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HomeMy WebLinkAboutRS_4th_Dim_GEO_TECH_190709_v1 13705 Bel-Red Rd – Bellevue, WA 98005 Phone: 425/649-8757 – Fax: 425/649-8758 GEO Geotechnical Engineers, Geologists, & Environmental Scientists Group Northwest, Inc. May 14, 2018 G-4661 Hari Ghadia 12505 Bel-Red Rd, Suite 212 Bellevue, WA 98005-2510 Send via: ghadia_hari@hotmail.com Subject: GEOTECHNICAL REPORT WITH INFILTRATION EVALUATION PROPOSED DEVELOPMENT 4502 NE 4TH ST RENTON, WASHINGTON Dear Hari Ghadia: In accordance with our March 8, 2018 contract with you we have investigated the soil and groundwater conditions at the subject property and prepared the following geotechnical report for the proposed commercial/residential development. SITE AND PROJECT DESCRIPTION The subject site consists of a developed parcel containing one building which is located at the south side of the lot as shown on the attached Plate 2 – Topographic Map. The existing building is a 1-story building with daylight basement which daylights toward the north. The property has an approximate area of 0.55 acres. There is an ecology block retaining wall which is located at the west side of the lot or at the adjacent west right-of-way which has an estimated height of around 12-feet and retains Bremerton Ave NE. The site topography includes a relative depression at the north-central portion of the lot with moderate to steep relatively minor slopes at the north and south sides of the lot. Based upon plans provided by Kaul Design Associates the subject site is proposed to be developed with a mixed-use building containing parking as shown on the attached Plate 2 – Site Plan. We understand that the development will consist of the following: May 14, 2018 G-4661 Geotechnical Report – 4502 NE 4th St, Renton, Washington Page 2 GEO Group Northwest, Inc. 1. The building and garage lowest level is roughly at the existing grade at the depression near the center of the lot. 2. At the "6600 sf commercial" portion of the building there will be main floor level (above the garage) which roughly matches the adjacent existing grades at the south side of the lot and then will have 3-stories of apartments above this level. 3. At the "structured garage" section we understand that the top of the garage roughly matches existing grade at the adjacent Bremerton Ave NE (also main floor level for the "6600 sf commercial" building). A ramp goes down to the lower garage level at the east side of the garage building. A detention or infiltration system may be constructed below the lowest level in the garage although depth/elevation information for this structure has not been provided. GEOLOGIC CONDITIONS The USGS geologic map 1 for the site vicinity indicates that the soils at the subject lot consist of Quaternary-age Ground Moraine deposits. These soils consist of ablation till overlying lodgement till. Till soils are generally described as a mixture of silt, sand and gravel which was both deposited and overridden by glacial ice at least 14,000 years ago. SUBSURFACE CONDITIONS On April 18, 2018 GEO Group Northwest explored the soil and groundwater conditions at the subject parcel by excavating five test pits by mini-excavator at the Test Pit locations noted on the attached Plate 2 – Site Plan. The test pits were excavated to depths of up to 9-feet 8-inches below ground surface (bgs). Soils observed at the test pits generally consist of variable density silty soils and fills which include brick, wood, organic material, concrete and asphalt debris overlying apparent dense and competent till and gravelly SAND and sandy GRAVEL. The depth of fills ranged from 2.5-feet at the test pit TP-5 to around 9-feet at the test pit TP-3. The observed underlying soils appear to match the USDA SCS soil description for Alderwood gravelly sandy loam (AgC) at the uplands and Everett very gravelly sandy loam (EvB) at the depression as shown on the attached Plate 4 – SCS Soil Map. 1 “Geologic Map of the Renton Quadrangle, King County, Washington”, USGS, D.R. Mullineaux, 1965. May 14, 2018 G-4661 Geotechnical Report – 4502 NE 4th St, Renton, Washington Page 3 GEO Group Northwest, Inc. Groundwater seepage was not encountered at the test pits TP-1 through TP-3. Significant groundwater seepage was encountered at a depth of 7-feet bgs at the test pit TP-4 and 5-feet bgs at the test pit TP-5. The results of our subsurface investigation are shown on the attached Appendix A - Test Pit Logs and USCS Soil Legend. GRADATIONAL ANALYSES We performed gradational analyses for soil samples collected at a depth of 2-feet at the test pit TP-4 and a depth of 2-feet 8-inches at the test pit TP-5. These analyses confirmed the soil descriptions at these levels as being gravelly silty SAND. The results of these analyses are attached as Plates 5 and 6 – Gradational Analysis. Extrapolation of the gradation curves indicates that the D10 value for the sampled soils at TP-4 and TP-5 to be 0.01 mm and 0.03 mm, respectively. INFILTRATION EVALUATION The USDA NRCS maps the site soils as Alderwood gravelly sandy loam AgC) and Everett very gravelly sandy loam (EvB) which appear to match the observed conditions at our test pits. The USDA NRCS online data indicates that for the AgC soil unit the capacity of the most limiting layer to transmit water (Ksat): very low to moderately low. Per NRCS for the EvB soil unit the capacity of the most limiting layer to transmit water (Ksat): is high. The soils observed at the uplands, overlying much of the site consist of silty soils which are relatively impermeable. The soils observed at the depression (lowland) portion of the site include where located below silty fills may have relatively high permeability, however, the groundwater level at this area is also relatively high, thereby reducing the effectiveness for an infiltration system. Groundwater seepage ranged in depth at test pits TP-4 and TP-5 from 5 to 7- feet below ground surface. This level may vary dependent upon the time of year, precipitation amounts and changed land use in the area. Due to the presence of silty soils overlying much of the site and the relatively high groundwater conditions we do not recommend attempting to infiltrate stormwater at the subject site. If site stormwater must be infiltrated then we recommend that infiltration rate testing be performed at the infiltration location and it may also May 14, 2018 G-4661 Geotechnical Report – 4502 NE 4th St, Renton, Washington Page 4 GEO Group Northwest, Inc. be necessary to install a monitoring well to determine the groundwater level conditions throughout the year. INFILTRATION RATE From Darcy’s Law for permeability and using the D10 grain size correlation we calculate that the initial infiltration rate for the soils at the TP-4 2’ and TP-5 2’-8”sampled depths is 0.14 to 1.3 inches/hour. These rates are relatively low, highly variable and subject to correction due to the potential for groundwater mounding and possible groundwater movement. As noted above we recommend performing infiltration rate testing at the infiltration rate location and depth if it is determined that infiltration must occur at the site. If stormwater infiltration is to occur for the subject development then we recommend that the designer apply appropriate correction factors to account for site variability, the test methodology and siltation/maintenance. Catchbasins with sump pits should be installed between the stormwater collection system and the infiltration system(s) and periodic maintenance should be required to clean-out the catchbasin sump pit(s). The infiltration system may eventually clog due to siltation and require replacement construction. This reason may also make it difficult to maintain an infiltration system below the proposed structure as currently proposed. SEISMIC DESIGN CRITERIA Based upon our subsurface investigation the project site has Site Class D soil (Stiff Soil) per the IBC based upon the observed subsurface soil conditions and provided that the buildings are constructed to bear on the competent soils as described herein. CONCLUSIONS AND RECOMMENDATIONS General Based upon the results of our study, it is our professional opinion that the site is geotechnically suitable for the proposed development. The primary geotechnical concern with regard to the design for the proposed building is the presence of loose and unacceptable fill soils which present risks of damage due to soil settlement, if not properly over-excavated and filled with compacted structural fill. It is our opinion that these risks can be mitigated by implementing a building pad improvement program. Building foundations should not be constructed to bear May 14, 2018 G-4661 Geotechnical Report – 4502 NE 4th St, Renton, Washington Page 5 GEO Group Northwest, Inc. directly on top of the loose fill soils or fill debris. An alternative to the building pad improvement program would be to construct the building on top of a pile foundation system. In the final recommendations section of this report we provide recommendations for an augercast pile foundation which may be used as an alternative to the recommended building pad improvement. It may also be an option for a portion of the building to be supported on a spread footing foundation, such as at the south side of the property, with the remainder of the building supported on a pile foundation, provided that competent soils are encountered at the foundation subgrades at the proposed spread footing areas. Site Preparation and General Earthwork The building pad areas should be stripped and cleared of surface vegetation and organic soils (forest duff). Silt fences should be installed around areas disturbed by construction activity to prevent sediment-laden surface runoff from being discharged off-site. Exposed soils that are subject to erosion should be compacted and covered with plastic sheeting. Temporary Excavation Slopes Under no circumstances should temporary excavation slopes be greater than the limits specified in local, state and national government safety regulations. Temporary cuts greater than four feet in height should be sloped at an inclination no steeper than 1H:1V (Horizontal:Vertical) in the overlying loose site soils. If seepage is encountered at the excavation slopes should have inclinations of no steeper than 2H:1V for the temporary construction time period. If excavations with the aforementioned slope inclinations encroach upon the adjacent properties or remove support for the existing ecology block wall at the west side of the site than shoring may be required. Structural Fill All fill material used to achieve design site elevations below the building areas and below non- structurally supported slabs, parking lots, sidewalks, driveways, and patios, should meet the requirements for structural fill. During wet weather conditions, material to be used as structural fill should have the following specifications: May 14, 2018 G-4661 Geotechnical Report – 4502 NE 4th St, Renton, Washington Page 6 GEO Group Northwest, Inc. 1. Be free draining, granular material containing no more than five (5) percent fines (silt and clay-size particles passing the No. 200 mesh sieve); 2. Be free of organic material and other deleterious substances, such as construction debris and garbage; 3. Have a maximum size of three (3) inches in diameter. All fill material should be placed at or near the optimum moisture content. The optimum moisture content is the water content in soil that enables the soil to be compacted to the highest dry density for a given compaction effort. Based upon our subsurface investigation the overlying apparent fill site soils consist of sandy SILT and silty SAND with some organic soils and debris. These soils are relatively silty and may be difficult to compact to meet the minimum structural fill compaction requirements. If work occurs during a period of wet weather it is likely that the native site soils may become too wet to achieve the compaction criteria. We recommend that the contractor take measures to protect the site soils from wet weather impacts such as using plastic sheeting to cover stockpiles. Additionally all unsuitable debris such as concrete, wood, organic soil, plastic sheeting and other deleterious materials must be removed from the site soils if they are to be used as structural fill. An imported granular fill material may provide more uniformity and be easier to compact to the required structural fill specification, especially if work occurs during periods of wet weather. Structural fill should be placed in thin horizontal lifts not exceeding ten inches in loose thickness. Structural fill under building areas (including foundation and slab areas), should be compacted to at least 95 percent of the maximum dry density, as determined by ASTM Test Designation D- 1557-91 (Modified Proctor). Structural fill under driveways, parking lots and sidewalks should be compacted to at least 90 percent maximum dry density, as determined by ASTM Test Designation D-1557-91 (Modified Proctor). Fill placed within 12-inches of finish grade should meet the 95% requirement. We recommend that GEO Group Northwest, Inc., be retained to evaluate the suitability of structural fill material and to monitor the compaction work during construction for quality assurance of the earthwork. May 14, 2018 G-4661 Geotechnical Report – 4502 NE 4th St, Renton, Washington Page 7 GEO Group Northwest, Inc. Building Pad Improvement The observed overlying loose soils and fills observed at the test pits present risks of soil settlement related damage to proposed building if the building is constructed to bear on top of these soils. Consequently we recommend that the building pad areas on which foundations and any building slabs are to be constructed be over-excavated to remove loose fills, organic soils and deleterious debris to a depth where the underlying medium dense to dense soils are encountered and then replaced with compacted structural fill as required, dependent upon the proposed building/floor elevations. These fills and unacceptable loose soils were observed at depths as deep as 9-feet below the existing ground surface at the test pit TP-3. Following removal of the unsuitable loose soils and debris fills the base of the excavation should be compacted by vibratory equipment to a firm and unyielding condition, approved by GEO Group Northwest and then backfilled with compacted structural fill placed in accordance with the Structural Fill section of this report. The over-excavation and fill placement at the building pad area is expected to require a significant amount of earthwork and may also present difficulties if loose and unsuitable fills are encountered at the base of the existing block wall at the west side of the site since this would necessitate shoring for the adjacent right-of-way. New building foundations and slabs may be constructed to bear on the compacted structural fill which is in turn placed on top of the competent medium dense to dense underlying soils. Spread Footing Foundations The proposed buildings can be supported on conventional spread footings bearing on top of an improved building pad constructed per our recommendations noted above and which has been approved by GEO Group Northwest, Inc., at the time of construction. Individual spread footings may be used for supporting columns and strip footings for bearing walls. Our recommended minimum design criteria for foundations bearing on improved building pad or on compacted structural fill placed on top of the improved building pad are as follows: - Allowable bearing pressure, including all dead and live loads Medium dense to dense native soils (competent soils) = 2,000 psf Compacted structural fill on top of competent soil (improved building pad) = 2,000 psf - Minimum depth to bottom of perimeter footing below adjacent final exterior grade = 18 inches May 14, 2018 G-4661 Geotechnical Report – 4502 NE 4th St, Renton, Washington Page 8 GEO Group Northwest, Inc. - Minimum depth to bottom of interior footings below top of floor slab = 18 inches - Minimum width of wall footings = 16 inches - Minimum lateral dimension of column footings = 24 inches - Estimated post-construction settlement = 1/4 inch - Estimated post-construction differential settlement; across building width = 1/4 inch A one-third increase in the above allowable bearing pressures can be used when considering short-term transitory wind or seismic loads. Lateral loads can also be resisted by friction between the foundation and the supporting compacted fill subgrade or by passive earth pressure acting on the buried portions of the foundations. For the latter, the foundations must be poured "neat" against the existing undisturbed soil or be backfilled with a compacted fill meeting the requirements for structural fill. Our recommended parameters are as follows: - Passive Pressure (Lateral Resistance) • 350 pcf equivalent fluid weight for compacted structural fill • 350 pcf equivalent fluid weight for native dense soil. - Coefficient of Friction (Friction Factor) • 0.35 for compacted structural fill • 0.35 for native dense soil We recommend that footing drains be placed around all perimeter footings. More specific details of perimeter foundation drains are provided below in the section titled: Subsurface Drainage. Conventional Retaining Walls and Basement Walls Based upon the preliminary plans we understand that conventional concrete retaining walls are proposed for the below-grade portions of the building and this may be at the lower level at the May 14, 2018 G-4661 Geotechnical Report – 4502 NE 4th St, Renton, Washington Page 9 GEO Group Northwest, Inc. south side of the site. These walls should be constructed on top of footings which bear on the building pad improvement discussed above or on top of augercast concrete piles. Permanent retaining walls restrained horizontally on top (such as basement walls) are considered unyielding and should be designed for a lateral soil pressure under the at-rest condition; while conventional reinforced concrete walls free to rotate on top should be designed for an active lateral soil pressure. Active Earth Pressure Conventional reinforced concrete walls that are designed to yield an amount equal to 0.002 times the wall height, should be designed to resist the lateral earth pressure imposed by an equivalent fluid with a unit weight of 35 pcf for level backfill; At-Rest Earth Pressure Walls supported horizontally by floor slabs are considered unyielding and should be designed for lateral soil pressure under the at-rest condition. The design lateral soil pressure should have an equivalent fluid pressure of 40 pcf for level backfill; Seismic Surcharge For the anticipated 100 year seismic event a horizontal surcharge load of 8H psf should be applied; Passive Earth Pressure 350 pcf equivalent fluid weight for compacted structural fill and native undisturbed soil; Base Coefficient of Friction 0.35 for compacted structural fill and native undisturbed soil; To prevent the buildup of hydrostatic pressure behind permanent concrete basement or conventional retaining walls, we recommend that a vertical drain mat, such as Miradrain 6000 or equivalent, be used to facilitate drainage behind such walls. The drain mat core should be placed against the wall(s) with the filter fabric side facing the backfill. The drain mat should extend from near the finished surface grade down to the footing drain system. Additionally all backfill placed between the excavation slopes or temporary shoring and the new basement/retaining walls should consist of free-draining fills having less than 5% passing the No. 200 sieve. Also, a May 14, 2018 G-4661 Geotechnical Report – 4502 NE 4th St, Renton, Washington Page 10 GEO Group Northwest, Inc. waterproofing layer should be placed between the drainage mat layer and the concrete wall, for moisture protection at all basement wall locations. The top 12 inches of backfill behind retaining or basement walls should consist of compacted and relatively impermeable soil. This cap material can be separated from the underlying more granular drainage material by a geotextile fabric, if desired. Alternatively, the surface can be sealed with asphalt or concrete paving. Where possible the ground surface should be sloped to drain away from the wall. GEO Group Northwest, Inc., recommends that backfill material which will support structures or improvements (such as patios, sidewalks, driveways, etc.) behind permanent concrete retaining walls and basement walls be placed and compacted consistent with the structural fill specifications in the Structural Fill section of this report. Slab-on-Grade Concrete Floors Slab-on-grade concrete floors may be constructed directly on top of the native medium dense to dense site soils or on top of compacted structural fills placed on top of the medium dense to dense site soils (building pad improvement). If structural fills are to be placed at these areas then they should be compacted in accordance with the specifications in the section titled: Structural Fill. To avoid moisture build-up on the subgrade, slab-on-grade concrete floors should be placed on a capillary break, which is in turn placed on the prepared subgrade. The capillary break should consist of a minimum of a six (6) inch thick layer of free-draining crushed rock or gravel containing no more than five (5) percent finer than the No. 4 sieve. A vapor barrier, such as a 10-mil plastic membrane, is recommended to be placed over the capillary break beneath the slab to reduce water vapor transmission through the slab. Two to four inches of sand may be placed over the barrier membrane for protection during construction. Subsurface Drainage We recommend that subsurface drains, footing drains, be installed around the perimeter of the foundation footings. The drains should consist of a four (4) inch minimum diameter perforated rigid drain pipe laid at or near the bottom of the footing with a gradient sufficient to generate flow. The drain line should be bedded on, surrounded by, and covered with a free-draining rock, May 14, 2018 G-4661 Geotechnical Report – 4502 NE 4th St, Renton, Washington Page 11 GEO Group Northwest, Inc. pea gravel, or other free-draining granular material. The drain rock and drain line should be completely surrounded by a geotextile filter fabric, Mirafi 140N or equivalent. Once the drains are installed, the excavation should be backfilled with a compacted fill material. The footing drains should be tightlined to discharge to the stormwater collection system. Under no circumstances should roof downspout drain lines be connected to the footing drainage system. All roof downspouts must be separately tightlined to discharge into the stormwater collection system. We recommend that sufficient cleanouts be installed at strategic locations to allow for periodic maintenance of the footing drains and downspout tightline systems. Augercast Concrete Pile Foundations An alternative to implementing the building pad improvement noted above the new building may be supported on augercast concrete piles that are embedded at least 10-feet into the underlying dense native soils which are anticipated at a depth of around 10-feet below the ground surface. Implementing this option may allow for less excavation at the site and may reduce the risk that temporary shoring will be necessary. Concrete grade beams should be used to connect the pile foundations and distribute the building loads. A structural concrete slab may be designed and constructed to support the slab loads and transfer these loads to the piling. Based upon the depth to competent soils at the test pits we estimate pile lengths may be on the order of 20-feet with 10- foot embedment into the competent dense soil. The piles should be designed with a minimum diameter of 14 inches. For concrete piles 14 to 18 inches in diameter embedded 10 feet into the underlying dense soils, the following allowable bearing capacities may be used: AUGERCAST CONCRETE PILE CAPACITIES Pile Diameter (Inches) Pile Embedment (Feet) Allowable Bearing (Tons) Allowable Uplift (Tons) 14 10 13 6.5 16 10 16 8 18 10 19 9.5 Note: Pile embedment length is based on the embedment depth below the top of the dense, native soil. May 14, 2018 G-4661 Geotechnical Report – 4502 NE 4th St, Renton, Washington Page 12 GEO Group Northwest, Inc. No reduction in pile capacity is required if the pile spacing is at least three times the pile diameter. A one-third increase in the above allowable pile capacities can be used when considering short-term transitory wind or seismic loads. Lateral forces can also be resisted by the passive earth pressures acting on the grade beams and friction with the subgrade. To fully mobilize the passive pressure resistance, the grade beams must be poured “neat” against compacted fill. Our recommended allowable passive soil pressure for lateral resistance is 350 pcf (pounds per cubic foot) equivalent fluid weight. A coefficient of friction of 0.35 may be used between the subgrade and the grade beams. We estimate that the maximum total post-construction settlement should be one-half (1/2) inch or less, and the differential settlement across building width should be one-quarter (1/4) inch or less. The performance of piles depends on how and to what bearing stratum the piles are installed. It is critical that judgement and experience be used as a basis for determining the embedment length and acceptability of a pile. Therefore, we recommend that GEO Group Northwest, Inc., be retained to monitor the pile installation operation, collect and interpret installation data, and verify suitable bearing stratum. We also suggest that the contractor’s equipment and installation procedure be reviewed by GEO Group Northwest, Inc., prior to pile installation to help mitigate problems which may delay work progress. ADDITIONAL SERVICES GEO Group Northwest, Inc., can provide additional exploration and testing services for the project such as infiltration rate testing if it is determined to be necessary. We recommend that GEO Group Northwest Inc. be retained to perform a general plan review of the final design and specifications for the proposed development to verify that the earthwork and foundation recommendations have been properly interpreted and implemented in the design and in the construction documents. We also recommend that GEO Group Northwest Inc. be retained to provide monitoring and testing services for geotechnically-related work during construction. This is to observe compliance with the design concepts, specifications or recommendations and to allow design changes in the event subsurface conditions differ from those anticipated prior to the start of construction. We anticipate the following construction monitoring inspections may be necessary: May 14, 2018 G-4661 Geotechnical Report – 4502 NE 4th St, Renton, Washington Page 13 GEO Group Northwest, Inc. 1. Site clearing and grubbing; 2. Grading of temporary excavation slopes; 3. Preparation of building foundation subgrades; 4. Over-excavation and structural fill placement at building pad improvement areas, removal of unsuitable fill soils; 5. Permanent subsurface drainage installation; 6. Installation of augercast piling, if implemented; LIMITATIONS This report has been prepared for the specific application to this site for the exclusive use of 4th Creek Meadows LLC and their authorized representatives. Any use of this report by other parties is solely at that party’s own risk. We recommend that this report be included in its entirety in the project contract documents for reference during construction. Our findings and recommendations stated herein are based on field observations, our experience and judgement. The recommendations are our professional opinion derived in a manner consistent with the level of care and skill ordinarily exercised by other members of the profession currently practicing under similar conditions in this area and within the budget constraint. No warranty is expressed or implied. In the event that soil conditions not anticipated in this report are encountered during site development, GEO Group Northwest, Inc., should be notified and the above recommendations should be re-evaluated. If you have any questions, or if we may be of further service, please do not hesitate to contact us. May 14, 2018 G-4661 Geotechnical Report – 4502 NE 4th St, Renton, Washington Page 14 GEO Group Northwest, Inc. Sincerely, GEO GROUP NORTHWEST, INC. Adam Gaston Project Engineer William Chang, P.E. Principal Attachments: Plate 1 – Vicinity Map Plate 2 – Site Plan Plate 3 – Topographic Map Plate 4 – SCS Soil Map Plates 5 – 6 – Gradational Analyses Appendix A – USCS Soil Legend and Test Pit Logs cc: Mr. Martin Reimer – Kaul Design Associates