Loading...
Geotechnical Letter Kiehn 10-29-2019.pdfSHMNON 6WILSON GEOTECHNICAL AND ENVIRONMENTAL CONSULTANTS October 29, 2019 Mr. Shane Kiehn 23615 991h Place W Edmonds, WA 98020 RE- GEOTECHNICAL EVALUATION OF STEEP SLOPE, 23615 99TH PLACE W, EDMONDS, WASHINGTON Dear Mr. Kiehn: This letter presents our observations and recommendations regarding the steep slope at 23615 99th Place W, Edmonds, Washington. The toe of the slope is about 30 feet east from the back of the residence. The slope is about 40 feet tall and slopes down to the west at an average slope of about 40%. Residences on 97th Place W are at the top of the slope. Because the slope is 40% or more, the City of Edmonds ("City") has classified the property as a landslide hazard area. Based on a phone call with you, we understand that the City requires a geotechnical evaluation of the slope so that you can proceed with cutting alder trees at the base of the slope. A certified arborist has recommended removing seven alder trees down to ground level and replanting the slope with two new trees for each tree removed (Option 3 in the arborist report). Our scope of services included: • Reviewing publicly available geologic data, including geologic maps, and previous subsurface explorations; • Visiting your property to evaluate the condition of the slope; ■ Qualitatively evaluating the impact of tree trimming on the stability of the slope; and ■ Preparing this letter. The following sections summarize our findings. EXISTING SUBSURFACE DATA We reviewed our own files and the Washington State Department of Natural Resources (DNR) online portal to evaluate the site subsurface conditions. We did not locate existing subsurface explorations within 500 feet of the property. Based on DNR geologic maps, the site sits near the contact of Vashon advance outwash and Vashon till. Advance outwash generally consists of very dense, relatively clean sand. Till is a very dense/hard, unsorted 400 North 34th Street ■ Suite 100 ■ PO Box 300303 ■ Seattle, Washington 98103-8636 ■ 206 632-8020 ■ Fax 206 695-6777 ■ www.shannonwilson.com ■ Mr. Shane Kiehn 0ctober29, 201 9 Page 2 of 5 IIISHANNON 6WILSON mixture of clay, silt, sand, and gravel. Both the advance outwash and till were overridden by the most recent glaciation. SITE VISIT We visited your property on September 23, 2019. The soil on the slope was typically gray silt/clay with sand and scattered cobbles (Exhibit 1). The soil appeared to be consistent with weathered Vashon till. The upper 6 to 12 inches of soil was soft and moist to wet; the soil density increased with depth. We did not observe seeps or springs on the slope. The slope was vegetated with shrubs, low brush, and trees (Exhibit 1). Trees on the slope ranged from about 6 to 24 inches in diameter. Most of the tree trunks were vertical and did not show signs of downslope soil creep; however, about four trees in the midslope were bowed downslope. The bottom 15 feet of the slope was about 45 degrees. The 45-degree section was confined to the property; to the north and south, the toe of the slope was retained by rockeries up to about 4 feet tall (Exhibit 2). Above the 45-degree section, the slope angle varied from about 23 to 35 degrees. Exhibit 1: Exposed soil (a) and typical vegetation (b), 104070-001 L1.docx/wp/Ikn 104070-001 Mr. Shane Kiehn 0ctober29 2019 Page 3 of 5 IIISHANNON 6WILSON Exhibit 2: Rockery at toe of slope to north of subject property (a) and toe of slope on subject property (b). We observed two pipelines on the slope. At the south side of the slope, we observed a sealed garden hose attached to an approximately 1/1—inch-diameter black hose that was buried in the slope. At the north side of the slope, we observed an approximately 2-inch- diameter black polyvinyl chloride pipe that extended downslope. The pipe was exposed for about 6 feet and otherwise buried. SLOPE STABILITY The very dense advance outwash and till typically form stable slopes at slopes of 50% and flatter. Consistent with this observation, the DNR maps do not show active landslides on or near the site. Based on our site observations, we did not see evidence of recent landsliding. 104070-001 L1.docx/wp/Ikn i[1L[iy(13i101 Mr. Shane Kiehn 0ctober29, 2019 Page 4 of 5 IIISHANNON 6WILSON We did observe indications of soil creep, which is the downslope movement of loose surficial soils under the effects of gravity that is typical for topsoil on steep slopes. IM PACTS O F TREE C UTTING O N SLO PE STABILITY The Edmonds Community Development Code (ECDC) 23.40.005 states that cutting or removing healthy trees within a landslide hazard area is considered an alteration of a critical area. ECDC 23.80.060 and 23.80.070 present development standards for alterations of critical areas. The development standards require that a registered professional engineer evaluate the impacts of the proposed tree cutting on the critical area and adjacent properties. Based on our site visit and review of geologic data, we have qualitatively evaluated the stability of the slope and the potential impacts of the proposed tree cutting and replanting. In our opinion, the proposed activities would likely not increase the landslide hazard at the site and would likely not affect adjacent properties. In our opinion, the proposed tree cutting and replanting conforms to the requirements in ECDC 23.80.060 and 23.80.070. CLOSURE We based our evaluation on our literature review and site visit. We did not perform subsurface explorations as part of this study. As with any site located on or near a slope, there are always risks of instability that present and future owners must be prepared to accept. In addition to natural factors (soil and groundwater conditions, steep topography, heavy rainfall events, etc.), the risks are linked to events such as: ■ Upslope water leaks, ■ Pipe breaks, ■ Improper drainage, ■ Lack of maintenance of drainage facilities, ■ Unwise excavations into the hillside, ■ Removal of trees and vegetation and/or poor maintenance of trees, and ■ Similar or unknown circumstances that may cause sliding. Because of these inherent risks, our evaluation described herein is not a guarantee or warranty of future stability. 104070-001 L1.docx/wp/Ikn i[1L[it(13i1111 Mr. Shane Kiehn October 29, 2019 Page 5 of 5 011SHANNON MLSON The conclusions and recommendations presented in this letter report are based on observed site conditions as they presently exist. Shannon & Wilson has prepared the enclosure, "Important Information About Your Geotechnical Report," to assist you and others in understanding the use and limitations of our reports. Sincerely, SHANNON & WILSON ,�XIWLgS of�W�ASy�'�' !97 LE Jeremy N. Butkovich, PE Associate JNB:WJP/jnb Enc. 1612qlzoll Important Information About Your Geotechnical Report 104070-001-u .docxwap/I kn 104070-001 SHANNON 6WILSON OEOTECHNICAL AND ENVIRONMENTAL CONSULTANTS Attachment to and part of Report: Date: October 29, 2019 To: Mr. Shane Kiehn Edmonds, Washington Important Information About Your Geotechnical Report CONSULTING SERVICES ARE PERFORMED FOR SPECIFIC PURPOSES AND FOR SPECIFIC CLIENTS. 104070-001 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. Page 1 of 2 1/2019 IIISHANNON 6WILSON 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 ASFE/Association of Engineering Firms Practicing in the Geosciences, Silver Spring, Maryland Page 2 of 2 1/2019