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HomeMy WebLinkAboutPettygrove's 1st Block 18 Lot 1 - Geotechnical Engineering Investigation - 1999.07.281 EEK ENGTNEERING & ENVIRONMENTAL,INC. CTVIL . GEOTECHMCAL . ENVIRONMENTAL July 28, 1999 Williams, LLC. P.O. Box 408 Port Townsend, WA 98368 3v? oa ' 6=r 5oVR)\9oxJ BELLINGTTA.\I 360{76_9589 80GE59-559? FAX 360-676-t6x Re Report - Geotechnical Engineering Investigation Lot l, Block l8 Petfygroves First Addition Maple Street Port Townsend, Washington Attn: Williams'Representative BEK Engineering, Inc. is pleased to present the results of our geotechnical engineering investigation at the aforementioned properby that is adjacent to Maple Street (Vacated) within the City of Port Townsend, Washington. The site is situated within Section 3, Township 30N, Range iW of the Willamette Meridian. The properfy is located to the west of Maple Street, north of K Skeet, and is sitr'rated on a westeriy-facing slope. It is our understanding that you intend to build a single-family residence with a 450 to 600 square foot footprint on the 50-foot (N-S) by 100-foot (E-VO rectangular properry (Site Plan, attached). It is our understanding that sewage will be hansmitted to the City sewer system, probably by pumping it to a sewer main that will be constructed along Maple Sheet, and that parking will consist of street side parking along Maple Street. SCOPE OF SERVICES The scope of this project included a reconnaissance level slope examination and subsurface investigation through hand excavation of test pits and collection of soil samples from the pits. No motorized subsurface investigation was condutted. Specifrcally, our scope of services included the following: i. Reviewed published maps regarding topographic and geologic conditions in the vicinity of the subj ect properry, including :. USGS Topoeraphic Map. Port Townsend North Ouadrangle. Washinston, United States Department of the Interior Geological Survey, 1953, Photorevised 1981. So Resion. Washinston, (U.q.Geological Strvey Map I-1198-F, Pessl, et al, 1989. City of PortTownsend Soil Surgey Map.. Soil Survey of Jefferson Counfy Area. Washinston. Soil Conservation Service. 2. Conducted a visual site inspection to evaluate the stability of the soil on the subject properly. 3. Excavated test pits on the slopes to determine soil types and subsurface groundwater conditioqs. 4. Measured slope gradients along the south and north lot lines of the 50-foolwide by lO0-foot-long properfy to construct slope profile drawings.5. Prepared a slope profile drawing and a site plan to illuskate points of interest. 6. Prepared this report describing the results of our investigation, including recommendations for site development while ensuring stability of the slopes at the site- i a c 2I3E HUMBOLDTSTREET -BELLINGHAM' \['.{, 9 8225 July 28, 1999 '-) Report - Geotechnical Engineerr..g'Investigation (Williams LLC. Lot l) REGTONAL GEOLOGIC, ITTDROGEOLOGIC, AI\D SOIL CONDITIONS ' According to the Surficial Geologic Map of the Port Townsend 30- by 60-Minute Ouadranele. Puset Sound Resion. Washineton (Pessl, et al, 1989), the subject properly is underlain by till (unit Qvt), that was deposited by the ice sheet during the Vashon stade glaciation- The Vashon Glacial Till consists of a poorly sorted mixhre of rock fragments deposited directly by the Vashon-age ice sheet. Finer portions of the soil consist of clay, silt, and sand. These fine particles comprise a moderately to highly compact matix in which coarser components (gravel, cobbles, and boulders) are firrnly imbedded. Our site observations confirm the presence of Till beneath the subject properfy The Ciry of Port Townsend Soil Survey map indicates two soil types adjacent to the subject property,.but only one soil type on the property. Clallam gravelly sandy loam on 15- to 3O-percent slopes (soil code = CN@) occupies the entire site, but Dick loamy sand on 0- to i5-% slopes (soil code DCC) is mapped as nearly touching the central area of the north properfy boundary. Our site investigation indicated that CMD soil may extend further to the north than it is mapped. The CMD soil is derived from weathered glacial till. The till reportedly extends many feet below the ground surface. The CMD soil is well drained and permeability is moderate above the cemented layer, which is located between 23 and 36 inches bgs (below ground surface). Runoff from the soil is medium and the hazard of water erosion is moderate. The Dick soil series is derived from slope wash or fine-grained particles that have been washed downldll from the Clallam series. The series is somewhat excessively drained and permeability is rapid. Runoff is slow and the hazard of water erosion is slight. SITE OBSERVATTONS A geotechnical engineer from BEK conducted a site visit (June 3, 1999) to observe and measure the site slopes and test the in-situ soils for density. A representative of Williams LLC. was present at the time of our site visit. Existing Slopes The slope along the south property line extends at a maximum slope of 39 percent over the east 50 feet, then increases to approximately 43 percent over the westem 50 feet. The north properly line slopes at an average of 37 percent over the eastem 30 feet, then slopes at approximately 41 percent to the northwest properly comer (Site Plan). There is negligible slope on the property along Maple Steet, but the west property line slopes slightly downward in a northerly direction- The slope on the S0-foot-wide property appears to be stable in both locations where it was measured, and other locations where cross-slope transects were made. Soil Conditions The soil on the slope is composed of a moist topsoil-duff layer to a depth of 0.35 feet. Between 0.35 and 2.35 feet the soil is composed of brown silty sand with gravel that is in a medium dense condition. The soil becomes darker and more dense befween2.35 and2-75 and becomes very dense and gray in colot at2.75 feet bgs. We interpret the soil between 235 and 3-0 feet (maximum exploration depth) to be glacially deposited till, and the overlying layers to be weathered till. Penetation resistance of a one-half inch bar that was driven with a 5-pound hammer was used as a surrogate measure of soil density. It is our opinion that the depth to the cemented till layer will be less on more steeply sloped sections of the prope4y and that the depth will be greater on less steeply sloped sections. As the slopes flatten, the soil will grade to the Dick series described earlier. There is an approximately 5-5-foot soil cut at the east side of Maple Sheet ancl directly upgradient from Lot 1. The soil cut has exposed approximately 3 feet of slightly weathered glacial tilt that is in a very dense 2BEKENCINEERING, INC.PROJECT:991618 July 28, 1999 /-) Report - Geotechnical Engineering Investigation (Williams LLC. Lot 1) condition just below the approximately 2.5-foot-thick weathered surface layer. The soil cut causes any subsurface ground water to surface and flow onto Maple Steet. Vegetation Lot I is vegetated by fir, cedar, and madrone trees with some red alder. The understory consists of ocean spray, red elderberry, sword fern, sala1, salmonberry, and some Oregon grape. A small amount of Himalayan blackberry, an exotic species, was encountered on the site. The trees on the slope do not exhibit a pistol grip-shaped growth form that is indicative of slopes with soil movement or creep. Accordingly, the vegetation on the property indicates that the slope is stable. Hydrology and Drainage No indications of surface or subsurface drainage were observed on the subject properly. The till layer observed at depth indicates that the slope drainage probably does not penetrate to lower soil horizons, but havels as shallow subsurface flows that follow the topography of the-site. The soil cut at the upper (east) side of Maple Street upgradient prevents the shallow ground water ftom travelling beyond that location and forces it to the surface of Maple Sheet. It is our understanding that a drainage ditch will be constucted on the east side of Maple Sheet that will collect the surfaced water and transmit it off-site to the north. The collection and hansmission of ground and surface water will reduce the potential for off-site drainage impacts to Lot 1. The hibutary area to the subject property following development of a roadside ditch on Maple Skeet will consist only of that precipitation that falls on the Lot and on the easement for Maple Street. If the surface of Maple Sheet were sloped to the east, potential runoff to Lot 1 would be firrther reduced. Uniform Building Code Site and Soil Categorization The subject properly, according to Figure 16-2 of the Uniform Building Code (IIBC, 1997), is within seismic zone 3, as is all of Westem Washington. The peak ground acceleration (PGA) for underlying bedrock at the site is approximately 0.29 g, for an earthquake with-a l0% probability of being exceeded in 50 years, according to the USGS National Seismic HazardMapping hoject for the latitude and longitude at the site (Lat. : 48.12, Lon. : 122.77). Soil typing is required by the Uliform Building Code to determine building response under seismic conditions (Section 1636.2,IJBC, i997). Our site investigation was conducted to a maximum depth of 3 feet below the surface, so we. are uncertain of the soil types that underlies the till layer. As a result, we recommend that Soil Type Sp be used when calculating horizontal and vertical accelerations at the site. . CONCLUSIONS It is our opinion that the soils at the site are stable and, due to their density and the existing slope angle, should not be subject to erosion during normal rainfall events or severe failure during design earthquake events (M6.5), provided the residence is founded on unweathered glacial till or struchral fill that is placed and compacted atop the till, and proper drainage techniques are utilized. Our slope measurements indicate that the house will be conshucted in areas that have less than 40-percent slopes, but that there will be little setback from the 4O-percent slope section of the properly. It is our opinion, based on ouf site investigation, that Lot 1 is categorized as a geologically hazardous area according to Chapter 19.05-100.8.5 (slope greater than 4O-percent) of the Port Townsend Municipal Code. JBEKENGINEERING,INC.PROJECT:991618 July 28, 1999 Report - Geotechnical Engineering Investigation (Williams LLC. Lot 1) RECOIIOIENDATIONS General Insofar as the square footage of the house has not been determined at this time, these recommendations are generalized and should be taken into account when sizing the residence for the properfy. All clearing, grading, drainage, and erosion conhol will be subject to Cify of Port Townsend regulations. Excavation and Grading Site grading and creation of impermeable surfaces should be minimized, and all spoils should be properly disposed of off-site, unless utilized for fill slopes. Cut or fill slopes should not exceed 2:1 (H:9 slope gradient or 4 feet in height. Retaining walls should be utilized to as great an extent as possible to create level areas around house. Footings for the residence and any retaining walls should be founded in the dense, unweathered glacial till that was encountered at 2.'15 feet below the ground surface. Footings should be stepped when extending in a downhill or uphill direction to'minimize the excavation of the unweathered till. We recommend that engineered grading plans be prepared for the residence. 4,nfit', t, All excavation and earthwork should be done during the dry season. *liUr17 ., ot-y. I Drainage and Erosion Control Site clearing should be minimized to reduce erosion potential and silt fence should be erected downgradient of all clearing or other land disturbance activities. We recommend that the clearing limitbe set a maximum distance of 10 feet horizontally on all sides of the proposed structure. Erection of silt fences downgradient of the construction area, in combination with construction access being limited to the east side of the properly, should eliminate any off-site sediment tansport from the lot during conshuction. Roof and footing drainage from the shucture should be collected in a drainage pipe conskucted along the west side of Lots 1 through 4 and conducted down the K or L Sheet Easement to appropriate drainage facilities along Fir Street, if possible. If it is impossible to conduct drainage to Fir Sfreet, a combined detention basin / level spreader can be constructed that will limit off site flow for the 2-year storm to pre development levels. Because the house has not been sized and the stormwater plan has not been completed, we carmot recommend a specific detention basin spreader length at this time, however we anticipate that the stmchre will consist cf an approximately 30-inch wide trench excavated to the upper edge of the unweathered glacial till (approximate depth 2.75 feet). To minimize damage to surface soi1, only the lower 1.75 feet of the tench will be used, which will provide a horizontal flow distanc e of 2.5 feet to the ground surface (on a 40-percent slope) at 2- year storm flow rates (see Detention Basin / Level Spreader, attached)- The detention basin / level spreader will be lined with a geotechnical filter fabric (Linq GTF-200, or equivalent) and filled with 2-inch drain rock to within 1 foot of the ground surface on the lower side and 2 feet below ground surface on the upper side. A 4-inch rigid perforated drainpipe will be placed in the upper leyel of the gravel and connected to the house roof and footing drains. The upper portion of the hench will be filled with native soil to mimic preexisting slope conditions. The void area of the rock (0.33) minus the area of the cross section of the pipe will result in a total void space of 1.0 cubic foot per linear foot of trench. The length of the trench will then be determined by the differential in the pre- and post-development runoff rates. We recommend that Maple Street and its attendant parking stip should be graded such that flow is toward the east, with a ditch constructed on the east side of the road- The ditch should be constructed to act as a biofiltration swale to clean road runoff prior to flowing into its ultimate receiving basin. I n#'' 4BEK ENCINEERINC, INC.PROJECT:991618 July 28, 1999 Report - Geotechnical Engineering Investigation (Williams LLC. Lot 1) Slope Restoration and Revegetation Following construction at the site, disturbed areas should be revegetated by planting an erosion control mixture consisting of native glasses. Additionally, landscaping should consist of planting deep-rooted native woody vegetation such as is found on the site. Plantings should be installed as soon as possible following the commencement of the autumn rains or as early as possible in the spring. Slope Maintenance No organic material, consisting of lawn clippings, hee trimmings, or brush should be placed near or on the hill slope. These materials may help hold water on the slope face and may increase the erosion rate of the slope. Organic materials should be properly composted or disposed of off-site- INDEMMFICATION AND LIMITATIONS This report has been prepared to aid in the development of the site from a geotechnical engineering standpoint. Our site observations indicate surface conditions at the dates and locations indicated. It is not warranted to assume that they are representative of conditions at other locations and times. The conclusions and recommendations contained in this report are based on surface and subsurface conditions observed during our site visit. We assume that the conditions observed are representative of the subsurface conditions throughout the property. If, duing further site developmenf different conditions from those described in this report are observed or appear to be present in excavations, we must be advised promptly so that we can review these conditions and reconsider an&or modify our conclusions and recommendations where necessary. The earthwork conhactor is responsible to perfbrm all work in conformance with all applicable WISH{OSHA regulations. There are inherent risks associated with construction on or adjacent to'steep slopes that the contractor and owner must assume. This report is intended to reduce, but not eliminate, uncertainty regarding the stability of the slope. We appreciate the opporfunity to be of service to you- Should you have any questions concerning this report or require fi.rther information, please contact Bob Bailey at our office at (360)-676-9589, or toll-free at (800)-8s9-5597 Sincerely, BEK ENGINEERTNG, INC. r'ffa" (ii:' -,, , .t'l'("tv" , .t..',t( il'\1 t.$"" t,P{ -5"SP G;q Thomas Civil RB:bb Attachments: P.E., Principal Topographic / Vicinity Map Site Plan Slope Profile A-A' Detention Basin / Level Spreader Warranty ErPrREs 11/211 zc(:,o RobertP. Bailey, M.S.C.E., P.E. Geotechnical Engineer 5 q-x1 u) 58 BEK ENGINEERINC, INC.PROJECT:99161B