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Report (64) ` c?,15 S STRATA ^ October 26, 2018 Troy Smith TW Smith Corporation Via Email: twsmithcorp@gmail.com SUBJECT: GEOTECHNICAL ASSESSMENT Subgrade Excavation for Swimming Pool 14504 SW Fern Road, Tigard, OR 97223 Introduction Per your request and recent authorization, Strata Design LLC (STRATA) has prepared this letter providing findings of our geotechnical site evaluation related to the excavation and subgrade acceptance criteria for the swimming pool structure at the subject residential site.The is located in Tigard, Oregon as shown in FIGURE 1, attached. Project Description We understand our evaluation pertains specifically to the installation of a pre-manufactured below grade swimming pool, with dimensions of 40 foot by 16 foot, with varying depths. The project is comprised of excavation into natural earth grade and will not involve significant placement of fill or embankment. Local Geology Our reviews of geologic mapping and study publication (Gannett, M.W., and Caldwell, R.R, 1998) indicates subsurface geology will consist of Columbia River basalt (map unit, Tcr)with a cap of silt loess. Based on the landslide study mapping (1), the property does not lie within a mapped boundary of known landslide. The attached Figure 2 the landslide mapping for the local vicinity. Site grades surrounding the swimming pool area are around 15 percent or less, considered to be relatively moderate. Site Assessment The undersigned professional with STRATA conducted a field reconnaissance of the excavation during the past week. We assessed the site soils as exposed by the rough excavated grades for the swimming pool, which varied between 2 and 12 feet of near vertical cut at the time of our visit. Our assessment included probing the excavated soil grades with a 4-foot T-probe with a metal tip. As is depicted in the attached in Figure 3 - Photographs, the excavation walls reveal the subsurface soil character to consist of very stiff silt loam. No groundwater table or water seepage was exposed throughout the depth of the 12 foot excavation. We observed no soil slumping, sloughing or caving of 1 SLIDO, maintained by the Oregon Department of Geology and Mineral Industries: http://www.gis.dogami.oregon.gov/maps/slido www.strata-design.com I (503)819-4423 RE: Geotechnical Evaluation-14504 SW Fern Road October 26,2018 Page 2 of 3 the excavation and see no interbedding or geologic unit contacts in the excavation. Geotechnical Recommendations Site Preparation As we are now observing to be the case, excavated grades are being covered by 4 to 6 inches of base rock.The base rock placement is observed to follow specifications listed below in the Structural Fill section. Existing topsoil and vegetation should be stripped and removed from all proposed improvement areas. Where trees and shrubs are present within structural areas, root balls should be grubbed out to the depth of the roots. Depending on the methods used to remove the root balls, considerable disturbance and loosening of the subgrade could occur during site grubbing. We recommend that soil disturbed during grubbing operations be removed to expose firm, undisturbed native subgrade soils. The resulting excavations should be backfilled with structural fill. Slope Stability To our knowledge, no known or mapped landslides are present at the site. Based on our own reconnaissance of the site and surrounding area, there does not appear to be any evidence of prior slope instability. Observed site slopes are generally smooth and uniform, consistent with stable slope conditions.The proposed building site is reasonably buffered from the top of the descending slopes present on site. Shallow Spread Foundations It is necessary that foundations bear on competent native soil subgrade. If soft, loose, or otherwise unsuitable soils are encountered, over-excavation and granular backfill may be recommended to mitigate those soils.The resulting over-excavation should be brought back to grade with imported granular material, following recommendations below in the Structural Fill section. Where utilizing shallow, spread footings for the column and perimeter foundations that are founded on competent, non-disturbed native soil or structural fill placed directly upon competent native soils, an allowable bearing capacity of 2,000 psf may be utilized in design.These bearing pressures are a net bearing pressure, applies to the total of dead and long-term live loads.The allowable pressure used for design and may be increased by a factor of 1.5 when considering seismic or wind loads. Structural Fill Imported granular structural fill should consist of crushed rock or gravel that is fairly well graded between coarse and fine particle sizes.The granular fill should contain no organic matter, debris, or particles larger than 4 inches, and have less than 5 percent material passing the U.S. Standard No. 200 Sieve. For the footing base pads,the percentage of fines can be increased to 12 percent of the material passing the U.S. Standard No. 200 Sieve if placed during dry weather, and provided the fill material is moisture-conditioned, as necessary, for proper compaction. For fine grading purposes such as in the case of the swimming pool base, the maximum particle size should be limited to no more than 1% inches for granular. RE: Geotechnical Evaluation-14504 SW Fern Road • October 26,2018 Page 3 of 3 Granular structural fill should be placed in lifts with a maximum compacted thickness of about 12 inches. Structural backfill should be compacted to a minimum of 95 percent of the material's maximum dry density as determined in general accordance with ASTM D1557 (Modified Proctor). Moisture conditioning should be expected in order to achieve adequate compaction. Wet Weather Considerations We recommend stabilizing the areas of the site experiencing heavily loaded construction traffic with a support layer of crushed rock with 3-inch or greater particle size. Silt fences, inlet protection, soil stockpile covers, etc., are required to reduce sediment transport during construction to acceptable levels. Measures to reduce erosion should be implemented in general accordance with project civil site plan and State, City, and County regulations regarding erosion control, where applicable. Limitations We have provided current design recommendations based on prior site explorations that indicate the soil conditions at only those specific locations and only to the depths explored. These observations do not account for potential variations in soil types,thickness,or water level that may exist between or away from the explorations. If subsurface conditions vary from those encountered in our site exploration, STRATA should be alerted to the change in conditions so that we may provide additional recommendations, if necessary.Observation by experienced geotechnical personnel should be considered an integral part of the construction process. The owner is responsible for insuring that the project designers and contractors implement our recommendations. This study consisted of visual examinations and a review of readily available geologic resources judged pertinent to the evaluation. Accordingly, the limitations of this study must be recognized. • Respectfully Submitted, STRATA DESIGN, LLC t�Ep P R©, 6, y i�' P •; A Yr 4111"� �A, S 11 �4 LI. S. Randall Goode. '.E. Principal Engineer Attachments: Figure 1—Vicinity Map Figure 2—Landslide Hazard Map Figure 3- Photographs RE: Geotechnical Evaluation-14504 SW Fern Road October 26,2018 Page 2 of 3 the excavation and see no interbedding or geologic unit contacts in the excavation. Geotechnical Recommendations Site Preparation As we are now observing to be the case, excavated grades are being covered by 4 to 6 inches of base rock.The base rock placement is observed to follow specifications listed below in the Structural Fill section. Existing topsoil and vegetation should be stripped and removed from all proposed improvement areas. Where trees and shrubs are present within structural areas, root balls should be grubbed out to the depth of the roots. Depending on the methods used to remove the root balls, considerable disturbance and loosening of the subgrade could occur during site grubbing. We recommend that soil disturbed during grubbing operations be removed to expose firm, undisturbed native subgrade soils. The resulting excavations should be backfilled with structural fill. Slope Stability To our knowledge, no known or mapped landslides are present at the site. Based on our own reconnaissance of the site and surrounding area, there does not appear to be any evidence of prior slope instability. Observed site slopes are generally smooth and uniform, consistent with stable slope conditions.The proposed building site is reasonably buffered from the top of the descending slopes present on site. Shallow Spread Foundations It is necessary that foundations bear on competent native soil subgrade. If soft, loose, or otherwise unsuitable soils are encountered, over-excavation and granular backfill may be recommended to mitigate those soils.The resulting over-excavation should be brought back to grade with imported granular material,following recommendations below in the Structural Fill section. Where utilizing shallow, spread footings for the column and perimeter foundations that are founded on competent, non-disturbed native soil or structural fill placed directly upon competent native soils, an allowable bearing capacity of 2,000 psf may be utilized in design.These bearing pressures are a net bearing pressure, applies to the total of dead and long-term live loads.The allowable pressure used for design and may be increased by a factor of 1.5 when considering seismic or wind loads. Structural Fill Imported granular structural fill should consist of crushed rock or gravel that is fairly well graded between coarse and fine particle sizes.The granular fill should contain no organic matter, debris, or particles larger than 4 inches, and have less than 5 percent material passing the U.S. Standard No. 200 Sieve. For the footing base pads, the percentage of fines can be increased to 12 percent of the material passing the U.S. Standard No. 200 Sieve if placed during dry weather, and provided the fill material is moisture-conditioned, as necessary, for proper compaction. For fine grading purposes such as in the case of the swimming pool base, the maximum particle size should be limited to no more than 1Y2 inches for granular.