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HomeMy WebLinkAboutEx06_Geotechnical_ReportApril 19, 2022 JN 22150 GEOTECH CONSULTANTS, INC. ARB c/o Broderick Architects 55 South Atlantic Street, Suite, 301 Seattle, Washington 98134 Attention: Kevin Broderick via email: kevin@broderickarchitects.com Subject: Geotechnical Engineering Study Proposed Additions to Existing Building 720 Rainier Avenue South Renton, Washington Dear Mr. Broderick: We are pleased to present this geotechnical engineering report for the planned addition to the existing building in Renton. The scope of our services consisted of assessing the current site conditions and reviewing available geologic information for the site vicinity, and then developing this report to provide recommendations for general earthwork and design criteria for foundations. This work was authorized by your acceptance of our Contract for Professional Services. As a part of an interior remodel of the existing building, a small covered entry will be created at the southwest corner of the current structure. An employee break room is to be added to the south side of the eastern half of the existing building. We expect that this addition will be one-story in height. A more substantial addition for vehicle repair/service will be constructed between the east side of the current building and the eastern property line. We expect that the additions will have slab-on- grade floors near the existing site grades. The southern portion of the property will remain paved for parking and drive lanes. If the scope of the project changes from what we have described above, we should be provided with revised plans in order to determine if modifications to the recommendations and conclusions of this report are warranted. SITE CONDITIONS SURFACE The subject property is located on the east side of Rainier Avenue South, three properties north of its intersection with South Grady Way. A building used as an automobile dealership occupies the northwestern portion of the property. The remainder of the irregularly-shaped lot is covered by pavement. The surrounding lots are, or have been, developed with commercial buildings used for automobile dealerships or automotive repair. There are no steep slopes on, or anywhere close to, the site. Consistent with the entire downtown Renton area, the subject property is essentially flat. DocuSign Envelope ID: 57102B24-44A9-4D9C-85FC-BE85E3655BCA RECEIVED 01/19/2023 AWeihs PLANNING DIVISION EXHIBIT 6DocuSign Envelope ID: D9B437AE-790D-4029-8662-F67F13AAFB4B ARB c/o Broderick Architects JN 22150 April 19, 2022 Page 2 GEOTECH CONSULTANTS, INC. The soils that underlie the site and the surrounding area have been deposited by various rivers that flowed through the deep valleys that remained after the last glaciers receded from the Puget Sound. These soils are known as alluvial deposits (alluvium). Published geologic maps confirm that the site vicinity is underlain by alluvium. Our firm has conducted borings for projects within one to two blocks of the subject site. These borings, which have extended to depths of at least 60 feet, found alluvial soils that typically consisted of finer-grained sediments (silt and silty, fine-grained sands) extending to depths of 20 to 25 feet. Below this depth, the alluvium becomes more coarse-grained, consisting of gravel and gravelly sand, having been deposited by faster flowing water. The groundwater table that underlies the entire area fluctuates seasonally, but often lies within 8 to 10 feet of the ground surface. CONCLUSIONS AND RECOMMENDATIONS GENERAL THIS SECTION CONTAINS A SUMMARY OF OUR STUDY AND FINDINGS FOR THE PURPOSES OF A GENERAL OVERVIEW ONLY. MORE SPECIFIC RECOMMENDATIONS AND CONCLUSIONS ARE CONTAINED IN THE REMAINDER OF THIS REPORT. ANY PARTY RELYING ON THIS REPORT SHOULD READ THE ENTIRE DOCUMENT. Based on existing subsurface information, the site is underlain by alluvial soils and a relatively shallow groundwater table. Older structures supported on lightly-reinforced conventional shallow foundations on top of these soils typically have varying, sometimes noticeable, amounts of settlement due to consolidation over time. The loose sandy soils beneath the level of the groundwater table are susceptible to liquefaction during the design seismic event. Heavily-reinforced shallow foundation systems are being used on newer construction in the surrounding area, in order to limit excessive post-construction settlement, but also to protect against foundation collapse in the event of soil strength loss (seismic liquefaction) that could occur during a large earthquake. The smaller entry and break room additions should be constructed using reinforced mat foundations. A mat foundation is essentially a heavily- reinforced, slab-on-grade foundation that is intended to distribute the building loads, reduce the necessary bearing capacity, bridge over any excessively soft areas of soil or localized soil liquefaction (sand boils), and reduce the amount of differential settlement across the new construction. For the eastern addition, where there will not be any interior supports, the perimeter walls can be supported on heavily-reinforced continuous footings that are designed similar to grade beams. If the floor slab of the addition is to support heavy loads, such as from vehicle lifts, the slab should be reinforced. Any loose or soft soils encountered in the foundation or slab excavations should be removed and be replaced with imported granular soils. We recommend that the project geotechnical engineer assess the exposed footing and slab subgrades prior to placement of forms and/or rebar. The project budget should account for the potential that removal and replacement of unsuitable shallow soils, such as old fill and/or topsoil, could be necessary. The use of mat foundations and heavily-reinforced continuous footings will result in more uniform settlement of the new construction as the underlying soils continue to undergo long- term secondary compression. However, movement between the new and existing construction may be evident over time. This may cause some cosmetic issues, but should not be of structural concern. Settlement-tolerant construction such as wood and metal framing and siding should still be used for the new construction. Masonry, stucco, tile and other settlement- DocuSign Envelope ID: 57102B24-44A9-4D9C-85FC-BE85E3655BCADocuSign Envelope ID: D9B437AE-790D-4029-8662-F67F13AAFB4B ARB c/o Broderick Architects JN 22150 April 19, 2022 Page 3 GEOTECH CONSULTANTS, INC. sensitive materials should be avoided, as they will more readily show the cosmetic effects of foundation settlement. Footing drains will not be needed for the new additions if their floors are at, or above, the surrounding grade. The erosion control measures needed during the site development will depend heavily on the weather conditions that are encountered. The site is generally flat, so the overall erosion potential should be low. The existing pavement areas should be maintained for truck loading and staging. Any soil spilled onto the pavements should be immediately cleaned up. Cut slopes and soil stockpiles should be covered with plastic during wet weather. Following clearing or rough grading, it may be necessary to mulch or hydroseed bare areas that will not be immediately covered with landscaping or an impervious surface. On most construction projects, it is necessary to periodically maintain or modify temporary erosion control measures to address specific site and weather conditions. Geotech Consultants, Inc. should be allowed to review the final development plans to verify that the recommendations presented in this report are adequately addressed in the design. Such a plan review would be additional work beyond the current scope of work for this study, and it may include revisions to our recommendations to accommodate site, development, and geotechnical constraints that become more evident during the review process. We recommend including this report, in its entirety, in the project contract documents. This report should also be provided to any future property owners so they will be aware of our findings and recommendations. SEISMIC CONSIDERATIONS In accordance with the International Building Code (IBC) and ASCE 7-16, the site class within 100 feet of the ground surface is best represented by Site Class Type E (Soft Soil). As noted in the USGS website, the mapped spectral acceleration value for a 0.2 second (Ss) and 1.0 second period (S1) equals 1.44g and 0.49g, respectively, for ASCE 7-16. The IBC and ASCE 7 require that the potential for liquefaction (soil strength loss) be evaluated for the peak ground acceleration of the Maximum Considered Earthquake (MCE), which has a probability of occurring once in 2,475 years (2 percent probability of occurring in a 50-year period). The site is underlain by loose, saturated, alluvial sand soils. These soils have been demonstrated to have a moderate to high potential for liquefaction during a large earthquake. Using procedures developed by Seed, Idriss, et al. we have calculated the approximate total ground settlement that could result if liquefaction were to occur in the saturated, loose to medium-dense soils as the result of the design earthquake. Based on this analysis, it is probable that soil liquefaction could extend down to a depth of 25 to 30 feet following an earthquake as strong as the MCE. Our calculations for sites with similar subsurface conditions have indicated that total ground settlement of at least 4 to 6 inches could theoretically result during the MCE. Differential settlements across the structures would be mitigated by the heavily-reinforced foundations such that we would predict differential dynamic settlements of 2 to 3 inches across the structures. DocuSign Envelope ID: 57102B24-44A9-4D9C-85FC-BE85E3655BCADocuSign Envelope ID: D9B437AE-790D-4029-8662-F67F13AAFB4B ARB c/o Broderick Architects JN 22150 April 19, 2022 Page 4 GEOTECH CONSULTANTS, INC. The foundation recommendations presented in this report are intended to prevent catastrophic foundation collapse of the new construction during the MCE. By preventing catastrophic settlement of the foundations, the safety of the occupants should be protected. The intent is not to prevent damage or ensure continued function of the structure after the design seismic event. REINFORCED FOUNDATIONS An allowable bearing pressure of 1,500 pounds per square foot (psf) should be used for the foundation design. A one-third increase in this design bearing pressure may be used when considering short-term wind or seismic loads. Heavily-reinforced continuous footings should be designed similar to grade beams, and should be sufficiently reinforced to be able to span a distance of at least 10 feet without soil support. This creates a rigid footing. The perimeter footings should extend to at least 18 inches below the surrounding grade. Mat foundations are typically designed using the approximate flexible method. Foundations designed using this method are also known as Winkler Foundations. For this analysis, we recommend using a coefficient of subgrade reaction of 90 pounds per cubic inch (lb/in3). Mat slabs should be thickened a minimum depth of 18 inches below the adjacent finish grade around the perimeter of the mat, and this thickened edge of the structural slabs should have a minimum width of 16 inches. Lateral loads due to wind or seismic forces may be resisted by friction between the foundation and the subgrade soil, or by passive earth pressure acting on the vertical, embedded portions of the foundation. For the latter condition, the foundation must be either poured directly against relatively level, undisturbed soil or be surrounded by level, well-compacted fill. We recommend using the following ultimate values for the foundation's resistance to lateral loading: PARAMETER ULTIMATE VALUE Coefficient of Friction 0.45 Passive Earth Pressure 250 pcf Where: pcf is Pounds per Cubic Foot, and Passive Earth Pressure is computed using the Equivalent Fluid Density. If the ground in front of a foundation is loose or sloping, the passive earth pressure given above will not be appropriate. We recommend maintaining a safety factor for the foundation's resistance to lateral loading, when using the above ultimate values. SLABS-ON-GRADE The mat foundations will serve as the either the floor slab, or the support for a topping slab for the lowest level of the buildings. Even where the exposed soils appear dry, water vapor will tend to naturally migrate upward through the soil and concrete to the new constructed space above it. This can affect moisture- DocuSign Envelope ID: 57102B24-44A9-4D9C-85FC-BE85E3655BCADocuSign Envelope ID: D9B437AE-790D-4029-8662-F67F13AAFB4B ARB c/o Broderick Architects JN 22150 April 19, 2022 Page 5 GEOTECH CONSULTANTS, INC. sensitive flooring, cause imperfections or damage to the slab, or simply allow excessive water vapor into the space above the slab. As noted by the American Concrete Institute (ACI) in the Guides for Concrete Floor and Slab Structures, proper moisture protection is desirable immediately below any on-grade slab that will be covered by tile, wood, carpet, impermeable floor coverings, or any moisture-sensitive equipment or products. ACI also notes that vapor retarders such as 6-mil plastic sheeting have been used in the past, but are now recommending a minimum 10-mil thickness for better durability and long term performance. A vapor retarder is defined as a material with a permeance of less than 0.3 perms, as determined by ASTM E 96. It is possible that concrete admixtures or topical applications to the surface of the slab may meet this specification, although the manufacturers of these products should be consulted. Where vapor retarders are used under slabs, their edges should overlap by at least 6 inches and be sealed with adhesive tape. The sheeting should extend to the foundation walls for maximum vapor protection. If no potential for vapor passage through the slab is desired, a vapor barrier should be used. A vapor barrier, as defined by ACI, is a product with a water transmission rate of 0.01 perms when tested in accordance with ASTM E 96. Reinforced membranes having sealed overlaps can meet this requirement. We recommend that the contractor, the project materials engineer, and the owner discuss these issues and review recent ACI literature and ASTM E-1643 for installation guidelines and guidance on the use of the protection/blotter material. LIMITATIONS If the subsurface conditions encountered during construction are significantly different from those anticipated, we should be advised at once so that we can review these conditions and reconsider our recommendations where necessary. Such unexpected conditions frequently require making additional expenditures to attain a properly constructed project. It is recommended that the owner consider providing a contingency fund to accommodate such potential extra costs and risks. This is a standard recommendation for all projects This report has been prepared for the exclusive use of ARB, and their representatives for specific application to this project and site. Our conclusions and recommendations are professional opinions derived in accordance with our understanding of current local standards of practice, and within the scope of our services. No warranty is expressed or implied. The scope of our services does not include services related to construction safety precautions, and our recommendations are not intended to direct the contractor's methods, techniques, sequences, or procedures, except as specifically described in our report for consideration in design. Our services also do not include assessing or minimizing the potential for biological hazards, such as mold, bacteria, mildew and fungi in either the existing or proposed site development. ADDITIONAL SERVICES Geotech Consultants, Inc. should be retained to provide geotechnical c onsultation, testing, and observation services during construction. This is to confirm that subsurface conditions are consistent with those indicated by our exploration, to evaluate whether earthwork and foundation construction activities comply with the general intent of the recommendations presented in this report, and to provide suggestions for design changes in the event subsurface conditions differ from those anticipated prior to the start of construction. However, our work would not include the DocuSign Envelope ID: 57102B24-44A9-4D9C-85FC-BE85E3655BCADocuSign Envelope ID: D9B437AE-790D-4029-8662-F67F13AAFB4B ARB c/o Broderick Architects JN 22150 April 19, 2022 Page 6 GEOTECH CONSULTANTS, INC. supervision or direction of the actual work of the contractor and its employees or agents. Also, job and site safety, and dimensional measurements, will be the responsibility of the contractor. During the construction phase, we will provide geotechnical observation and testing services when requested by you or your representatives. Please be aware that we can only document site work we actually observe. It is still the responsibility of your contractor or on-site construction team to verify that our recommendations are being followed, whether we are present at the site or not. Please contact us if you have any questions regarding this report, or if we can be of further assistance. Respectfully submitted, GEOTECH CONSULTANTS, INC. 4/19/2022 Marc R. McGinnis, P.E. Principal MRM:kg DocuSign Envelope ID: 57102B24-44A9-4D9C-85FC-BE85E3655BCADocuSign Envelope ID: D9B437AE-790D-4029-8662-F67F13AAFB4B