Loading...
HomeMy WebLinkAboutBLD2000-00182 Geotechnical Report CERTIFIED PROFESSIONAL GEOLOGIST CRAIG K. WHITE, .INC. CERTIFIED:AAPG AIPG: 2671 GEOSCIENCES CONSULTING DIVISION OF ENVIRONMENTAL GEOSCIENCES GEOLOGICAL SITE EVALUATION Parcel#998200325 ib>' Tala Shore#3, Lot 26 Section 15, T28N, R1E, W.M. 1114)' , Jefferson County, Washington ( , Property Location and Description: The subject property is located at 710 Tala Shore Drive near the community of Port Ludlow, Washington, where it occupies a portion of the tidelands and high-bank, east-facing bluffs overlooking Admiralty Inlet and the entrance to the Hood Canal. Access to the property is from Tala Shore Drive, approximately 0.8 miles north of its intersection with Ludlow-Paradise Rd. Access to the beach along the base of the bluff was achieved by descending a steel stairway on the adjacent property to the south. The subject property is bounded by other private lands on its north and south sides, by Tala Shore Drive on the west and the marine waters of the Hood Canal to the east. Property size is approximately 0.65 acres. A Location Map, Slope Stability Map and Detail Plat showing general drainage and slope features within the area of study are included with this report as Attachments 1, 2 and 3, respectively. Topography and Drainage: Slope angles were measured on the subject property using a Brunton hand P.O. BOX 3398, SILVERDALE,WA 98383(360)830-0718 Geological Site Evaluation Mr. Mario Reyes April 22, 2000 clinometer and laser-sighting device. Along the top of the bluff, the ground surface at the subject property slopes gently toward the east and southeast in the general direction of the bluff face. Slope angles of approximately 7 degrees (east) were recorded on the upland portion of the property, near Tala Shore Dr., decreasing to 5 - 6 degrees in the direction of the bluff, near the proposed development site. Slope measurements along the steeper, lower bluff face averaged as much as 55 - 60 degrees in areas where vegetation is absent and the ground materials are exposed. Along the top of the bluff, where vegetation becomes more well-established, slope angles decrease to approximately 35 45 degrees. The overall heghtof the bluff at the subject r property, from the beach to the upper bluff ecie is roughly 60 feet_ Although this site investigation was conducted during a relatively dry period with only light rainfall, a number of small springs were identified along the bluff face in the vicinity of the subject property. Closer study reveals that most of these springs occur along the contact between the soil layer and the underlying glacial tills. Water that percolates downward through the more porous and permeable soils accumulates on the top of the impermeable till where it builds in volume and finally escapes to the surface along the bluff face. Although water flow was relatively minor at the time of this investigation it is likely that these springs become much more active during periods of heavy rainfall, increasing the volume of runoff along the bluff face and raising the risk of ground movements. In several areas of the bluff, immediately below the upper bluff edge, indications of 2 • Geological Site Evaluation Mr. Mario Reyes April 22, 2000 minor erosion from surface water runoff are also present. Again, although water flow was not observed during this relatively dry period, this area along the upper bluff is judged vulnerable to erosion and slides from unchecked runoff during heavy winter rains, underscoring the need to provide protection from such damage. Vegetation: Large-growth vegetation in the vicinity of the subject property consists of conifers (Douglas fir, Western hemlock and Western red cedar) as well as several varieties of broadleaf trees (Bigleaf maple, Red alder and Pacific madrone). Lower-growth plants include Vine maple, Swordfern, Himalayan blackberry, Nootka rose and English ivy (along portions of the bluff face) as well as grasses and ornamental plants in landscaped areas. Vegetation in the upland area of the subject property has been mostly removed in the area of the proposed structure relocation. Quite unfortunately, this clearing effort has also removed lower-growth plants and surface soils in the area immediately behind the upper bluff edge, where preservation of a natural vegetation buffer is desirable. Remediating this area.vailuxstnwegetation.shotild-be-a-tenmefie° developmeni. Vegetation along the bluff face is fairly well established along the upper portion of the bluff where the low slope angle and presence of a relatively thick soil cover allows 3 Geological Site Evaluation Mr. Mario Reyes April 22, 2000 deep-rooting plants to develop and provide a healthy, dense cover. Along the lower part of the bluff, vegetation is patchy or altogether absent due to the effects of water erosion from natural springs that occur, here, and the presence of steeper slopes which discourages the retention of soils and vegetation. In several places where slides have occurred involving vegetation and soils from the upper bluff face, these plants and ground materials have become temporarily re-established along the toe of the slope where they will probably help resist wave erosion for a period of time. Geology and Ground Materials: A review of existing geologic maps of the area indicates that, while older rocks are present along the beach to the north, the geologic units in the vicinity of the subject property are principally glacial and interglacial deposits of Quaternary age, most of which are less than 20,000 years old. The oldest unit exposed at the subject property is the Vashon till, a layer of compact cobbles and coarse sand in a binder of silt and clay 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. In many places, the weight of the ice lobe compacted these sediments into a concrete-like mixture that is almost always impermeable and considerably more resistant to erosion than other sediment types. This unit is commonly overlain by a regolith of weathered till, capped with surface soils of varying thickness. Examination of the ground materials at the subject property supports the above 4 Geological Site Evaluation Mr. Mario Reyes April 22, 2000 geologic interpretation. Exposures of cliff-forming, highly-compacted, glacial till form the lower part of the bluff, here, extending upward to an elevation of approximately 35 - 40 feet above the beach. This unit, in turn, is overlain by a well-weathered mantle of regolith and sandy loam, 20 to 25 feet thick that produces a more gently-sloping bluff face above the till. The impermeable nature of the till is supported by the presence of a number of natural springs and seeps that occur along the upper boundary of this unit where water accumulates. Although the compact nature of Vashon till generally provides an excellent foundation for structures and the soils derived from this unit are commonly sandy and well-drained, the till, itself, is often, locally, impervious to the downward percolation of surface water which, during periods of heavy rainfall, can accumulate in low spots on the surface and around foundations, creating drainage problems. Landslide Hazards: On maps and documents prepared by the Jefferson County Department of Community Development, as a part of the Interim Critical Areas Ordinance (ICAO), the bluff along the eastern 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", with some areas having experienced both "Old" (Uos) and "Recent" (Urs) slides on Washington State Department of Ecology Coastal Zone Atlas maps (Jefferson County volume) (see Attachment 2). 5 Geological Site Evaluation Mr. Mario Reyes April 22, 2000 Direct examination of the shoreline and east-facing bluffs in the vicinity of the subject property reveals a number of areas where recent slides have occurred, as evidenced by the presence of trees, stumps, soil material and other debris along the lower bluff and upper shoreface of the beach. Above the slide debris, along the bluff face, exposed ground materials occur in a series of patchy escarpments where vegetation and surface soils have been stripped away by periodic erosion and sloughing. As mentioned earlier in this report, active springs and seeps along these exposed areas of the bluff are common and water from these sources has probably been a factor in precipitating the slide events. Along a portion of the bluff face, near the south property line, the upper bluff edge bends inland for a distance of at least 20 feet from its position near the north property line, creating a bowl-shaped feature that extends onto the adjacent lot to the south. This feature probably represents a large slump or slide that may have occurred a number of years ago as water buildup along the top of the impermeable glacial till destabilized the overlying, unconsolidated regolith and surface soil layers. Similar features along other parts of the bluff in this area suggest this style of large-scale slumping is not uncommon, here, and will likely occur in the future, probably during periods of heavy rainfall and water buildup in the shallow subsurface ground materials. Conclusions and Recommendations: The observations and interpretations outlined in this report support the 6 • Geological Site Evaluation '' Mr. Mario Reyes April 22, 2000 classification of the shoreline bluffs in the vicinity of the subject property as unstable. While-evidence• PrePertr indicates these have been mostly shallow-founded and relatively limited in size, the presence of impermeable layers (that promote water buildup in the overlying ground materials) coupled with certain observed geomorphic features characteristic of older slumping, indicates the potential for larger-scale events along the bluff face does exist. It is our opinion, however, that the area of the proposed relocation site, inland from the east-facing bluff is generally stable, underlain by compacted glacial tills and well-drained surface soils. Accordingly, with proper setbacks for the structure, control of surface and near-surface water runoff, careful monitoring of slope conditions and implementation of the slope stabilization measures outlined, below, we believe the proposed structure relocation should not increase the potential for slides, erosion or sediment discharge on this or other adjacent properties. Specific recommendations are as follows: 1. It is recommended that a minimum setback distance of 60 feet from the upper edge of the south facing bluff(measured from the center of the property)be observed for all structures. Within this setback area, we also recommend that a 30-foot vegetation buffer along the upper bluff edge be re-established with deep-rooting, native plants. Any large trees within this area that may threaten the safety of structures may be cut or topped, as deemed necessary, however, 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 • Mr. Mario Reyes April 22, 2000 2. Efforts should be made to promote and maintain a healthy growth of dense vegetation wherever possible along the top and face of the bluff 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. 3. The large Douglas fir along the upper bluff edge near the south property line may become unstable and susceptible to wind-throw during winter storms. Such an event could result in the loss of a significant portion of the upper bluff face and serious consideration should be given to its removal. As outlined above, the stump and roots of this tree should be left in place to help retain the ground materials, here. • n 4. The accumulation of construction materials, stumps, branches, grass cuttingsor other yard debris on the bluff face should be discouraged, as these materials may inhibit the growth of healthy vegetation. Unnecessary climbing or walking along the steeper portions of the bluff, 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 structures with water tightlined to the beach at the base of the bluff. Buried portions of tightlines should utilize 4- or 6-inch, smoothwall sewer/drainpipe, tightly cemented at the joints. Lawns should be avoided in the area of the upper bluff edge in favor of landscaping designed to minimize the need for heavy watering. 8 Geological Site Evaluation Mr. Mario Reyes April 22, 2000 6. In no case should water from any source be allowed to discharge onto the bluff face, and areas where excessive surface water flow occurs should be protected by catchment boxes or curtain drains that collect and shunt the water away from these areas. Low areas along the upper bluff edge where water may be ponded during heavy rains should be identified and filled.inthe`event that-a cu n- ain'is in bluff-edge,udh lation should not be closer Than 20 feet froril tfie116ff Also, care should be taken to avoid the use of heavy equipment near the bluff, as the vibrations and shaking from this equipment may disturb sensitive ground materials. 7. Periodic examinations of the bluff face should be made, especially after storms or periods of heavy rainfall, to determine the location of any new springs or areas of excessive water runoff and to identify any unstable areas Recording the appearance of the bluff in photographs, taken each year, and comparing them with current conditions may assist in this evaluation. Summary: Although the development of shoreline bluff 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 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 9 lir 1 Geological Site Evaluation . Mr. Mario Reyes April 22, 2000 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 Mr. Mario Reyes 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. lllllllllllllllll a4,,,,, _„. .4*, .a;/ / .' % -0, 4001*or & e' - I G K. WO 00; Cr K. White, C.P.G. i-1461 usI i r►?,'\. '� pril 22, 2000 Attachments: 1. Location Map 2. Slope Stability 3. Detail: Area of Study 10 3 �; Q, 'i' 1 N. 4j N. Proposed 30'Vegetation Buffer 917* / Z \ Old Slide/Slump Area 60' ® 4 — Top of Bluff ,Douglas fir(unstable) WI e i _ ,, ... ,,,„, I /6-gd ,..._ ... .„ 00-20- 1 1 a s cu t_ ci: 1 , Drainfield A O 6. 1 Cik a.) .E -1:1•-, (— — — — — A o Ho. et . c 1. Vi = 4� L 'V© Top of Slope z . or AO Vashon Regolith,Surf Soils r 1 v . e li � S imp smeable Surface 60' • • , • - ft. , O Q I Eosed Vashon till b • I. 55-60 degrees e' • c cti Toe of slope � Attachment 3 f. .4 � . Profile • • ' - - J Detail: Area of Study Recent Slide Debris •