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HomeMy WebLinkAbout965800007 Geotech AssessmentSTRATUM GROUP 1451 Grant Street, Bellingham, WA' 98225 Phone (360) 714-9409 January 12, 2000 George Harper 2201 Washington-Irvine Pearland, TX 77581 Re; Geotechnical Report, Shoreline Bluff Lot 7, Kala Heights Kala Heights Drive, Kala Point, WA Dear Mr. Harper: We are pleased to present the results of our engineering geology reconnaissance of the above referenced property adjacent to a shoreline bluff The purpose of this geology evaluation was to 1) determine the suitability of the property for the siting of residence, 2) qualitatively evaluate the risk of slope failures, and 3) provide general site development and maintenance recommendations for development of a building site adjacent to a potentially unstable bluff This evaluation was limited to a visual inspection of the property and vicinity, a visual inspection of the bluff face, review of available geologic mapping in the area, and inspection of the shoreline at the base of the bluff The property is located on an approximately 180-foot high bluff on the west side of the Quimper Peninsula. The location of the property is indicated on Figure 1. The property is approximately ~4(acres in size and is located in the northeast 1/4 of Section 27, Township 29N, Range lW W.M. SCOPE OF SERVICES The scope of our services included the following: 1) Conducted a site visit to visually inspect the subject property including the bluff face slope conditions, shoreline conditions, and relevant conditions in the vicinity of the property. 2) Observed surface soil conditions on the bluff face and on the uplands above the bluff. 3) Prepared this report summarizing our findings, including an evaluation of the feasibility of building a residence on the upland portion of the subject property, a qualitative evaluation of the shoreline bluff stability in regards to building a residence on the property, recommendations for site development, and recommendations for further investigation, if necessary. January 12, 2000 Harper Property, Lot 7 Kala Heights Drive Geologic Evaluation of Shoreline Bluff GENERAL GEOLOGY Northwestern Washington has been occupied by continental glaciers at least four times during the Pleistocene Epoch (1.6 million to 10,000 years ago). During these glacial and accompanying interglacial periods, the underlying bedrock was eroded and a relatively thick layer of glacial related and interglacial fluvial sediments were deposited over the underlying bedrock in the vicinity of the subject property. The Surficial Geologic Map of the Port Townsend 30- by 60-Minute Quadrangle, Puget Sound Region, Washington (Pessl, Dethier, Booth and Minard, 1989) and the Geologic Map of Northeastern Jefferson County_, Washington (Birdseye, 1976) indicate the shoreline and bluff exposed on the subject property is underlain glacial till and advanced outwash deposits. The glacial till is described as consisting of a mixture of clay, silt, sand and gravel and is in a very compact cOndition. The advance outwash deposits consist of sand, gravel, silt and clay deposited by meltwater flowing from the advancing Puget lobe of the Fraser glaciation. The advance outwash sediments have been overridden and consolidated by glacial ice. Our observations of the bluff face on the subject property and upland areas on the subject property and vicinity of the property are consistent with the mapping described above. We observed that the glacial till at the top of the shoreline bluff in the ~icinity of the property varied from approximately 10 feet thick to 40 feet thick. At the subject property, the till is at least 15 feet thick. The advance outwash deposits onthe bluff consist primarily of sand with occasional lenses of gravel and silt. SPECIFIC SITE OBSERVATIONS The subject property is approximately cres in size. The property is bound on the southwest by Kala Heights Drive. Residences are located northwest and southeast of the property. The upland portion of the property is located between Kala Heights Drive and the top of the steep shoreline bluff located on the northeast side of the property. The distance between Kala Heights Drive and the top of the shoreline bluff is approximately 145 feet. The northeast boundary of the property is located approximately 20 feet down the shoreline bluff slope. The bluff height is approximately 185 feet. The upland portion of the property slopes gently to the northeast, towards the top of the shoreline bluff. A 30-foot wide buffer of native brash and trees lines the top edge of steep slope. The buffer consists primarily of salal, wood rose, and trailing blackberry. A couple of mature Douglas firs are located within the buffer as well. The rest of the upland portion of the property is brash- and grass-covered. Stratum Group File: 11.30.99 January 12, 2000 Harper Property, Lot 7 Kala Heights Drive Geologic Evaluation of Shoreline Bluff A community seWer collection tank is located on the upland portion of the property 50 feet from the edge of the bluff. The tank receives sewage from several homes in the vicinity, and will receive sewage from the proposed residence as well. Sewage from the tank is pumped to a community sanitary drainfield located approximately 400 feet south of the subject property. The shoreline bluff is approximately 185 feet high. The overall average slope from the top of the steep bluff to the beach is approximately 38 degrees (78%). However, a couple portions of the bluff are much steeper with the majority of the bluff sloping at approximately 35 degrees (70%). The uppermost 12 feet is nearly vertical and an approximately 20-foot section of the bluff slope near the base of the bluffis slopes at an angle of approximately 70 degrees. Both north and south of the subject property the bluff slope is similar for a distance of approximately 500 feet, but south and north of this area, the bluff is generally steeper. Most of the bluff on/he subject property is covered with a stand of approximately 20 to 25 year old red alder trees with an understory of ferns, trailing blackberry, salmon berry and snow berry. Numerous small western red cedar and Douglas firs were growing in the understory. A few mature big leaf maples, red alders, Douglas fir, and western red cedar are also present on the slope. The mature trees all appear to have been disturbed by past slope failures, and appear to be "survivors" fi'om past slope failures. The only unvegetated portion of the bluff slope was the upper 10 feet of the slope. This portion of the slope is nearly vertical and is underlain by very compact glacial till. Numerous slope failures are readily apparent all along the shoreline bluff north and south of the subject property. All the slope failures appear to relatively shallow soil failures due to the steepness of the slope. The primary cause of slope failures is from undermining of the slope by wave action at the beach. As the slope is undermined the lower portions of the slope fail first and the slope failures progress up the slope until the top of the bluff fails as well. We observed the full range of these slope failures at various stages all along the approximately 2 miles of shoreline bluff in the vicinity of the subject property. It appears that the top portion of the. bluff at the subject property and adjoining properties failed approximately 25 years ago (based on the assumed age of the majority of the red alders growing on the slope). The slope failure at the top of the slope would have had sufficient energy to slide all the way to the beach and knock over or strip most of the vegetation growing in the landslide path. Landslide debris consisting of failed soil and trees has been deposited at the base of the buff on the beach. The landslide debris acts as a natural protection for the base of the bluff until over time the debris is washed away and the process of progressive slope failures up the shoreline bluff repeats itselfi. Based on the presence of 100 year old or older Douglas firs on most of the bluff slope on this stretch of coast line and the consistent geology along this portion of the coast, it appears that the Stratum Group File: I 1.30.99 January 12, 2000 Harper Property, Lot 7 Kala Heights Drive Geologic Evaluation of Shoreline Bluff frequency of slope failures that remove all the trees has been less frequent than 100 years. In addition, we observed that the top of the bluff is fairly linear, that is, we did not observe any significant convergent topography. This is consistent with our observations that the slides that have been occurring on this bluff are shallow soil failures. Soils exposed on the steep portions of the bluff slope consist primarily of!ayered very compact silty sand and sandy silt with occasional lenses of gravel. The uppermost portion of the bluff at the subject property is underlain by very compact massive silty sand with gravel, cobbles and boulders. We interpret the layered sands and silts to be pre-Fraser advance oUtwash fluvial deposits and the massive silty sand with gravel, cobbles and boulders to be glacial till. We did not observe any water seeps anywhere on the bluff face of the subject property or anywhere else on the shoreline bluff in the vicinity of the property at the time of our site visit on December 18, 1999. CONCLUSIONS AND RECOMMENDATIONS Based on our visual inspection of the subject property and vicinity, we conclude that a residence can be located on the upland portion of the property (as indicated in Figure 2) such that bluff erosion or landsliding would not threaten the structure within the expected life of the structure (75 years) as long as our recommendations are followed. ~ We recommend that any buildings constructed on the subject property be set back from the top edge of the steep shoreline bluff slope at least 60 feet. It is our opinion from an engineering geology standpoint that this distance will be adequate to allow for expected shallow bluff face failures that will result from the continued erosion of the base of the bluff by wave action and raveling of the exposed soils on the bluff face. We do not anticipate that the development of the subject property will cause any negative impacts on the stability of the slope or the shoreline bluff as long as our recommendations are followed. Because the bluff is an erosional bluff, slope failures should be expected to occur on a periodic basis. The only way to prevent continued erosion of the base of the bluff is to construct hard armoring at the shoreline at the base of the bluff. However, the eroding bluff acts as a feeder bluff (erosion of the bluff provides sediment) for the beaches and Kala Point, a sand and gravel spit, southeast of the subject property. Any shoreline armoring will have a negative impact on properties down drift from the bluff. The construction of shoreline armoring on other properties may cause an increase of erosion of the subject property. The owner of the subject property should contact Jefferson County officials regarding any proposed shoreline protection projects along the bluff or any shoreline protection construction. With the exception of a rock rip rap Stratum Group Fil~ 11.30.99 January 12, 2000 Harper Property, Lot 7 Wa Heights Drive Geologic Evaluation of Shoreline Bluff bulkhead at the base of the trail at Fort Townsend State Park approximately 3/4 of a mile northwest of the subject property, there are currently no shoreline-armored areas along this stretch of coastline. In addition to the above setback criteria, we make the following general recommendations: Native vegetation, particularly tress and low native brush on the slope between the building site and the shoreline bluff', should be disturbed as little as possible. No tr~s or grading should be performed on the bluff' face. If trees are thinned or limbed for view purposes, no debris should be placed within 30 feet of the top of the bluff. Some thinning of trees growing between the top of the bluff face and the proposed home site should not cause any problems as long as the native brush understory is le~ in place. Thinning and trimming of trees on the bluff slope should be limited to topping of alders and maples and limbing for view corridors of Douglas fir and western red cedar. Any debris from thinning on the bluff' slope itself should be removed from the slope. Sanitary sewage will be collected in a community tank located on the subject property 50 feet from the top of the bluff. The sewage is pumped from the tank to a community drainfield approximately 400 feet fi-om the subject property. All building structures should have roof gutters and a down spout system that collects water in a smooth wall tightline pipe that transmits the water to beach level. The roof and footing drain systems should connect to a catch basin located 30 to 40 feet from the bluff crest. The most vulnerable portion of a water collection system is the tightline section that transmits water to the beach. We recommend using a 6-inch diameter High Density Polyethylene (HDPE) pipe to transmit collected water from the catch basin to beach level by tightline. We recommend that the HDPE pipe be butt welded by certified personnel. Certified welds are guaranteed to be as strong as or stronger than the pipe. We recommend a concrete deadman anchor be constructed around the pipe and catch basin to hold the system in place. The deadman anchor should contain 2 to 3 cubic yards of concrete. Alternatively, other anchor systems could be proposed for review and approval by Stratum Group. The pipe should be supported directly on the ground surface down the slope as much as is practical and the pipe should terminate into an energy dissipater. The energy dissipater can consist of a gabion basket system or simply a pile of coarse stones (1-foot to 4-foot diameter). For a single family home, a pile of coarse stones should be sufficient. Where the pipe transitions from the uplands to the slope, we recommend that a bentonite amended soil plug be placed at the slope crest (10 feet long minimum) to prevent ground seepage from following along the pipe trench and exiting on the slope. Behind the bentonite water stop plug the trench may be baclffilled with trench spoils provided the spoils are not placed in a wet condition and provided adequate compaction can be achieved (jumping jack compactor). For a project of this magnitude a pile 8traasm Group File: 11.30.99 January 12, 2000 Harper Property, Lot 7 Kala Heights Drive Geologic Evaluation of Shoreline Bluff of coarse stones should be adequate. If county inspectors are used to document construction of the project, they should obtain written certification for the pipe welds. They should also insure that the pipe is properly bedded, and the trench water plug is properly installed, check trench baelcfill, and insure that the pipe dead man anchor (catch basin) is properly constructed. The inspector would also need to verify that the pipe is adequately secured to the slope and that the energy dissipater (probably a coarse stone pile) will disperse heavy water flows at beach level. Building structures may be founded directly on the native very compact silty sand and gravel or on compacted structural fill placed directly on the native soils. If structural fill is used, we recommend a relatively clean sandy gravel with less than 5% passing the #200 sieve based on a wet sieve analysis of that portion passing the sA-inch sieve (GW by USCS Classification). Structural fill, if used, should be placed on a horizontal surface in 8 to l0 inch loose lifts and compacted to 95% of maximum dry density as determined by ASTM-D1557 test procedure. Structural fill placed beneath footings should extend beyond the edges of the footings a distance equal to the thickness of the fill beneath and footings. Continuous and spread footings should have minimum dimensions of 16 and 24 inches respectively, and should' be founded a minimum of 18 inches below final grade for frost protection. If large column footings are planned the soil bearing pressure should be limited to 2,000 psf. This value may be increased by 1/3 for transient loads (wind & seismic). A footing drain system is recommended around the perimeter of the proposed house, which should consist of a 4-inch perforated pipe surrounded by washed rock as indicated previously. Based on soil conditions observed at the site we recommend you do not use a drainage geotextile around the footing drain system. The pipe, which transmits downspout water to the catch basin, should be composed of a minimum 4-inch diameter rigid smooth wall pipe. It is important that this pipe goes directly to the catch basin and not into the footing drain system as this could cause backup of water into the footing drain. Establishing and maintaining the healthy vegetation cover both on the uplands and on the slope, will best control long term erosion. Site grading material should not be stockpiled or disposed of on or near the slope and yard waste and other deleterious material should not be placed on or over the slope. Such material smothers vegetation, retains water and acts to destabilize the slope. Erosion control during construction of the proposed facilities can be accomplished through placement of proper sedimentation control facilities. We recommend siltation control $tr'amm Group File:. 11.30.99 January 12, 2000 Harper Property, Lot 7 Kala Heights Drive Geologic Evaluation of Shoreline Bluff facilities, consisting of either hay bales or silt fences that are fabricated around the construction areas. Typical details for siltation control facilities using either hay bales or silt fences are attached to this report. Siltation devices should be placed down gradient (bluff crest) of all construction areas and cleared areas to provide siltation control during construction. All siltation control devices should be maintained in operable condition during construction, and lef~ in operable condition until the site has been revegetated and siltation is no longer a threat. At that time the siltation facilities should be removed. CLOSURE Please note that there are inherent risks associated with building on lots near or adjacent to steep slopes. These are risks that the building owner should recognize and be willing to accept. If conditions appear different than those described in this report, or other concerns arise, we request that we be notified so we can review those areas and modify our recommendations as required. We appreciate the opportunity to be of service to you. Should you have any questiOns regarding our reconnaissance please contact our office at (360) 714-9409. Sincerely yoUrs, Stratum Group McShane, M.S. Senior Geologist · Ted Hammer ~ ~ < ~-~' '/-~" "'~ ~-Q Geotechnical Engineer File:. 11.30.99 . Mill ... o I/2 1 HtE 1C~ 0 I0~ ~ ~000 4000 FT. ET Pz~.ted £x~m TOPO! ©I.g~ Wildflowez T~d, uctio~ (w~v.topo.com) 1.. q~..~ON Or.: