Loading...
HomeMy WebLinkAbout09A- Geotech Report 400 North 34th Street, Suite 100 | PO Box 300303 | Seattle, Washington 98103-8636 | 206-632-8020 www.shannonwilson.com August 19, 2022 Ms. Tanya Sharp 1161 Edmonds Street Edmonds, WA 98020 RE: GEOLOGICAL LETTER FOR THE PROPOSED LOT 4 RESIDENTIAL DEVELOPMENT, MATS VIEW ROAD, PORT LUDLOW, WASHINGTON Dear Ms. Sharp: This letter presents the results of our site reconnaissance and seismic design parameters for the proposed Lot 4 residential development located on Mats View Road, near Port Ludlow. We understand you recently purchased the lot and plan to build a single-family residence (SFR) on the property. In 1996, Shannon & Wilson (S&W) prepared a geotechnical report for the development of ½-acre lots in the subdivision. The 1996 study included ten test pit excavations to evaluate subsurface conditions within the subdivision. Subsurface conditions were fairly uniform throughout the subdivision; however, none of the test pits were located on Lot 4. Our scope of services included:  Reviewing our 1996 geotechnical report, aerial photos, and subsurface information from publicly available resources.  Reviewing the Critical Areas Map.  Performing a one-day site reconnaissance.  Characterizing the soil and compare with the conditions encountered in the 1996 geotechnical report.  Addressing the geologic hazards contained in the Jefferson County Code regarding Critical Areas.  Preparing geotechnical conclusions and recommendations,  Preparing this report. PROJECT AND SITE DESCRIPTION The project site is located in the northern portion of the Mats View Meadow Subdivision and designated as Lot 4 in Port Ludlow, Washington, as shown in the Vicinity Map, Figure 1. The tax parcel number for the lot is 970800004. The lot is an irregular-shaped lot Ms. Tanya Sharp August 19, 2022 108265-001 Page 2 of 7 108265-001-L1.docx/wp/lkn and approximately 1.3 acres in area. Mats View Road extends across the south and eastern end of the lot. The ground surface from the south is relatively flat and gently slopes from the west end down to the northeast, towards Mats Bay. The lot is covered in grass with approximately 2- foot-diameter alder, maple, fir, and cedar trees with an undergrowth of sword fern in the north and northeastern section of the lot. Lot 4 is characterized as a geologically hazardous area from the Washington Department of Natural Resources Geologic Hazard Maps. The two geologic hazards identified on this lot are seismic hazard and a shoreline slope stability hazard, as shown in Exhibit 1. Exhibit 1: Lot 4 from Jefferson County GIS Parcel Viewer SITE RECONNAISSANCE On March 22, 2022, S&W representatives visited the site and performed hand borings to assess the subsurface conditions. They walked the property to observe any evidence of slope instability or erosional features. There was no evidence of erosional features or slope instabilities, such as ground cracking, landslide features, or bulges. Seismic Hazard Shoreline Slope Stability: Intermediate slope, Stable slope Ms. Tanya Sharp August 19, 2022 108265-001 Page 3 of 7 108265-001-L1.docx/wp/lkn SUBSURFACE EXPLORATIONS S&W evaluated the subsurface conditions at the project site by performing three hand borings on March 22, 2022. The approximate exploration locations are shown in Figure 2. The hand borings were advanced using a bucket auger and were sampled to depths of between about 3.8 and 5 feet below the existing ground surface (bgs). Very dense soil encountered at depth in the hand borings prevented advancing the borings further. Summary logs of the explorations are enclosed. SUBSURFACE CONDITIONS The subsurface conditions encountered in the explorations generally consisted of topsoil overlying non-glacial and glacial sediment and glacial till. The subsurface conditions are further described as follows:  Topsoil at the site was between about 1.2 and 2.7 feet thick and consisted of loose, Silty Sand (SM) and Silt (ML) with organics.  Underlying the topsoil was glacial and non-glacial sediment consisting of medium dense to dense, Silty Sand (SM); Silty Sand with Gravel (SM); and Poorly Graded Sand (SP), locally with organics and wood.  Glacial till consisting of very dense, Silty Sand (SM) to Silty Sand with Gravel (SM) was encountered in the bottom of all of the hand borings at depths of between about 3.8 and 5 feet bgs. CONCLUSIONS The site and subsurface conditions observed from the site reconnaissance are consistent with the S&W 1996 report. The recommendations provided in the S&W 1996 report are valid for the design of a SFR on this lot. However, the S&W 1996 report did not address the geologic hazardous areas on this lot. Geotechnical recommendations addressing the geologic hazards are presented below. Seismic Hazard The seismic hazard area is shown in Exhibit 1 and is located in the north and northwest corner of the lot. We reviewed the potential seismic hazards, including historical seismic activity and liquefaction susceptibility. Ms. Tanya Sharp August 19, 2022 108265-001 Page 4 of 7 108265-001-L1.docx/wp/lkn Historical Seismic Activity The site is located in a seismically active region that is characterized by large earthquakes associated with subduction interface, subduction intraplate, and shallow crustal sources. Several large earthquakes have occurred regionally over the past 300 years. Historically documented earthquakes with magnitude greater than 6.0 are listed in Exhibit 2. Exhibit 2: Historic Earthquakes Magnitude >6.0 in Washington Region Earthquake Name Distance from Project Limits (miles) Suspected Source Fault Estimated Magnitude Documented Consequences 1700 Cascadia Subduction Zone ~160 to West Cascadia Megathrust ~9.0 Tsunami, landslides 1872 Lake Chelan ~140 to East Unknown ~6.8 Landslide into Columbia River, Geyser near Chelan Falls 1946 Vancouver Island ~150 to Northwest Beaufort Range Fault 7.4 Landslides and building damage on and near Vancouver Island 1949 Puget Sound/Olympia ~65 to Southeast Intraplate 7.0 Several deaths, dozens injured, damaged buildings from Seattle to Chehalis, landslides 1965 Puget Sound ~53 to Southeast Intraplate 6.5 Several deaths, damaged buildings from Seattle to Olympia 2001 Nisqually ~65 to Southeast Intraplate 6.8 One death, 400 injuries, landslides, lateral spreading, damaged buildings from Seattle to Olympia References: Washington Department of Natural Resources, 1986 and 2021; University of Washington, 2001 Liquefaction Potential Liquefaction is a seismic hazard involving severe loss of soil resistance in saturated soils during and shortly after severe ground shaking. The following conditions must be present for liquefaction to occur:  Loose to medium dense, cohesionless, granular soils (e.g., sands and sometimes gravel) or nonplastic to low plasticity, fine-grained soils (e.g., silts); Ms. Tanya Sharp August 19, 2022 108265-001 Page 5 of 7 108265-001-L1.docx/wp/lkn  Saturated soil (soils below the groundwater table and spatially consistent zones of soils saturated by perched groundwater); and  Strong shaking for a relatively long duration (i.e., strong earthquakes). The consequences of liquefaction include soil shear strength loss, ground settlement, down- lateral spreading toward free faces, potential instability of slopes on or containing liquefiable soils, and uplift of buoyant buried structures such as pipes and vaults. We reviewed the Liquefaction Susceptibility Map of Jefferson County, Washington, and the site is mapped as very low to low liquefaction susceptibility. Based on the subsurface conditions, we agree that liquefaction susceptibility is low. Shoreline Slope Hazard The area is also characterized as a shoreline slope hazard area. The area is shown in the northeast corner in Exhibit 1. The shoreline slope hazard is marked as stable intermediate slope. During our site reconnaissance we did not observe any indications of slope instability. RECOMMENDATIONS We referenced the 2018 International Building Code (IBC) to provide seismic design parameters as they have been updated since the 1996 report. We considered the effects of a 1,000-year return period design level earthquake. The peak ground acceleration (PGA) expected during this earthquake is provided in seismic acceleration maps, which are included in the Washington State Department of Transportation Geotechnical Design Manual. This PGA is multiplied by a site coefficient dependent on seismic site class to determine the design PGA designated As. Based on the observed subsurface soil conditions and the Site Class Map of Jefferson County, Washington (2004), it is our opinion that site class D is appropriate for the soils underlying the subject property. We determined As for the subject property is 0.492 times the acceleration of gravity. Exhibit 3: Seismic Design Parameters Site Class Spectral Acceleration at 0.2 sec (g) Spectral Acceleration at 1.0 sec (g) Site Coefficients Design Spectral Response Parameters Fa Fv SDS SD1 D 1.31 0.472 1.2 1.83 1.572 0.86 Ms. Tanya Sharp August 19, 2022 108265-001 Page 6 of 7 108265-001-L1.docx/wp/lkn The above response spectra parameters for Site Class D were evaluated based on mapped seismological inputs and the tabulated site coefficients provided in the 2018 IBC and American Society of Civil Engineers (ASCE) 7-16. These parameters do not represent a site- specific site response analysis. In accordance with the 2018 IBC Section 1612.2.3 and ASCE 7-16 Section 11.4.8, a ground motion hazard analysis (site-specific site response analysis) is required for Site Class D sites where S1 is greater than or equal to 0.2 g, as occurs at this site. However, the guidance of Section 11.4.8 allows the use of tabulated site coefficients if the value of the seismic response coefficient, CS, is determined by ASCE 7-16 Equation 12.8-2 for values of T ≤ 1.5 Ts and taken as equal to 1.5 times the value computed in accordance with either ASCE 7-16 Equation 12.8-3 for TL ≥ T > 1.5 Ts or ASCE 7-16 Equation 12.8-4 for T > TL (see ASCE 7-16 Section 11.4.8 Exception 2). Application of the response spectrum parameters in Exhibit 3 requires determination of CS using the equation as described. CLOSURE This letter was prepared for the exclusive use of the property owner, Ms. Sharp for seismic design of the proposed SFR on Lot 4. The professional opinions, recommendations, and conclusions contained in this geological letter are valid for a period not greater than five years from the date of this letter. This time limitation is included in recognition that the site conditions can and do change with time. This letter should not be used without our approval if any of the following occurs:  Conditions change due to natural forces or human activity under, at, or adjacent to the site.  Assumptions stated in this geological letter have changed.  Project details change or new information becomes available such that our evaluations, analyses, conclusions, and recommendations may be affected.  If the site ownership or land use has changed.  More than five years has passed since the date of this letter. For any site located on or near a slope, there are slope instability risks that present and future owners have to accept, including, but not limited to:  Natural factors: soil and groundwater conditions, steep topography, heavy rainfall events, erosion, and vegetation conditions.  Human-related factors: water leaks, pipe breaks, improper drainage, lack of maintenance of vegetation or drainage facilities, fill or debris placement, excavation, and/or removal of trees/vegetation. Ms. Tanya Sharp August 19, 2022 108265-001 Page 7 of 7 108265-001-L1.docx/wp/lkn Similar circumstances or other unknown conditions may also affect slope stability. Our evaluation and recommendations described herein are not a guarantee or warranty of future stability. We have prepared the document “Important Information About Your Geotechnical/ Environmental Report” to assist you and others in understanding the use and limitations of this geotechnical report. Please read this document to learn how you can lower your risks for this project. Sincerely, SHANNON & WILSON Andrew Tangsombatvisit Geotechnical Staff Martin W. Page, PE, LEG Vice President Geotechnical Engineer AXT:DPO:MWP/axt Enc. Figure 1 – Vicinity Map Figure 2 – Site and Exploration Plan Summary Hand Boring Logs Important Information About Your Geotechnical/Environmental Report VICINITY MAP PROJECT LOCATION Sharp Residence Lot 4, Mats View Road Port Ludlow, Washington Aerial imagery downloaded from Google Earth Pro on 08-18-22. August 2022 108265-001 Filename: E:\J\_SEA\108265\001\108265-001 Fig 1 - Vicinity Map.dwg Date: 08-19-2022 Login: SACSeattlePortLudlow Washington Project Location 90 5 0 2 4 Approximate Scale in Miles FIG. 1 Puget Sound Port LudlowOak Bay Rd 525 116 19 104 104 Chimicum Port Gamble Squamish Harbor Hansville Mutiny Bay Freeland Marrowstone SITE AND EXPLORATION PLAN Sharp Residence Lot 4, Mats View Road Port Ludlow, Washington Aerial imagery downloaded from Google Earth Pro on 08-18-22. August 2022 108265-001 Filename: E:\J\_SEA\108265\001\108265-001 Fig 2 - Site Plan.dwg Date: 08-19-2022 Login: SAC0 80 160 Approximate Scale in Miles FIG. 2 HA-1 Hand Auger Designation and Approximate Location LEGEND NOTE Lot boundaries and proposed structures derived from Mats View Meadows Plat Map prepared by David Cummins & Associates on March 2002. HA-1 HA-2 HA-3 Proposed Residence Location Proposed Driveway Lot 3 Lot 4 Mats V i e w R d MarianneMeadow Har o l d s H o l l o wMats View Rd 108265-001-L1-Summary.docx/wp/lkn 108265-001 HAND AUGER LOGS – LOT 4, MATS VIEW ROAD HA-1 Depth Below Existing Ground Surface, Feet Soil Description 0 – 2.7 Loose, dark brown, Silty Sand (SM); moist; fine to medium sand; nonplastic; organics. 2.7 – 4 Medium dense, gray-blue, Poorly Graded Sand (SP); wet; few gravel; fine to medium sand; nonplastic. 4 – 4.3 Dense, gray, Poorly Graded Sand (SP); wet; fine to medium sand; nonplastic. > 4.3 Very dense, gray, Silty Sand (SM); moist; trace of gravel; fine to medium sand; nonplastic; diamict. HA-2 Depth Below Existing Ground Surface, Feet Soil Description 0 – 1.2 Loose, dark brown, Silty Sand (SM); moist; fine to medium sand; nonplastic; roots. 1.2 – 2 Medium dense, gray-brown, Silty Sand (SM); moist; fine to medium sand; few coarse, rounded gravel; nonplastic. 2 – 2.5 Medium dense, mottled gray-brown, Silty Sand (SM); wet; fine to medium sand; nonplastic. 2.5 – 5 Medium dense to dense, gray-blue to gray, Poorly Graded Sand (SP); moist; fine to medium sand, nonplastic, becomes dense at 4 feet. > 5 Very dense, gray-brown, Silty Sand (SM); moist; few coarse, rounded gravel; fine to medium sand; nonplastic; diamict. HA-3 Depth Below Existing Ground Surface, Feet Soil Description 0 – 1.9 Loose, dark brown, Silt (ML); wet; few fine to medium sand; nonplastic; organics, few roots; 3-inch-diameter round cobble at 1 foot. 1.9 – 3 Dense, blue-gray, Silty Sand (SM); wet; few medium to coarse, rounded gravel; fine to medium sand; nonplastic. 3.3 – 3.8 Dense, gray, Silty Sand (SM); moist; fine sand; nonplastic; organics/wood > 3.8 Very dense, gray-brown, Silty Sand with Gravel (SM); moist; few coarse, rounded gravel; fine to medium sand; nonplastic; diamict. 108265-001 Page 1 of 2 August 19, 2022 (1/2022) Ms. Tanya Sharp Edmonds, Washington IMPORTANT INFORMATION ABOUT YOUR GEOTECHNICAL/ENVIRONMENTAL REPORT CONSULTING SERVICES ARE PERFORMED FOR SPECIFIC PURPOSES AND FOR SPECIFIC CLIENTS. Consultants prepare reports to meet the specific needs of specific individuals. A report prepared for a civil engineer may not be adequate for a construction contractor or even another civil engineer. Unless indicated otherwise, your consultant prepared your report expressly for you and expressly for the purposes you indicated. No one other than you should apply this report for its intended purpose without first conferring with the consultant. No party should apply this report for any purpose other than that originally contemplated without first conferring with the consultant. THE CONSULTANT’S REPORT IS BASED ON PROJECT-SPECIFIC FACTORS. A geotechnical/environmental report is based on a subsurface exploration plan designed to consider a unique set of project-specific factors. Depending on the project, these may include the general nature of the structure and property involved; its size and configuration; its historical use and practice; the location of the structure on the site and its orientation; other improvements such as access roads, parking lots, and underground utilities; and the additional risk created by scope-of-service limitations imposed by the client. To help avoid costly problems, ask the consultant to evaluate how any factors that change subsequent to the date of the report may affect the recommendations. Unless your consultant indicates otherwise, your report should not be used (1) when the nature of the proposed project is changed (for example, if an office building will be erected instead of a parking garage, or if a refrigerated warehouse will be built instead of an unrefrigerated one, or chemicals are discovered on or near the site); (2) when the size, elevation, or configuration of the proposed project is altered; (3) when the location or orientation of the proposed project is modified; (4) when there is a change of ownership; or (5) for application to an adjacent site. Consultants cannot accept responsibility for problems that may occur if they are not consulted after factors that were considered in the development of the report have changed. SUBSURFACE CONDITIONS CAN CHANGE. Subsurface conditions may be affected as a result of natural processes or human activity. Because a geotechnical/environmental report is based on conditions that existed at the time of subsurface exploration, construction decisions should not be based on a report whose adequacy may have been affected by time. Ask the consultant to advise if additional tests are desirable before construction starts; for example, groundwater conditions commonly vary seasonally. Construction operations at or adjacent to the site and natural events such as floods, earthquakes, or groundwater fluctuations may also affect subsurface conditions and, thus, the continuing adequacy of a geotechnical/environmental report. The consultant should be kept apprised of any such events and should be consulted to determine if additional tests are necessary. MOST RECOMMENDATIONS ARE PROFESSIONAL JUDGMENTS. Site exploration and testing identifies actual surface and subsurface conditions only at those points where samples are taken. The data were extrapolated by your consultant, who then applied judgment to render an opinion about overall subsurface conditions. The actual interface between materials may be far more gradual or abrupt than your report indicates. Actual conditions in areas not sampled may differ from those predicted in your report. While nothing can be done to prevent such situations, you and your consultant can work together to help reduce their impacts. Retaining your consultant to observe subsurface construction operations can be particularly beneficial in this respect. Ms. Tanya Sharp Edmonds, Washington 108265-001 Page 2 of 2 August 19, 2022 (1/2022) A REPORT’S CONCLUSIONS ARE PRELIMINARY. The conclusions contained in your consultant’s report are preliminary, because they must be based on the assumption that conditions revealed through selective exploratory sampling are indicative of actual conditions throughout a site. Actual subsurface conditions can be discerned only during earthwork; therefore, you should retain your consultant to observe actual conditions and to provide conclusions. Only the consultant who prepared the report is fully familiar with the background information needed to determine whether or not the report’s recommendations based on those conclusions are valid and whether or not the contractor is abiding by applicable recommendations. The consultant who developed your report cannot assume responsibility or liability for the adequacy of the report’s recommendations if another party is retained to observe construction. THE CONSULTANT’S REPORT IS SUBJECT TO MISINTERPRETATION. Costly problems can occur when other design professionals develop their plans based on misinterpretation of a geotechnical/environmental report. To help avoid these problems, the consultant should be retained to work with other project design professionals to explain relevant geotechnical, geological, hydrogeological, and environmental findings, and to review the adequacy of their plans and specifications relative to these issues. BORING LOGS AND/OR MONITORING WELL DATA SHOULD NOT BE SEPARATED FROM THE REPORT. Final boring logs developed by the consultant are based upon interpretation of field logs (assembled by site personnel), field test results, and laboratory and/or office evaluation of field samples and data. Only final boring logs and data are customarily included in geotechnical/environmental reports. These final logs should not, under any circumstances, be redrawn for inclusion in architectural or other design drawings, because drafters may commit errors or omissions in the transfer process. To reduce the likelihood of boring log or monitoring well misinterpretation, contractors should be given ready access to the complete geotechnical engineering/environmental report prepared or authorized for their use. If access is provided only to the report prepared for you, you should advise contractors of the report’s limitations, assuming that a contractor was not one of the specific persons for whom the report was prepared, and that developing construction cost estimates was not one of the specific purposes for which it was prepared. While a contractor may gain important knowledge from a report prepared for another party, the contractor should discuss the report with your consultant and perform the additional or alternative work believed necessary to obtain the data specifically appropriate for construction cost estimating purposes. Some clients hold the mistaken impression that simply disclaiming responsibility for the accuracy of subsurface information always insulates them from attendant liability. Providing the best available information to contractors helps prevent costly construction problems and the adversarial attitudes that aggravate them to a disproportionate scale. READ RESPONSIBILITY CLAUSES CLOSELY. Because geotechnical/environmental engineering is based extensively on judgment and opinion, it is far less exact than other design disciplines. This situation has resulted in wholly unwarranted claims being lodged against consultants. To help prevent this problem, consultants have developed a number of clauses for use in their contracts, reports, and other documents. These responsibility clauses are not exculpatory clauses designed to transfer the consultant’s liabilities to other parties; rather, they are definitive clauses that identify where the consultant’s responsibilities begin and end. Their use helps all parties involved recognize their individual responsibilities and take appropriate action. Some of these definitive clauses are likely to appear in your report, and you are encouraged to read them closely. Your consultant will be pleased to give full and frank answers to your questions. The preceding paragraphs are based on information provided by the Geoprofessional Business Association (https://www.geoprofessional.org)