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HomeMy WebLinkAbout21-101265-SU-Geotechnical (Soils) Report-03-31-2021-V1 13705 Bel-Red Road, Bellevue, Washington 98005 Phone 425/649-8757 · Fax 425/649-8758 October 2, 2020 G-5261 Mr. Hamid Korasani Sazei Design Group 6608 110th Ave NE Kirkland, WA 98033 Subject: GEOTECHNICAL REPORT PROPOSED VODA HOUSE SHORT PLAT 35027 – 21ST AVE SW FEDERAL WAY, WASHINGTON Dear Mr. Korasani: In accordance with our September 1, 2020 contract with you we have prepared the following geotechnical report for the proposed development. SITE AND PROJECT DESCRIPTION The project site consists of a 0.68-acre lot at the subject address as shown on the attached Plate 1 - Vicinity Map. The lot is currently developed with a single-family residence and accessory structures and it is accessed via a gravel driveway running west from 21st Ave SW. It is bounded by residentially zoned properties and associated roads. Based upon information provided by the project owner we understand that the proposed development at the subject lot will consist of subdividing the lot into two lots for eventual residential development. The name of the short plat is proposed to be the Voda_House Short Plat. A site plan indicating the short plat/development configuration has not been provided. We understand that a new house will be developed at the existing yard area west of the existing residence. GEO Group Northwest has prepared the attached Plate 2 – Site Plan using the online King County Imap in order to illustrate the current development and topographic conditions at the subject site. In general, the site has level to slightly sloping topography with slopes facing toward the east. October 2, 2020 G-5261 Geotechnical Report – 35027 – 21st Ave SW, Federal Way, WA Page 2 GEO Group Northwest, Inc. GEOLOGIC CONDITIONS The geologic map1 for the site indicates that the subject site is underlain by Quaternary-age Vashon Till (Qvt). Vashon Till typically consists of a mixture of silt, sand and gravel which was deposited and overridden by glacial ice during the most recent period of glaciation, the Vashon Stade. SUBSURFACE CONDITIONS GEO Group Northwest explored the subsurface soil and groundwater conditions by excavating three test pits labeled TP-1 through TP-3 on September 17, 2020. The approximate test pit locations are shown on the attached Plate 2 – Site Plan. The soils encountered at the test pits consist of the competent medium dense to dense Vashon Till soils. Soils are primarily fine sandy SILT with varying amounts of gravel, cobbles and boulders. At the test pits TP-1 and TP-3 near the base of the pits gravelly silty SAND and gravelly SAND with some silt was encountered. It is possible that these soils are the underlying Advance Outwash soil deposit. But they may also be a sandier zone of glacial till. Loose soils were not encountered at the test pits. Groundwater seepage was not encountered at the test pits. The results of our subsurface investigation are shown on the attached Appendix A – Test Pit Logs and USCS Soil Legend. SEISMIC DESIGN CRITERIA Based upon the subsurface investigation it is our opinion that the overlying 100-foot thickness of soils at the project site may be characterized as Site Class C soil (Very Dense Soil and Soft Rock) and may be designed accordingly for seismic loads per the IBC. According to an online Seismic Hazard tool the seismic coefficients are as follows: Ss = 1.293g S1 = 0.499g 1 “Geologic Map of the Des Moines 7.5’ Quadrangle, King County, Washington”, USGS, Booth et al, 2004. October 2, 2020 G-5261 Geotechnical Report – 35027 – 21st Ave SW, Federal Way, WA Page 3 GEO Group Northwest, Inc. CONCLUSIONS AND RECOMMENDATIONS Based upon our subsurface investigation the proposed development is acceptable for the subject site soil conditions. We recommend that the new building is constructed to bear on top of the underlying native and competent medium dense to dense Vashon Till site soils or compacted structural fills placed on top of the competent soils. Additionally, we recommend that all foundation subgrades are approved by GEO Group Northwest at the time of construction, prior to foundation pour(s). If loose soils are encountered at building foundation locations then a program of over-excavation and replacement with compacted structural fill shall be implemented and approved by GEO Group Northwest. Based upon our subsurface investigation it is anticipated that competent medium dense to dense Vashon Till soils are present at typical foundation subgrade elevations (18-inches below ground surface). We recommend that the following recommendations and design parameters be incorporated into the design for the development. Site Preparation and General Earthwork The proposed development areas should be stripped and cleared of surface vegetation, organic soils (topsoil), loose soils and fill debris. Silt fences should be installed around areas disturbed by construction activity to prevent sediment-laden surface runoff from being discharged off-site. Exposed soils that are subject to erosion should be compacted and covered with plastic sheeting. Stockpiled soils should be covered with plastic sheeting if work is done during wet weather in order to mitigate off-site sedimentation risks. The site soils include relatively fine-grained silty soils which are moisture sensitive. Therefore, if grading work occurs during wet weather there are risks related to soil softening as a result of construction equipment. Yielding soils should be removed from structural subgrades (footing and slab areas). If a significant portion of the work may occur during the wet season (Nov-April) then it may be beneficial to construct a working pad consisting of crushed rock immediately following the building pad excavation. We recommend that a construction pad for the building consist of a minimum 6-inch thickness of clean crushed rock with a layer of filter fabric such as Mirafi 500X placed on top of the non-yielding and competent native site soils. Alternatively, the crushed rock may have a minimum thickness of 12-inches without the layer of filter fabric. October 2, 2020 G-5261 Geotechnical Report – 35027 – 21st Ave SW, Federal Way, WA Page 4 GEO Group Northwest, Inc. Temporary Excavation Slopes and Permanent Slopes Under no circumstances should temporary excavation slopes be greater than the limits specified in local, state and national government safety regulations. Temporary cuts greater than four feet in height should be sloped at an inclination no steeper than 1H:1V (Horizontal:Vertical) in the overlying loose soils. Excavations within the underlying dense to very dense and cemented silty soils (un-weathered Vashon Till) may be sloped having inclinations of no steeper than 1H:2V provided seepage is not encountered. If seepage is encountered at the excavation, then the excavation work should be halted and the temporary excavation slopes should be evaluated by GEO Group Northwest prior to proceeding to a deeper level. We recommend that permanent graded slopes shall be sloped no steeper than 3H:1V and that any fills placed at these areas where inclinations are steeper than 4H:1V are compacted to meet the structural fill compaction requirements. Structural Fill All fill material used to achieve design site elevations below the building areas and below non- structurally supported slabs, parking lots, sidewalks, driveways, and patios, should meet the requirements for structural fill. During wet weather conditions, material to be used as structural fill should have the following specifications: 1. Be free draining, granular material containing no more than five (5) percent fines (silt and clay-size particles passing the No. 200 mesh sieve); 2. Be free of organic material and other deleterious substances, such as construction debris and garbage; 3. Have a maximum size of three (3) inches in diameter. All fill material should be placed at or near the optimum moisture content. The optimum moisture content is the water content in soil that enables the soil to be compacted to the highest dry density for a given compaction effort. Based upon our subsurface investigation the overlying site soils consist of silty soils. The site soils are moisture sensitive and meeting the compaction requirements may be difficult during wet weather. These soils may be used for structural fill provided that they are compacted to meet October 2, 2020 G-5261 Geotechnical Report – 35027 – 21st Ave SW, Federal Way, WA Page 5 GEO Group Northwest, Inc. the compaction criteria noted below. It may be beneficial to import a granular fill material meeting the specifications noted above, especially if work is to be performed during a period of wet weather. Structural fill should be placed in thin horizontal lifts not exceeding ten inches in loose thickness. Structural fill under building areas (including foundation and slab areas) and behind earth reinforced retaining walls (segmental block retaining walls and geo-grid reinforced rockery walls), should be compacted to at least 92 percent of the maximum dry density, as determined by ASTM Test Designation D-1557-91 (Modified Proctor). Structural fill under driveways, parking lots and sidewalks should be compacted to at least 90 percent maximum dry density, as determined by ASTM Test Designation D-1557-91 (Modified Proctor). Fill placed within 12-inches of finish grade should meet the 92% requirement. We recommend that GEO Group Northwest, Inc., be retained to evaluate the suitability of structural fill material and to monitor the compaction work during construction for quality assurance of the earthwork. Spread Footing Foundations The proposed new building foundations may consist of conventional spread footings bearing on top of the underlying medium dense to dense competent Vashon Till soils or on compacted structural fill which is placed and compacted on top of the competent soils. The footings should not be constructed on top of loose soils. If loose soils are encountered at footing areas then they should be removed from foundation support areas and the excavation deepened until the competent soils are encountered. Over-excavation areas may be backfilled with compacted structural fill provided that it is placed on top of the competent medium dense to dense native site soils. We recommend that all foundation subgrades are approved by GEO Group Northwest at the time of construction, prior to the foundation pour, in order to confirm that the subgrades have been properly prepared. Individual spread footings may be used for supporting columns and strip footings for bearing walls. Our recommended minimum design criteria for foundations bearing on the medium dense to dense competent Vashon Till soils or on compacted structural fill placed on top of these soils are as follows: October 2, 2020 G-5261 Geotechnical Report – 35027 – 21st Ave SW, Federal Way, WA Page 6 GEO Group Northwest, Inc. - Allowable bearing pressure, including all dead and live loads Medium dense to dense native Vashon Till soils = 2,500 psf Compacted structural fill on top of the medium dense to dense Vashon Till soils = 2,500 psf - Minimum depth to bottom of perimeter footing below adjacent final exterior grade = 18 inches - Minimum depth to bottom of interior footings below top of floor slab = 18 inches - Minimum width of wall footings = 16 inches - Minimum lateral dimension of column footings = 24 inches - Estimated post-construction settlement = 1/4 inch - Estimated post-construction differential settlement; across building width = 1/4 inch A one-third increase in the above allowable bearing pressures can be used when considering short-term transitory wind or seismic loads. Lateral loads can also be resisted by friction between the foundation and the supporting compacted fill subgrade or by passive earth pressure acting on the buried portions of the foundations. For the latter, the foundations must be poured "neat" against the existing undisturbed soil or be backfilled with a compacted fill meeting the requirements for structural fill. Our recommended parameters are as follows: - Passive Pressure (Lateral Resistance) • 350 pcf equivalent fluid weight for compacted structural fill • 350 pcf equivalent fluid weight for native dense soil. - Coefficient of Friction (Friction Factor) • 0.35 for compacted structural fill • 0.35 for native dense soil October 2, 2020 G-5261 Geotechnical Report – 35027 – 21st Ave SW, Federal Way, WA Page 7 GEO Group Northwest, Inc. Drainage Considerations We recommend that footing drains be constructed at the perimeter of the new foundations and at the base of all retaining walls. Footing drains are recommended to consist of a minimum 4-inch diameter perforated rigid PVC pipe laid in a bed of gravel, surrounded with gravel and separated from finer grained material with a layer of filter fabric. The footing drain pipes should be tightlined to the stormwater drainage system and downspout drains shall not drain into the footing drain piping. Slab-on-Grade Concrete Floors Slab-on-grade concrete floors may be constructed directly on top of the competent medium dense to very dense in-situ site soils or on top of compacted structural fills placed on top of the competent site soils provided that the subgrade is not yielding at the time of concrete pour. Slab- on-grade floors should not be constructed on top of loose soils or on top of wet yielding soils. We recommend that we are retained to observe the condition of the slab subgrades prior to the concrete pour to verify that they consist of medium dense or dense soils and are non-yielding. A proof-roll by large construction equipment may be used to evaluate the condition of slab subgrade areas. Over-excavation and replacement with compacted structural fill may be necessary if loose or yielding soils are encountered at the slab subgrade. If structural fills are to be placed at these areas then they should be compacted in accordance with the specifications in the section titled: Structural Fill. To avoid moisture build-up on the subgrade, slab-on-grade concrete floors should be placed on a capillary break, which is in turn placed on the prepared subgrade. The capillary break should consist of a minimum of a six (6) inch thick layer of free-draining crushed rock or gravel containing no more than five (5) percent finer than the No. 4 sieve. To reduce moisture vapor transmission through the slab we recommend installing a minimum 10-mil thick vapor retarder, such as Moistop Ultra® 10, by Fortifiber Building Systems Group®, between the capillary break and concrete floor slab. Moistop Ultra 10 is a polyolefin film with a water vapor permeance of 0.02 perms. It is puncture and tear resistant, meets ASTM E-1745 Class A, B and C requirements for underslab vapor retarders and is suitable for residential and commercial applications. Boots are available for sealing around pipes, conduit and other penetrations. We recommend these be installed in accordance with the manufacturer’s recommendations. October 2, 2020 G-5261 Geotechnical Report – 35027 – 21st Ave SW, Federal Way, WA Page 8 GEO Group Northwest, Inc. ADDITIONAL SERVICES We recommend that GEO Group Northwest Inc. be retained to perform a general plan review of the final design and specifications for the proposed development to verify that the earthwork and foundation recommendations have been properly interpreted and implemented in the design and in the construction documents. We also recommend that GEO Group Northwest Inc. be retained to provide monitoring and testing services for geotechnically-related work during construction. This is to observe compliance with the design concepts, specifications or recommendations and to allow design changes in the event subsurface conditions differ from those anticipated prior to the start of construction. We anticipate that geotechnical construction monitoring inspections may be necessary for the following construction tasks: 1. Foundation and building pad preparation including over-excavation and structural fill placement at areas where loose soils are encountered; 2. Structural fill placement and compaction; 3. Subsurface drainage installation; 4. Slab-on-grade floor subgrade preparation observation/approval. LIMITATIONS This report has been prepared for the specific application to this site for the exclusive use of Sazei Design Group and their authorized representatives. Any use of this report by other parties is solely at that party’s own risk. Our findings and recommendations stated herein are based on field observations, our experience and judgement. The recommendations are our professional opinion derived in a manner consistent with the level of care and skill ordinarily exercised by other members of the profession currently practicing under similar conditions in this area and within the budget constraint. No warranty is expressed or implied. In the event that soil conditions not anticipated in this report are encountered during site development, GEO Group Northwest, Inc., should be notified and the above recommendations should be re-evaluated. If you have any questions please do not hesitate to contact us. October 2, 2020 G-5261 Geotechnical Report – 35027 – 21st Ave SW, Federal Way, WA Page 9 GEO Group Northwest, Inc. Sincerely, GEO GROUP NORTHWEST, INC. Adam Gaston Project Engineer William Chang, P.E. Principal Attachments: Plate 1 – Vicinity Map Plate 2 – Site Plan Appendix A – Test Pit Logs and USCS Soil Legend APPENDIX A TEST PIT LOGS AND USCS SOIL LEGEND G-5261 CLEAN GRAVELS GW (little or no fines)GP DIRTY GRAVELS GM (with some fines)GC CLEAN SANDS SW (little or no fines)SP < 5% Fine Grained: GW, GP, SW, SP DIRTY SANDS SM > 12% Fine Grained: GM, GC, SM, SC (with some fines)SC 5 to 12% Fine Grained: use dual symbols Liquid Limit < 50%ML Liquid Limit > 50%MH Liquid Limit < 30%CL Liquid Limit > 50%CH Liquid Limit < 50%OL Liquid Limit > 50%OH Pt Sieve Size (mm)Sieve Size (mm) SILT / CLAY #200 0.075 SAND 0 - 4 0 -15 Very Loose < 2 < 0.25 Very soft FINE #40 0.425 #200 0.075 4 - 10 15 - 35 26 - 30 Loose 2 - 4 0.25 - 0.50 Soft MEDIUM #10 2 #40 0.425 10 - 30 35 - 65 28 - 35 Medium Dense 4 - 8 0.50 - 1.00 Medium Stiff COARSE #4 4.75 #10 2 30 - 50 65 - 85 35 - 42 Dense 8 - 15 1.00 - 2.00 Stiff GRAVEL > 50 85 - 100 38 - 46 Very Dense 15 - 30 2.00 - 4.00 Very Stiff FINE 19 #4 4.75 > 30 > 4.00 Hard COARSE 76 19 COBBLES BOULDERS ROCK FRAGMENTS ROCK PLATE A1 CONTENT OF FINES EXCEEDS 12% Cu = (D60 / D10) greater than 6 Cc = (D302 ) / (D10 * D60) between 1 and 3 Cu = (D60 / D10) greater than 4 Cc = (D302 ) / (D10 * D60) between 1 and 3 SILTY & CLAYEY SOILS Blow Counts N Unconfined Strength qu, tsf Description Bellevue, WA 98005 NOT MEETING ABOVE REQUIREMENTS Fax (425) 649-8758Phone (425) 649-8757 Blow Counts N Relative Density % Friction Angle N, degree Description > 76 mm >0.76 cubic meter in volume 13240 NE 20th Street, Suite 10 DETERMINE PERCENTAGES OF GRAVEL AND SAND FROM GRAIN SIZE DISTRIBUTION CURVE COARSE GRAINED SOILS ARE CLASSIFIED AS FOLLOWS:SANDS (More Than Half Coarse Grains Smaller Than No. 4 Sieve) SILTS (Below A-Line on Plasticity Chart, Negligible Organic) CLAYS (Above A-Line on Placticity Chart, Negligible Organic) HIGHLY ORGANIC SOILS NOT MEETING ABOVE REQUIREMENTS UNIFIED SOIL CLASSIFICATION SYSTEM (USCS) LEGEND OF SOIL CLASSIFICATION AND PENETRATION TEST CONTENT OF FINES EXCEEDS 12% ATTERBERG LIMITS BELOW "A" LINE. or P.I. LESS THAN 4 ATTERBERG LIMITS ABOVE "A" LINE. or P.I. MORE THAN 7 CLAYEY GRAVELS, GRAVEL-SAND-CLAY MIXTURES TYPICAL DESCRIPTION LABORATORY CLASSIFICATION CRITERIA ATTERBERG LIMITS BELOW "A" LINE with P.I. LESS THAN 4 ATTERBERG LIMITS ABOVE "A" LINE with P.I. MORE THAN 7 U.S. STANDARD SIEVE SOIL PARTICLE SIZE FRACTION Passing Retained GENERAL GUIDANCE OF SOIL ENGINEERING PROPERTIES FROM STANDARD PENETRATION TEST (SPT) SANDY SOILS INORGANIC CLAYS OF HIGH PLASTICITY, FAT CLAYS ORGANIC SILTS AND ORGANIC SILTY CLAYS OF LOW PLASTICITY ORGANIC CLAYS OF HIGH PLASTICITY PEAT AND OTHER HIGHLY ORGANIC SOILS CLAYEY SANDS, SAND-CLAY MIXTURES INORGANIC SILTS, ROCK FLOUR, SANDY SILTS OF SLIGHT PLASTICITY INORGANIC SILTS, MICACEOUS OR DIATOMACEOUS, FINE SANDY OR SILTY SOIL INORGANIC CLAYS OF LOW PLASTICITY, GRAVELLY, SANDY, OR SILTY CLAYS, CLEAN CLAYS WELL GRADED SANDS, GRAVELLY SANDS, LIITLE OR NO FINES POORLY GRADED SANDS, GRAVELLY SANDS, LITTLE OR NO FINES SILTY SANDS, SAND-SILT MIXTURES WELL GRADED GRAVELS, GRAVEL-SAND MIXTURE, LITTLE OR NO FINES POORLY GRADED GRAVELS, AND GRAVEL- SAND MIXTURES LITTLE OR NO FINES SILTY GRAVELS, GRAVEL-SAND-SILT MIXTURES > 203 mm FINE-GRAINED SOILS More Than Half by Weight Smaller Than No. 200 Sieve MAJOR DIVISION GROUP SYMBOL 76 mm to 203 mm ORGANIC SILTS & CLAYS (Below A-Line on Placticity Chart) More Than Half by Weight Larger Than No. 200 Sieve COARSE- GRAINED SOILS GRAVELS (More Than Half Coarse Grains Larger Than No. 4 Sieve) 0 10 20 30 40 50 60 0 10 20 30 40 50 60 70 80 90 100 110PLASTICITY INDEX (%)LIQUID LIMIT (%) CL-ML CL or OL MH or OH OL or ML A-LinePLASTICITY CHART FOR SOIL PASSING NO. 40 SIEVE 7 4 CH or OH Geotechnical Engineers, Geologists, & Environmental Scientists GEO Group Northwest, Inc. TEST PIT NO. TP-1 LOGGED BY AG TEST PIT DATE: DEPTH SAMPLE Water OTHER TESTS/ ft. USCS SOIL DESCRIPTION No. % COMMENTS 5 10 15 TEST PIT NO. TP-2 LOGGED BY AG TEST PIT DATE:09/17/2020 DEPTH SAMPLE Water OTHER TESTS/ ft. USCS SOIL DESCRIPTION No. % COMMENTS 5 10 15 TEST PIT LOGS PROPOSED DEVELOPMENT 35027 - 21ST AVE SW FEDERAL WAY, WA DATE 9/16/20 PLATE A2JOB NO. G-5261 09/17/2020 Total depth of test pit = 7 feet below ground surface (bgs) No groundwater seepage observed Geotechnical Engineers, Geologists, & Environmental Scientists GEO Group Northwest, Inc. S-1 Total depth of test pit = 6 feet below ground surface (bgs) No groundwater seepage observed Tan gravelly fine sandy SILT, dry, medium dense to dense ML Whitish tan gravelly fine sandy SILT with some cobbles, dry, cemented below 30" bgs, dense Tan gravelly fine sandy SILT, moist, dense, cemented below 18" bgsML S-1 S-2 S-2 Gray cemented gravelly fine sandy SILT, moist, dense to very dense (unweathered till) ML ML Probe 6-8" Probe 1" Probe 1/2" Probe 1/4" Probe 3" Probe 1-2" Gray with mottling gravelly sandy SILT, moist, cemented, dense to very dense (unweathered till) ML Probe 1/4" Probe 1/4" Probe 1" Probe 1-2" S-3 becoming gravelly silty SAND below 6' bgs (unweathered till)SM Probe 1/4" - occasional cobble at 18" bgs TEST PIT NO. TP-3 LOGGED BY AG TEST PIT DATE: DEPTH SAMPLE Water OTHER TESTS/ ft. USCS SOIL DESCRIPTION No. % COMMENTS 5 10 15 TEST PIT NO. LOGGED BY TEST PIT DATE: DEPTH SAMPLE Water OTHER TESTS/ ft. USCS SOIL DESCRIPTION No. % COMMENTS 5 10 15 TEST PIT LOGS PROPOSED DEVELOPMENT 35027 - 21ST AVE SW FEDERAL WAY, WA DATE 9/16/20 PLATE A3 09/17/2020 JOB NO. G-5261 Total depth of test pit = 8 feet - 9-inches below ground surface (bgs) No groundwater seepage observed Geotechnical Engineers, Geologists, & Environmental Scientists GEO Group Northwest, Inc. S-1 Tan gravelly fine sandy SILT, dry, dense ML Tan gravelly fine sandy SILT with some cobbles and the occasional boulder, dry, cemented below 3' bgs, dense S-2 Gray cemented gravelly fine sandy SILT, moist, dense to very dense (unweathered till) ML ML Probe 1" Probe 1/2" Probe 1/4" Probe 1/4" Probe 1" Probe 1" S-3 - becoming gravelly silty SAND below 6' bgs (unweathered till)SM S-4 Probe 1/4" - becoming gravelly SAND with some silt below 7.5' bgs, dense to very dense (unweathered till) SM/ SP