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HomeMy WebLinkAboutRS_th geotech investig_20150225_kmr highlights CITY OF RENTON Technical Memorandum No. 4 Geotechnical Investigation Thunder Hills Sewer Interceptor February 13, 2015 Prepared By: Phil A. Haberman, P.G., P.E.G. Senior Engineering Geologist Gopal A. Singam, P.E. Senior Geotechnical Engineer Stantec Consulting Services Inc. 11130 NE 33rd Place Suite 200 Bellevue, WA 98004 425.869.9448 TECHNICAL MEMORANDUM NO. 4 February 13, 2015 rs v:\2002\active\2002003607\civil\design\report\mem_th geotech investig_20150210.docx i Contents 1.0 INTRODUCTION ............................................................................................................. 1 2.0 PROJECT DESCRIPTION ................................................................................................. 1 3.0 SITE DESCRIPTION .......................................................................................................... 1 4.0 SUBSURFACE DATA ........................................................................................................ 3 5.0 SOIL AND GROUNDWATER CONDITIONS ..................................................................... 3 6.0 GEOLOGIC HAZARDS ................................................................................................... 5 7.0 DISCUSSION .................................................................................................................. 8 8.0 RECOMMENDATIONS .................................................................................................... 8 9.0 CONSTRUCTION FIELD REVIEWS.................................................................................. 15 10.0 CLOSURE ...................................................................................................................... 16 Appendices APPENDIX A: STATEMENT OF GENERAL CONDITIONS APPENDIX B: FIGURES  Vicinity Map (Figure 1)  Site Plans (Figures 2 & 3)  Cross Sections (Figures 4-9)  Gravity Wall Schematics (Figures 10 & 11) APPENDIX C: BORING LOGS TECHNICAL MEMORANDUM NO. 4 February 13, 2015 rs v:\2002\active\2002003607\civil\design\report\mem_th geotech investig_20150210.docx ii This page intentionally left blank TECHNICAL MEMORANDUM No. 4 February 13, 2015 rs v:\2002\active\2002003607\civil\design\report\mem_th geotech investig_20150210.docx 1 1.0 Introduction In accordance with authorization by the City of Renton, Stantec has completed a geotechnical investigation for the Thunder Hills Creek Sewer Alternatives Analysis located in Renton, Washington. Figure 1 shows the project area. The purpose of our investigation was to evaluate the geologic conditions in the most critical geologic hazard areas of the existing sewer line and to provide preliminary geotechnical recommendations and opinions for protection of the existing or new sewer line, manholes, and access roadway. The scope of work for the study consisted of multiple levels of field investigations and document reviews followed by cross section development, slope stability analyses, and engineering analyses to prepare this report. Preliminary recommendations presented herein pertain to various geotechnical aspects of the proposed project and provide an overview of the geologic conditions and hazards. 2.0 Project Description The proposed alternatives include replacement of the existing sewer line that extends along Thunder Hills Creek from approximately Grant Avenue South to Interstate 405 (I- 405), diverting flow from this basin into sewer infrastructure west of Talbot Road South, or rehabilitating (lining) the existing line along Thunder Hills Creek. For the alternatives located in the Thunder Hills Creek basin, construction may include adding sanitary sewer manholes where none currently exist, creating/rehabilitating an access roadway to the area near I-405 for maintenance equipment; and potentially constructing retaining structures to support the existing access roadway and sewer line. 3.0 Site Description The Thunder Hills Creek project area is located between I -405 and Grant Avenue South, just east of the Berkshire Apartment Home development (Figure 1). For the purposes of this study, we narrowed the site area to include the most geologically hazardous areas within the project area. This area is located from I -405 upstream for approximately 600 feet (Figures 2 and 3). For our study, we also consider the site to include all of the steep slopes located east and west of Thunder Hills Creek (approximately 150-200 feet wide) in this area. Localized steep slope areas, drainage zones, and areas where the existing sewer line crosses Thunder Hills Creek along the alignment south of the main site area were also reviewed. Conclusions and opinions regarding these areas, along with geotechnical recommendations pertaining to slope stability and access roadway re-construction are included. TECHNICAL MEMORANDUM No. 4 February 13, 2015 rs v:\2002\active\2002003607\civil\design\report\mem_th geotech investig_20150210.docx 2 The site consists of the existing sewer interceptor alignment through the Thunder Hills Creek valley along with a gravel improved access roadway located adjacent to the stream for much of the alignment. The access roadway has been damaged significantly by soil movement and erosion near Station 5+00 (Figure 3). North of this location, the roadway is improved (partially) with quarry rock north to a flat area near I - 405. Attempts have been made to locate the existing sewer pipe. The pipe type varies along the alignment and the pipe depth is known only at existing manholes and has been interpolated between manholes. Near Station 11+80, the sewer line extends beneath Thunder Hills Creek at an estimated depth of 7 feet. Thunder Hills Creek appears to flow continuously throughout the year and there are numerous intermittent, seasonal streams that join the creek. The site plans show the locations of various streams that join Thunder Hills Creek from the east, as well as wetland areas located in and around the creek. Minor springs and seeps seasonally add volume to the creek throughout the valley. There are numerous residential yard drains that add stormwater to the stream. Near Station 5+30, a relatively large creek joins Thunder Hills Creek from the southeast, approximately doubling the volume of runoff. South of Station 5+50, variable amounts of sediment are present in the stream channel. North of this area, the stream has incised into the underlying sandstone (Renton Formation). Four to 12 inch sized quarry rock is present in the stream bed and banks in many areas. Larger quarry rock, generally ½ to 4 man sized basalt, has been used to stabilize the stream banks and/or to prevent ongoing stream erosion. Specifically in the vicinity of Station 5+00, large quarry rock has been used to fill the stream channel. Rock filled gabion walls, generally 4 to 6 feet in height, are located between the access roadway/path and Thunder Hills Creek north of Station 4+00. For the most part, the gabion baskets have deteriorated significantly and in places the walls are somewhat overturned. The gabion walls appear to have limited functionality as retaining structures for the roadway and sewer line. The slopes extending downward into the Thunder Hills Creek valley between Stations 0+25 and 5+40 are very steep, with magnitudes of 100 to 150 percent. There are localized slope areas that are near vertical (200 percent magnitude) to overturned due to excavation, sloughing, and/or landslide activity. Several rockeries are located along the east and west sides of the access roadway between Stations 3+80 and 5+20. The rockeries are comprised of 1 to 2 man sized basalt and are up to 7 feet in height. The rockeries are loosely constructed. There is evidence that shallow landslide activity occurs periodically along portions of the slope west of the access roadway. Several large, but shallow, landslides have occurred within the last several years north of Station 2+50. The slides appear to consist of the upper colluvium (1 to 4 feet thick) sliding off of the underlying sandstone. The slides extend upslope between 10 and 50 feet and are up to 70 feet wide. TECHNICAL MEMORANDUM No. 4 February 13, 2015 rs v:\2002\active\2002003607\civil\design\report\mem_th geotech investig_20150210.docx 3 The site is bordered to the north by I-405, to the west by the Berkshire Apartment Homes, to the east by undeveloped land (easements) and single family residences, and to the south by Thunder Hills Creek, easements, and residential developments. 4.0 Subsurface Data 4.1.1 Site Investigation Program The geotechnical field investigation program was completed on October 17th and 20th, 2014 and included drilling and sampling four hollow stem auger borings drilled by a Stantec subcontractor using a limited access drill rig. The borings were located at or near pre-determined locations and extended approximately 5 to 25 feet below the existing site grades. The soils encountered were logged in the field during the exploration and are described in accordance with the Unified Soil Classification System (USCS). Disturbed soil samples were obtained by using a 140 pound hammer free falling a vertical distance of 30 inches for the borings. The summation of hammer-blows required to drive the sampler the final 12-inches of an 18-inch sample length is defined as the Standard Penetration Resistance, or N-value for a 140 pound hammer and 2 inch outside diameter split spoon sampler. The uncorrected blow count is presented graphically on the boring logs in Appendix C. The resistance, or “N” value, provides a measure of the relative density of granular soils and the consistency of cohesive soils. Our report discussions regarding soil density as well as engineering parameters are based on the N values. A Stantec field representative directed the drilling program, collected disturbed soil samples from split spoon sampler tubes, classified the encountered soils, kept a detailed log of each auger hole, and observed and recorded pertinent site features. The results of the drilling and sampling are presented on the boring logs enclosed in Appendix C. We also reviewed six boring logs from a geotechnical investigation conducted by Soil and Environmental Engineers, Inc. (S&EE) in 2011. This report was conducted to develop solutions to retain/protect the existing sewer line in the lower portion of the Thunder Hills Creek valley. 5.0 Soil and Groundwater Conditions 5.1.1 Area Geology The site lies within the Puget Lowland. The lowland is part of a regional north-south trending trough that extends from southwestern British Columbia to near Eugene, Oregon. North of Olympia, Washington, this lowland is glacially carved, with a TECHNICAL MEMORANDUM No. 4 February 13, 2015 rs v:\2002\active\2002003607\civil\design\report\mem_th geotech investig_20150210.docx 4 depositional and erosional history including at least four separate glacial advances/retreats. The Puget Lowland is bounded to the west by the Olympic Mountains and to the east by the Cascade Range. The lowland is filled with glacial and nonglacial sediments consisting of interbedded gravel, sand, silt, till, and peat lenses. The Geologic Map of King County, indicates that the site is located near the contacts between Vashon Glacial Till and Tertiary Bedrock. Vashon Glacial Till is typically characterized by an unsorted, nonstratified mixture of clay, silt, sand, gravel, cobbles and boulders in variable quantities. These materials are typically dense and relatively impermeable. The poor sorting reflects the mixing of the materials as these sediments were overridden and incorporated by the glacial ice. Tertiary Bedrock in this area consists of the Renton Formation. The Renton Formation includes feldspathic fine to medium grained sandstone with beds of coal, carbonaceous siltstone, and claystone. Tertiary Bedrock locally outcrops south of I -90 and the Seattle Fault Zone due to uplift associated with seismic activity. 5.1.2 Soil Conditions Details of the encountered soil conditions are presented on the boring logs in Appendix C. The detailed soil description on these logs should be referred to in preference to the generalized descriptions below. Boring B-1 In Boring B-1, we encountered approximately 6 inches of topsoil and vegetation underlain by approximately 5 feet of medium dense to dense, silty-fine to medium grained sand with variable amounts of gravel and debris (Fill). This layer was underlain by stiff to very stiff silt with variable amounts of sand and woody debris (Fill). The silt layer was underlain by stiff silt with variable amounts of sand, gravel, and trace amounts of woody debris (Highly Weathered Renton Formation). These materials were underlain by medium dense, silty-sand with clasts of weathered sandstone (Weathered Renton Formation), which continued to the termination depth of the boring. Borings B-2 and B-3 In Borings B-2 and B-3, we encountered approximately 10 to 12 inches of vegetation and topsoil underlain by approximately 5 feet of medium dense, silty- fine to medium grained sand with variable amounts of gravel (Fill). This layer was underlain by dense to very dense, silty-fine to medium grained sand with variable amounts of gravel (Glacial Till), which continued to the termination depths of these borings. Boring B-4 In Boring B-4, we encountered approximately 8 inches of angular rock underlain by approximately 4 feet of loose to medium dense, silty-fine to medium grained TECHNICAL MEMORANDUM No. 4 February 13, 2015 rs v:\2002\active\2002003607\civil\design\report\mem_th geotech investig_20150210.docx 5 sand with variable amounts of gravel (Fill). This layer was underlain by hard sandstone (Renton Formation), which continued to the termination depth of the boring (refusal). Hand Borings All of the hand borings, generally located on steep slopes east and west of Thunder Hills Creek, encountered several inches of topsoil and vegetation underlain by 1 to 3 feet of loose, silty-sand to silt with sand (Colluvium derived from Renton Formation). These materials were underlain by hard sandstone (Slightly Weathered and Weathered Renton Formation). Previous Borings and Hand Borings The borings and hand borings conducted in May, 2010 by S & EE encountered similar materials at similar depths and densities as those in our investigation. We have utilized some of this data to complete our slope stability analyses and gravity wall design in the northern portion of the site, where their explorations were focused. Groundwater At the time of our investigation, groundwater was encountered in Boring B-4 at approximately 4.5 feet below the existing site grade. Groundwater was not encountered in any of the other explorations at the date of our investigation. We anticipate that groundwater in the Thunder Hills Creek valley is primarily influenced by area streams and surface water runoff/infiltrating surface waters. There are areas of the site near I-405 where surface water and groundwater is at the same level (ground surface) and areas where groundwater is not encountered below stream depths due to stream channel confinement within the Renton Formation sandstone (Station 5+50 to 6+50 area). We anticipate low to moderate seasonal seepage from the valley/channel sidewalls contributing to the overall volume in Thunder Hills Creek. A majority of volume contribution to the stream is from drainage conveyance from nearby residential developments and tributary streams. Water levels at the time of the field investigation may be different from those encountered during the construction phase of the project. 6.0 Geologic Hazards 6.1.1 Landslide Hazard & Preliminary Slope Stability Analyses Typically, slopes with magnitudes greater than about 40 percent and vertical relief of at least 10 feet can be classified as geologically hazardous (steep slope/landslide hazards). A majority of the slopes that extend into the Thunder Hills Creek valley meet these criteria. TECHNICAL MEMORANDUM No. 4 February 13, 2015 rs v:\2002\active\2002003607\civil\design\report\mem_th geotech investig_20150210.docx 6 During our field assessment, we traversed the subject property, where accessible. As we conducted the traverses, we looked for any signs that would indicate past slope failures or features indicating possible future instability. We observed several relatively recent landslides in the area north of Station 2+50, primarily along the steep slopes along the west side of Thunder Hills Creek. These slides appear to be relatively shallow, consisting of colluvium sliding off of the underlying sandstone. We did not observe evidence of deep-seated landslide activity within the valley. It is our opinion that the contributing causes for landslide activity in the north portion of Thunder Hills Creek valley include excavations for access roadway construction, surface water and spring/seep activity along the slopes, and the presence of loose colluvium over relatively impermeable, hard sandstone at steep inclinations. The commercially available slope stability computer program Slope/W was used to preliminarily evaluate the local stability of the lower portion of the existing slopes west of Thunder Hills Creek extending east toward the creek at each of six cross sections (Figures 4 through 9). The slope stability was analyzed under static and seismic (pseudo- static method) conditions for existing conditions and with a proposed gravity retaining wall in place between the access roadway and creek. The computer program calculates factors of safety for potential slope failures and generates the potential failure planes. This software calculates the slope stability under seismic conditions using pseudo-static methods. The stability of the described configuration was analyzed by comparing observed factors of safety to minimum values as set by standard geotechnical practice. A factor of safety of 1.0 is considered equilibrium and less than 1.0 is considered failure. A typical minimum factor of safety for global stability is 1.3 to 1.5 for static conditions and 1.1 to 1.2 for seismic conditions. In accordance with typical engineering standards, we used a horizontal peak ground acceleration of 0.2g. Based on the presence of loose fill and colluvium over hard sandstone and relatively recent slide activity, we would expect that the current factors of safety are approximately at equilibrium or a 1.0 factor of safety for much of the upper portions of the area slopes. Slope stability analyses are not warranted for the upper portions of the slopes extending into the Thunder Hills Creek valley as their relative stability can be visually assessed. Factors that influence the relative factors of safety along the slope areas include surface water, vegetation and root systems, colluvium/fill density and thickness, and slope magnitude. We anticipate lower factors of safety and higher probability of landslide activity to occur from approximately November to May when precipitation is highest. We conducted slope stability analyses for the lower portion of the slope west of the access roadway extending east through the creek. Our initial analyses indicate that adequate factors of safety currently exist in the area between the access roadway and TECHNICAL MEMORANDUM No. 4 February 13, 2015 rs v:\2002\active\2002003607\civil\design\report\mem_th geotech investig_20150210.docx 7 Thunder Hills Creek under static and seismic conditions. It is our opinion that the sewer line is more susceptible to movement caused by long-term stream erosion rather than shallow landslide activity in the area. 6.1.2 Erosion Hazard The Natural Resources Conservation Services (NRCS) maps for King County indicate that the project area and directly adjacent side slopes are underlain by Alderwood and Kitsap soils (very steep), Alderwood gravelly sandy loam (8 to 30 percent slopes) and Beausite gravelly sandy loam (15 to 30 percent slopes). Since the project is located within an actively incising stream environment adjacent to very steep slope areas, all soils should be considered to have “Severe” to “Very Severe” erosion potential. It is our opinion that soil erosion potential at this project site, if grading activities are proposed, can be reduced through surface water runoff control and local removal of problem soil areas (discussed in Section 8.1). Typically erosion of exposed soils will be most noticeable during periods of rainfall and may be controlled by the use of normal temporary erosion control measures, such as silt fences, hay bales, mulching, control ditches and diversion trenches. The typical wet weather season, with regard to site grading, is from October 31st to April 1st. Erosion control measures should be in place before the onset of wet weather. While erosion of the sandstone that underlies the site between Stations 0+00 and 5+50 will occur at a low to very low rate over the lifespan of the sewer line (80 years), large storm events and long term erosion of the sandstone could erode the existing gabion walls and slope between the stream and sewer line. To reduce adverse effects of soil and slope erosion caused by Thunder Hills Creek, permanent erosion prevention systems should be constructed. Large rock buttressing or gravity walls embedded adequately into the unweathered sandstone should provide adequate protection for the sewer line and access roadway. 6.1.3 Seismic Hazard We encountered generally medium dense to very dense soils and locally soft rock at the project site. The overall subsurface profile corresponds to a Site Class D as defined by Chapter 20 of ASCE 7 (Table 20.3-1) and referenced in Table 1613.3.2 of the 2012 International Building Code (2012 IBC). A Site Class D applies to an overall profile consisting of medium dense/stiff to very dense/hard materials within the upper 100 feet. Areas of the site, including areas directly underlain by soft bedrock (sandstone), would be considered as a Site Class C, soft rock profile. We do not anticipate the need to utilize seismic parameters from this profile as part of the currently proposed project and therefore they have not been included. We referenced the U.S. Geological Survey (USGS) Earthquake Hazards Program Website (seismic calculator) to obtain values for SS, S1, Fa, and Fv. The USGS website includes the most updated published data on seismic conditions. The site specific seismic design parameters and adjusted maximum spectral response acceleration parameters are as follows: TECHNICAL MEMORANDUM No. 4 February 13, 2015 rs v:\2002\active\2002003607\civil\design\report\mem_th geotech investig_20150210.docx 8 PGA (Peak Ground Acceleration, in percent of g) 32.24 (10% Probability of Exceedence in 50 years) 62.52 (2% Probability of Exceedence in 50 years) SS 141.10% of g S1 48.30% of g Additional seismic considerations include liquefaction potential and amplification of ground motions by soft/loose soil deposits. The liquefaction potential is highest for loose sand with a high groundwater table. The dense to very dense, glacially consolidated materials and bedrock that underlie the site have a very low potential for liquefaction. 7.0 Discussion 7.1.1 General It may not be economically feasible to construct preventative structures to eliminate shallow landslide activity which originates higher up the steep slopes on the proposed/existing access roadway or stream. At a minimum, we recommend performing remedial excavation work to reduce the likelihood and adverse effects of shallow colluvial slides on the proposed/existing access path along with select hazard tree removal and drainage improvements below the slope. Gravity retaining walls or rock buttresses should be constructed between the access roadway and stream to prevent erosion/undercutting of the roadway and sewer line by Thunder Hills Creek over the design lifespan of the sewer line (approximately 80 years). It is our opinion that rock buttresses or gravity walls will be the most economical method to prevent lateral soil movement and significant stream erosion in the area between the existing (or proposed/new) sewer line and Thunder Hills Creek. We anticipate that significant grading may be necessary in the vicinity of Station 4+70 to 5+40 to create roadway grades suitable for the maintenance equipment. In this area, additional excavation work may be necessary to repair sloughed soils and erosion channels. Also, gravity walls in this area may be required to support the access roadway. Depending on grading configurations, these walls may be over 8 feet in height. Once the final grading plans and road width/locations have been determined, we can provide specific wall construction recommendations. 8.0 Recommendations 8.1 Site Preparation In general, site preparation should consist of vegetation and topsoil removal from proposed excavation/improvement areas. Based on observations from the site investigation program and site reconnaissance work, it is anticipated that the stripping TECHNICAL MEMORANDUM No. 4 February 13, 2015 rs v:\2002\active\2002003607\civil\design\report\mem_th geotech investig_20150210.docx 9 depth will generally be less than 12 inches where topsoil and vegetation are present. The excavated material is not suitable as structural fill but could be used as fill material in non-settlement sensitive areas such as landscaping. In these non-settlement sensitive areas, the fill should be placed in maximum 12 inch thick lifts that should be compacted to at least 90 percent of the modified proctor (ASTM D 1557 Test Method) maximum dry density. As needed, leaning trees and other trees designated as hazard trees located in critical areas, may be removed during site preparation. It may be useful to leave root systems in place depending on the location of the hazard trees. We can provide recommendations on which trees are suitable for full removal or partial removal upon request. The existing roadway alignment is underlain by approximately 4 to 14 feet of fill. The fill consists of silty-sand and silt with variable amounts of sand. Areas of woody debris and other fill debris occur locally. These materials are generally considered suitable for use as structural fill provided they are within 3 percent of the optimum moisture content. It should be noted that these materials are typically suitable for structural fill only during the summer months and are highly moisture sensitive due to their fines content. Imported structural fill should consist of a sand and gravel mixture with a maximum grain size of 3 inches and less than 5 percent fines (material passing the U.S. Standard No. 200 Sieve). Structural fill should be placed in maximum lift thicknesses of 12 inches and should be compacted to a minimum of 95 percent of the modified proctor maximum dry density, as determined by the ASTM D 1557 test method. 8.2 Temporary Excavations Based on our understanding of the project, grading associated with access road and retaining wall construction could include significant cuts and/or fills. Preliminarily, temporary excavations should be sloped no steeper than 1.5H:1V (Horizontal:Vertical) in medium dense fill soils and 1H:1V in medium dense to dense native soils. Locally, steeper cuts may be feasible; however, Stantec should be on site to provide specific recommendations at that time. If an excavation is subject to heavy vibration or surcharge loads, we recommend that the excavation be sloped no steeper than 2H:1V and 1.5H:1V, respectively as above, where room permits. If groundwater is encountered, lower declinations may be required. In general, excavations in slightly weathered sandstone may be stable up to a vertical condition; however, we do not anticipate the need to excavate into sandstone other than removing loose colluvium from existing slopes along the west side of the proposed access roadway. In these areas, we recommend scraping the loose materials and any loose sandstone that readily comes free from the rock faces only. Again, Stantec TECHNICAL MEMORANDUM No. 4 February 13, 2015 rs v:\2002\active\2002003607\civil\design\report\mem_th geotech investig_20150210.docx 10 should be on site to observe the conditions during construction and provide location- specific recommendations. All temporary cuts should be in accordance with the Washington Administrative Code (WAC) Part N, Excavation, Trenching, and Shoring. The temporary slopes should be visually inspected daily by a qualified person during construction activities and the inspections should be documented in daily reports. The contractor is responsible for maintaining the stability of the temporary cut slopes and reducing slope erosion during construction. The temporary cut slopes should be covered with visqueen to help reduce erosion during wet weather, and the slopes should be closely monitored until the permanent retaining systems or slope configurations are complete. Materials should not be stored or equipment operated within 10 feet of the top of any temporary cut slope. Soil conditions may not be completely known from the geotechnical investigation. In the case of temporary cuts, the existing soil conditions may not be completely revealed until the excavation work exposes the soil. Typically, as excavation work progresses, the maximum inclination of the temporary slopes will need to be re-evaluated by the geotechnical engineer so that supplemental recommendations can be made. Soil and groundwater conditions can be highly variable. Scheduling for soil work will need to be adjustable, to deal with unanticipated conditions, so that the project can proceed and required deadlines can be met. If any variations or undesirable conditions are encountered during construction, Stantec should be notified so that supplemental recommendations can be made. If room constraints or groundwater conditions do not permit temporary slopes to be cut to the maximum angles allowed by the WAC, temporary shoring systems may be required. The contractor should be responsible for developing temporary shoring systems, if needed. We recommend that Stantec and the project structural engineer review temporary shoring designs prior to installation, to verify the suitability of the proposed systems. 8.3 Erosion and Sediment Control Erosion and sediment control (ESC) is used to reduce the transportation of eroded sediment to wetlands, streams, lakes, drainage systems, and adjacent properties. Erosion and sediment control measures should be implemented and these measures should be in general accordance with local regulations. At a minimum, the following basic recommendations should be incorporated into the design of the erosion and sediment control features for the site:  Schedule the soil, foundation, utility, and other work requiring excavation or the disturbance of the site soils, to take place during the dry season (generally June through September). However, provided precautions are taken using Best Management Practices (BMP’s), certain grading activities can be completed during the wet season in areas of the site upstream from the I-405 to AB-1 area (generally October through April). TECHNICAL MEMORANDUM No. 4 February 13, 2015 rs v:\2002\active\2002003607\civil\design\report\mem_th geotech investig_20150210.docx 11  All site work should be completed and stabilized as quickly as possible.  Additional perimeter erosion and sediment control features may be required to reduce the possibility of sediment entering the surface water. This may include additional silt fences, silt fences with a higher Apparent Opening Size (AOS), construction of a berm, or other filtration systems.  Any runoff generated by dewatering discharge should be treated through construction of a sediment trap if there is sufficient space. If space is limited, other filtration methods will need to be incorporated. Specifically for this project, site grading should only be performed during the summer months (late June through mid-September) when the creek is at it lower levels and surface waters will be less prevalent. Additional erosion control measures will likely be required between the proposed roadway and Thunder Hills Creek during construction due to the presence of wetlands. There are areas where loose colluvium overlies hard sandstone at steep angles between Station 0+50 and 4+00. Depending on the roadway grading plans, access roadway setback from the toe of the slope, and project/maintenance requirements, some soil removal may be necessary. The removal of soils and trees from site slopes should be observed by the geotechnical engineer as slope stability above these locations could be adversely affected. Replacement of loose soils with quarry rock may be warranted. In general, we recommend removal of trees and colluvium only where necessary. 8.4 Retaining Walls As needed, primarily in the area between Stations 1+00 to 5+50 and locally south of the site area extending toward Grant Avenue South, gravity retaining walls or rock buttresses may be utilized to support the access roadway and prevent lateral pipe movements and/or erosion/undercutting of the sewer line. However, any type of retaining structure will only be effective if they are embedded into the underlying sandstone. In other words, fill material or soil deposits that are used to support retaining structures could be easily eroded by the stream during storm events when flows are high in volume and velocity, potentially causing the structures to fail (as observed with the current gabion walls). Based on the existing site grades, we anticipate that the gravity walls in the lower site area will range from 4 to 8 feet in height, with a typical height of 6 feet. Tiered systems may be utilized if there is adequate lateral space. The new gravity walls may also be constructed between the existing gabion walls and access roadway if there is adequate space. There are numerous types of retaining structures that could be utilized in these areas. We can provide alternative options upon request. We anticipate that Ultra block or ecology block walls will be the most economical for the proposed construction. TECHNICAL MEMORANDUM No. 4 February 13, 2015 rs v:\2002\active\2002003607\civil\design\report\mem_th geotech investig_20150210.docx 12 Gravity Retaining Walls For 2.5 foot by 2.5 foot by 5 foot long Ultra blocks, we recommend a minimum of 1.5 feet of embedment in dense to very dense native soils or hard sandstone. A rock breaker attachment to a trackhoe may be necessary to achieve embedment grades. The wall keyway should be cut to create a 8-10V:1H (Vertical to Horizontal) batter for the walls with level backfill conditions. A maximum 4-inch thick gravel base (leveling course) should be placed and compacted prior to block placement (1-1/4 to 1-1/2 inch minus crushed rock). For an exposed wall height of 6 feet, the lowest level of blocks (1st row) should be placed perpendicular to the face of the wall. The two rows above this level should be placed parallel to the wall direction and interlocking. Figures 10 and 11 shows the layout and generalized wall profiles. The keyway, drainage system, backfill, and block placement should be verified by the geotechnical engineer. A minimum 1.5 feet thick drainage/backfill zone should extend behind the back of the wall down to the base of the excavation. Mirafi N140 filter fabric (or equivalent) should be placed between the excavated area of roadway and drainage/backfill zone and extend over the drainage zone. This area should be backfilled with clean angular rock, 2 to 8 inches in size. A minimum 4-inch diameter perforated PVC pipe (Schedule 40) should be installed behind the wall at its base. At least 6 inches of clean washed rock (1 to 2 inches in size) should surround the pipe and the pipe should be sloped to drain from the behind the walls utilizing perpendicular drains placed below the wall approximately every 25 feet along the length of the wall. Additional drainage systems could be installed across the access roadway and connected to the wall drains if desired. Stantec should be on site during all wall construction activities, including keyway excavation work, drainage placement, block and quarry rock placement, and slope grading. Rock Buttresses In lieu of constructing gravity walls, large quarry rock could be used to stabilize eroding areas and replace failing gabion walls. The usefulness of the existing gabion walls over time is limited. Since their construction is unknown, we recommend removing the gabion walls prior to rock buttress placement. Much of the angular rock backfill may be utilized between large rocks. We recommend that quarry rock used as buttress material be 4-man sized or larger for applications between Stations 4+50 and 5+50. North of Station 4+50, smaller rock may be utilized. Once the location of the access roadway (width, elevation, and setback) has been determined, we can provide specific buttress recommendations. In general, buttressing should consist of the following:  Loose soil removal and temporary excavation creation (generally between 1H:1V and 2H:1V) TECHNICAL MEMORANDUM No. 4 February 13, 2015 rs v:\2002\active\2002003607\civil\design\report\mem_th geotech investig_20150210.docx 13  Excavation of a keyway at the toe of the buttress (at least 18 inches into underlying dense soils or weathered sandstone)  Placement of large angular basalt with interstitial quarry rock (as void fill) Other Gravity Retaining Walls Shorter gravity walls or rockeries (where soil conditions are suitable), on the order of 8 feet or less, are necessary along the upslope side (east of the access roadway south of this site area. During our field assessments, we observed undermined cuts and local near-vertical excavations along the upslope (east) side of the access roadway at several locations and we understand that this roadway may be widened in the near future. General locations of necessary gravity walls are as follows: Location Anticipated Height 15+80 to 18+10 Up to 8 feet 19+55 to 20+65 4 feet or under 21+30 to 23+25 4 feet 24+10 to 25+60 Up to 6 feet Additionally, short gravity walls or rockeries may be needed between Stations 12+70 and 13+10 and 18+10 and 19+00 depending on the final grading plans. We can provide final wall recommendations during the next phase of the project. 8.5 Utilities Sewer line trenches should be excavated according to accepted engineering practices following OSHA (Occupational Safety and Health Administration) standards, by a contractor experienced in such work. The contractor is responsible for the safety of open trenches. Traffic and vibration adjacent to trench walls should be reduced; cyclic wetting and drying of excavation side slopes should be avoided. Depending upon the location and depth of some utility trenches, groundwater flow into open excavations could be experienced, especially during or shortly following periods of precipitation. In general, silty and gravelly fill soils, as well as fine-grained native soils were encountered at shallow depths in the explorations at this site. At this site, these soils have variable density and minimal cohesion and will have a tendency to cave or slough in excavations. Shoring or sloping back trench sidewalls is required within these soils. If sewer line excavations extend deep enough to encounter sandstone (not anticipated), rock chipping/breaking equipment would likely be required to allow for excavation. Excavations in sandstone should be adequately safe to remain vertical for a significant amount of time. TECHNICAL MEMORANDUM No. 4 February 13, 2015 rs v:\2002\active\2002003607\civil\design\report\mem_th geotech investig_20150210.docx 14 All utility trench backfill should consist of imported structural fill. Certain on site soils may be suitable for use as backfill in landscaping areas during the summer months; however, we should evaluate these soils at that time to determine their moisture levels. The upper 5 feet of utility trench backfill placed in pavement areas should be compacted to at least 95 percent of the maximum dry density based on ASTM Test Method D1557. Below 5 feet and in landscaping areas, utility trench backfill in pavement areas should be compacted to at least 90 percent of the maximum dry density based on ASTM Test Method D1557. Pipe bedding should be in accordance with the pipe manufacturer's recommendations. The contractor is responsible for removing all water-sensitive soils from the trenches regardless of the backfill location and compaction requirements. Depending on the depth and location of the proposed utilities, we anticipate the need to re-compact existing fill soils below the utility structures and pipes. The contractor should use appropriate equipment and methods to avoid damage to the utilities and/or structures during fill placement and compaction procedures. 8.6 Utility Protection Between Stations 1+00 and 5+50, the proposed gravity retaining wall(s) or rock buttresses will be effective in protecting the sewer line from stream erosion. South of Station 5+50, the Thunder Hills Creek valley has variably incised sides (to near vertical) and the channel is filled with sediments (to variable depths). Erosion hazard potential and recommendations related to sewer interceptor protection south of Station 5+50 can be found in our Preliminary Erosion Hazard Evaluation report dated February 13, 2015. The existing sewer line crosses Thunder Hills Creek near Station 11+80 at an estimated depth of 7 feet. For sewer line locations that are at or above bottom of stream channel elevations, additional backfill erosion measures may be necessary, depending on the alternative that is chosen. Once an alignment and profile have been developed, we can finalize our recommendations; however, the following options may be considered as part of erosion protection for the sewer line:  Angular rock backfill placement (2 to 4 inch sized quarry rock)  Lean mixed concrete placement around sewer lines  Localized ecology/Ultra block wall placement along sewer lines In general, Thunder Hills Creek could adversely affect the sewer line through severe erosion and undercutting in localized areas. Our observations and analyses indicate that there are areas where groundwater is not present below stream levels in the vicinity of the existing sewer line, indicating that the channel is confined and not likely to create issues for the sewer line. For the alternative that includes lining of the existing sewer line, we do not anticipate the need to add specific erosion protection other than gravity walls/buttresses between TECHNICAL MEMORANDUM No. 4 February 13, 2015 rs v:\2002\active\2002003607\civil\design\report\mem_th geotech investig_20150210.docx 15 the access roadway and stream from Stations 0+00 to 5+50 (this site area). Analysis of stream erosion effects on the existing sewer line south of 5+50 can be found in our Preliminary Erosion Hazard Evaluation report. If a new sewer line is proposed, there is a potential need for utility protection in areas where the sewer line crosses Thunder Hills Creek, or a combination of being located within 10 feet laterally of Thunder Hills Creek and at a depth of 3 feet below stream level or deeper. We should review the preliminary plans in order to provide comments regarding utility erosion protection recommendations. 8.7 Groundwater Influence on Construction At the time of our investigation, groundwater was encountered in Boring B-4 at approximately 4.5 feet below the existing site grade. Groundwater was not encountered in any of the other explorations at the date and time of our investigation. We anticipate that groundwater in the Thunder Hills Creek valley is primarily influenced by area streams and surface water runoff/infiltrating surface waters. There are areas of the site near I-405 where surface water and groundwater is at the same level (ground surface) and areas where groundwater is not encountered below stream depths due to stream channel confinement within the Renton Formation sandstone (Station 5+50 to 6+50 area). We would expect groundwater to be locally encountered at sewer line depths south of Station 7+00 and north of Station 5+00. If groundwater is encountered in trenches south of Station 7+00, we would expect it to be found at or near the elevations of Thunder Hills Creek. There may be areas where groundwater is not encountered due to channel confinement, similar to our observations in the area of Station 6+00. When groundwater is encountered, we would expect moderate to heavy seepage. North of Station 5+00, we would anticipate light to moderate seepage from trench sidewalls extending from the ground surface down to the level of the sandstone (upper 4 to 5 feet). If temporary dewatering systems to remove groundwater are used, their design should be the responsibility of the contractor. We should review any dewatering design prior to their use on site. 9.0 Construction Field Reviews Stantec should be retained to provide part time field review during construction in order to verify that the soil conditions encountered are consistent with our design assumptions and that the intent of our recommendations is being met. This will require field and engineering review to:  Observe all aspects of gravity wall, rock buttress, and access roadway construction  Monitor temporary excavations, slope stability, and grading activities TECHNICAL MEMORANDUM No. 4 February 13, 2015 rs v:\2002\active\2002003607\civil\design\report\mem_th geotech investig_20150210.docx 16  Density testing to verify compaction of structural fills Geotechnical design services should also be anticipated during the subsequent final design phase to support the structural design and address specific issues arising during this phase. Field and engineering review services will also be required during the construction phase in order to provide a Final Letter for the project. 10.0 Closure This report was prepared for the exclusive use of the City of Renton and their appointed consultants. Any use of this report or the material contained herein by third parties, or for other than the intended purpose, should first be approved in writing by Stantec. The recommendations contained in this report are based on limited data from provided test holes, and preliminary proposed construction. Additional exploration work is warranted and necessary to provide engineering parameters and recommendations for sewer line placement. Use of this report is subject to the Statement of General Conditions provided in Appendix A. It is the responsibility of the City of Renton who is identified as “the Client” within the Statement of General Conditions, and its agents to review the conditions and to notify Stantec should any of these not be satisfied. TECHNICAL MEMORANDUM NO. 4 February 13, 2015 APPENDIX A Statement of General Conditions rs \\us1314-f01\workgroup\2002\active\2002003607\civil\design\report\mem_th geotech investig_20150210.docx TECHNICAL MEMORANDUM No. 4 February 13, 2015 rs \\us1314-f01\workgroup\2002\active\2002003607\civil\design\report\mem_th geotech investig_20150210.docx TECHNICAL MEMORANDUM NO. 4 February 13, 2015 Statement of General Conditions USE OF THIS REPORT: This report has been prepared for the sole benefit of the Client or its agent and may not be used by any third party without the express written consent of Stantec Consulting Services, Inc. and the Client. Any use which a third party makes of this report is the responsibility of such third party. BASIS OF THE REPORT: The information, opinions, and/or recommendations made in this report are in accordance with Stantec Consulting Services, Inc.’s present understanding of the site specific project as described by the Client. The applicability of these is restricted to the site conditions encountered at the time of the investigation or study. If the proposed site specific project differs or is modified from what is described in this report or if the site conditions are altered, this report is no longer valid unless Stantec Consulting Services, Inc. is requested by the Client to review and revise the report to reflect the differing or modified project specifics and/or the altered site conditions. STANDARD OF CARE: Preparation of this report, and all associated work, was carried out in accordance with the normally accepted standard of care in the state of execution for the specific professional service provided to the Client. No other warranty is made. INTERPRETATION OF SITE CONDITIONS: Soil, rock, or other material descriptions, and statements regarding their condition, made in this report are based on site conditions encountered by Stantec Consulting Services, Inc. at the time of the work and at the specific testing and/or sampling locations. Classifications and statements of condition have been made in accordance with normally accepted practices which are judgmental in nature; no specific description should be considered exact, but rather reflective of the anticipated material behavior. Extrapolation of in situ conditions can only be made to some limited extent beyond the sampling or test points. The extent depends on variability of the soil, rock and groundwater conditions as influenced by geological processes, construction activity, and site use. VARYING OR UNEXPECTED CONDITIONS: Should any site or subsurface conditions be encountered that are different from those described in this report or encountered at the test locations, Stantec Consulting Services, Inc. must be notified immediately to assess if the varying or unexpected conditions are substantial and if reassessments of the report conclusions or recommendations are required. Stantec Consulting Services, Inc. will not be responsible to any party for damages incurred as a result of failing to notify Stantec Consulting Services, Inc. that differing site or sub-surface conditions are present upon becoming aware of such conditions. PLANNING, DESIGN, OR CONSTRUCTION: Development or design plans and specifications should be reviewed by Stantec Consulting Services, Inc., sufficiently ahead of initiating the next project stage (property acquisition, tender, construction, etc), to confirm that this report completely addresses the elaborated project specifics and that the contents of this report have been properly interpreted. Specialty quality assurance services (field observations and testing) during construction are a necessary part of the evaluation of sub-subsurface conditions and site preparation works. Site work relating to the recommendations included in this report should only be carried out in the presence of a qualified geotechnical engineer; Stantec Consulting Services, Inc. cannot be responsible for site work carried out without being present. rs \\us1314-f01\workgroup\2002\active\2002003607\civil\design\report\mem_th geotech investig_20150210.docx TECHNICAL MEMORANDUM No. 4 February 13, 2015 rs \\us1314-f01\workgroup\2002\active\2002003607\civil\design\report\mem_th geotech investig_20150210.docx TECHNICAL MEMORANDUM No. 4 February 13, 2015 APPENDIX B: FIGURES Vicinity Map (Figure 1) Site Plans (Figures 2 & 3) Cross Sections (Figures 4 – 9) Gravity Wall Schematics (Figures 10 & 11) rs \\us1314-f01\workgroup\2002\active\2002003607\civil\design\report\mem_th geotech investig_20150210.docx TECHNICAL MEMORANDUM NO. 4 February 13, 2015 rs \\us1314-f01\workgroup\2002\active\2002003607\civil\design\report\mem_th geotech investig_20150210.docx SITE N VICINITY MAP FIGURE 1 11130 NE 33rd Place, Suite 200 Bellevue, WA 98004 (425) 869-9448 (425) 869-1190 (Fax) www.stantec.com Project Location Renton WASHINGTON Thunder Hills Creek Renton, Washington Dec., 2014 2002003607 SITE PLAN FIGURE 2 11130 NE 33rd Place, Suite 200 Bellevue, WA 98004 (425) 869-9448 (425) 869-1190 (Fax) www.stantec.com PB-1 PB-2 PB-3 3015015 (In Feet) 1 inch = 30 feet Approximate Graphic Scale A B C A’ B’ C’ B-4 B-3 Exposed Sandstone Exposed Sandstone Exposed Sandstone Exposed Sandstone & Near Vertical Slopes Older Slide Older Slide Approximate Thunder Hills Creek Channel Approximate Boring Location Approximate Boring Location (P&EE) Large Area of Exposed Sandstone B-1 PB-1 Cross Section A A’ Thunder Hills Creek Renton, Washington Dec., 2014 2002003607 N Wetland E I-405 1+00 4+00 11130 NE 33rd Place, Suite 200 Bellevue, WA 98004 (425) 869-9448 (425) 869-1190 (Fax) www.stantec.com SITE PLAN FIGURE 3 3015015 (In Feet) 1 inch = 30 feet Approximate Graphic Scale PB-6 PB-5 PB-4PB-3 B-1 B-2 D D’ E E’ F F’ Exposed Sandstone “New” Stream Joining THC Exposed Sandstone Approximate Thunder Hills Creek Channel Approximate Boring Location Approximate Boring Location (P&EE) Large Area of Exposed Sandstone B-1 PB-1 Cross Section A A’ Thunder Hills Creek Renton, Washington Dec., 2014 2002003607 N 6+00 7+004+00 11130 NE 33rd Place, Suite 200 Bellevue, WA 98004 (425) 869-9448 (425) 869-1190 (Fax) www.stantec.com CROSS SECTION A - A’ FIGURE 4 Thunder Hills Creek Renton, Washington Dec., 2014 2002003607 10505 (In Feet) 1 inch = 10 feet Approximate Graphic Scale A 80 90 100 Elevation (Feet) 110 120 130 C Colluvium (Primarily Loose Silty Sand (SM)) F Fill/Debris (Loose to Medium Dense Silty Sand (SM) and Silt with Sand (ML)) T Vashon Glacial Till (Generally Medium Dense to Dense Silty Sand with Gravel (SM)) W Highly Weathered Renton Formation (Generally Medium Dense Silty Sand (SM) to Stiff Sandy Silt (ML)) R Renton Formation (Slightly Weathered Sandstone) C C RR F T Approximate Sewer Line Location A’ Observed/Interpreted Surface/Stream Water Elevation Interpreted Ground Surface Ground Surface (Surveyed) Approximate Geologic Contact Approximate Location of Existing Gabion Wall 11130 NE 33rd Place, Suite 200 Bellevue, WA 98004 (425) 869-9448 (425) 869-1190 (Fax) www.stantec.com CROSS SECTION B - B’ FIGURE 5 Thunder Hills Creek Renton, Washington Dec., 2014 2002003607 10505 (In Feet) 1 inch = 10 feet Approximate Graphic Scale 80 90 100 Elevation (Feet) B’ 110 120 130 140 C Colluvium (Primarily Loose Silty Sand (SM)) F Fill/Debris (Loose to Medium Dense Silty Sand (SM) and Silt with Sand (ML)) T Vashon Glacial Till (Generally Medium Dense to Dense Silty Sand with Gravel (SM)) W Highly Weathered Renton Formation (Generally Medium Dense Silty Sand (SM) to Stiff Sandy Silt (ML)) R Renton Formation (Slightly Weathered Sandstone) R C/W C T R F Approximate Sewer Line Location Observed/Interpreted Surface/Stream Water Elevation Interpreted Ground Surface Ground Surface (Surveyed) Approximate Geologic Contact Approximate Location of Existing Gabion Wall 11130 NE 33rd Place, Suite 200 Bellevue, WA 98004 (425) 869-9448 (425) 869-1190 (Fax) www.stantec.com CROSS SECTION C - C’ FIGURE 6 Thunder Hills Creek Renton, Washington Dec., 2014 2002003607 10505 (In Feet) 1 inch = 10 feet Approximate Graphic Scale 100 Elevation (Feet) 110 120 130 140 C C’ C Colluvium (Primarily Loose Silty Sand (SM)) F Fill/Debris (Loose to Medium Dense Silty Sand (SM) and Silt with Sand (ML)) T Vashon Glacial Till (Generally Medium Dense to Dense Silty Sand with Gravel (SM)) W Highly Weathered Renton Formation (Generally Medium Dense Silty Sand (SM) to Stiff Sandy Silt (ML)) R Renton Formation (Slightly Weathered Sandstone) T R C F C R T Approximate Sewer Line Location Observed/Interpreted Surface/Stream Water Elevation Interpreted Ground Surface Ground Surface (Surveyed) Approximate Geologic Contact Approximate Location of Existing Gabion Wall 11130 NE 33rd Place, Suite 200 Bellevue, WA 98004 (425) 869-9448 (425) 869-1190 (Fax) www.stantec.com CROSS SECTION D - D’ FIGURE 7 Thunder Hills Creek Renton, Washington Dec., 2014 2002003607 10505 (In Feet) 1 inch = 10 feet Approximate Graphic Scale Elevation (Feet) 110 120 130 140 150 D’ C Colluvium (Primarily Loose Silty Sand (SM)) F Fill/Debris (Loose to Medium Dense Silty Sand (SM) and Silt with Sand (ML)) T Vashon Glacial Till (Generally Medium Dense to Dense Silty Sand with Gravel (SM)) W Highly Weathered Renton Formation (Generally Medium Dense Silty Sand (SM) to Stiff Sandy Silt (ML)) R Renton Formation (Slightly Weathered Sandstone) T R C F R C T Approximate Sewer Line Location Observed/Interpreted Surface/Stream Water Elevation Interpreted Ground Surface Ground Surface (Surveyed) Approximate Geologic Contact Approximate Location of Existing Gabion Wall 11130 NE 33rd Place, Suite 200 Bellevue, WA 98004 (425) 869-9448 (425) 869-1190 (Fax) www.stantec.com CROSS SECTION E - E’ FIGURE 8 Thunder Hills Creek Renton, Washington Dec., 2014 2002003607 10505 (In Feet) 1 inch = 10 feet Approximate Graphic Scale 130 140 Elevation (Feet) 150 E’ C Colluvium (Primarily Loose Silty Sand (SM)) F Fill/Debris (Loose to Medium Dense Silty Sand (SM) and Silt with Sand (ML)) T Vashon Glacial Till (Generally Medium Dense to Dense Silty Sand with Gravel (SM)) W Highly Weathered Renton Formation (Generally Medium Dense Silty Sand (SM) to Stiff Sandy Silt (ML)) R Renton Formation (Slightly Weathered Sandstone) T F W R R W T C/W Approximate Sewer Line Location Observed/Interpreted Surface/Stream Water Elevation Interpreted Ground Surface Ground Surface (Surveyed) Approximate Geologic Contact F 11130 NE 33rd Place, Suite 200 Bellevue, WA 98004 (425) 869-9448 (425) 869-1190 (Fax) www.stantec.com CROSS SECTION F - F’ FIGURE 9 Thunder Hills Creek Renton, Washington Dec., 2014 2002003607 Elevation (Feet) 140 150 160 F’ C Colluvium (Primarily Loose Silty Sand (SM)) F Fill/Debris (Loose to Medium Dense Silty Sand (SM) and Silt with Sand (ML)) T Vashon Glacial Till (Generally Medium Dense to Dense Silty Sand with Gravel (SM)) W Highly Weathered Renton Formation (Generally Medium Dense Silty Sand (SM) to Stiff Sandy Silt (ML)) R Renton Formation (Slightly Weathered Sandstone) T W R F W T R Approximate Sewer Line Location Observed/Interpreted Surface/Stream Water Elevation Interpreted Ground Surface Ground Surface (Surveyed) Approximate Geologic Contact 10505 (In Feet) 1 inch = 10 feet Approximate Graphic Scale 11130 NE 33rd Place, Suite 200 Bellevue, WA 98004 (425) 869-9448 (425) 869-1190 (Fax) www.stantec.com GRAVITY WALL SCHEMATIC FIGURE 10 11130 NE 33rd Place, Suite 200 Bellevue, WA 98004 (425) 869-9448 (425) 869-1190 (Fax) www.stantec.com Min. 4 Inch Diameter Perforated PVC Pipe (Schedule 40, Sloped to Drain Mirafi 140N Filter Fabric or Equivalent 1 to 2 Inch Diameter Washed Rock 2 to 4 Inch Angular Quarry Rock 1-1/4 Inch Crushed Rock Base (4 Inch Thickness) 1 Min. 8 Max. 1.5 Ft. Min. Not to Scale 6 Inches Minimum Thunder Hills Creek Renton, Washington Dec., 2014 2002003607 NOTES: For use with 2.5’x2.5’x5’ interlocking concrete blocks Intended for use between Thunder Hills Creek and Sewer Line/Access Roadway Rock base to be compacted to at least 95% of the modified proctor (ASTM D1557 Test Method) Stantec to verify keyway, drainage, backfill, soil conditions, and block installation during construction Medium Dense Road Fill (Silty Sand/Rock) Max. 2H:1V Slope 2’ Max. Ht. Sandstone Thunder Hills Creek 11130 NE 33rd Place, Suite 200 Bellevue, WA 98004 (425) 869-9448 (425) 869-1190 (Fax) www.stantec.com 11130 NE 33rd Place, Suite 200 Bellevue, WA 98004 (425) 869-9448 (425) 869-1190 (Fax) www.stantec.com GRAVITY WALL SCHEMATIC FIGURE 11 11130 NE 33rd Place, Suite 200 Bellevue, WA 98004 (425) 869-9448 (425) 869-1190 (Fax) www.stantec.com Min. 4 Inch Diameter Perforated PVC Pipe (Schedule 40, Sloped to Drain Mirafi 140N Filter Fabric or Equivalent 1 to 2 Inch Diameter Washed Rock 2 to 4 Inch Angular Quarry Rock Medium Dense Road Fill (Silty Sand/Rock) Sandstone 1-1/4 Inch Crushed Rock Base (4 Inch Thickness) 1 Min. 8 Max. NOTES: For use with 2.5’x2.5’x5’ interlocking concrete blocks Intended for use between Thunder Hills Creek and Sewer Line/Access Roadway Rock base to be compacted to at least 95% of the modified proctor (ASTM D1557 Test Method) Stantec to verify keyway, drainage, backfill, soil conditions, and block installation during construction Thunder Hills Creek 1.5 Ft. Min. Not to Scale 6 Inches Minimum Max. 2H:1V Slope 2’ Max. Ht. Thunder Hills Creek Renton, Washington Dec., 2014 2002003607 TECHNICAL MEMORANDUM NO. 4 February 13, 2015 APPENDIX C Boring Logs rs \\us1314-f01\workgroup\2002\active\2002003607\civil\design\report\mem_th geotech investig_20150210.docx Vegetation/Topsoil SM; Medium dense to dense, silty-sand with variable amounts of gravel and debris, dark yellowish brown to grayish brown, moist to very moist. (Fill) ML; Stiff to very stiff, silt with variable amounts of sand, trace gravel, trace debris, trace woody debris, grayish brown to olive gray, moist to very moist. (Fill) ML; Stiff, silt with variable amounts of sand, trace gravel, trace woody debris, olive gray, moist. (Highly Weathered Renton Formation) SM; Medium dense, silty-sand, tan to yellow clasts of highly weathered sandstone, moist. (Renton Formation - Slightly Weathered) Borehole terminated at 25 feet. SM ML ML SM 7 6 4 2 16 17 3 3 3 7 11 12 2 4 3 3 4 6 2 3 5 5 4 7 INITIAL DTW (ft):Not Encountered WELL CASING DIA. (in):--- STARTED B-1 EXCAVATION COMPANY:CN EQUIPMENT:Limited Access DEPTH (ft):25.0 BORING NO.: SAMPLING EQUIPMENT:Split Spoon CHECKED BY:GS Time&Depth(feet)5 10 15 20 25 PROJECT NUMBER:2002003607 PROJECT:Thunder Hills Interceptor STATIC DTW (ft):Not Encountered LONG:LAT:COMPLETED:10/17/14 10/17/14 LOGGED BY:PH GROUND ELEV (ft): TOC ELEV (ft): LOCATION:Renton, WA Description NORTHING (ft): EASTING (ft): PAGE 1 OF 1 USCSGraphicLogMETHOD:HSA SIZE:6 WELL DEPTH (ft):--- EXCAVATION / INSTALLATION:GEOFORM304THUNDERHILLS.GPJSTANTECENVIROTEMPLATE010509.GDT2/12/15MeasuredRecov.(feet)Depth(feet)5 10 15 20 25BlowCountSampleHeadspacePID(units)Time Sample ID Topsoil/Vegetation SM; Medium dense, silty-sand with variable amounts of gravel, trace debris, yellowish brown, moist. (Fill) SM; Dense, silty-sand with variable amounts of gravel, sandstone remnants at 8.5-9 feet, yellowish brown to grayish brown, moist. (Glacial Till) Borehole terminated at 9 feet. SM SM 2 3 9 10 15 11 5 14 19 16 17 22 INITIAL DTW (ft):Not Encountered WELL CASING DIA. (in):--- STARTED B-2 EXCAVATION COMPANY:CN EQUIPMENT:Limited Access DEPTH (ft):9.0 BORING NO.: SAMPLING EQUIPMENT:Split Spoon CHECKED BY:GS Time&Depth(feet)5 10 PROJECT NUMBER:2002003607 PROJECT:Thunder Hills Interceptor STATIC DTW (ft):Not Encountered LONG:LAT:COMPLETED:10/20/14 10/20/14 LOGGED BY:PH GROUND ELEV (ft): TOC ELEV (ft): LOCATION:Renton, WA Description NORTHING (ft): EASTING (ft): PAGE 1 OF 1 USCSGraphicLogMETHOD:HSA SIZE:6 WELL DEPTH (ft):--- EXCAVATION / INSTALLATION:GEOFORM304THUNDERHILLS.GPJSTANTECENVIROTEMPLATE010509.GDT2/12/15MeasuredRecov.(feet)Depth(feet)5 10BlowCountSampleHeadspacePID(units)Time Sample ID Topsoil/Vegetation SM; Medium dense, silty-sand with variable amounts of gravel, trace debris, yellowish brown, moist. (Fill) SM; Dense, silty-sand with variable amounts of gravel, yellowish brown to grayish brown, moist. (Glacial Till) Borehole terminated at 9 feet. SM SM 2 3 5 10 11 10 8 14 15 18 20 22 INITIAL DTW (ft):Not Encountered WELL CASING DIA. (in):--- STARTED B-3 EXCAVATION COMPANY:CN EQUIPMENT:Limited Access DEPTH (ft):9.0 BORING NO.: SAMPLING EQUIPMENT:Split Spoon CHECKED BY:GS Time&Depth(feet)5 10 PROJECT NUMBER:2002003607 PROJECT:Thunder Hills Interceptor STATIC DTW (ft):Not Encountered LONG:LAT:COMPLETED:10/20/14 10/20/14 LOGGED BY:PH GROUND ELEV (ft): TOC ELEV (ft): LOCATION:Renton, WA Description NORTHING (ft): EASTING (ft): PAGE 1 OF 1 USCSGraphicLogMETHOD:HSA SIZE:6 WELL DEPTH (ft):--- EXCAVATION / INSTALLATION:GEOFORM304THUNDERHILLS.GPJSTANTECENVIROTEMPLATE010509.GDT2/12/15MeasuredRecov.(feet)Depth(feet)5 10BlowCountSampleHeadspacePID(units)Time Sample ID Quarry rock SM; Loose to medium dense, silty-sand with variable amounts of gravel, trace debris, yellowish brown, moist to wet. (Fill) SM; Dense to hard, slightly weathered sandstone, yellowish brown to tan, moist. (Renton Formation) Borehole terminated at 6 feet. SM SM 2 5 4 8 12 11 50 INITIAL DTW (ft):Not Encountered WELL CASING DIA. (in):--- STARTED B-4 EXCAVATION COMPANY:CN EQUIPMENT:Limited Access DEPTH (ft):5.0 BORING NO.: SAMPLING EQUIPMENT:Split Spoon CHECKED BY:GS Time&Depth(feet)5 PROJECT NUMBER:2002003607 PROJECT:Thunder Hills Interceptor STATIC DTW (ft):Not Encountered LONG:LAT:COMPLETED:10/20/14 10/20/14 LOGGED BY:PH GROUND ELEV (ft): TOC ELEV (ft): LOCATION:Renton, WA Description NORTHING (ft): EASTING (ft): PAGE 1 OF 1 USCSGraphicLogMETHOD:HSA SIZE:6 WELL DEPTH (ft):--- EXCAVATION / INSTALLATION:GEOFORM304THUNDERHILLS.GPJSTANTECENVIROTEMPLATE010509.GDT2/12/15MeasuredRecov.(feet)Depth(feet)5BlowCountSampleHeadspacePID(units)Time Sample ID