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HomeMy WebLinkAbout702224002 Geotech Assessmentd~~ off- 5~2 GEOTECHNICAL REPORT, LANDSLIDE HAZARD AREA PENNY CREEK QUARRY, QUILCENE, WA I. Introduction This report provides information regarding geologic conditions, slope stability and landslide hazard designation in reference to Jefferson County UDC 3.6.10.g. This site has been designated as a moderate landslide hazard area on the Jefferson County GIS Landslide Hazards Layer. This designation is from a review of topographic contours indicating steep slopes in excess of 15% (Jefferson County GIS Landslide Hazard Layer Description). II. Background Penny Creek Quarry is an operating basalt mine located on and adjacent to slopes up to and exceeding 100%. Vertical relief is up to 650 feet measured from Penny Creek Road at the base of the quarry to the crest of a bluff bounding north-northwest of the site. This report is a supplement to a forthcoming mine and reclamation plan intended to satisfy Revised Code of Washington 78.44 governing surface mine reclamation through the Washington Department of Natural Resources Surface Mine Reclamation Program. A reclamation plan application will follow guidelines set forth by RCW 78.44 including stable final slope configurations. III. Geologic Conditions Ecological Land Services, Inc. (ELS) conducted site reconnaissance to observe geomorphic expression and on-site geologic materials in preparation for the reclamation plan. Additional site specific conditions have been recorded for this report including fracture patterns expressed in exposed rock to determine landslide hazard potential. Material observed on site consists of Eve, Eocene Crescent Formation Volcanics with overlying glacial sediments on other slopes within the region. The region was subjected to extensive Pleistocene glaciation during the V ashon Stade. The site is located in an area mapped as Crescent Formation rocks bounded to the east and southeast by Vashon Recessional Outwash along the base of slopes (Jefferson County GIS geologic map). Steep slopes in the project vicinity are likely a result of glacial erosion upon basalt foothills of the Quilcene Range, and of fluvial erosion through lodgement till by Penny Creek on glacial retreat. IV. Slope Stability and Landslide Hazard Slopes on and around Penny Creek Quarry are steep, averaging 45 degrees (100%) and steeper in basalt bedrock. Fractures within the rock mass likely resulted from tectonism: emplacement and faulting. One fault is identified on the South Seattle Quadrangle geologic map roughly 3 miles east of the quarry. Jefferson County Landslide Hazard designation as "moderate" for slopes in and adjacent to the quarry was likely based on topography alone. Information from Jefferson County's on-line Landslide Hazard Layer description states: "this is a composite layer produced from the SCS Soils Source coverage for soils types in East Jefferson County, the Department of Ecology Coastal Zone Atlas of Jefferson County, and a review of topographic contours indicating steep slopes. This layer does...give an attribute with a rating factor which is a weighted probable rating based on severity of slope.... In addition, properties with known land slide problems and properties with areas in excess of 15% slope have been added (categorically 15%-30%, 30%-60%, >60%) (htt ://www.co.'efferson.wa.us/idms/metadata/Landslide%24Hazard.shtml.)." The Department of Ecology Coastal Zone Atlas of Jefferson County does not provide coverage of Penny Creek Quarry. Evidence of unstable slopes in the project vicinity such as slumps, scarps, cut-off drainage or ponding, or pistol butt trees was not observed during multiple field reconnaissance conducted by ELS. Mine operations as proposed will remove basalt in a north by west direction into the hill side. Mining will be conducted in a manner resulting in final slope angles roughly 45 degrees (100%) including benches and chutes to blend with surrounding natural topography. Mining will not permanently create slopes greater than pre-mining conditions. Fracture pattern measurements indicate that fractures are generally perpendicular to slope angles and direction. A fault zone evident in the quarry face is oriented roughly parallel to slope and steeper (higher angle) than slope angles. Rock stability varies with inclination of discontinuity surfaces, such as faults, joints, and bedding planes, within the rock mass. Inclined planar discontinuities which daylight at the toe of a rock slope can cause instability when they are inclined at a steeper angle than the angle of friction of the rock surfaces (Hoek and Bray 1981). Fractures at Penny Creek do not appear to daylight into the working face of the quarry or toe. V. Conclusions • Penny Creek Quarry is located in basalt bedrock. • Slopes in the vicinity of Penny Creek Quarry were created by glacial and fluvial erosion. • Slopes in the vicinity of Penny Creek Quarry vary, up to and greater than 100%. • Landslide activity was not observed in or around Penny Creek Quarry. • Slopes in and adjacent to Penny Creek Quarry appear stable. • Excavation at Penny Creek Quarry will not create permanent slopes greater than those naturally occurring. • Primary fractures in the rock mass of the quarry are roughly perpendicular to slope and angled similarly to the slope. • In their current conditions, slopes in and around Penny Creek Quarry demonstrate overall stability. • Soft sediment slope instability is not evident on or around this site. • From observation, it appears that large blocks of loose bedrock that could produce toppling or sliding hazard are no longer present on the quarry working face or adjacent land. VI. Recommendations Fractures will continue to be encountered through excavation. When final cut slope configurations are achieved we recommend that any loose, fractured rock remaining on cut slopes be removed to minimize likelihood of potential block sliding or toppling. VII. Limitations The services described in this report were performed consistent with generally accepted professional consulting principles and practices. There: are no other warranties, express or implied. The services performed were consistent with our agreement with our client. This report is prepared solely for the use of our client and may not be used or relied upon by a third parry for any purpose. Any such use or reliance will be at such party's risk. The opinions and recommendations contained in this report apply to conditions existing when services were performed. ELS is not responsible for the impacts of any changes in environmental standards, practices, or regulations after the date of this report. ELS does not warrant the accuracy of supplemental information incorporated in this report that was supplied by others. VIII. References Hoek, E., Bray, J.W., 1981, Rock Slope Engineering, Spon Press, 358 p.