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HomeMy WebLinkAbout22-Geotech Report 7-31-18 SW Sunset Boulevard Residenceassociated earth sciences incorporated Associated Earth Sciences, Inc. 911 5th Avenue Kirkland, WA 98033 P (425) 827 7701 F (425) 827 5424 Subsurface Exploration, Geologic Hazard, and Preliminary Geotechnical Engineering Report SW SUNSET BOULEVARD RESIDENCE Renton, Washington Prepared For: PLAN TO PERMIT C/O WSCO Project No. 180038E001 July 31, 2018 SUBSURFACE EXPLORATION, GEOLOGIC HAZARD, AND PRELIMINARY GEOTECHNICAL ENGINEERING REPORT SW SUNSET BOULEVARD RESIDENCE Renton, Washington Prepared for: Plan to Permit c/o WSCO PO Box 956 Kirkland, Washington 98083 Prepared by: Associated Earth Sciences, Inc. 911 5th Avenue Kirkland, Washington 98033 425-827-7701 Fax: 425-827-5424 July 31, 2018 Project No. 180038E001 Subsurface Exploration, Geologic Hazard, SW Sunset Boulevard Residence and Preliminary Geotechnical Engineering Report Renton, Washington Project and Site Conditions July 31, 2018 ASSOCIATED EARTH SCIENCES, INC. TG/ld - 180038E001-2 - Projects\20180038\KE\WP Page 1 I. PROJECT AND SITE CONDITIONS 1.0 INTRODUCTION This report presents the results of our subsurface exploration, geologic hazards assessment, and geotechnical engineering study for the proposed residential project at the subject property. In the event that any changes in the nature, design, or layout of the project are planned, the conclusions and recommendations contained in this report should be reviewed and modified, or verified, as necessary. 1.1 Purpose and Scope The purpose of this study was to provide subsurface soil and groundwater data to be utilized in the design of the subject project. Our study included reviewing available geologic literature, drilling of two exploration borings, and performing a geologic study to assess the type, thickness, distribution, and physical properties of the subsurface sediments and shallow groundwater conditions. A geologic hazards assessment and geotechnical engineering studies were also completed to determine suitable geologic hazard mitigation techniques, the type of suitable foundations, allowable foundation soil bearing pressures, anticipated foundation settlements, erosion considerations, drainage considerations, and temporary excavation recommendations. This report summarizes our current fieldwork and offers geologic hazard mitigation and preliminary development recommendations based on our present understanding of the project. 1.2 Authorization Written authorization to proceed with this study was granted on June 15, 2018 by means of our signed scope of work and cost proposal. Our study was accomplished in general accordance with our proposal, dated January 25, 2018. This report has been prepared for the exclusive use of Plan to Permit, c/o WSCO and their agents for specific application to this project. Within the limitations of scope, schedule, and budget, our services have been performed in accordance with generally accepted geotechnical engineering and engineering geology practices in effect in this area at the time our report was prepared. No other warranty, express or implied, is made. Subsurface Exploration, Geologic Hazard, SW Sunset Boulevard Residence and Preliminary Geotechnical Engineering Report Renton, Washington Project and Site Conditions July 31, 2018 ASSOCIATED EARTH SCIENCES, INC. TG/ld - 180038E001-2 - Projects\20180038\KE\WP Page 2 2.0 PROJECT AND SITE DESCRIPTION The subject site is a rectangular-shaped undeveloped parcel 4,900 square foot (0.09 acre) in size, located along SW 4th Place in Renton, Washington (King County Parcel No. 2143701845) as shown on the “Vicinity Map” (Figure 1). The property is bounded by SW Sunset Boulevard to the south, SW 4th Place to the north, and small, undeveloped parcels to the east and west. The property is undeveloped, and is vegetated with mature trees and shrubs at the rear property line. Site topography slopes gently downward to the east with overall vertical relief across the site on the order of 15 feet and maximum site gradients over an elevation change of at least 10 feet of less than 15 percent. We anticipate that the proposed structure will be a three-story or less, single-family dwelling with an attached garage. Site access may be from SW Sunset Boulevard, or SW 4th Place. If project plans are changed from those on which this report is based, we should be allowed to review our recommendations and make any revisions that may be required as a result of the changes. 3.0 SITE EXPLORATION Our field study included drilling two exploration borings on June 27, 2018. The various types of sediments, as well as the depths where the characteristics of the sediments changed, are indicated on the exploration logs presented in the Appendix. The depths indicated on the logs where conditions changed may represent gradational variations between sediment types. If changes occurred between sample intervals in our exploration borings, they were interpreted. Our explorations were approximately located in the field by measuring from known site features and the above-referenced site plan. The site and the approximate locations of the subsurface explorations referenced in this study are presented on the “Existing Site and Exploration Plan” (Figure 2). The conclusions and recommendations presented in this report are based, in part, on the exploration borings completed for this study. The number, locations, and depths of the explorations were completed within site and budgetary constraints. Because of the nature of exploratory work below ground, extrapolation of subsurface conditions between field explorations is necessary. It should be noted that differing subsurface conditions may sometimes be present due to the random nature of deposition and the alteration of topography by past grading and/or filling. The nature and extent of variations between the field explorations may not become fully evident until construction. If variations are observed at that time, it may be necessary to re-evaluate specific recommendations in this report and make appropriate changes. Subsurface Exploration, Geologic Hazard, SW Sunset Boulevard Residence and Preliminary Geotechnical Engineering Report Renton, Washington Project and Site Conditions July 31, 2018 ASSOCIATED EARTH SCIENCES, INC. TG/ld - 180038E001-2 - Projects\20180038\KE\WP Page 3 3.1 Exploration Borings The exploration borings for this study were completed by advancing a 6-inch, outside-diameter, hollow-stem auger using a rubber track-mounted drill. During the drilling process, samples were generally obtained at 2½- to 5-foot-depth intervals. The borings were continuously observed and logged by a geologist from our firm. The exploration logs presented in the Appendix are based on the field logs, drilling action, and observation of the samples collected. Disturbed but representative samples were obtained by using the Standard Penetration Test (SPT) procedure in accordance with American Society for Testing and Materials (ASTM) D-1586. This test and sampling method consists of driving a standard 2-inch, outside-diameter, split-barrel sampler a distance of 18 inches into the soil with a 140-pound hammer free-falling a distance of 30 inches. The number of blows for each 6-inch interval is recorded, and the number of blows required to drive the sampler the final 12 inches is known as the Standard Penetration Resistance (“N”) or blow count. If a total of 50 is recorded within one 6-inch interval, the blow count is recorded as the number of blows for the corresponding number of inches of penetration. The resistance, or N-value, provides a measure of the relative density of granular soils or the relative consistency of cohesive soils; these values are plotted on the attached exploration boring logs. The samples obtained from the split-barrel sampler were classified in the field and representative portions placed in watertight containers. The samples were then transported to our laboratory for further visual classification and laboratory testing, as necessary. 4.0 SUBSURFACE CONDITIONS Subsurface conditions on the project site were inferred from the field explorations conducted for this study, visual reconnaissance of the site, and a review of applicable geologic literature. As shown on the exploration logs, soils encountered at the site consisted of surficial topsoil, fill soils, and bedrock formations. The deposits encountered at the site were generally consistent with those mapped in the site area on the Geologic Map of the Renton Quadrangle, King County, Washington, U.S. Geological Survey (USGS), Geologic Quadrangle Map GQ-405, compiled by D.R. Mullineaux, dated 1965. The published geologic map shows the project site mapped as Renton Formation bedrock with Tukwila Formation bedrock mapped nearby. The following section presents more detailed subsurface information on the sediment types encountered at the site. Subsurface Exploration, Geologic Hazard, SW Sunset Boulevard Residence and Preliminary Geotechnical Engineering Report Renton, Washington Project and Site Conditions July 31, 2018 ASSOCIATED EARTH SCIENCES, INC. TG/ld - 180038E001-2 - Projects\20180038\KE\WP Page 4 4.1 Stratigraphy Topsoil A surficial layer of organic topsoil was encountered at the locations of both exploration borings. This organic layer was observed to be approximately 6 inches in thickness at both locations. Due to their high organic content, these materials are not considered suitable for foundation, slab-on-grade floor support, nor for use in a structural fill. Existing Fill In exploration location EB-2, we encountered fill soils (soils not naturally placed) consisting of medium dense, light brown, very weathered sandstone and fine sand that was “soil-like” and had a disturbed texture. Fill soils at EB-2 extended to approximately 4.5 feet below the ground surface. Existing fill is also expected in unexplored areas of the site, such as existing utility trenches that cross the site. Due to their variable density, the existing fill soils are not suitable for foundation support. Bedrock Weathered bedrock of the Renton and Tukwila Formations were encountered below the topsoil horizon in EB-2 and below the existing fill in EB-1. Bedrock deposits generally consisted of medium dense to very dense, light brown to light oxidized brown, very weathered sandstone with occasional coal that was loose to medium dense in the upper few feet of the unit. The Renton Formation bedrock was highly weathered to the point of being “soil-like” near the contact with the overlying fill. With depth the weathered Renton Formation bedrock became increasingly dense and had less fines content. The Tukwila Formation bedrock was observed in both of our borings below the Renton Formation bedrock at a depth of 10 feet and 17 feet in EB-1 and EB-2, respectively, and consisted of very dense, gray to dark gray, moderately weathered sandstone with occasional silt and clay zones. Weathered sandstone associated with the Renton and Tukwila bedrock formations typically possess high-strength and low-compressibility attributes that are favorable for support of foundations, floor slabs, paving, and structural fill with proper preparation. In the areas where the weathered bedrock is at or near the ground surface, the density of the upper several feet of the unit was typically loose to medium dense. Where the weathered bedrock consists of a loose to medium dense soil, it may be suitable for reuse as structural fill, if specifically allowed by project plans and specifications, and provided they can be properly moisture-conditioned and compacted to project specifications. Preparation of excavated weathered bedrock for use in structural fills will require the moisture-conditioning of the material and restricting grading operations to dry weather conditions. Moderately weathered bedrock that is not “soil-like,” Subsurface Exploration, Geologic Hazard, SW Sunset Boulevard Residence and Preliminary Geotechnical Engineering Report Renton, Washington Project and Site Conditions July 31, 2018 ASSOCIATED EARTH SCIENCES, INC. TG/ld - 180038E001-2 - Projects\20180038\KE\WP Page 5 with a “blocky” texture should be broken into pieces no greater than 6 inches in diameter for use in structural fills. Compaction of the sandstone material should be performed using a segmented pad foot or sheepsfoot roller. 4.2 Hydrology Groundwater was not encountered in exploration boring EB-1, however groundwater in the form of seepage was encountered at 9 feet in EB-2. We did not observe groundwater emanating from the slope. We expect groundwater seepage across much of the site to be limited to interflow. Interflow occurs when surface water percolates down through the surficial weathered or higher-permeability sediments and becomes perched atop underlying, lower- permeability sediments. It should be noted that the occurrence and level of groundwater seepage at the site may vary in response to such factors as changes in season, precipitation, and site use. Subsurface Exploration, Geologic Hazard, SW Sunset Boulevard Residence and Preliminary Geotechnical Engineering Report Renton, Washington Geologic Hazards and Mitigations July 31, 2018 ASSOCIATED EARTH SCIENCES, INC. TG/ld - 180038E001-2 - Projects\20180038\KE\WP Page 6 II. GEOLOGIC HAZARDS AND MITIGATIONS The following discussion of potential geologic hazards is based on the geologic, slope, and ground and surface water conditions, as observed and discussed herein. 5.0 LANDSLIDE HAZARDS AND MITIGATIONS It is our opinion that the risk of damage to the proposed structure by landsliding is low due to gentle slope inclinations and the presence of medium dense to dense soils observed at relatively shallow depths beneath the surface of the site. No detailed slope stability analyses were completed as part of this study, and none are warranted, in our opinion. Based on our review of the City of Renton Landslide Vulnerability Map, it does not appear that the site contains areas that are considered to be governed by regulations associated with Landslide Hazard Areas. 6.0 SEISMIC HAZARDS AND MITIGATIONS Earthquakes occur regularly in the Puget Lowland. The majority of these events are small and are usually not felt by people. However, large earthquakes do occur, as evidenced by the 1949, 7.2-magnitude event; the 1965, 6.5-magnitude event; and the 2001, 6.8-magnitude event. The 1949 earthquake appears to have been the largest in this region during recorded history and was centered in the Olympia area. Evaluation of earthquake return rates indicates that an earthquake of the magnitude between 5.5 and 6.0 is likely within a given 20-year period. Generally, there are four types of potential geologic hazards associated with large seismic events: 1) surficial ground rupture, 2) seismically induced landslides, 3) liquefaction, and 4) ground motion. The potential for each of these hazards to adversely impact the proposed project is discussed below. 6.1 Surficial Ground Rupture The nearest known fault trace to the project site is the Seattle Fault Zone (SFZ) located approximately 3 to 4 miles to the north. Studies of the SFZ by the USGS have provided evidence of surficial ground rupture along a northern splay of the Seattle Fault. According to the USGS studies, the latest movement of this fault was about 1,100 years ago when about 20 feet of surficial displacement took place. This displacement can presently be seen in the form of raised, wave-cut beach terraces along Alki Point in West Seattle and Restoration Point at the south end Subsurface Exploration, Geologic Hazard, SW Sunset Boulevard Residence and Preliminary Geotechnical Engineering Report Renton, Washington Geologic Hazards and Mitigations July 31, 2018 ASSOCIATED EARTH SCIENCES, INC. TG/ld - 180038E001-2 - Projects\20180038\KE\WP Page 7 of Bainbridge Island. The recurrence interval of movement along this fault system is still unknown, although it is hypothesized to be in excess of several thousand years. Based on the distance between the site and the SFZ, and the suspected long recurrence interval, the risk of damage to the subject project by surficial ground rupture along the SFZ is considered to be low during the expected life of the proposed structure and no mitigation efforts beyond complying with the current (2015) International Building Code (IBC) are recommended. 6.2 Seismically Induced Landslides It is our opinion that the risk of damage to the proposed structure by seismically induced landsliding is low due to the presence of medium dense to dense soils observed at depth beneath the surface of the site and the lack of steep slopes on the site. 6.3 Liquefaction Liquefaction is a process through which unconsolidated soil loses strength as a result of vibrations, such as those which occur during a seismic event. During normal conditions, the weight of the soil is supported by both grain-to-grain contacts and by the fluid pressure within the pore spaces of the soil below the water table. Extreme vibratory shaking can disrupt the grain-to-grain contact, increase the pore pressure, and result in a temporary decrease in soil shear strength. The soil is said to be liquefied when nearly all of the weight of the soil is supported by pore pressure alone. Liquefaction can result in deformation of the sediment and settlement of overlying structures. Areas most susceptible to liquefaction include those areas underlain by non-cohesive silt and sand with low relative densities, accompanied by a shallow water table. The observed site soils were medium dense to dense and unsaturated and are thus not expected to be prone to liquefaction. A detailed liquefaction hazard analysis was not performed as part of this study, and none is warranted, in our opinion. 6.4 Ground Motion Structural design of the building should follow 2015 IBC standards using Site Class “D” as defined in Table 20.3-1 of American Society of Civil Engineers (ASCE) 7 - Minimum Design Loads for Buildings and Other Structures. Subsurface Exploration, Geologic Hazard, SW Sunset Boulevard Residence and Preliminary Geotechnical Engineering Report Renton, Washington Geologic Hazards and Mitigations July 31, 2018 ASSOCIATED EARTH SCIENCES, INC. TG/ld - 180038E001-2 - Projects\20180038\KE\WP Page 8 7.0 EROSION HAZARDS AND MITIGATIONS The sediments underlying the site generally contain sand and sand with silt and will be sensitive to erosion. In order to reduce the amount of sediment transported off the site during construction, the following recommendations should be followed. 1. Silt fencing should be placed around the lower perimeter of all cleared area(s). The fencing should be periodically inspected and maintained as necessary to ensure proper function. 2. To the extent possible, earthwork-related construction should proceed during the drier periods of the year and disturbed areas should be revegetated as soon as possible. Temporary erosion control measures should be maintained until permanent erosion control measures are established. 3. Areas stripped of vegetation during construction should be mulched and hydroseeded, replanted as soon as possible, or otherwise protected. During winter construction, hydroseeded areas should be covered with clear plastic to facilitate grass growth. 4. If excavated soils are to be stockpiled on the site for reuse, measures should be taken to reduce the potential for erosion from the stockpile. These could include, but are not limited to, covering the pile with plastic sheeting, the use of low stockpiles in flat areas, and the use of straw bales/silt fences around pile perimeters. 5. Interceptor swales with rock check dams should be constructed to divert stormwater from construction areas and to route collected stormwater to an appropriate discharge location. 6. A rock construction entrance should be provided to reduce the amount of sediment transported off-site on truck tires. 7. All stormwater from impermeable surfaces, including driveways and roofs, should be tightlined into approved facilities and not be directed onto or above steeply sloping areas. Subsurface Exploration, Geologic Hazard, SW Sunset Boulevard Residence and Preliminary Geotechnical Engineering Report Renton, Washington Preliminary Design Recommendations July 31, 2018 ASSOCIATED EARTH SCIENCES, INC. TG/ld - 180038E001-2 - Projects\20180038\KE\WP Page 9 III. PRELIMINARY DESIGN RECOMMENDATIONS 8.0 INTRODUCTION It is our opinion that, from a geotechnical standpoint, the property is suitable for the proposed development provided the recommendations contained herein are properly followed. The site is underlain by medium dense to very dense weathered bedrock. Conventional spread footing foundations bearing on either the medium dense to very dense weathered bedrock or on structural fill placed over these sediments are capable of providing suitable building support. 9.0 SITE PREPARATION Site preparation of building and paving areas should include removal of all grass, trees, brush, debris, and any other deleterious materials. Any existing fill should be removed. Where any existing loose fill or natural sediments are relatively free of organics and near their optimum moisture content for compaction, they can be segregated and considered for reuse as structural fill. As noted previously, the weathered bedrock encountered in our explorations may be above optimum moisture content, may need to be crushed down into smaller rock, and will require preparation to be reused as structural fill. Erosion and surface water control should be established around the perimeter of the excavation to satisfy City of Renton requirements. Any utilities on the site that cross the building footprint should be relocated to outside of the building footprint at the time of construction. 9.1 Temporary Cut Slopes Based on our understanding of the current project, excavation for the basement level of the proposed home can be accomplished by the use of temporary cut slopes. Associated Earth Sciences, Inc. (AESI) should review the project plans once they have been developed to provide additional recommendations regarding temporary slopes, if necessary. In our opinion, stable construction slopes should be the responsibility of the contractor and should be determined during construction based on the conditions encountered at that time. For estimating purposes, however, we anticipate that temporary, unsupported cut slopes in undisturbed dense to very dense weathered bedrock sediments can be planned at a maximum slope of 1H:1V (Horizontal:Vertical). Temporary, unsupported cut slopes in medium dense fill or weathered bedrock sediments can be planned at 1.5H:1V. Temporary cut slopes may need to be adjusted in the field at the time of construction based on the presence of groundwater. This should be determined in the field by the geotechnical engineer. As is typical with earthwork operations, some sloughing and raveling may occur, and cut slopes may have to be adjusted in the field. In addition, WISHA/OSHA regulations should be followed at all times. If steeper or deeper cuts are Subsurface Exploration, Geologic Hazard, SW Sunset Boulevard Residence and Preliminary Geotechnical Engineering Report Renton, Washington Preliminary Design Recommendations July 31, 2018 ASSOCIATED EARTH SCIENCES, INC. TG/ld - 180038E001-2 - Projects\20180038\KE\WP Page 10 required, then temporary shoring may be necessary. We are available to provide recommendations for temporary shoring, if needed. 9.2 Site Disturbance The on-site soils contain a variable percentage of fine-grained material, which makes them moisture-sensitive and subject to disturbance when wet. The contractor must use care during site preparation and excavation operations so that the underlying soils are not softened, particularly during wet weather conditions. If disturbance occurs in areas of conventional footings, the softened soils should be removed and the area brought to grade with clean crushed rock fill or the footings should be extended deeper. Because of the moisture-sensitive nature of the soils, we anticipate that wet weather construction would significantly increase the earthwork costs over dry weather construction. 10.0 STRUCTURAL FILL Structural fill may be necessary to establish desired grades or to backfill around foundations and utilities. All references to structural fill in this report refer to subgrade preparation, fill type, placement, and compaction of materials, as discussed in this section. If a percentage of compaction is specified under another section of this report, the value given in that section should be used. After overexcavation/stripping has been performed to the satisfaction of the geotechnical engineer/engineering geologist, the upper 12 inches of exposed ground should be recompacted to a firm and unyielding condition. If the subgrade contains too much moisture, adequate recompaction may be difficult or impossible to obtain and should probably not be attempted. In lieu of recompaction, the area to receive fill should be blanketed with washed rock or quarry spalls to act as a capillary break between the new fill and the wet subgrade. Where the exposed ground remains soft and further overexcavation is impractical, placement of an engineering stabilization fabric may be necessary to prevent contamination of the free-draining layer by silt migration from below. After stripping and subgrade preparation of the exposed ground is approved, or a free-draining rock course is laid, structural fill may be placed to attain desired grades. Structural fill is defined as non-organic soil, acceptable to the geotechnical engineer, placed in maximum 8-inch loose lifts, with each lift being compacted to 95 percent of the modified Proctor maximum density using ASTM D-1557 as the standard. The contractor should note that any proposed fill soils must be evaluated by AESI prior to their use in fills. This would require that we have a sample of the material at least 3 business days in Subsurface Exploration, Geologic Hazard, SW Sunset Boulevard Residence and Preliminary Geotechnical Engineering Report Renton, Washington Preliminary Design Recommendations July 31, 2018 ASSOCIATED EARTH SCIENCES, INC. TG/ld - 180038E001-2 - Projects\20180038\KE\WP Page 11 advance to perform a Proctor test and determine its field compaction standard. Soils in which the amount of fine-grained material (smaller than the No. 200 sieve) is greater than approximately 5 percent (measured on the minus No. 4 sieve size) should be considered moisture-sensitive. Use of moisture-sensitive soils in structural fills should be limited to favorable dry weather conditions. The on-site soils contain moderate amounts of silt and are considered moisture-sensitive. Therefore, we expect that this material may be difficult to compact to structural fill specifications, particularly during the following wet weather. If fill is placed during wet weather, or if proper compaction cannot be obtained, a select on-site and/or import material consisting of a clean, free-draining gravel and/or sand should be used. Free-draining fill consists of non-organic soil with the amount of fine-grained material limited to 5 percent by weight when measured on the minus No. 4 sieve fraction and at least 25 percent greater than the No. 4 sieve. A representative from our firm should inspect the stripped subgrade and be present during placement of structural fill to observe the work and perform a representative number of in-place density tests. In this way, the adequacy of the earthwork may be evaluated as filling progresses and any problem areas may be corrected at that time. It is important to understand that taking random compaction tests on a part-time basis will not assure uniformity or acceptable performance of a fill. As such, we are available to aid the owner in developing a suitable monitoring and testing frequency. 11.0 FOUNDATIONS 11.1 Allowable Soil Bearing Pressure Spread footings may be used for building support when founded either directly on the medium dense to very dense, weathered bedrock, or on structural fill placed over these materials. Sediments suitable for foundation support in the area of the proposed building were encountered in our explorations at depths of approximately 3 to 5 feet, but may be locally deeper. Therefore, overexcavation and replacement of loose fill with structural fill may be needed. If structural fill is placed below footing areas, the structural fill should extend horizontally beyond the footing edges a distance equal to or greater than the thickness of the fill. For footings founded either directly upon the medium dense to very dense weathered bedrock, or on structural fill as described above, we recommend that an allowable bearing pressure of 2,500 pounds per square foot (psf) be used for design purposes, including both dead and live loads. We recommend that the footing subgrade be recompacted to a firm and unyielding condition prior to footing placement. An increase in the allowable bearing pressure of one-third may be used for short-term wind or seismic loading. Subsurface Exploration, Geologic Hazard, SW Sunset Boulevard Residence and Preliminary Geotechnical Engineering Report Renton, Washington Preliminary Design Recommendations July 31, 2018 ASSOCIATED EARTH SCIENCES, INC. TG/ld - 180038E001-2 - Projects\20180038\KE\WP Page 12 11.2 Footing Depths Perimeter footings for the proposed building should be buried a minimum of 18 inches into the surrounding soil for frost protection. No minimum burial depth is required for interior footings; however, all footings must penetrate to the prescribed stratum, and no footings should be founded in or above loose, organic, or existing fill soils. 11.3 Footings Adjacent to Cuts 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 edges of steps or cuts in the bearing soils. 11.4 Footing Settlement Anticipated settlement of footings founded as described above should be on the order of 1 inch or less. However, disturbed soil not removed from footing excavations prior to footing placement could result in increased settlements. 11.5 Footing Subgrade Bearing Verification All footing areas should be observed by AESI prior to placing concrete to verify that the exposed soils can support the design foundation bearing capacity and that construction conforms with the recommendations in this report. Foundation bearing verification may also be required by the governing municipality. 11.6 Foundation Drainage Perimeter footing drains should be provided as discussed under the “Drainage Considerations” section of this report. 12.0 LATERAL WALL PRESSURES All backfill behind retaining walls or around foundation units should be placed as per our recommendations for structural fill and as described in this section of the report. Horizontally backfilled retaining walls that are free to yield laterally at least 0.1 percent of their height may be designed using an equivalent fluid equal to 35 pounds per cubic foot (pcf). Fully restrained, horizontally backfilled, rigid walls that cannot yield should be designed for an equivalent fluid Subsurface Exploration, Geologic Hazard, SW Sunset Boulevard Residence and Preliminary Geotechnical Engineering Report Renton, Washington Preliminary Design Recommendations July 31, 2018 ASSOCIATED EARTH SCIENCES, INC. TG/ld - 180038E001-2 - Projects\20180038\KE\WP Page 13 of 50 pcf. If roadways, parking areas, or other areas subject to vehicular traffic are adjacent to retaining walls, a surcharge equivalent to 2 feet of soil should be added to the wall height in determining lateral design forces. Retaining walls that retain sloping backfill at a maximum angle of 2H:1V should be designed using an equivalent fluid pressure of 55 pcf for yielding conditions or 75 pcf for fully restrained conditions. In accordance with the 2015 IBC, retaining wall design should include seismic design parameters. Based on the site soils and assumed wall backfill materials, we recommend a seismic surcharge pressure in addition to the equivalent fluid pressures presented above. A rectangular pressure distribution of 5H and 10H psf (where H is the height of the wall in feet) should be included in design for “active” and “at-rest” loading conditions, respectively. The resultant of the rectangular seismic surcharge should be applied at the midpoint of the walls. The lateral pressures presented above are based on the conditions of a uniform horizontal backfill consisting of the on-site, weathered bedrock sediments or imported sand and gravel compacted to 90 percent of ASTM D-1557. A higher degree of compaction is not recommended, as this will increase the pressure acting on the wall. Footing drains must be provided for all retaining walls, as discussed under the “Drainage Considerations” section of this report. It is imperative that proper drainage be provided so that hydrostatic pressures do not develop against the walls. This would involve installation of a minimum, 1-foot-wide blanket drain to within 1 foot of the ground surface using imported, washed gravel against the walls placed to be continuous with the footing drain. 12.1 Passive Resistance and Friction Factors Lateral loads can be resisted by friction between the foundation and the competent natural sediments or supporting structural fill soils, and/or by passive earth pressure acting on the buried portions of the foundations. The foundations must be backfilled with compacted structural fill to achieve the passive resistance provided below. We recommend the following allowable design parameters. • Passive equivalent fluid = 300 pcf • Coefficient of friction = 0.35 13.0 FLOOR SUPPORT Slab-on-grade floors may be constructed either directly on the medium dense to very dense weathered bedrock sediments, or on structural fill placed over these materials. Areas of the Subsurface Exploration, Geologic Hazard, SW Sunset Boulevard Residence and Preliminary Geotechnical Engineering Report Renton, Washington Preliminary Design Recommendations July 31, 2018 ASSOCIATED EARTH SCIENCES, INC. TG/ld - 180038E001-2 - Projects\20180038\KE\WP Page 14 slab subgrade that are disturbed (loosened) during construction should be recompacted to an unyielding condition prior to placing the pea gravel, as described below. If moisture intrusion through slab-on-grade floors is to be limited, the floors should be constructed atop a capillary break consisting of a minimum thickness of 4 inches of washed pea gravel or washed crushed rock. The pea gravel/crushed rock should be overlain by a 10-mil (minimum thickness) plastic vapor retarder. 14.0 DRAINAGE CONSIDERATIONS All retaining and perimeter footing walls should be provided with a drain at the footing elevation. Drains should consist of rigid, perforated, polyvinyl chloride (PVC) pipe surrounded by washed pea gravel or drain rock. The level of the perforations in the pipe should be set approximately 2 inches below the bottom of the footing and should be constructed with sufficient gradient to allow gravity discharge away from the structure. Daylight basement level footing drains are expected to have a suitable gravity outfall location. If basement excavations do not have a suitable gravity outfall location, a sump pump system would be needed to remove water from the basement footing drain system. All retaining walls should be lined with a minimum, 12-inch-thick, washed gravel blanket provided over the full height of the wall that ties into the footing drain. Roof and surface runoff must not discharge into the footing drain system, but should be handled by a separate, rigid, tightline drain. In planning, exterior grades adjacent to walls should be sloped downward away from the structure to achieve surface drainage. All collected runoff must be tightlined to a City-approved location. 15.0 PROJECT DESIGN AND CONSTRUCTION MONITORING We recommend that AESI perform a geotechnical review of the plans prior to final design completion. In this way, our recommendations may be properly interpreted and implemented in the design. This plan review is not included in the current scope of work and budget. We are also available to provide geotechnical engineering and monitoring services during construction. The integrity of the earthwork and foundations depends on proper site preparation and construction procedures. In addition, engineering decisions may have to be made in the field in the event that variations in subsurface conditions become apparent. Construction monitoring services are not part of this current scope of work. Copyright:© 2013 National Geographic Society, i-cubed 0 20001000 FEET ± NOTE: BLACK AND WHITEREPRODUCTION OF THIS COLORORIGINAL MAY REDUCE ITSEFFECTIVENESS AND LEAD TOINCORRECT INTERPRETATION VICINITY MAP PROJ NO. DATE: FIGURE:180038E001 7/18 1Document Path: G:\GIS_Projects\aaY2018\180038 SW Sunset Blvd Res\mxd\180038E001 F1 VM_SW_SunsetBlvdRes.mxdDATA SOURCES / REFERENCES:USGS: 7.5' SERIES TOPOGRAPHIC MAPS, ESRI/I-CUBED/NGS 2013KING CO: STREETS, PARCELS, CITY LIMITS 1/18 LOCATIONS AND DISTANCES SHOWN ARE APPROXIMATE KitsapCounty Snohomish County Pierce County King CountyThomas Ave SWSW 3rd P l SR 900 EarlingtonAveSWSW4thPl !( ¬«167 ¬«515 SITE ¥405 SW SUNSET BOULEVARD RESIDENCE RENTON, WASHINGTON !( !( !( !( SR 900 RAYMOND PL SW SW 3 R D P L EARLINGTON AVE SWSW4THPL THOMAS AVE SWEB-1 EB-2 130 120 10070 40 160 110150 90 8070 190 80 50 170 150 14 0 180 180 170 7060144 132 128126124142 136 122188186184182180176174172178 170 108106104102989690110 94 82 76 84 58 158 156 154 152162160 7262 168166116112118 11486 82 80 148 146182180 76747219219 0 154 152 88 84 82 7868 4038138 13492 52 150 70 6856 164 7472184 70184 168 168 36 194 1 5 0 140 130124 12 0 100 80 78 76 50 48BLACK AND WHITE REPRODUCTION OF THIS COLOR ORIGINAL MAY REDUCE ITSEFFECTIVENESS AND LEAD TO INCORRECT INTERPRETATION DATA SOURCES / REFERENCES:PSLC: KING COUNTY 2016, GRID CELL SIZE IS 3'.DELIVERY 2 FLOWN 2/25/16 - 3/28/16WA STATE PLANE NORTH (FIPS 4601), NAD83(HARN)NAVD88 GEOID03 (GEOID03), US SURVEY FEET.CONTOURS FROM LIDARKING CO: STREETS, PARCELS 1/18,AERIAL: KINGCO, PICTOMETRY INT. 2015 LOCATIONS AND DISTANCES SHOWN ARE APPROXIMATE EXISTING SITE AND EXPLORATION PLAN SW SUNSET BOULEVARD RESIDENCE RENTON, WASHINGTON Document Path: G:\GIS_Projects\aaY2017\170600 Overlake School\mxd\180038E001 F2 SM_SunsetBlvdRes.mxd0 10050 FEET ±KitsapCounty SnohomishCounty Pierce County King County LEGEND: SITE !(EXPLORATION BORING CONTOUR 10 FT CONTOUR 2 FT PROJ NO. DATE: FIGURE:180038E001 7/18 2 APPENDIX Topsoil - 6 inches Renton Formation BedrockLight brown sand drill cuttings.Erratic drilling observed 1 to 2.5 feet. Slightly moist, lightly oxidized brown, SANDSTONE; very weathered; thinlylaminated; blocky texture. Color becomes slightly lighter, moist. Hard/stiff drilling 5 to 7.5 feet. As above. Hard/stiff drilling 7.5 to 10 feet. Sample A (upper ~3 inches): Color turns darker red brown. Tukwila Formation Bedrock Sample B (lower ~6 inches): Slightly moist, gray with minor oxidation,SANDSTONE; very weathered; thinly laminated; minor mica flakes.Hard/stiff drilling at 10 feet. Some drill chatter starting at 12 feet. Drill cuttings turn to light brownish gray at 13 feet. Very slow drilling at 14 feet. Slightly moist, gray, SANDSTONE; moderately weathered; disturbed texture;clay zones. 667 2250/5" 50/5" 3850/3" 50/6" S-1 S-2 S-3 S-4 S-5 Bottom of exploration boring at 15.5 feetNo groundwater encountered. 1 of 1 NAVD 88 Sheet Depth (ft)Exploration Number180038E001 M - Moisture 6 inches 40 Datum S T Graphic10 Other TestsHole Diameter (in) DESCRIPTION Location Water Level ()Approved by: 30 Blows/Foot Driller/Equipment Blows/6"Geologic Drill / Mini-Track Well5 10 15 20 25 Water LevelProject Name EB-1 SymbolTG2" OD Split Spoon Sampler (SPT) 3" OD Split Spoon Sampler (D & M)JHSCompletionSamples Ground Surface Elevation (ft) Grab Sample 6/27/18,6/27/18 Logged by: Shelby Tube Sample 140# / 30" Ring Sample No Recovery Water Level at time of drilling (ATD) SW Sunset Boulevard Residence 125 Project Number 20 Renton, WA Date Start/Finish Hammer Weight/Drop Sampler Type (ST): Exploration Log AESIBOR 180038.GPJ June 29, 20181313 5050/5" 5050/5" 5050/3" 5050/6" Topsoil - 6 inches Fill Slightly moist, light brown, SANDSTONE, and fine SAND; very weathered anddisturbed; "soil-like". Renton Formation Bedrock Slightly moist, dark oxidized red brown; minor mica flakes. Moist to very moist, light oxidized brown, SANDSTONE; very weathered; minorcoal. Slightly harder/stiffer drilling at 9 feet. Moist to very moist, light oxidized brown to light gray, SANDSTONE; highlyweathered; minor mica flakes; "soil-like"; tip of sampler is wet from seepagefrom above. Harder/stiffer drilling at 12.5 feet. Moist, as above; tip of sampler is wet, seepage from above. Tukwila Formation Bedrock Slightly moist to moist, gray to dark gray, SANDSTONE; moderately weathered;minor mica flakes; clay zones; some horizontal lamination of darkerminerals/grains.Drill cuttings become light brown and tan at 21 feet. Much harder/stiffer drilling at 22 feet. Some chattering in drill action 23 to 25 feet. Grinding drill action at 24.5 feet, could not advance drill.Sampler over driven to obtain sample, no recovery. 566 666 327 9910 163249 131150/5" 100/3" S-1 S-2 S-3 S-4 S-5 S-6 S-7 Bottom of exploration boring at 24.5 feetGroundwater encountered at 9 feet. 1 of 1 NAVD 88 Sheet Depth (ft)Exploration Number180038E001 M - Moisture 6 inches 40 Datum S T Graphic10 Other TestsHole Diameter (in) DESCRIPTION Location Water Level ()Approved by: 30 Blows/Foot Driller/Equipment Blows/6"Geologic Drill / Mini-Track Well5 10 15 20 25 Water LevelProject Name EB-2 SymbolTG2" OD Split Spoon Sampler (SPT) 3" OD Split Spoon Sampler (D & M)JHSCompletionSamples Ground Surface Elevation (ft) Grab Sample 6/27/18,6/27/18 Logged by: Shelby Tube Sample 140# / 30" Ring Sample No Recovery Water Level at time of drilling (ATD) SW Sunset Boulevard Residence 113 Project Number 20 Renton, WA Date Start/Finish Hammer Weight/Drop Sampler Type (ST): Exploration Log AESIBOR 180038.GPJ June 29, 20181212 1212 99 1919 81 5050/5" 100/3"