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Home My WebLink AboutRS_Geotechnical_Report_230120.pdf 13705 Bel-Red Road · Bellevue, Washington 98005 Phone 425/649-8757 · Fax 425/649-8758 November 15th, 2022 Project No. G-5804 Mr. Mac Kowalczyk 4124 Lincoln Ave NE Renton, Washington Email: mackowal@yahoo.com Subject: Recommendations for Tree Removal Proposed tree Removal 4124 Lincoln Ave NE Renton, Washington Reference: Comments Email City of Renton “Arborist Report: Washington Forestry Consultants Inc. Tree Assessment Kowalczyk property, August 17, 2022, Galen M Wright, Joshua Sharpes.” To whom it may concern, GEO Group Northwest, Inc., has reviewed the comments from the City of Renton pertaining to the removal of trees on the above-subject property. We understand that the client is seeking a Routine Vegetation Management Permit, from the City of Renton to remove up to all trees on the property, and that the City of Renton has requested a brief geotechnical report to review the arborist’s recommendations, assess geologic hazardous areas onsite and to provide any recommendations for mitigation as it relates to removing trees on the slope. SITE DESCRIPTION The above referenced property is located in north Renton. The property is surrounded by another single-family residential property to the south, Lincoln Avenue NE to the west, and a large, short November 15th 2022 G-5804 Proposed Tree Removal Page 2 GEO Group Northwest, Inc. plat development that is currently under construction stretching over multiple parcels to the north and east of the subject property. The site is approximately 63,850 square feet, and has a single- story home on the flatter, west portion that was built in 1946. The property is primarily flat with a gradual incline from the center of the property to the east most property line. There is a City of Renton low slope hazard covering the east side of the property, a small stretch of this slope is designated as sensitive and the north-east corner of the site is mapped as a protected slope. Please see Plate 1 – Site Location. Proposed Tree Removal Based on the above-referenced Arborist report, a total of 56 significant sized trees were found on the subject parcel. Thirty-three of those trees are in ‘Fair’ or better condition and the remaining 23 trees are in ‘Poor’ or worse conditions. The report states that healthy trees recommended for removal are tree numbers 12, 24, 26, 30, 31, 36, and 39. The City of Renton requires at least 26 trees to be retained on the parcel based on the square footage of the lot, however the client is seeking a Routine Vegetation Management Permit to remove all the trees onsite. Some of the trees to be removed are located on the west, flat side of the lot, however the majority of the trees to be removed are located in the 15-25% City of Renton Slope designation as well as the sensitive and protected slope areas. SITE INVESTIGATION Geology Based on a review of the geologic maps for the area, the surficial soil in the area is mapped as Glacial Till (Qvt). Glacial Till deposits typically consist of very compact mixtures of sand, silt, clay, and gravel that were overridden and over-consolidated by glacial ice during the Fraser glaciation period which ended approximately 13,000 years ago. Geotechnical Investigation: On November 3rd, 2022, a Staff Geologist from our office visited the site to observe subsurface conditions and locations of trees with proximity to steep slopes to assess general stability. The site was forested and had a thick ground covering of leaves. Onsite we also observed remnants of blackberries and plant debris. Both locations mapped as protected and sensitive slopes were probed and observed to be dense. November 15th 2022 G-5804 Proposed Tree Removal Page 3 GEO Group Northwest, Inc. To observe subsurface conditions, we used an excavator to dig two test pits onsite. Test Pit 1 (TP-1) was dug on the north-eastern side of the gentle slope. Below a thick ground cover of plant debris, we observed dense, tan silty sand with a high silt content to a depth of 2.5 feet. The soil appeared to lighten downwards and increase in density with depth. Towards the south-east corner of the site, Test Pit 2 (TP-2) was dug tot total depth of 2 feet below ground surface. This tan silty sand also lightened downwards, was dense, and had a very high silt content. We interpret these soils to be dense, native soils. No groundwater, channels or signs of erosion were observed onsite. For a more detailed description of subsurface soils please see Appendix A – Test Pit Logs. Geologic Hazardous Areas – Steep Slope The east half of the site is where the majority of trees are proposed to be removed. This east half of the site is mapped as a City of Renton Slope hazard and it gradually slopes up towards the east property line. The east side of the property is primarily a steady, terraced, 15%-25% slope that is separated from the flat side of the property by a sensitive 25%-40% slope. The only area onsite mapped as a protected slope greater than 40% is a small area less than 100 square feet in the north-east corner of the site. In our subsurface investigation, we observed that the site is underlain with dense to very dense native soils approximately 2 feet below the ground surface. We also probed the sensitive and protected slope and found them to be underlain with dense soils. In our professional opinion, because of the dense native soils observed onsite, removing any and all trees on the sensitive and protected slope areas is unlikely to affect slope stability. Conclusions and Recommendations In our opinion, due to the dense to very dense native soils that underly the site, the gentle angle of the slope and the small size of the sensitive and protected slope areas, the removal of trees will be unlikely to have a negative impact on slope stability and will not increase the risk of landslides or erosion in the area. Removing the current trees, leaving the roots in the protected slope area, and planting new, native species will likely promote stability on the slope and mitigate the risk of erosion and landslides on the slope. The trees should be removed in the order of priority as designated by the arborist with the dead, very poor, poor and fair condition trees removed first. The roots of the trees should remain behind in the protected slope area, and the client should revegetate, mulch and replant native species on locations designated as sensitive and protected slopes. November 15th 2022 G-5804 Proposed Tree Removal Page 4 GEO Group Northwest, Inc. Minimum Risk Statement In our opinion, provided that the included detail and recommendations provided in our report and this letter are properly implemented, it is our opinion the proposed development will not increase the potential for soil movement; and the risk of damage to the proposed development or from the proposed development to the adjacent properties from soil instability will be minimal. Please contact us if you have any questions. 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 – Site Plan Plate 3 – Critical Areas Appendix A – USCS Soil Classification Legend & Soil Boring Logs Source: King County iMap, 2021 SCALE NONE DATE 11/3/2022 MADE SH CHKD WC JOB NO.G-5804 PLATE 1 SITE LOCATION MAP PROPOSED TREE REMOVAL 4124 LINCOLN AVE NE RENTON, WASHINGTON Geotechnical Engineers, Geologists, & Environmental Scientists GEO Group Northwest, Inc. PROJECT SITE N SCALE:NO SCALE DRAWN:SH CHECKED:WC DATE: 11/3/2022 PROJECT NO.:G-5804 PLATE 2 SITE PLAN PROPOSED TREE REMOVAL 4124 LINCOLN AVE NE RENTON, WASHINGTONGeotechnical Engineers, Geologists, & Environmental Scientists GEO Group Northwest, Inc. N TP-1 TP-2 LEGEND TESPT PIT LOCATIONTP-1 SCALE:NO SCALE DRAWN:SH CHECKED:WC DATE: 11/3/2022 PROJECT NO.:G-5804 PLATE 2 HAZARDOUS SITE PLAN PROPOSED TREE REMOVAL 4124 LINCOLN AVE NE RENTON, WASHINGTONGeotechnical Engineers, Geologists, & Environmental Scientists GEO Group Northwest, Inc. N LEGEND SUBJECT PROPERTY November 15th 2022 G-5804 Proposed Tree Removal Page 5 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 More Than Half by Weight Larger Than No. 200 Sieve SANDS WELL GRADED SANDS, GRAVELLY SANDS, LITTLE OR NO FINES 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 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 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 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 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) 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 MH or OH U-Line PLASTICITY CHART FOR SOIL PASSING NO. 40 SIEVE 7 4 CH Geotechnical Engineers, Geologists, & Environmental Scientists GEO Group Northwest, Inc. ML A-Line ML or OL TEST PIT TP-1 LOGGED BY:SH DATE EXCAVATED: 11-3-2022 SURFACE ELEV: DEPTH USCS SOIL DESCRIPTION SAMPLE Water OTHER TESTS/ ft. No. % COMMENTS Probe 2" 1 Probe 1" 2 SM Probe 0.5" 3 4 5 6 7 8 9 10 11 12 TEST PIT LOG JOB NO.G-5804 DATE 11/9/2022 PLATE A1 PROPOSED TREE REMOVAL 4124 LINCOLN AVE NE RENTON, WASHINGTON Bottom of test pit at 2 feet 5 inches below ground surface. Groundwater not encountered. Hydraulically restrictive soils at approximately 2 feet 8"-1' of tan topsoil observed in forested area below shrubbery, roots, bark and leaves Geotechnical Engineers, Geologists, & Environmental Scientists GEO Group Northwest, Inc. lightening downwards SILTY SAND, come cobbles, light tan, slightly damp, dense, very high silt content, SILTY SAND, some cobbles, brown, slightly damp, dense, very high silt content 1 TEST PIT TP-2 LOGGED BY:SH DATE EXCAVATED: 11/3/2022 SURFACE ELEV: DEPTH USCS SOIL DESCRIPTION SAMPLE Water OTHER TESTS/ ft. No. % COMMENTS 1 SM 2 Probe 0.5" 3 4 5 6 7 8 9 10 11 12 TEST PIT LOG JOB NO.G-5804 DATE 9/11/2022 PLATE A2 PROPOSED DETACHED RESIDENCE 4124 LINCOLN AVE NE SEATTLE, WASHINGTON Bottom of test pit at 2 feet below ground surface. Groundwater not encountered. Topsoil 8", debris, roots, wood and leaves Geotechnical Engineers, Geologists, & Environmental Scientists GEO Group Northwest, Inc. lightening downwards SILTY SAND, come cobbles, light tan, slightly damp, dense, very high silt content, SILTY SAND, some cobbles, brown, slightly damp, dense, very high silt content