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HomeMy WebLinkAboutR-408505L, LGEOTECHNICAL RECOMMENDATIONS C) 411 O 1 0 LLI CONCLUSIONS AND RECOMMENDATIONS Based on the results of our field exploration program, laboratory testing, and engineering analysis, we U) conclude that development of the proposed development can be accomplished as planned. A summary of w primary geotechnical considerations for the site development and design of the proposed development is C provided in the subsequent sections. OSummary z ■ The planned townhomes site is classified as Site Class C, in accordance with the 2015 International Building Code (IBC). Odense ■ The planned townhomes may be supported on conventional spread footings bearing on dense to very undisturbed glacial till or on structural fill placed over these soils. Footings bearing on dense to very dense undisturbed glacial till may be designed using an allowable soil bearing value of 6,000 pounds per square foot (psf). Footings bearing on structural fill placed over undisturbed dense v� to very dense glacial till may be designed using an allowable bearing value of 3,000 psf. All existing fill, LLI highly weathered glacial till or otherwise unsuitable soils should be removed from below foundations priorto constructing foundations or placing structural fill. The allowable bearingvalue may be increased U by one-third for short duration loads such as wind or seismic events. ■ Lateral foundation loads may be resisted by resistance on the sides of the footings and by passive Ofriction on the base of the footings. For footings supported and surrounded by either dense native soils J or compacted structural fill, a coefficient of friction of 0.35 and a passive resistance of 350 pounds per J cubic foot (pcf) may be used. ■ A subgrade modulus of 100 pounds per cubic inch (pci) may be used for design of the slabs -on -grade for the townhomes. Concrete slabs -on -grade should be supported on a 4-inch-thick capillary break layer CY) overlain by a vapor retarder (in conditioned spaces or enclosed rooms, such as mechanical or storage 0 space). 6i ■ The pavement section extending to the west and north from the southeast corner of the project site may be supported on existingfill soils provided thatthe upper 2 feet of the fill is placed and compacted as structural fill. A proof roll test can also be conducted to reduce excavation costs if native soils are encountered within the upper 2 feet and to check that the soils can perform adequately under planned loads. If any soft spots are observed, the upper 2 feet of subgrade soils should be removed and replaced with at least 2 feet of structural fill compacted to at least 95 percent of the maximum dry density (MDD) per ASTM International (ASTM) D 1557. Suitable on -site soils may be used as structural GEoENGINEERS� November 15, 2018 Page 3 File No. 23656-001-00 Prior to placing the gravel layer, the subgrade should be proofrolled as described previously in the "Earthwork" section of this report. If necessary, the building slab subgrades should be recompacted to a firm and unyielding condition. We recommend that concrete slabs -on -grade be constructed on a gravel layer to provide uniform support and drainage and to act as a capillary break. The gravel layer below slabs -on -grade should consist of at least 4 inches of clean crushed gravel with a maximum particle size of 1 inch and negligible sand or silt in accordance with Washington State Department of Transportation (WSDOT) Standard Specification 9-03.1(4)C American Association of State Highway and Transportation Officials (AASHTO) Grading No. 67. If prevention of moisture migration through the slab is essential, a vapor retarder such as heavy plastic sheeting should be installed between the slab and the gravel layer. It may also be prudent to apply a sealer to the slab to further retard the migration of moisture through the floor. We recommend that the plastic sheet be placed over the capillary break layer. Pavement Recommendations Recommendations for typical pavements (asphalt and concrete) are provided in the following sections. The City of Renton may have standard pavement sections that could apply to the site, therefore the project civil engineer should review the City's standards, if applicable. Subgrade Preparation We recommend the subgrade soils in new pavement areas be prepared and evaluated as described in the "Earthwork" section of this report. All new pavement and hardscape areas should be supported on subgrade soils that have been proof rolled or probed as described in the "Clearing and Site Preparation" section of this report. If the exposed subgrade soils are loose or soft, it may be necessary to excavate localized areas and replace them with structural fill or gravel base course. Pavement subgrade conditions should be observed during construction and prior to placing the subbase materials in order to evaluate the presence of zones of unsuitable subgrade soils and the need for overexcavation and replacement of these zones. New Hot Mix Asphalt Pavement In light -duty pavement areas (e.g., automobile parking), we recommend a pavement section consisting of at least a 21/2 -inch thickness of 1/2-inch hot -mix asphalt (HMA) per WSDOT Sections 5-04 and 9-03, over a 4-inch thickness of densely compacted crushed surfacing base course (CSBC) per WSDOT Section 9-03.9(3). In heavy-duty pavement areas (such as the driveway), we recommend a pavement section consisting of at least a 3-inch thickness of 1/2-inch HMA over a 6-inch thickness of densely compacted CSBC. The base course should be compacted to at least 95 percent of the MDD obtained using ASTM D 1557. We recommend that proof rolling of the subgrade and compacted base course be observed by a representative from ourfirm priorto paving. Soft or yielding zones observed during proof rolling may require overexcavation and replacement with compacted structural fill. The pavement sections recommended above are based on our experience. Thicker asphalt sections may be needed in accordance with the City of Renton or based on the actual traffic data, truck loads, and intended use. All paved and landscaped areas should be graded so that surface drainage is directed to appropriate catch basins. GEOENGINEERS� w i a z w LL November 15, 2018 Page 7 File No. 23656-001-00 fill under the planned road and associated hardscape provided that earthwork is accomplished during dry weather conditions in the summer months. Earthquake Engineering We evaluated the site for seismic hazards including liquefaction, lateral spreading, fault rupture and earthquake -induced landsliding. Our evaluation indicates that the site does not have liquefiable soils present and, therefore, also has no risk of liquefaction -induced lateral spreading. In addition, the site has a low risk of fault rupture and earthquake -induced landsliding. 2015 IBC Seismic Design Information For the planned townhomes, we recommend the IBC 2015 parameters for average field standard penetration resistance, site class, short period spectral response acceleration (Ss), 1-second period spectral response acceleration (Si), and seismic coefficients FA and Fv presented in Table 1. TABLE 1. 2015 IBC SEISMIC PARAMETERS 203.5 IBC Parameter Recommended Value Average Field Standard Penetration Resistance >50 Site Class C Short Period Spectral Response Acceleration, Ss (percent g) 143.2 1-Second Period Spectral Response Acceleration, Si. (percent g) 53.8 Seismic Coefficient, FA 1.000 Seismic Coefficient, Fv 1.300 Liquefaction Potential Liquefaction is a phenomenon where soils experience a rapid loss of internal strength as a consequence of strong ground shaking. Ground settlement, lateral spreading and/or sand boils may result from soil liquefaction. Structures supported on liquefied soils could suffer foundation settlement or lateral movement that could be severely damaging to the structures. Conditions favorable to liquefaction occur in loose to medium dense, clean to moderately silty sand, which is below the groundwater level. Based on our evaluation of the subsurface conditions observed in the explorations, it is our opinion that potentially liquefiable soils are not present at the project site. Ground Rupture Ground rupture from lateral spreading is associated with liquefaction. Lateral spreading involves lateral displacements of large volumes of liquefied soil, and can occur on near -level ground as blocks of surface soils displace relative to adjacent blocks. In our opinion, ground rupture resulting from lateral spreading at the site is unlikely because potentially liquefiable soils are not present at the site as discussed above. Because of the thickness of the Quaternary sediments below the site, which are commonly more than 1,000 feet thick, the potential for surface fault rupture is considered low. GEOENGINEERS� November 15, 2018 Page 4 File No. 23656-001-00 Portland Cement Concrete Pavement Portland cement concrete (PCC) sections may be considered for areas where concentrated heavy loads may occur. We recommend that these pavements consist of at least 6 inches of PCC over 6 inches of CSBC. A thicker concrete section may be needed based on the actual load data for use of the area. If the concrete pavement will have doweled joints, we recommend that the concrete thickness be increased by an amount equal to the diameter of the dowels. The base course should be compacted to at least 95 percent of the MDD. We recommend PCC pavements incorporate construction joints and/or crack control joints spaced at maximum distances of 12 feet apart, center -to -center, in both the longitudinal and transverse directions. Crack control joints may be created by placing an insert or groove into the fresh concrete surface during finishing, or by saw cutting the concrete after it has initially set up. We recommend the depth of the crack control joints be approximately one fourth the thickness of the concrete; or about 11/2 inches deep for the recommended concrete thickness of 6 inches. We also recommend the crack control joints be sealed with an appropriate sealant to help restrict water infiltration into the joints. Asphalt -Treated Base If pavements are constructed during the wet seasons, consideration may be given to covering the areas to be paved with asphalt -treated base (ATB) for protection. Light -duty pavement areas should be surfaced with 3 inches of ATB, and heavy-duty pavement areas should be surfaced with 6 inches of ATB. Thicker ATB sections may be needed based on construction equipment loads. Prior to placement of the final pavement sections, we recommend the ATB surface be evaluated and areas of ATB pavement failure be removed and the subgrade repaired. If ATB is used and is serviceable when final pavements are constructed, the CSBC can be eliminated, and the design PCC or asphalt concrete pavement thickness can be placed directly over the ATB. Earthwork Based on the subsurface soil conditions encountered in the explorations, we expect that the soils at the site may be excavated using conventional heavy-duty construction equipment. Very dense glacial till was encountered at relatively shallow depths at the planned building locations; therefore, glacial till soils within deeper portions of excavations may require a large excavator to accomplish the excavations. Cobbles were observed in most of the test pits and glacial till deposits in the area commonly contain boulders that may be encountered during excavation. Accordingly, the contractor should be prepared to deal with boulders, if encountered. The glacial till contains sufficient fines (material passing the U.S. standard No. 200 sieve) to be highly moisture -sensitive and susceptible to disturbance, especially when wet. Ideally, earthwork should be undertaken during extended periods of dry weather when the surficial soils will be less susceptible to disturbance and provide better support for construction equipment. Dry weather construction will help reduce earthwork costs and increase the potential for using the native soils as structural fill. Trafficability on the site is not expected to be difficult during dry weather conditions. However, the fill and native soils will be susceptible to disturbance from construction equipment during wet weather conditions and pumping and rutting of the exposed soils under equipment loads may occur. GWENGINEERS� November 15, 2018 Page 8 File No. 23656-001-00 Landslides Because site topography is relatively flat and dense to very dense glacial till deposits occur at shallow depths, it is our opinion that landsliding as a result of strong ground shaking is unlikely at the site. Foundations We recommend that the buildings be supported on shallow spread footings founded on the dense to very dense native glacial till soil encountered in the explorations, or on properly compacted structural fill extending down to medium dense to dense glacial till. The following recommendations for the buildings are based on the subsurface conditions observed in the explorations and the site survey. Foundation Design For shallow foundation support, we recommend widths of at least 18 and 24 inches, respectively, for continuous wall and isolated column footings supporting the proposed townhomes. The design frost depth in the Puget Sound area is 12 inches, therefore, we recommend that the footings be founded at least 18 inches below lowest adjacent finished grade. Unsuitable soils consisting of fill, topsoil, and/or highly weathered glacial soils will vary across the site and must be removed from below planned footings. Based on our explorations, up to 31/2 feet of fill and/or looser weathered native soils exist under the proposed north building unit (GEI-TP-1 and GEI-TP-2), approximately 3 feet under the central building unit (GEI-TP-3 and GEI-TP-4) and 21/2 feet under the south building unit (GEI-TP-5 and GEI-TP-6). Therefore, depending on the foundation locations and depths, overexcavation under the footings may be necessary. For foundations supported on medium dense native glacial till or structural fill extending down to medium dense to dense native glacial till, we recommend footings be designed using a maximum allowable bearing pressure of 3,000 psf. A maximum allowable bearing pressure of 6,000 psf may be used in design where foundations are bearing on dense to very dense relatively unweathered glacial till or on controlled density fill (CDF) extending down to the dense to very dense native till. All existing fill and looser native soils should be removed from below planned footings. These allowable bearing pressures apply to the total dead and long-term live loads and may be increased up to one-third for short-term live loads such as wind or seismic forces. The overexcavated areas should be backfilled with: (1) CDF having a design strength of at least 200 pounds per square inch (psi) where 6,000 psf bearing pressures are used or, (2) imported gravel borrow where 3,000 psf is used. Where structural fill is placed below footings, the fill should extend beyond the edges of the foundations by the depth of the overexcavation, while the CDF should extend beyond the edges of the foundations by half the depth of the excavation. Foundation Settlement We estimate that the post -construction settlement of footings founded on the very dense glacial till or structural fill extending to the medium dense to very dense till, as recommended above, will be between 1/2 and 1 inch. Differential settlement between comparably loaded column footings or along a 25-foot section of continuous wall footing should be less than 1/2 inch. We expect most of the footing settlements will occur as loads are applied. Loose or disturbed soils not removed from footing excavations prior to placing concrete will result in additional settlement. GEoENGINEERS November 15, 2018 Page 5 File No. 23656-001-00 Clearing and Site Preparation Areas to be developed or graded should be cleared of surface and subsurface deleterious matter including any debris, shrubs, trees and associated stumps and roots. Graded areas should be stripped of organic soils. The organic soils can be stockpiled and used later for landscaping purposes or may be spread over disturbed areas following completion of grading. If spread out, the organic strippings should be in a layer less than 1-footthick, should not be placed on slopes greater than 3HAV (horizontal to vertical) and should be track -rolled to a uniformly compacted condition. Materials that cannot be used for landscaping or protection of disturbed areas should be removed from the project site. Undocumented fill may be present in various areas of the site and will be required to be removed under building foundations and within the upper two feet of pavement, hardscape and slab subgrade levels. Where existing fill and looser native soils are removed, they may be reused and recompacted as structural fill, if conditions allow. If medium dense to dense native soils are encountered below slab subgrade, additional excavation is not required. If old fill is encountered below slab subgrade, the fill should be evaluated and possibly removed up to 2 feet below slab subgrade or until medium dense to dense native soils are encountered (less than 2 feet below slab subgrade). Excavations for slab subgrade preparation likely do not need to extend more than 2 feet below slab subgrade. The upper two feet below pavement subgrade should also be removed and replaced as structural fill; however, if existing fill soils are suitable and adequately compacted based on evaluations after the pavement is removed, the contractor can perform a proof roll on the exposed surface at or below slab subgrade level, and if approved by the geotechnical engineer, the fill may be left in place. Subgrade Preparation Prior to placing new fills, pavement base course materials or gravel below on -grade floor slabs, subgrade areas should be proof rolled to locate any soft or pumping soils. Prior to proof rolling, all unsuitable soils should be removed from below the building footprints. Proof rolling can be completed using a piece of heavy tire -mounted equipment such as a loaded dump truck. During wet weather, the exposed subgrade areas should be probed to determine the extent of soft soils. If soft or pumping soils are observed, they should be removed and replaced with compacted structural fill. If deep pockets of soft or pumping soils are encountered outside the building areas, it may be possible to limit the depth of overexcavation by placing a non -woven geotextile fabric such as TenCate Mirafi 50OX (or equivalent) on the overexcavated subgrade prior to placing structural fill. The geotextile will provide additional support by bridging over the soft material and will help reduce fines contamination into the structural fill. After completing the proof rolling, the subgrade areas should be recompacted to a firm and unyielding condition, if possible. The degree of compaction that can be achieved will depend on when the construction is performed. If the work is performed during dry weather conditions, we recommend that all subgrade areas be recompacted to at least 95 percent of the MDD in accordance with the ASTM D 1557 test procedure (modified Proctor). If the work is performed during wet weather conditions, it may not be possible to recompact the subgrade to 95 percent of the MDD. In this case, we recommend that the subgrade be compacted to the extent possible without causing undue heaving or pumping of the subgrade soils. GEOENGINEERS� November 15, 2018 Page 9 File No. 23656-001-00 Lateral Resistance Lateral loads can be resisted by passive resistance on the sides of the footings and by friction on the base of the footings. Passive resistance should be evaluated using an equivalent fluid density of 350 pcf where footings are poured neat against native soil or are surrounded by structural fill compacted to at least 95 percent of MDD, as recommended. Resistance to passive pressure should be calculated from the bottom of adjacent floor slabs and paving or below a depth of 1 foot where the adjacent area is unpaved, as appropriate. Frictional resistance can be evaluated using 0.35 for the coefficient of base friction against footings. The above values incorporate a factor of safety of about 1.5. If soils adjacent to footings are disturbed during construction, the disturbed soils must be recompacted, otherwise the lateral passive resistance value must be reduced. Construction Considerations Immediately prior to placing concrete, all debris and loose soils that accumulated in the footing excavations during forming and steel placement must be removed. Debris or loose soils not removed from the footing excavations will result in increased settlement. If wet weather construction is planned, we recommend that all footing subgrades be protected using a lean concrete mud mat or 3 inches of compacted crushed base course. The mud mat or base course should be placed the same day that the footing subgrade is excavated and approved for foundation support. We recommend that all completed footing excavations be observed by a representative of our firm prior to placing mud mat, reinforcing steel, and structural concrete. Our representative will confirm that the bearing surface has been prepared in a manner consistent with our recommendations and that the subsurface conditions are as expected. Footing Drains We recommend that perimeter footing drains be installed around each building. The perimeter drains should be installed at the base of the exterior footings. The perimeter drains should be provided with cleanouts and should consist of at least 4-inch-diameter perforated pipe placed on a 3-inch bed of drainage material, and surrounded by 6 inches of drainage material enclosed in a non -woven geotextile fabric such as TenCate Mirafi 140N (or approved equivalent) to prevent fine soil from migrating into the drain material. We recommend against using flexible tubing for footing drainpipes. The perimeter drains should be sloped to drain by gravity, if practicable, to a suitable discharge point, preferably a storm drain. We recommend that the cleanouts be covered, and be placed in flush -mounted utility boxes. Water collected in roof downspout lines must not be routed to the footing drain lines. Slab -on -Grade Floors We expect that the lower level concrete slab -on -grade can be supported on the medium dense to very dense native soil encountered in our explorations or on properly compacted structural fill. A subgrade modulus of 100 pci may be used for design of the slabs -on -grade at the site. We recommend that an appropriate capillary break and vapor retarder be installed below concrete slabs to reduce the risk of moisture migration through the floor slab. This is especially important since zones of groundwater seepage may be present at the planned floor slab level in more permeable layers above the dense native glacial till or in looser soils on top of the dense glacial till. GEOENGINEERS� November 15, 2018 Page 6 File No. 23656-001-00 R-408505 IN COMPLIANCE WITH CITY OF RENTON STANDARDS DEVELOPMENT ENGINEERING Nathan Janders 01 /03/2020 thirdplace design • co-operative where architecture meets community J\ CITY CORRECTIONS KC 11.08.19 SURVEYED: SCALE: N.T.S.N,T,S, ONTAL:NAVD1968 HORIZONTAL: DATUM CITY OF RENTON Planning/Building/Public Works Dept. DMOND CREST E R NHON EDMONDS AVE NE, RENTON WA COVER SHEET: GEOTECHNICAL RECOMMENDATIONS 7.29.19 DESIGNED: PAGE:BOOK: °��" KC ONE INCH DRAWING N0: T—O'O� CHECKED: ATFULLSCALE IF NOTONE INCH SCALE ACCORDINGLY NO. REVISION BY DATE APPR APPROVED: SHEET OF: co co T 411 O T U T C) O 0 6 T Q J co T 0 O O 1 0) T i