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gurneegeo.pdfEDM 06-10 EB 15rinks Ill( Bruce- M.S.C.E., M.B.p_' j 7 Copy r.E. Geotechnical/Civil Engineer March 29,200e( RE:CE:jV 't) City of Urn ds FEB I 0 2oo6 do Gurnee Co Rany Builder^ dr_va OPr�Enrr � P.O. Box 3446 c+ry a� AMSONDV,CES CTR.s Lynnwood, WA 9804a Subject Geotechnical Evaluation — Foundation Recommendations Proposed Two New Residences 18216. Homeview Dr., Edmonds, Washington This engineering report presents the results of a geotechnical evaluation of the Gurnee Company Builders' property at 1$2xx Homeview Dr., Edmonds, Washington. This evaluation was required due to owner / contractor concerns, as well as City of Edmonds requirements. REFERENCES: • Property Survey with topographic contours by LSA dated November 29, 2004 • Proposed House Site Plan (furnished by owner) a Photographs dated March 5 and March 10, 2005 by D. Bruce, P.E. BACKGROUND: The overall property is irregular in shape with significant curved frontage along Homeview Dr. (see Survey Map). The overall property is founded on the south portion by Ridgeway road. The property has never been developed. The overall property contains significant slopes with numerous discontinuities of grade (see topographic map). It is understood that Gurnee Company Builders proposes to construct two new single-family residences (one in the southerly portion of the property. and the second in the northern portion of the property). NOTE: This engineer understands that the overall property is divided into two parcels (see Survey Map). SOILS - FOUNDAT/ONS • SITE DEVELOPMENT - INSPEC77ON - DRAINAGE - DESIGN & PERMIT - LEGAL P.O. Box 55502 • Shoreline. WA 98155 • 12061 545-W17 e FAX r91)A1'rA"AA1> City of Edmonds clo Gurnee Company Builders March 29, 2005 Page 2 Visual evaluation of the property reveals no evidence of any geotechnical distress: no slides, no settlements, no soil tension cracks, or any evidence of erosional degradation. The adjacent property to the west consists of a single-family residence on similarly sloped property. EVALUATION: In order to augment the existing site geotechnical information, 4 soil test pits were dug under -this engineer's observation on March 10, 2005 using a rubber -tired backhoe (see photographs). Two test pits were dug in the probable vicinity of the proposed southerly residence. Two test pits were dug in the probable vicinity of the proposed northerly residence. All 4 test pits revealed similar sub -grade conditions, namely: 0" to 8" Forest duff, organics and organic silt 8" to 8' (bottom of pits) Dense to very dense sands with gravels No water was encountered in any of the 4 test pits- All test pit walls remained vertical and stable. No sloughing or caving occurred. CONCLUSIONS / RECOMMENDATIONS: Based on the findings of this investigation, and experience with similar sites in the area, the property at 182xx Homeview Dr., Edmonds, Washington is eotechnically approved for the proposed two new residences, subject to the following: • Soil cuts steeper than 1:1 and greater than 5 feet in depth will require temporary shoring consisting of Ecology -Blocks, per on-site geotechnical engineer. • Standard reinforced continuous and spread footings. Allowable bearing pressure: 3,000 p.s.f. • Equivalent fluid pressure of 35 p.c.f. is recommended for any retaining wall design provided drainage zone is inspected and verified by this engineer. • For retaining wall design, use friction factor of 0.55 and passive pressure of 350 p.c.f. City of Edmonds c!o Gurnee Company Builders March 29, 2005 Page 3. • Geotechnical inspections by this engineerrip or 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 Structural Fill 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 ground surface. 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 three thousand (3,000) 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 orie-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 Design Value Coefficient of Friction 0.55 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. City of Edmonds c/o Gurnee Company Builders March 29, 2005 Page 4 We recommend that a safety factor of at least 1.5 be used for design of the foundation's resistance to lateral loading. SLABS -ON -GRADE: 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. PERMANENT FOUNDATION AND RETAINING WALLS: Retaining walls backfilled on one side only should be designed to resist lateral earth pressures imposed by the soils retained by these structures. The following recommended design parameters are for walls less than twelve (12) feet in height, which restrain level backfill: Parameter Active Earth Pressure* Passive Earth Pressure Coefficient of Friction Soil Unit Weight Where - Design Value 35 p.c.f. 350 p:c.f. 0.55 125 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 defect 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). 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 City of Edmonds cfo Gurnee Company Builders March 29, 2005 Page 5 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. Heavy construction equipment should not be operated behind retaining and foundation walls within a distance equal to the 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 General Earthwork and Structural Fill contains recommendations regarding placement and compaction of structural fill behind retaining and foundation walls. EXCAVATION AND SLOPES: At the time of this report, this engineer understands that temporary cuts will be shallower than 1:1. On-site geotechnical inspections to verify. if soil cuts are deeper than 5 feet in total depth and cannot be made at a 211:1 V slope, then temporary shoring consisting of Ecology -Blocks will be required. On-site geotechnical engineering inspections to verify. 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 four (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 City of Edmonds c/o Gurnee Company Builders March 29, 2005 Page 6 steeper than 2:1 (H:V). Fill slopes should not exceed 21-1:1 V. 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. DRAINAGE 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 pits during the fieldwork. 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 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. GENERAL EARTHWORK AND STRUCTURAL FILL: The proposed building and pavement areas should be stripped and cleared of all surface vegetation, all organic matter, and other deleterious material. The stripped or removed materials should not be mixed with any materials to be used as structural fill. Structural fill is defined as any fill placed under the building, behind permanent retaining or foundation walls, or in other areas where the underlying soils needs to support loads. This engineer should observe site conditions during and after excavation prior to placement of any structural fill. City of Edmonds clo Gurnee Company Builders March -29, 2005 Page 7 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 which results in the greatest compacted dry density. The moisture content of fill soils 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, compaction equipment, and the number of passes made to compact the lift. In no case should the lifts exceed twelve (12) inches in loose thickness. The following table presents recommended relative compaction for structural fill: Location of Fill Placement Beneath footings, slabs or walkways Behind retaining walls Beneath pavements Minimum Relative Compaction _ 95% 90% 95% for upper 12 inches of Sub -grade, 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-1557-78 (Modified Proctor). Use of ©n -Site Soils If grading activities take place during wet weather, or when the sandy, on-site soils are extremely wet, site preparation costs may be higher because of delays due to rains and the potential need to import additional 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 (5) percent sift 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. -' City of Edmonds . cfo Gurnee Company Builders March 29, 2005 Page 8 The use of "some" on-site soils for fill material may be acceptable if the upper organic materials are segregated and moisturecontents are monitored by engineering inspection. DRAINAGE CONTROLS: No drainage problems were evident with the overall property at 182xx Homeview Dr. As stated, the property contains significant contours with numerous discontinuities. No evidence of any drainage course or "stream" was found on the property. Obviously, there are high points and low points within the overall contours. The clean, permeable sub -grade sands allow for on-site infiltration. This engineer recommends a drainage criteria permeability rate of 13 inches per hour for design purposes. Once specific house plans have been completed, this engineer is available to provide infiltration design and specific criteria. ACCESS TO LOTS: At the time of this report, this engineer understands that access to the southern residence will be off Ridgeway Rd. at the western end of the property. This access location is aeotechnicaliy approved, subject to specific inspections by this engineer. It is understood that the northern residence will have access off of Homeview. Dr. in the vicinity of the "flatter" section of the property. Both these locations are geotechnically viable, subject to on-site inspections. CONCRETE: All foundation concrete (footings, stem walls, slabs, any retaining walls, etc.) shall have a minimum cement content of 5-1/2 sacks percubic yard of concrete mix_ SLOPES -- SETBACKS — BUFFERS: There are no geotechnical conditions that limit or restrict the location of either of the two proposed new residences. As stated, the actual excavation depths and proximity to slopes mai r► eauire temporary shoring consisting of Ecology -Blocks. City of Edmonds c/o Gurnee Company Builders March 29, 2005 Page 9 The recommendations of this report are only valid when key geotechnical aspects are inspected by this engineer during construction. • Soil cuts • Temporary Eco -Block shoring (if required) • Foundation sub -grade vera ication • Any retaining wall, or rockery placement • Any fill placement • Subsurface drainage installation • Temporary and final erosion control SUMMARY: The proposed two new residences at 182xx Homeview Dr., Seattle, Washington by Gurnee Company Builders are geotechnically viable when constructed in accordance with the recommendations herein, compliance with City of Edmonds approved plans and requirements, and key geotechnical inspections daring construction. GEOTECHNICAL REVIEW OF FINAL FLANS: At the time of this investigation and report, final house plans were not available for review. As stated in this report, the proposed two new residences may be constructed on conventional footings, subject to geotechnical inspections. Actual excavation criteria may require use of temporary Eco -Block shoring. Prior to final permit issuance, this engineer should review the final house plans to vqfi& compliance with the recommendations of this report. Upon satisfactory review, a "Statement of Minimal Risk" will be issued. 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 City of Edmonds c/o Gurnee Company Builders March 29, 2005 Page 10 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. DMB:abj l Dennis M. Bruce, P.E. - Geotechnical 1 Civil Engineer 1 L'gT •F1-b�i-a-\i F �.