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HomeMy WebLinkAboutR-408506LO co 0 1 O 1 0 LLI 6 W i a Z W LL GEOTECHNICAL RECOMMENDATIONS (CONTINUED) Subgrade disturbance or deterioration could occur if the subgrade is wet and cannot be dried. If the subgrade deteriorates during proof rolling or compaction, it may become necessary to modify the proof rolling, compaction criteria, or methods. Structural Fill All fill, whether existing on -site glacial till soil or imported soil, that will support building foundations and floor slabs, pavement and hardscape areas, or be placed in utility trenches should generally meet the criteria for structural fill presented below. The suitability of soil for use as structural fill depends on its gradation and moisture content. Materials Materials used on the project site, under buildings, pavement, hardscape areas, and to backfill utility trenches are classified as structural fill for the purpose of this report. Structural fill material quality varies depending upon its use as described below: 1. Structural fill placed below all building elements (except footing designed for greater than 3,000 psf bearing pressure) and during wet weather conditions should consist of imported Gravel Borrow, as described in Section 9-03.14(1) of the 2018 WSDOT Standard Specifications, with the additional restriction that the fines content be limited to no more than 5 percent. On -site soils may be used as structural fil provided it is placed during the summer months, is properly moisture conditioned to within 2 percent of the optimum moisture content, and can be compacted to at least 95 percent of the MDD. 2. CDF having a design strength of at least 200 psi should be used under all foundations designed for greater than 3,000 psf bearing pressure. 3. Structural fill placed to construct embankment and parking areas and to backfill utility trenches may consist of on -site fill and glacial till provided that the soils are moisture conditioned for the required compaction. On -site till soils may be suitable for use as structural fill during dry weather conditions in areas needing 95 percent compaction. If structural fill is placed during wet weather, the structural fill should consist of imported gravel borrow. 4. Structural fill placed as CSBC below pavements should conform to Section 9-03.9(3) of the 2018 WSDOT Standard Specifications. 5. Structural fill placed as capillary break below slabs should consist of 1-inch minus clean crushed gravel with negligible sand or silt in conformance with Section 9-03.1(4)C, grading No. 67 of the 2018 WSDOT Standard Specifications. Reuse of On -site Native Soils The existing fill and till soils contain a high percentage of fines and will be sensitive to changes in moisture content and difficult to handle and compact during wet weather. The existing fill (free of organic debris) and till deposits are expected to be suitable for structural fill in areas requiring compaction to at least 95 percent of MDD (per ASTM D 1557), provided the work is accomplished during the normally dry season (June through September) and that the soil can be properly moisture conditioned to within 2 percent of the optimum moisture content. Imported structural fill consisting of sand and gravel (WSDOT gravel borrow) should be planned under all building foundation elements, especially if construction occurs during wet weather. GEoENGINEERS� November 15, 2018 Page 10 File No. 23656-001-00 of the excavation is not affected. We recommend that this distance be at least 5 feet from the top of the cut for temporary cuts made at 1HAV or flatter, and no closer than a distance equal to one-half the height of the slope for cuts made steeper than 1H:IV. Water that enters the excavation must be collected and routed away from prepared subgrade areas. We expect that this may be accomplished by installing a system of drainage ditches and sumps along the toe of the cut slopes. Some sloughing and raveling of the cut slopes should be expected. Temporary covering, such as heavy plastic sheeting with appropriate ballast, should be used to protect these slopes during periods of wet weather. Surface water runoff from above cut slopes should be prevented from flowing over the slope face by using berms, drainage ditches, swales or other appropriate methods. If temporary cut slopes experience excessive sloughing or raveling during construction, it may become necessary to modify the cut slopes to maintain safe working conditions. Slopes experiencing problems can be flattened, regraded to add intermediate slope benches, or additional dewatering can be provided if the poor slope performance is related to groundwater seepage. Drainage Considerations We anticipate shallow groundwater seepage may enter excavations for utilities depending on the time of year construction takes place, especially in the winter months. However, we expect that this seepage water can be handled by digging interceptor trenches in the excavations and pumping from sumps. The seepage water, if not intercepted and removed from the excavations, will make it difficult to place and compact structural fill and may destabilize cut slopes. All paved and landscaped areas should be graded so that surface drainage is directed away from the buildings to appropriate catch basins. Water collected in roof downspout lines must not be routed to the footing drain lines. Collected downspout water should be routed to appropriate discharge points in separate pipe systems. Infiltration Considerations Sieve analyses were performed on selected soil samples collected from the test pits that were completed as part of this study. The soil samples typically consisted of native weathered or relatively unweathered glacial till. The design infiltration value described below is based on the results of the grain size analyses, the United States Department of Agriculture (USDA) Textural Triangle, and the Washington State Department of Ecology Storm Water Management Manual (2005). The grain size analyses are presented in Appendix B. Based on our analysis, it is our opinion that the on -site native glacial till soils have a very low infiltration capacity. The majority of the soils across the site contain significant fines, which limits the infiltration capacity. The results of the sieve analyses indicated that the fines content (material passing the U.S. No. 200 sieve) typically ranges from 30 to 35 percent. Due to the density, high fines content, and relative impermeability of the glacial till, infiltration should be assumed to be very low when designing infiltration systems. We recommend a preliminary infiltration rate of not more than 0.2 inches per hour be used for design of the infiltration facilities. Depending on the depth of proposed infiltration facilities, the infiltration rate will vary; however, we recommend site specific pilot infiltration testing be performed to determine the design infiltration rate if specific infiltration facilities are being considered. GEOENGINEERS� November 15, 2018 Page 14 File No. 23656-001-00 The use of existing on -site fill and till soils as structural fill during wet weather should be planned only for areas requiring compaction to 90 percent of MDD, as long as the soils are properly protected from wet weather, not placed during periods of precipitation, and that they can be dried if needed to achieve proper compaction. The contractor should plan to cover and maintain all fill stockpiles with plastic sheeting if it will be used as structural fill. The reuse of on -site soils is highly dependent on the skill of the contractor and schedule, and we will work with the design team and contractor to maximize the reuse of on -site till soils during the wet and dry seasons. Fill Placement and Compaction Criteria Structural fill should be mechanically compacted to a firm, non -yielding condition. Structural fill should be placed in loose lifts not exceeding 12 inches in thickness when using heavy compaction equipment and not more than 6 inches when using hand -operated compaction equipment. The actual thickness will be dependent on the structural fill material used and the type and size of compaction equipment. Each lift should be moisture conditioned to within about 2 percent of the optimum moisture content to achieve proper compaction to the specified density before placing subsequent lifts. Compaction of all structural fill at the site should be in accordance with the ASTM D 1557 (modified Proctor) test method. Structural fill should be compacted to the following criteria: 1. Structural fill placed below floor slabs and foundations should be compacted to 95 percent of the MDD. 2. Structural fill in new pavement and hardscape areas, including utility trench backfill, should be compacted to at least 90 percent of the MDD, except that the upper 2 feet of fill below final subgrade should be compacted to at least 95 percent of the MDD, see Figure 3, Compaction Criteria for Trench Backfill. 3. Structural fill placed as CSBC below pavements should be compacted to 95 percent of the MDD. 4. Non-structural fill, such as fill placed in landscape areas, should be compacted to at least 90 percent of the MDD. Weather Considerations Disturbance of near -surface soils should be expected if earthwork is completed during periods of wet weather. During dry weather, the soils will: (1) be less susceptible to disturbance, (2) provide better support for construction equipment, and (3) be more likely to meet the required compaction criteria. The wet weather season generally begins in October and continues through May in western Washington; however, periods of wet weather may occur during any month of the year. For earthwork activities during wet weather, we recommend that the following steps be taken: ■ The ground surface in and around the work area should be sloped so that surface water is directed away from the work area. The ground surface should be graded so that areas of ponded water do not develop. Measures should be taken by the contractor to prevent surface water from collecting in excavations and trenches. Measures should be implemented to remove surface water from the work area. ■ Earthwork activities should not take place during periods of moderate to heavy precipitation. ■ Slopes with exposed soils should be covered with plastic sheeting. ■ The contractor should take necessary measures to prevent on -site soils and soils to be used as fill from becoming wet or unstable. These measures may include the use of plastic sheeting, sumps with pumps, and grading. The site soils should not be left uncompacted and exposed to moisture. Sealing the surficial soils by rolling with a smooth -drum roller prior to periods of precipitation will help reduce the extent that these soils become wet or unstable. ■ The contractor should cover all soil stockpiles that will be used as structural fill with plastic sheeting. ■ Construction traffic should be restricted to specific areas of the site, preferably areas that are surfaced with the existing asphalt or working pad materials not susceptible to wet weather disturbance. ■ Construction activities should be scheduled so that the length of time that soils are left exposed to moisture is reduced to the extent practical. Routing of equipment on the existing fill and native till subgrade soils during the wet weather months will be difficult and the subgrade will likely become highly disturbed and rutted. In addition, a significant amount of mud can be produced by routing equipment directly on the glacial soils in wet weather. Therefore, to protect the subgrade soils and to provide an adequate wet weather working surface for the contractor's equipment and labor, we recommend that the contractor protect exposed subgrade soils with sand and gravel, crushed gravel, or ATB. Permanent Cut and Fill Slopes We recommend that permanent cut or fill slopes be constructed at inclinations of 2HAV or flatter, and be blended into existing slopes with smooth transitions. To achieve uniform compaction, we recommend that fill slopes be overbuilt slightly and subsequently cut back to expose well compacted fill. It is our experience that permanent cut slopes made in dense to very dense glacial till are difficult to establish vegetation on and difficult to place and maintain topsoil on. Therefore, 3H:1V or flatter permanent cut slopes should be considered for landscape purposes if site conditions allow for their use. To reduce erosion, newly constructed slopes should be planted or hydroseeded shortly after completion of grading. Until the vegetation is established, some sloughing and raveling of the slopes should be expected. This may necessitate localized repairs and reseeding. Temporary covering, such as clear heavy plastic sheeting, jute fabric, or erosion control blankets (such as American Excelsior Curlex 1 or North American Green SC150) could be used to protect the slopes during periods of rainfall. Utility Trenches Trench excavation, pipe bedding, and trench backfilling should be completed using the general procedures described in the 2018 WSDOT Standard Specifications or other suitable procedures specified by the project civil engineer. The native glacial deposits and fill soils encountered at the site are generally of low corrosivity based on our experience in the Puget Sound area. Utility trench backfill should consist of structural fill and should be placed in loose lifts not exceeding 12 inches in thickness when using heavy compaction equipment and not more than 6 inches when using hand -operated compaction equipment such that adequate compaction can be achieved throughout the lift. Each lift must be compacted prior to placing the subsequent lift. The backfill should be compacted in accordance with the criteria discussed above. Figure 3 illustrates recommended trench compaction criteria under pavement and non-structural areas. GWENGINEERS� November 15, 2018 Page 11 GEOENGINEERS File No. 23656-001-00 third place design co-operative where architecture meets community J\ CITYCORRECTIONS KC 11.08.19 rDU RVEYED: +__ SIGNED: NO. REVISION DRAWN: CHECKED: BY DATE APPR APPROVED: SCALE: N.T.S. KC ONE INCH ATFULLSCALE IF NOTONE INCH SCALE ACCORDINGLY VERTICAL: NAVD 1988 HORIZONTAL: NAD 1983/1991 DATUM November 15, 2018 Page 12 File No. 23656-001-00 CITY OF RENTON Planning/Building/Public Works Dept. Sedimentation and Erosion Control In our opinion, the erosion potential of the on -site soils is low to moderate. Construction activities including stripping and grading will expose soils to the erosional effects of wind and water. The amount and potential impacts of erosion are partly related to the time of year that construction actually occurs. Wet weather construction will increase the amount and extent of erosion and potential sedimentation. Erosion and sedimentation control measures may be implemented by using a combination of interceptor swales, straw bale barriers, silt fences and straw mulch for temporary erosion protection of exposed soils. All disturbed areas should be finish graded and seeded as soon as practicable to reduce the risk of erosion. Erosion and sedimentation control measures should be installed and maintained in accordance with the requirements of the City of Renton. Excavations We anticipate that excavations are limited and will be primarily associated with footing excavations and underground utilities. These cuts can likely be made as temporary open cut slopes depending on site constraints. The stability of open cut slopes is a function of soil type, groundwater seepage, slope inclination, slope height and nearby surface loads. The use of inadequately designed open cuts could impact the stability of adjacent work areas, existing utilities, and endanger personnel. The contractor performing the work has the primary responsibility for protection of workmen and adjacent improvements. In our opinion, the contractor will be in the best position to observe subsurface conditions continuously throughout the construction process and to respond to variable soil and groundwater conditions. Therefore, the contractor should have the primary responsibility for deciding whether or not to use open cut slopes for much of the excavations rather than some form of temporary excavation support, and for establishing the safe inclination of the cut slope. Acceptable slope inclinations for utilities and ancillary excavations should be determined during construction. Because of the diversity of construction techniques and available shoring systems, the design of temporary shoring is most appropriately left up to the contractor proposing to complete the installation. Temporary cut slopes and shoring must comply with the provisions of Title 296 WAC, Part N, "Excavation, Trenching and Shoring." The excavations for the buildings and utilities will be completed primarily in loose to medium dense fill and dense to very dense glacial till deposits. The following sections summarize the general excavation recommendations. Temporary Cut Slopes For planning purposes, temporary unsupported cut slopes more than 4 feet high may be inclined at 1H:1V maximum steepness within the dense to very dense glacial till (below a depth of about 3 feet) and 11/2H:IV maximum steepness in the overlying fill (upper 3 feet). If significant seepage is present on the cut face then the cut slopes may have to be flattened. However, temporary cuts should be discussed with the geotechnical engineer during final design development to evaluate suitable cut slope inclinations for the various portions of the excavation. The contractor should scale slopes cut at 1HAV to remove loose materials and cobbles. The above guidelines assume that surface loads such as traffic, construction equipment, stockpiles or building supplies will be kept away from the top of the cut slopes a sufficient distance so that the stability GEOENGINEERS November 15, 2018 Page 13 File No. 23656-001-00 R-408506 IN COMPLIANCE WITH CITY OF RENTON STANDARDS DEVEL:J�l ENT ENGINEERING Nathannders 01 /03/2020 WILLOWCREST TOWNHOMES EDMONDS AVE NE, RENTON, WA COVER SHEET: GEOTECHNICAL RECOMMENDATIONS DATE: 7.29.19 FIELDBOOK: PAGE: DRAWING NO: T-0.04 SHEET: OF: co co T O O T U T co C) O O 1 6 T Q J co N T 0 O O 1 T