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235th GeoTech Report.pdfOEOrrIECH CONSULTAN-FS, INC. 13256 Northeast 20th Street, Suite 16 Bellevue, Washington 98005 (425) 747-5618 FAX (425) 747-8561 October 30, 2006 Carmen Crispeno JN 05413 22232 — 17th Avenue Southeast Bothell, Washington 98021 Subject: Transmittal Letter — Geotechnical Engineering Study RECNEIVED Proposed Residential Short -Plat NOV 2 0 2006 98)(X — 235th Place Southwest Edmonds, Washington PERMIT COUNTER Dear Mr. Crispeno: We are pleased to present this geotechnical engineering report for the two proposed residences to be constructed 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 and retaining walls, This work was authorized by your acceptance of our proposal, P-6880, dated October 12, 2005- 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. ZJM/DRW: jyb Respectfully submitted, GEOTECH CONSULTANTS, INC. 0"W. e-�- D. Robert Ward, P.E. Principal GEOTECH CONSULTANTS, INC- Attachment 5 GEOTECHNICAL ENGINEERING STUDY Jwo Proposed Residences 98XX — 235th Place Southwest Edmonds, Washington This report presents the findings and recommendations of our geotechnical engineering study for the site of the two proposed residences to be located in Edmonds, Washington. We were initially provided with a topographic map of the site- Recently we were provided with a "Frontage Improvement Plan", which was prepared by DVG Enterprises. Based on the topographic map and the plan, we understand that the existing residential lot will be separated into three lots, with the existing residence and pool on the pastern side of the property remaining in one lot- The western portion of the site is currently undeveloped, and it will be plafted into two residential lots. The residences will be located on the western side of the new lots, located as close as 5 feet from the western property line. Both residences will have a lower level garage, and will be set back the minimum required setback distance of 5 feet. A main and upper level will be located above the garage. The main floor will be the lowest floor on the eastern, upslope sides of the two residences. Cuts of up to 12 feet of are proposed on the eastern side of the garage, while cuts of approximately 6 feet are proposed on the eastern side of the main level. 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 existing residential site in Edmonds. The residential property is rectangular in shape, located on the southern side of the right-of-way of 235th Place Southwest. The site is relatively flat on the eastern side, but slopes down on the western slope. At the top of the slope are an existing two-story residence and a pool. The residence and pool will remain after the lot is short-plafted. The western portion of the site is generally undeveloped, and the vegetation is relatively light. The slope in the middle of the property nearest the flat portion is very steep, with an inclination of approximately 64 percent slope on the northern side and a 71 percent slope on the southern side. This very steep slope is about 25 feet tall. Although the slope is steep, we did not observe indications of soil instability. The slope then flattens on the western side of the property to approximately 25 to 30 percent. An existing paved driveway is located on the western edge of the property that provides access to a residence to the South - SUBSURFACE The subsurface conditions were explored by excavating four test pits and two 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. GEOTECH CONSULTANTS, INC- Caanen Crispeno JN 05413 October 30, 2006 Page 2 The test pits were excavated on November 1, 2005 with a rubber -tired backhoe. A geotechnical engineer from our staff observed the excavation process, logged the test pits, and obtained representative -samples of the soil encountered. "Grab" samples of selected subsurface soil were collected from the backhoe bucket. The Test Pit Logs are attached to this report as Plates 3 and 4. Test Boring 1 was drilled on October 27, 20qf using a track -mounted, hollow -stem auger drill. While Test Boring 2 was drilled on November 10, 2005 using a portable Acker drill. This drill system utilizes a small, gasoline -powered engine to advance a hollow -stem auger to the sampling depth. 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 5 and 6. Soil Conditions The two test borings were drilled on the top of the slope, west of the existing single-family residence. Four test pits were excavated on the lower, flatter portion of the site. The test borings encountered approximately 5 to 8 feet of loose to medium -dense, silty sand and gravel. Most of this soil is likely native, but a small portion at the ground surface may be fill soil. Below these depths, the silty sand with gravel became dense to very dense to the maximum explored depth of 20.5 feet. The dense to very dense soil is known as glacial till. Test Pit 1 encountered 2 feet of loose fill overlying medium -dense silt, while weathered, relatively loose, silty sand with gravel was revealed in the other test pits near the ground surface. Dense to very dense glacial till was revealed in the test pits at depths ranging from approximately 1 to 4 feet below the ground surface. No obstructions were revealed by our explorations. However, debris and buried utilities will probably be encountered in the soil that has been placed on the site during the development of the surrounding properties. Although our explorations did not encounter cobbles or boulders, they are often found in soils that have been deposited by glaciers. Groundwater Conditions No groundwater seepage was observed during our explorations. The test pits and 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. It should be noted that groundwater levels vary seasonally with rainfall and other factors, and wet zones were encountered near 25 feet in Boring 1. During the normally wet winter and spring months, we anticipate that groundwater could be found in more permeable soil layers within the glacial till and/or between the near -surface weathered soil and the underlying glacial till. 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. If a transition in soil type occurred between samples in the GEOTECH CONSULTANTS, INC- Carmen Crispeno JN 05413 October 30, 2006 Page 3 borings, the depth of the transition was interpreted. The relative densities and moisture descriptions indicated on the test pit and boring logs are interpretive descriptions based on the conditions observed during excavation and drilling. The compaction of backfill was not in the scope of our services. Loose soil will therefore be found in the area of the test pits. If this presents a problem, the backfill will need to be removed and replaced with structural fill during construction - 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 pits and borings conducted for this study encountered dense to very dense glacial till underlying loose to medium -dense weathered and fill soils at depths ranging from approximately 1 to 8 feet below the ground surface- The glacial till appears to generally exist at depths less than 4 feet in the two residence areas- Based on the soil conditions encountered in our explorations, it is our opinion that the new proposed residences can be supported on conventional continuous and spread footings bearing on the dense to very dense glacial till. Depending on final site grades some overexcavation may be required to expose the competent, native glacial till. Prior to pouring the footings a geotechnical engineer from our firm should observe the footing subgrade conditions to ensure that suitable bearing soils have been exposed. The glacial till soil is silty and thus is moisture sensitive. It may be necessary to hand clean bearing surfaces during periods of wet weather or protect them with a mat of imported, granular fill. Per the Edmonds Community Development Code (ECDC) section 23.80.020, the steep portion of the site, which is on the eastern side of the two proposed residential lots, is designated as a Landslide Hazard Area because its inclination is greater than 40 percent over a height of greater than 10 feet. Even though it is designated as such, we did not observe any indications of soil instability of this slope. In addition, due to the silty nature of the site soil, any portion of the site that is inclined steeper than 15 percent is an Erosion Hazard Area. Per the development standards of the ECDC, no building on Erosion Hazard Areas is recommended. If this recommendation were strictly followed, very little if any of the two residences would be possible. For Landslide Hazard Areas, a minimum buffer of 50 feet from the steep slope is first recommended, but can be reduced to 10 feet. However, even the use of a minimum buffer of 10 feet, and a minimal required building setback from the western property lines of 5 feet, the width of the proposed residences would be unreasonable. The two residences are proposed in Erosion Hazard Areas of the site, in the buffers, and also slightly into the Landslide Hazard Area on the sites; we believe this is very feasible from a geotechnical engineering standpoint because the core soils of the site is dense to very dense glacial till provided recommendations in this study are followed. An Alteration of an Erosion Hazard Area and Landslide Hazard Area and/or buffers can occur per the ECDC is a hazards analysis is submitted- The following requirements must be met for an Alteration to be allowed (our comments regarding these requirements are shown in italics)- GEOTECH CONSULTANTS, INC. Carmen Crispeno October 30, 2006 JN 05413 Page 4 a) The development will not increase surface water discharge or sedimentation to adjacent properties beyond pre-devetopment conditions. A stormwater drainage system will be designed- for all impervious surfaces on the site, and the sites will be landscaped. Therefore, this requirement is met in our professional opinion. b) The development will not decrease slope stability on adjacent properties. The foundations of the residences will be designed as retaining walls that will support the hillside above. Therefore, the residences will somewhat increase the stability of the property above. The development will have no impact on slope stability of the north and south adjacent properties because excavations will not be close to the adjoining property lines. The development will have no impact on slope stability of the property to the west because that property is below the residences and Well away from the residences. c) Such alterations will not adversely impact other critical areas. It appears that the only adjacent critical areas are Erosion Hazard Areas to the north and south. As noted in b) no excavations will be made near the north and south property lines, therefore the adjacent critical areas will not be adversely impacted. Development standards are also discussed in the ECDC. Seven standards need to be followed. The standard and our comments regarding how we believe the standards are being maintained are as follows: a) The proposed development shall not decrease the factor of safety for landslide occurrence below the limit of 1.5 for static conditions and 1.2 for dynamic conditions. The core of the site is comprised of dense to very dense glacial till. These soils have an existing factor of safety against landslide occurrence of well over these limits. In addition, foundation and retaining walls on the site will be designed to meet these standards once our recommendations given in this study are followed. b) Structures and improvements shall be clustered to avoid geologically hazardous areas and other critical areas. The residences are located as far west as is required, which is on the flattest portion of the site. The residences are located as close to each other as allowed by land use setbacks. c) Structures and improvements shall minimize alteration to the natural contour of the slope, and d) Foundation shall be tiered where possible to preserve the most critical portions of the site and its natural landforms and vegetation. Most of the residences will be located in the flatter portion of the site, A minimum length garage is proposed for the lower level of the residences; the main level steps up above the eastern side of the garage. Thus, the residences are being tiered as much as possible. Although the western portion of the steep slope will be built on, the eastern upper portion will not. e) The proposed development shall not result in a greater risk or need for increased buffers on neighboring properties. The steep slope is basically contained on the site, therefore we cannot see any reason the neighboring properties will be affected in this way. GEOTECH CONSULTANTS, INC. Carmen Crispeno October 30, 2006 JN 05413 Page 5 f)i The use of retaining walls that allow the maintenance of existing natural slope areas is preferred over graded artific,ial slopes. The foundation walls will be used to support the steep eastern slope,- that slope, will not be artificially graded- g) Development shall be designed to minimize impervious lot coverage. The residences have minimal driveways and the residence size is consistent with those in the neighborhood. 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. Existing catch basins in, and immediately downslope of, the planned work areas should be protected with pre -manufactured silt socks- Cut slopes and soil stockpiles should be covered with plastic during wet weather. Following rough grading, it may be necessary to mulch or hydroseed bare areas that will not be immediately covered with landscaping or an impervious surface. Other measures may be needed that are in accordance with Best Management Practices. The drainage and/or waterproofing recommendations presented in this report are intended only to prevent active seepage from flowing through concrete walls or slabs. Even in the absence of active seepage into and beneath structures, water vapor can migrate through walls, slabs, and floors from the surrounding soil, and can even be transmitted from slabs and foundation walls due to the concrete curing process. Water vapor also results from occupant uses, such as cooking and bathing. Excessive water vapor trapped within structures can result in a variety of undesirable conditions, including, but not limited to, moisture problems with flooring systems, excessively moist air within occupied areas, and the growth of molds, fungi, and other biological organisms that may be harmful to the health of the occupants. The designer or architect must consider the potential vapor sources and likely occupant uses, and provide sufficient ventilation, either passive or mechanical, to prevent a build up of excessive water vapor within the planned structure. 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 Table 1615.1.1 of the 2003 International Building Code (IBC), the site soil pro- file within 100 feet of the ground surface is best represented by Soil Profile Type C (Very Dense Soil). The site soils are not susceptible to seismic liquefaction because of their dense nature and the absence of near -surface groundwater. GEOTECH CONSULTANTS, INC. Carmen Crispeno October 30, 2006 CONVENTIONAL FOUNDATIONS JN 05413 Page 6 The proposed structure can be supported on conventional continuous and spread footings bearing on undisturbed, dense to very dense, native glacial till. Depending on the final site grades, overexcavation may be required below the footings to expose this soil. 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. An allowable bearing pressure of 3,000 pounds per square foot (psf) is appropriate for footings supported on dense to very dense glacial till soil. A one-third increase in this design bearing pressure 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 dense to very dense glacial till soil will be approximately one-half inch, with differential settlements on the order of less than half an 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 structural fill. We recommend using the following ultimate values for the foundation's resistance to lateral loading: ULTINIATE PARAMETER VALUE oe icient 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. 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. 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 backfill: GEOTECH CONSULTANTS, JNC_ Carmen Crispeno October 30, 2006 PARAMETER VALUE Active Earth Pressure * 35 pcf - level backslope Active Earth Pressure * 50 pcf - backslope inclined between 2:1 (H:V) and 3:1 (H:V) 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 10 psf times the height of the wall should be added to the above active equivalent fluid pressure. JN 05413 Page 7 The values given above are to be used to design permanent foundation and retaining walls only- It is not appropriate to back -calculate soil strength parameters from the earth pressures and soil unit weights presented in the table. The passive pressure given is appropriate for the depth of level structural fill placed in front of a retaining or foundation wall only. 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 comers 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 density. WaH Pressures Due to Seismic Forces A dynamic analysis of the structure and retaining walls should be conducted. To model the surcharge wall loads that could be imposed by the design earthquake, we recommend 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. GEOTECH CONSULTANTS. INC. Carmen Crispeno October 30, 2006 JN 05413 Page 8 Heavy construction equipment should not be operated behind retaining and foundation walls within a distance equal to the height of -...a wall, unless the walls are designed for the additional lateral pressures resulting from the equipment- 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. Retaininq Wall Backfill and Waterproofin 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 on -site soil is used as backfill, a minimum 12-inch width of free -draining gravel and 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 later section entitled Drainage Considerations should also be reviewed for recommendations related to subsurface drainage behind foundation and retaining walls. 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. The section entitled General Earthwork and Structural Fill contains recommendations regarding the placement and compaction of structural fill behind retaining and foundation walls - The above recommendations are not intended to waterproof below -grade walls, or to prevent the formation of mold, mildew or fungi in interior spaces. Over time, the performance of subsurface drainage systems can degrade, subsurface groundwater flow pattems can change, and utilities can break or develop leaks. Therefore, waterproofing should be provided where future seepage through the walls is not acceptable. This typically includes limiting cold -joints and wall penetrations, and using bentonite panels or membranes on the outside of the walls. There are a variety of different waterproofing materials and systems, which should be installed by an experienced contractor familiar with the anticipated construction and subsurface conditions. Applying a thin coat of asphalt emulsion to the outside face of a wall is not considered waterproofing, and will only help to reduce moisture generated from water vapor or capillary action from seeping through the concrete. As with any project, adequate ventilation of basement and crawl space areas is important to prevent a build up of water vapor that is commonly transmitted through concrete walls from the surrounding soil, even when seepage is not present. This is appropriate even when waterproofing is applied to the outside of foundation and retaining walls. We recommend that you contact a specialty consultant if detailed recommendations or specifications related to waterproofing design, or minimizing the potential for infestations of mold and mildew are desired. GEOTECH CONSULTANTS, INC. Carmen Crispeno October 30, 2006 JN 05413 Page 9 The General, Slabs -On -Grade, and Drainage Considerations sections should be reviewed for additional recom mendations related to the control of groundwater and excess water vapor for the anticipated construction. SLABS -ON -GRADE The building floors can be constructed as slabs -on -grade atop native glacial till, or on structural fill. 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. All interior slabs -on -grade must 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, are typically used. A vapor retarder is defined as a material with a permeance of less than 0.3 US perms per square foot (pso per hour, 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 plastic sheeting is used under slabs, joints 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 0-00 perms per square foot per hour 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, should be placed over the vapor retarder or barrier for protection of the retarder or barrier 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 GEOTECH CONSULTANTS, INC- Carmen Crispeno JN 05413 October 30, 2006 Page 10 on the use of the protection/blotter material. Our opinion is that with impervious surfaces that all means should be undertaken to reduce water vapor transmission. The General, Permanent Foundation and Retaining Waits, and Drainage Considerations sections should be reviewed for additional recommendations related to the control of gro.undwater and excess water vapor for the anticipated construction. 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 glacial till soil at the subject site would generally be classified as Type A. Therefore, temporary cut slopes greater than 4 feet in height in this soil 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 upper, looser weathered till and fill soils at the site would generally be classified as Type B. Thus, temporary cut slopes greater than 4 feet in height in this soil should not be excavated steeper than 1:1 (H:V), extending continuously between the top and the bottom of a cut. The above-recorn mended temporary slope inclinations are 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 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 permanent cuts into native soil should be inclined no steeper than 2:1 (H-V). Compacted fill slopes should also 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. Topsoil is often placed on regraded slopes to promote growth of vegetation. Proper preparation of the regraded surface, and use of appropriate topsoil is necessary to prevent the topsoil from sliding off the slope. This is most likely to occur following extended wet weather if a silty topsoil is used. On steeper slopes, it may be necessary to "track walk" the slope or cut small grooves across the slope prior to placing the topsoil- GEOTECH CONSULTANTS, INC. Carmen Crispeno October 30, 2006 DRAINAGE CONSIDERATIONS JN 05413 Page 11 Foundation drains should be used where (1) crawl spaces or basements will be below a structure, (2) a slab is below the outside grade, (3) the outside grade does not slope downward from a building, or where an interior foundation exists between two floor levels. Drains should also be placed at the base of all earth -retaining walls. These drains should be surrounded by at least 6 inches of 1-inch-minus, washed rock and then wrapped in 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 bottom of a slab floor or the level of a crawl space, and it should be sloped for drainage- All roof and surface water drains must be kept separate from the foundation drain system. A typical drain detail is attached to this report as Plate 7. For the best long-term performance, perforated PVC pipe is recommended for all subsurface drains. 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, however, wet zones were encountered during drilling. 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. Additionally, a drainage Swale should be provided upslope of the buildings to intercept surface run-off and direct it into the storm drains. Water from roof, storm water, and foundation drains should not be discharged onto slopes-, it should be tightlined to a suitable outfall located away from any slopes. GENERAL EARTHWORK AND STRUCTURAL FILL All building and pavement areas should be stripped of surface vegetation, topsoil, organic soil, and other deleterious material. 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. Fills placed on sloping ground should be keyed into the native soils. This is typically accomplished by placing and compacting the structural fill on level benches that are cut into the competent soils. 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 GEOTECH CONSULTANTS, INC. Carmen Crispeno October 30, 2006 JN 05413 Page 12 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: Beneath footings, stabs 95% or walkways Filled slopes and behind 90% retainina walls I 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). Use of On -Site Soil If grading activities take place during wet weather, or when the silty, on -site soil is wet, site preparation costs may be higher because of delays due to rain and the potential need to import granular fill. The on -site soil is generally silty and therefore moisture sensitive. Grading operations will be difficult during wet weather, or when the moisture content of this soil exceeds the optimum moisture content. The moisture content of the silty, on -site soil must be at, or near, the optimum moisture content, as the soil cannot be consistently compacted to the required density when the moisture content is significantly greater than optimum. The moisture content of the on -site soil was generally above the estimated optimum moisture content at the time of our explorations. The on -site glacial till underlying the topsoil could be used as structural fill, if grading operations are conducted during hot, dry weather, when drying the wetter soil by aeration is possible. During excessively dry weather, however, it may be necessary to add water to achieve the optimum moisture content. Moisture -sensitive soil may also be susceptible to excessive softening and "pumping" from construction equipment, or even foot traffic, when the moisture content is greater than the optimum moisture content. It may be beneficial to protect subgrades with a layer of imported sand or crushed rock to limit disturbance from traffic - 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. LiMfTATIONS 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 pits and borings are representative of subsurface conditions on the site- If GEOTECH CONSULTANTS, INC. Carmen Crispeno JN 05413 October 30, 2006 Page 13 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 pits and 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 residences 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 owner must ultimately accept the possibility that some slope movement could occur on the steep slope outside of site development areas. This report has been prepared for the exclusive use of Carmen Crispeno and his 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. 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. GEOTECH CONSULTANTS, INC. Carmen Crispeno October 30, 2006 JN 05413 Page 14 The following plates are attached to Complete this report: Plate I Vicinity Map Plate 2 Site Exploration Plan Plates 3 - 6 Test Pit and Boring Logs Plate 7 Typical Footing Drain 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 - ZJM/DRW: jyb Respectfully submitted, GEOTECH CONSULTANTS, INC. Zachary J. Munstermann Geotechnical Engineer ;A-2-? E04RES -Z- D. Robert Ward, P.E. Principal GEOTECH CONSULTANTS, INC- A a uul RD AV X L up w 4. 14 A, WOOLDWAL w 113TIC3 te AV 1 vwd TIM R 11 IT r: e AV a !ize iTn AV & 107TH w _�316 '�:j MMER LN T '7�v.% Nw Th I �WZj !'I , MID ILrT—V5 I" 3 FL V. I 13 AV �ITH Nw 104TH AV W A co Zo TH AV T x N) I AV NW �G 4AI !Z M x 100 4!' NW 1\j TIrg r-.r-w w -AV 9e 0 A' 4 9nH C) :3 ul CL �* :31 g 3 l AV 3RD A �INW W AV 1w w Wo lw 91TH 91TH PL W 95TH PL w AV W AV r EF AV ­1�f 93RO C) (D Lo 0 C) cn PHINN N nr T7 AV e AV w 4 0 !=XNA I 'AV Al. -h �9. Tll PL -5 67T 0 s FL w SLF4 5 =3 Al AVr 6, r� 86TH p 7 AV 85TH 867� Al —7- 5u" AV I., (D L[9XN .1 4 ' 17Z�'7i�e � L , t4 AURORA r V , Cr) Tmo AV 84T H I A" v lvw I Al 1410VA Al d MDO 16IN j..1-1; MIDVAL Llll�! Al 1� 1 7 k M 0 w , 2 V A, !::STO E I N t� ' ;z )Q�.4 �A c _AK 1—e- ; 82NO AV w to a L a B-1 Existing Proposed House Residence TP-1 t ITP-4 appfoxtripate toe ot steep slope (designated as landslide hazard per ECDC Chapter 23) Proposed B-2 Existing Residence Pool isting Pool TP-2 TP-3 Existing Shed . . .. . .. . .. . .. Legend: Test boring location Test pit location GEOTECH CONSULTANTS, INC. SITE EXPLORATION PLAN 98xx - 235th Place Southwest Edmonds, Washington Job No.- Date: platel. 05413 1 Oct. 2006 1 No Scale 1 2 TEST PIT I CP Description FILL I Brown, silty SAND with gravel, medium- to fine-grained, moist, loos; (FILL) Is Light blue -gray, mottled with orange SILT, non -plastic, bedded, moist, denseA s F-.—.l I Gray, silty SAND with gravel, medium- to fine-grained, moist, very dense 5 Test Pit was terminated at 4 feet on November 1, 2005. No groundwater seepage was observed during excavation. No caving was observed during excavation. 10- 1X Q, 08� TEST PIT 2 ab 'eO Description Gray, silty SAND with gravel, medium- to fine-grained, moist, very dense [S MI 5 Test Pit was terminated at 4 feet on November 1, 2005. No groundwater seepage was observed during excavation. No caving was observed during excavation. 10 15 GEOTECH Jt4 CONSULTANTS, INC. TEST PIT LOG 98xx - 235th Place Southwest Edmonds, Washington 7 b 15ate: ILogged by: 1PIate: 05413 December 20051 zim 3 141 10 15 61 10 15 TEST PIT 3 Description Brown, silty SAND and gravel, medium- to fine-grained, moist, loose to medium - dense 'SM]- becomes li�ht brown, dense with SAND and SILT pockets - becomes gray and very dense • Test Pit was terminated at 6 feet on November 1, 2005. • No groundwater seepage was observed during excavation. • No caving was observed during excavation. 6� TEST PIT 4 Description Gray, silty SAND with gravel, medium- to fine-grained, moist, dense - becomes orangish-brown with organics, medium -dense SM F - no organics - becomes gray, very dense • Test Pit was terminated at 4.5 feet on November 1, 2005. • No groundwater seepage was observed during excavation. • No caving was observed during excavation. GEOTECH CONSULTANTS, INC. TEST PIT LOG 98xx - 235th Place Southwest Edmonds, Washington b ate: I Logged by: Plate: Di 1705413 Fecember 20051 ZJM 4 0 BORING 1 0 cll� Description Oran ish-tan, silty SAND with gravel, medium- to fine -grained, moist, loose J�Mmff 36 1 2 1 - becomes gray, dense 15r- 1 1 50/5" 1 3 111! M'' .. I I - becomes very dense SM SM 20 �— 1 150/3" 1 4 WME Ul".-Illllllll 35 40 * Test boring was terminated at 30.5 feet during drilling on October 27, 2005- * No groundwater seepage was encountered during drilling. GEOTECH CONSULTANTS, INC. BORING LOG 98xx - 235th Place Southwest Edmonds, Washington IJob 00.1 Logged by. Plate: 05413 IDDeacteen:ber2 zim 5 j 10 15 20 25 30 35 40 BORING 2 Vo Descn*pbon Brown, silty SAND with gravel, slightly moist 14 - 11 jj:j:j;j:j:j:j:jj Brown to gray, silty SAND, slightly moist, medum-dense 56# 1 2 Gray, sandy SILT with gravel, slightly moist, very dense Mmi-loginig; • Test boring was terminated at 10 feet during drilling on November 10, 2005. • Groundwater seepige was not encountered during drilling. # Blows may be overstated due to rocks. GEOTECH CONSULTANTS, INC. BORING LOG 98xx - 235th Place Southwest Edmonds, Washington Job Date: Logged by. 054131 Nov.20051 DLB 1plate: 6