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HomeMy WebLinkAboutRS_Geotechnical_Report_241007_v1 13705 Bel-Red Road · Bellevue, Washington 98005 Phone 425/649-8757 · Fax 425/649-8758 July 27th 2022 G-5715 Mr. Tristan Bull Tristan.m.bull@gmail.com Subject: Geotechnical Engineering Investigation Proposed New Detached Garage 5424 NE 10th St, Renton, Washington Dear Mr. Bull: At your request, GEO Group Northwest, Inc., conducted a geotechnical engineering investigation of the above-subject location for a proposed new detached garage on your property in Renton, Washington. We understand that the proposed garage will be detached from the current residence and be centered on the property adjacent to a north-south running creek. The scope of our services included a review of the geologic maps of the area and providing a characterization of the subsurface soil and groundwater conditions encountered, preparation of boring logs (attached) and preparation of this geotechnical report. SITE CONDITIONS Site Description The project site is located in the north east portion of the city of Renton, Washington, as illustrated in Plate 1 – Site Location Map. The parcel is rectangle-shape and consists of 54,885 square feet. The site is bounded by other single family residential properties to the north, east, and west and NE 10th Street to the south. According to the city of Renton GIS, there is a high erosion hazard covering 40% of the lot, a City of Renton 15-25% regulated slope critical area mapped along the retaining wall of the current house and a small creek running north-south through the mid-west side of the property that will require a 15-foot setback from any structure. July 27th 2022 G-5715 Tristan Bull Page 2 GEO Group Northwest, Inc. The new garage is proposed to be built on the south-east portion of the lot between the creek and the existing house. The existing house was built in 1980 and has 2 stories and a finished basement with a total living space of 3240 square feet. The existing house is located on the south-east side of the lot. Description of Proposed Development We understand that the proposed project involves building a new detached garage to the west side of the existing house, to the south of the existing deck where there is currently a paved driveway. The creek to the west is mapped as being approximately 24 feet from the edge of the proposed garage at its closest point. The proposed two car garage will be 720 square feet in size and rectangular in shape. The garage will be made of timber framing with a pile supported slab floor. Please see Plate 2 – Site Plan. SITE INVESTIGATION Geologic Overview Based on a review of the geologic map for the area, the surficial soil in the area is mapped as a small peat deposit (Qlp) overlaying glacial till (Qgt). Peat contains minor amounts of silt and clay chiefly as basal beds, and a thin, widespread layer of volcanic ash. Thickness varies from 3 to 23 feet at the greatest depth and is locally very soft and under load my readily flow laterally. Glacial till, which is mapped to underly the peat deposit, is a dense mix of cemented and consolidated silt, gravel and sand that was deposited, overridden and over-consolidated by glacial ice during the Fraser glaciation period which ended approximately 13,000 years ago. Subsurface Investigation On June 14th 2022, a Staff Geologist from our firm, visited the site to perform a visual reconnaissance of the site and investigate the subsurface soil conditions. A single boring was drilled using a licensed drill excavation service. The boring location is illustrated on Plate 2 – Site Plan. Borehole 1 (BH-1) was drilled at the location of the south west corner of the proposed garage, adjacent to the pavement. The soils encountered were loose to a depth of 15 feet where they became dense throughout the rest of the 25-foot boring. July 27th 2022 G-5715 Tristan Bull Page 3 GEO Group Northwest, Inc. The first foot and a half of BH-1 consisted of a 3-inch layer of topsoil below a grass lawn that was underlaid by brown silty sand with organic material and some staining present. At a depth of 2.5 feet, the silty sand transitioned to a grey color increasing in organic content downwards. Below the silty sand at 5 to 10 feet below the ground surface the soil transitioned to lose, brown, peat with a moisture content between 200 and 300%. Water was encountered at approximately 7.5 feet below the ground surface. Below 15 feet the soil became dense and at 15 feet dense, grey, wet, silty sand was once again observed. A gravel lens was encountered at approximately 18 feet below ground surface, followed by the same grey silty sand to 25 feet. At a final depth of 25 feet the silty sand was observed to be stained and a reddish-tan color, the boring reached refusal this depth either due to an obstruction, or very dense soil conditions. For a more detailed description of the soils encountered, please refer to the boring logs in Appendix A. ENVIRONMENTALLY CRITICAL AREAS As per the City of Renton GIS, the following critical areas are mapped onsite High Erosion Hazard The project site is located within a high erosion hazard critical area which is mapped to cover approximately 40% of the site covering the north-east corner. The proposed project will not intercept the mapped erosion hazard and is on one of the flattest parts of the lot. The existing house and the fully vegetated backyard both intercept this erosion hazard. There were no signs of erosion observed on site and the area is well vegetated. In our professional opinion, due to the flat topography of the project location, and because the location of the proposed garage is outside the mapped hazard, the risk of the project being susceptible to erosion is low. Please see Plate 3 – Critical areas. City of Renton Regulated Slope (15-25%) The slope in question is mapped along the west side of the existing house, along the driveway, and adjacent to where the proposed garage will be located. This slope is mapped as being in the lowest risk category at 15-25%. The only part of this small slope that the project will disturb has already been leveled and paved. The rest of the slope has had retaining walls installed along it or July 27th 2022 G-5715 Tristan Bull Page 4 GEO Group Northwest, Inc. has been graded for the existing home. The project is will be constructed on level ground where the current pavement is located, thus not disturbing the slope at all. For these reasons it is our opinion this project poses no risk to the mapped critical area slope. City of Renton Regulated Shoreline - Stream There is a stream onsite running north south through the west side of the property. The stream runs through a small natural depression and is surrounded by a small marshy area. The city of Renton requires a 15-foot set back from all structures, and in our opinion, as long as the set back and the following recommendations outlined below are adhered to the proposed project will not impact the stream. CONCLUSIONS AND RECOMMENDATIONS The site is underlain with loose soils and peat to a depth of 15 feet below ground surface. Below this peat, to a depth of 25 feet dense silty Sand soils with a high silt content were encountered and interpreted to be glacial till. The garage will be constructed on a reinforced slab. We recommend the slab be supported with number 4 rebar,12 inches on center in a grid. A stable pad for equipment to drive on to access the site is also recommended for the construction site due to the soft and spongy nature of the soil. Soils that are anticipated to be acceptable for building support were encountered at approximately 15-20 feet below ground surface. In consideration of these conditions, we recommend the garage be supported on a system of 2-to-4-inch diameter galvanized pipe piles. Our recommendations for installation are outlined below. Seismicity Evaluation Please see referenced slope stability analysis for further modeling of seismic factors in slope stability. In accordance with the 2018 International Building Code, the site classification is Site Class D (stiff soil). Glacially consolidated soils have a high shear strength and the potential for landslides, liquefaction and/or lateral spreading is negligible during a strong motion earthquake. In our opinion, the site is stable and the risk of a surface rupture, resulting from a large magnitude seismic event, is very low. No seismic mitigation measures are recommended. Based on the assigned Site Class D the design calculations per the 2018 IBC, are as follows: Ss = 1.395g Sms = 1.674 Sds = 1.116 S1 = 0.478g Sm1 = null Sd1 = null July 27th 2022 G-5715 Tristan Bull Page 5 GEO Group Northwest, Inc. The peak ground acceleration for the site, adjusted for the assigned site class, is 0.594 g based on USGS seismic hazard design mapping per the 2018 IBC. Foundations Small-Diameter Pipe Pile System Based on the findings from the borings drilled from this study supplemented with previous subsurface data nearby, suitable bearing soils are expected to be present at a depth of 15 to 20 feet below ground surface at the project area of the site. In consideration of these conditions, it is our opinion that the proposed attached garage should be supported on small-diameter steel pipe piles. The pipe piles should be driven until the condition is reached where the resistance of the subsurface soils sufficiently retards or terminates the advancement of the pile; this condition commonly is called refusal. The depth at which refusal is achieved is dependent upon the type of pipe and driving hammer that are used, and the characteristics of the subsurface soils that the pile encounters. The following table presents design criteria for commonly-available combinations of driving hammers and pipe sizes. The following capacities include a factor of safety of 2. Pipe Pile Design Criteria Pipe Diameter Pipe Wall Thickness Hammer Weight Class Hammer Type Refusal Criteria* Allowable Capacity 2 inch Schedule 80 140 pound Rhino 60 sec/inch 3 tons‡ 3 inch Schedule 40 650 pound TB225† 12 sec/inch 6 tons** 3 inch Schedule 40 850 pound TB325† 10 sec/inch 6 tons** 4 inch Schedule 40 850 pound TB325† 16 sec/inch 10 tons** July 27th 2022 G-5715 Tristan Bull Page 6 GEO Group Northwest, Inc. 4 inch Schedule 40 1100 pound TB425† 10 sec/inch 10 tons** * = Maximum penetration rate to be sustained through at least 3 time cycles of continuous driving. ‡ = Capacity is limited to 2 tons in City of Seattle soil liquefaction and peat settlement critical areas, and length is limited to 30 feet for projects in City of Seattle. † = Teledyne hydraulic hammer model number, or equivalent. ** = Load testing of 3% of total number of piles (min. of 1, max. of 5 piles) required during installation in City of Seattle. Based on findings from our subsurface investigation, we estimate that finished pile lengths can be more than 30 feet below ground surface and can be checked with a test pile. Groundwater was encountered approximately 7.5 feet below ground surface. Therefore, we recommend that the piles consist of galvanized pipe. We estimate that the maximum settlement of the pipe piles should be one-quarter (1/4) inch or less. No reduction in the pile capacities is required if the pile spacing is at least three times the pile diameter; otherwise, the capacities should be re-evaluated to account for group effects. A one-third increase in the above allowable pile capacities can be used when considering short- term transitory wind or seismic loads. By themselves, pipe piles do not generate lateral capacities. Lateral forces can be resisted by moment frame action between the piles and the structural beams and passive pressure against adjacent silt. To fully mobilize the passive pressure resistance, the footings or grade beams should be poured neat against competent native soil or should be backfilled with compacted structural fill. Under such conditions, the footings can be designed for an allowable passive soil pressure of 350 pcf equivalent fluid weight for lateral resistance. A coefficient of friction of 0.35 can be used between the subgrade soils and the footings or grade beams. The performance of pipe piles is dependent on how and to what bearing stratum the piles are installed. Since a completed pile in the ground cannot be observed, it is critical that judgment and experience be used as a basis for determining the driving refusal and acceptability of a pile. Therefore, we recommend that GEO Group Northwest, Inc. be retained to monitor the pile installation operation, collect and interpret installation data, and verify suitable bearing stratum. We also suggest that the contractor’s equipment and installation procedures be reviewed by us prior to pile installation to help mitigate problems which may delay the progress of the work. Grading and Earthwork July 27th 2022 G-5715 Tristan Bull Page 7 GEO Group Northwest, Inc. Site Clearing and Erosion Control The area where construction work will be performed should be cleared of vegetation, topsoil, organics, debris, and any other deleterious materials that are found. These materials should be hauled off site or used for landscaping, as appropriate; they should not be used as structural fill or basement wall backfill for the project. Temporary erosion and sedimentation controls (TESCs) such as silt fences should be installed as part of site clearing activities. The silt fences or other barrier controls should be placed along the cross-slope boundaries of the disturbed areas to prevent sediment-laden runoff from being discharged off site. Exposed soils, including stockpiled soils, should be covered with plastic sheeting when they are not being worked. Subgrade Preparation Soils in areas to receive structural fill, concrete slabs, or pavements, should be prepared to a firm, unyielding condition. The prepared subgrade should be observed and approved by the geotechnical engineer. Any detected soft spots or disturbed areas should be compacted or excavated and replaced with compacted structural fill or crushed rock as directed by the geotechnical engineer. Structural Fill Structural fill is typically defined as earthen material that is placed below buildings (including foundations and on-grade slab floors), sidewalks, driveways, or other structures, and provides support to those structures. Soils that meet the material specifications for structural fill as presented below in this report, or are otherwise approved by the geotechnical engineer, can be used for structural fill. Structural fill material should be placed and compacted in accordance with the recommendations provided below or as otherwise approved by the geotechnical engineer during construction. Material Specifications Materials to be used as structural fill should not contain deleterious materials nor rocks or lumps larger than 3 inches in its greatest dimension. During wet weather, we recommend that the material have a fines content (passing a #200 sieve) of less than 5 percent. All material should be placed at or near its optimum moisture content. If the material is too wet to be compacted to July 27th 2022 G-5715 Tristan Bull Page 8 GEO Group Northwest, Inc. the required degree, it will be necessary to dry the material by aeration (which may be difficult) or replace the material with an alternative suitable material, in order to be capable of achieving the required compaction. The site soils are unlikely to be useable under most conditions due to their silty character and higher than optimum moisture content. Compaction Specifications Structural fill material should be compacted to at least 92 percent of its maximum dry density as determined by ASTM D1557 (Modified Proctor Test), unless otherwise authorized by the geotechnical engineer, and with the following exceptions. Structural fill material under exterior slabs or pavements should be compacted to at least 90 percent of its maximum dry density, except for the top 12 inches of the material, which should be compacted to at least 95 percent of its maximum dry density as determined by ASTM D1557. Structural fill material should be spread and compacted in lifts that are 10 inches or less in thickness in an un-compacted state. The compacted fill material should be field tested by using ASTM Designations D2922 and D3017, Nuclear Probe Method, to verify that the required degree of compaction has been achieved. Slab-on-Grade Floors The garage concrete slab should be constructed as a structurally supported slab. We recommend the slab consist of a 6-inch slab reinforced with number 4 rebar,12 inches on center in a grid. During preparation of the slab subgrade, any areas of the subgrade that have been disturbed by construction activity should be either re-compacted to form a stable subgrade. The pin piles should be on 5-foot center to support the structural slab. To avoid moisture build-up on the subgrade, floor slabs should be placed on a capillary break, which is in turn placed on the prepared subgrade. The capillary break should consist of a layer, at least 6 inches thick, of free-draining crushed rock or gravel containing no fines and no more than five percent material finer than a No. 4 sieve. A vapor barrier should be placed over the capillary break to reduce upward transmission of water vapor through the slab, if such transmission is undesirable. July 27th 2022 G-5715 Tristan Bull Page 9 GEO Group Northwest, Inc. LIMITATIONS The findings and recommendations stated herein are based on field observations, our experience on similar projects and our professional judgment. The recommendations presented herein are our professional opinions 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 project schedule and budget constraints. No warranty is expressed or implied. In the event that site conditions are found to differ from those described in this report, we should be notified so that the relevant recommendations in this report can be reevaluated and modified if appropriate. CLOSING We appreciate the opportunity to provide you with geotechnical engineering services for this project. Please do not hesitate to contact us if you have any questions regarding this report. Sincerely, GEO Group Northwest, Inc. Sophie Holt, G.I.T. William Chang, P.E. Staff Engineering Geologist Principal Engineer Attachments: Plate 1 – Site Location Map Plate 2 – Borehole Location Plate 3 – Site Plan Plate 4 – Critical Areas July 27th 2022 G-5715 Tristan Bull Page 10 GEO Group Northwest, Inc. Plate 4 – Proposed Development Appendix A – USCS Soil Classification Legend & Soil Boring Logs Source: King County iMap, 2021 SCALE NONE DATE 7/22/2022 MADE SH CHKD WC JOB NO.G-5715 PLATE 1 SITE LOCATION MAP PROPOSED NEW GARAGE 5424 NE 10TH ST RENTON, WASHINGTON Geotechnical Engineers, Geologists, & Environmental Scientists GEO Group Northwest, Inc. PROJECT SITE N SCALE NONE DATE 7/22/2022 MADE SH CHKD WC JOB NO.G-5715 PLATE 2 PROPOSED DEVELOPMENT PROPOSED NEW GARAGE 5424 NE 10TH ST RENTON, WASHINGTON Geotechnical Engineers, Geologists, & Environmental Scientists GEO Group Northwest, Inc. N LEGEND BOREHOLE LOCATION PROPOSED DEVELOPMENT BH-1 SCALE:1" = 20' DRAWN:SH CHECKED:WC DATE: 7/23/2022 PROJECT NO.:G-5715 PLATE 1 PROPOSED DEVELOPMENT PROPOSED NEW GARAGE 5424 NE 10TH ST RENTON, WASHINGTONGeotechnical Engineers, Geologists, & Environmental Scientists GEO Group Northwest, Inc. 20 400 SCALE: 1 INCH = 6 FEET N LEGEND BOREHOLE LOCATION BH-1 SCALE: DRAWN:SH CHECKED:WC DATE: 7/22/2022 PROJECT NO.:G-5715 PLATE 3 ENVIRONMENTALLY CRITICAL AREAS PROPOSED NEW GARAGE 5424 NE 10TH ST RENTON, WASHINGTONGeotechnical Engineers, Geologists, & Environmental Scientists GEO Group Northwest, Inc. N LEGEND Erosion Hazard High July 27th 2022 G-5715 Tristan Bull Page 11 GEO Group Northwest, Inc. APPENDIX A USCS Soil Classification Legend & Soil Boring Logs CLEAN GRAVELS GW (little or no fines)GP DIRTY GRAVELS GM (with some fines)GC CLEAN SANDS SW (little or no fines)SP DIRTY SANDS SM (with some fines)SC Liquid Limit < 50%ML Liquid Limit > 50%MH Liquid Limit < 50%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 2.00 #40 0.425 10 - 30 35 - 65 28 - 35 Medium Dense 4 - 8 0.50 - 1.00 Medium Stiff COARSE #4 4.75 #10 2.00 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 0.75"19 #4 4.75 > 30 > 4.00 Hard COARSE 3"76 0.75"19 COBBLES BOULDERS ROCK FRAGMENTS ROCK PLATE A1 COARSE- GRAINED SOILS GRAVELS (More Than Half Coarse Fraction is Larger Than No. 4 Sieve) WELL GRADED GRAVELS, GRAVEL-SAND MIXTURE, LITTLE OR NO FINES CONTENT OF FINES BELOW 5% SOIL CLASSIFICATION & PENETRATION TEST DATA EXPLANATION UNIFIED SOIL CLASSIFICATION SYSTEM (USCS) MAJOR DIVISION GROUP SYMBOL TYPICAL DESCRIPTION LABORATORY CLASSIFICATION CRITERIA Cu = (D60 / D10) greater than 4 Cc = (D30)2 / (D10 * D60) between 1 and 3 POORLY GRADED GRAVELS, AND GRAVEL-SAND MIXTURES LITTLE OR NO FINES CLEAN GRAVELS NOT MEETING ABOVE REQUIREMENTS SILTY GRAVELS, GRAVEL-SAND-SILT MIXTURES CONTENT OF FINES EXCEEDS 12% GM: ATTERBERG LIMITS BELOW "A" LINE. or P.I. LESS THAN 4 CONTENT OF FINES BELOW 5% Cu = (D60 / D10) greater than 6 Cc = (D30)2 / (D10 * D60) between 1 and 3 (More Than Half Coarse Fraction is Smaller Than No. 4 Sieve) POORLY GRADED SANDS, GRAVELLY SANDS, LITTLE OR NO FINES CLEAN SANDS NOT MEETING ABOVE REQUIREMENTS SILTY SANDS, SAND-SILT MIXTURES CONTENT OF FINES EXCEEDS 12% ATTERBERG LIMITS BELOW "A" LINE with P.I. LESS THAN 4 CLAYEY SANDS, SAND-CLAY MIXTURES ATTERBERG LIMITS ABOVE "A" LINE with P.I. MORE THAN 7 CLAYEY GRAVELS, GRAVEL-SAND-CLAY MIXTURES GC: ATTERBERG LIMITS ABOVE "A" LINE. or P.I. MORE THAN 7 INORGANIC CLAYS OF LOW PLASTICITY, GRAVELLY, SANDY, OR SILTY CLAYS, LEAN CLAYS Less Than Half by Weight Larger Than No. 200 Sieve (i.e., fines) INORGANIC CLAYS OF HIGH PLASTICITY, FAT CLAYS ORGANIC SILTS & CLAYS (Below A-Line on Plasticity Chart) ORGANIC SILTS AND ORGANIC SILTY CLAYS OF LOW PLASTICITY ORGANIC CLAYS OF HIGH PLASTICITY FINE-GRAINED SOILS SILTS (Below A-Line on Plasticity Chart, Negligible Organics) INORGANIC SILTS, ROCK FLOUR, SANDY SILTS OF SLIGHT PLASTICITY INORGANIC SILTS, MICACEOUS OR DIATOMACEOUS, FINE SANDY OR SILTY SOIL CLAYS (Above A-Line on Plasticity Chart, Negligible Organics) More Than Half by Weight Larger Than No. 200 Sieve SANDS WELL GRADED SANDS, GRAVELLY SANDS, LITTLE OR NO FINES HIGHLY ORGANIC SOILS PEAT AND OTHER HIGHLY ORGANIC SOILS SOIL PARTICLE SIZE GENERAL GUIDANCE FOR ENGINEERING PROPERTIES OF SOILS, BASED ON STANDARD PENETRATION TEST (SPT) DATA FRACTION U.S. STANDARD SIEVE Passing Retained SANDY SOILS SILTY & CLAYEY SOILS 76 mm to 203 mm > 203 mm > 76 mm 13705 Bel-Red Road >0.76 cubic meter in volume Phone (425) 649-8757 E-mail: info@geogrourpnw.com Description Bellevue, WA 98005 Blow Counts N Relative Density, % Friction Angle N, degrees Description Blow Counts N Unconfined Strength qu, tsf 0 10 20 30 40 50 60 0 10 20 30 40 50 60 70 80 90 100PLASTICITY INDEX (%)LIQUID LIMIT (%) CL-ML CL MHor OH U-Line PLASTICITY CHART FOR SOIL PASSING NO. 40 SIEVE 74 CH Geotechnical Engineers, Geologists, & Environmental Scientists GEO Group Northwest, Inc. ML A-Line ML or OL (Form: BORELOG.XLS) Logged By:SH Date Drilled:6/14/2022 Surface Elev. Drilled By: Depth USCS Description ft.Code Loc.No. 1 Brown SILTY SAND, some organics, 15.9sand is fine to very fine, some mottling 2 SM Grey SILTY SAND, some organics, spongy 1,1,2 17.5texture, soft, sand is fine to very fine, some mottling (N=3) 5 3 Pt Brown peat, loose, organics 1,1,1 300.6 (N=2) 4 Pt Dark brown peat, loose, organics, 1,0,1 (N=1)238.7 10 5 Pt Dark brown peat, loose, organics, 1 242.3 (N=1) 15 6 SM Grey SILTY SAND, medium dense, wet, sand is 5,8,9 21.6medium to fine grained (N=17) Gravel lense encountered 20 7 SM Grey SILTY SAND with gravel, wet, medium dense 5,8,10 19.0Grey silt lense up top (N=18) 25 LEGEND:2" O.D. SPT Sampler Water Level noted during drilling 3" O.D. California Sampler Water Level measured at later time, as noted JOB NO. G-DATE PLATE A2 5424 NE 10TH ST SEATTLE, WASHINGTON 6/14/2022 BORING LOG PROPOSED GARAGEElevationBORING NO. B - 1 Page 1 of 1 Approx. 206' CN Drilling Other Tests/ Comments Water Content % SPT Blow Counts Sample Geotechnical Engineers, Geologists, & Environmental Scientists GEO Group Northwest, Inc. (Form: BORELOG.XLS) Logged By:SH Date Drilled:2/14/2022 Surface Elev. Drilled By: Depth USCS Description ft.Code Loc.No. 25 8 SM Tan-reddish SILTY SAND, staining, discoloration 22, 50 medium dense, sand is fine grained,(N=50) 30 35 40 45 50 LEGEND:2" O.D. SPT Sampler Water Level noted during drilling 3" O.D. California Sampler Water Level measured at later time, as noted JOB NO. G-5634 DATE PLATE A2 22-50 for 5.5ElevationBORING NO. B - 1 Page 1 of 1 Approx. 206' Geologic Drill 615 NE 55TH ST SEATTLE, WASHINGTON 3/10/2022 BORING LOG PROPOSED SINGLE-FAMILY RESIDENCE Other Tests/ Comments Water Content % SPT Blow Counts Sample Geotechnical Engineers, Geologists, & Environmental Scientists GEO Group Northwest, Inc. Depth of boring: 26.5 feet. Drilling Method: Hollow-stem auger. Sampling Method: 2"-O.D. standard penetration test sampler driven with 140 lb. hammer and cathead. Groundwater encountered during drilling.