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Geotech.pdf February 21, 2017 Prepared for BHC Consultants, LLC 130 2nd Avenue South Edmonds, WA 98020 (425) 778-0907 Landau Associates 4!",% /& #/.4%.43 Page 1.0 INTRODUCTION .............................................................................................................................. 1-1 1.1 Project Description ............................................................................................................. 1-1 1.2 Scope of Services ................................................................................................................ 1-1 2.0 SITE CONDITIONS ........................................................................................................................... 2-1 2.1 Geologic Setting .................................................................................................................. 2-1 2.2 Surface Conditions .............................................................................................................. 2-1 2.3 Subsurface Soil Conditions ................................................................................................. 2-1 2.4 Groundwater Condition...................................................................................................... 2-2 3.0 CONCLUSIONS AND RECOMMENDATIONS .................................................................................... 3-1 3.1 Utility Construction ............................................................................................................ 3-1 3.1.1 Construction Dewatering ................................................................................3-1 3.1.2 Trenching and Excavation Support ..................................................................3-1 3.1.3 Pipe Foundation Support ................................................................................3-3 3.1.4 Pipe Bedding and Trench Backfill ....................................................................3-3 3.1.5 Reuse of Site Soils ..........................................................................................3-4 3.1.6 Wet Weather Earthwork .................................................................................3-4 3.2 Trenchless Construction ..................................................................................................... 3-4 3.2.1 Horizontal Directional Drilling .........................................................................3-5 3.2.2 Pipe Reaming .................................................................................................3-6 3.3 Critical Areas and Associated Permitting Implications ....................................................... 3-6 3.4 Geotechnical Implications of Critical Area Work ................................................................ 3-6 3.5 Decommissioning of Piezometers ...................................................................................... 3-7 4.0 REVIEW OF DOCUMENTS AND CONSTRUCTION OBSERVATIONS ................................................. 4-1 5.0 USE OF THIS REPORT ...................................................................................................................... 5-1 6.0 REFERENCES ................................................................................................................................... 6-1 FIGURES Figure 1. Vicinity Map Figure 2. Site and Exploration Plan APPENDICES Appendix A. Field Explorations Appendix B. Laboratory Soil Testing City of Edmonds 1073015.010.011 2017 Sanitary Sewer Replacement Project i February 21, 2017 Landau Associates This page intentionally left blank. City of Edmonds 1073015.010.011 2017 Sanitary Sewer Replacement Project ii February 21, 2017 Landau Associates 1.0 This report summarizes the results of geotechnical engineering services conducted by Landau Associates, Inc. (LAI) for the City of (City) proposed 2017 Sanitary Sewer Replacement Project in Edmonds, Washington. The general project location is shown on the Vicinity Map (Figure 1). The general configuration of the project alignment is shown on the Site and Exploration Plan (Figure 2). This report has been prepared based on our discussions with BHC Consultants, LLC (BHC); a base map of the project alignment provided by BHC; data collected during our field investigation; our familiarity with geologic conditions within the vicinity of the project area; and our experience on similar projects. 1.10±®©¤¢³ $¤²¢±¨¯³¨®­ LAI understands that the City plans to replace about 1,500 lineal feet of old sewer mains within Yost Park and approximately 1,400 lineal feet outside the park (2,900 lineal feet total). The sewer mains that will be replaced are located along portions of Pioneer Way, Shell Valley Road, and Maple Street. Existing sewer mains that follow existing paths within Yost Park will also be replaced. We understand that the sewer mains that will be replaced are generally located between about 8 and 10 ft below the ground surface (bgs); however, there are a few manholes that are 13 ft deep and one that is 21.5 ft deep. We also understand that trenchless installation techniques such as horizontal directional drilling (HDD) and pipe reaming will be considered during the design process. 1.23¢®¯¤ ®¥ 3¤±µ¨¢¤² BHC retained LAI to provide geotechnical engineering services to support design of the project. Our services were provided in general accordance with the terms of a Subconsultant Services Agreement between BHC and LAI dated July 12, 2016luded the following specific tasks: Compiling and reviewing geologic and geotechnical information for the project area Developing an exploration plan Developing a traffic control plan Obtaining a Right-of-Way (ROW) Use Permit from the City and providing the necessary traffic ROW Arranging for underground utility location prior to performing field activities Advancing eight exploratory borings to characterize soil and groundwater conditions along the project alignment Installing piezometers in three of the exploratory borings to monitor groundwater levels over time Collecting representative soil samples at selected depth intervals City of Edmonds 1073015.010.011 2017 Sanitary Sewer Replacement Project 1-1 February 21, 2017 Landau Associates Logging the exploratory borings and recording pertinent information, including soil sample depths, stratigraphy, soil engineering characteristics, and groundwater occurrence Conducting a geotechnical laboratory testing program Performing geotechnical engineering analyses and evaluating data derived from the subsurface investigation and laboratory testing program Developing geotechnical engineering conclusions and recommendations to support design and construction of the proposed sewer line Preparing and submitting this written report summarizing our findings, conclusions, and recommendations for the project. This report includes: a site plan showing the locations of the borings completed for this project results of laboratory testing and summary logs of the exploratory borings a discussion of the near-surface soil and groundwater conditions observed along the project alignment a discussion related to possible critical areas (i.e., landslide hazard areas) within or adjacent to the project area along with a discussion related to associated permitting implications an assessment of the geologic characteristics of the soils, sediments, and/or rock of the project area and potentially affected adjacent properties, and a review of the site history regarding landslides, erosion, and prior grading in accordance with the requirements in Chapter 23.80.050 of the Edmonds City Code and Community Development Code a qualitative evaluation of the suitability of using the pipe reaming method of installation to install new sewer mains a qualitative evaluation of the feasibility of using the HDD method of installation to install the new sewer mains an evaluation of the moisture sensitivity of the soils along the project alignment conclusions regarding the anticipated need to dewater temporary excavations and a discussion related to dewatering methods that could be used if dewatering will be required recommendations related to trench excavation and temporary shoring recommendations related to maximum allowable slopes for temporary excavations recommendations for pipe foundation support, pipe bedding, and initial backfill materials an evaluation of the suitability of excavated soil for use as trench backfill trench backfill compaction criteria a discussion related to the need to properly decommission the piezometers that were installed for this project either prior to or during construction of the project recommendations for monitoring and testing during construction. City of Edmonds 1073015.010.011 2017 Sanitary Sewer Replacement Project 1-2 February 21, 2017 Landau Associates 2.0 This section discusses the general geologic setting of the project area and describes the surface and subsurface conditions observed along the project alignment at the time of our field investigation. Interpretations of the site conditions are based on the results of our review of available information, and the results of our site reconnaissance, subsurface explorations, and laboratory testing. 2.1Geologic Setting General geologic information for the project area was obtained from the DĻƚƌƚŭźĭ ağƦ ƚŅ ƷŷĻ 9ķƒƚƓķƭ 9ğƭƷ ğƓķ tğƩƷ ƚŅ ƷŷĻ 9ķƒƚƓķƭ ‘ĻƭƷ vǒğķƩğƓŭƌĻƭͲ ‘ğƭŷźƓŭƷƚƓ (Minard 1983), published by the U.S. Geological Survey, which indicates that the project alignment is underlain by glacial till and advance outwash. Soil defined as glacial till typically consists of a heterogeneous, non-sorted mixture of subrounded boulders, cobbles, gravel, and sand in a matrix of silt and clay. The heterogeneous nature of the till is a result of it being mixed and transported before being deposited, overridden, and compacted by the weight of an advancing glacier. This unit typically exhibits low permeability and high shear strength. Soil defined as advance outwash typically consists of mostly clean sand with varying amounts of gravel. Fine grained sand and some silt can occur in isolated, discrete deposits within this unit. Advance outwash was deposited by meltwater from the advancing front of a glacier and as such has been glacially overconsolidated. This unit typically exhibits moderate permeability. Though fill was not shown in the vicinity of the project alignment on the above-referenced geologic map, fill associated with prior development along and/or adjacent to the project alignment should be anticipated. 2.2Surface Conditions The project alignment is located within Edmonds, Washington. Development in the vicinity of the project alignment is primarily residential in nature along Maple Street, Shell Valley Road, and Pioneer Way. Between Maple Street and Shell Valley Road, the project alignment traverses the central portion of Yost Park. This section of the project alignment is located along an unpaved path that is situated on a slope that is located along the southern bank of Shell Creek. The project alignment generally slopes downward from east to the west with an elevation difference of approximately 10 ft. 2.3Subsurface Soil Conditions Subsurface conditions along the project alignment were explored on August 29 and 30, 2016. The exploration program consisted of advancing and sampling eight exploratory borings (B-1 through B-8) at the approximate locations illustrated on Figure 2. The exploratory borings were advanced to depths ranging from about 16.5 to 31.5 ft bgs. A discussion of field exploration procedures, together with City of Edmonds 1073015.010.011 2017 Sanitary Sewer Replacement Project 2-1 February 21, 2017 Landau Associates summary logs of the exploratory borings, is presented in Appendix A. A discussion of laboratory test procedures and the test results are presented in Appendix B. Soils encountered along the project alignment were generally observed to consist of advance outwash that in areas is overlain by top soil and/or fill. Along the western portion of the project alignment, between 10 th Avenue South and the intersection of Shell Valley Road and Pioneer Way, the soils encountered in our explorations generally consisted of very loose to loose sand with silt that is underlain by medium dense to very dense sand with silt and gravel. Near the intersection of Shell Valley Road and Pioneer Way, very loose to loose material was not observed to be present and the medium dense to very dense advance outwash was gravelly. Along Pioneer Way, the soils encountered generally consisted of medium stiff to very stiff clay and medium dense to very dense clayey sand. 2.4Groundwater Condition At the time of our field investigation in late August 2016, no groundwater was observed in any of our borings. However, at the time of drilling, piezometers were installed in borings B-1, B-4, and B-8. Three rounds of groundwater measurements were made between September 8, 2016 and February 17, 2017. The groundwater measurements that were recorded are presented in the following table. Depth to Groundwater (ft bgs) Boring Designation 9/8/2016 12/6/2016 2/17/2017 Not Encountered B-1 Not Encountered Not Encountered Not Encountered B-4 Not Encountered Not Encountered Not Encountered B-8 28.5 28.5 The groundwater conditions reported herein are for the specific locations and dates indicated, and therefore, may not necessarily be indicative of other locations and/or times. Furthermore, it is anticipated that groundwater conditions along the project alignment will vary depending on local subsurface conditions, the weather, and other factors. It is likely that the highest groundwater levels will occur in the winter/spring months. City of Edmonds 1073015.010.011 2017 Sanitary Sewer Replacement Project 2-2 February 21, 2017 Landau Associates 3.0 Based on the results of the field exploration, laboratory testing, and engineering analyses performed, it is our opinion that subsurface conditions along the project alignment are suitable for the proposed 2017 Sanitary Sewer Replacement project, provided the recommendations contained herein are incorporated into the project design. The following sections present geotechnical conclusions and recommendations related to construction dewatering; trenching and excavation support, pipe foundation support, pipe bedding and trench backfill, reuse of site soils, wet weather earthwork, trenchless construction methods, critical areas implications, and piezometer decommissioning. 3.15³¨«¨³¸ #®­²³±´¢³¨®­ The following sections provide geotechnical recommendations for design and construction of new underground sewers. The following recommendations are based on our understanding that the maximum depth of the sewer mains is about 13 ft, with the exception of one manhole that has a proposed maximum depth of about 21.5 ft. 3.1.1#®­²³±´¢³¨®­ $¤¶ ³¤±¨­¦ At the time of our field investigation in late August 2016, groundwater was not observed within the depths explored along the project alignment; however, in December 2016 and February 2017, groundwater was observed at a depth of about 28.5 ft bgs in the piezometer that was installed in Boring B-8. As a result, the need for temporary construction dewatering along the project alignment is not anticipated. However: depending on the depth of utilities and possible groundwater fluctuations over time, some construction dewatering may be required to control groundwater seepage into excavations along the project alignment and the contractor should be prepared to handle groundwater seepage into utility trenches should it occur. If minor groundwater seepage is encountered during construction, it is expected that open sump pumping from the excavation will be sufficient, assuming the excavation walls remain stable. Wells or well points may be necessary along the project alignment if substantial pockets of groundwater are encountered and/or the excavation walls become unstable. Well points are a viable option to a depth of about 17 ft. The contractor should be responsible for the design, installation, monitoring, and maintenance of any required dewatering system(s). If wells or well points are necessary, the contractor should be required to submit a dewatering plan prepared by a registered professional 3.1.24±¤­¢§¨­¦  ­£ %·¢ µ ³¨®­ 3´¯¯®±³ It is anticipated that excavations for the replacement sewer mains will be generally in either loose to very dense sand with varying amounts of silt, clay, and gravel or medium stiff to very stiff clay and medium dense to very dense clayey sand. A heavy-duty, hydraulic excavator with sufficient reach should be able to excavate the trenches to the depths planned. Though not observed in our borings, City of Edmonds 1073015.010.011 2017 Sanitary Sewer Replacement Project 3-1 February 21, 2017 Landau Associates cobbles and boulders may be present along the project alignment and the contractor should be prepared to handle and dispose of such oversized material if encountered. The final trench bottom should be firm and free of loose and disturbed soil. We recommend that a smooth-bladed bucket be used to clean the trench bottom of loose and/or disturbed soil prior to placing the bedding material. Trench excavation should conform to the requirements of Section 7-08.3(1)A of the 2016 Washington State Department of Transportation (WSDOT) {ƷğƓķğƩķ {ƦĻĭźŅźĭğƷźƚƓƭ ŅƚƩ wƚğķͲ .ƩźķŭĻ ğƓķ aǒƓźĭźƦğƌ /ƚƓƭƷƩǒĭƷźƚƓ (WSDOT 2016). Actual trench configurations and maintenance of safe working conditions, including temporary excavation stability, should be the responsibility of the contractor. All applicable local, state, and federal safety codes should be followed. Temporary excavations in excess of 4 ft should either be shored or sloped in accordance with {ğŅĻƷǤ {ƷğƓķğƩķƭ ŅƚƩ /ƚƓƭƷƩǒĭƷźƚƓ ‘ƚƩƉͲ tğƩƷ b, located in Chapter 296-155 of the Washington Administrative Code (WAC). In the absence of groundwater seepage, the soils anticipated within the trench zone along the project alignment generally classify as Type C soil per Chapter 296-155 of the WAC. The prescriptive maximum allowable excavation slope for Type C soils is 1½H:1V (horizontal to vertical). If groundwater seepage is present, flatter slopes, temporary shoring, and/or dewatering may be required. Trench boxes are a worker safety device but do provide some lateral support of the adjacent soil. Where a trench box is used to support excavations, one or both sides of the trench may cave against the box, especially if the soil is not properly dewatered. Caving may extend out on either side of the trench for a distance approximately equal to the depth of the trench box. The potential for caving can be reduced by routing stormwater away from the excavation and limiting vehicular traffic or vibrations next to the trench. When the trench box is moved, precautions should be taken to minimize disturbance of the pipe, underlying bedding materials, and surrounding soil. Trench boxes should meet the requirements in {ğŅĻƷǤ {ƷğƓķğƩķƭ ŅƚƩ /ƚƓƭƷƩǒĭƷźƚƓ ‘ƚƩƉͲ tğƩƷ bͲ located in Chapter 296-155 of the WAC. Additional bracing or sheeting may be required where the near edge of the trench will be closer than 1.5 times the trench depth to settlement sensitive utilities or structures. If bracing is needed to support the trench walls, the temporary bracing system should be designed by a structural engineer licensed in the state of Washington. The parameters provided in the following table can be utilized for design of temporary shoring. Temporary shoring typically consists of steel plates with internal bracing. Surcharge loads on trench support systems due to construction equipment, stockpiled material, and vehicle traffic should be included in the design. The temporary shoring design should be submitted to the . City of Edmonds 1073015.010.011 2017 Sanitary Sewer Replacement Project 3-2 February 21, 2017 Landau Associates RECOMMENDED SOIL PARAMETERS FOR DESIGN OF TEMPORARY BRACING Moist Unit Cohesion Internal Angle of Weight (pcf) (psf) Friction (degrees) 125 0 34 pcf = Pounds per cubic foot psf = Pounds per square foot 3.1.30¨¯¤ &®´­£ ³¨®­ 3´¯¯®±³ Based on conditions observed at our exploration locations, soils at the bottom of the proposed utility trenches are anticipated to primarily consist of either medium dense to very dense sand with varying amounts of silt, clay, and gravel or medium stiff to very stiff clay and medium dense to very dense clayey sand. These soil types should provide adequate foundation support for the proposed sewer mains, provided the soil remains in a relatively undisturbed condition and the trench is properly dewatered. It is anticipated that the soil at the trench bottom could be easily disturbed by construction activities, and in a disturbed condition will generally provide poor foundation support for the new sewer mains. If the trench bottom becomes disturbed due to excavation and/or foot traffic during laying of the pipe, the trench bottom may need to be overexcavated to expose undisturbed foundation soil. Removal and replacement of unsuitable foundation material should be in accordance the Ci ǤƦźĭğƌ ƩĻƓĭŷ {ĻĭƷźƚƓ CƚǒƓķğƷźƚƓ {ƷğĬźƌźǩğƷźƚƓ {ƷğƓķğƩķ 5ĻƷğźƌ. 3.1.40¨¯¤ Bedding  ­£ 4±¤­¢§ " ¢ª¥¨«« To provide uniform support of buried utility pipes, the pipe should be bedded in accordance with Section 7-08 of the 2016 WSDOT {ƷğƓķğƩķ {ƦĻĭźŅźĭğƷźƚƓƭ using gravel backfill material specified in Section 9.03.16 of the City aƚķźŅźĭğƷźƚƓƭ Ʒƚ {ƷğƓķğƩķ {ƦĻĭźŅźĭğƷźƚƓƭ and the City{ƷğƓķğƩķ 5ĻƷğźƌ 9ΏЍ͵Ћ. Pipe zone backfill should be in accordance with Section 7-08.3(3) of the 2016 WSDOT {ƷğƓķğƩķ {ƦĻĭźŅźĭğƷźƚƓƭ and the City{ƷğƓķğƩķ 5ĻƷğźƌ 9ΏЍ͵Њ͵ Excavated soils may be reused as trench backfill provided they are free of organic material, rubbish, or other deleterious materials. Additionally, native soils used as utility trench backfill should meet gradational requirements in Section 9.03.19 of the City of Edmonds aƚķźŅźĭğƷźƚƓƭ Ʒƚ {ƷğƓķğƩķ {ƦĻĭźŅźĭğƷźƚƓƭ. While not encountered with frequency in our borings, coarse gravel and cobbles are likely to be present in the site soils given the geologic setting. As a result, significant effort (i.e., screening or manual picking) may be required by the contractor to process some of the onsite soils to meet the specified gradational requirements. If an insufficient amount of onsite soil is available for use as trench backfill, import fill consisting of Bank Run Gravel for Trench Backfill meeting the requirements of Section 9.03.19 of the City aƚķźŅźĭğƷźƚƓƭ Ʒƚ {ƷğƓķğƩķ {ƦĻĭźŅźĭğƷźƚƓƭ should be utilized for trench backfill. City of Edmonds 1073015.010.011 2017 Sanitary Sewer Replacement Project 3-3 February 21, 2017 Landau Associates Backfilling of trenches should be in accordance with the requirements of Section 2.09.3(E) of the City aƚķźŅźĭğƷźƚƓƭ Ʒƚ {ƷğƓķğƩķ {ƦĻĭźŅźĭğƷźƚƓƭ and the City{ƷğƓķğƩķ 5ĻƷğźƌ 9ΏЍ͵Ћ. Trench backfill should be placed in 6- to 8-inch loose lifts and compacted to a relative density of at least 95 percent of the maximum dry density (Class D Backfill of the City aƚķźŅźĭğƷźƚƓƭ Ʒƚ {ƷğƓķğƩķ {ƦĻĭźŅźĭğƷźƚƓƭ). Determination of the maximum dry density should be in accordance with the requirements of Section 2-03.3(14)D of the 2016 WSDOT {ƷğƓķğƩķ {ƦĻĭźŅźĭğƷźƚƓƭ. Alternatively, the maximum dry density may be determined using ASTM Test Method D 1557 (modified Proctor). Flooding and/or jetting of backfill should not be used as a means to consolidate or compact trench backfill. Hand-operated compaction equipment, or other approved methods, should be used to compact the first 18 inches of trench backfill above the pipe. 3.1.52¤´²¤ ®¥ 3¨³¤ 3®¨«² Soils along portions of the project alignment (e.g., along Pioneer Way) exhibit a high fines content, which makes them highly sensitive to moisture conditions. Moisture sensitivity can make obtaining proper compaction difficult and may increase the contractoreffort required to reuse the soil, or limit when earthwork activities can be performed (i.e., not during periods of wet weather). Soils along the western portion of the project alignment are anticipated to consist of sand with limited fines. This -rich materials and reused as trench backfill, provided it can be properly compacted. If onsite soils cannot be reused as trench backfill, the onsite soils should be disposed of at a suitable offsite location. 3.1.67¤³ 7¤ ³§¤± % ±³§¶®±ª If earthwork must be performed during wet weather or under wet conditions, the contractor could reduce soil disturbance by: Performing earthwork in small areas to minimize subgrade disturbance and exposure to additional moisture. Placing fill material immediately following excavation and subgrade preparation. Implementing surface water control measures to prevent run-on into excavations and fill soil stockpiles. Removing wet surficial soil prior to commencing fill placement each day. Sealing the exposed ground surface by rolling with a smooth drum compactor or rubber-tired roller at the end of each working day. 3.2T±¤­¢§«¤²² Construction The following sections provide geotechnical recommendations for design and construction of the proposed replacement sanitary sewer line using trenchless construction techniques such as HDD and City of Edmonds 1073015.010.011 2017 Sanitary Sewer Replacement Project 3-4 February 21, 2017 Landau Associates pipe reaming. The following recommendations are based on our understanding that the maximum depth of the proposed new underground utilities will be about 21.5 ft. 3.2.1(®±¨¹®­³ « $¨±¤¢³¨®­ « $±¨««¨­¦ Soil conditions along the planned bore path are expected to consist of medium dense to very dense sand with silt and varying amounts of gravel and clayey sand with gravel, overlain in some areas by very loose to loose sand with silt. At the time of our exploration, no groundwater was observed in the exploratory borings LAI advanced along the project alignment. Based on our experience on similar projects, the likely HDD construction sequence would be to drill a pilot hole, which will likely vary from 4 to 6 inches in diameter, depending on the type of bit and soil conditions; push or back ream the hole to an uniform diameter that is about 50 percent greater than the diameter of the replacement pipe; and connect to and pull back the replacement pipe. It is our opinion that installing a replacement sanitary sewer main by means of the HDD installation technique is generally feasible from a geotechnical standpoint. However, when evaluating this method of installation, the following considerations should be taken into account. Advantages of the HDD method of installation include the relatively small construction impacts, the speed at which the pipe can be installed, and the minor amount of surface and pavement restoration required. Disadvantages of the HDD method include relatively high costs (when compared to the cost of the conventional open-trench technique) and the fact that current HDD technology with regard to control of line and grade is not satisfactory for construction of gravity sewers with tight line and grade tolerances (i.e., 2 percent or less). If there is sufficient fall between entrance and exit points, it might be possible to design the new sewer main with a grade in excess of 2 percent. However, even if the pipe is installed using the HDD technique with a specified grade in excess of 2 percent, it should be recognized that the pipe could be installed with deviations from constant grade. These deviations could result in along the pipe. If the new sewer main is installed with bellies, solids could collect within the pipe. Consequently, the potential for solids collecting within the new sewer main should be considered when evaluating the HDD installation method. Finally, it should be recognized that because the drilling fluid used to advance a HDD borehole is under significant pressure, it has the capability of expanding and fissuring the soil surrounding the borehole. This fissuring could result in a hydraulic connection between the borehole and the ground surface (frac out), which could result in an environmental problem if the drilling fluid enters a nearby environmental sensitive area (e.g., Shell Creek). This issue is typically addressed by specifying a minimum amount of soil cover over the borehole. A general rule of thumb is to maintain a soil cover thickness over the borehole that is at least 5 times the borehole diameter. When using this rule of thumb, the borehole diameter should be taken as 50 percent greater than the diameter of the replacement pipe. Because a portion of the project alignment that traverses a slope located within Yost Park, the horizontal distance between the borehole and the face of the slope will need to be City of Edmonds 1073015.010.011 2017 Sanitary Sewer Replacement Project 3-5 February 21, 2017 Landau Associates considered slope face should be anticipated in order to help prevent frac out along the face of the slope. 3.2.20¨¯¤ 2¤ ¬¨­g The pipe reaming method of installation is a pipe replacement technique that uses a HDD machine that has been slightly modified. After pushing the drill rods through the old pipeline and connecting the rods to a special reamer, the new pipe string is attached to the reamer via a swivel and towing head. As the drill rig rotates and simultaneously pulls back, the old pipe is ground up and replaced by the new pipe. Removal of the old pipe is accomplished by mixing the ground up material with the drilling fluid and transferring it to an exit point for removal via a vacuum truck. Advantages of this method of installation include the fact that pipe diameter/capacity may be increased, the relatively small construction impacts, the speed at which the pipe can be installed, and the minor amount of surface restoration required. Disadvantages include the need for sewage bypass during installation and the inability to correct large defects in grade and alignment, if present. The City of Edmonds GIS web map application indicates that the pipes to be replaced are all 10-inch diameter concrete pipe. The backfill material around the pipe has been assumed to be reused site soils, which are expected to consist of medium dense to dense, sand with silt and varying amounts of gravel and clayey sand with gravel. Plain concrete pipe is suitable for the pipe reaming method. Reinforced concrete pipe can also be successfully replaced using the pipe reaming method, provided it is not heavily reinforced or if it is substantially deteriorated. Based on the soil characteristics observed along the project alignment, and assuming the observed conditions are representative of trench backfill conditions along the existing sanitary sewer line, it is our opinion that the pipe reaming method of installation is generally feasible from a geotechnical perspective. However, similar to the HDD method of installation, the pipe reaming method of installation involves the use of drilling fluids that are under significant pressure, and as a result, a minimum amount of soil cover over the borehole will be required to prevent frac out. The general rule of thumb that is presented in Section 3.2.1 of this report related to the minimum amount of soil cover needed to prevent frac out is also applicable to the pipe reaming method of installation. 3.3#±¨³¨¢ « !±¤ ²  ­£ !²²®¢¨ ³¤£ 0¤±¬¨³³¨­¦ )¬¯«¨¢ ³¨®­² According to the City of Edmonds GIS web map application, approximately 1,400 ft of the project alignment passes through areas that have been identified as being a landslide or erosion hazard area. No portion of the project alignment was identified as being in a seismic hazard area. 3.4'¤®³¤¢§­¨¢ « )¬¯«¨¢ ³¨®­² ®¥ #±¨³¨¢ « !±¤  7®±ª No obvious signs of slope instability were observed along the project alignment during our site visits. However, because a portion of the project alignment passes through a landslide or erosion hazard City of Edmonds 1073015.010.011 2017 Sanitary Sewer Replacement Project 3-6 February 21, 2017 Landau Associates area, it will be necessary to consider the potential impact that the proposed construction activities could have on the stability of the slopes in the vicinity of the work areas. The purpose of this project is to replace an existing sewer main with a new sewer main that is located along a similar alignment and at similar depths to the original main. It appears that the original sewer main was installed without any adverse impacts on the stability of the slope within Yost Park. Given that, it is our opinion that it should be possible to install the replacement sewer main without negatively impacting slope stability. In order to do this, the contractor will need to control stormwater runoff and not stockpile soil and construction materials at the crest of the slope. It will also be important to limit the removal and disturbance of vegetation along the project alignment. 3.5$¤¢®¬¬¨²²¨®­¨­¦ ®¥ 0¨¤¹®¬¤³¤±² Decommissioning of the piezometers installed as part of this project may be performed at any time; however, it may be beneficial to the contractor to leave the wells installed until completion of construction. The decommissioning should be performed in accordance with WAC 173-160-460 (2)(a), which specifies that the well shall be decommissioned by filling from bottom to top with bentonite, bentonite slurry, neat cement grout, or neat cement. City of Edmonds 1073015.010.011 2017 Sanitary Sewer Replacement Project 3-7 February 21, 2017 Landau Associates 4.0 We recommend that LAI be retained to review the geotechnical-related portions of the project plans and specifications to determine if they are consistent with the recommendations presented in this report. We also recommend that monitoring, testing, and consultation be provided during construction to confirm that the conditions encountered are consistent with those indicated by our explorations, to provide recommendations should conditions be revealed during construction that differ from those anticipated, and to evaluate whether geotechnical-related construction activities comply with project plans/specifications and the recommendations contained in this report. City of Edmonds 1073015.010.011 2017 Sanitary Sewer Replacement Project 4-1 February 21, 2017 Landau Associates 5.0 LAI prepared this report for the exclusive use of BHC Consultants, LLC and the City of Edmonds for specific application to the design of the proposed 2017 Sanitary Sewer Replacement project in Edmonds, Washington. . Within the limitations of scope, schedule, and budget, our services have been conducted in accordance with generally accepted practices of the geotechnical engineering profession; no other warranty, express or implied, is made as to the professional advice included in this report. The conclusions and recommendations contained in this report are based in part upon the subsurface data obtained from the explorations completed for this study. There may be some variation in subsurface soil and groundwater conditions along the project alignment, and the nature and extent of the variations may not become evident until construction. Accordingly, a contingency for unanticipated conditions should be included in the construction budget and schedule. If variations in subsurface conditions are encountered during construction, LAI should be notified for review of the recommendations in this report, and revision of such if necessary. If there is a substantial lapse of time between submission of this report and the start of construction, or if conditions change due to construction operations at or adjacent to the project alignment, we recommend that we review this report to determine the applicability of the conclusions and recommendations contained herein. We appreciate the opportunity to provide geotechnical services on this project and look forward to assisting you during the construction phase of the project. If you have any questions or comments regarding the information contained in this report, or if we may be of further service, please call. City of Edmonds 1073015.010.011 2017 Sanitary Sewer Replacement Project 5-1 February 21, 2017 Landau Associates 6.0 Minard, J.P. 1983. DĻƚƌƚŭźĭ ağƦ ƚŅ ƷŷĻ 9ķƒƚƓķƭ 9ğƭƷ ğƓķ tğƩƷ ƚŅ ƷŷĻ 9ķƒƚƓķƭ ‘ĻƭƷ vǒğķƩğƓŭƌĻƭͲ ‘ğƭŷźƓŭƷƚƓ. U.S. Geological Survey. WSDOT. 2016. M 41-10: {ƷğƓķğƩķ {ƦĻĭźŅźĭğƷźƚƓƭ ŅƚƩ wƚğķͲ .ƩźķŭĻͲ ğƓķ aǒƓźĭźƦğƌ /ƚƓƭƷƩǒĭƷźƚƓ. Washington State Department of Transportation. City of Edmonds 1073015.010.011 2017 Sanitary Sewer Replacement Project 6-1 February 21, 2017 Lynndale Park Puget Sound ST 524 Project Location Edmonds Edmonds City Park Woodway Woodway ek ST 104 ST 99 Esperance Esperance Project Location Everett ! Edmonds ! ! ! Spokane Seattle 00.51 Tacoma ! ! Olympia Washington Washington Miles Data Source: Esri 2012 Figure City of Edmonds 2017 1 Vicinity Map Sanitary Sewer Replacement Edmonds, Washington < < < < < < < < S evA ht01 < APPENDIX A !00%.$)8 ! &)%,$ %80,/2!4)/.3 Subsurface conditions along the project alignment were explored on August 28 and 30, 2016. The exploration program consisted of advancing and sampling eight exploratory borings (B-1 through B-8) at the approximate locations illustrated on the Site and Exploration Plan (Figure 2 of this report). The borings were advanced to depths ranging from about 16½ to 31½ ft below the existing ground surface (BGS) using the hollow-stem auger exploration technique. Geologic Drill Exploration, Inc. of Spokane, Washington advanced the hollow-stem auger borings under subcontract to Landau Associates. The locations of the exploratory borings were located approximately in the field using a handheld global positioning system (GPS) unit. The ground surface elevations at the exploration locations were not determined. The field exploration program was coordinated and monitored by Landau Associates personnel who also obtained representative soil samples, maintained a detailed record of the observed subsurface soil and groundwater conditions, and described the soil encountered by visual and textural examination. Each representative soil type observed in our exploratory borings was described using the soil classification system shown on Figure A-1, in general accordance with ASTM International (ASTM) D 2488, {ƷğƓķğƩķ wĻĭƚƒƒĻƓķĻķ tƩğĭƷźĭĻ ŅƚƩ 5ĻƭĭƩźƦƷźƚƓ ƚŅ {ƚźƌƭ ːźƭǒğƌΏağƓǒğƌ tƩƚĭĻķǒƩĻΜ. Logs of the exploratory borings are presented on Figures A-2 through A-9. These logs represent our interpretation of subsurface conditions identified during the field exploration program. The stratigraphic contacts shown on the summary logs represent the approximate boundaries between soil types; actual transitions may be more gradual. The soil and groundwater conditions depicted are only for the specific dates and locations reported and, therefore, are not necessarily representative of other locations and times. A further discussion of the soil and groundwater conditions observed is contained in the text portion of this report. Disturbed samples of the soil encountered in the exploratory borings were obtained at selected intervals using a 1.5-inch inside-diameter split-spoon sampler. The sampler was driven up to 18 inches into the undisturbed soil ahead of the drill bit with a 140-lb hammer falling a distance of approximately 30 inches. The number of blows required to drive the sampler for the final 12 inches of soil penetration, or part thereof, is noted on the boring logs, adjacent to the appropriate sample notation. Samples collected in this manner were taken to our laboratory for further examination and testing. A discussion of laboratory test procedures and the laboratory test results are presented in Appendix B. Upon completion of drilling and sampling, the boreholes were decommissioned in general accordance with the requirements of Washington Administrative Code (WAC) 173-160. LA ANDAU SSOCIATES 2/21/17 P:\\1073\\015\\R\\Final\\COE 2017 Sewer Improv_apa.doc A-1 APPENDIX B !00%.$)8 " ,!"/2!4/29 3/), 4%34).' Soil samples obtained from the exploratory borings were taken to our laboratory for further examination and testing. Laboratory tests were performed on representative soil samples to characterize certain engineering and index properties of the soils along the project alignment. The laboratory testing program, which was performed in general accordance with the ASTM International (ASTM) standard test procedures noted below, was limited to visual inspection to confirm our field soil descriptions, and determination of natural moisture content, Atterberg Limits, and grain size distribution. A brief description of the testing performed is presented below. Natural Moisture Content The natural moisture contents of selected soil samples obtained from the exploratory borings were determined in general accordance with ASTM D 2216 test procedures. The results from the moisture content determinations are indicated adjacent to the corresponding samples on the summary logs presented in Appendix A. Atterberg Limits Analysis The Atterberg limits test was performed on selected soil samples obtained from the exploratory borings in general accordance with ASTM D 4318 test procedures. The purpose of the test was to determine the liquid limit, plastic limit, and plasticity index of fine-grained material. Atterberg limits results are summarized on Figure B-1. Grain Size Analysis Grain size analyses were conducted on selected soil samples obtained from the exploratory borings in general accordance with ASTM D 422 test procedures. The results of these analyses are presented in the form of grain size distribution curves on Figure B-2. LA ANDAU SSOCIATES 2/21/17 P:\\1073\\015\\R\\Final\\COE 2017 Sewer Improv_apb.doc B-1