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HomeMy WebLinkAboutBLD2016-00564 - 09A Geotech ReportStratum Group File: 6.1.22 1 PO Box 2546, Bellingham, Washington 98227 Phone: (360) 714-9409 June 7, 2022 Emily Scavarda Estes Builders Re: Site Inspection 3225 Highway 20 Port Townsend, WA Jefferson County Parcel 901171004 BLD2016-00564 and 2018-00266 I visited the site on May 24, 2022 and found that the home and garage had been located and developed consistent with our recommendations we made in our Geology Hazard Assessment dated May 9, 2013. The buildings are setback from the base of the slope as recommended and from the top of the shoreline bluff slope as recommended. Native vegetation has been retained as much as practical around the site consistent with our recommendations and stormwater dispersion has been installed consistent with our recommendations. Stratum Group appreciates the opportunity to be of service to you. Should you have any questions please contact our office at (360) 714-9409. Sincerely yours, Stratum Group Dan McShane, L.E.G., M.Sc. Licensed Engineering Geologist RECEIVED BY DCD 6/16/2022 PO Box 2546, Bellingham, Washington 98227 Phone: (360) 714-9409 June 18, 2021 Angie Robar Client Experience Director angie@estesbuilders.com Re: Geology Hazard Assessment Jefferson County Parcel: 901171004 Discovery Bay, Washington This geologic hazard evaluation was conducted to assess the risk of landslide and erosion at the property. I had previous completed a geology hazard assessment at the site in 2013 for a different owner and it is my understanding that Jefferson County has requested and updated report for the site for the proposed home and ADU. Jefferson County Public Land Records maps indicate that portions of the property are within an intermediate landslide hazard area and a portion is within an area mapped as unstable recent slide. The shoreline stability map indicates that portions of the property are within an area mapped as intermediate stability with an area mapped as unstable recent slide. It is my understanding that the proposed building area for a home will be on the southwest area of the property where there is a wide bench area adjacent to a low shoreline bank. Access to this site is via a gravel road that angles down the slopes above this area from the highway above. Based on my geology hazard assessment, it is my opinion that the proposed building area on the southwest portion of the site will not be at risk from landslides or erosion. Furthermore, development of the site as proposed will not increase the risk of landslides or erosion on or off the site. This geology hazard assessment included a review of available geologic mapping, lidar (light detecting and ranging) imagery and our own notes and observations we have made in the vicinity of the site and at locations with similar geologic conditions. GENERAL GEOLOGY Several geology maps cover the area where the property is located. The Geologic Map of Northeastern Jefferson County, Washington (Gayer, 1976) indicates that the site is underlain by thin drift over basalt. The Bedrock Geologic and Quaternary Tectonic map of the Port Townsend Area, Washington (Gower, 1980) includes a detail map of the bedrock of the area (Figure 1). June 18, 2021 Tax Parcel 901171004, Jefferson County, WA Geology Hazard Assessment Stratum Group File: 4.25.13 Subsequent maps the Bedrock Geologic Map of the Port Townsend 30 by 60 Quadrangle, Puget Sound Region, Washington (Whetten and others, 1988) and the most recent geology map of the area 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) (Figure 2) are compilation maps of larger scale that incorporate the mapping of Gower (1980) but are at a scale that cannot fully incorporate all the bedrock units on the Gower map. Whetten and others (1988) indicate the bedrock underlying the site and vicinity consists of Crescent Formation conglomerate. Schasse and Slaughter (2005) indicate the site is underlain by a landside deposit and conglomerate of the Crescent Formation. Figure 1. Clipped and annotated geologic map by Gower (1980) of Anderson Lake Area Bedrock Geologic and Quaternary Tectonic map of the Port Townsend Area, Washington. R Tevc = Crescent Formation Conglomerate, Teu = unnamed sedimentary rocks (Eocene), Teub = basalt, Tlu = upper part of the Lyre Formation sandstone, Tll = lower part of the Lyre Formation conglomerate, Tav = andesite volcanics, Tlus = upper part of Lyre Formation siltstone, Qg = glacial sediment, Qls = landslides (Gower does not have a description of the landslide feature). Heavy black lines are faults with “D” on the down side and “U” on the up side. Site June 18, 2021 Tax Parcel 901171004, Jefferson County, WA Geology Hazard Assessment Stratum Group File: 4.25.13 Figure 2. Clip of 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) showing approximately the same area as Figure 1. Bedrock My observations of bedrock on and in the vicinity of the property site are generally consistent with the bedrock descriptions and mapping of Gower (1980) and the subsequent adoption of Gower (1980) by Schasse and Slaughter (2005). I observed basalt consistent with Crescent Formation on the subject property and have observed siltstone, conglomerate, sandstone, andesite (adakite) and basalt in the vicinity of the site consistent with Gower (1980). Observations of some of the units in the area are that much of the bedrock mapped as Unnamed sedimentary rocks and Crescent Formation conglomerate is very deeply weathered and some basalt-rich units I interpreted to be palogonite (shattered basalt from erupting into water). Weathering of some of the bedrock in the vicinity is extensive enough that it difficult to discern if the rock is sedimentary or highly weathered basalt (Figures 3 and 4 from an area upslope of the property). I suspect that the much of the unit Gower (1980) called Crescent conglomerate may be deeply weathered basalt and/or palogonite. Gower (1980) noted an 18-meter thick layer of saprolite (soil derived from very weathered bedrock) along Highway 20. I have observed the Site June 18, 2021 Tax Parcel 901171004, Jefferson County, WA Geology Hazard Assessment Stratum Group File: 4.25.13 same feature and observed much of the basalt dominated rock was deeply weathered. Hard, lightly weathered sandstone and conglomerate (Figure 5) is located on the portions of the slope areas above the property. This sedimentary rock is consistent with the Lyre Formation in that the sedimentary rock contains clasts of chert and metamorphic pebbles consistent with Lyre Formation descriptions. I also observed andesitic volcanic rock on the slopes above the property. Hahn and others (2004) further describe these volcanic rocks as adakite, a chemically unique volcanic rock associated with specific subduction volcanism. Figure 3. Deeply weathered basalt bedrock and possible palogonite June 18, 2021 Tax Parcel 901171004, Jefferson County, WA Geology Hazard Assessment Stratum Group File: 4.25.13 Figure 4. Deeply weathered bedrock Figure 5. Hard conglomerate consistent with Lyre Formation June 18, 2021 Tax Parcel 901171004, Jefferson County, WA Geology Hazard Assessment Stratum Group File: 4.25.13 Potential Deep-Seated Large Landslides Gayer (1976) maps of the geology and slope stability included slope stability designations that have been incorporated into Jefferson County’s landslide hazard maps. Gayer (1976) did not identify the slide areas indicated by Gower (1980) as unstable. Gower (1980) provides no explanation or description of the landslide features on the map located on and near the property. Schasse and Slaughter (2005) simply adopted Gower (1980) in this area (personal communication with Slaughter) and also provide no explanation to the landslide designation. I speculate that the bench-like features and undulating terrain combined with topographic maps of the area may have led Gower (1980) to interpret the landforms in the area as landslide areas. Regardless there is no explanation whether these map areas are areas of shallow slides or deep slides. I have conducted several geology detailed assessments throughout the area mapped by Gower (1980) including on the subject property and properties to the south, north and east. It is my opinion that there are no landslides in the area as indicated by Gower (1980) and subsequently also designated on the geology map of Schasse and Slaughter (2005). Minor soil creep and root throw are present on some of the steep slopes and there are areas on the steep slopes that may have had small shallow slides in the past. However, intact bedrock and bedrock units are not consistent with that area being the lower part of a large landslide complex as indicated on the Schaess and Slaughter (2005) map as adopted from Gower (1980). My interpretation of lidar bare earth imagery of the area (Figure 6) combined with on the ground observations throughout the area is not consistent with large scale landslides indicated on the Gower (1980) and Schasse and Slaughter (2005) maps. The rather undulating topography superficially resembles landslide topography. However the steep slope just east of Highway 20 that may have been interpreted to be a large scarp consists of very hard unweathered sandstone bedrock and is different than the bedrock on the bench-like areas west of Highway 20 which consist of deeply weathered basalts and glacial sediments. The bench-like areas and undulating topography are a result of a combination of highly variable bedrock hardness and weathering combined with late glacial processes that shaped the topography in the vicinity including former shorelines, ice margin deposits or glacial recessional processes that eroded bench-like features in the bedrock that has variable erosion resistance. These features can give a superficial appearance of landslide landforms; however, the more accurate digital elevation models (DEM) available through lidar as well as broader interpretations of late glacial features in the area are not consistent with these slopes being larges scale landslides. During the late stages of the last glacial period local relative sea-level was higher than present due to the mass of glacial ice having pressed the local land surface downward 100s of feet. Mapping immediately to the south by Polenz and others (2014) and my own observations along the slopes of Discovery Bay at numerous locations have noted numerous features associated with ice margin features, former June 18, 2021 Tax Parcel 901171004, Jefferson County, WA Geology Hazard Assessment Stratum Group File: 4.25.13 elevated shorelines, variable bedrock hardness, fault zones (also mapped by Gower (1980)) and weathering features that have formed discontinuous bench-like landform features and depressions. Polenz and others (2014) noted that the area was inundated with marine water to an elevation of 130 feet and further concluded that the continental ice margin was in close proximity during that inundation. Figure 6. Lidar bare earth imagery (Jefferson County) Proposed Building Area Fill is present underlying the proposed building area and along most of the lowermost portion of the subject property. A railroad was formerly located along the lowermost portion of the site and fill for constructing the railroad underlies this lower area. SPECIFIC SITE OBSERVATIONS The lot is accessed via a gravel road off of State Route 20. The lot slopes down to Port Undulating and bench-like topography Deep slide area associated with creek Ravine slopes subject to shallow slides Steep stream slopes subject to shallow slides Steep slopes with shallow slide potential but no delivery potential Building site June 18, 2021 Tax Parcel 901171004, Jefferson County, WA Geology Hazard Assessment Stratum Group File: 4.25.13 Discovery from State Route 20. The upper, east end of the property adjacent to State Route 20 is at elevation of approximately 200 feet and the west end of the property is at the shoreline of Port Discovery. The overall slope of the property is approximately 13 degrees based on the topographic map. However, the slopes on the property vary from gentle slopes to as steep as 45 degrees with a few cliff areas on the site. The west side of the property at the shoreline consists of a former railroad grade that followed the shoreline along Port Discovery. The level railroad grade is approximately 10 feet above the high tide line. Most of the railroad grade along the property has been armored with rock with some concrete debris. The beach consists of boulders and cobbles and sand areas. Some minor erosion has taken place in areas not armored with rock and concrete debris. The proposed home site is located on the southwest portion of the site along an area where the railroad grade as well as adjoining areas is as wide as 110 feet between the low bank to the beach and the base of the slope that slopes up to the east. The proposed home site is accessed via a gravel road that switches backs down to the site across the slope to the east. The slope above the proposed building site slopes up gently for approximately 200 feet and then steepens to a bench area approximately 80 feet in elevation above the slope. Above this bench area the slope steepens to approximately 45 degrees up to another bench area at approximately 180 feet elevation with a gentle slope from there to the highway on the east side of the site. The lower part of the slope on the property above the proposed building area is underlain by glacial till and colluvial soils derived from the slopes above. The bench area is underlain by reworked glacial till consistent with the area being a former short duration shoreline that existed during the late stages of the last glacial period. The steep slope above is underlain by a mix of glacial till and weathered bedrock. The bedrock consisting of basalt fragments consistent with conglomerate derived from basalt cobbles. The road that accesses the building site is a full bench cut road and follows contours such that the slopes crossed by the road are stable and cut slopes are minimal. Most of the cut slopes are cut into bedrock or glacial till. The lower part of the road does cross an area of convergent topography with seeps and wet areas on the slope. This section of road required some over excavation of soft wet soils and fill placement over the low area. According to a letter provided by Doug Reeves in 2013 when I first assessed this particular location, the road was constructed with rock fill that allows any perched water to pass under the road. A culvert was installed at a level above the low point to transmit any excess flow that may develop during severe storms or rain on snow events. Observations of the fill below and above the road are consistent with the description of the road construction by June 18, 2021 Tax Parcel 901171004, Jefferson County, WA Geology Hazard Assessment Stratum Group File: 4.25.13 Mr. Reeves. We examined this area again as part of the update to this hazard assessment and found that the road was still in good condition and the drainage was working as intended. Tree stumps on the slope above the proposed building area are straight as are most of the trees on the slope. Other than minor soil creep on the steepest slope sections, there is no indication of slope movement. There is no indication of old or potential landsliding on the slopes above the proposed home site. The shoreline to the west of the proposed home site is lined with rocks and trees and no appreciable erosion is evident along the shoreline. The proposed building site has been undergoing site preparation for building and the fill soils have been exposed consistent with our initial observations that the level building area is underlain by old fill associated with the former railroad grade. CONCLUSIONS AND RECOMMENDATIONS Based on our visual inspection of the slopes on the subject property and area, we conclude that a residence can be located on the level building area on the southwest portion of the site such that erosion or landsliding will not threaten the structure within the expected life of the structure (100 years). In addition the development of the site will not cause an increase risk of landsliding or erosion off of the site. Development elsewhere along the former railroad grade is acceptable, but the width of the grade is narrower thus constraining development. Recommendations: Setbacks: The home should be setback from the base of the slope on the east side of the lot at least 15 feet. The home should be set back at least 25 feet from the low shoreline bank to the west of the site. Other setback requirements associated with shoreline rules or fish and wildlife may be applicable. Vegetation: Native vegetation should be left in place as much as practical on the slopes exceeding 35 degrees and within the buffer between the building area and the slope to the east and the shoreline low bank to the west. It is acceptable to thin and limb the trees for view purposes. Drainage: Storm water should be directed to a dispersion/infiltration area to the north and south of the home site. The topography of the area is such that this can be readily accomplished. Foundation Considerations: The proposed home site is located in an area that was filled June 18, 2021 Tax Parcel 901171004, Jefferson County, WA Geology Hazard Assessment Stratum Group File: 4.25.13 previously for the construction of a railroad. This is not a landslide or erosion risk, but the quality of the fill is not known. At the time of our most recent assessment deeper foundation elements were being installed at the building site via overexcavated areas backfilled with structural fill and the installation of pipe foundation. Stratum Group appreciates the opportunity to be of service to you. If conditions appear different than those described in this report, or other concerns arise, we request that we be notified so we can review those areas and modify our recommendations as required. Should you have any questions regarding our reconnaissance please contact our office at (360) 714-9409. Sincerely yours, Dan McShane, L.E.G., M.Sc. Geoff Malick, L.G., M.Sc. Licensed Engineering Geologist Licensed Geologist