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Home My WebLink AboutRS- Geotech_Nielsen Short Plat_201021_v1.pdfNielsen Short Plat July 5, 2018 3835 Lincoln Avenue NE (Parcel # 3345700130) Project #16040 Renton, Washington South Fork Geosciences, pllc Page 1 PO Box 1275 North Bend, WA 98045 425-831-2023 | andrew@sfgeo.com Subsurface Exploration, Geologic Hazards, and Preliminary Design Recommendations Introduction The purpose of this study was to gain subsurface information to be utilized in the design and construction of a short plat subdivision on the subject property. The proposed construction will consist of grading, roadway and utility construction, foundation subgrade preparation, building construction, and landscaping. Authorization to proceed with this study was provided by Kris Nielsen on January 25, 2017. This report was prepared for Ms. Nielsen and her agents for specific use in support of this project. The location of the subject property is shown in the attached Vicinity Map. The approximate locations of the subsurface explorations logged for this study are shown on the attached Site and Exploration Plan. Site Conditions The subject property was located at 3835 Lincoln Avenue NE (Parcel # 3345700130) in Renton, Washington. The subject property was irregularly shaped and according to the King County Department of Assessments, it was 0.92 acres in size. The eastern portion of the property was generally flat to gently sloping and was developed in association with the existing home on the property. There was a break in slope near the center of the property down to the western portion. The western portion of the property sloped steeply down to the west and was not developed. Based on our review of the site plan and the information available on the King County iMap Interactive Mapping Tool, this slope was on the order of 50 to 60 percent and was 45 to 50 feet in vertical height. This review also indicated that there was approximately 55 feet of total relief on the subject property. The property was bordered on the east by Lincoln Avenue NE, to the north and south by other residential properties, and to the west by vacant land. The subject property had been developed in association with the existing single-family home on the property. This development included driveways and yard areas around the existing home and a detached garage. There were some larger evergreen trees scattered on the developed portion of the property. The western portion of the property was covered with typical second growth forest vegetation, consisting of fir, cedar, maple, and alder trees, and ground cover/brush. Though there were some minor signs of surface soil creep due to the steep slope, we did not observe any signs of accelerated soil erosion or deep-seated soil movement during our site reconnaissance. Subsurface Conditions Subsurface conditions on the property were inferred from a visual reconnaissance of the property, a review of the referenced geologic map (Booth et al, 2007) and four subsurface exploration pits. The exploration pits were excavated with a subcontracted track-mounted excavator provided by Northwest Excavating and Trucking Company, Inc., they were logged by the undersigned licensed engineering geologist, and all the exploration pits were immediately backfilled. The approximate locations of the subsurface explorations are shown in the Site and Nielsen Short Plat July 5, 2018 3835 Lincoln Avenue NE (Parcel # 3345700130) Project #16040 Renton, Washington South Fork Geosciences, pllc Page 2 PO Box 1275 North Bend, WA 98045 425-831-2023 | andrew@sfgeo.com Exploration Plan and the exploration logs are attached to this report. Our interpretation of the near surface, native soils as Vashon recessional outwash sediments differs with the referenced geologic map (Booth et al, 2007). We attribute our different interpretation to the site-specific nature of our study. Stratigraphy Fill Soils Besides the small amount of trench backfill encountered associated with the pipe in EP-3, we did not encounter any fill soils in the exploration pits performed. We presume that there are fill soils associated with the construction of the existing home. Any fill soils encountered in areas where structures are to be placed will need to be removed, reworked, or replaced as structural fill as specified in the “Site Preparation and Grading” section of this report. Vashon Recessional Outwash Vashon recessional outwash sediments were encountered in al the exploration pits performed for this study from the ground surface to the maximum depth explored. The recessional outwash sediments generally consisted of medium dense, damp to moist, brown fine to coarse SAND with gravel and varying amounts of silt (SW to SW-SM). The Vashon recessional outwash sediments were deposited from meltwater streams and channels from the receding Vashon glacial ice sheet. Vashon recessional outwash sediments were not consolidated by the glacial ice, but due to their generally coarse grain size and subsequent exposure to water, they are typically consolidated to a medium dense condition, which is adequate for support of lighter structures. Due to their relatively coarse grain size distribution, the recessional outwash sediments are generally moderately to highly permeable and moderately resistant to erosion, except when subjected to concentrated water flows. Vashon Lodgement Till Based on the geologic mapping and the geologic setting, we presume that the Vashon recessional outwash sediments are underlain by Vashon lodgement till sediments at greater depth. Though not shown in our explorations, our observations of the ground surface near the toe of the western slope revealed surface soils and groundwater conditions that are indicative of the presence of lodgement till at greater depth. Lodgement till is a dense, poorly sorted mixture of clay, silt, sand, and gravel that was deposited at the sole of the Vashon glacial ice. Vashon lodgement till sediments, due to their glacially consolidated nature, are relatively dense in place, have low permeability, and moderately to highly resistant to erosion. Hydrology There was no groundwater seepage observed in any of the explorations performed for this study. There were no signs of natural ponding water or natural flowing surface water observed on the developed (eastern) portion of the site, but we did observe water coming out of the ground approximately 15 feet west of the water meter during our site reconnaissance. This seepage was obviously due to a water line leak, based on the proximity to the water meter/water supply line and since the water meter was indicating current water flow without Nielsen Short Plat July 5, 2018 3835 Lincoln Avenue NE (Parcel # 3345700130) Project #16040 Renton, Washington South Fork Geosciences, pllc Page 3 PO Box 1275 North Bend, WA 98045 425-831-2023 | andrew@sfgeo.com any water use at the time it was observed. Groundwater conditions are subject to change depending on seasonal weather conditions, but based on our observations, we do not anticipate encountering adverse groundwater conditions during construction. During our slope reconnaissance, we did observe some minor groundwater seepage on the lower portion of the slope. We presume that this is due to water infiltrating through the Vashon recessional outwash sediments and then perching atop the underlying Vashon lodgement till sediments and seeping out along this geologic contact. Though the seepage was present, we did not observe signs of slope movement , past rotational slope failure, or accelerated erosion related to the groundwater seepage. The groundwater seepage ran downslope in small, self-armored rills to the flatter area below, and these rills appear to be naturally stabilized to the peak flow levels that they receive. Geologic Hazards The following discussion of potential geologic hazards is based on the visual reconnaissance of the site, the subsurface explorations, review of aerial photographs and regional topographic and geologic maps of the area, and review of the applicable City of Renton Municipal Code and maps that are available online. Based on our comparison of the City of Renton Municipal Code and referenced hazard maps to the topographic and geologic features of the subject property, the western slope on the subject property fulfills the criteria for an erosion hazard area and a high landslide hazard area. The following sections of this report will set forth our recommendations for geologic hazard mitigation with respect to the proposed development and address general seismic risks and design parameters associated with a seismic event. Erosion Hazard Area The western slope on the subject property is considered an erosion hazard. However, the proposed development will not disturb the existing slope and surface water runoff will not be routed toward the slope. We did not observe signs of recent accelerated erosion during our site reconnaissance. Based on the proposed development and our observations, it is our opinion that no special mitigations are required for the erosion hazard area. Though special mitigations are not necessary, a temporary erosion and sediment control (TESC) plan should be created and implemented during site construction. It is our opinion that implementation of a relatively basic erosion control plan will prevent off site sediment transport. The proper use of “best management practices” (BMPs) should be utilized during earthwork activities to reduce the potential for erosion and sediment transport off the property. The following is a partial list of BMPs that should be implemented: • Phasing and scheduling major earthwork activities during dry conditions • Rock construction entrances • Straw mulch • Plastic sheeting Nielsen Short Plat July 5, 2018 3835 Lincoln Avenue NE (Parcel # 3345700130) Project #16040 Renton, Washington South Fork Geosciences, pllc Page 4 PO Box 1275 North Bend, WA 98045 425-831-2023 | andrew@sfgeo.com • Perimeter controls, as required • Keeping exposed soils and stockpiles covered when not actively worked • Establish temporary/permanent vegetation as soon as possible after foundation and utility installation Implementation of a TESC plan will likely be a requirement of the clearing and grading permit or the building permit. South Fork Geosciences is available to assist in the design of the TESC plan and to provide any required inspections during construction. Landslide Hazard Areas The western slope on the subject property is also considered a high landslide hazard area. We have reviewed the proposed development plan and the location of the addition elements to the existing home with respect to the top of slope. Since the proposed development does not disturb the slope or place foundation loads any closer to the top of slope than the existing home on the property, it is our opinion that no additional setbacks are required for the proposed development. The top of slope will serve as the delineation of the landslide hazard area and an additional buffer and setback will not be needed , provided that the new foundation loads on the west lot are as shown in the referenced site plan (Rich Design Group LLC, Nielsen Short Plat, Sheet C1, received via e-mail on May 5, 2018). With respect to landslide hazard areas, it is our opinion that no special mitigations are required for the proposed development of the eastern lot. Since these recommendations are premised on the locations of structures with respect to the top of the slope, South Fork Geosciences should review the final plans to be submitted prior to permitting and construction. Seismic Concerns In general, there are four elements of hazard associated with large seismic events: ground rupture; seismically induced landslides; liquefaction; and ground motion. The potential for each of these to impact the subject property is discussed below. Ground Rupture Most large earthquakes in the Puget Sound area are sub-crustal events with epicenters ranging from 50 to 70 kilometers in depth. The subject property lies within the Seattle Fault Zone (SFZ). Though the subject property lies within the SFZ, we are unaware of any direct evidence of surface fault rupture on or in the immediate vicinity of the subject property. Also, research has estimated the recurrence interval on some fault strands of the SFZ to be on the ord er of 200 to 12,000 years (Johnson, et al., 2004). Based on the lack of evidence of past ground rupture in the immediate vicinity of the subject property and the estimated recurrence interval of the fault strands, it is our opinion that the probability of ground surface rupture impacting the subject property is low, and no mitigations are necessary. Though it is our opinion that site-specific mitigation measures are not required for the proposed structure, it should be understood that there are inherent and incalculable risks with building within a potentially active fault zone. Nielsen Short Plat July 5, 2018 3835 Lincoln Avenue NE (Parcel # 3345700130) Project #16040 Renton, Washington South Fork Geosciences, pllc Page 5 PO Box 1275 North Bend, WA 98045 425-831-2023 | andrew@sfgeo.com Seismically Induced Landslides Due to the medium dense native soils encountered and the lack of evidence of accelerated erosion or recent slope failure, it is our opinion that the potential for seismically induced slope failures on or adjacent to the site is low and the setback of the existing home and planned addition elements provide adequate mitigation for seismically induced landslides. Liquefaction Liquefaction is a condition where loose, saturated, fine sandy soils lose their shear strength due to rapid pore pressure build-up when subjected to high intensity cyclic loads, such as occur during earthquakes. Due to the medium dense, well-graded, native soils encountered, the liquefaction potential of this site is low, and no mitigations are necessary. Ground Motion Seismic hazards that will affect the structure would likely be due to the intensity and duration of the ground shaking. The structural design of the project should be consistent with 2015 International Building Code (2015 IBC) guidelines (Section 1613). Based on the results of our subsurface explorations and our estimation of soil properties at depth utilizing available geologic data, Site Class “C” may be used for the design of the project, as defined by ASCE 7 “Minimum Design Loads for Buildings and Other Structures”, Chapter 20. Preliminary Design Recommendations Complete architectural and civil engineering plans were not available for review at the ti me this report was prepared. The design recommendations in this report should be considered preliminary, and South Fork Geosciences should review future architectural and civil engineering plans to ensure that our recommendations have been properly incorp orated into the design of the structures and site earthwork. Based on our subsurface exploration and site reconnaissance, it is our opinion that the subject property is suitable for the proposed development, provided the risks discussed are accepted and the recommendations contained herein are properly followed. The near surface, medium dense, native Vashon recessional outwash sediments will be capable of providing support for the proposed structures. To aid in the design of the project, the following general recommendations for site development are provided. South Fork Geosciences is available for further consultation with respect to specific design items, if needed. Site Preparation and Site Grading It is likely that structural fill soils will be required to establish grades for the project. Any fill soil placed beneath roadways, parking areas, foundations, retaining walls or concrete slabs must be constructed as a structural fill. In areas that will provide structural support, any existing fill soils or loose soils should be removed and replaced with structural fill as described below. Structural fill is defined as non-organic soil, placed in horizontal loose lifts, with each lift being compacted to at least 95 percent of the maximum dry density, using the modified Proctor test (ASTM: D1557) as the standard. Prior to placing any structural fill, the exposed soils must either be undisturbed or be compacted to a dense, non-yielding condition and be approved for Nielsen Short Plat July 5, 2018 3835 Lincoln Avenue NE (Parcel # 3345700130) Project #16040 Renton, Washington South Fork Geosciences, pllc Page 6 PO Box 1275 North Bend, WA 98045 425-831-2023 | andrew@sfgeo.com structural fill placement. In the case of utility trench filling, the trench backfill should be placed and compacted in accordance with the applicable municipal or utility company standards. Fill soils should be predominantly free of organics and other deleterious material and should be appropriately moisture conditioned when placed and compacted. Placement and compaction of the structural fill should be monitored by a competent field technician. In situ density testing should be performed during fill placement to verify proper compaction of the fill soil. A sample of the planned structural fill soil will need to be available at least 48 hours prior to compaction verification testing for laboratory analysis. Temporary Cut Slopes Temporary cut slopes may be necessary to allow the installation of the foundations and for utility installation. Temporary cut slopes should be limited to 1H:1V in the fill and recessional outwash sediments. Stability of the cut slopes is the responsibility of the on-site contractor and a safe work environment should be maintained at all times. A trench shield or trench box should be used to install all utilities over 4 feet in depth or the sides of trenches and excavations should be sloped back per OSHA/WISHA standards. Permanent Slopes Due to the relatively flat topography of portion of the site to be developed, we do not anticipate large constructed slopes to be associated with the proposed development. As a standard of practice, permanent cut slopes in the native sediments and slopes in structural fill soils should be limited to a maximum of 2H:1V and landscape fill slopes comprised of organic rich soil or non-structurally compacted mineral soil should be limited to 3H:1V. Foundations Spread footings may be used for building support when founded on medium dense native soils or approved structural fill soils. We recommend that an allowable bearing pressure of 2,000 pounds per square foot (psf) be utilized for design purposes, including both dead and live loads. An increase of one-third may be used for short-term wind or seismic loading. Perimeter footings should be buried at least 18 inches into the surrounding soil for frost protection; interior footings require only 12 inches burial. However, all footings must penetrate to the prescribed bearing stratum, and no footing should be founded in or above loose, organ ic, or existing fill soils. It should be noted that the area bounded by lines extending downward at 1H:1V from any footing must not intersect another footing or intersect a filled area that has not been compacted to at least 95 percent of ASTM: D 1557. In addition, a 1.5H:1V line extending down from any footing must not daylight because sloughing or raveling may eventually undermine the footing. Thus, footings should not be placed near the edge of steps or near vertical cuts in the bearing soils. Anticipated settlements of footings founded on the medium dense native soils or approved structural fill should be less than 1-inch. Loose or disturbed surface soils, excessive moisture present or poor foundation subgrade preparation could result in large r settlements. South Fork Nielsen Short Plat July 5, 2018 3835 Lincoln Avenue NE (Parcel # 3345700130) Project #16040 Renton, Washington South Fork Geosciences, pllc Page 7 PO Box 1275 North Bend, WA 98045 425-831-2023 | andrew@sfgeo.com Geosciences should perform a foundation bearing evaluation prior to concrete placement to verify that the design bearing capacity of the soil has been attained. A foundation bearing evaluation will likely be required by the City of Renton as a condition of the building permit. Perimeter footing drains should be provided as discussed under the section titled Drainage Considerations. Lateral loads can be resisted by friction between the foundation and the supporting soils, and/or by passive earth pressure acting on the buried portions of the foundations. The spread footings must be backfilled with structural fill compacted to a dense, non -yielding condition to achieve the passive resistance provided below. The structural fill must extend horizontally outward from the embedded portion of the foundation a distance equal to at least three times the embedment depth over which the passive resistance is applied. We recommend the following design parameters. • Passive equivalent fluid = 300 pcf • Coefficient of friction = 0.30 The above values are allowable and include a factor of safety of at least 1.5. Retaining Walls/Rockeries Based on the relatively flat topography of the portion of the site that will be developed, we do not anticipate the need for significant retaining walls to be used for grade separation on the project. South Fork Geosciences should be contacted to review any retaining walls or rockeries that are in excess of 4 feet in height. Floor Support We anticipate that the proposed homes will utilize slab-on-grade floors or a combination of slab-on-grade floors and structural/crawlspace floors. Slab-on-grade concrete floors should be cast atop a prepared subgrade of structural fill soil compacted to a minimum of 90 percen t of the modified Proctor maximum dry density. A capillary break with a minimum thickness of 4 inches should be placed atop the prepared subgrade. The capillary break material should be a gap graded material consisting of pea gravel, ¾-inch washed drain rock, or clean crushed rock with less than 5 percent fines (material passing the No.200 sieve). The capillary break will reduce the potential for moisture wicking through the floor slab. A 10-mil thick plastic vapor barrier should also be placed atop the capillary break material. All concrete placement should follow the guidelines set forth by the American Concrete Institute (ACI). In areas that structural/crawl space-type floors are used, provisions should be made to slope internal crawlspace areas to lows that may be drained to the footing drain system around the perimeter of the foundation to prevent the accumulation of standing water within the crawlspace areas. The soil surface should be covered with a minimum 10-mil thick moisture barrier. Nielsen Short Plat July 5, 2018 3835 Lincoln Avenue NE (Parcel # 3345700130) Project #16040 Renton, Washington South Fork Geosciences, pllc Page 8 PO Box 1275 North Bend, WA 98045 425-831-2023 | andrew@sfgeo.com Drainage Considerations Foundation Drainage In general, a perimeter foundation drain should be established around the new structures to protect the floor slab and internal crawlspace areas from ground water intrusion. However, due to the free draining native soils present, footing drains may be eliminated at the time of construction if the soil conditions are favorable. South Fork Geosciences is available to make this assessment during construction. If foundation drainage is required, the level of the foundation drain should be set at, or slightly below, the base of the footing elevation. The drain should consist of 4-inch diameter, rigid, perforated, PVC drain pipe and should be set to allow for gravity discharge. The drainpipe should be surrounded by a minimum of 6 inches of pea gravel or washed drain rock. Roof drains should not tie into the footing drain but should be collected in a separate, tightline drain. The drain lines should be set to discharge via gravity to an approved stormwater drainage. Foundation drainage for the addition elements to the existing home should match the existing foundation drainage to the extent possible, as appropriate. As a standard of practice, final exterior grades should slope away from foundations. Stormwater Management The Vashon recessional outwash soils encountered near the ground surface are generally conducive to stormwater infiltration. We presume that infiltration facilities to receive stormwater generated from new impervious surfaces will be designed using “Ap pendix C Small Project Drainage Requirements” of the 2009 King County Surface Water Design Manual (KCSWDM). The native recessional outwash sediments should be classified as “medium sands” for the sake of the prescriptive designs in the KCSWDM. Any drywells or infiltration trenches should be located 10-feet away from building foundations. We recommend construction of two infiltration facilities per lot, in different relative locations, to limit the amount of impervious surface runoff that is routed to each infiltration facility and to increase the extent of the area in which runoff is infiltrated. South Fork Geosciences is available to review the location of the proposed infiltration facilities if required or requested. Conclusion Our findings and recommendations provided in this report were prepared in accordance with generally accepted principles of engineering geology as practiced in the Puget Sound area at the time this report was submitted. We make no other warranty, either expressed or implied. Our observations, findings, and opinions are a means to identify and reduce the inherent risks to the owner. We are available to provide additional design recommendations and consultation throughout the development of this project. South Fork Geosciences should be contact ed to review any architectural or grading plans and to provide additional geotechnical information and input prior to construction. We are also available to provide construction monitoring services during Nielsen Short Plat July 5, 2018 3835 Lincoln Avenue NE (Parcel # 3345700130) Project #16040 Renton, Washington South Fork Geosciences, pllc Page 9 PO Box 1275 North Bend, WA 98045 425-831-2023 | andrew@sfgeo.com construction for earthwork quality control and to help ensure that the recommendations in this report are properly implemented. We have enjoyed working with you and we are confident that this report will aid in the design of your project. If there are any questions, please feel free to contact us. Sincerely, Andrew L. Glandon, LEG Engineering Geologist / Owner South Fork Geosciences, PLLC Attachments: Vicinity Map Site and Exploration Plan City of Renton Landslide Hazard Area Mapping Exploration Pit Logs References: “Geologic Map of King County” Compiled by Derek B. Booth, Kathy A. Troost & Aaron P. Wisher, March 2007 King County Department of Assessments http://blue.kingcounty.com/Assessor/eRealProperty/Dashboard.aspx?ParcelNbr=3345700130 Renton Municipal Code http://www.codepublishing.com/WA/Renton/ City of Renton Interactive Mapping Application http://rp.rentonwa.gov/HTML5Public/Index.HTML?viewer=CORMaps United States Department of Agriculture Natural Resources Conservation Service, http://websoilsurvey.nrcs.usda.gov USGS U.S. Quaternary Faults and Folds Database Map Viewer, https://usgs.maps.arcgis.com/apps/webappviewer/index.html?id=db287853794f4555b8e93e42290e9716 Johnson, S.Y., Blakely, R.J., Brocher, T.M., Haller, K.M., Barnett, E.A., Bucknam, R.C., Haeussler, P.J., Pratt, T.L., Nelson, A.R., Sherrod, B.L., Wells, R.E., Lidke, D.J., Harding, D.J., and Kelsey, H.M., compilers, 2016, Fault number 570, Seattle fault zone, in Quaternary fault and fold database of the United States: U.S. Geological Survey website, https://earthquakes.usgs.gov/hazards/qfaults, accessed 07/05/2018 04:00 PM Nielsen Short Plat July 5, 2018 3835 Lincoln Avenue NE (Parcel # 3345700130) Project #16040 Renton, Washington South Fork Geosciences, pllc Page 10 PO Box 1275 North Bend, WA 98045 425-831-2023 | andrew@sfgeo.com Source: City of Renton GIS Vicinity Map Not to Scale North Project Site Nielsen Short Plat July 5, 2018 3835 Lincoln Avenue NE (Parcel # 3345700130) Project #16040 Renton, Washington South Fork Geosciences, pllc Page 11 PO Box 1275 North Bend, WA 98045 425-831-2023 | andrew@sfgeo.com Source: “A House Remodel and Addition, Sheet C1” Rich Design Group, LLC Site and Exploration Plan EP-1 EP-3 EP-2 EP-4 North is Right Not to Scale Nielsen Short Plat July 5, 2018 3835 Lincoln Avenue NE (Parcel # 3345700130) Project #16040 Renton, Washington South Fork Geosciences, pllc Page 12 PO Box 1275 North Bend, WA 98045 425-831-2023 | andrew@sfgeo.com Source: City of Renton GIS North is Up Linear Scale is Valid City of Renton Landslide Hazard Area Mapping Landslide Hazard Area (High) Nielsen Short Plat July 5, 2018 3835 Lincoln Avenue NE (Parcel # 3345700130) Project #16040 Renton, Washington South Fork Geosciences, pllc Page 13 PO Box 1275 North Bend, WA 98045 425-831-2023 | andrew@sfgeo.com Exploration Pit Logs EP-1 1-30-2017 Sod @ surface 0-4in – brown, organic topsoil 4in-6.5ft – medium dense, damp, brown fine to coarse SAND with gravel (SW) [Vashon Recessional Outwash] Becomes light brown-gray @ 1.5ft Total Depth = 6.5ft No groundwater seepage observed, minor caving due to non-cohesive soils EP-2 1-30-2017 Sod @ surface 0-12in – loose to medium dense, damp brown fine to medium SAND with gravel and silt (SP- SM) [Vashon Recessional Outwash] 12in-4ft – medium dense, damp, brown to light brown fine to coarse SAND with gravel (SW) [Vashon Recessional Outwash] Becomes light brown-gray @ 1.5ft Total Depth = 4ft No groundwater seepage observed, minor caving due to non-cohesive soils EP-3 1-30-2017 0-4in – Topsoil 4in-4ft – medium dense, damp light brown to light brown gray fine to medium SAND with gravel (SP) [Vashon Recessional Outwash] • at back of pit a 4in PVC pipe was exposed at 3ft, did not break Total Depth = 4ft No groundwater seepage observed, no caving observed EP-4 1-30-2017 Sod @ surface 0-6in – loose, damp, dark brown fine to medium SAND with gravel and silt (SP -SM) [Topsoil] 6in-3ft – medium dense, damp, light brown to light brown-gray fine to coarse SAND with gravel (SW) [Vashon Recessional Outwash] Total Depth = 3ft No groundwater seepage observed, no caving observed