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HomeMy WebLinkAboutJefferson Street Geologic Hazard Assessment Report - Stratum Group - 2018.01.16Stratum Group File: 1.1.18 1 STRATUM GROUP P.O. Box 2546 Bellingham, WA 98227 January 16, 2018 Brandon Maxwell and Samantha Harper City of Port Townsend- Public Works Department 250 Madison Street Port Townsend, WA 98368 Re: Geology Hazard Assessment Jefferson Street – Taylor Street to Washington Street TIB Project No. P-W-151(P07)-1 Dear Mr. Maxwell and Ms. Harper: This geologic hazard assessment was conducted to assess the potential risk of landslide and erosion hazards for the proposed Jefferson Street - Taylor to Washington Street Transportation Investment Board (TIB) project. It is our understanding that the City of Port Townsend Public Works Department is planning on constructing sidewalks along the southeast side of Jefferson Street and the southwest side of Quincy Street. The project will also include widening the paved street area to accommodate public street parking as well as removing the existing cement lined stormwater drainage ditch along the southeast side of Jefferson Street. It is our understanding that the proposed improvements will not be located on the steep slope that meets the Port Townsend Municipal Code (PTMC) criteria as a geologically hazardous area. However, because the proposed project will be located within 50 feet of a steep inland bluff slope, a geotechnical report is required per PTMC 19.05.100. The scope of our work included an inspection of the project site and vicinity including the inland bluff slopes, observations of soil and geology units via hand dug test pits on and at the top of the steep slope and at nearby exposures, review of draft plans and conceptual designs for the project, review of available geologic maps, historic aerial photographs and historic maps, and review of lidar (light detecting and ranging) derived imagery. Past observations at similar sites including areas in the vicinity of the subject property aided in our interpretations. After our initial geology assessment, we met on site with Public Works staff familiar with the project to discuss the proposed project and the proximity of the proposed project to the top edge of the slope. Based on our geology hazard assessment, it is our opinion that the proposed project will not be at risk of landslide or erosion hazards as long as the recommendations made in this report are followed. Furthermore, the proposed project will not increase the risk of landslides or erosion on or off the site as long as the recommendations of this report are followed. Stratum Group File: 1.1.181.18 2 January 16, 2018 Jefferson Street – Taylor Street to Washington Street TIB Project No. P-W-151(P07)-1 Geologic Hazard Assessment GENERAL GEOLOGY The Geologic Map of the Port Townsend South and Part of the Port Townsend North 7.5-minute Quadrangles, Jefferson County, Washington (Schasse and Slaughter, 2005) indicates that there are three geologic units located in the vicinity of the project: Vashon recessional outwash sand, Vashon lodgement till, and fill (Figure 1). Vashon recessional outwash sand is unconsolidated stratified sand with pebbles and silt. In the Port Townsend area, Vashon recessional outwash typically forms a thin veneer over thicker deposits of till. The outwash was deposited during the late stages of the last glacial period approximately 14,000 years ago as the glacial ice melted out of the area. It has been our experience that this unit is in places poorly sorted and appears to have been in part derived from ice contact and ice wasting. Vashon lodgment till is a compact and dense diamicton consisting of very poorly sorted mix of clay, silt, sand, gravel, cobbles, and scattered boulders. The till was deposited directly by glacial ice when the area was covered by glacial ice between 18,000 and 14,000 years ago. The fill consists of a mix of sediments, organic material, rip-rap, and debris emplaced to reshape topography. Figure 1. Lidar imagery (Jefferson County, 2012) with geology (Schasse and Slaughter, 2005) overlay for the Jefferson Street Sidewalks Project site. Our site observations differ from the mapped geologic units and contacts. Stratum Group File: 1.1.181.18 3 January 16, 2018 Jefferson Street – Taylor Street to Washington Street TIB Project No. P-W-151(P07)-1 Geologic Hazard Assessment While the geologic units mapped by Schasse and Slaughter (2005) are present in the near vicinity, our site observations are not consistent with the above described mapping. This discrepancy is primarily due to the scale of the mapping. We hand dug multiple test pits on the site including the steep slope and observed moderately compact fine sand with gravel and scattered cobbles (SW) to fine sand with trace silt (SP) (Figures 2, 3). The unit is consistent with Vashon advance outwash. The deposit is moderately cohesive and well drained, owing to its limited silt and clay content. The advance outwash was deposited by meltwater streams as glacial ice advanced towards the area approximately 20,000 to 18,000 years ago. The unit is compact due to the mass of glacial ice that overrode the area after deposition. Figure 2. Photograph of fine sand excavated from a hand dug test pit on the steep slope. Figure 3. Photograph of pebbles within the fine sand unit from a hand dug test pit on the steep slope. Stratum Group File: 1.1.181.18 4 January 16, 2018 Jefferson Street – Taylor Street to Washington Street TIB Project No. P-W-151(P07)-1 Geologic Hazard Assessment We also observed Vashon till at a stratigraphically higher position on the road cut exposure on the northwest side of Jefferson Street. This observation is consistent with the proposed project location and steep slope below being underlain by advance outwash rather than recessional deposits as indicated by Schasse and Slaughter (2005). Some of the material we observed near the surface of our test pits may be side cast material sourced from when the road cut was excavated and graded. However, based on the test pit excavations, the slope and the area immediately above the slope is underlain primarily of undisturbed advance outwash that consists predominantly of fine sand in a dense condition. SITE SPECIFIC OBSERVATIONS The proposed project is located on the southeast side of Jefferson Street and the southwest side of Quincy Street (Figure 4). Figure 4. Aerial view of project area indicated with red line (modified Jefferson County GIS map) The proposed sidewalk project will link a currently existing sidewalk along the south side of Jefferson Street that terminates at Adams Street with a sidewalk on the north side of Washington Street at Quincy Street. The project will also widen portions of the road to accommodate parking and remove an existing concrete lined stormwater ditch along the current outer edge of Jefferson Street (Figure 5). Stratum Group File: 1.1.181.18 5 January 16, 2018 Jefferson Street – Taylor Street to Washington Street TIB Project No. P-W-151(P07)-1 Geologic Hazard Assessment Figure 5. Photograph of Jefferson Street showing the car lane, shoulder, parking area and concrete-lined stormwater ditch. The steep slope is to the right obscured by ivy and brush. A portion of the project is located near the top edge of a steep slope that slopes down to the southeast from Jefferson Street and down to the southwest from Quincy Street. The current topography in the project area, including the street corridor and steep slope are the result of prior excavation and grading projects. The street was cut into an existing bluff and graded to accommodate the road by at least the 1870s (from historic maps of Port Townsend). This is apparent in lidar imagery (Figures 1 and 6). The slope on the southeast (downhill) side of Jefferson Street appears to have been somewhat modified from its original slope by excavation into the lower slope from below. Project area Steep slope obscured by ivy Concrete lined drainage ditch to be removed Stratum Group File: 1.1.181.18 6 January 16, 2018 Jefferson Street – Taylor Street to Washington Street TIB Project No. P-W-151(P07)-1 Geologic Hazard Assessment Figure 6. Lidar derived bare-earth image of project area indicated with red line (modified Jefferson County GIS) The steep slope near the southeast side of Jefferson Street ranges in height between 22 and 38 feet with the highest point near the intersection of Adams Street and the slope height gradually becoming lower as the road descends to the northeast. The measured slope angles along this section range from 28 degrees (53 percent) to 40 degrees (84 percent) with an estimated average slope close to 32 degrees (62 percent). Portions of the slope that exceed 40 percent are greater than 10 feet and therefore, this slope meets the criteria for a geologically hazardous area per PTMC 19.05.100.B.5. The steep slope near the southwest side of Quincy Street has a maximum height of 16 feet and gradually becomes lower as the street descends towards Washington Street (Figure 6). The slope angles on this slope range from 34 to 45 degrees (67 to 100 percent) with an average of 41 degrees (87 percent). The slope areas that exceed 40 percent also exceed 10 feet in height and therefore, the portions of this slope above 10 feet in height meet the criteria for a geologically hazardous area per PTMC 19.05.100.B.5. As noted in the geology section, soils underlying the steep bluff consist of dense fine sand to gravely sand that we interpret to be advance glacial outwash. The sand is dense and well drained and where not obscured by thin colluvium top soil extends to the base of the bluff. No springs or seeps were observed on the slope. No areas of sliding or mass wasting other than minor surface soil ravel was observed anywhere on the slopes. Steep slope area Stratum Group File: 1.1.181.18 7 January 16, 2018 Jefferson Street – Taylor Street to Washington Street TIB Project No. P-W-151(P07)-1 Geologic Hazard Assessment Vegetation on the slopes consists of low brush with significant areas covered with English ivy. A few trees are present on the slope as well including big leaf maple. We noted that the English ivy is growing on and up several trees and appears to have covered much of the low growing brush such as rose and snowberry on the slopes. There are several maple trees lining the southwest side of Quincy Street between the street and the steep slope. Figures 7 and 8 show the slope vegetation cover. Figure 7. View of the steep slope below the southeast side of Jefferson Street. Figure 8. View of the steep slope adjacent to the southwest side of Quincy Street. Stratum Group File: 1.1.181.18 8 January 16, 2018 Jefferson Street – Taylor Street to Washington Street TIB Project No. P-W-151(P07)-1 Geologic Hazard Assessment On the Jefferson Street section near the steep slope, the current edge of the travelled car lane (solid white line) is located between 19 feet and 33 feet from the top edge of the steep slope. There is a 16-foot long section of road where the top edge is less than 22 feet or less from the edge of travelled car lane. Along the 90-degree turn from Jefferson Street to Quincy Street, the edge of the travelled car lane is between 10 and 12 feet from the edge of the steep slope. On the Quincy Street section, the edge of the travelled car lane is between 10 and 13 feet from the top edge of the slope. A row of maple trees is located between the edge of Quincy Street and the top edge of the steep slope. PORT TOWNSEND GEOLOGICALLY HAZARDOUS AREAS EVALUATION As per PTMC 19.05.100. B 1 through 6, the following classification criteria were evaluated to determine if the proposed project location is in a geologically hazardous area. Based on this evaluation and the conditions described in the above sections as well as our understanding of the proposed street improvements, the proposed project will not be located within a geologically hazardous area. However, the proposed project area is located within 50 feet of the top of an inland geologically hazardous slope (including but not limited to an inland bank of greater than 40 percent that exceeds a vertical height of at least 10 feet) and is therefore within an “inland bank management zone” and requires a geologic hazard evaluation per PTMC 19.05.100.D.2.f.iii. 19.05.100.B.1: Any area containing soil or soil complexes described or mapped within the United States Department of Agriculture/Soil Conservation Service Soil Survey for Jefferson County as having a severe to very severe erosion hazard potential. Soils underlying the project area are mapped as Townsend gravelly loam 0 to 15 percent slopes. Townsend gravelly loam is described as being “moderately well drained” by the USDA. These soils do not have a severe to very severe erosion hazard potential. A small area of the lower slope near Washington Street is mapped as cut and fill land and described as “somewhat excessively drained” and not subject to severe to very severe erosion hazards. Obviously, the steep slope does not match the slope indicated on the soil map as the steep slopes are up to 100 percent. Furthermore, the description of the Townsend gravelly loam does not match our site soil observations on the site. The soils map indicates that the parent material is till and the unit is predominantly gravel. Our soil observations found the soil to be sand and the parent material to be advance glacial outwash. The soils we observed are well drained and will not be subject to severe erosion unless stormwater flows are concentrated on the steep slope. 19.05.100B2: Areas with all three of the following characteristics: a. Slopes greater than 15 percent; b. Hillsides intersection geologic contacts with a relatively permeable sediment Stratum Group File: 1.1.181.18 9 January 16, 2018 Jefferson Street – Taylor Street to Washington Street TIB Project No. P-W-151(P07)-1 Geologic Hazard Assessment overlying a relatively impermeable sediment or bedrock; and c. Springs or ground water. Although the slope below the proposed project area exceeds 15 percent, no geologic contacts or impermeable boundaries were observed during our site investigation. No perched water or standing water was observed on or near the slope during our site investigation. 19.05.100B3: Any area potentially subject to mass movement due to a combination of geologic, topographic, hydrologic factors, but not limited to those areas mapped or described by the Soil Conservation Service, Washington State Department of Ecology, Department of Natural Resources or U.S. Geologic Service. The classification may be based on performance standards rather than mapping. Based on our inspection of the steep slope, it is our professional opinion that the steep slope is not at risk of mass movement other than minor surface slope ravel on the steepest slope sections. The proposed project will not be located on the steep slope and will not be at risk from mass movement from the steep slope. 19.05.100B4: Any area potentially unstable or subject to erosion or sloughing as a result of rapid stormwater runoff, soil saturation or undercutting by wave action. There is no evidence of rapid stormwater runoff or soil saturation on the steep slope. As stated above, the underlying soil is well drained and compact. As long as stormwater is not directed onto the steep slope in a concentrated manner, the steep slope will not be subject to erosion hazards. 19.05.100B5: Any slope of 40 percent or steeper that exceeds a vertical height of 10 feet over a 25-foot horizontal run. The steep slope to the southeast of Jefferson Street and the steep slope southwest of Quincy Street have slopes that are greater than 40 percent and exceed a vertical height of 10 feet. The proposed project will not be located on this steep slope area, but the proposed project will be located within an “inland bank management zone” per PTMC 19.05.100.D.2.f.iii. because the proposed project is within 50 feet of the top of an inland geologically hazardous slope (including but not limited to an inland bank of greater than 40 percent that exceeds a vertical height of at least 10 feet). 19.05.100B6: Areas subject to severe risk of damage as a result of earthquakes, slope failure, settlement, soil liquefaction or faulting. These areas are identified by the presence of poorly drained soils (greater than 50 percent silt and less than 35 percent coarse material) loose sand or gravel, peat, artificial fill and landslide materials or soils with a high organic content. Soils within the proposed project are well drained, compact, and are not susceptible to earthquake induced mass movement, liquefaction, or faulting. Some shallow surface soil movement may take place on the steep slope during a large seismic event. Stratum Group File: 1.1.181.18 10 January 16, 2018 Jefferson Street – Taylor Street to Washington Street TIB Project No. P-W-151(P07)-1 Geologic Hazard Assessment RECOMMENDATIONS The proposed project will not be located in a geological hazardous area, but will be located within 50 feet of an identified inland bluff that meets the PTMC criteria as a geologic hazard area as described in the previous section. This section addresses the relevant applicable development standards in PTMC 19.05.100.D 2. and 19.100.05.E to the proposed project and provides specific recommendations for the proposed project near the steep slope along Jefferson Street and Quincy Street so that the proposed project will not be at risk from landslides or erosion and the proposed project will not increase the risk of landslides on or off the site. Buffers and setbacks Port Townsend CAO 19.05.100 E describes buffers and setbacks requirements for geologically hazardous areas. Port Townsend CAO 19.05.100 E 1 states Within the management zones established for marine bluffs and inland banks under subsections (D)(2)(f)(ii) and (iii) of this section, the buffer width shall be established by an approved geotechnical report prepared by an engineering geologist with a Washington specialty license in engineering geology as specified in Chapter 18.220 RCW. The report shall be based upon the best available science, existing and proposed uses, risks of slope failure, and coastal erosion rates, if applicable. Dan McShane is a licensed engineering geologist in the State of Washington and this report is based on the use of the best available science, the interpretation of slope and geologic conditions and the expected impacts of construction and the proposed use. Based on our assessment of the steep slope, it is our opinion that the proposed project will not be subject to landslides or rapid erosion. Some shallow slope ravel may be expected on the steepest portions of the slope. Therefore, it is our professional opinion that no buffer or setback is needed between the sidewalk and the top edge of the 40 percent slopes. Although no setback is required from the slope that meets the geologically hazardous criteria, two other considerations may result in a setback from the top edge of the slope: 1) Placing the sidewalk directly adjacent to the edge of the steep slope could cause some differential settlement along the outer (nearest to the slope) portion of the side walk, and 2) Placing the sidewalk adjacent to the slope may require a railing to be located along the sidewalk for safety purposes. In order to address the potential of differential settlement, we recommend that the sidewalk be set at least 2 feet back from the top edge of the steep slope in order to avoid the potential for differential settlement of the sidewalk. It is our understanding after walking the project site and discussing our results that the sidewalk will be located 2 feet back from the top edge of the steep slope. If the sidewalk is constructed within 2 feet of the edge of the slope, differential settlement is possible and the project may require grading or a structure to support the outer edge of the sidewalk. Stratum Group File: 1.1.181.18 11 January 16, 2018 Jefferson Street – Taylor Street to Washington Street TIB Project No. P-W-151(P07)-1 Geologic Hazard Assessment Stormwater Control Currently street run off flows along the side of the two streets. Along Jefferson Street, between the street and the slope, there is currently a concrete lined stormwater drainage ditch. The project will add very minimally to the existing impervious area. The proposed project will capture all stormwater from the street and sidewalk and transmit the stormwater to the City stormwater system. The result will reduce the amount of water that reaches the slope and will reduce the risk of stormwater discharging onto the slope in a concentrated manner. Erosion Control Soils underlying the proposed project area consists of well drained sand. The project area will take place in an area that consists gravel, bare compacted soil, a concrete lined stormwater ditch and grass and some brush consisting mostly of English ivy. No disturbance of the geologically hazardous slope will take place. If work takes place during wet weather, stormwater from the street area should be managed to prevent concentrated discharge onto the steep slopes. Vegetation No vegetation removal will take place on the steep slopes. The existing ground along the proposed project is gravel, a concrete lined ditch, compact bare soils, grass and English ivy with minimal other vegetation. A row of trees along the edge of Quincy Street are to be left in place. The project will cover most of the area with pavement. A couple of feet of soil along the edge of the sidewalk will likely be disturbed and should be planted with grasses or low growing ground cover. Post project, trimming back English ivy from encroaching on the sidewalk area will not cause slope stability problems. Heavy machinery operation: Heavy machinery should not be operated on the steep slope itself. Stratum Group File: 1.1.181.18 12 January 16, 2018 Jefferson Street – Taylor Street to Washington Street TIB Project No. P-W-151(P07)-1 Geologic Hazard Assessment CONCLUSIONS The proposed sidewalk expansion project is located near the edge of a steep slope that meets the criteria for a geologically hazardous area per PTMC 19.05. Although the proposed project will not be located within the area that meets the criteria of a designated geologically hazardous area, it is located within an “inland bank management zone” due to its close proximity to the critical area. Based on our geology hazard assessment, the proposed project will not be at risk from landslides or erosion and will not increase the risk of landslides or erosion as long as the recommendations of this report are followed. Stratum Group appreciates the opportunity to be of service to you. Should you have any questions regarding our assessment please contact our office at (360) 714-9409. Sincerely yours, Stratum Group Dan McShane, L.E.G., M.Sc. Geoff Malick, G.I.T., M.Sc. Licensed Engineering Geologist Geologist-in-Training