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HomeMy WebLinkAbout938400525 Geotech AssessmentCERTIFIED PROFESSIONAL GEOLOGIST AIPG: 2671 CRAIG K. WHITE, INC. GEOSCIENCES CONSULTING CERTIFIED: MPG DIVISION OF ENVIRONMENTAL GEOSCIENCES June 6, 2000 GEOLOGICAL SITE EVALUATION L0.t.~ 26 ....and 46 Replatted as Lots A and B Block 5, Division III, Plat of Cape George Colony Section 12, T30N, R2W W.M. Jefferson County, Washington Property_ Location and Description: The subject property is located on S. Rhododendron Dr., approximately 0.3 miles from its intersection with Palmer Rd. in the community of Cape Geor~ge Colony near Port Townsend, Washington, where it occupies a portion of the hillside slopes overlooking Discovery Bay (see Attachment 1). Access to the property from S. Rhododendron Dr. is along an unimproved driveway that leads to the center of the property near the top of the south-facing hillside slope. Access to the property along the base of the slope is along a secondary easement that extends north from Vancouver Dr. (see Attachment 3). The subject property, is bounded on all sides by other private lands. Given this fact, and the odd shape of the parcel we are uncertain about what the county will determine as the front, rear and side lot lines, which may be critical in developing this property. Lot size is approximately 35~000 s.f. or about 0.7 acres. No structures are eurremly located on the property, however, a bulldog site has been prepared by grading and leveling the area along, the top of the slope and further enlarged by the emplacement of fill along the upper slope edge. A septic tank and drainfield has been installed near the base of the slope. - P.O. BOX3398, SILVERDALE, WA 98383 (360) 830-0718 Geological Site Evaluation Ms. Naomi Nachun, Mr. Laurence Corm June 6, 2000 A Location Map, Slope Stability Map and Detail Plat showing general drainage and slope features within the Area of Study are included with tiffs report as Attachments 1, 2 and 3, respectively. Topography and Drainage: Drainage on the subject property is generally to the south, over the edge of the slope, toward a broad stream valley that drains west into Discovery Bay. Components of drainage also occur to the southeast (near the east property line) and to the southwest (near the west property line). At the base of the slope, runoff water drains into several low areas that eventually flow southwest, toward Vancouver Dr. Slope angles were measured on the subject property using a Brunton hand clinometer. On the middle and lower portion of the natural, south-facing slope, measured angles ranged from 25 degrees to as much as 40 degrees. However, along the upper portion of the slope where fill has been emplaced, the steepness of the slope has been increased to as much as 50 degrees, in places. This oversteepened condition occurs to an elevation of nearly 30 feet below the upper slope edge. Overall relief along the slope face is approximately 75 feet. Above the top of the slope, in the area of the proposed building site, the ground surface is nearly level, with the western portion of the property becoming lower in elevation. Slope angles, here, ranged from level to less than 7 degrees. This site investigation was conducted in late spring during a week with 2 Geological Site Evaluation Ms. Naomi Nachun, Mr. Laurence Conn June 6, 2000 intermittent periods of dry days and light rainfall. No springs or seeps were identified on the slope face, however, evidence of moderate to heavy scouring and surface erosion was present in several areas along the upper slope face, indicating that the volume of water moving down the slope is probably considerable during the wetter periods of the year. Vegetation: Large-growth vegetation on the subject property consists of conifers (mostly Douglas fir) as well as several varieties ofbroadleaftrees (Red Alder, willow and Pacific madrone). Lower-growth plants commonly include Himalayan blackberry, Salal, Oregon grape, Nootka rose, Scots broom and various grasses and weeds. In most cases, these species reflect the presence of well-drained surface soils. Vegetation in the upland area of the property was mostly removed during grading and preparation of the building site. Quite unfortunately, this clearing and the emplacement of fill along the upper slope edge has resulted in a loss of the lower-growth vegetation and surface soils in the area immediately behind the bluff top, where preservation of a natural vegetation buffer is desirable. Remediating this area with new, low-growth plants should be a condition of the proposed development. Vegetation along the bluff'face is fairly well established along the lower part of the bluff`, where the lower slope angle and presence of a relatively thick soil cover has allowed trees and deep-rooting plants to develop. Along the upper part of the slope, vegetation is Geological Site Evaluation Ms. Naomi Nachun, Mr. Laurence Conn June 6, 2000 patchy and more sparse due to the emplaced fill in this area, the effects of surface erosion and the presence of steeper slopes which discourage the retention of soils and vegetation. Geology and Ground Materials: A review of existing geologic maps of the area indicates that the geologic units 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 principal 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 almost always impermeable but considerably more stable and resistant to erosion than other sediments in this area. Examination of the ground materials at the subject property supports the above geologic interpretation. Exposures of moderately well-compacted glacial till occur in several localities where excavations have been made along the base of the slope, immediately west of the subject property, and in four soil log test pits on the subject property, itself. Soils and regoliths derived from tills are also present in the upland portion of the property along an excavation cut near the north property line, indicating these ground materials probably occur at relatively shallow depths below the emplaced fill at the Geological Site Evaluation Ms. Naomi Nachun, Mr. Laurence Conn June 6, 2000 building site. It should be emphasized that, although the compact nature of glacial tills generally provides an excellent foundation for structures, and the soils derived from this unit are usually sandy and well-drained, the till, itself, is frequently impervious to the downward percolation of surface water, which, during periods of heavy rainfall, can accumulate within the overlying soils, destabilizing them on steep slopes and sometimes causing slides. Slope Stability: On maps prepared by the Jefferson County Department of Community Development,, as a part of the Interim Critical Areas Ordinance (ICAO), the slope along the southern part of the subject property has been identified as a Geologically Hazardous Area, based on slope steepness, the nature of the ground materials, and potential surface or near-surface water. This slope has also been identified as Unstable ("U") on Washington State Department of Ecology Coastal Zone Atlas maps (Jefferson County volume) (see Attachment 2). Direct examination of the south-facing slope at the subject property supports the classification of this area as potentially unstable. Along the middle and lower part of the slope, several areas were identified where uncontrolled runoff' across the face of the slope has loosened soils and vegetation, resulting in minor sloughing and exposure of the underlying ground materials. Although this portion of the slope is not overly steep, the Geological Site Evaluation Ms. Naomi Nachun, Mr. Laurence Conn June 6, 2000 hazards posed from possible slides during periods of heavy water buildup on the impermeable till layer will always be present. Near the base of the slope, undercutting as a result of excavation for the septic tank and drainfield has oversteepened and potentially destabilized several areas, further increasing the chances of ground movements. Emplacement of fill along the upper slope edge, in order to increase the size of the building site, has also resulted in oversteepening along the upper part of the slope. While it is not known whether these materials were engineered and compacted, the presence of added weight in an oversteepened condition along the upper slope edge increases our concerns for the stability of this area. Conclusions and Recommendations: The observations and interpretations outlined in this report support the classification of the hillside slope on the subject property as potentially unstable. Although present slope activity within the study area appears to be confined to minor erosion and sloughing of surface soils, there can be no guarantee that a larger movement will not occur sometime in the future. However, it is our opinion that the natural ground materials within the area behind the top of the slope are generally stable and, with proper setbacks for construction, control of surface and near-surface water runoff and careful monitoring of slope conditions, we see no reason why the proposed development of this property should not occur. Geological Site Evaluation Ms. Naomi Nachun, Mr. Laurence Conn June 6, 2000 Our specific recommendations are as follows: 1. It is recommended that a minimum setback distance of 15 feet from the upper edge of the south-facing bluff (as shown by the anchored surveyor's flagging) be observed for all structures. This recommendation is conditioned on one of the following situations: a. that the emplaced fill along the upper slope edge can be documented as having been engineered and compacted under the supervision of a licensed professional engineer for the purposes of supporting a structure, or b. that the proposed structure is founded directly on natural ground materials, rather than on the emplaced fill, or c. that the proposed structure is situated on an engineered foundation, such as piling, designed by a licensed professional engineer, and anchored in natural ground materials. 2. Within the proposed 15-foot structural setback area, we also recommend that a natural vegetation buffer be re-established along the upper bluff edge by planting deep-rooting, native plants to reduce the effects of runoff and erosion along the bluff face. Large trees within this area and along the upper part of the slope that may threaten the safety of structures or that impede a limited view corridor, may be cut or topped, as deemed necessary, but the stumps and root systems of such trees should be left in place as they will resist erosion for a period of time. 7 Geological Site Evaluation Ms. Naomi Nachun, Mr. Laurence Conn June 6, 2000 3. Efforts should be made to promote and maintain a healthy growth of dense vegetation along the face of the slope to aid in retaining surface soils and reduce the effects of erosion. A list of recommended deep-rooting plants is enclosed with this report, or you may wish to contact a local landscape architect or Certified Washington State Nurseryman. 4. The accumulation of construction materials, stumps, branches, grass cuttings or other yard debris on the slope face should be discouraged, as these materials may inhibit the growth of healthy vegetation. Unnecessary climbing or walking along the steeper portions of the slope, which may loosen or disturb ground materials, should also be avoided in favor of established stairways or paths. 5. 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 oftightlines should utilize 4- or 6-inch, smoothwall sewer/drain pipe, tightly cemented at the joints. Lawns should be avoided in the area of the upper slope edge in favor of landscaping designed to minimize the need for heavy watering. 6. In no case should water from any source be allowed to discharge directly onto the slope face, and areas where excessive water flow occurs should be protected by catchment boxes or curtain drains that collect and shunt the water away from these areas. Low spots along the upper slope edge where water may be ponded during periods of heavy rainfall should be identified and filled. Geological Site Evaluation Ms. Naomi Nachun, Mr. Laurence Conn June 6, 2000 7. Frequent inspections of the slope should be made, especially after periods of heavy rainfall, to determine the location of any new springs, areas of excessive water runoff or erosion, and to identi~ any unstable areas. Recording the appearance of the slope in photographs, taken each year, and comparing them with current conditions may assist in this evaluation. Summary: Although the development of hillside properties 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 runoffthat 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 periodic, localized slides and sloughing of 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 not be ignored. This report has been prepared for the exclusive use of Ms. Naomi Nachun and Mr. Laurence Conn for specific application to the referenced site, and the conclusions and recommendations presented, herein, should be applied in their entirety. Within the limits of 9 Geological Site Evaluation Ms. Naomi Nachun, Mr. Laurence Conn June 6, 2000 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 recom- mendations 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. 6,2000 1. Location Map 2. Slope Stability. 3. Detail: Area of Study 10 Cape Geo~ ett P / ? u~ Stable Intermediate Unstable JUt~ 14 ~000 DEPT. OF C / Attachment 2 Slope Stability Unstable Bluff Unstable Recent Slide Unstable Old Slide Modified Slope Source: Washington State Deptartment Of Ecology Coastal Zone' Atlas B~s~ o~ Slc~ -- Vancouver Dr. NoAh Measured Slope Angle (Degrees) .Arrows Indicate Approximate Direction of Drainage Note: Sketch is not to scale and distances and relative positions of objects and structures am approximate. -Attachment 3 Detail: Area of Study Base of Slope - -- -: Drah~eld Vancouver Dr. No~h Measured Slope Angle (Degrees) Arrows Indicate Approximate Direction of Drainage Note: Sketch is not to scale and distances and relative positions of objects and structures are approximate. .Attachment 3 Detail: Area of Study