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HomeMy WebLinkAboutTrail Crest - Geotechnical Letter Report Addendum - 2021.06.30June 30, 2021 Bruce Seton, Jr. Seton Construction 4640 South Discovery Road Port Townsend, WA 98368 Re: Geological Letter Report Addendum Trail Crest Residential Development Cook Avenue Port Townsend, Washington Project No. 200262-03 Dear Mr. Seton: This letter report addendum summarizes Aspect Consulting, LLC’s (Aspect) observations, conclusions, and recommendations made during an updated geotechnical evaluation for the planned residential development (Project). The Project is located on the northwest side of Cook Avenue, which runs from southwest to northeast, in Port Townsend, Washington (Site). We previously provided a geotechnical letter report for the development of the Site, included as Appendix A. The City of Port Townsend (City) has since requested an update to the geotechnical letter report in support of revisions to the two proposed Trail Crest plans dated May 7, 2021 (DEA, 2021a and 2021b). The purpose of this addendum is to evaluate the stability of two proposed steep cut slopes adjacent to Cook Avenue with consideration of a newly proposed sidewalk and updates to the utility plans. The addendum also provides recommendations and opinions regarding installation of a proposed storm drain pipe and watermain on the steep slopes. Project Understanding In preparing this report addendum, we discussed the Project with Adam Stricker of David Evans and Associates, Inc. (DEA), reviewed the updated utility layouts (DEA, 2021a and 2021b), and reviewed our previous documents for the Project (Appendix A). It is our understanding that a proposed storm drain pipe and a watermain have been re-routed along the east side of Lot 50. The utilities will be installed down the steep slope on the northwest side of Cook Avenue in separate directions to tie into the systems along the roadway. We also understand that an 8-foot-wide sidewalk is proposed along the northwest side of Cook Avenue, which will be created by cutting into the existing steep slope. This report addendum aims to support the Project by using the existing subsurface information, a previous reconnaissance of the area, and the updated utility layout drawings. The recommendations and conclusions presented in our 2020 report (Appendix A) remain applicable unless specifically addressed herein. earth +water Aspect Consulting, LLC 350 Madison Avenue N. Bainbridge Island, WA 98110 206.780.9370 www.aspectconsulting.com Seton Construction June 30, 2021 Project No. 200262-03 Page 2 Site Conditions Please refer to our previous Geotechnical Letter Report (Appendix A) for our detailed site observations, earthwork recommendations, and geologic hazard analyses. The updated plans include utility re-routes and a proposed 8-foot-wide sidewalk. Utilities within Cook Avenue will include a 12-inch-diameter, polyvinyl chloride (PVC) storm drain with at least 3 feet of soil cover; an 8-inch-diameter, PVC sewer main with about 6 feet of soil cover; and dry utility conduits directly beneath the sidewalk (DEA, 2021a and 2021b). Sanitary sewer manholes will be installed up to every 300 feet and extend up to 7.5 feet below Cook Avenue. Catch basins for the storm drain pipe will be installed up to every 200 feet and extend up to 6.5 feet below the northwest curb and gutters of Cook Avenue. These updates result in two design modifications to the proposed 1 horizontal to 1 vertical (1H:1V) steep cut slope adjacent to Cook Avenue. First, the slope will be set back roughly 10 additional feet to accommodate the sidewalk. Second, temporary excavations will be performed at the toe of the cut slope to install the utilities up to 8 feet below ground surface (bgs). The excavations will be supported using trench boxes to install the catch basins and sanitary sewer manholes. Utilities planned on the steep slope itself northwest of Cook Avenue will include a 12-inch-diameter PVC storm drain pipe and an 8-inch-diameter ductile iron watermain (DEA, 2021b). The storm drain pipe will descend the existing slope west of the proposed steep cut slope via a 128 linear foot pipe segment inclined 18 percent from horizontal. The watermain will descend the existing slope along the crest of the proposed 1:1 cut slope over approximately 410 linear feet with an average inclination of 21 percent. The watermain will tie into a water booster station located on Cook Avenue northeast of the 1:1 cut slope (DEA, 2021b). The proposed booster station area is located on an existing slope. To create a 35-foot by 54-foot flat bench, the area will be cut back using 2H:1V slope. Conclusions and Recommendations We reviewed the elements related to the relevant geotechnical aspects and have found the plans, design, and utility locations to be in general accordance with our previous geotechnical engineering recommendations. The modifications to the planned Project work along Cook Avenue will not negatively affect the stability of the slopes at and near the Site. The Project can be constructed in a stable manner, provided the recommendations contained herein and in our previous report (Appendix A) are incorporated into the design and construction of the Project. Cook Avenue Cut Slope Stability Analyses We performed a slope stability analysis on the planned 1H:1V cut slope at the tallest location with the planned temporary cuts during static conditions at the toe of the slope for the planned stormwater and sanitary sewer utility installations. We performed our slope stability analyses using the computer program Slide (Rocscience, 2018) and Spencer’s method, as shown in the figures presented in Appendix B. The stability analysis computes a factor of safety against sliding activity, which is the ratio of the forces that resist sliding (internal soil strength, topographic buttressing, lack of groundwater, etc.) to the forces that contribute to sliding (gravity, presence of groundwater, loads above the slope, etc.). A factor of safety of one indicates a “just-stable” condition, and a factor of safety less than one would indicate unstable conditions. Seton Construction June 30, 2021 Project No. 200262-03 Page 3 We also performed a stability analysis to assess the long-term performance of the slope during normal static and design seismic conditions. The stability analysis was performed using conservative traditional infinite slope methods. The infinite slope method is applicable to situations where a thin layer of soil overlies a much harder stratum (Vashon lodgment till, glacially overridden materials), as is the case with the Site slope. Strength parameters were assumed based on the previous geotechnical report for the underlying soils found within the slope. These are shown in Table 1: Table 1. Summary of Soil Engineering Properties Used in Slope Stability Analyses Geologic Unit Total Unit Weight Moist (pcf) Strength Parameters Cohesion (psf) Friction Angle (degree) Fill 120 0 32 Topsoil/Colluvium 110 0 28 Weathered Vashon lodgment till 130 250 38 Vashon lodgment till 135 500 42 Notes: (1) pcf = pounds per cubic foot, and psf = pounds per square foot. We developed two cross-sections of the slope (short-term and long-term conditions) using the updated drawings (DEA, 2021a). The short-term scenario determined the factor of safety for the temporary slope, while trench boxes are used to excavate and install proposed utilities under static conditions. The long-term scenario was used to evaluate the factor of safety after the utilities are installed and backfilled using the materials recommended in Appendix A (Aspect, 2020) under both static and seismic conditions. The International Building Code (IBC) seismic design is based on the “Maximum Considered Earthquake (MCE)” with a 2 percent probability of exceedance in 50 years (2,475-year return period; ICC, 2015). Seismic design should be completed with ground motion parameters assuming that the proposed roadway is Risk Category III and site soils are class D (stiff soil). We used the Site adjusted peak ground acceleration (PGA) of 0.635g to produce a uniform horizontal seismic coefficient (kh = ½ x PGA) of 0.318g, where g is the force of gravity. The results of our stability analyses for the proposed conditions are summarized in Table 2 below. Table 2. Summary Slope Stability Analysis Results Figure Number Condition Calculated Factor of Safety Minimum Required Factor of Safety B-1 Short-Term Static 1.86 1.25 B-2 Long-Term Static 2.01 1.50 B-3 Long-Term Seismic 1.29 1.10 The calculated factors of safety exceed the minimum required values. Seton Construction June 30, 2021 Project No. 200262-03 Page 4 Water Booster Station Cut Slope Stability Evaluation The cut slope to the water booster station will be more gradual and shorter (less than 15 feet high) than the planned 1H:1V cut slope along Cook Avenue. Based on available test pit data and our understanding of geologic conditions (Appendix A), the stratigraphy in the cut slope areas is roughly the same; therefore, we conclude that the 2H:1V slope will have factors of safety greater than those calculated for the 1H:1V cut slope in the previous section. Design of Utilities on Steep Slopes Because the stormwater and sewer lines will be installed in trenches and provided they are backfilled with good quality structural fill, as discussed herein, they can be installed without special anchorage (such as is commonly required for surface-mounted outfall pipes extending down bluff faces). Backfill for utility trenches on the steep slope northwest of Cook Avenue should consist of 3/4-inch clear crushed rock that is free draining and has high internal friction angles when properly compacted. We recommend installing check dams in the utility trenches every 50 feet to reduce the velocity of water flowing through the pervious trench backfill if groundwater infiltrates the trench. The check dams should extend the full width of the utility trench and have heights three times greater than the diameter of the pipe. From the bottom of the utility trenches, check dam heights will be 3 feet for the storm drain pipe and 2 feet for the watermain. Check dams should be constructed using a low-permeability material, such as on-Site Vashon lodgment till that contains a significant percentage of fines (silt and clay). Alternatively, the check dams could be constructed using controlled density fill (CDF) or lean concrete. Backfill within the utility trenches, including check dams, should be placed and compacted to a dense and unyielding condition. We recommend using small, hand-operated compaction equipment on the Site slopes to reduce the possibility of damaging utilities, structures, or slopes while backfilling. Using this equipment, fill lifts should not exceed 6 inches in loose thickness. Excavation and placement of fill on the Site slopes should be observed by Aspect to verify that all unsuitable materials are removed, and suitable compaction is achieved. Recommendations for Continuing Geotechnical Services We are available to provide geotechnical engineering and monitoring services during construction under separate scope and budget. The integrity of the geotechnical elements depends upon proper Site preparation and construction procedures. In addition, subsurface conditions may vary from those observed in the subsurface explorations; we would appreciate the opportunity to observe Site conditions during construction and provide appropriate field recommendations as needed. During the construction phase of the Project, we recommend that Aspect be retained to observe and evaluate temporary excavations, subgrade preparation, structural backfill placement and compaction, and installation of the geosynthetic reinforced fill wall. The purpose of our observations is to verify compliance with design concepts and recommendations, and to allow design changes or evaluation of appropriate construction methods in the event that subsurface conditions differ from those anticipated prior to the start of construction. Seton Construction June 30, 2021 Project No. 200262-03 Page 5 References Aspect Consulting, LLC, 2020, Geological Letter Report – Trail Crest Residential Development, November 6, 2020. David Evans and Associates, Inc. (DEA), 2021a, Trail Crest Cook Avenue Frontage Improvements Plans, SW 1/4 Sec. 33, TW 31 N, R.1 W, W.M., Drawings, May 7, 2021. David Evans and Associates, Inc. (DEA), 2021b, Trail Crest Divisions 2 and 3 SPD, SW 1/4 Sec. 33, TW 31 N, R.1 W, W.M., Drawings, May 7, 2021. International Code Council (ICC), 2015, International Building Code (IBC), Prepared by International Code Council, January 2015. Rocscience, 2018, Slide 8.019 and 8.020 Analysis Program, Build date May 18, 2021. Limitations Work for this project was performed for Seton Construction (Client), and this report was prepared consistent with recognized standards of professionals in the same locality and involving similar conditions, at the time the work was performed. No other warranty, expressed or implied, is made by Aspect Consulting, LLC (Aspect). Recommendations presented herein are based on our interpretation of site conditions, geotechnical engineering calculations, and judgment in accordance with our mutually agreed-upon scope of work. Our recommendations are unique and specific to the project, site, and Client. Application of this report for any purpose other than the project should be done only after consultation with Aspect. Variations may exist between the soil and groundwater conditions reported and those actually underlying the site. The nature and extent of such soil variations may change over time and may not be evident before construction begins. If any soil conditions are encountered at the site that are different from those described in this report, Aspect should be notified immediately to review the applicability of our recommendations. Risks are inherent with any site involving slopes and no recommendations, geologic analysis, or engineering design can assure slope stability. Our observations, findings, and opinions are a means to identify and reduce the inherent risks to the Client. It is the Client's responsibility to see that all parties to this project, including the designer, contractor, subcontractors, and agents, are made aware of this report in its entirety. At the time of this report, design plans and construction methods have not been finalized, and the recommendations presented herein are based on preliminary project information. If project developments result in changes from the preliminary project information, Aspect should be contacted to determine if our recommendations contained in this report should be revised and/or expanded upon. The scope of work does not include services related to construction safety precautions. Site safety is typically the responsibility of the contractor, and our recommendations are not intended to direct the contractor’s site safety methods, techniques, sequences, or procedures. The scope of our work Seton Construction June 30, 2021 Project No. 200262-03 Page 6 also does not include the assessment of environmental characteristics, particularly those involving potentially hazardous substances in soil or groundwater. All reports prepared by Aspect for the Client apply only to the services described in the Agreement(s) with the Client. Any use or reuse by any party other than the Client is at the sole risk of that party, and without liability to Aspect. Aspect’s original files/reports shall govern in the event of any dispute regarding the content of electronic documents furnished to others. Please refer to Appendix C titled “Report Limitations and Guidelines for Use” for additional information governing the use of this report. We appreciate the opportunity to perform these services. If you have any questions please call Alison Dennison, Project Manager, (206) 780-7717. Sincerely, Aspect consulting, LLC Alison J. Dennison, LEG Senior Engineering Geologist adennison@aspectconsulting.com Jane Gregg, PE Project Geotechnical Engineer jgregg@aspectconsulting.com Erik O. Andersen, PE Principal Geotechnical Engineer eandersen@aspectconsulting.com Attachments: Appendix A – 2020 Geotechnical Letter Report by Aspect Appendix B – Slope Stability Analysis Results Appendix C – Report Limitations and Guidelines for Use cc: Adam Stricker and Al Fure; David Evans and Associates, Inc. V:\200262 Trail Crest Pt Townsend\Deliverables\Addendum\Trail Crest - Geotechnical Letter Report Addendum_06.30.2021.docx 6.30.2021 6.30.2021 APPENDIX A 2020 Geotechnical Letter Report by Aspect November 6, 2020 Bruce Seton, Jr. Seton Construction 4640 South Discovery Road Port Townsend, WA 98368 Re: Geological Letter Report Trail Crest Residential Development Cook Avenue Port Townsend, Washington Project No. 200262-02 Dear Mr. Seton: This letter report summarizes Aspect Consulting, LLC’s (Aspect) observations, conclusions, and recommendations made during an engineering geological assessment for the planned residential development (Project) along the north side of Cook Avenue in Port Townsend, Washington (Site), as shown on Figure 1, Site Location Map. Aspect performed a slope reconnaissance, a review of subsurface investigations previously advanced by others on the Site, and a geological assessment in accordance with the scope of work that you authorized on October 2, 2020. This letter should be used in conjunction with the two previous reports for this Site that were issued by AGRA Earth & Environmental, Inc. (AGRA) and EnvriroSound Consulting Inc. (ESC), which are included in Appendix A and Appendix B, respectively. Project Understanding It is our understanding that as part of a minor preliminary plat modification, the City of Port Townsend (City) has requested a review of a geologically hazardous area to meet the City’s code requirement (19.05.100; City, 2018). The eastern boundary of the Site, along some of the planned single-family residential lots, slopes down to the east with moderate (15 to 40 percent) to steep slopes (greater than 40 percent). The City’s standard development buffer from a steep slope is 25 feet plus a 15-foot building setback, if the slope height exceeds 10 feet. The development will meet these standard setback and buffer distances for the planned residences. Near the northeast corner of the Site, a fill slope constructed as a roadway embankment is planned. The fill embankment will be up to about 25 feet tall, and a portion of it will cross into the steep slope buffer area. Near the southeast corner of the Site, a utility trench will cross through the slopes. The utility trench will contain a water main and a stormwater system. The trench will be excavated up to 4 feet deep and 8 feet wide and will cross several steep slopes. earth +water Aspect Consulting, LLC 350 Madison Avenue N. Bainbridge Island, WA 98110 206.780.9370 www.aspectconsulting.com Seton Construction November 6, 2020 Project No. 200262-02 Page 2 Observations Aspect conducted a field reconnaissance on October 14, 2020. To supplement our field observations, we reviewed historical aerial photos, historical topographic maps, geologic maps, nearby subsurface investigations, and well logs of the Site and its vicinity. Site Conditions The Site covers about 46 acres and is located on the north side of Cook Avenue in an area developed for single-family residential use. The Site is located west of a newly developed 14-lot residential development along Trail Crest Drive. Mass grading, utility installation, and roadway construction has been completed for the development; however, no residences have been built. This development is located at the toe of a natural slope being evaluated for this Project. This slope is estimated to be about 60 to 80 feet in height and ranges in inclinations between 10 to 32 degrees (18 to 53 percent). Limited areas of greater than 40 percent slopes were observed to be between 10 and 30 feet in height. A gravel, recreation trail about 5 feet wide passes through the sloped area, generally in a north to south direction. Vegetation The majority of the slope is vegetated with low brush, woody shrubs, grasses, several mature evergreens ranging from 12 to 24 inches in diameter at breast height (DBH) and scattered deciduous trees. The evergreens on the slope exhibited little to no trunk curvature, suggesting that surficial creep or earth movement is not actively occurring and the underlying soils are stable. Drainage No standing water, seepage, or surface flow was observed at the time of our reconnaissance. The underlying materials are relatively impermeable. Surface drainage conditions, as well as groundwater conditions at the Site, will vary with fluctuations in precipitation, Site usage (such as irrigation), and off-Site land use. Geology The geologic map for the area (Schasse and Slaughter, 2005) indicates the surficial soils are underlain by Vashon lodgment till, which was deposited during the most recent glacial period. Vashon lodgment till is typically a very dense diamict of silt, sand, and gravel. Vashon lodgment till was compacted under the weight of advancing glacial ice and therefore it is typically very dense and exhibits high shear strength. Previous explorations advanced on the Site indicate that the subsurface conditions are consistent across the Site and consist of 0.5 to 1 foot of topsoil, underlain by 3 to 4 feet of a weathered Vashon lodgment till, and ultimately underlain by an unweathered Vashon till (AGRA, 1995). The explorations were advanced 3.5 to 7 feet below the existing ground surface (bgs). This is consistent with the geologic map. Erosion Hazard In our opinion, weathered Vashon till has a low to moderate erosion potential when exposed during construction and the unweathered Vashon till has a low erosion potential. The erosion risk increases on sloped areas, whether natural or excavated during construction. It is our opinion that the erosion Seton Construction November 6, 2020 Project No. 200262-02 Page 3 hazard at the Site can be addressed through standard temporary erosion and sedimentation control (TESC) and best management practices (BMPs) during construction. Liquefaction Hazard Liquefaction occurs when loose, saturated, and relatively cohesionless soil deposits temporarily lose strength from seismic shaking. The primary factors controlling the onset of liquefaction include intensity and duration of strong ground motion, characteristics of subsurface soil, in situ stress conditions, and the depth to groundwater. The Site is mapped as having very low liquefaction susceptibility (Palmer, 2004). Given the relative density, grain size distribution, and geologic origin of the soils at the Site, we do not consider liquefaction to be a significant hazard for the Project. Landslide Hazard Two types of landslides are common on interior steep slopes in the Puget Sound area: deep-seated rotational landslides and shallow flows (also known as surficial or colluvial landslides; Varnes, 1978). Landslides may be triggered by natural events, such as extended, heavy precipitation, freeze-thaw cycles, an earthquake, or by manmade features, such as broken water pipes or improperly managed stormwater flow. Landslide hazard maps of the Site indicate that this slope is an “Intermediate” (Ecology, 1979), because it is generally steeper than 15 percent. We reviewed newest publicly available light detection and ranging (LiDAR) data (DNR, 2005) and aerial photographs (Google, 2020 and NETR, 2020) of the Site local area from 1952 through 2018 and did not identify landslide headscarps or evidence of recent landslides (loss of vegetation) on the slope. Deep-Seated Rotational Landslides Rotational landslides consist of deep-seated failures that typically involve slip along a curved shear plane. Rotational landslides may transport large masses of semi-intact soil downslope, resulting in alternating steep headscarps along the upper portion of the failure plane, with more gently sloping benches composed of displaced soil. Based on the results of our reconnaissance, the slope appears stable. Judging from the relatively uniform and consistent topography, the shape and size of the mature evergreen trees, and the results of previous subsurface observations, the risk of deep-seated landslide activity at or near the Site is low. Shallow Flows Shallow flows consist of relatively shallow failures that typically involve sliding of loose colluvial soil and overlying vegetation that typically mantles steep slopes. Shallow flows are typically triggered by a significant increase in the moisture content within the upper soil layer of a slope, and commonly result from periods of extended or heavy precipitation, groundwater seepage, or concentrated surface water discharge onto a slope. Shallow flows can also occur over time, in a process called ‘creep,’ in which surficial soils slowly move downslope. Surface creep is typically evidenced by curvatures in shade-intolerant trees on the slope. Shallow flows occur within the upper several feet of a slope and typically do not extensively affect the deep-seated or overall stability of a slope. Seton Construction November 6, 2020 Project No. 200262-02 Page 4 We did not observe evidence of recent or incipient shallow flow activity at the Site. The Site is vegetated with evergreen trees that have no noticeable lean or trunk curvature. Conclusions Based on our evaluation, the proposed Project can be constructed in a stable manner and will not negatively affect the stability of the slopes at and near the Site, provided the recommendations contained herein are incorporated into the design and construction of the Project. Geologically Hazardous Area Considerations The Site contains moderate to steep slopes measured greater than 40 percent. We did not observe any groundwater seepage, springs, adverse permeable/impermeable geologic strata sequence, concentrated erosion/scour, signs of past landslides, tension cracks, leaning/bowed vegetation, or other evidence of potential instability. Due to the stability of the slopes, it is our opinion that the standard 25-foot steep slope buffer plus 15-foot building setback distance (40 feet total) for the planned residences is adequate, and the Project will not pose a hazard to the slope stability and can be constructed as proposed. It is also our opinion that the utility trench planned near the southeast corner of the Site and the roadway fill embankment planned in the northwest corner of the site can encroach into the steep slope buffer, can be constructed safely using TESC and BMPs during construction, and will not pose a hazard to the slope stability. Slope Management The most likely impact from slope instability to the Site would be shallow flow landslides triggered by saturation of the near-surface soils. These failure-types are typically limited to the upper 2 to 3 feet of soil on the slope. Table 1 includes factors that can affect stability of the near-surface soil layer: Table 1. Stabilizing and Destablizing Factors for Slope Management Stabilizing Factors Destabilizing Factors Root Reinforcement Roots mechanically reinforce a soil by transfer of shear stresses in the soil to tensile resistance in the roots. Surcharge Weight of vegetation on a slope exerts both a downslope (destabilizing) stress and a stress component perpendicular to the slope, which tends to increase resistance to sliding. Soil Moisture Modification Evapotranspiration and interception in the foliage lower soil moisture content. Root Wedging Alleged tendency of roots to invade cracks, fissures, and channels in a soil and thereby cause local instability by a wedging or prying action. Buttressing and Arching Anchored and embedded stems can act as buttress piles or arch abutments in a slope, counteracting shear stresses. Windthrowing Destabilizing influences from an overturning moment exerted on a slope as a result of strong winds blowing downslope through trees. (Gray and Leiser, 1982) Seton Construction November 6, 2020 Project No. 200262-02 Page 5 Uncontrolled runoff or surface water should never be allowed to flow across the Site slopes. We recommend maintaining dense vegetative groundcover on the Site slopes. If soils on or within 25 feet of the slopes become exposed through erosion and/or shallow failures, we recommend immediately covering and aggressively revegetating the exposed area. This may require the placement of plastic sheeting replaced by a woven jute-mat to provide temporary ground cover while vegetation takes root. For specific vegetation recommendations, Ecology has several good publications on the subject, including:  Vegetation Management: A Guide for Puget Sound Bluff Property Owners, Ecology Publication 93-31, at https://fortress.wa.gov/ecy/publications/SummaryPages/9331.html.  Slope Stabilization and Erosion Control Using Vegetation: A Manual of Practice for Coastal Property Owners, Ecology Publication 93-30, at https://fortress.wa.gov/ecy/publications/SummaryPages/9330.html. Earthwork Basic excavation and grading necessary for the Project can be completed with standard construction equipment. Contractors should be prepared to encounter cobbles or boulders contained in the Vashon lodgment till. Excavation activities would be significantly easier and more cost-effective during the drier summer months. Appropriate erosion control measures should be implemented prior to beginning earthwork activities in accordance with the local regulations. Temporary and Permanent Slopes Maintenance of safe working conditions, including temporary excavation stability, is the responsibility of the contractor. All temporary cuts in excess of 4 feet in height that are not protected by trench boxes, or otherwise shored, should be sloped in accordance with Part N of Washington Administrative Code (WAC) 296-155 (WAC, 2009), as shown in Table 2 below. Table 2. Temporary Excavation Cut Slope Recommendations Soil Unit WAC Soil Classification Maximum Temporary Slope Maximum Height (ft) Fill Type C 1.5H:1V2 20 Topsoil/Colluvium Type C 1.5H:1V 20 Weathered Vashon lodgment till Type A 0.75H:1V 20 Vashon lodgment till Type A 0.75H:1V 20 Notes: 1. H:V = Horizontal to Vertical The estimated maximum cut slope inclinations are provided for planning purposes only and are applicable to excavations without groundwater seepage or runoff and assume dewatered conditions. Flatter slopes will likely be necessary in areas where groundwater seepage exists, or where construction equipment surcharges are placed in close proximity to the crest of the excavation. Seton Construction November 9, 2020 Project No. 200262-02 Page 6 With time and the presence of seepage and/or precipitation, the stability of temporary unsupported cut slopes can be significantly reduced. We recommend planning the construction schedule to have excavation occur during the summer months and to minimize the amount of time that the temporary slopes will be unsupported during construction. The contractor should monitor the stability of the temporary cut slopes and adjust the construction schedule and slope inclination accordingly. Vibrations created by traffic and construction equipment may cause caving and raveling of the face of the temporary slopes. Structural Fill Soil within the Site area is generally expected to be topsoil overlying very dense Vashon lodgment till that contains a significant percentage of fines (silt and clay). From a geotechnical standpoint, the Vashon lodgment till can be suitable for reuse as structural fill provided the materials are well-compacted near optimum moisture content, placed in the dryer summer months, density testing is completed on regular intervals and overseen by Aspect, and that the materials are screened to ensure they are free of organics, cobbles, boulders, and other deleterious debris. Generally, loosely compacted soils are a result of poor construction technique or improper moisture content. Contractors must take extra care when using lodgment till as structural fill. Soils with a high percentage of silt or clay, such as the Vashon lodgment till, are particularly susceptible to becoming too wet for proper compaction. Silty or clayey soils with a moisture content too high for adequate compaction should be dried as necessary, or moisture conditioned by mixing with drier materials, or other methods. Excavation and placement of structural fill near the Site slopes should be observed by Aspect to verify that all unsuitable materials are removed, and suitable compaction is achieved. Backfill within the utility trench and the roadway embankment fill should be placed as structural fill and be placed within 3 percent of its optimum moisture content and compacted to at least 95 percent of the MDD (ASTM, 2018). If the on-site material becomes inadequate, imported structural fill should consist of relatively freely draining, uniformly graded sand and gravel. We recommend Gravel Borrow, as specified in Section 9 03.14(1) of the Standard Specifications (WSDOT, 2020), be specified for imported structural fill. If the site conditions don’t allow the use of on-Site fill material, as specified above, the roadway embankment fill should be constructed using a well-graded mixture of freely-draining sand and gravel. Gravel Borrow per Section 9-03.14 of the WSDOT Standard Specifications should be utilized. The ground surface should be prepared by stripping all vegetation and organic-rich topsoil. Embankment fill material should be systematically placed in horizontal layers not exceeding 18 inches in loose lift thickness, and compacted to at least 95 percent of the Modified Proctor MDD (ASTM D 1557). The fill slope should be constructed with a maximum slope of 2H:1V (horizontal:vertical). The procedure to achieve the specified minimum relative compaction depends on the size and type of compacting equipment, the number of passes, thickness of the layer being compacted, and certain soil properties. Structural fill should be placed and compacted in lifts with a loose thickness no greater than 12 inches when using relatively large compaction equipment, such as a vibrating plate attached to an excavator (hoe pack) or drum roller. If small, hand-operated compaction equipment is used to compact structural fill, lifts should not exceed 6 inches in loose thickness. A Seton Construction November 9, 2020 Project No. 200262-02 Page 7 sufficient number of in-place density tests should be performed as the fill is placed to verify the required relative compaction is being achieved. The frequency of the in-place density testing can be determined by Aspect at the time of construction, when more details of the Project grading and backfilling plans are available. Additional Project Design and Construction Monitoring At the time of this report, site plans, site grading, structural plans, and construction methods for the residential development have not been finalized, and the recommendations presented herein are based on current Project information. If Project developments result in changes to the assumptions made herein, we should be contacted to determine if our recommendations should be revised. We recommend that once design plans are fully developed, Aspect is consulted in order to verify that our recommendations were properly interpreted and applied. This report is issued with the understanding that the information and recommendations contained herein will be brought to the attention of the appropriate design team personnel and incorporated into the Project plans and specifications, and that the necessary steps will be taken to verify that the contractor and subcontractors carry out such recommendations in the field. We do not direct the contractor’s operations, and we cannot be responsible for the safety of personnel other than our own on the Site; the safety of others is the responsibility of the contractor. The contractor should notify the property owner if he considers any of the recommended actions presented herein unsafe. We recommend a pre-construction meeting be organized at the start of construction including the contractor and Aspect. During this meeting, we will understand the goals and schedule to be upheld during construction. We will also discuss effective lines of communication. The integrity of the structural fill compaction, foundation, and overall Site stability depends on proper Site preparation and construction procedures. In addition, engineering decisions may have to be made in the field in the event that variations in subsurface conditions become apparent. References AGRA Earth & Environmental, Inc. (AGRA), 1995, Results of Subsurface Exploration Program – Proposed John Henry Doll Estates – Port Townsend, Washington, Prepared for: Washington Land Design, Project No. 11-10599-00, October 25, 1995. ASTM International (ASTM), 2018, 2018 Annual Book of ASTM Standards, West Conshohocken, Pennsylvania. City of Port Townsend, 2018, Port Townsend Municipal Code, Title 19 Environmental Protection, Chapter 19.05 Critical Areas, Effective December 27, 2018. EnvriroSound Consulting Inc. (ESC), 2010, Geologically Hazardous Critical Areas Review – Proposed Trail Crest Development – Cook Avenue – Port Townsend, Jefferson County, Washington – LIP08-091, Trail Crest Subdivision, Prepared for: Global Investments, Ltd, Project No. ESC10-G014, June 21, 2010. Google, 2019, Google Earth Pro Program, Years reviewed: 1990, 2004, 2006, 2009, 2011, 2013, 2014, 2015, 2016, 2017, and 2018, accessed June 16, 2020. Seton Construction November 6, 2020 Project No. 200262-02 Page 8 Gray, D.H., and A.T. Leiser, 1982, Biotechnical Slope Protection and Erosion Control, Van Nostrand Reinhold, New York. Nationwide Environmental Title Research, LLC (NETR), 2020, Historical Aerials, Years reviewed: 1951 and 1980, https://www.historicaerials.com/, accessed June 16, 2020. Palmer, S.P., S.L. Magsino, E.L. Bilderback, J.L. Poelstra, D.S. Folger, and R.A. Niggemann, 2004, Liquefaction Susceptibility and Site Class Maps of Washington State, By County, Washington State Department of Natural Resources (DNR), Washington Division of Geology and Earth Resources Open File Report 2004-20, September 2004. Schasse, H.W. and Slaughter, S.L., 2005, Geologic Map of the Port Townsend South and Part of the Port Townsend North 7.5-minute Quadrangles, Jefferson County, Washington, Washington State Department of Natural Resources, Division of Geology and Earth Resources, Geologic Map GM-57, Scale 1:24,000, June 2005. Varnes, D.J., 1978, Slope movement types and processes, in Schuster, R.L., and Krizek, R.J., eds., Landslides—Analysis and control: National Research Council, Washington, D.C., Transportation Research Board, Special Report 176, p. 11–33. Washington State Department of Natural Resources (DNR), 2005, Washington Lidar Portal, lidarportal.dnr.wa.gov, accessed June 16, 2020. Washington State Department of Transportation (WSDOT), 2020, Standard Specifications for Road, Bridge and Municipal Construction, Document M 41-10. Washington State Legislature, 2009, Washington Administrative Code (WAC), April 1, 2009. Limitations Work for this project was performed for Seton Construction (Client), and this report was prepared consistent with recognized standards of professionals in the same locality and involving similar conditions, at the time the work was performed. No other warranty, expressed or implied, is made by Aspect Consulting, LLC (Aspect). Recommendations presented herein are based on our interpretation of site conditions, geotechnical engineering calculations, and judgment in accordance with our mutually agreed-upon scope of work. Our recommendations are unique and specific to the project, site, and Client. Application of this report for any purpose other than the project should be done only after consultation with Aspect. Variations may exist between the soil and groundwater conditions reported and those actually underlying the site. The nature and extent of such soil variations may change over time and may not be evident before construction begins. If any soil conditions are encountered at the site that are different from those described in this report, Aspect should be notified immediately to review the applicability of our recommendations. Risks are inherent with any site involving slopes and no recommendations, geologic analysis, or engineering design can assure slope stability. Our observations, findings, and opinions are a means to identify and reduce the inherent risks to the Client. It is the Client's responsibility to see that all parties to this project, including the designer, contractor, subcontractors, and agents, are made aware of this report in its entirety. At the time of Seton Construction November 6, 2020 Project No. 200262-02 Page 9 this report, design plans and construction methods have not been finalized, and the recommendations presented herein are based on preliminary project information. If project developments result in changes from the preliminary project information, Aspect should be contacted to determine if our recommendations contained in this report should be revised and/or expanded upon. The scope of work does not include services related to construction safety precautions. Site safety is typically the responsibility of the contractor, and our recommendations are not intended to direct the contractor’s site safety methods, techniques, sequences, or procedures. The scope of our work also does not include the assessment of environmental characteristics, particularly those involving potentially hazardous substances in soil or groundwater. All reports prepared by Aspect for the Client apply only to the services described in the Agreement(s) with the Client. Any use or reuse by any party other than the Client is at the sole risk of that party, and without liability to Aspect. Aspect’s original files/reports shall govern in the event of any dispute regarding the content of electronic documents furnished to others. Please refer to Appendix B titled “Report Limitations and Guidelines for Use” for additional information governing the use of this report. We appreciate the opportunity to perform these services. If you have any questions please call Alison Dennison, Project Manager, (206) 780-7717. Sincerely, Aspect consulting, LLC Alison J. Dennison, LEG Senior Engineering Geologist adennison@aspectconsulting.com Erik O. Andersen, PE Principal Geotechnical Engineer eandersen@aspectconsulting.com Attachments: Figure 1 – Site Location Map Figure 2 – Site Plan Appendix A – 1995 Report by AGRA Earth & Environmental, Inc. Appendix B – 2010 Report by EnvriroSound Consulting Inc. Appendix C – Report Limitations and Guidelines for Use cc: Al Fure and Adam Stricker, David Evans and Associates, Inc. V:\200262 Trail Crest Pt Townsend\Deliverables\Geotechnical Letter Report\Final\Trail Crest Development - Geotechnical Letter Report_11062020.docx November 6, 2020 November 6, 2020 FIGURES ^Pug e t S o u ndGIS Path: Q:\_GeoTech\200262 Trail Crest Residential Development\2020-10 Geotechnical Report\GIS\01 Site Location Map.mxd || Coordinate System: NAD 1983 StatePlane Washington North FIPS 4601 Feet || Date Saved: 10/29/2020 || User: scudd || Print Date: 10/29/2020Site Location Map Geological Letter Report Trail Crest Residential Development Cook Avenue Port Townsend, Washington FIGURE NO.1OCT-2020 PROJECT NO.200262-02 BY:AJD / RAP / SBM REVISED BY:SCC 0 2,000 4,000 Feet ! ! ! #! ! ! ! !( W A S H I N G T O N Bellingham Olympia Port Angeles Seattle Spokane Tacoma Wenatchee Yakima ! ! ! !( Eaglemount Fort Casey Port Townsend Basemap Layer Credits || Esri, HERE, Garmin, (c) OpenStreetMap contributors, and the GIS user communitySources: Esri, HERE, Garmin, Intermap, increment P Corp., GEBCO, USGS, FAO, NPS, NRCAN, GeoBase, IGN, Kadaster NL, Ordnance Survey, Esri Japan, METI, Esri China (Hong Kong), (c) OpenStreetMap contributors, and theGIS User Community SITE LOCATION SITELOCATION SITELOCATION Approximate Location of Planned Utility Trench Approximate Location of Planned Fill Slope toSupport New Roadway CAD Path: Q:\_GeoTech\200262 Trail Crest Residential Development\2020-10 Geotechnical Report\200262-02.dwg 02 Site Plan || Date Saved: Nov 06, 2020 11:04am || User: scuddGeological Letter Report Trail Crest Residential Development Cook Avenue Port Townsend, Washington 2 BY:AJD/SCC Site Plan Nov-2020 REVISED BY:-PROJECT NO.200262 FIGURE NO.Feet 0 150 300 Source: Base map provided by David Evans and Associates, Inc. on November 5, 2020. APPENDIX A 1995 Report by AGRA Earth & Environmental, Inc. APPENDIX B 2010 Report by EnvriroSound Consulting Inc. APPENDIX C Report Limitations and Guidelines for Use REPORT LIMITATIONS AND GUIDELINES FOR USE Geoscience is Not Exact The geoscience practices (geotechnical engineering, geology, and environmental science) are far less exact than other engineering and natural science disciplines. It is important to recognize this limitation in evaluating the content of the report. If you are unclear how these "Report Limitations and Guidelines for Use" apply to your project or property, you should contact Aspect Consulting, LLC (Aspect). This Report and Project-Specific Factors Aspect’s services are designed to meet the specific needs of our clients. Aspect has performed the services in general accordance with our agreement (the Agreement) with the Client (defined under the Limitations section of this project’s work product). This report has been prepared for the exclusive use of the Client. This report should not be applied for any purpose or project except the purpose described in the Agreement. Aspect considered many unique, project-specific factors when establishing the Scope of Work for this project and report. You should not rely on this report if it was: • Not prepared for you; • Not prepared for the specific purpose identified in the Agreement; • Not prepared for the specific subject property assessed; or • Completed before important changes occurred concerning the subject property, project, or governmental regulatory actions. If changes are made to the project or subject property after the date of this report, Aspect should be retained to assess the impact of the changes with respect to the conclusions contained in the report. Reliance Conditions for Third Parties This report was prepared for the exclusive use of the Client. No other party may rely on the product of our services unless we agree in advance to such reliance in writing. This is to provide our firm with reasonable protection against liability claims by third parties with whom there would otherwise be no contractual limitations. Within the limitations of scope, schedule, and budget, our services have been executed in accordance with our Agreement with the Client and recognized geoscience practices in the same locality and involving similar conditions at the time this report was prepared Property Conditions Change Over Time This report is based on conditions that existed at the time the study was performed. The findings and conclusions of this report may be affected by the passage of time, by events such as a change in property use or occupancy, or by natural events, such as floods, earthquakes, slope instability, or groundwater fluctuations. If any of the described events may have occurred following the issuance of the report, you should contact Aspect so that we may evaluate whether changed conditions affect the continued reliability or applicability of our conclusions and recommendations. ASPECT CONSULTING Geotechnical, Geologic, and Environmental Reports Are Not Interchangeable The equipment, techniques, and personnel used to perform a geotechnical or geologic study differ significantly from those used to perform an environmental study and vice versa. For that reason, a geotechnical engineering or geologic report does not usually address any environmental findings, conclusions, or recommendations (e.g., about the likelihood of encountering underground storage tanks or regulated contaminants). Similarly, environmental reports are not used to address geotechnical or geologic concerns regarding the subject property. We appreciate the opportunity to perform these services. If you have any questions, please contact the Aspect Project Manager for this project. APPENDIX B Slope Stability Analysis Results 1.8631.8631.8631.86338Material NameColorUnit Weight(lbs/Ō3)Strength TypeCohesion(psf)Phi(deg)WaterSurfaceRuTopsoil110Mohr‐Coulomb028None0Weathered Vashon Till130Mohr‐Coulomb25038None0Vashon Till135Mohr‐Coulomb50042None0Fill120Mohr‐Coulomb032None0SidewalkRoadwayManhole or Catch Basin Excavation Area(Depth and Extents Exaggerated for Conservatism)Dry Utility Conduit Excavation Area(Depth and Extents Exaggerated for Conservatism)Ground Surface (approx.)10.05.02402202001806080100120140160180200APPENDIX:B-1REVIEWED BY:AJDBY:JRGPROJECT NO.2002626/17/2021Geotechnical Engineering Report AddendumTrail Crest, Cook AvenuePort Townsend, WashingtonSlope Stability AnalysisP:\_GEOTECH\Trail Crest - Port Townsend\Data\Analyses\2021Analyses\SSA\2021.05.18_rev2_TrailCrest_200262_SSA.slmdSCALE: 1:180SLIDEINTERPRET 8.032Short-Term StabilityStatic Conditions 2.0082.0082.0082.00838Material NameColorUnit Weight(lbs/Ō3)Strength TypeCohesion(psf)Phi(deg)WaterSurfaceRuTopsoil110Mohr‐Coulomb028None0Weathered Vashon Till130Mohr‐Coulomb25038None0Vashon Till135Mohr‐Coulomb50042None0Fill120Mohr‐Coulomb032None0SidewalkRoadwayManhole or Catch Basin Excavation Area(Depth and Extents Exaggerated for Conservatism)Dry Utility Conduit Excavation Area(Depth and Extents Exaggerated for Conservatism)Ground Surface (approx.)10.05.02402202001806080100120140160180200APPENDIX:B-2REVIEWED BY:AJDBY:JRGPROJECT NO.2002626/17/2021Geotechnical Engineering Report AddendumTrail Crest, Cook AvenuePort Townsend, WashingtonSlope Stability AnalysisP:\_GEOTECH\Trail Crest - Port Townsend\Data\Analyses\2021Analyses\SSA\2021.05.18_rev2_TrailCrest_200262_SSA.slmdSCALE: 1:180SLIDEINTERPRET 8.032Long-Term StabilityStatic Conditions 1.2901.2901.2901.29038Material NameColorUnit Weight(lbs/Ō3)Strength TypeCohesion(psf)Phi(deg)WaterSurfaceRuTopsoil110Mohr‐Coulomb028None0Weathered Vashon Till130Mohr‐Coulomb25038None0Vashon Till135Mohr‐Coulomb50042None0Fill120Mohr‐Coulomb032None0SidewalkRoadwayManhole or Catch Basin Excavation Area(Depth and Extents Exaggerated for Conservatism)Dry Utility Conduit Excavation Area(Depth and Extents Exaggerated for Conservatism)Ground Surface (approx.)10.05.0 0.3182402202001806080100120140160180200220APPENDIX:B-3REVIEWED BY:AJDBY:JRGPROJECT NO.2002626/17/2021Geotechnical Engineering Report AddendumTrail Crest, Cook AvenuePort Townsend, WashingtonSlope Stability AnalysisP:\_GEOTECH\Trail Crest - Port Townsend\Data\Analyses\2021Analyses\SSA\2021.05.18_rev2_TrailCrest_200262_SSA.slmdSCALE: 1:200SLIDEINTERPRET 8.032Long-Term StabilitySeismic Conditions APPENDIX C Report Limitations and Guidelines for Use REPORT LIMITATIONS AND GUIDELINES FOR USE Geoscience is Not Exact The geoscience practices (geotechnical engineering, geology, and environmental science) are far less exact than other engineering and natural science disciplines. It is important to recognize this limitation in evaluating the content of the report. If you are unclear how these "Report Limitations and Guidelines for Use" apply to your project or property, you should contact Aspect Consulting, LLC (Aspect). This Report and Project-Specific Factors Aspect’s services are designed to meet the specific needs of our clients. Aspect has performed the services in general accordance with our agreement (the Agreement) with the Client (defined under the Limitations section of this project’s work product). This report has been prepared for the exclusive use of the Client. This report should not be applied for any purpose or project except the purpose described in the Agreement. Aspect considered many unique, project-specific factors when establishing the Scope of Work for this project and report. You should not rely on this report if it was: • Not prepared for you; • Not prepared for the specific purpose identified in the Agreement; • Not prepared for the specific subject property assessed; or • Completed before important changes occurred concerning the subject property, project, or governmental regulatory actions. If changes are made to the project or subject property after the date of this report, Aspect should be retained to assess the impact of the changes with respect to the conclusions contained in the report. Reliance Conditions for Third Parties This report was prepared for the exclusive use of the Client. No other party may rely on the product of our services unless we agree in advance to such reliance in writing. This is to provide our firm with reasonable protection against liability claims by third parties with whom there would otherwise be no contractual limitations. Within the limitations of scope, schedule, and budget, our services have been executed in accordance with our Agreement with the Client and recognized geoscience practices in the same locality and involving similar conditions at the time this report was prepared. Property Conditions Change Over Time This report is based on conditions that existed at the time the study was performed. The findings and conclusions of this report may be affected by the passage of time, by events such as a change in property use or occupancy, or by natural events, such as floods, earthquakes, slope instability, or groundwater fluctuations. If any of the described events may have occurred following the issuance of the report, you should contact Aspect so that we may evaluate whether changed conditions affect the continued reliability or applicability of our conclusions and recommendations. ASPECT CONSULTING Geotechnical, Geologic, and Environmental Reports Are Not Interchangeable The equipment, techniques, and personnel used to perform a geotechnical or geologic study differ significantly from those used to perform an environmental study and vice versa. For that reason, a geotechnical engineering or geologic report does not usually address any environmental findings, conclusions, or recommendations (e.g., about the likelihood of encountering underground storage tanks or regulated contaminants). Similarly, environmental reports are not used to address geotechnical or geologic concerns regarding the subject property. We appreciate the opportunity to perform these services. If you have any questions, please contact the Aspect Project Manager for this project.