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HomeMy WebLinkAbout821333042 Geotech Assessment (2022) August 26, 2022 William Gaudet 6925 SW Alta Vista Port Orchard, WA 98367 Attn: Kathy Swenson kathyswenson5@gmail.com (253) 228-8090 Geological Letter Proposed Single-Family Residence xxx – Teal Lake Road Jefferson County, Washington PN: 821333042 Doc ID: Gaudet.TealLakeRd.GHA INTRODUCTION This Geological Letter summarizes our assessment of potential geologic hazards for the proposed single-family residence to be constructed on the above referenced parcel. The unaddressed parcel is located near the intersection of State Highway 104 and Teal Lake Road in the Port Ludlow area of Jefferson County, Washington. The general location of the site is shown on the attached Site Location Map, Figure 1. Our understanding of the project is based on our conversations with you, our August 12, 2022 site visit, our understanding of the Jefferson County Code (JCC) Section 18.22.500, and our experience in the area. We understand that you are looking at several different locations on the property to potentially build a residence. We anticipate that a residence at the site will be a one- or two-story wood framed structure supported by conventional shallow foundations. According to Jefferson County, the site is located within a potentially geologically hazardous area. As such, Jefferson County is requiring a Geologic Assessment be prepared in accordance with the JCC, Chapter 18.22.540. This report addresses the County’s requirements. SCOPE The purpose of our services was to evaluate the site conditions basis for assessing potential adverse impacts to and from the slopes located within the site vicinity. Specifically, the scope of services will include the following: 1. Reviewing the available geologic, hydrogeologic, and geotechnical data for the site area; 2. Performing a site reconnaissance; and, 3. Preparing a Geological Letter (a critical areas assessment that does not include geotechnical design recommendations, unless listed above) to address the Jefferson County Code, Title 18.22.540. Gaudet.TealLakeRd.GHA August 26, 2022 page | 2 The above scope of work was summarized in our Proposal for Geotechnical Engineering Services dated August 5, 2022. We received written authorization to proceed with our scope of work from you on the same day. SITE CONDITIONS Surface Conditions The subject parcel is located 3 parcels north of the intersection of State Highway 104 and Teal Lake Road in the Port Ludlow area of Jefferson County, Washington. The parcel is irregular in shape, measuring approximately 325 to 390 feet wide (west to east) by 550 to 740 feet long (south, to north), and encompasses about 5.74 acres. The site is bounded by undeveloped parcels to the north, south, and west and by existing large-lot residential development to the east. Access is gained to the site from Teal Lake Rd via a gravel road which passes through the parcel to the east. According to topographic information obtained from Jefferson County GIS and our site observations, the ground surface of the site generally slopes up to the north and northwest. There is a shared access driveway that bisects the site from the southeast to the northwest. From the southern portion of the site, the ground surface slopes up to the northwest at approximately 15 to 30 percent. These slopes continue throughout the central and northwestern portion of the site. In the northeast portion of the site, on the east side of the bisecting driveway, the ground surface slopes steeply up to the north at approximately 40 to 50 percent over a topographic relief of approximately 45 feet before gently sloping at approximately 10 to 15 percent. The total topographic relief of the site is on the order of approximately 130 feet. The existing site configuration and topography are shown on the Site Vicinity Map, Figure 2. Vegetation across the site generally consists of a moderate stand of coniferous and deciduous trees with a dense understory of native and invasive plants and shrubs. No seeps, springs, or standing water were observed at the time of our site reconnaissance. No signs of slope instability or surficial erosion were observed at the time of our site reconnaissance. Site Soils The USDA Natural Resources Conservation Survey (NRCS) Web Soil Survey i ndicates that the site is underlain by Carlsborg gravelly loamy sand (CaC and CaD) soils. Detailed descriptions of these soils are included below. An excerpt of the NRCS map for the site area is included as Figure 3. • Carlsborg gravelly loamy sand (CaC and CaD): The Carlsborg gravelly loamy sand soils are derived from alluvium and glacial outwash and are included in the hydrologic soils group A. The CaD soils mapped as underlying the majority of the site, form on slopes of 15 to 30 percent, and are considered to have a “moderate to severe” erosion hazard when exposed. The CaC soils are mapped as underlying the southwestern portion of the site form on slopes of 0 to 15 and are considered to have a “moderate” erosion hazard when exposed. Site Geology The Geologic map of the Lofall 7.5-minute quadrangle, Jefferson and Kitsap Counties, Washington, By Contreras, et Al. (2013) maps the site as being located within an area that is underlain by Vashon Recessional Outwash Sand (Qgos). No landslides, mass wasting deposits, or alluvial fans are mapped Gaudet.TealLakeRd.GHA August 26, 2022 page | 3 on or within the site vicinity. A detailed description of the above listed geologic unit is included below. An excerpt of the above referenced geologic map is attached as Figure 4. • Latest Vashon recessional outwash (Qgos): The outwash soils were deposited during the Vashon Stade of the Fraser Glaciation, some 12,000 to 15,000 years ago. The recessional outwash was deposited by meltwater streams emanating from the retreating ice mass and is considered normally consolidated. As such, the recessional outwash generally has moderate strength and compressibility characteristics w here undisturbed. The infiltration potential of outwash is generally favorable. We also reviewed the Washington State Department of Ecology (DOE) Co astal Atlas, which maps the southern and central portions of the site as “unstable” and the northern portion of the site as “intermediate”. These designations are likely because of the height and inclination of the slopes. An excerpt of the WA DOE Coastal Zone Atlas map for the site area is included as Figure 5. ENGINEERING CONCLUSIONS AND RECOMMENDATIONS Based on the results of our data review, site reconnaissance, and our experience in the area, it is our opinion that an active landslide hazard does not exist on or within the site vicinity. Pertinent conclusions and geotechnical recommendations regarding the proposed development are presented below. Geologically Hazardous Areas – per Jefferson County Code Chapter 18.22.510 Jefferson County Title 18.22.510 defines geologically hazardous areas based on the following classification/designation: (1) The following are geologically hazardous areas and subject to the standards of this article when mapped as high or moderate geologically hazardous areas: A. Erosion hazard areas (as defined in JCC 18.10.050). (i) “Erosion hazard areas” has the same meaning as in WAC 365-190-030(5). "Erosion hazard areas" are those areas containing soils which, according to the United States Department of Agriculture Natural Resources Conservation Service Soil Survey Program, may experience significant erosion. Erosion hazard areas also include coastal erosion-prone areas and channel migration zones. B. Landslide hazard areas (as defined in JCC 18.10.120). Landslide hazard areas include any areas susceptible to landslide because of any combination of bedrock, soil, slope (gradient), slope aspect, structure, hydrology, or other factors, as follows: (i) Areas of historic failures, such as: (A) Areas delineated by United States Department of Agriculture, Natural Resources Conservation Service as having a significant limitation for building site development; (B) Coastal areas mapped by the Washington Department of Ecology Coastal Atlas as unstable, unstable old slides, and unstable recent slides; or Gaudet.TealLakeRd.GHA August 26, 2022 page | 4 (C) Areas designated and mapped as quaternary slumps, earthflows, mudflows, lahars, or landslide hazards by the Washington Department of Natural Resources or the United States Geological Survey. (ii) Areas where all three of the following conditions occur: (A) Slopes are steeper than 15 percent; (B) Hillsides intersecting geologic contacts with a relatively permeable sediment overlying a relatively impermeable sediment or bedrock; and (C) Spring or groundwater seepage. (iii) Areas that have shown movement during the Holocene epoch (from 10,000 years ago to present) or have been underlain or covered by mass wastage debris of this epoch. (iv) Areas with slopes that are parallel or subparallel to planes of weakness (such as bedding planes, joint systems, and fault planes) in subsurface materials. (v) Areas with slopes having gradients steeper than 80 percent subject to rockfall during seismic shaking. (vi) Areas that are potentially unstable as a result of rapid stream incision, stream bank erosion, and undercutting by wave action, including stream channel migration zones. (vii) Areas that show evidence of, or are at risk from, snow avalanches. (viii) Areas located in a canyon or on an active alluvial fan, presently or potentially subject to inundation by debris flows or catastrophic flooding. (ix) Areas with a slope of 40 percent or steeper and with a vertical relief of 10 or more feet, except areas composed of bedrock. C. Seismic hazard areas (as defined in JCC 18.10.190). (i) “Seismic hazard areas” has the same meaning as in WAC 365-190-030(18) "Seismic hazard areas" are areas subject to severe risk of damage as a result of earthquake induced ground shaking, slope failure, settlement, soil liquefaction, debris flows, lahars, or tsunamis. D. Channel migration zones (CMZs) (as defined in JCC 18.10.030). E. Seiche and landslide generated wave hazard areas (as defined in JCC 18.10.190). (i) “Seiche and landslide generated wave hazard areas” means lake and marine shoreline areas susceptible to flooding, inundation, debris impact, or mass wasting as the result of a seiche or landslide generated waves. No known best available science is currently available to characterize potential seiche hazards in Jefferson County. F. Tsunami hazard areas (as defined in JCC 18.10.200). (i) “Tsunami hazard areas” means coastal areas susceptible to flooding, inundation, debris impact, or mass wasting as the result of a tsunami generated by seismic events. [Ord. 5-20 § 3 (Appx. A); Ord. 3-20 § 1 (Appx. A); Ord. 8-06 § 1] G. Other geologic events, such as coal mine hazards and volcanic hazards, are not a known risk in Jefferson County, and are therefore not subject to review as part of the permitting process. [Ord. 5-20 § 2 (Appx. A)] Gaudet.TealLakeRd.GHA August 26, 2022 page | 5 A – Erosion Hazard Areas The site is mapped by the NRCS as being underlain by Carlsborg gravelly loamy sandy (CaC and CaD) soils which have a “moderate” and “moderate to severe” erosion hazard, respectively. The site is not located along the shoreline or waterbody. Furthermore, no signs of active or ongoing erosion were observed at the time of our site reconnaissance. Accordingly, it is our opinion that the site does not meet the definition of an erosion hazard area and no prescriptive buffer for erosion hazard areas should be required by Jefferson County. B – Landslide Hazard Areas The Geologic map of the Lofall 7.5-minute quadrangle, Jefferson and Kitsap Counties, Washington, By Contreras, et Al. (2013) does not map the site or site vicinity as being underlain by a landslide deposit. The WA Department of Ecology Slope Stability maps the site has “intermediate” to “unstable” but does not map any historic or prehistoric failures on or within the site vicinity. Similarly, the Washington Department of Natural Resources Landslide Compilation and Inventory does not map any landslide or mass wasting deposits on or in the immediate vicinity of the site. There are slopes steeper than 15 percent at the site, however we do not interpret the site to be underlain by an adverse geologic contact, nor did we observe any seeps or springs along slope faces. We do not interpret the site to have slopes that are parallel or su bparallel to planes of weakness. No slopes of 80 percent or more are present at or within the site vicinity. The site is not located along a stream or shoreline, therefore, there is no potentially unstable areas due to stream migration or wave erosion. We do not interpret the site to be at risk on snow avalanches . There are slopes steeper than 40 percent with 10 feet or more of vertical relief to the south of the site, however, we did not observe any signs of slope instability during our site reconnaissance. While the site does have one of the indicators of a landslide hazard area (slopes steeper than 40 percent with a vertical relief of 10 or more feet) as defined by Jefferson County Title 18.22.510, it is our opinion that the slope on and adjacent to the site does not meet the definition of a landslide hazard area and no prescriptive buffer or development limitation should be imposed by Jefferson County. C – Seismic Hazard Areas The Washington State Department of Natural Resources Geology Portal maps the site as being located approximately 630 feet north of the Lofall Fault. The Geology Portal also maps the site as having a “low” susceptibility to liquefaction, ho wever, areas to the southwest of the site are mapped as having a “high” susceptibility to liquefaction. No evidence of ground fault rupture was observed at the time of our site visit. Additionally, based on the mapped soils at the site, we interpret the site to be at a low risk for liquefaction. Based on the above, it is our opinion that the site is not in a seismic hazard area. An excerpt of the DNR Liquefaction Susceptibility map is included as Figure 6. D – Channel Migration Zones The site is not located within a channel migration zone. Therefore, there is no risk associated with channel migration zones for the proposed development. Gaudet.TealLakeRd.GHA August 26, 2022 page | 6 E – Seiche and Landslide Generated Wave Hazard Areas The site is not located along the shoreline, therefore is at no risk of seiche or landslide generated wave hazards. F – Tsunami Hazard Areas The site is not mapped by the WA DNR Tsunami Inundation Maps. Given the elevation of the site (130 feet to 250 feet) and distance from the shoreline, it is our opinion that the site is at low risk of inundation via tsunami. Recommend Setback While it is our opinion that Jefferson County should not impose critical area buffers at the site, a setback from slopes steeper than 33 percent in accordance with the 2018 International Building Code (IBC) will be required. The 2018 IBC section 1808.7 requires a building setback from slopes that are steeper than 3H:1V (Horizontal:Vertical) unless evaluated and reduced and/or a structural setback is provided by a licensed geotechnical engineer. The setback di stance is calculated based on the vertical height of the slope. The typical IBC setback from the top of the slope equals the lesser of one third the height of the slope, or 40 feet, and the lesser of half the height of the slope from the toe of the slope, or 15 feet. The steeply sloping area in the northeast portion of the site has slopes steeper than 3H:1V with a topographic relief of approximately of approximately 45 feet. Therefore, per the 2018 IBC, any structures should be setback at least 15 feet from the top of the slope and at least 15 feet from the base of this slope. We can provide additional information regarding setbacks upon request. LIMITATIONS We have prepared this report for use by William Gaudet and other members of the design team, for use in the design of a portion of this project. The data used in preparing this report and this report should be provided to prospective contractors for their bidding or estimating purposes only. Our report, conclusions and interpretations are based on our subsurface explorations, data from others and limited site reconnaissance, and should not be construed as a warranty of the subsurface conditions. Variations in subsurface conditions are possible between the explorations and may also occur with time. A contingency for unanticipated conditions should be included in the budget and schedule. Sufficient monitoring, testing and consultation should be provided by our firm during construction to confirm that the conditions encountered are consistent with those indicated by the explorations, to provide recommendations for design changes should the conditions revealed during the work differ from those anticipated, and to evaluate whether earthwork and foundation installation activities comply with contract plans and specifications. If there are any changes in the loads, grades, locations, configurations or type of facilities to be constructed, the conclusions and recommendations presented in this report may not be fully applicable. If such changes are made, we should be given the opportunity to review our recommendations and provide written modifications or verifications, as appropriate. ◆ ◆ ◆ Gaudet.TealLakeRd.GHA August 26, 2022 page | 7 We have appreciated the opportunity to be of service to you on this project. If you have any questions or comments, please do not hesitate to call at your earliest convenience. Respectfully submitted, GeoResources, LLC Davis Carlsen, GIT Eric W. Heller, PE, LG Staff Geologist Senior Geotechnical Engineer DC:KSS:KEB/dc DocID: Gaudet.TealLakeRd.GHA Attachments: Figure 1: Site Location Map Figure 2: Site Vicinity Map Figure 3: NRCS Soils Map Figure 4: Geologic Map Figure 5: WA DOE Coastal Atlas – Slope Stability Figure 6: Seismic Hazards Map Approximate Site Location Map created from Jefferson County Public GIS (https://jeffcowa.maps.arcgis.com/apps/webappviewer/) Not to Scale Site Location Map Proposed Single-Family Residence xxx – Teal Lake Road Jefferson County, Washington PN: 821333042 DocID: Gaudet.TealLakeRd.F August 2022 Figure 1 Approximate Site Location Map created from Jefferson County Public GIS (https://jeffcowa.maps.arcgis.com/apps/webappviewer/) Not to Scale Site Vicinity Map Proposed Single-Family Residence xxx – Teal Lake Road Jefferson County, Washington PN: 821333042 DocID: Gaudet.TealLakeRd.F August 2022 Figure 2 Approximate Site Location Map created from Web Soil Survey (http://websoilsurvey.sc.egov.usda.gov/App/WebSoilSurvey.aspx) Soil Type Soil Name Parent Material Slopes, % Erosion Hazard Hydrologic Soils Group CaC Carlsborg gravelly loamy sand Alluvium and/or glacial outwash 0 to 15 Moderate A CaD 15 to 30 Moderate to Severe Not to Scale NRCS Soils Map Proposed Single-Family Residence xxx – Teal Lake Road Jefferson County, Washington PN: 821333042 DocID: Gaudet.TealLakeRd.F August 2022 Figure 3 Approximate Site Location Map created from the Geologic map of the Lofall 7.5-minute quadrangle, Jefferson and Kitsap Counties, Washington by T.A. Contreras, K.A. Stone, and G.L. Paulin (2013) Qgoaf Vashon Alluvial Fan deposits Qgos Vashon Recessional Outwash Sand Qgic Vashon ice-contact deposits Qco Olympia nonglacial deposits Not to Scale Geologic Map Proposed Single-Family Residence xxx – Teal Lake Road Jefferson County, Washington PN: 821333042 DocID: Gaudet.TealLakeRd.F August 2022 Figure 4 Approximate Site Location Map created from the Washington State Coastal Atlas Map (https://fortress.wa.gov/ecy/coastalatlas/tools/Map.aspx) Not to Scale WA DOE Coastal Atlas – Slope Stability Proposed Single-Family Residence xxx – Teal Lake Road Jefferson County, Washington PN: 821333042 DocID: Gaudet.TealLakeRd.F August 2022 Figure 5 Approximate Site Location Map Created from Department of Natural Resources Geologic Maps (https://geologyportal.dnr.wa.gov/2d-view) Not to Scale Seismic Hazards Map Proposed Single-Family Residence xxx – Teal Lake Road Jefferson County, Washington PN: 821333042 DocID: Gaudet.TealLakeRd.F August 2022 Figure 6 Lofall Fault