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Report (139) t ji !7 0Csj RECEIVED \5c L\ c MAR 2 9 2018 C9TV : r, .I GT . 31 POING DIVISION 15974 SW 76th Ave, Tigard Modify LIDA from a rain garden to flow-through planters + soakage trench Flow-through planters meet the LIDA requirement. The soakage trench is the outflow destination from the planters. The geotechnical report shows a soil infiltration rate of 16"/hr, more than adequate to house a soakage trench to dissipate the water from the planters. The soakage trench resides in the same location as the original permitted rain garden, but is underground, while the planters will be along the side yard abutting the foundation (permissible per LIDA guidelines). With this revised layout, I am able to put grass over the buried soakage trench and provide the home a backyard. Combined area of planters is 95 sq ft which is 6% of impervious areas ( roof) • Hydro Geo&Environmental,Inc 8525 SW 67"'Ave., • Portland.OR 97223 Phone:503.892.2000 • c-mail.h,u,u C,ti com�usc.,.- • Hydro Geo & Environmental, Inc. July 13.2017 Project# 2388.17 Attn: Diane RE: SUMMARY OF SOIL INFILTRATION TESTING FOR SITE @ 15970 SW 76th AVE.,TIGARD, OREGON In accordance with your authorization Hydro Geo & Environmental, Inc., (HGE) has completed soil infiltration testing at the subject property in the approximate locations indicated on the attached site plan. The purpose of the infiltration testing was to provide field infiltration rates for use in design of a storm water disposal system for the proposed development. The scope of work for this project consisted of excavation of one shallow test pit, and then angering of exploratory borehole from the bottom of the test pit, evaluation of field infiltration rates using standardized methods and equipment, and preparation of this letter report. This letter report has been prepared for exclusive use of the owner and their agents, for specific application to the referenced scope of services, in accordance with generally accepted geotechnical engineering practices. No other warranty, expressed or implied, is made. hi the event that changes in the nature, usage, or layout of the proposed site improvements are made the conclusion contained in this report shall not be considered valid unless the changes are reviewed by HGE in writing. The field-work was conducted on July 11. 2017 consisting of excavating of one shallow test pit and conducting the percolation tests at the depth of 4.5 feet. The hand-augered exploratory borehole was advanced then from the bottom of test pit to a depth of 7 feet and was terminated at that depth due to full auger refusal. An engineering associate from HGE coordinated and observed the subsurface conditions and infiltration testing. The site is located at 15970 SW 76°' Ave., at the north corner of intersection with SW Durham Rd., in Tigard, OR (Figurel). The site is fairly level at 147-150 above Mean Sea Level (Beaverton Quad, USGS 7.5 Minute Map) and slopes slightly eastward. The existing 2372 s.f. single residential house was demolished prior of our site explorations. The site is bounded on the west by paved SW 7601 Ave., on the south, north and east by adjacent residential houses (see Figure 1,Vicinity Map). GEOLOGY & SUBSURFACE CONDITIONS The near-surface geology of the project area consists of Late Pleistocene age (recent to Juh'13, 2017 Page approximately t._i n,ill;i,ii yi;5 ut �) v � wkoli :01, mnrn tionneitivi lw Mt d11111}lic. floods of the Columbia River. These glacial floods originated in the Missoula valley of Montana, passed through eastern Washington, and followed the Columbia River downstream. When these large floods entered the Portland Basin they flowed up the Willamette River and its tributaries flooding most of the Willamette and Tualatin Valley up to approximately elevation 350. These episodes of flooding deposited thick layers of silt, sand and gravel that mantle the Willamette Valley and Portland Basin. Based on a review of the Soil Survey of Washington County, the near-surface soils at the site are mapped as Hillsboro Loam, 0 to 3 percent slopes. This well drained soil is on broad valley terraces that formed in mixed silty and loamy, old alluvium on terraces. At the time of our visit we hand dug one test pit to a depths 4.5 feet. Subsurface materials represented in the test pit # 1 consisted of a 1.5- foot surface layer of dark-brown moist organic silt fill with some grass and tree roots. Underlying the fill and organic topsoil, the native soil consists of yellowish-brown, moist stiff, low plasticity sandy silt, underlain at the depth of 4.5 feet by yellowish-brown damp to moist, dense fine-grained silty sand, in turn, underlain at the depth of 7 feet by dark-grey basalt rock fragments. Our test boring was terminated at the depth of 7 feet due to full auger refusal No evidence of perched or static groundwater or seasonal perched subsurface water was encountered in the test pit and exploratory boring. Based on Oregon Water Resources Department data, groundwater in the vicinity was encountered in 196 feet deep water well at the depth of 25 feet below the surface. (See Washington County Water Well Report # 011634 attached). INFILTRATION TESTS- The test was conducted by driving a six-inch diameter infiltrometer stand pipe into the soil at the above pointed interval. The infiltration test was conducted as Encased Falling Head tests based on methodology of Portland Stormwater Management Manual —January 2014. We have embedded a solid 6-inch diameter casing into the native medium-grained sand soil at the elevation of 4.5 feet below the surface. That embedment has provided a good seal around the pipe casing so that percolation was limited to the 6-inch plug of the material within the casing. The pipe with clean water approximately of 12 inches above the soil was maintained at this depth for at least 4 hours to presoak the native fine-grained sand soil. Total of three trials of infiltration test were conducted during our study. After each trial, the water level was readjusted to the 12 inch level. The water level was measured to the nearest 0.01 foot (/ inch) at 10-minute intervals for a total period of 2 hours. Successive trials were run until the percent change in measured infiltration rate between two successive trials is minimal. All test results are summarized in the data table. The infiltration rate noted below is a last of three observations of actual infiltration rate measured in the field in undisturbed medium-grained sand soil and do not include a factor of safety. Location Soil Test depth Field infiltration rate TP-1 sand 4.5 feet 16 inches/hour July 13, 2017 Page 3 After infiltration testing, test pit was further augured to verify soil conditions beneath the test location. The soils observed below the test depth appeared consistent with sandy soil above and across the site. In accordance with AASHTO classification, tested soils refer to A-2 and A-3 groups of this classification. In accordance with Unified Soil Classification System, tested soil refer to SM class group symbol (well-graded fine-grained sand). CONCLUSIONS- Based on the results of the infiltration test, observation of subsurface conditions, and our office review, the native site soils appear to have moderate permeability at the depth of sandy soils encountered and are suitable for subsurface discharge of storm water. We believe that the onsite infiltration capability has not been compromised by the past construction use on the site and is suitable for subgrade of pervious pavement. Field infiltration rates recorded during this study generally correspond to the range of permeability values reported in the Soil Survey of Washington County,Oregon. Differences in infiltration test results noted above may be due to slight areal and depth variations in soil gradation,density, and in-situ moisture content. In addition, it has been observed that the permeability of undisturbed native soils such as those found on this site can be substantially different than soils that have been disturbed by construction activities. It is recommended that HGE be contacted to observe subsurface conditions at the time of construction to correlate actual soil conditions with those observed during this study. It is also advisable to test the infiltration system to confirm adequate capacity. We appreciate the opportunity to assist you on this project. If you have any questions or would like additional information please feel free to contact the undersigned at(503) 892-2000. Truly yours, Hydro Geo& Environmental,Inc. 1 i�' i c I BERG 'NEM t& 1E17• MA r'-- 1 kik .,y Mike Go berg, .G Principal Engineer' g Geologist Y Ir`-i-'a /P/307 July 13, 2017 Page 4 TROT PIT 1 (V Hydro Geo& Environmental, Inc Project# 2388.17 8525 SW 67th Ave., Portland,OR 97223 Site Address: 15970 SW 76`x' Ave.,Tigard, OR Test Pit/borehole# 1 Depth, 7 feet. 6/08/17 Depth(ft) Soil Description: 0.0-1.5 Top soil-organic silt fill with tree roots and wood debris 1.5-4.5' Native yellowish-brown, damp to moist low plasticity stiff sandy silt 4.5-7.0' Yellowish-brown damp to moist, dense, well-graded, fne-grained silty sand 7.0- Dark-grey weathered basalt rock fragments No seepage Test pit was terminated at the depth of 4.5 feet and then hand-augered test boring was advanced to a depth of 7 feet.Test boring was terminated at the depth of 7 feet due to full auger refusal Test pit backfilled upon infiltration test completion. Completed by(name and title) A` M.Golberg,Engineering Geologist