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HomeMy WebLinkAboutGeological Site Assessment - GEO General - 10/15/2001 DTANNING GEOLOGICAL SITE ASSESSMENT Prepared for: Mr. and Mrs. Scott Trammell Prepared by: Craig K. White, Inc. Geosciences Consulting P.O. Bog 3398 Silverdale, Washington 98383 October 15, 2001 hite,Inc 1GG0k&S C wsumma 41 Table of Contents Property Location and Description 1 Topography and Drainage 2 Vegetation 3 Geology and Ground Materials 4 Slope Stability 6 Seismic Hazards 7 Conclusions and Recommendations 7 Summary and Limitations 10 Appendixes: Location Map Appendix 1 Potential Seismic Hazards Appendix 2 Detail: Area of Study Appendix 3 CERTIFIED PROFESSIONAL GEOLOGIST CRAIG K. WHITE, INC. CERTIFIED:AAPG AIPG: 2671 GEOSCIENCES CONSULTING DIVISION OF ENVIRONMENTAL GEOSCIENCES October 15,2001 GEOLOGICAL SITE ASSESSMENT Tax Parcel# 12230-75-00200 Tract 20 of Survey 2/108 Section 30, T22N, Rl W W.M. Mason County, Washington Property Location and Description: The Subject property is located on East Sherwood Hills Rd., near the community of Allyn, Washington, where it occupies a portion of the uplands and west-facing hillside slopes between Case Inlet and the Hood Canal. The property is bounded on the north, south and west by other private lands, and on the east by East Sherwood Hills Rd. Property size is approximately 250,000 s.f or about 5.7 acres. A Location Map, Seismic Hazards Map, and Detail Plat showing drainage and slope features within the area of study are included with this report as Appendixes 1, 2 and 3, respectively. Access to the subject property is from State Highway 3 onto Sherwood Creek Rd. to E. Sherwood Hill Rd. for a distance of approximately %2 mile. Thereupon, left on an unnamed gravel road for a distance of 0.2 miles to the subject property.The property slopes gently to the northwest and has been cleared of vegetation to the base of the slope. No existing structures are currently located on the property, however, a proposed building P.O. BOX 3398,SILVERDALE,WA 98383(360)830-0718 Geological Site Assessment Mr. And Mrs. Scott Trammell October 15, 2001 site has been staked along the upper part of the slope and a location for the proposed drainfield was identified with flagging at the base of the slope. Three soil log test pits were also open for examination at the time of this site visit. Topography and Drainage: Drainage on the subject property is generally to the southwest, from the top of the slope along E Sherwood Hill Rd., toward a broad bench at the base of the slope. Minor components of drainage also occur to the north in several areas near the toe of the slope (see Appendix 3, Detail, Area of Study). Slope angles were measured at several locations on the subject property using a Brunton hand clinometer and laser-sighting device. On the southwest-facing slope, measured slope angles ranged from 19 degrees, immediately below the proposed building site, to as much as 21 degrees(on the roadway near the north property line). Slope angles flattened to as little as 5 degrees above the top of the slope near the east property line and to less than 3 degrees at the base of the hill. This site investigation was conducted in early fall during a relatively dry period and 2 Geological Site Assessment Mr. And Mrs. Scott Trammell October 15, 2001 no seeps or springs were noted along the slope face. Several areas along the slope showed evidence of minor erosion from surface water runoff resulting from rainfall that had occurred since the area was cleared of vegetation, underscoring the need for good water management as a part of the development plan. Vegetation: Large-growth vegetation on the subject property consists of conifers(Douglas fir, Western red cedar and Western hemlock) as well as several varieties of broadleaf trees (Red alder, Bigleaf maple and willow). Lower-growth plants include Swordfern, Salal, Oregon grape, Red and Black huckleberry, vine maple and Himalayan blackberry. Natural vegetation on the northwest-facing slope in the vicinity of the subject property is well-developed with a dense understory of lower-growth plants consisting of species that generally reflect the presence of well-drained soils. Vegetation in the area of the proposed building site has been cleared and the stumps and root systems of trees and plants removed in preparation for site development. Because of the present, exposed condition of the surface soils and their vulnerability to erosion, we recommend that landscaping or replanting of the slope be undertaken as early in the construction phase as possible. 3 Geological Site Assessment Mr. And Mrs. Scott Trammell October 15, 2001 Geology and Ground Materials: A review of existing geologic maps of the area(Molenaar, Deeter and others) indicates the geologic units present in the vicinity of the subject property are glacial and interglacial deposits of Quaternary age, most of which are less than 20,000 years old. The oldest geologic unit within the area of study is the Esperance (Colvos) sand, a sequence of fluvial sands and interbedded gravels with occasional lenses of clay, and a basal"blue clay" (sometimes mapped with the underlying Kitsap formation). The unconsolidated nature of the Esperance sands makes them highly vulnerable to surface erosion where exposed along steep coastal bluffs in the Puget Sound region, Also, their high porosity and permeability readily admits water which may build up on the surfaces of clay lenses within and at the base of the unit, sometimes increasing pore pressures to the point that the sand grains become unstable and occasionally resulting in deep-seated slope failures. Esperance sands commonly achieve thicknesses of up to 30 feet and the unit may serve as a primary aquifer where it occurs below the water table. The upland portion of the study area is capped by the Vashon till, a layer of compact cobbles and coarse sand in a binder of clay and silt that commonly ranges from a few feet to as much as 100 feet in thickness. This unit is comprised of materials deposited directly beneath a massive layer of glacial ice that once overrode the Puget Sound region. 4 Geological Site Assessment Mr.And Mrs. Scott Trammell October 15, 2001 In many places, the weight of the ice lobe compacted these sediments into a concrete-like mixture that is almost always impermeable, but considerably more resistant to erosion than the underlying, unconsolidated sands and gravels. Examination of the ground materials in the vicinity of the subject property supports the above geologic interpretation. Exposures of buff-colored sand (Esperance sands) occur in a broad roadcut along East Sherwood Hill Rd. These sands are capped at the top of the hill with a reddish-brown, poorly-sorted mixture of cobbles, sand, silt and clay (Vashon till). At the subject property, three open soil log test pits reveal the soil and subsoil ground materials to depths as great as 60 inches. The soil materials are principally unsorted, weakly-consolidated, tan to reddish-brown, cobbly to pebbly, sandy loam, typical of glacially-derived soils.Near the base of the pits, the soils become well-indurated as they transition downward into hardpan. The principal soil type in the area of the subject property is Alderwood gravelly sandy loam, 5 to 15 percent slopes (Ab). This is the most extensive soil of the Alderwood series. In undisturbed areas, the surface soil consists of a friable, brown, medium acid, gravelly, sandy loam 8 - 13 inches deep. Below the surface soil, to depths ranging from 18 5 Geological Site Assessment Mr. And Mrs. Scott Trammell October 15, 2001 to 24 inches, is a pale brown, gravelly, sandy loam that is very friable. Between this layer and the cemented till is a 3- to 10-inch layer of very pale brown, gravelly, sandy loam The cemented till consists of light gray, gravelly, sandy loam and normally occurs at depths ranging from 24 to 32 inches. It is impermeable to roots and very slowly permeable to water. Although compacted glacial till usually provides a stable foundation for structures, we should note that its low permeability often results in increased surface water runoff in areas where it lies close to the surface, underscoring the need for good water management. Slope Stability: On Washington State Department of Ecology Coastal Zone Management maps (Mason County volume), portions of the steeper slopes along Sherwood Creek are shown as having intermediate (I) stability. The subject property, itself, is not depicted on this map series which usually does not extend more than a mile inland from the coastline. Direct examination of the steep roadcut along the west side of East Sherwood Hill Rd. shows evidence of several previous slides that have occurred within the Esperance sand due to oversteepening of the hillside and exposure of the ground materials to heavy water erosion. Examination of the gentler, west-facing, hillside slope at the subject 6 Geological Site Assessment Mr.And Mrs. Scott Trammell October 15, 2001 property shows no evidence of instability or recent ground movements. Although some indications of erosion due to heavy surface water runoff are present along portions of the slope, we believe that this may be controlled by proper landscaping and revegetation of the exposed areas. The location of the drainfield, near the base of the slope, will eliminate the potential risk of ground movements on the slope face due to increased water from effluent. Seismic Hazards: The Puget Lowland is classified as a Seismic Zone III by the Uniform Building Code (UBC). Crustal earthquakes, which occur along surface faults, have been documented in several areas of the Puget Sound region(see Appendix 2: Seismic Hazards Map). However, the largest earthquakes in this region have been subcrustal events, ranging in depth from 30 to 60 miles or more. These deeper-focus earthquakes are not related to surface faults but are usually caused by movements along plate boundaries or within deeper sub-crustal slabs. Although the mapping of faults and study of earthquakes in the Puget Sound region is a continuing effort, the potential effects of a large seismic event on hillsides and slopes in this area are still not completely understood. Conclusions and Recommendations: 7 Geological Site Assessment Mr. And Mrs. Scott Trammell October 15, 2001 The observations and interpretations outlined in this report support the classification of the hillside slopes on the subject property as generally stable. Although recent slope activity on the property appears to be limited to minor surface erosion due to water runoff,we cannot guarantee that a larger-scale ground movement will not occur at some time in the future. However, with proper building site preparation and foundation design, management of surface water runoff and landscaping or revegetation to protect exposed soils from erosion, we see no reason why the development of this property, as proposed, should not occur. Specific recommendations are as follows: 1. It is recommended that the foundation design for the structure at the proposed building site be prepared or approved by a licensed professional engineer. In the unlikely event that excavation for the foundation exposes the poorly-consolidated(Esperance) sand unit beneath the till hardpan, mechanical compaction of these sediments or backfilling with engineered materials may be desirable. 2. An effective water-collection system(gutters and downspouts) should be installed on all new structures with water tightlined to the base of the slope. Buried portions of new tightlines should utilize 4- or 6-inch, smoothwall, sewer/drain pipe, tightly 8 Geological Site Assessment Mr. And Mrs. Scott Trammell October 15, 2001 cemented at the joints. 3. Curtain drains should be installed on the uphill sides of all structures to intercept surface and near-surface runoff water from the uphill slopes that might otherwise collect against the foundation. The outlet ends of these drains may be connected to the tightline system A curtain drain may also be required on the uphill side of the drainfield to prevent its flooding during periods of heavy rainfall. 4. Efforts should be made to promote and maintain a healthy growth of deep- rooting vegetation on all slopes, wherever possible, to aid in retaining surface soils and reduce the effects of erosion. Rock retaining walls and other landscape structures may also be used where their design and installation is appropriate to retain ground materials. Shrubs and plants used on slopes should be selected to minimize the need for heavy watering and the planting of lawn grasses should be restricted to relatively level ground. A list of recommended, deep-rooting plants is available, on request, or you may wish to contact a local landscape architect or Certified Washington State Nurseryman. 5. Frequent inspections of the hillside slope should be conducted, especially following periods of heavy rainfall, to determine the location of any new springs, seeps, or areas of excessive erosion, and to identify any unstable conditions. 9 Geological Site Assessment Mr. And Mrs. Scott Trammell October 15, 2001 Summary and Limitations: Although the development of properties along hillside slopes in the Puget Sound region is common, it should be acknowledged by property owners that these areas may be inherently unstable and involve higher risks than other areas due to the steepness of slope faces and the unconsolidated nature of the ground materials. Frequent and severe winter storms commonly produce high volumes of water runoff that may cause extensive surface erosion, saturate ground materials and destabilize slopes, resulting in ground movements that often occur without warning. Given these conditions, it should be expected that erosion and periodic sloughing of ground materials on the subject property may be a continuing problem. While the potential for larger-scale movements, such as may be precipitated by a seismic event, are not well-understood in this area, the hazards that may be posed by such an occurrence should also not be ignored. This report has been prepared for the exclusive use of Mr. and Mrs. Scott Trammell for specific application to the referenced site, and the conclusions and recommendations presented, herein, should be applied in their entirety. Within the limits of scope and budget, this study was conducted in accordance with generally-accepted practices employed at the time this work was done. No other warranty of conditions is expressed or implied. You should also be aware that these conclusions and recommendations are based on a general knowledge of this area and the interpretations of surface and subsurface conditions as they are believed to exist. These conditions may, in fact, be different than interpreted and events may inevitably occur that were not predicted. Clients should also be aware that there is a practical limit to the usefulness of this 10 Geological Site Assessment Mr. And Mrs. Scott Trammell October 15, 2001 report and recommendations without a site re-evaluation and critical review. Although this time limit is somewhat arbitrary, it is suggested that 2 years be considered a reasonable limit for the usefulness of this report. *.OF PROF 1 y p Craig . White, C.P.G. °ft K iO a ` Oc r 15, 2001 tt APPENDIXES Appendix 1: Location Map Appendix 2: Potential Seismic Hazards Appendix 3: Detail: Area of Study N • a CV s�, r•-•� ("d .--� 5T XCN ��• w LLI o Pt 0 s it a 0 o 4t ?�• • • O as . d � N ICA • ' • + O • a s 0 00 �- f a O On a 1714 N a g� 'Devils Mtn.Fault Zone f \l SKAGIT Fault Point l t 0 JSLA p G Port. �.} `n• lalr• �. . p� L Ati ar 9Xndertdf a , non r�sr 10 t at 'S ti • rN !�r u NO ! o l � •r o � Okla � ` � �QE t s f' ^oY ram'•`' F 4 `+•r- r � _ l �C - J LAKE ASHINGTON � � F f ~ r � �j_� ��rt ��"h � � • S A Seattle Fault Zone L _ 7 n i ort Orcharrt� l Lako ' • r ( IASHON p , .r K • c_ ,rry 4 Potential Earthquake Faults rL L t Ml in the , ` Puget Sound Region `��/� 7 r Source:U.S.Geological Survey 'C3 A ° � 1 eToQ o � �cn ytvo N � o m A u a� a ^c F- � O o � a L � Q � � i. u a 0 a u