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20060728_Geotech.pdfb p, Dennis M. Bruce, P.E. N,s.G.E„ Eu1.s.A Coonioi/Civil Engineer July 28, 2006 RECEIVED AUG 2 12006 PERMIT COUNTER City of Edmonds QCT 16 2015 cto Dennis Walcker P.O. Box 996 EVELOPMENT SER Lynnwood, WA 98046 pVICE Subject: Geotechnical Evaluation — Foundation and Shoring Recommendations Proposed New Residence at 742 Daley St., Edmonds, Washington This engineering report presents the results of a geotechnical evaluation of the Dennis Walcker propertyat 742 Daley St., Edmonds, Washington. This evaluation was required due to owner concerns, as well as City of Edmonds requirements for steeply - sloped lots. REFERENCES: • Property Topographic Survey by Tri -County dated October 2006 • Proposed New Residence Plans by Geigle Designs dated May 2006 • Structural Engineering Analysis and Designs by Reed and Associates dated May 3, 2006 a Grading calculations by Donna Breske, P.E. dated June 7, 2006 m Buffer mitigation plan by Landau Associates dated May 3, 2006 • Site photographs BACKGROUND: The overall property at 742 Daley St. is rectangular in shape with approximately 90 feet of frontage (along Daley St.) and approximately 118 feet of depth (see Topographic Survey). A stream (year-round flowing) exists at the northeast corner of the property. It is understood that the buffer mitigation study has recommended a setback distance from the centerline of the creek / stream and that this distance is indicated on the project plans. The lot contains a significant slope with a total elevation differential of approximately 30 feet (see Topographic Map). SOILS FOUNDATIONS + SITE DEVELOPMENT - INSPE"DTMN - DRAINAGE - DESIGN & PERMIT - LEGAL P.O. Box 55502 - shoreline, WA 98155 - (206) 546-9217 • FAX (206) 546-844A2TTAC;HMENT 6 Attachment 23 City of Edmonds c/o Dennis Walcker July 28, 2006 Page 2 Currently, the lot is undeveloped and contains a few mature trees, shrubbery and grasses. Visual evaluation of the properly reveals no evidence of any geotechnical distress: no slides, no soil tension cracks or any evidence of erosional degradation. It is understood that Dennis Walcker proposes to construct a new residence (3 stories — see plans) at the southeast portion of the site, with driveway access from the north (at the western property line zone),.. See plans. Construction of the proposed house will require excavations of varying depths (see plans). This engineer has provided extensive geotechnical investigations and construction inspections for the 3 new homes (be Eaglewood Homes) immediately to the west. EVALUATION; In order to augment the existing site geotechnical information, 4 soil test holes were hand dug by this engineer on July 12, 2006 (see Site Plan for locations). All 4 test holes revealed similar sub -grade conditions, namely: 0" to 6" Organics, roots, and forest duff 6" to 34" (bottom of test holes) Dense, relatively clean sand No water was encountered in any of the 4 test holes. All test hole walls remained vertical and stable. No sloughing or caving occurred. Based on the findings of this investigation, and experience with similar sites in the immediate area, the Walcker property at 742 Daley St., Edmonds, Washington is aeotechnically gooroved for the proposed new residence, subject to the following: Temporary shoring required for excavation cuts deeper than 5 feet. See section on "Temporary Shoring". Standard reinforced continuous and spread footings. Allowable bearing pressure: 4,000 p.s.f. City of Edmonds c/o Dennis Walcker July 28, 2006 Page 3 Equivalentof • re • any retaining wall design provided drainage zone is inspected and verified by this engineer. For retaining wall design, use friction factor of 0.60 and passive pressure of 350 p.c.f. Geotechnical inspections by this engineer prigr to any foundation concrete placement. The proposed structure can be supported on conventional continuous and spread footings bearing on undisturbed native soils or on structural fill placed above native soils. See the later sub -section entitled General Earthwork and Stnural ill for structural fill placement and compaction recommendations. Continuous and individual spread footings should have minimum widths of eighteen (18) and twenty-four (24) inches, respectively, and should be bottomed; at least eighteen (18) Inches below the lower adjacent finish groundsurface. Depending on the final site grades„ some over -excavation may be required below footings to expose competent native soils. Unless lean concrete is used to fill the over excavated hole, the width of the over -excavation at the bottom must be at least as wide as the sum of two times the depth of the over -excavation and the footing width. For example, an over -excavation extending two feet below the bottom of a three-foot wide footing must be at least seven feet wide at the base of the excavation.. Footings constructed according to the above recommendations may be designed for an allowable soil bearing pressure of four thousand (4,000) pounds per square foot (p.s.f.). 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 total post -construction settlement of footings founded on competent, native soils (or on structural fill up to five (5) feet in thickness) will be about one-half inch, with differential settlements on the order of one-quarter inch. Lateral loads due to wind or seismic forces may be resisted by friction between the foundations and the bearing soils, or by passive earth pressure acting on the vertical, embedded portions of the foundations. For the latter condition, the foundations must either be poured directly against undisturbed soil or the backfill placed around the outside of the foundation must be level structural fill. We recommend the following design values be used for the foundation's resistance to lateral loading: Parameter Desl n clue Coefficient of Friction 0.60 City of Edmonds c/o Dennis Walcker July 28, 2006 Page 4 Passive Earth Pressure 350 p.c.f. Where: (1) p.c.f. is pounds per cubic foot. (2) Passive earth pressure is computed using the equivalent fluid density, We recommend that a safety factor of at least 1.5 be used for design of the foundation's resistance to lateral loading. Slab -on -grade floors may be supported on undisturbed, competent native soils or on structural fill. The slabs may be supported on the existing soils provided these soils can be re -compacted prior to placement of the free -draining sand or gravel underneath the slab. This sand and gravel layer should be a minimum of four (4) inches thick. We also recommend using a vapor barrier such as 6 -mil. plastic membrane beneath the slab with minimum overlaps of 12 inches for sealing purposes. Retaining walls backfilled; on one side only should be designed to resist lateral earth pressures imposed by the soils retained by - structures. recommendedheight,12) feet in which restrain level backfill: Active Earth Pressure" Passive Earth Pressure Coefficient of Friction Soil Unit Weight Where: 30 p.c.f. 350 p.c.f. 0.60 125 p.c.f. (1) p.c.f. is pounds per cubic foot (2) Active and passive earth pressures are computed using equivalent fluid densities. For restrained walls which cannot deflect at least 0.002 times the wall height, a uniform lateral pressure of one hundred (100 p.s.f. should be added to the active equivalent fluid pressure). City of Edmonds clo Dennis Walcker July 28, 2006 Page 5 The values given above are to be used for design of permanent foundation and retaining walls only. An appropriate safety factor should be applied when designing the walls. We recommend using a safety factor of at least 1.5 for overturning and sliding. The above design values do not include the effects of any hydrostatic pressures behind the walls and assume that no surcharge slopes or loads will be placed above the walls. If these conditions exist, then those pressures should be added to the above lateral pressures. Also, if sloping backfill is desired behind the walls, then we will need to be given the wall dimensions and slope of the backfill in order to provide the appropriate design earth pressures. Leavy construction equipment should not be operated behind retaining and foundation walls within a distance equal to thae height of the wall, unless the walls are designed for the additional lateral pressures resulting from the equipment. Placement and compaction of retaining wall backfill' should be accomplished with hand -operated equipment. Retaining Wall Backfill Backfill placed within eighteen (18) inches of any retaining or foundation walls should be free -draining structural fill containing no organics. This backfill should contain no more than five (5) percent silt or clay particles and have no particles greater than four (4) inches in diameter. The percentage of particles passing the No. 4 sieve should be between twenty-five (25) and seventy (70) percent. Due to their high silt content, if the native soils are used as backfill, a drainage composite, such as Mirafi and Enkadrain, should be placed against the retaining walls. The drainage composites should be hydraulically connected to the foundation drain system. The purpose of these backfill requirements is to assure that the design criteria for the retaining wall is not exceeded because of a build-up of hydrostatic pressure behind the wall. The subsection entitled Gene(i Earthwork and Structural bill contains recommendations regarding placement and compaction of structural fill behind retaining and foundation walls. At the time of this investigation and report, it is understood that the majority of the soil cuts will be approximately 8 to 10 feet below grade. Temporary shoring is re lred. for any soil cuts deeper than 5 feet below grade. The dense, stable sub -grade sands allow for temporary Eco -Block shoring to be utilized up to a maximum of 12 feet (a minimum of 6 -inch embeddment with 11.5 feet shoring above). See attached typical sketch. City of Edmonds c/o Dennis Walcker July 28, 2006 Page 6 Some portions of the site may require excavation depths deeper than 11.5 feet. This engineer limits Eco -Block shoring to 6 blocks (each block 2 -feet high). The dense, sub -grade sands allow for a temporary "back cut" at 1H:1V declination (see attached sketch). In no case should excavation slopes be greater than the limits specified in local, state and national government safety regulations. Temporary cuts up to a height of four (4) feet deep in unsaturated soils may be vertical. For temporary cuts having a height greater than flour (4) feet, the cut should have an inclination no steeper than 1:1 (Horizontal:Vertical) from the top of the slope to the bottom of the excavation. Under specific recommendations by the geotechnical engineer, excavation cuts may be modified for site conditions. All permanent cuts into native soils should be inclined no steeper than 2:1 (H:V). Fill slopes should not exceed 2HAV. It is Important to note that sands do cave suddenly, and without warning. The contractors should be made aware of this potential hazard. 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 stability of the surficial layer of soil. DRAIN GE CONSIDERATIONS: Footing drains are recommended at the base of all footings and retaining walls. These drains should be surrounded by at least six (6) inches of one -inch -minus washed rock wrapped in non -woven geotextile filter fabric (Mirafi 140N, Supac 4NP, or similar material). At the highest point, the perforated pipe invert should be at least as low as the bottom of the footing and it should be sloped for drainage.. All roof and surface water drains must be kept separate from the foundation drain system. No groundwater was observed in any of the 4 test holes during the fieldwork, nor was any groundwater encountered at the adjacent (westerly) recent home projects. Seepage into the planned excavation is possible, and likely if excavation occurs during winter months, and if encountered should be drained away from the site by use of drainage ditches, perforated pipe, French drains, or by pumping from sumps interconnected by shallow connector trenches at the bottom of the excavation. The excavation of the 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. Any exposed slopes to be covered with plastic to minimize erosion. Final site grading in areas City of Edmonds c/o Dennis Walcker July 28, 2006 Page 7 adjacent to buildings should be sloped at least two (2) percent away from the building, except where the area adjacent to the building is paved. w • M 1 � • ' . ly a MIT M « « « Beneath footings, slabs or walkways Behind retaining walls Beneath pavements 95% 90% 916% for upper 12 inches of Sub -grade, 90% below that level Where: Minimum relative compaction is the ratio, expressed in percentages,Of densitythe compacted dry rt City of Edmonds c/o Dennis Walcker July 28, 2006 Page 8 Use of On -Site Soils If grading activities take place during wet weather, or when the sandy, on-site soils are very wet, site preparation costs may be higher because of delays due to rains and the potential need to import granular fill. The on-site soils are generally sandy and thus are not highly moisture sensitive. Grading operations will be difficult when the moisture content of these soils greatly exceeds the optimum moisture content. Moisture sensitive soils will also be susceptible to excessive softening and "pumping"' from construction equipment traffic when the moisture content is greater than the optimum moisture content. Ideally, structural fill, which is to be placed In wet weather, should consist of a granular soil having no more than five (6) percent silt or clay particles. The percentage of particles passing the No. 200 sieve should be measured from that portion of the soil passing the three -quarter -inch sieve. The use of "some" on-site soils for fill material mpy be agge fable if the upper organic materials are segregated and moisture contents are monitored by engineering inspection. DRAINAGE CONTROLS: No drainage problems were evident with the Walcker property at 742 Daley St. As stated, the sub -grade sands allow for excellent infiltration. Any surficial runoff would flow down-slope to the northeast into the existing creek. On-site storm water infiltration is geotechnically possible, if desired by owner. Subsequent design criteria and recommendations for submittal to City of Edmonds available upon request. CONCRETE: All foundation concrete (footings, stem walls, slabs, any retaining walls, etc.) shall have a minimum cement content of 5-1/2 sacks per cubic yard of concrete mix. 0 OTECHNICAL !3F -VIEW OI; STRU'C U AL DESIGNS: This engineer has reviewed the structural designs by Reed and Associates. These designs utilized a default' soil value of 2,000 p.s.f. in determining retaining wall dimensions and other structural elements. The actual soil conditions are "stronger' than City of Edmonds c/o Dennis Walcker July 28, 2006 Page 9 the default values utilized. Thus, the structural analysis and design criteria are more than sufficient for the site geotechnical conditions, subject to inspections. As stated, this site contains significant slopes, which will pose a construction and logistic challenge for the driveway access, foundation excavation, temporary shoring and foundation construction. it is essential that a qualified geotechnical engineer provided ongoing inspections to verify site conditions and assure the Integrity of the structural elements. The recommendations of this report are only valid when key geotechnical aspects are inspected by this en inset Burin construction. • Soil cuts • Temporary shoring installation (Eco -Blocks up to 12 feet maximum) • Foundation sub -grade verification • Any retaining wall, or rockery placement • Any fill placement • Subsurface drainage installation • Temporary and permanent erosion control measures k The proposed new Walcker residence at 742 Daley St., Edmonds, Washington is geotechnically viable when constructed in accordance with the recommendations herein, compliance with City of Edmonds approved plans and requirements, and key geotechnical inspections &LmA construction. The plans and specifications for theWsalcker residence at 74 Daley St., Edmonds, Washington have been reviewed by this engineer and conform to the recommendations of the analysis and report and, provided that those conditions and recommendations are satisfied during the construction and use, and inspected and verified by this engineer, the area disturbed by construction will be stabilized and remain stable and will not increase the potential for soil movement, and the risk of damage to the proposed development and from the development to adjacent properties from soil instability will be minimal. City of Edmonds c/o Dennis Walcker July 28, 2006 Page 10 CLOSURE: The findings and recommendations of this report were prepared in accordance with generally accepted professional engineering principles and practice. No other warranty, either express or implied, is made. The conclusions are based on the results of the field exploration and interpolation of subsurface conditions between explored locations. If conditions are encountered during construction that appear to be different than those described in this report, this engineer should be notified to observe the situation and review and verify or modify the recommendations. If there are any questions, do not hesitate to call. 1 409 r DMB:abjwss 1 cc: Dennis Walcker r � Dennis M. Bruce, P.E. Geotechnical / Civil Engineer