand would be the southern side of the building, although we do expect the overexcavation to be minor. The dense native sand is well suited for the support of the proposed building. Based on our investigation, the proposed new drugstore building may be constructed using conventional foundations supported on the competent, dense native sand. As noted above, in the area of the proposed bank building, loose soil (possibly fill) was encountered to a depth of approximately 7 feet. This loose soil is not suitable for supporting the loads imposed by the new bank building because settlement would occur. The foundation for this building should bear on or into the competent native sand. The entire extent of the loose soil is not known, as only one test boring was done in that areas; however, we anticipate that the loose soil may exist under a majority of the proposed bank foundation. Several options exist for foundation construction that will allow for the loads to be adequately transferred to the competent native sand, including: 1) Remove the loose soil down to the dense sand placing footings on the sand. 2) Remove the loose soil down to the dense sand and replacing it with imported structural fill. The fill could consist of structural fill soil or lean -mix concrete as noted in Conventional Foundations section of this report. 3) Avoid the overexcavation and use a deep foundation system. A very adequate system based on the likely loads of the bank in our opinion would be small diameter steel pipe piles that are driven into the underlying, competent native soil. A significant geotechnical engineering consideration of the project is the large excavation and subsequent retaining wall needed at the northeastern corner. The depth or excavation and GEOTECH CONSULTANTS, INC. Seven Hills Properties A 12034 February 28, 2012 Page 4 subsequent wall will vary from approximately 20 to 35 feet. Due to the depth of excavation, and its location on the steep slope and relatively close to property lines, excavation shoring will be needed for most of the tall wall. One exception could be along the eastern side of the property where a conventional retaining wall could possibly be constructed if temporary excavation easements can be obtained from the neighboring eastern property owner. The shoring should likely be incorporated into the permanent retaining wall system. Based on the soils observed in our test borings, the height and location of the shoring wall, we feel two options are feasible for construction of the shoring wall that is taller than approximately 15 feet; that being a soil nail wall or a tied -back soldier pile wall. More simple, cantilevered soldier piles shoring could be used for shorter excavations. Further recommendations regarding the design and construction of the potential shoring systems can be found in the subsequent sections of this report. We point out two significant items regarding the proposed shoring walls, especially the northern wall: 1) the design of nails or tie -backs must illustrate that these structures will not extend across the property lines; if they do cross property lines, easements will be needed and 2) due to the very step inclination of the northern slope, at least 2 feet of catchment should be included in the final wall design. This is because though the northern slope has a core soil of very dense sand, the outer, weathered surface of the slope is relatively loose (this is typical for any steep slope in the Puget Sound area). Based on The Edmonds Community Development Code (ECDC), Chapter 23 (Geologically Hazardous Areas), the steep northern slope would be classified as a critical area susceptible to two specific types of geological hazards. One is a Landslide Hazard Area due to its steeper than 40 percent slope, and greater than 10 foot vertical relief. The other is an Erosion Hazard Area. The ECDC suggests a minimum development buffer of 50 feet from any landslide area, although this buffer can be minimized to 10 feet. In our professional opinion, this minimum buffer of 10 feet is warranted. However, the ECDC further states that a reduction of the buffer, and alteration or development within Geologically Hazard Areas and their associated buffers, is allowed if supported by a geotechnical report and if certain requirements are followed. We will provide information regarding the requirements below. However, we first want to state that it is our professional opinion from a geotechnical engineering standpoint that the project can be built as planned, whereby development occurs in the Landslide and Erosion Hazard area and the associated buffers, because of two main points: 1) the core soil at the site is dense to very dense, glacially - consolidated, native sand and 2) permanent retaining walls that are designed to modern standards will be placed where steep, unsupported slopes currently exist, and 3) water from a stormwater pipe that currently discharges water onto the northern slope will be repaired so that the water does not discharge onto the slope. Based on ECDC 23.80.060A, an alteration to a Geologically Hazardous Area and associated buffer may occur for activities that: 1. Will not increase the threat of the geologic hazard to adjacent properties beyond predevelopment conditions; 2. Will not adversely affect other critical areas 3. Are designed so that the hazard to the project is eliminated or mitigated to a level equal to or Less that predevelopment conditions; and 4. Are certified as safe as designed and under anticipated conditions by a qualified engineer licensed in the State of Washington. The slope on the northern portion of the site is unsupported and extends directly down to the base of an existing building. The project will include the use of a large retaining wall, designed to current standards at this slope. This wall will also include catchment. It is our professional opinion that this wall will provide more stability for the slope and area in comparison to the current unsupported GEOTECH CONSULTANTS, INC. Seven Hills Properties JN 12034 February 28, 2012 Page 5 slope. In addition, as noted above, water from a stormwater pipe that currently discharges water onto the northern slope will be repaired so that the water does not discharge onto the slope. Lastly, if the recommendations contained in this report are followed, we strongly believe that the project is safe as designed under anticipated conditions. For all these significant reasons, it is our professional opinion that the four points noted in ECDC 23.80.060A are satisfied. In addition, ECDC 23.80.070A2 indicates that alterations of an Erosion or Landslide Hazard Area and buffer may occur for activities for which a hazards analysis is submitted and certifies that: a) The development will not increase surface water discharge or sedimentation to adjacent properties beyond predevelopment conditions; b) The development will not decrease slope stability on adjacent properties; and c) Such alterations will not adversely impact other critical areas. This project will decrease, not increase amount of surface water discharge or sedimentation to the adjacent property, so a) is most definitely satisfied. As for b), as noted above, we believe that the construction of new retaining wall will may increase, not decrease slope stability. Lastly, the only other critical area is the steep slope above the proposed retaining wall; this slope will be positively, not adversely affected because of the wall in our opinion. The infiltration of stormwater is being considered for this project. We understand that using rain gardens in the west parking lot area is one consideration. As noted earlier, dense to very dense sand was revealed near the ground surface in the test borings, especially at the northeastern portion of the site. Because of this denseness, it is our professional opinion that infiltration in this area is extremely limited; therefore, stormwater infiltration in that area is not prudent. It is possible that low infiltration rates could be achieved in the areas of the proposed rain gardens. Therefore, the use of the proposed rain gardens appears feasible. Rain gardens have overflow pipes buried within them if and when the infiltration rate of the soil is exceeded. We would expect some water to infiltrate, but not an excessive amount. Storm detention/retention facilities and other utilities are often installed below, or near, structures. The walls of storm vaults must be designed as either cantilever or restrained retaining walls, as appropriate. Wall pressures for the expected soil conditions are presented in the Permanent Foundation and Retaining Walls section of this report. It is important that the portion of the structure above the permanent detained water level be backfilled with free -draining soil, as recommended for retaining walls. Should drainage not be provided, the walls must be designed for hydrostatic forces acting on the outside of the structure. The backfill for all underground structures must be compacted in lifts according to the criteria of this report. Trenches for underground structures and utilities should not cross a line extending downwards from a new or existing footing at an inclination of (1:1) (Horizontal:Vertical), or a line extending downwards from a property line at an inclination of (1:1) (H:V). We should be consulted if these excavation zones will be exceeded for installation of storm facilities or other utilities. The erosion control measures needed during the site development will depend heavily on the weather conditions that are encountered. We anticipate that a silt fence will be needed around the downslope sides of any cleared areas. Rocked construction access roads should be extended into the site to reduce the amount of soil or mud carried off the property by trucks and equipment. Wherever possible, these roads should follow the alignment of planned pavements, and trucks should not be allowed to drive off of the rock -covered areas. Cut slopes and soil stockpiles should be covered with plastic during wet weather. Following rough grading, it may be necessary to mulch GEOTECH CONSULTANTS, INC. Seven Hills Properties JN 12034 February 28, 2012 Page 6 or hydroseed bare areas that will not be immediately covered with landscaping or an impervious surface. As with any project that involves demolition of existing site buildings and/or extensive excavation and shoring, there is a potential risk of movement on surrounding properties. This can potentially translate into noticeable damage of surrounding on -grade elements, such as foundations and slabs. However, the demolition, shoring, and/or excavation work could just translate into perceived damage on adjacent properties. Unfortunately, it is becoming more and more common for adjacent property owners to make unsubstantiated damage claims on new projects that occur close to their developed lots. Therefore, we recommend making an extensive photographic and visual survey of the project vicinity, prior to demolition activities, installing shoring, and/or commencing with the excavation. This documents the condition of buildings, pavements, and utilities in the immediate vicinity of the site in order to avoid, and protect the owner from, unsubstantiated damage claims by surrounding property owners. Additionally, any adjacent structures should be monitored during construction to detect soil movements. To monitor their performance, we recommend establishing a series of survey reference points to measure any horizontal deflections of the shoring system. Control points should be established at a distance well away from the walls and slopes, and deflections from the reference points should be measured throughout construction by survey methods. Geotech Consultants, Inc. should be allowed to review the final development plans to verify that the recommendations presented in this report are adequately addressed in the design. Such a plan review would be additional work beyond the current scope of work for this study, and it may include revisions to our recommendations to accommodate site, development, and geotechnical constraints that become more evident during the review process. We recommend including this report, in its entirety, in the project contract documents. This report should also be provided to any future property owners so they will be aware of our findings and recommendations. SEISMIC CONSIDERATIONS In accordance with the International Building Code (IBC), the site soil profile within 100 feet of the ground surface is best represented by Site Class C (Very Dense Soil). The site soils have a low potential for seismic liquefaction because of their dense nature and the absence of near -surface groundwater. This statement regarding liquefaction includes the knowledge of the determined peak ground acceleration noted below. As noted in the USGS website, the mapped spectral acceleration value for a 0.2 second (Ss) and 1.0 second period (Si) equals 1.2g and 0.4g, respectively. The International Building Code (IBC) states that a site-specific seismic study need not be performed provided that the peak ground acceleration be equal to SDs/2.5, where SDS is determined in ASCE 7. It is noted that SDs is equal to 2/3SMs. SMs equals Fa times Ss, where Fa is determined in Table 11.4-1. For our site, Fa = 1.0. Thus, the calculated peak ground acceleration that we utilized for the seismic -related parameters of this report equals 0.32g. CONVENTIONAL FOUNDATIONS GEOTECH CONSULTANTS, INC. Seven Hills Properties A 12034 February 28, 2012 Page 7 The proposed structures can be supported on conventional continuous and spread footings bearing on undisturbed, medium dense to dense native sand soil, or on structural fill placed above this competent native soil. See the section entitled General Earthwork and Structural Fill for recommendations regarding the placement and compaction of structural fill beneath structures. Adequate compaction of structural fill should be verified with frequent density testing during fill placement. Prior to placing structural fill beneath foundations, the excavation should be observed by the geotechnical engineer to document that adequate bearing soils have been exposed. We recommend that continuous and individual spread footings have minimum widths of 12 and 16 inches, respectively. Exterior footings should also be bottomed at least 18 inches below the lowest adjacent finish ground surface for protection against frost and erosion. The local building codes should be reviewed to determine if different footing widths or embedment depths are required. Footing subgrades must be cleaned of loose or disturbed soil prior to pouring concrete. Depending upon site and equipment constraints, this may require removing the disturbed soil by hand, or re - compaction and moisture conditioning of the bearing surfaces. As discussed in the general section, overexcavation may be required below the footings in some areas to expose competent native soil. Unless lean concrete is used to fill an overexcavated hole, the overexcavation must be at least as wide at the bottom as the sum of the depth of the overexcavation and the footing width. For example, an overexcavation extending 2 feet below the bottom of a 2 -foot -wide footing must be at least 4 feet wide at the base of the excavation. If lean concrete is used, the overexcavation need only extend 6 inches beyond the edges of the footing. If this option is chosen, it may be prudent to conduct the excavation/filling work in short sections to greatly reduce the amount of time the excavations need to remain open. This is because some caving of the loose upper soil is possible. The following allowable bearing pressures are appropriate for footings constructed according to the above recommendations: Placed directly on competent, 5,000 psf native soil or lean -mix concrete placed above the dense native soil Supported on structural fill 2,500 psf placed above the dense native soil Where: (i) psf is pounds per square foot. A one-third increase in these design bearing pressures may be used when considering short-term wind or seismic loads. For the above design criteria, it is anticipated that the total post -construction settlement of footings founded on competent native soil, or on structural fill up to 5 feet in thickness, will be less than one -inch, with differential settlements on the order of one -half-inch in a distance of 50 feet along a continuous footing with a uniform load. Lateral loads due to wind or seismic forces may be resisted by friction between the foundation and the bearing soil, or by passive earth pressure acting on the vertical, embedded portions of the foundation. For the latter condition, the foundation must be either poured directly against relatively level, undisturbed soil or be surrounded by level, compact fill. We recommend using the following ultimate values for the foundation's resistance to lateral loading: GEOTECH CONSULTANTS, INC. Seven Hills Properties February 28, 2012 ■ ULTIMATE VALUE Coefficient of Friction 0.50 Passive Earth Pressure 300 pcf Where: (i) pcf is pounds per cubic foot, and (ii) passive earth pressure is computed using the equivalent fluid density. JN 12034 Page 8 If the ground in front of a foundation is loose or sloping, the passive earth pressure given above will not be appropriate. We recommend maintaining a safety factor of at least 1.5 for the foundation's resistance to lateral loading, when using the above ultimate values. PIPE PILES As discussed in the general section, small diameter steel pipe piles could be used to support the portion of the new bank foundation that is underlain by a layer of loose sand soil. Three- or 4 -inch - diameter pipe piles driven with a 650- or 800- or 1,100 -pound hydraulic jackhammer to the following final penetration rates may be assigned the following compressive capacities. INSIDE PILE FINAL FINAL, FINAL ALLOWABLE DIAMETER DRIVING DRIVING DRIVING RATE COMPRESSIVE RATE RATE (1,100 -pound CAPACITY (650 -pound (800 -pound hammer) hammer) - , - hammer) • - • tell - Note: The refusal criteria indicated in the above table are valid only for pipe piles that are installed using a hydraulic impact hammer carried on leads that allow the hammer to sit on the top of the pile during driving. If the piles are installed by alternative methods, such as a vibratory hammer or a hammer that is hard -mounted to the installation machine, numerous load tests to 200 percent of the design capacity would be necessary to substantiate the allowable pile load. The appropriate number of load tests would need to be determined at the time the contractor and installation method are chosen. As a minimum, load tests on 20 percent of the piles is typical where alternative pile installation methods are used. As a minimum, Schedule 40 pipe should be used. The site soils should not be highly corrosive. Considering this, it is our opinion that standard "black" pipe can be used, and corrosion protection, such as galvanizing, is not necessary for the pipe piles. Pile caps and grade beams should be used to transmit loads to the piles. Isolated pile caps should include a minimum of two piles to reduce the potential for eccentric loads being applied to the piles. Subsequent sections of pipe can be connected with slip or threaded couplers, or they can be welded together. If slip couplers are used, they should fit snugly into the pipe sections. This may require that shims be used or that beads of welding flux be applied to the outside of the coupler. GEOTECH CONSULTANTS, INC. Seven Hills Properties JN 12034 February 28, 2012 Page 9 Lateral loads due to wind or seismic forces may be resisted by passive earth pressure acting on the vertical, embedded portions of the foundation. For this condition, the foundation must be either poured directly against relatively level, undisturbed soil or surrounded by level, compact fill. We recommend using a passive earth pressure of 300 pounds per cubic foot (pcf) for this resistance. If the ground in front of a foundation is loose or sloping, the passive earth pressure given above will not be appropriate. We recommend a safety factor of at least 1.5 for the foundation's resistance to lateral loading, when using the above ultimate passive value. PERMANENT FOUNDATION AND RETAINING WALLS Retaining walls backfilled on only one side should be designed to resist the lateral earth pressures imposed by the soil they retain. The following recommended parameters are for walls that restrain level backfill (parameters for shoring -type walls are given in a subsequent section of this report): Where: (i) pcf is pounds per cubic foot, and (ii) active and passive earth pressures are computed using the equivalent fluid pressures. * For a restrained wall that cannot deflect at least 0.002 times its height, a uniform lateral pressure equal to 10psf times the height of the wall should be added to the above active equivalent fluid pressure. The values given above are to be used to design only permanent foundation and retaining walls that are to be backfilled, such as conventional walls constructed of reinforced concrete or masonry. It is not appropriate to use the above earth pressures and soil unit weight to back -calculate soil strength parameters for design of other types of retaining walls, such as soldier pile, reinforced earth, modular or soil nail walls. The passive pressure given is appropriate only for a shear key poured directly against undisturbed native soil, or for the depth of level, compact fill placed in front of a retaining or foundation wall. The values for friction and passive resistance are ultimate values and do not include a safety factor. We recommend a safety factor of at least 1.5 for overturning and sliding, when using the above values to design the walls. Restrained wall soil parameters should be utilized for a distance of 1.5 times the wall height from corners or bends in the walls. This is intended to reduce the amount of cracking that can occur where a wall is restrained by a corner. The design values given above do not include the effects of any hydrostatic pressures behind the walls and assume that no surcharges, such as those caused by slopes, vehicles, or adjacent foundations will be exerted on the walls. If these conditions exist, those pressures should be added to the above lateral soil pressures. Where sloping backfill is desired behind the walls, we will need to be given the wall dimensions and the slope of the backfill in order to provide the appropriate design earth pressures. The surcharge due to traffic loads behind a wall can typically be accounted for by adding a uniform pressure equal to 2 feet multiplied by the above active fluid GEOTECH CONSULTANTS, INC. Active Earth Pressure * 35 pcf Passive Earth Pressure 300 pcf Coefficient of Friction 0.50 Soil Unit Weight 135 pcf Where: (i) pcf is pounds per cubic foot, and (ii) active and passive earth pressures are computed using the equivalent fluid pressures. * For a restrained wall that cannot deflect at least 0.002 times its height, a uniform lateral pressure equal to 10psf times the height of the wall should be added to the above active equivalent fluid pressure. The values given above are to be used to design only permanent foundation and retaining walls that are to be backfilled, such as conventional walls constructed of reinforced concrete or masonry. It is not appropriate to use the above earth pressures and soil unit weight to back -calculate soil strength parameters for design of other types of retaining walls, such as soldier pile, reinforced earth, modular or soil nail walls. The passive pressure given is appropriate only for a shear key poured directly against undisturbed native soil, or for the depth of level, compact fill placed in front of a retaining or foundation wall. The values for friction and passive resistance are ultimate values and do not include a safety factor. We recommend a safety factor of at least 1.5 for overturning and sliding, when using the above values to design the walls. Restrained wall soil parameters should be utilized for a distance of 1.5 times the wall height from corners or bends in the walls. This is intended to reduce the amount of cracking that can occur where a wall is restrained by a corner. The design values given above do not include the effects of any hydrostatic pressures behind the walls and assume that no surcharges, such as those caused by slopes, vehicles, or adjacent foundations will be exerted on the walls. If these conditions exist, those pressures should be added to the above lateral soil pressures. Where sloping backfill is desired behind the walls, we will need to be given the wall dimensions and the slope of the backfill in order to provide the appropriate design earth pressures. The surcharge due to traffic loads behind a wall can typically be accounted for by adding a uniform pressure equal to 2 feet multiplied by the above active fluid GEOTECH CONSULTANTS, INC. Seven Hills Properties February 28, 2012 density. Heavy construction equipment should not walls within a distance equal to the height of a wall, lateral pressures resulting from the equipment. Wall Pressures Due to Seismic Forces JN 12034 Page 10 be operated behind retaining and foundation unless the walls are designed for the additional The surcharge wall loads that could be imposed by the design earthquake can be modeled by adding a uniform lateral pressure to the above -recommended active pressure. The recommended surcharge pressure is 8H pounds per square foot (psf), where H is the design retention height of the wall. Using this increased pressure, the safety factor against sliding and overturning can be reduced to 1.2 for the seismic analysis. Retaining Wall Backfill and Waterproofing Backfill placed behind retaining or foundation walls should be coarse, free -draining structural fill containing no organics. This backfill should contain no more than 5 percent silt or clay particles and have no gravel greater than 4 inches in diameter. The percentage of particles passing the No. 4 sieve should be between 25 and 70 percent. If the native sand is used as backfill, a minimum 12 -inch width of free -draining gravel or a drainage composite similar to Miradrain 6000 should be placed against the backfilled retaining walls. The drainage composites should be hydraulically connected to the foundation drain system. Free -draining backfill or gravel should be used for the entire width of the backfill where seepage is encountered. For increased protection, drainage composites should be placed along cut slope faces, and the walls should be backfilled entirely with free -draining soil. The purpose of these backfill requirements is to ensure that the design criteria for a retaining wall are not exceeded because Of a build-up of hydrostatic pressure behind the wall. The top 12 to 18 inches of the backfill should consist of a compacted, relatively impermeable soil or topsoil, or the surface should be paved. The ground surface must also slope away from backfilled walls to reduce the potential for surface water to percolate into the backfill. It is critical that the wall backfill be placed in lifts and be properly compacted, in order for the above -recommended design earth pressures to be appropriate. The wall design criteria assume that the backfill will be well -compacted in lifts no thicker than 12 inches. The compaction of backfill near the walls should be accomplished with hand -operated equipment to prevent the walls from being overloaded by the higher soil forces that occur during compaction. The section entitled General Earthwork and Structural Fill contains additional recommendations regarding the placement and compaction of structural fill behind retaining and foundation walls. SHORING As recommended in the general section, regardless of the type of shoring wall constructed, a catchment wall with a freeboard height of at least 2 feet above the final grade on the uphill side of the wall should be constructed above the northern retaining wall. This catchment freeboard height will need to be maintained to provide adequate protection from any shallow sloughing of near - surface soils upslope of the wall. These recommendations are directed towards protecting the proposed development and areas downslope from soil only. Trees sometimes accompany even shallow slides as they occur on slopes. Trees can cause significant damage to structures, even GEOTECH CONSULTANTS, INC. Seven Hills Properties JN 12034 February 28, 2012 Page 11 heavily reinforced concrete walls. Removal of trees from areas above and on steep slopes is a heavily debated issue. While removal of the tree can eliminate the threat of the trunk and branches causing damage to the structure, the healthy root system can provide near -surface soil stabilization benefits. We generally recommend that any unhealthy or undermined trees be removed above the stump. Trees should be evaluated by a professional arborist on a case-by-case basis. The construction of a catchment wall as described above would protect only areas downslope from soil movement, and would not provide protection from trees or other debris. A variety of shoring systems are feasible for use at this site. This section presents design considerations for cantilevered and tied -back soldier -pile walls, and for nailed walls. Since the most suitable choice is primarily dependent on a number of factors under the contractor's control, we suggest that the contractor work closely with the structural engineer during the shoring design. As discussed above, the sensitivity of adjacent buildings and utilities must be considered in the design to reduce the risk of causing settlement of these adjacent elements. Regardless of the system used, all shoring systems will deflect in toward the excavation. Therefore, there is always a risk of noticeable settlement occurring on the ground behind the shoring wall. These risks are reduced, but not entirely eliminated, by using more rigid shoring systems, such as soldier piles. Depending on the required length of tieback anchors, easements may need to be obtained in order to install the anchors onto adjacent properties. The shoring design should be submitted to Geotech Consultants, Inc. for review prior to beginning site excavation. We are available and would be pleased to assist in this design effort. As discussed in the General section of this report, a 2 -foot -tall catchment should be placed at the top of the northern shoring wall, regardless of the wall type. An active pressure of 80 pcf should be used for this catchment wall. Cantilevered and Tied -Back Soldier Piles Cantilevered and tied -back soldier pile systems have proven to be an efficient and economical method for providing excavation shoring. Tied -back walls are typically more economical than cantilevered walls where the depth of excavation is greater than 15 feet. Soldier -Pile Installation Soldier -pile walls would be constructed after making planned cut slopes, and prior to commencing the mass excavation, by setting steel H -beams in a drilled hole and grouting the space between the beam and the soil with concrete for the entire height of the drilled hole. We anticipate that the holes could be drilled without casing, but the contractor should be prepared to case the holes or use the slurry method if caving soil is encountered. Excessive ground loss in the drilled holes must be avoided to reduce the potential for settlement on adjacent properties. If water is present in a hole at the time the soldier pile is poured, concrete must be tremied to the bottom of the hole. As excavation proceeds downward, the space between the piles should be lagged with timber, and any voids behind the timbers should be filled with pea gravel, or a slurry comprised of sand and fly ash. Treated lagging is usually required for permanent walls, while untreated lagging can often be utilized for temporary shoring GEOTECH CONSULTANTS, INC. Seven Hills Properties February 28, 2012 JN 12034 Page 12 walls. Temporary vertical cuts will be necessary between the soldier piles for the lagging placement. The prompt and careful installation of lagging is important, particularly in loose or caving soil, to maintain the integrity of the excavation and provide safer working conditions. Additionally, care must be taken by the excavator to remove no more soil between the soldier piles than is necessary to install the lagging. Caving or overexcavation during lagging placement could result in loss of ground on neighboring properties. Timber lagging should be designed for an applied lateral pressure of 30 percent of the design wall pressure, if the pile spacing is less than three pile diameters. For larger pile spacings, the lagging should be designed for 50 percent of the design load. Soldier -Pile Wall Design Permanent soldier -pile shoring that is cantilevered or restrained by one row of tiebacks, and that has a level backslope, should be designed for an active soil pressure equal to that pressure exerted by an equivalent fluid with a unit weight of 30 pounds per cubic foot (pcf). At the northern side of the site where the slope of approximately 40 degrees is located, the active pressure should increase to 60 pcf. To design northern tied -back shoring with more than one row of tiebacks, we recommend assuming that the lateral active soil pressure on the wall, expressed in pounds per square foot (psf), is equal to 40H, where H is the total height of the excavation in feet. Slopes differing from the 40 degree backslope angle above the shoring walls may also exert additional surcharge pressures. These surcharge pressures may vary from the above recommendations, depending on the configuration of the cut slope and shoring wall. We should review recommendations regarding slope and building surcharge pressures when the preliminary shoring design is completed. Catchment should be included in the shoring design. It is important that the shoring design provides sufficient working room to drill and install the soldier piles, without needing to make unsafe, excessively steep temporary cuts. Cut slopes should be planned to intersect the backside of the drilled holes, not the back of the lagging. Lateral movement of the soldier piles below the excavation level will be resisted by an ultimate passive soil pressure equal to that pressure exerted by a fluid with a density of 600 pcf. A safety factor of 1.5 should be included in a design of This soil pressure is valid only for a level excavation in front of the soldier pile; it acts on two times the grouted pile diameter. Cut slopes made in front of shoring walls significantly decrease the passive resistance. This includes temporary cuts necessary to install internal braces or rakers. The minimum embedment below the floor of the excavation for cantilever soldier piles should be equal to the height of the "stick-up." Tied -back soldier piles should be embedded no less than 12 feet below the lowest point of the excavation, including footing and utility excavations. The vertical capacity of soldier piles to carry the downward component of the tieback forces will be developed by a combination of frictional shaft resistance along the embedded length and pile end -bearing. GEOTECH CONSULTANTS, INC. Seven Hills Properties February 28, 2012 DESIGN VALUE Pile Shaft Friction 1,500 psf Pile End -Bearing 20,000 psf Where: (i) psf is pounds per square foot. JN 12034 Page 13 The above values assume that the excavation is level in front of the soldier pile and that the bottom of the pile is embedded a minimum of 10 feet below the floor of the excavation. For the pile end -bearing to be appropriate, the bottom of the drilled holes must be cleaned of loosened soil. The shoring contractor should be made aware of this, as it may affect their installation procedures. The concrete surrounding the embedded portion of the pile must have sufficient bond and strength to transfer the vertical load from the steel section through the concrete into the soil. TIEBACK ANCHORS General considerations for the design of tied -back or braced soldier -pile walls are presented on Plate 10. We recommend installing tieback anchors at inclinations between 20 and 30 degrees below horizontal. The tieback will derive its capacity from the soil -grout strength developed in the soil behind the no-load zone. The minimum grouted anchor length should be 10 feet. The no-load zone is the area behind which the entire length of each tieback anchor should be located. To prevent excessive loss -of -ground in a drilled hole, the no-load section of the drilled tieback hole should be backfilled with a sand and fly ash slurry, after protecting the anchor with a bond breaker, such as plastic casing, to prevent loads from being transferred to the soil in the no-load zone. The no-load section could be filled with grout after anchor testing is completed. During the design process, the possible presence of foundations or utilities close to the shoring wall must be evaluated to determine if they will affect the configuration and length of the tiebacks. Based on the results of our analyses and our experience at other construction sites, we suggest using an adhesion value of 2,000 psf in the (very dense sand) to design temporary anchors, if the mid -point of the grouted portion of the anchor is more than 10 feet below the overlying ground surface. This value applies to non -pressure - grouted anchors. Pressure -grouted or post -grouted anchors can often develop adhesion values that are two to three times higher than that for non -pressure - grouted anchors. These higher adhesion values must be verified by load testing. Soil conditions, soil -grout adhesion strengths, and installation techniques typically vary over any site. This sometimes results in adhesion values that are lower than anticipated. Therefore, we recommend substantiating the anchor design values by load -testing all tieback anchors. At least two anchors in each soil type encountered should be performance -tested to 200 percent of the design anchor load to evaluate possible anchor creep. Wherever possible, the no-load section of these tiebacks should not be grouted until the performance tests are completed. Unfavorable GEOTECH CONSULTANTS, INC. Seven Hills Properties February 28, 2012 JN 12034 Page 14 results from these performance tests could require increasing the lengths of the tiebacks. The remaining anchors should be proof -tested to at least 135 percent of their design value before being "locked off." After testing, each anchor should be locked off at a prestress load of 80 to 100 percent of its design load. If caving or water -bearing soil is encountered, the installation of tieback anchors will be hampered by caving and soil flowing into the holes. It will be necessary to case the holes, if such conditions are encountered. Alternatively, the use of a hollow - stem auger with grout pumped through the stem as the auger is withdrawn would be satisfactory, provided that the injection pressure and grout volumes pumped are carefully monitored. All drilled installations should be grouted and backfilled immediately after drilling. No drilled holes should be left open overnight. Soil Nailing Soil nailing is a relatively new shoring system where closely spaced, tieback anchors (nails) are grouted into drilled holes in the cut face as the excavation proceeds, thereby reinforcing the cut face. More anchors are required for this system than for conventional systems, but steel soldier piles and timber lagging are eliminated. The anchored or nailed system essentially operates as a reinforced soil wall or a gravity wall, with the nails tying the soil mass together. We recommend that an allowable adhesion value of 2,000 pounds per square foot (psf) be used for the design of the soil nails. Due to the steep nature of the northern slope, the initial, upper row of anchors should be placed before any cuts into the slope are made. Then, 4- to 6 -foot vertical cuts may be made in the shoring area followed immediately by the placement of anchors. The cut face is then covered with a wire mesh, and shotcrete is placed over the mesh and soil face. Generally, no temporary, unsupported excavations for soil -nail walls should be allowed to stand longer than 12 hours without the acceptance of the geotechnical engineer. Once the shotcrete has hardened, the excavation again proceeds and the nails are placed. A geotextile drainage composite must be placed over the face of the cut prior to shotcreting to prevent buildup of hydrostatic pressures behind the shotcrete facing. As the excavation progresses downward, the drainage composite strips are extended, until reaching the base of the excavation, where weep holes are placed through the shotcrete to be tied into an acceptable conveyance system. Because soil nails are passive elements (they are not pre -stressed as tiebacks are), soil -nail walls will typically deflect more than a soldier -pile wall. This involves more risk of causing damage to adjoining utilities, streets, and other on -grade elements. The shoring designer should provide an estimate of the lateral deflection that is anticipated for the soil nail wall. Caving of loose or granular soils, or in zones of seepage, can require that the shoring contractor modify their installation techniques. This can increase the cost and time necessary to install the nailed wall. We recommend that the shoring contractor be consulted regarding potential difficulties and modifications that can occur during the construction of a soil -nailed wall. This adhesion value should be substantiated by load -testing at least two anchors in each soil type to at least 200 percent of their design capacity, prior to installing production GEOTECH CONSULTANTS, INC. Seven Hills Properties February 28, 2012 JN 12034 Page 15 anchors. During shoring construction, at least 5 percent of the production anchors should be proof -tested to 130 percent of the design anchor capacity. The shoring designer will likely utilize one of several commercially available computer programs to design the nailed walls. We recommend that the following soil strength parameters be used in the nail wall design: Soil Type Moist Unit Effective Internal miction Effective Cohesion (psf), Weight (pcf) Angle (degrees) 1 Medium Dense native sand ••3 - feet) The shoring designer must take into consideration the steepness of the northern slope (40 percent and the need for 2 feet of catchment. Consideration of the loose condition of the near -surface soils must also be considered in the design and construction of the system. Excavation and Shoring Monitoring As with any shoring system, there is a potential risk of greater -than -anticipated movement of the shoring and the ground outside of the excavation. This can translate into noticeable damage of surrounding on -grade elements, such as foundations and slabs. Therefore, we recommend making an extensive photographic and visual survey of the project vicinity, prior to demolition activities, installing shoring or commencing excavation. This documents the condition of buildings, pavements, and utilities in the immediate vicinity of the site in order to avoid, and protect the owner from, unsubstantiated damage claims by surrounding property owners. Additionally, the shoring walls should be monitored during construction to detect soil movements. To monitor their performance, we recommend establishing a series of survey reference points to measure any horizontal deflections of the shoring system. Control points should be established at a distance well away from the walls and slopes, and deflections from the reference points should be measured throughout construction by survey methods. At least four points should be established on top of the shoring wall and should be monitored during construction. Additionally, benchmarks installed on any surrounding buildings should be monitored for at least vertical movement. We suggest taking the readings at least once a week, until it is established that no deflections are occurring. The initial readings for this monitoring should be taken before starting any demolition or excavation on the site. SLABS -ON -GRADE The building floors can be constructed as slabs -on -grade atop the native soils underlying the surface of the site, or on structural fill, or on previously placed fill that has been re -compacted. The subgrade soil must be in a firm, non -yielding condition at the time of slab construction or underslab fill placement. Any soft areas encountered should be excavated and replaced with select, imported structural fill. Even where the exposed soils appear dry, water vapor will tend to naturally migrate upward through the soil to the new constructed space above it. This can affect moisture -sensitive flooring, cause imperfections or damage to the slab, or simply allow excessive water vapor into the space above GEOTECH CONSULTANTS, INC. Seven Hills Properties JN 12034 February 28, 2012 Page 16 the slab. All interior slabs -on -grade should be underlain by a capillary break or drainage layer consisting of a minimum 4 -inch thickness of gravel or crushed rock that has a fines content (percent passing the No. 200 sieve) of less than 3 percent and a sand content (percent passing the No. 4 sieve) of no more than 10 percent. As noted by the American Concrete Institute (ACI) in the Guides for Concrete Floor and Slab Structures, proper moisture protection is desirable immediately below any on -grade slab that will be covered by tile, wood, carpet, impermeable floor coverings, or any moisture -sensitive equipment or products. ACI also notes that vapor retarders, such as 6 -mil plastic sheeting, have been used in the past, but are now recommending a minimum 10 -mil thickness. A vapor retarder is defined as a material with a permeance of less than 0.3 perms, as determined by ASTM E 96. It is possible that concrete admixtures may meet this specification, although the manufacturers of the admixtures should be consulted. Where vapor retarders are used under slabs, their edges should overlap by at least 6 inches and be sealed with adhesive tape. The sheeting should extend to the foundation walls for maximum vapor protection. If no potential for vapor passage through the slab is desired, a vapor barrier should be used. A vapor barrier, as defined by ACI, is a product with a water transmission rate of 0.01 perms when tested in accordance with ASTM E 96. Reinforced membranes having sealed overlaps can meet this requirement. In the recent past, ACI (Section 4.1.5) recommended that a minimum of 4 inches of well -graded compactable granular material, such as a 5/8 -inch -minus crushed rock pavement base, be placed over the vapor retarder or barrier for their protection, and as a "blotter" to aid in the curing of the concrete slab. Sand was not recommended by ACI for this purpose. However, the use of material over the vapor retarder is controversial as noted in current ACI literature because of the potential that the protection/blotter material can become wet between the time of its placement and the installation of the slab. If the material is wet prior to slab placement, which is always possible in the Puget Sound area, it could cause vapor transmission to occur up through the slab in the future, essentially destroying the purpose of the vapor barrier/retarder. Therefore, if there is a potential that the protection/blotter material will become wet before the slab is installed, ACI now recommends that no protection/blotter material be used. However, ACI then recommends that, because there is a potential for slab cure due to the loss of the blotter material, joint spacing in the slab be reduced, a low shrinkage concrete mixture be used, and 'other measures" (steel reinforcing, etc.) be used. ASTM E-1643-98 "Standard Practice for Installation of Water Vapor Retarders Used in Contact with Earth or Granular Fill Under Concrete Slabs" generally agrees with the recent ACI literature. We recommend that the contractor, the project materials engineer, and the owner discuss these issues and review recent ACI literature and ASTM E-1643 for installation guidelines and guidance on the use of the protection/blotter material. We recommend proof -rolling slab areas with a heavy truck or a large piece of construction equipment prior to slab construction. Any soft areas encountered during proof -rolling should be excavated and replaced with select, imported structural fill. EXCAVATIONS AND SLOPES Excavation slopes should not exceed the limits specified in local, state, and national government safety regulations. Temporary cuts to a depth of about 4 feet may be attempted vertically in unsaturated soil, if there are no indications of slope instability. However, vertical cuts should not be made near property boundaries, or existing utilities and structures. Based upon Washington Administrative Code (WAC) 296, Part N, the dense to very dense sand soil at the subject site GEOTECH CONSULTANTS, INC. Seven Hills Properties JN 12034 February 28, 2012 Page 17 would generally be classified as Type A. Therefore, temporary cut slopes greater than 4 feet in height should not be excavated at an inclination steeper than 0.75:1 (Horizontal: Vertical), extending continuously between the top and the bottom of a cut. The above -recommended temporary slope inclination is based on the conditions exposed in our explorations, and on what has been successful at other sites with similar soil conditions. It is possible that variations in soil and groundwater conditions will require modifications to the inclination at which temporary slopes can stand. Temporary cuts are those that will remain unsupported for a relatively short duration to allow for the construction of foundations, retaining walls, or utilities. Temporary cut slopes should be protected with plastic sheeting during wet weather. It is also important that surface water be directed away from temporary slope cuts. The cut slopes should also be backfilled or retained as soon as possible to reduce the potential for instability. Please note that sand or loose soil can cave suddenly and without warning. Excavation, foundation, and utility contractors should be made especially aware of this potential danger. These recommendations may need to be modified if the area near the potential cuts has been disturbed in the past by utility installation, or if settlement -sensitive utilities are located nearby. All new permanent cuts into native soil should be inclined no steeper than 1.5:1 (H:V). Fill slopes should not be constructed with an inclination greater than 2:1 (H:V). To reduce the potential for shallow sloughing, fill must be compacted to the face of these slopes. This can be accomplished by overbuilding the compacted fill and then trimming it back to its final inclination. Adequate compaction of the slope face is important for long-term stability and is necessary to prevent excessive settlement of patios, slabs, foundations, or other improvements that may be placed near the edge of the slope. Water should not be allowed to flow uncontrolled over the top of any temporary or permanent slope. All permanently exposed slopes should be seeded with an appropriate species of vegetation to reduce erosion and improve the stability of the surficial layer of soil. Any disturbance to the existing slope outside of the project limits may reduce the stability of the slope. Damage to the existing vegetation and ground should be minimized, and any disturbed areas should be revegetated as soon as possible. Soil from the excavation should not be placed on the slope, and this may require the off-site disposal of any surplus soil. DRAINAGE CONSIDERATIONS We anticipate that permanent foundation walls will be constructed against the shoring walls. Where this occurs, a plastic -backed drainage composite, such as Miradrain, Battledrain, or similar, should be placed against the entire surface of the shoring prior to pouring the foundation wall. Weep pipes located no more than 6 feet on -center should be connected to the drainage composite and poured into the foundation walls or the perimeter footing. A footing drain installed along the inside of the perimeter footing will be used to collect and carry the water discharged by the weep pipes to the storm system. Isolated zones of moisture or seepage can still reach the permanent wall where groundwater finds leaks or joints in the drainage composite. This is often an acceptable risk in unoccupied below -grade spaces, such as parking garages. However, formal waterproofing is typically necessary in areas where wet conditions at the face of the permanent wall will not be tolerable. If this is a concern, the permanent drainage and waterproofing system should be designed by a specialty consultant familiar with the expected subsurface conditions and proposed construction. GEOTECH CONSULTANTS, INC. Seven Hills Properties JN 12034 February 28, 2012 Page 18 Footing drains placed inside the building or behind backfilled walls should consist of 4 -inch, perforated PVC pipe surrounded by at least 6 inches of 1 -inch -minus, washed rock wrapped in a non -woven, geotextile filter fabric (Mirafi 140N, Supac 4NP, or similar material). At its highest point, a perforated pipe invert should be at least 6 inches below the level of a crawl space or the bottom of a floor slab, and it should be sloped slightly for drainage. Plate 9 presents typical considerations for footing drains. All roof and surface water drains must be kept separate from the foundation drain system. As a minimum, a vapor retarder, as defined in the Slabs -On -Grade section, should be provided in any crawl space area to limit the transmission of water vapor from the underlying soils. Also, an outlet drain is recommended for all crawl spaces to prevent a build up of any water that may bypass the footing drains. No groundwater was observed during our field work. If seepage is encountered in an excavation, it should be drained from the site by directing it through drainage ditches, perforated pipe, or French drains, or by pumping it from sumps interconnected by shallow connector trenches at the bottom of the excavation, The excavation and site should be graded so that surface water is directed off the site and away from the tops of slopes. Water should not be allowed to stand in any area where foundations, slabs, or pavements are to be constructed. Final site grading in areas adjacent to buildings should slope away at least 2 percent, except where the area is paved. Surface drains should be provided where necessary to prevent ponding of water behind foundation or retaining walls. PAVEMENT AREAS The pavement section may be supported on competent, native soil, on structural fill compacted to a 95 percent density. The pavement subgrade must be in a stable, non -yielding condition at the time of paving. Granular structural fill or geotextile fabric may be needed to stabilize soft, wet, or unstable areas. To evaluate pavement subgrade strength, we recommend that a proof roll be completed with a loaded dump truck immediately before paving. In most instances where unstable subgrade conditions are encountered, an additional 12 inches of granular structural fill will stabilize the subgrade, except for very soft areas where additional fill could be required. The subgrade should be evaluated by Geotech Consultants, Inc., after the site is stripped and cut to grade. Recommendations for the compaction of structural fill beneath pavements are given in the section entitled General Earthwork and Structural Fill. The performance of site pavements is directly related to the strength and stability of the underlying subgrade. The pavement for lightly loaded traffic and parking areas should consist of 2 inches of asphalt concrete (AC) over 4 inches of crushed rock base (CRB) or 3 inches of asphalt -treated base (ATB). We recommend providing heavily loaded areas with 3 inches of AC over 6 inches of CRB or 4 inches of ATB. Heavily loaded areas are typically main driveways, dumpster sites, or areas with truck traffic. Increased maintenance and more frequent repairs should be expected if thinner pavement sections are used. Water from planter areas and other sources should not be allowed to infiltrate into the pavement subgrade. The pavement section recommendations and guidelines presented in this report are based on our experience in the area and on what has been successful in similar situations. (( We can provide recommendations based on expected traffic loads and California Bearing Ratio (CBR) tests, if requested.)) As with any pavements, some maintenance and repair of limited areas can be GEOTECH CONSULTANTS, INC. Seven Hills Properties JN 12034 February 28, 2012 Page 19 expected as the pavement ages. Cracks in the pavement should be sealed as soon as possible after they become evident, in order to reduce the potential for degradation of the subgrade from infiltration of surface water. For the same reason, it is also prudent to seal the surface of the pavement after it has been in use for several years. To provide for a design without the need for any maintenance or repair would be uneconomical. GENERAL EARTHWORK AND STRUCTURAL FILL All building and pavement areas should be stripped of surface vegetation, topsoil, organic soil, and other deleterious material. It is important that existing foundations be removed before site development. The stripped or removed materials should not be mixed with any materials to be used as structural fill, but they could be used in non-structural areas, such as landscape beds. Structural fill is defined as any fill, including utility backfill, placed under, or close to, a building, behind permanent retaining or foundation walls, or in other areas where the underlying soil needs to support loads. All structural fill should be placed in horizontal lifts with a moisture content at, or near, the optimum moisture content. The optimum moisture content is that moisture content that results in the greatest compacted dry density. The moisture content of fill is very important and must be closely controlled during the filling and compaction process. The allowable thickness of the fill lift will depend on the material type selected, the compaction equipment used, and the number of passes made to compact the lift. The loose lift thickness should not exceed 12 inches. We recommend testing the fill as it is placed. If the fill is not sufficiently compacted, it can be recompacted before another lift is placed. This eliminates the need to remove the fill to achieve the required compaction. The following table presents recommended relative compactions for structural fill: LOCATION Or, iMINIMUM PLACE ME NT COM PACTION Beneath footings, slabs 95% or walkways Filled slopes and behind 90% retaining walls 95% for upper 12 inches of Beneath pavements subgrade; 90% below that level Where: Minimum Relative Compaction is the ratio, expressed in percentages, of the compacted dry density to the maximum dry density, as determined in accordance with ASTM Test Designation D 1557-91 (Modified Proctor). The sand soil at the site could very likely be used as structural fill provided it does not contain organics and/or is not excessively wet or dry. The sand will need to be compacted using vibratory equipment, preferably large equipment. Structural fill that will be placed in wet weather should consist of a coarse, granular soil with a silt or clay content of no more than 5 percent. The percentage of particles passing the No. 200 sieve should be measured from that portion of soil passing the three -quarter -inch sieve. GEOTECH CONSULTANTS, INC. Seven Hills Properties February 28, 2012 LIMITATIONS JN 12034 Page 20 The conclusions and recommendations contained in this report are based on site conditions as they existed at the time of our exploration and assume that the soil and groundwater conditions encountered in the test borings are representative of subsurface conditions on the site. If the subsurface conditions encountered during construction are significantly different from those observed in our explorations, we should be advised at once so that we can review these conditions and reconsider our recommendations where necessary. Unanticipated soil conditions are commonly encountered on construction sites and cannot be fully anticipated by merely taking soil samples in test borings. Subsurface conditions can also vary between exploration locations. Such unexpected conditions frequently require making additional expenditures to attain a properly constructed project. It is recommended that the owner consider providing a contingency fund to accommodate such potential extra costs and risks. This is a standard recommendation for all projects. The recommendations presented in this report are directed toward the protection of only the proposed development from damage due to slope movement. Predicting the future behavior of steep slopes and the potential effects of development on their stability is an inexact and imperfect science that is currently based mostly on the past behavior of slopes with similar characteristics. Landslides and soil movement can occur on steep slopes before, during, or after the development Of property. The use of a catchment wall will deter such movement from reaching the development. This report has been prepared for the exclusive use of Seven Hills Properties, and its representatives, for specific application to this project and site. Our recommendations and conclusions are based on observed site materials, and selective laboratory testing and engineering analyses. Our conclusions and recommendations are professional opinions derived in accordance with current standards of practice within the scope of our services and within budget and time constraints. No warranty is expressed or implied. The scope of our services does not include services related to construction safety precautions, and our recommendations are not intended to direct the contractor's methods, techniques, sequences, or procedures, except as specifically described in our report for consideration in design. Our services also do not include assessing or minimizing the potential for biological hazards, such as mold, bacteria, mildew and fungi in either the existing or proposed site development. ADDITIONAL SERVICES In addition to reviewing the final plans, Geotech Consultants, Inc. should be retained to provide geotechnical consultation, testing, and observation services during construction. This is to confirm that subsurface conditions are consistent with those indicated by our exploration, to evaluate whether earthwork and foundation construction activities comply with the general intent of the recommendations presented in this report, and to provide suggestions for design changes in the event subsurface conditions differ from those anticipated prior to the start of construction. However, our work would not include the supervision or direction of the actual work of the contractor and its employees or agents. Also, job and site safety, and dimensional measurements, will be the responsibility of the contractor. GEOTECH CONSULTANTS, INC. Seven Hills Properties February 28, 2012 J N 12034 Page 21 During the construction phase, we will provide geotechnical observation and testing services when requested by you or your representatives. Please be aware that we can only document site work we actually observe. It is still the responsibility of your contractor or on-site construction team to verify that our recommendations are being followed, whether we are present at the site or not. The following plates are attached to complete this report: Plate 1 Vicinity Map Plate 2 Site Exploration Plan Plates 3 - 8 Test Boring Logs Plate 9 Typical Footing Drain Detail Plate 10 Tied -Back Shoring Detail We appreciate the opportunity to be of service on this project. If you have any questions, or if we may be of further service, please do not hesitate to contact us. JLH/DRW: jyb Respectfully submitted, GEOTE , -I :,ON ' . 'TS, INC. ion L. Hinds eotechnical Engineer 2-/L �/► 'z - D. Robert Ward, P.E. Principal GEOTECH CONSULTANTS, INC. I(Source: Microsoft Streets and Trips, 2004) GEOTECH CONSULTANTS, INC. VICINITY MAP 9801 Edmonds Way Edmonds, Washington Job No: Date: Plate: 12034 1 Feb. 2012 1 1 1 it varles GEOTECH CONSULTANTS, INC. SITE EXPLORATION PLAN 9801 Edmonds Way Edmonds, Washington Job No: Date: Plate: 2 12034 Feb. 2012 5 10 `R K11 25 BORING 1 Description Approximate Elevation 345' * Test boring log continued on next page. GEOTECH CONSULTANTS, INC. ;e TEST BORING LOG 9801 Edmonds Way Edmonds, Washington Job Date: Logged by: I Plate: 12034 Feb. 2012 JLH :3] BORING 1 (continued) 4 C' Description 25 30 35 40 45 50 71 15 -wet, clean sand lense SP 91 7 M Ex -with only trace gravel, no silt -becomes mostly fine grained, no gravel, dense * Test boring was terminated on February 14, 2012 at 46.5 feet. * Groundwater was not encountered during drilling. GEOTECH CONSULTANTS, INC. TEST BORING LOG 9801 Edmonds Way Edmonds, Washington Job Date: Logged by. I Plate: 12034 1 Feb. 2012 1 JLH 4 61 10 15 K11 25 BORING 2 Description Approximate Elevation 335' i est coring was terminatea on reoruary -14, zu-iz at zb.o reet. * Groundwater was not encountered during drilling. GEOTECH CONSULTANTS, INC. TEST BODING LOG 9801 Edmonds Way Edmonds, Washington Job Date: Logged by: Plate: 5 12034 Feb. 2012 JLH °0� ����o�\oy��a��p� �e��e JSC) MIE 65 13 10 Description Approximate Elevation 325' 1 4 inches of asphalt pavement over; Light gray to brown SAND with gravel and trace silt, fine to medium grained, moist, dense -less silt -with some coarse grained sand 63 4 -becomes mostly fine grained, with no gravel 15- 41 5 =: -with some coarse grained sand, and gravel * Test boring was terminated on February 15, 2012 at 16.5 feet. * Groundwater was not encountered during drilling. PRI 25 GEOTECH CONSULTANTS, INC. TEST BORING LOG 9801 Edmonds Way Edmonds, Washington Job Date: Logged by: I Plate: 6 12034 Feb. 2012 JLH 5 IN 15 20 25 1 �, °1Pe 0° \e G� i 27 13 IMPME ■ BORING 4 Description Approximate Elevation 323.5'1 4 inches of asphalt pavement over; Orange brown, slightly silty SAND with gravel, fine to medium grained, with black, orange and gray sand, very moist, loose to medium dense (Possible FILL) Light gray to light brown SAND with gravel, medium grained, moist, dense -with some coarse sand and trace silt • Test boring was terminated on February 15, 2012 at 16.5 feet. * Groundwater was not encountered during drilling, GEOTECH CONSULTANTS, INC. TEST BORING LOG 9801 Edmonds Way Edmonds, Washington Job Date: Logged by. I Plate: 12034 Feb. 2012 JLH :7] 5 10 15 20 25 BORING 5 ��� ��a�'�eQ,�°� e�� 5��� J5�'� Description Approximate Elevation 327' 4 inches of topsoil over; Light gray SAND with gravel and trace silt, fine grained to medium grained, moist, dense 40 1 -less silt 39 12 ff-11111111 I * Test boring was terminated on February 15, 2012 at 16.5 feet. * Groundwater was not encountered during driling. GEOTECH CONSULTANTS, INC. TEST BORING LOG 9801 Edmonds Way Edmonds, Washington Job Date: Logged by: I Plate: 8 12034 Feb. 2012 JLH Slope backfill away from foundation. Provide surface drains where necessary. Tightline Roof Drain (Do not connect to footing drain) Backfill (See text for requirements) 4" min. o o IIIIII [EIJI Vapor Retarder/Barrier and Capillary Break/Drainage Layer (Refer to Report text) 4" Perforated Hard PVC Pipe (Invert at least 6 inches below slab or crawl space. Slope to drain to appropriate outfall. Place holes downward.) NOTES: (1) In crawl spaces, provide an outlet drain to prevent buildup of water that bypasses the perimeter footing drains. (2) Refer to report text for additional drainage, waterproofing, and slab considerations. GEOTECH CONSULTANTS, INC. FOOTING DRAIN DETAIL 9801 Edmonds Way Edmonds, Washington Job No: Date: Plate: 12034 1 Feb. 2012 1 1 9 Nonwoven Geotextile 1=filter Fabric Washed Rock Possible Slab (7/8" min. size) oQ oQ p O p p a po.0 0 °oopoo. Qp0 poo. . 'uO poo.. 'Jeop. 09 �o oo Q Lo Q CCa OQp�o 0,ao 0p�a 0Qo00 ���•.O Q ;i 00OdQ p00��o.do e °0oO� OeoO O,O p O Q Q Q oQv 4" min. o o IIIIII [EIJI Vapor Retarder/Barrier and Capillary Break/Drainage Layer (Refer to Report text) 4" Perforated Hard PVC Pipe (Invert at least 6 inches below slab or crawl space. Slope to drain to appropriate outfall. Place holes downward.) NOTES: (1) In crawl spaces, provide an outlet drain to prevent buildup of water that bypasses the perimeter footing drains. (2) Refer to report text for additional drainage, waterproofing, and slab considerations. GEOTECH CONSULTANTS, INC. FOOTING DRAIN DETAIL 9801 Edmonds Way Edmonds, Washington Job No: Date: Plate: 12034 1 Feb. 2012 1 1 9 See text for design pressure on catchment portion of wall 2ft Lowest Excavation Elevation (Assumed to be Level) W D (10min) 600(D) (psf) Passive Pressure Notes: No Load Zone +/-25- ' 40(H) ' (psf) Active Pressure 600 Existing Slope 0.15H Locate All Anchors Behind This Line 0.15H Tieback Anchors (2,000 psf Allowable Adhesion) (1) The report should be referenced for specifics regarding design and installation. (2) Active pressures act over the pile spacing. (3) Passive pressures act over twice the grouted soldier pile diameter or the pile spacing, whichever is smaller. (4) It is assumed that no hydrostatic pressures act on the back of the shoring walls. (5) Slopes, traffic loads, and/or adjacent building foundations positioned above or behind shoring (differing from report recommendations) will exert additional pressures on the shoring wall. (6) See report for recommendations regarding soldier pile walls with single row of tieback anchors. GEOTECH CONSULTANTS, INC. TIE BACK SHORING DETAIL 9801 Edmonds Way Edmonds, Washington Job No: Date: Plate: 10 12034 Feb. 2012