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HomeMy WebLinkAboutExhibit 4 - 09A- GEOTECH REPORTStratum Group File: 8.17.20c 1 PO Box 2546, Bellingham, Washington 98227 Phone: (360) 714-9409 February 10, 2022 Jeff Bohman Olympic Discovery Trail Re: Engineering GeologyAssessment Proposed Hal Baetz Viewpoint Jefferson County, Washington This engineering geology assessment was conducted to assess the risk of landslide and erosion hazards and the geology related soil conditions related to the proposed viewpoint work at the site. It is my understanding based on the proposed site plans provided (dated 12/1/21) is that the existing driveway will be used as a construction access and a new driveway entrance will be built approximately 150 feet to the north and a parking area, walkway, shelter and vault restroom will be built on the site. The Jefferson County Public Land Records shoreline slope stability map maps the area to the east of the property as unstable slopes. The County seismic hazard map has the property located in a seismic hazard area. Based on my engineering geology assessment of the site the proposed development will not be at risk from landslides or erosion and will not increase the risk of landslides or erosion on or off the site. The seismic hazard at the site from ground shaking will not be substantially different than other sites in the region. There is a risk that the site will be impacted by a tsunami associated with a major magnitude earthquake on the Cascadia Subduction zone; however, given the proposed use of the site and time for evacuation of the area to higher ground, this risk is deemed acceptable. Upper soils throughout the site have been altered from past site activities and the test pits indicated soils were fill or had been graded as there was little developed top soil and the soil structure throughout the site is consistent with modified land with significant areas of imported soil. Soils throughout the site have been altered from past site use that included the use of the site as pasture, development of the site with several buildings with drive and parking areas and the use of the site as a staging area for an estuary restoration project at Snow Creek that included and use of the site for placement of dredged and excavated soils. As such soils underlying the site are highly variable in composition even over short distances. Recommendations for accomplishing the development of the site with these soil conditions in mind are provided in the conclusions and recommendations of this report. Exhibit 4 February 10, 2022 Geologic Assessment Stratum Group File: 1.10.22 2 This geology assessment included a field inspection of the subject property and vicinity and the excavation of multiple hand dug test pits at the site to determine site soil engineering geology parameters. The assessment included review of proposed plans for the site, review of documents associated with the Snow Creek Estuary Restoration, review of historic aerial photographs dating back to 1939, available geologic mapping, lidar (light detecting and ranging) bare earth imagery from 2005, 2012 and 2019, and our own notes and observations we have made in the vicinity of the site and at locations with similar geologic conditions. GENERAL GEOLOGY The Lidar Revised Geologic Map of the Uncas 7.5-minute Quadrangle, Clallam and Jefferson Counties, Washington (Tabor and others, 2011) indicates (see Figure 1) that the property is underlain by Holocene age alluvial sediment. These are alluvial sediments deposited after the last glacial period. Figure 1. Clipped and annotated geologic map showing the approximate subject property location. Qvt = Vashon till; Qa5 = Holocene alluvial sediments, Qval = advance outwash lake deposits, Qvrf = fine grained recessional glacial marine deposits or glacial lacustrine deposits, Qb = beach deposits, Qf3 = Pleistocene alluvial fan deposits. Dashed blue line hachure between till and glacial recessional units demarcates a former shoreline late ice age shoreline. My observations are generally consistent with the above described mapping. However, the subject property has been significantly modified with site grading and placement of excavated and dredged soils from the Snow Creek Estuary Restoration project that was done 2015. As such that the site is now modified land. The modifications took place after the geology map was produced. Dense silt and clay with more consistent with the fine grained recessional sediment mapped across the road appears to extend at least under the eastern portion of the site and likely extends at least partially under the eastern portion of the site. This difference from the map is in part based on the scale of the mapping but also may be the result of the lower resolution 2005 lidar Qvt February 10, 2022 Geologic Assessment Stratum Group File: 1.10.22 3 used for the map by Tabor and others (2011). The better 2012 lidar resolution suggests that the eastern portion of the subject property being underlain by glacial recessional deposits. Some of the test pits excavated on the eastern potion of the upper level area of the site encountered very hard dense silt/clay with rocks consistent with glacial till. The presence of till below the recessional deposits is not unexpected as the recessional sediments overlie the glacial till. In addition the subject property has been modified by previous development with buildings and roads and the associated grading and drainage. The buildings have since all been removed and the site was used for staging a construction project for the Snow Creek Estuary Restoration. Additional grading and placement of fill took place on the site such that the site is at least in part modified land. SITE OBSERVATIONS The property is accessed via a dirt access road off of Fairmont Road. The upper part of the property is level and grass covered with some brush and weeds. At the time of my site visit in mid December 2020, there were some wet areas with water less than an inch deep on this level area (Figure 2). Figure 2. Level upper portion of property I excavated six hand dug test pits on the upland area. The test pits all encountered dense soils throughout the upland area. Test pit depths ranged from 2 to 4 feet deep. The more easterly pits encountered very hard silt clay with rocks consistent with undisturbed glacial till at depths of 3 feet and digging was very difficult due to the hard soil. February 10, 2022 Geologic Assessment Stratum Group File: 1.10.22 4 Towards the west part of the upland the test pits encountered medium hard silt with the pebbles and rocks to depths of 4 feet. No developed top soil layer was present in any of the test pits on the upland area. The soil was variably mottled with poorly developed mottles to no mottling some of the soil was dark brown. The soils in all of the test pits appeared to be well compacted fill. These observations of compacted silty soils with gravel are consistent the past use of the site observed in historic photographs and in construction design plans and narratives for the Snow Creek Estuary Restoration. The level upland area is bound on the west by a moderate slope down to a pond area and on the northwest by a moderate slope down to a saltwater estuary of grassy wetlands (Figure 3). The slopes are on order of 15 to 20 percent. The slopes are mostly grass covered with some brush and recent native plant plantings in blue tubes. Figure 3. View of slope on the site Test pits were excavated on these slope areas. Soils encountered on the slopes were highly variable. Sand and gravel was encountered in most of the test pits, with the some test pits on the northwest being all sand and gravel to a depth of 4 feet. Pits on the upper slopes on the west were either silt with some gravel or had a foot or two of sand and gravel with hard dense silt below. Soils in all of the test pits were dense and compact. History of the Site Based on test pits the soils throughout the site have been altered from past site activities and the test pits indicated soils were fill or had been graded as there was little developed top soil and the soil structure throughout the site is consistent with modified land with significant areas of imported soil. February 10, 2022 Geologic Assessment Stratum Group File: 1.10.22 5 Historic aerial images of the site were reviewed to assess the past site modifications relevant to soil stability on the site. The property was used as a pasture from at least 1939 until some in the 1950s when a home was built on the site and other small outbuildings were built. That home was removed and a home and out buildings were built on the site in the 1970s. The home and buildings were removed in 2010. The Snow Creek Estuary Restoration project took place in 2015. The project entailed removing the former railroad grade across the upper estuary and removal of accumulated sediment in part of the estuary in order to open up tidal flow paths after past flow restrictions associated with the old rail grade. The Snow Creek Estuary Restoration Final Design Report indicated that the estuary sediment consists of silt to coarse gravel. This is consistent with observations in the test pits particularly those where medium dense silt had pebbles and gravel embedded in the fill suggesting that the excavation would have been mixed from excavation, grading and compaction. The specific types of soils used at the site for fill is not available. The design plans projected fill placement on the site to vary from 0 to 15 feet and indicated that the level area would be raised several feet from existing graded and made larger by creating the slopes now on the site. The plans called for the fill to be compacted to 85% compaction. The oblique aerial images from 2006 before the restoration project and from 2016 after the restoration project show the extent of the fill (Figures 4 and 5). February 10, 2022 Geologic Assessment Stratum Group File: 1.10.22 6 Figure 4. Oblique aerial view of site in 2006. Figure 5. Oblique view of site in 2016 post fill placement GEOLOGIC HAZARD EVALUATION Landslides The slopes above the property across Fairmount Road are not subject to landslides and there are no indications of landslides or unstable slopes in that area. The slopes on the subject site are moderate slopes that have compacted fill. These slopes will not be subject to landslides. Excavation areas Removed rail grade Note driveway position ends before power pole due to raised ground surface from fill February 10, 2022 Geologic Assessment Stratum Group File: 1.10.22 7 Seismic Hazard The seismic hazard at the site from ground shaking will not be substantially different than other sites in the region. The fill soils underlying the site have been compacted and are in a dense condition and will not be subject to liquefaction during a seismic event. Deeper soils under the fill are old alluvial sediments that even if portions the old alluvial sediments are of the grain size susceptible for liquefaction, they are old deposits that have undergone multiple previous seismic events and therefore have as well as time to compact and will not be susceptible to liquefaction. Furthermore the type of development proposed for the site does not warrant mitigation for seismic hazards associated with location in an area of alluvial sediments. Tsunami Tsunami modeling (Washington Geologic Survey, 2022) indicates that for an L1 magnitude 9.0 earthquake on the Cascadia Fault, inundation at High 101 near the subject property would be 33 feet (Figure 6). The model is for an extreme seismic event and may not reflect the actual event results, but the shape and orientation of Discovery results in elevated tsunami inundation at the head of the bay that with multiple tsunami events indicated in tidal sediments (Garrison-Laney and Miller, 2017). Given the proposed land use as well as the ample to evacuate to higher ground and the low probability of the L1 magnitude 9 event, no mitigation is warranted for the tsunami potential. CONCLUSIONS AND RECOMMENDATIONS Based on my geology assessment, it is my opinion that site is not at risk from landslides and the proposed development will not increase the risk of landslides on or off the property. The seismic hazard at the site from ground shaking will not be substantially different than other sites in the region. There is a risk that the site will be impacted by a tsunami associated with a major magnitude earthquake on the Cascadia Subduction zone; however, given the proposed use of the site and time for evacuation of the area to higher ground, this risk is deemed acceptable. The following recommendations are in response to the items requested to be covered in Steve Durant’s email on January 8, 2021: - HMA Pavement, gravel surfacing and PCC (vehicular loads, pedestrian loads) recommendations (e.g. sections), including geotextile recommendation. - Subgrade preparation and embankment material to transition grade from Fairmount Road (approximately 5' grade transition), including geotextile recommendation Pavement design and drive surface design should be done by a licensed engineer specializing in design of drive and parking for anticipated loads and traffic (I am an engineering geologist, so this outside my area of qualification). February 10, 2022 Geologic Assessment Stratum Group File: 1.10.22 8 The above said, based on soil conditions encountered at the proposed new access drive ‘ramp’ down onto the level area of the site will not require unusual construction methods. Settlement should be minor as long any build up of the ramp is underlain by well drained subgrade below the driving surface material. The fill soils at this location are not as thick as elsewhere on the site and the underlying glacial marine drift or glacial lacustrine sediment at this location was elevated relative to other areas on the level prior to fill soils emplacement. The glacial sediment at this elevated area would have undergone self consolidation from wetting and drying cycles and thus settlement is a lower concern at this location. Selection of the parking surface material should consider that long term differential settlement is a likely condition at this site due to 1) the highly variable fill soils from the excavated estuary and former rail grade, 2) fill soils vary in thickness across the site, 3) Potential for wet glacial recession marine or lake sediments that underlie the fill will take many years to compact from heavy loads such as the fill soils that were placed at variable depths over the site 4), potential presence of buried zones of soft soils or buried organics at depth that can cause surface settlement as these soils compact over time. - Provide preliminary recommendations for site preparation and earthwork including depth of stripping, suitability of on-site soils for use as structural fill, constraints for wet weather construction, and fill placement and compaction (structural fill) requirements. Stripping depth to remove organic upper soils within the area proposed for development will generally be just a few inches to remove the top layer thin top soil. Soils throughout the site had been compacted and compact soils were encountered at 4 inches or less throughout the site. On site soils are not suitable for structural fill as the silt content is such that compaction for structural fill requirements will be very difficult for moisture sensitive soils such as the silt soils (ML) encountered at the site. Due to the silty soils throughout the proposed parking and drive areas wet weather work is not advisable as soils will become very soft and muddy during disturbance and compaction of the subgrade will be difficult to achieve. A woven polyester geotextile16oz biaxial fabric or equivalent should be laid over the compacted subgrade soils of the parking area. 6 inches of crushed drain rock and compacted base class 1 base coarse should underlie the drive surface material. The specifics of the base coarse should be determined by the engineer depending on the drive surface used. - Foundation recommendations for vault toilet, and memorial structure The vault toilet and memorial structure are relative small structures and should be supported with rigid foundations such that will allow for the differential settlement. Soil bearing values should be 1,500 psf due. Higher values are not recommended for this site due to the silt soils and possible presence of soft glacial marine and glacial lake sediments at depths below the fill. Sediments. February 10, 2022 Geologic Assessment Stratum Group File: 1.10.22 9 - French drain section recommendation, including geotextile recommendation - Assumes Jefferson County will not require infiltration testing (PIT). Geotech still to provide discussion of site soil properties and soil analysis (grain size distribution; moisture content determinations), as well as geotech's assessment that site soils not conducive to infiltration BMPs other than dispersion. - Groundwater relative to proposed improvements and recommendations for mitigation (French drain?) Site soils at the surface and to the depths of the test pits were consistently silt with some gravel (ML). These soils will not perform well with infiltration as the silt content is high. If infiltration is used an infiltration rate of 0.13 inches per hour should be used (Massman, 2003) and per Western Washington Stormwater Manual. The glacial marine or lake sediments underlying the fill are silt/clays and hence deep French drains will not be successful at this site. In the design plans for the Snow Creek Estuary Restoration, exiting drains were noted in the plans associated with the former buildings on the site that routed perched water off the site. These drains were called out for removal but indicated that perched water was an issue at the for home site area now covered with fill soils. Based on soil conditions, stormwater plans should designed for dispersal rather than on site infiltration. The proposed dispersion trenches will allow for most water to infiltrate and any overflow will be across well vegetated moderate slopes that will not be subject to erosion and water will infiltrate where coarser grained soils are resent on the lower slopes. The proposed swale between the proposed development and Fairmont Road will direct any perched water that develops on the silty fill soils down the moderate slope to a discharge at a French drain. The slopes below this area very moderate and vegetated and any water will disperse across this area. Stratum Group appreciates the opportunity to be of service to you. Should you have any questions regarding this assessment please contact our office at (360) 714-9409. Sincerely yours, Stratum Group Dan McShane, L.E.G., M.Sc. Licensed Engineering Geologist Stratum Group File: 8.17.20c 10 1939 February 10, 2022 Geologic Assessment Stratum Group File: 1.10.22 11 1950 February 10, 2022 Geologic Assessment Stratum Group File: 1.10.22 12 1956 February 10, 2022 Geologic Assessment Stratum Group File: 1.10.22 13 1957 February 10, 2022 Geologic Assessment Stratum Group File: 1.10.22 14 1965 February 10, 2022 Geologic Assessment Stratum Group File: 1.10.22 15 1977 1979 February 10, 2022 Geologic Assessment Stratum Group File: 1.10.22 16 1990 February 10, 2022 Geologic Assessment Stratum Group File: 1.10.22 17 1996 February 10, 2022 Geologic Assessment Stratum Group File: 1.10.22 18 1997 February 10, 2022 Geologic Assessment Stratum Group File: 1.10.22 19 2002 February 10, 2022 Geologic Assessment Stratum Group File: 1.10.22 20 2004 February 10, 2022 Geologic Assessment Stratum Group File: 1.10.22 21 2006 February 10, 2022 Geologic Assessment Stratum Group File: 1.10.22 22 2009 February 10, 2022 Geologic Assessment Stratum Group File: 1.10.22 23 2016