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HomeMy WebLinkAboutSWP272312(1) TERRA ASSOCIATES Inc. ' Consultants in Geotechnical Engineering, Geology • ' and - Environmental Earth Sciences July 12, 1990 Project No. T-1416 Mr. Ron Deverman Wilsey and Ham Pacific 1980 - 112th Avenue NE P.O. Box C-97304 Bellevue, Washington 98009 - 7730 Subject: Geotechnical Engineering Study Forrest Creste Apartments 12200 - 12400 Block 138th Avenue SE Renton, Washington Dear Mr. Deverman: As requested, we have conducted a geotechnical engineering study for the proposed Forrest Creste Apartments in Renton. The location of the project is shown on Figure 1. The purpose of our study was to explore subsurface and groundwater conditions at the site in order to provide geotechnical information on the feasibility of developing this site, recommendations for site preparation, foundations for the proposed structures and site drainage. We also prepared information to be incorporated in an Environmental Impact Statement (EIS). The scope of our study included site reconnaissance, excavation of several test pits and probes across the site, laboratory testing of representative soil samples, engineering analyses and the preparation of this report. This report presents the - results of our observations and studies along with supporting field and laboratory test data. SUMMARY -- Our study indicates that the site is underlain at a shallow depth by loose to medium dense silty sand overlying dense, gravelly, silty sand till soils. Conventional spread footings may be used as foundation support for the proposed apartments. These footings may bear on the undisturbed, competent, native soils existing at the site or on compacted fills placed above the competent, native soils. Old fill soils up to eight feet thick are present in the northwestern corner of the site. These fills should be removed from building and parking areas. 12525 Willows Road, Suite 101, Kirkland, Washington 98034 • Phone (206) 821-7777 • FAX 821-4334 Mr. Ron Deverman July 12, 1990 Moderate groundwater seepage was noted in several of the test pits in the western portion of the site. The groundwater appears to be perched on top of the till and will produce seepage in two to three foot cuts. To minimize difficulties in working with the moisture sensitive native soils, we suggest that grading be performed during the dry season and when soil moistures are reduced. This will improve the chances for using the native materials as fill material. If grading is to be performed in wet weather, you should plan _ on importing significant quantities of clean granular soils for use as structural fill. _ PROJECT DESCRIPTION We understand that the 11.73 acre site is planned to be developed as a 200 unit, multi- family apartment complex. Twenty-one residential buildings with separate carports and one recreation building are planned for the project. The structures will probably be two and three story, wood-frame buildings with slab-on-grade floors. Some daylight basements may be constructed. At the time of our study, a site plan showing building and roadway layout superimposed on topography was provided to us. No grading plans or building details were provided to us. However, we expect minor cuts and fills will be required to provide building pads, basement excavations and pavement areas on the gently sloping site. Considering the existing topography on the site, the cuts and fills may have magnitudes on the order of five to ten feet or so. Basement walls are likely for the buildings in the deeper cuts. Access to the site will be from 138th Avenue SE and also from the property to the south. When project plans are finalized, Terra Associates should be notified so that we can review the building and grading plans and prepare final recommendations. FIELD EXPLORATION AND LABORATORY TESTING The subsurface exploration for this study was conducted on June 13 and 18, 1990. Subsurface conditions on the site were explored by excavating 10 test pits using a backhoe provided by All Seasons Construction of Duvall, Washington. In addition, five probes were performed using hand auger. The test pits were excavated at the approximate locations shown on the Test Pit Location Map, Figure 2. The locations of these test pits and probes were approximately determined by pacing from assumed property corners. Elevations at test pit and probe locations were determined by interpolating between contours shown on the topographic survey provided to us. _ The field exploration was monitored continuously by our geologist who classified the soils encountered, maintained a log of each test pit and probe, obtained representative soil samples and observed pertinent site features. All samples were visually classified in _ accordance with the Unified Soil Classification System described on Figure 3. The logs of the test pits are attached to this report as Figures 4 through U. The probe logs are presented in Table A. Project No. T-1416 Page No. 2 Mr. Ron Deverman July 12, 1990 - The soil classifications shown on these logs represent our interpretation of the field logs and reflect the results of visual examinations as well as laboratory tests performed on samples obtained from the test pits. Representative soil samples collected from the test pits were returned to our laboratory for further examination and testing. Moisture contents were determined for all samples. Sieve analyses were performed on selected samples. The results of our laboratory testing are shown on the test pit logs and on the grain size analysis plots, Figure 9. SITE CONDITIONS Surface The project area occupies 11.73 acres of gently sloping terrain east of 138th Avenue SE and south of SE 122nd Street in Renton, Washington. The topographic relief across the - site is approximately 60 feet, rising from Elev. 395 feet in the southwest corner to Elev. 455 feet in the northeast portion of the site. In general, the site slopes toward the southwest at inclinations of 10 to 20 percent. Some old fill piles are present in the extreme northwestern portion of the site. The site is vegetated with dense, middle-aged fir, cedar, maple and alder. The - underbrush in the southwestern portion of the site becomes very thick. The extreme northwestern portion of the site has been utilized as horse pasture and is mostly barren of vegetation. A horse stable is present in this area. A small stream crosses the northern portion of the site. The origin of this flow appears to be, in part, from drainage off the development to the northeast of the site. This drainage has been routed onto the site from near the projected intersection of 140th Avenue SE and SE 122nd Street. Some erosion has occurred a fair distance downslope from the outfall. The site is bounded on the west by 138th Avenue SE, across which is an undeveloped site. The southern property boundary adjoins an undeveloped, forested land. The eastern - margin of the site lies adjacent to forested property and sparse single family lots. Across the northern boundary of the site is a single family residential area. Subsurface The U.S. Geological Survey has mapped the geology of the study site as Vashon till. The till was deposited about 15,000 years ago along the base of the Puget Lobe during the Fraser Glaciation. In general, our findings support the USGS determination. In each test pit, an average of about 6 inches of duff and topsoil were found to overlie reddish brown, silty sand with gravel. This deposit is underlain at depths of two to four feet by olive to gray, medium dense to dense, gravelly, silty sand till. This till material extends to depths exceeding 10 feet, the maximum depth explored. Project No. T-1416 Page No. 3 - Mr. Ron Deverman July 12, 1990 Groundwater Groundwater was encountered in many of the test pits and probes excavated on the site. -- Light to heavy seepage was noted from the top of the till unit at depths of two to four feet throughout the entire western and northwestern portions of the site. The perched groundwater may be more prominent in the winter months when precipitation is greater. Groundwater conditions in the area surrounding the site were evaluated using well logs on file at the Department of Ecology. From these logs, there is evidence of a regional groundwater table at approximate Elevation 350 having a gradient to the west. Slope Stability The slopes on the site are gentle to moderate. The steepest slopes on the site were approximated at 15 to 20 percent. Given that the entire site is underlain by dense glacial till, the slopes should remain in their presently stable condition, provided erosion prevention measures are taken. The seismic hazard associated with glacial till soils on gentle to moderate slopes is low. The area lies outside Class III Seismic Hazard zones as delineated in the King County Sensitive Area Map Folio. DISCUSSION AND RECOMMENDATIONS Based on our subsurface exploration and field observations, it is our opinion that the site is suitable for the proposed apartment complex. Buildings may be founded on conventional spread footings bearing on firm native soils or on compacted structural fill. - If native soils become disturbed or wet, they should be removed and replaced with clean crushed gravel or structural fill. Roadways may be built on recompacted native soils after removal of fills and organic rich soils or on structural fill. Due to the moisture sensitivity and relatively low permeability of the site soils, we suggest you plan to conduct the earthwork during the dry season and when surficial soils are not excessively moist. In wet weather, it will be very difficult to compact the onsite soils. In dry weather, the likelihood of using the onsite soils for fill will improve substantially. If grading work must be performed in wet weather, you should allow for import of -- significant quantities of clean granular soil for use as fill. We anticipate that minor cuts and fills will be required for most roads and building pads. Permanent cut slopes should be graded to 2:1 (Horizontal:Vertical). Temporary cuts up to ten feet high may be made at 1:1. Fills should be constructed in accordance with recommendations in the Site Preparation and Grading section of this report and should _ be made at inclinations of 2:1. Project No. T-1416 Pape No. 4 • • -- Mr. Ron Deverman July 12, 1990 - If cuts encounter seepage during the initiation of earthwork, interceptor drains should be constructed on the uphill side of roadways and building pads to prevent the working area from becoming wet. Proper drainage should be installed around the perimeter of all - foundations. This report has been prepared in accordance with generally accepted geotechnical engineering practices. We do not guarantee project performance in any respect, only that our work meets normal standards of professional care. No other warranty, expressed or implied, is made. This report is the property of Terra Associates and is intended for -- specific application to this project and for your exclusive use. The following sections of this report describe our recommendations in greater detail. Foundations Apartment buildings to be constructed on the project site may be supported on continuous and/or isolated spread footings bearing on the competent, native soils present below the topsoil and organic-rich layers or on compacted, structural fill placed above competent, native soils. The near surface soils below the topsoil are loose. Hence, depending on the depth of the excavation required to reach design footing grade, the native soils may need to be recompacted in place. If soils become disturbed and softened by excessive moisture due to seepage into excavations or rain, these materials should be removed and clean gravel should be placed in the footing excavations. The gravel mat should extend beyond the edge of the footing a distance equal to its depth. Continuous and individual spread footings for the proposed buildings may be designed for an allowable bearing pressure of 3,000 pounds per square foot. Footings should extend to a minimum depth of 18 inches below the lowest, adjacent, outside finish-grade. A minimum width of 12 inches should be used for continuous footings and individual spread - footings should have a minimum size of 18 inches. A one-third increase in the above bearing pressures may be used when considering wind or seismic loads. All footings should be provided with steel reinforcement in accordance with structural requirements. Old fills are present in the northwestern corner of the site. For any structures to be constructed in this area, foundations and slabs should bear on native soils beneath the -- existing fills. Alternatively, the fills may be removed and replaced by structural fill. Settlements We anticipate that the total settlements for the apartment buildings supported on the competent, native soils, bedrock, or on compacted, structural fill will be less than one-half inch. Long-term differential settlement of buildings between the center and the edges should be less than one-quarter inch. The majority of the settlements should occur during construction. Project No. T-1416 Page No. 5 Mr. Ron Deverman July 12, 1990 Slabs-On-Grade Concrete floor slabs, if used, may be constructed as slabs on grade supported either on - the competent, native soils or on structural fill. We recommend that four inches of a free- draining gravel, such as 1/4 to 3/8 inch pea gravel, be placed below the slab to act as a capillary break. In addition, a plastic membrane with a thickness of ten to twelve mils should be placed above the gravel to act as a vapor barrier for additional moisture protection. Basement and Retaining Walls If lower level basements are planned, or if retaining walls are needed at grade changes on the site, the walls should be designed to resist the lateral pressure imposed by an equivalent fluid weighing 40 pounds per cubic foot (pcf). If walls are restrained from free movement at the top, they should be designed for an additional uniform lateral pressure of 100 pounds per square foot. These pressures assume a maximum height of ten feet and that no surcharge loads will occur. Please contact us for supplementary recommendations if conditions are expected to be different. The basement walls and retaining walls should be provided with a continuous blanket of free-draining material at least twelve inches wide. A perforated pipe should be placed at the footing level to collect water and discharge it to the storm drain system. Horizontal Loads Horizontal structural loads carried to the foundations may be resisted by both friction forces on the base of foundations and passive resistance on the sides of foundations. A coefficient of 0.35 may be used between concrete and soil. Resistance to lateral loads may also be computed as passive earth pressures exerted by the soils adjacent to the foundations. We recommend using an equivalent fluid weight of 300 pounds per cubic foot where the foundations are poured neat against undisturbed soil, or where the backfill is compacted in accordance with the requirements for structural fill. Site Drainage Surface gradients across the site should be created to direct runoff away from the apartment buildings and towards suitable discharge facilities. If cuts encounter seepage during the initiation of earthwork, provisions should be made to install interceptor drains or ditches along the uphill side of road alignments and building cuts. These drains will prevent shallow subsurface drainage from reaching the work area and creating unfavorable soil conditions. Once detailed grading plans have been prepared, we would be pleased to review them and provide our input for additional drainage requirements, if needed. Project No. T-1416 PauP Nn. 0 Mr. Ron Deverman July 12, 1990 Perimeter foundation drains should be installed and tightlined away from the apartment buildings. Drains should be at the levels of footing bottoms or crawl space levels, whichever is lower. Roof gutter drains should be separately tightlined away from the buildings. All drains should be discharged into the storm drain system. The drainage entering the site from the northeast should be routed into the storm drain -- system proposed for the site. Site Preparation and Grading The site soils are very moisture sensitive and for this reason, we recommend conducting earthwork only when soil moisture contents are low. The building and pavement areas should be stripped and cleared of vegetation and topsoil. The stripped topsoils may be used as berms or in nonstructural areas. Old fill should also be removed from building and roadway areas. Following stripping, any loose areas noted should be over excavated and replaced with structural fill or crushed rock to a depth that will provide a stable base. If the over- excavated area remains soft and wet, a stable subgrade may be constructed by placing a geotextile in the bottom of the excavation and placing clean, crushed rock over it. Permanent cut slopes should be made at stable inclinations of 2:1 (Horizontal:Vertical). Fill slopes should also be made at inclinations of 2:1. Temporary cuts up to ten feet high may be made at inclinations of 1:1. Cuts in till soils may be made at a 1:1 inclination. Once planned cut locations are known, we suggest review by us so that we can provide you with supplementary information regarding appropriate cut slopes. All permanent exposed slopes should be vegetated to reduce the potential for erosion. Slope areas should be properly prepared prior to placing fills. A keyed toe and horizontal benches should be cut into native soils and the fill placed in horizontal lifts. Structural fill should be placed in thin layers and compacted to at least 95 percent of the maximum dry density in accordance with ASTM Test Designation D-1557 (Modified Proctor). All on-site soils are high in fines content making them difficult to compact in - moist conditions, during rainy weather or when placed over existing wet conditions. Import fills, if needed for use in wet weather construction, should be predominantly granular with a maximum size of three inches and no more than five percent fines passing - the No. 200 sieve. To avoid excessive earthwork costs, we recommend conducting grading operations after - the site soils achieve workable moisture levels during the dry season. Project No. T-1416 Page No. 7 Mr. Ron Dever-man July 12, 1990 Pavement Areas Roadways may be constructed on the recompacted, native soils after stripping the surface of vegetation, topsoil, and old fill or on compacted structural fill depending on the depth of cuts or fills required to reach design grades. The upper twelve inches of the subgrade should be compacted to 95% of the maximum dry density (ASTM D-1557). If native soils cannot be compacted due to excessive moisture contents, they should be overexcavated and replaced with a thickness of clean, gravel pit run or crushed rock suitable to stabilize the subgrade. For fills more than a foot thick, a compactive effort of 90% is adequate below the top foot. All subgrade areas should be in a stable, non-yielding condition prior to paving. For the anticipated use, a pavement section consisting of two inches of Class B asphalt over four inches of gravel base should be sufficient for parking areas. Heavy traffic lanes should be constructed with two inches of Class B asphalt over six inches of gravel base. Utilities Where utility lines are to be excavated and installed in paved areas, we recommend that all bedding and backfill be placed in accordance with APWA specifications. Backfill placement and compaction should be in accordance with the recommendations given earlier in this report under Site Preparation and Grading. Significant seepage may be encountered in any of the excavations. The onsite silty soils will be difficult to adequately compact if they become very moist. In addition, deep utility trenches will be difficult to excavate once till is encountered. The contractor should be prepared to deal with this likelihood. Additional Services It is recommended that we be provided the opportunity for a review of the site layout, final design and specifications in order that earthwork and foundation recommendations - may be properly interpreted and implemented in the design and construction. The analyses and recommendations submitted in this report are based upon the data - obtained from the test pits and probes excavated on the site. The nature and extent of variations in the test pits may not become evident until construction. If variations then appear evident, we should be requested to reevaluate the recommendations presented in this report prior to proceeding with the construction. It is also recommended that we be retained to provide geotechnical services during - construction. This is to observe compliance with the design concepts, specifications and recommendations, and to allow design changes in the event subsurface conditions differ from those anticipated prior to the start of construction. Project No. T-1416 Pay,e No. 8 Mr. Ron Deverman July 12, 1990 -- We request that a minimum of two working days notice be given to schedule our services during construction. -- The following figures are included and complete this report: Figure 1 Vicinity Map Figure 2 Test Pit Location Map Figure 3 Soil Classification Chart Figure 4 through 8 Test Pit Logs Figure 9 Grain Size Analyses Table A Probe Hole Data Appendix A EIS Summary - We trust the information presented herein is adequate for your requirements. If you need additional information or clarification, please call. Sincerely yours, TERRA ASSOCIATES, INC. A Anil Butail, P.E. President JJ/AB:tc Project No. T-1416 Page No. 9 FIER- +rSt MST _ Sf rl SE f S( t - < k S., AJb •.F < u —� 'I s V N .: ]MSTIS ImS sIL Y^SE�a1 } r '� F SE I TN T I:f If 1 T I z z N•'1 >f,OE'EEfr.y.. y s a E '� T tit YrrX zr I ✓t rN sT 1.. of < I s y' N'S3`�0UAtl s f S S;"E" « I I op i'� f , 1_. 71l--T.PL E'll NZ PC 3<� T L U' ,, SE ❑2TN I SE 1r21N T f'�', 'OFNER M S N Sf Illtn PL - �II(NS!T < lit 3 ;NAIEN Sf <III Tn'� ST IQTN4 NS r NE { SE IIErN Pl'a SEr-111TN ST t� MICE 1. N 'IOM�P ' W.LN SE 1161N SIE _— + KI ANISE I ,jTN N T 5 --_ lES g{ 1 S[rI9rN LS ✓j S e N[E R > r^ < 1 C e iT(N /4. :I f _SE_. 1211fN ST I NITS2TN. ST S� jiN T r� f MIT ST I III SST N ITM,P/r ETN R � '•I ST S M� N SSNE l M.11't rl. ,� p Ir E t z +Y:� � J�'• r AV NIT 4' l £ 1 .RY T_O_(�_P , .X T - HITH ST @ SE I21TN ST r L�GRCL1YMip00 > F.�, - �„I SE;.291. St C 1T I i pAT X.I Eawns �.., 'IMKIS/TEI •� ..L YSE_ _4-E2Npr <I Sr SE. !T2N0 S �5..�- SNf 1ST PIN / 1 I I 2q St z zr a ' r9ETN I � • �l -f i+ Si inr.SE RI s I]erX _St.: S'.56T.IT iZ•>f o t<T,i !,� SSF,NTr,"S: 1 _ �+t• Oiz PARK mIN J WN lr SE y S[•+Otw N ` ` n<. El�. _ •�,. tF\ - sIWIN E'n 7t SE ,51>t < N ,.('(:j,IMAPSEN000:•'.:'1 '^ aE:I�a2ND F t lNo �I'r��1II' ~< _ 1."TR PL 91[•N OR Y S S. I I — REF: Thomas Brothers Maps, 1989. VICINITY MAP TERRA FORREST CRESTE APARTMENTS ASSOCIATES RENTON, WASHINGTON r Figure 1 Geotechnical Consultants Proj. No. 1416 Date 6/90 1>, Fg n �.0 ■pir <v Ja8 �t� t •• �� /:rw array �! Mill, _,,`,�' ��� �r �•� sa ■p � ,.c .art,. � �. i[. � / _ tea�^^iKT".:SiL st.;,nr'-X�• �—I�I.IO SOIL CLASSIFICATION SYSTEM LETTER GRAPH -- MAJOR DIVISIONS SYMBOL SYMBOL TYPICAL DESCRIPTION o:: GRAVELS Clean L° well-graded o. g gravels, grave;-sand mixtures, U) Gravels GW o: little or no fines. J o) More than 50% of (less than GP ; Poorly-graded gravels, gr.:el-sand mixtures 9 coarse fraction 5/° fines). I ; . . . ; ;• little or no fines_ _ GM Silty gravels, gravel-sane-silt mixtures, C) r is larger than Gravels non-plastic fines. — W with. fines. Z �' `� No. 4 sieve. — EGC Clayey gravels, gravel-sand-clay r�ixtures. �, _ plastic fines. Clean ' Well-graded sands, gravelly sands, -- oo SANDS Sands SW 1:::. iii`......... little or no tines. U) N ° (less than SP Poorly-graded sands or gravelly sa,ds, .rzz More ti�an �0% of 5% fines). little or no fines. Q 0 coarse fraction v Silty sands, sand-silt mixtures, Uis smaller than with fines.ands SM non-plastic fines. No. 4 sieve. SC Clayey sands, sand-clay miytures, plastic fines. � SILTS AND CLAYS Inorganic silts and very fine sands, rock flour, .J ro ML siltshg or ciaye fine sands or clayey sil s v✓ith o Liquid limit is less than 50% Inorganic clays of Ipw to.medium plasticity, M o CL ravelly clays, sandy clays, silty clays, lean 0 E (',j I S. W o o OLi�,�,�i� Organic silts and organic clays of low Z o z plasticity. to ro SILTS AND CLAYS MH Inorganic silts, micaceous or diatomaceous c° °i fine sandy or silty soils, elastic. Lia) Q)• Liquid limit is greater than 50%. CH Inorganic clays of higl, plasticity, fat clays. Z 0 E T / �- OH , ' I : i Oroar,ic clays of medium to high plast,cr.y, o11 rganic silts. HIGHLY ORGANIC SOILS PT Peat and other highly organic soils. DEFINITION OF TERMS AND SYMBOLS 2" OUTER DIAMETER I SPLIT SPOON SAMPLER C TORVANE READING, tsf 2.4" INNER DIAMETER RING SAMPLER j� OR SHELBY TUBE SAMPLER qU PENETROMETER READING, tsf P SAMPLER PUSHED W MOISTURE, percent of dry weight SAMPLE NOT RECOVERED PCf DRY DENSITY, pounds per cubic foot WATER LEVEL (DATE) LL LIQUID LIMIT,percent WATER OBSERVATION WELL PI PLASTIC !NDr=X — N STANDARD PENETRATION, blows per foot TERRA FORREST CRESTE APARTMENTS _ ASSOCIATES RENTON, WASHINGTON .y Geotechnical Consultants Proj. No. 1416 Date 6/90 Figure 3 TEST PIT NO. TP- 1 Logged By .JJ -- Date 6-13-90 Elev. 412+ Depth W (ft.) USCS Soil Description (0/0) 0 SM Red—tan, silty SAND, trace gravel, rootlets, 17 very moist, loose to medium dense. - SM/ Olive, gravelly, silty SAND and SAND, moist SP to wet, medium dense. 5 — 11 SM Gray, gravelly, silty SAND, moist to very moist, dense to very dense. (TILL) 10 Test Pit completed at 9 feet. No groundwater seepage encountered. No caving. 15 Logged By JJ TEST PIT NO. TP-2 Date 6-13-90 417+ Elev. 0 an Red—brown, silty SAND with some gravel 1.8 _. .` SM and charcoal, very moist, loose. Grades to medium dense. 5 SM Olive—gray, gravelly, silty SAND, moist, dense to very dense. 11 (TILL) 10 Test Pit completed at 9 feet. - No groundwater seepage encountered. No caving. 15 `TEST PIT LOGS TERRA 1 FOM EST CIF ESTE AJIMIRTIVIENTS ASSOCIATES i-; Renton, Washington Geotechnical Consultants Proj. No. 1416 Date 6/90 Figure 4 TEST PIT NO. TP-3 Logged By JJ Date 6-13-90 Elev. 432± Depth W (ft.) USCS Soil Description (%) 8" DUFF 0 and TOPSOIL Reddish-tang si wit some ty grave an SM roots to 18 moist, loose to medium dense. SM Olive, gravelly, silty SAND, moist, medium dense to dense. (Weathered Till) 5 SM Gray, gravelly, silty SAND, moist to very 12 moist, dense to very dense. (TILL) Occasional cobbles. 10 Test Pit completed at 92 feet. No groundwater seepage encountered. No caving. 15 Logged By JJ TEST PIT NO. TP-4 Date 6-13-90 Elev. 420± 0 SI'[ Reddish- ryr, silty withgrav , trace c , roots to 22 very moist to wet, loose. SM Olive, gravel, silty SAND, very moist to wet, 15 medium dense. _ 5 Gray, gravelly, silty SAND, very moist, S1,11 dense. (TILL) . 11 10 Test Pit completed at 9 feet. Light groundwater seepage encountered at S feet. No caving. 15 'TEST PIT LOGS . TERRA sr cF:Ms-rS ASSOCIATES Renton, Washington Geotechnicai Consultants Proj. No. 1416 Date 6/90 Figure 5 TEST PIT NO. TP-5 . Logged By JJ Date 6-13-90 - Elev. 398± Depth W (ft.) USCS Soil Description (0/0) 8" DUFF -and TUP6UIL ..T.g.; Reddish-brown, silty SAND, abundant roots to SM 18", very moist to wet, loose. Olive to gray, gravelly, silty SAND, moist to SM very moist, dense to very dense. 10 (TILL) Test Pit completed at 812 feet. 10— Moderate to heavy groundwater seepage at — 2 feet. No caving. 15 - Logged By JJTEST PIT NO. TP-6 Date 6-13-90 Elev. 436+ sm Reddish-brown, silty SAND with gravel, wet to very moist, loose to medium dense. 7TT- 5— SM Olive to gray, gravelly, silty SAND, very 1% moist, dense to very dense. 16 10— Test Pit compelted at 812 feet. Moderate groundwater S eepage encountered at 3 feet. No caving. 15 `TEST PIT LOGS TERRA HJFWIE r CXNES-M AA534TIVISYM ASSOCIATES Renton, Washington Geotechnical Consultants Proj. No. 1416 1 Date 6/90 Figure 6 TEST PIT NO. TP-7 Logged By JJ Date 6-13-90 Elev. 444± Depth W (ft.) USCS Soil Description (%) 0 Reddish-brown, silty SAND with gravel, roots SM to 2' , moist to wet, loose to medium dense. Olive to gray, gravelly, silty SAND, wet to 5 moist, medium dense to very dense. SM (TILL) 10 Test Pit completed at 9 feet. Light groundwater seepage encountered at 3 feet. No caving. 15 Logged By JJ TEST PIT NO. TP-8 Date 6-13-90 Elev. 419± 0 8 DUFF and TOPSOIL Reddish-brown, silty SAND with some gravel, SM roots to 3' , very moist to wet, loose to medium dense. _ 5 Olive to gray, gravelly, silty SAND, wet to SM very moist, medium dense to very dense. (TILL) 10 Test Pit completed at 9 feet. Heavy groundwater seepage at 3 feet. No caving. 15 r-- `PEST PIT LOGS TERRA ASSOCIATES ^"3 Renton, Washington Geotechnical Consultants Proj. No. 1416 1 Date 6/90 Figure 7 TEST PIT NO. TP-9 Logged By JJ Date 6-13-90 Elev. 422± Depth W (ft.) USCS Soil Description (%) 0 and SM Reddish—brown, silty SAND, moist to wet, 21 loose to medium dense. Roots to 2 feet. 5 Olive to gray, gravelly, silty SAND, very 13 SM moist, dense. (TILL) 10 Test Pit completed at 92 feet. Light seepage at 4-5 feet. No caving. 15 Logged By TEST PIT NO. TP- 10 JJ Date 6-13-90 Elev. 391± 0 S M Keddish brown, silty SAND, with some gravel, roots to SM Tangravelly, silty SAND wet medium dense. Gray, gravelly SAND, occasional cobble, very - 5 SM moist, very dense. 11 Test Pit completed at 6 feet. Moderate seepage at 3 feet. No caving. 10 15 `TEST PIT LOGS TERRA r-cowns LaRems ASSOCIATES Renton, Washington Geotechnical Consultants Proj. No. 1416 Date 6/90 Figure 8 I SIEVE ANALYSIS HYDROMETER ANALYSIS E SIZE OF OPENING IN INCHES NUMBER rF MESH PER INCH, U.S. STANDARD GRAIN SIZE IN MM p N O V 7 N O O O O O O n •i cl N _ _ . N v p N n O 0 O O O O O O O p t00� 0 90 10 d0 , 20 77 T � — C� m - m m .a --- — ---— :1D m -t--f 77 f'l C) 0 40 m _ — --� -n z� c � m 50 50 i- O z' C ➢ 1 m— —� G7 C 30 70 < m 20 -n o —' — --- 90 OI -- -- mDD - t ------ ----— - ---- — — — — -- 00 CO --, Z --I y CDC CD b O O O O O CO ID c c7 N CD cD V O N '- O O CD O O O O O O O O CD I � z N - GRAIN SIZE IN MILLIMETERS Z CO COARSE FINE COARSE MIEDIUM FINE FINES � m I COBBLES ( RAVEL SAND (O > ➢ m =0 CO D z Boring or Dep111 Moisture L` PL Z -u ➢ Key USCS Description Content (°o) . > r Test Pit (ft.) j O Z m M TP-6 5 SM Gravelly, silty SAND. 16 z TP-9 1.5 SM Gravelly, —�-- y, silty SAND. 21 CD TP-2 2 SM Silty, gravelly SAND. 18 Table A Probe Hole Data Forrest Creste Renton,Washington Depth Probe Hole inches Soil Description P-11 0.0 - 9.0 Dark brown, silty SAND with charcoal and organics. 9.0 -30.0 Reddish brown, silty SAND with gravel, moist, loose to medium dense. 30.0 - 36.0 Olive, gravelly, silty SAND, wet to very moist, medium dense to dense. (TILL) P-12 0.0 - 12.0 Dark brown, organic rich topsoil. 12.0 - 24.0 Reddish brown, gravelly, silty SAND, wet, loose to medium dense. Heavy seepage at 18.0" - 24.0" - P-13 0.0 - 8.0 Duff and Topsoil. 8.0 - 24.0 Reddish brown, silty SAND with gravel, - roots to 18", moist, loose to medium dense. 24.0 - 26.0 Gray, gravelly, silty SAND, moist, very dense. (TILL) P-14 0.0 - 8.0 Duff and Topsoil. 8.0 - 30.0 Reddish brown, , silty SAND with gravel, roots to 18", moist to very moist, loose to medium dense. 30.0 - 36.0 Olive, gravelly, silty SAND, very moist to wet, medium dense to dense. (Weathered Till). Moderate`seepage at 24". Project No. T-1416 Table A (cont) Probe Hole Data Depth Probe Hole inches Soil Description P-15 0.0 - 3.0 Topsoil 3.0 - 36.0 Reddish brown, , silty SAND trace gravel, roots to 18", moist, medium dense. 36.0 - 48.0 Olive, silty SAND and SAND, wet, medium dense. 48.0 - 49.0 Gray, gravelly, silty SAND, very moist to moist, dense to very dense. (TILL) Light seepage at 4 feet. Project No. T-1416 APPENDIX A EIS SUMMARY r- i i i I APPENDIX A EIS SUMMARY Surface The project area occupies 11.73 acres of gently sloping terrain east of 138th Avenue SE and south of SE 122nd Street in Renton, Washington. The topographic relief across the site is approximately 60 feet, rising from Elev. 395 feet in the southwest corner to Elev. 455 feet in the northeast portion of the site. In general, the site slopes toward the southwest at inclinations of 10 to 20 percent. Some piles of old fill are present in the - extreme northwestern portion of the site. The site is vegetated with dense, middle-aged fir, cedar, maple and alder. The underbrush in the southwestern portion of the site becomes very thick. The extreme northwestern portion of the site has been utilized as horse pasture and is mostly barren of vegetation. A horse stable is present in this area. A small stream crosses the northern portion of the site. The origin of this drainage appears to be, in part, from drainage off the development to the northeast of the site. This drainage has been routed onto the site from near the projected intersection of 140th Avenue SE and SE 122nd Street. Some erosion has occurred a fair distance downslope from the outfall. The site is bounded on the west by 138th Avenue SE, across which is an undeveloped site. The southern property boundary adjoins an undeveloped, forested land. The eastern - margin of the site lies adjacent to forested property and sparse single family lots. Across the northern boundary of the site is a single family residential area. Subsurface The U.S. Geological Survey has mapped the geology of the study site as Vashon till. The r tills was deposited about 15,000 years ago along the base of the Puget Lobe during the Fraser Glaciation. In general, our findings support the USGS determination. In each test pit, an average of about 6 inches of duff and topsoil were found to overlie reddish brown, silty sand with gravel. This deposit is underlain at depths of two to four feet by olive to gray, medium dense to dense, gravelly, silty sand till. This till material extends to depths exceeding 10 feet, the maximum depth explored. _ Groundwater Groundwater was encountered in many of the test pits and probes excavated on the site. Light to heavy seepage was noted from the top of the till unit at depths of two to four feet throughout the entire western and northwestern portions of the site. The perched groundwater may be more prominent in the winter months when precipitation is greater. Groundwater conditions in the area surrounding the site was evaluated using well logs on file at the Department of Ecology. From these logs, there is evidence of a regional groundwater table at approximate Elevation 350 having a gradient to the west. Slope Stabilitv The slopes on the site are gentle to moderate. The steepest slopes on the site were approximated at 15 to 20 percent. Given that the entire site is underlain by dense glacial till, the slopes should remain in their presently stable condition, provided erosion prevention measures are taken. The seismic hazard associated with glacial till soils on gentle to moderate slopes is low. The area lies outside Class III Seismic Hazard zones as delineated in the King County Sensitive Area Map Folio. ENVIRONMENTAL IMPACTS Some effect on the environment will be unavoidable during and after the development of this site, as construction of roadways and buildings cannot be performed without modifications to the existing ground surface. Roadway areas and building areas will be stripped of topsoil and materials unsuitable for support of structures. If rains occur during earthwork operations, some erosion of the barren soils may occur. The existing shallow groundwater table will be interrupted by earthwork and utility installation. This may be considered an unavoidable impact. IMPACT MITIGATION The effects of the development of this site on the environment may be minimized, in large part, by careful construction practices. Bare soil, particularly on slope areas, should be protected with jute mats and/or immediately seeded with appropriate vegetation to prevent erosion. Stormwater should not be allowed to flow over site slopes, but should be contained in retention/detention ponds or be tightlined to a suitable collector system or ditchline.