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Home My WebLink AboutGEO Geological Review - 6/14/1999 s CERTIFIED PROFESSIONAL GEOLOGIST CRAIG K. WHITE, INC. CERTIFIED:AAPG AIPG: 2671 GEOSCIENCES CONSULTING DIVISION OF ENVIRONMENTAL GEOO� �aSCIE/NCES®`' pry JUN 2 8 1999 MASON GO.PLAWNG DEPT GEOLOGICAL SITE ASSESSMENT Lot 118, Wooten Lake Development Volume 4 of Plats,Page 41, Section 19, T23N,R2W,W.M. Mason County,Washington 22`f-) J. E. rya Property Location and (Description: 0 3 4 f-.Q" V �r The subject property is located at NE 2241 Tahuya-Blacksmith Rd. approximately eight miles northeast of Belfair, Washington,where it occupies a portion of the wooded uplands area above the west shore of Wooten Lake. Access to the property from Tahuya-Blacksmith Rd. is along a gravel driveway that leads to a level parking area above the roadway. The property is bounded on the north,west and south sides by other private lands and on the east side by Tahuya-Blacksmith Rd. Existing structures or developments include a 2-car garage, septic system and drainfield. A proposed single-family residence is sited along the west side of the property, above the drainfield. Property size is 42,906 square feet, or slightly less than one acre. Topography and(Drainage: Drainage at the subject property is generally to the east, toward Tahuya- Blacksmith Rd. Slope angles were measured at several locations using a Brunton hand-clinometer and laser-sighting device. Slope angles of 8 to 11 degrees(approximately 14 to 20 percent)were recorded along the steeper portion of the slope, near the drainfield. In the area of the proposed building site the slope angles decrease to approximately 5 degrees(about 9 percent)and become nearly level along the cleared trail near the west property line. Excavation into the hillside of the area behind the garage has produced an oversteepened slope with a height of up to 6 feet that is nearly vertical, in places, and will require stabilization to reduce the potential for sloughing and erosion of the exposed ground materials. Although this site investigation was conducted following several days of light to moderate rainfall,the soils appeared well-drained and relatively dry with no indications seeps, springs or heavy surface runoff. P.O. BOX 3398,SILVERDALE,WA 98383 (360)830-0718 __ ------__ _m_ �..-.a._.._,.�,-�.�-�,-.,.R .--.,,� ,•„_� .T._.,, R�.�,..,,..,,. — _ .�z--�., �- � ..,,- -�_.-_ ... �— ..ter. - —. _ - Geological Site Assessment Mr. Bill Lewis June 14, 1999 Vegetation: Large-growth vegetation on the subject property consists of conifers(Douglas fir, Western hemlock and Shore pine) as well as broadleaf trees (Red alder and Black cottonwood). Lower-growth plants include Vine maple, Salal, Himalayan blackberry, huckleberry and wild rhododendron. Vegetation on the western(uphill) portion of the property is well-developed and represented by species that reflect the presence of well-drained soils. In the vicinity of the garage and drainfield, vegetation has mostly been removed as a result of development. Geology and Ground Materials: A review of existing geologic maps of the area(Carson and others)indicates the geologic units present in the vicinity of the subject property are glacial and interglacial deposits, most of which are less than 15,000 to 20,000 years old. The principal geologic unit that comprises the subsoil materials at the subject property is 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 composed of materials deposited directly beneath a massive layer of glacial ice that once overrode the Puget Sound region. In many places, the weight of the ice lobe compacted these sediments into a concrete-like mixture that is often impermeable and considerably more resistant to erosion than the unconsolidated gravels, sands and clays that underlie other parts of the Puget Sound area. The compact, dense nature of this material also enables it to provide a good foundation base for structures in places where it is at least several feet in thickness. The presence of Vashon till at the subject property is supported by direct examination of the ground materials in several areas along Tahuya-Blacksmith Rd. and on the property, in the excavated bank adjacent to the garage. Exposures of well-rounded cobbles, pebbles and coarse sand, loosely-to moderately-well cemented in a matrix of clay and silt confirm the presence of these rocks with a projected thickness well in excess of ten feet. It should be noted that, although compacted glacial till near the surface usually provides a good foundation for structures, its low-permeability often makes it highly resistant to the infiltration of water, thus increasing the volume of runoff and the potential for erosion. For this reason, careful planning for surface water management at the subject property is strongly recommended. Conclusions and Recommendations: The observations and interpretations outlined in this site assessment support the County's classification of the slope at the subject property as a designated landslide hazard area, based on the presence of slopes steeper than 8.5 degrees (15 percent) and the 2 Geological Site Assessment Mr. Bill Lewis June 14, 1999 unconsolidated nature of the surface soils and shallow subsurface ground materials. However, with appropriate grading and site preparation, control of surface and near-surface water runoff, monitoring of slope conditions and implementation of the slope stabilization measures outlined below,we see no reason why the proposed development of . the subject property should not occur. Specific recommendations are as follows: 1. Although the ground materials exposed in the excavated area behind the garage appear to be moderately well-compacted, their exposure to normal weathering processes will likely result in accelerated erosion and sloughing along this oversteepened bank. The installation of a rock wall or bulkhead is recommended to stabilize these steep faces. In the event sloughing and erosion continue to be a problem, a poured concrete retaining wall may be required. - 2. It is our opinion that slope angles at the location of the proposed building site are gentle enough to permit construction with a conventional foundation. However, due to the low permeability of the ground materials,we strongly recommend implementing the following methods of controlling surface water runoff and preventing excessive erosion. a. The foundation of the proposed structure should be coated with an impermeable sealant to prevent water from seeping through cracks and pores.Backfill against the foundation should be clean, porous fill with a minimum of fine clay and silt. Grading of backfill away from the structure should be adequate to prevent water from accumulating against the foundation. b. An effective water-collection system(gutters and downspouts) should be installed on all structures with runoff water tightlined to the base of the slope, along the roadway. We recommend that all buried tightlines be constructed with 4- or 6-inch smoothwall drain/sewer pipe, securely cemented at the joints. c. Installation of a curtain drain along the western(uphill) side of the proposed building site should be considered to intercept near-surface water that may drain from the adjacent property on the west toward the structure. Outlets from this drain may be tied into the tightline. d. In no case should water be allowed to discharge freely down the slope face and areas where excessive water flow occurs should be identified and remediated. Efforts should be made to promote a healthy growth of deep-rooting plants along the steeper parts of the slope to aid in retaining surface soils and reduce the effects of erosion. A list of recommended, deep-rooting plants is available on request, or you may wish to consult a local landscape architect or Certified Washington State Nurseryman. t Geological Site Assessment Mr. Bill Lewis June 14, 1999 e. Periodic examinations should be made along the slope face, especially after periods of heavy rainfall, to determine the location of springs or excessive water runoff and identify any unstable areas. Summary: Although the development of properties along slopes in the Puget Sound area 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, causing ground movements that often occur without warning. Given these conditions, it should be expected that localized erosion and periodic sloughing of ground materials may be a continuing problem. While the potential for larger-scale earth 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 event should not be ignored. This report has been prepared for the exclusive use of Mr. Bill Lewis 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 at the time the 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 believe to exist. These conditions may, in fact,be different than interpreted and events may inevitably occur that were not predicted. of PNOFE ' Cr ' K. White, C.P.G. �F� E JIGK. kt"�- Attachments: 1. Location Map of study area showing general topography 4 �_-- -.-ram"=:..c:_.:::-.-_-..;�' �-..-�'::-•:::......,.';::. . : -_ ::_�.:. ;,: -:�� _� ': - - . i ta MCA l 3 PQ -� a rr, _ r IF CN z J � X = f u. FT .max' 'I