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HomeMy WebLinkAbout902131006 Geotech Assessment (2003)GEt}TECHNICAL REPQRT Prepared For Dan 4'Donneli May 30, 2003 ~~. may` ~. :3 ~ .y k 3 S~ ~. _: ;_,; E ~.,___ ti ~,~ F .._ . _:. _._ ___ .._ . ~ si E i f"~~ f ` ~ !~1 ~ tp ryy~t, r-~{ i I t p€~! {qi i l _ t ~' , ~ ~ 7 t s ~ Y _.., x For the Property described as ~ .__.~_~._~..~~. Tax Parcel # 902131006, Located in Discovery Bay Village Section 13, Township 29 North, Range 2 West, W.M. Jefferson County, Washington ~ ~~ ~~~~ Prepared by NQRTHVYESTERN TERRITQRtES, lNC. 717 S. Peabody Street Port Angeles, Washington 98362 Phone 360-452-8491 Fax 364-452-8498 Web Site www.nti4u.com E-mail info@nti4u.com ~/ ~ NORTHWESTERN TERRITORIES, INC. r ~ A JLS GROUP COMPANY ' 717 SOUTH PEABODY STREET, PORT ANGELES, WA 98362 ^ Engineers ^ Land Surveyors ^ Geologists N ~, ^ Construction Inspection ^ Materials Testing (3G0) 452.8497 FAX 452-8498 www_nli4u-can E-Mail: info c(?i rUi4u.com JLS GROUP, INC. Geotechnical Report Tax Parcel # 902131006, Located in Discovery Bay Village May 30, 2003 Dan O'Donnell 3016 38th Ave. SW Seattle, WA 98126 Subject: Geotechnical report for Tax Parcel # 902131006, Located in Discovery Bay Village, Section 13, Township 29 North, Range 2 West, W.M., Jeffersori County, WA Dear Mr. O'Donnell: Background At your request, Bill Payton, Engineering Geologist with Northwestern Territories Inc. (NTI) conducted a bluff stability inspection at the above mentioned property on -April 2, 2003. The purpose of this inspection was to examine the slope at the subject site by visual means in order to de#ermine the relative stability of the slope and make recommendations in regards to the proposed construction of a single family residence on the upper bench (Photo 1). An additional site visit was made on April 24, 2003 to conduct a test pit investigation in order to verify fill depths in the area of the proposed house. Site Description The subject property is located off of Fager Hill Road in Discovery Bay Village south of Highway 101 near Gardiner (Figure 1). The property is currently undeveloped except for the previous construction of two cut/fill benches for homesites and a shared drainfield. The proposed home will be constructed on the upper bench, with the lower bench possibly used for a future guest cabin. The upper bench trends in anortheast-southwest direction and is generally 55' to 70' in width. The property is well vegetated with mature trees and brush, except for the benches and cut/fill slopes. The fill slopes are mostly vegetated with young trees and brush. There is a bare spot on the fill slope at the west end of the upper bench due to a recent shallow slide. Large trees on top of the cut slope above the upper bench may pose a hazard to the proposed home if they were to fall toward the bench. There are gullies on the north and south ends of the upper bench. The gullies were-dry at the time of the site visit. However, horsetail ferns were present on the fill slope in the area of the north gully, which suggests that water does move through the area. It appears that the driveway and bench were constructed over the north gully. Gabion baskets and drainpipes were present below the bench in this area, however no culvert was noticed above the bench in the gully to convey water through the bench fill. It is unclear where the drainpipes originate. The gully is not dammed by the bench, but any water flowing down the gully would flow across the bench or through the bench soils if no culvert is present, which may explain the presence of the horse tail ferns. Slope measurements taken from topographic maps of the area indicate an overall slope angle of roughly 14 degrees {24%) towards the southeast {Figure 2}. The fill slopes below the benches have slope angles around 34 to 36 degrees. The cut slope above the upper bench is at about 68 to 70 degrees above a debris slope at the toe, which is at 34 degrees. These slopes (except for the cut slope above the upper bench) are generally within or below.the "angle of repose" which is defined as the maximum angle at which loose, cohesionfess material remains stable {assuming no destabilizing forces). This angle commonly ranges between 33 and 37 degrees. The cut slope above the upper bench appears to be composed of compact glacial till and has been standing at its current steep angle since the benches were constructed (reported to be several years ago).jlt has sloughed some as evidenced by the talus at the toe. However, it is common for slopes composed of this material to stand at steep angles for quite some time, as evidenced by the nearly vertical bluff along Water Street in Downtown Port Townsend, which according to the Coastal Zone Atlas is the same soil type. Site Geology The Washington State Department of Ecology's Coastal Zone Atlas maps the area of the subject property as Vashon lodgement till {Qvt1), a compact mixture of boulders, cobbles, pebbles, sand, silt and clay, generally overlain by 1-5 feet of ablation till. The Atlas also describes this soil as being excellent for foundation stability and good for seismic stability. The Atlas maps the slope stability in the area of the property as Intermediate (I). According to the Soil Survey of Jefferson County Area, Washington (United States Department of Agriculture, 1975), the subject site is in an area mapped as soil type Clallam gravelly sandy loam (CmC, CmD). This soil formed in glacial till. The Survey 2 describes this soil as being well drained and moderately permeable above the cemented layer, which is at a depth of 20 to 40 inches below the surface. Runoff is slow to medium, and the hazard of water erosion is slight to moderate. This soil is saturated part of the time during the rainy season, and at times water moves laterally, in places, above the cemented layer. Under the Unified Soil Classification System, this soil is classified as Silty Sand (SM) or Silty Gravel (GM). Visual observations made at exposed areas and in the test pits were generally consistent with the above soil descriptions except that a layer of compact silt was present in TP-2 below the fill. Test Pit Investigation As mentioned above, a test pit investigation was also conducted at the upper bench in order to determine the depth of fill in the area of the proposed home. Two test pits were excavated to depths of about 10 feet with a rubber tired backhoe. Each test pit was about 2 feet wide and 20 feet long. The approximate test pit locations and depths of fill are shown on Figure 3. TP-1 sloughed in during the excavation, thus trenches may need to be stabilized in order to protect workers. All codes or regulations should be followed with regard to trench excavations. Upon completion, the test pits were backfilled but not compacted. if footings will be placed over a test pit, the disturbed soil should be overexcavated and replaced in 8 inch maximum compacted lifts such that settlement under the footings will be prevented. Footings should extend at least two feet into native undisturbed soil below the fill. Figure 3 can be used as a guide to determine the depths necessary for the footings. Conclusions and Recommendations The subject property appears grossly stable at present and the proposal seems feasible from a geotechnical perspective. This report however, cannot guarantee that a slide will not occur upon the property. We recommend that a 20 foot minimum building setback be maintained from the foundation of the proposed home to the descending slope on the east side of the bench and a 30 foot minimum building setback be maintained from the slope on the south side of the bench. Also, the north end of the bench should be avoided due to the potential of wet soil conditions mentioned above. In order to protect the home from possible sloughing of the upper cut slope, it may be necessary to construct a low retaining wall between the house and the toe of the slope. This wall could be constructed of large boulders or concrete jersey barriers, etc. An example of the use of this type of wall can be seen along the toe of the bluff on Water Street in Port Townsend. The existing small trees at the toe of this slope could also provide limited protection from minor raveling of the slope. The talus soi{ at the toe of 3 the cut slope should be left in place as it provides stability to the slope. If this talus were removed, then the retaining wall would be even more necessary. The following recommendations should also be considered with regards to the proposal: It will be necessary to maintain ground cover to reduce erosion from surface runoff. Any bare areas that develop (including the slide scar mentioned above) should be revegetated. Native deep-rooted vegetation that requires little or no irrigation would be the most beneficial. Erosion control blankets might be useful in order to assist in the establishment of vegetation in the area of the slide scar or other steep areas. 2. Vegetation on the slope face provides stabilization to the slope face soils. The vegetation on the slope face should be left in as natural state as possible. If an enhanced view is desired, trees should be pruned in such a way that they are not damaged. It may be worthwhile to consult a tree expert to ensure that the trees are not damaged during any pruning operations. 3. The trees on the cut slope above the bench that could possibly damage the proposed house if they fell in that direction should be removed. It is understood that these trees are not on the subject property and that an agreement with the property owner would need to be reached. 4. The silty soil found in TP-2 was relatively dry and compact at the time of the investigation. However, this type of soil is highly moisture sensitive and can become mucky and unsuitable for foundations if allowed to become saturated. We therefore recommend that construction activities be conducted during the dry summer months. _: 5. Heavy irrigation or other activities that would contribute large quantities of water to the soil should be avoided. 6. Surface runoff from hard surfaces such as roofs, driveways, walkways and patios should be tightlined to an appropriate drainage control system such that surface water discharge to adjacent properties does not significantly exceed predevelopment conditions. The tightline should extend below the fill slopes and other steep slopes. 7. Surface runoff should not be allowed to flow over the face of the slope and erode the slope face. Surface flows can be redirected to drainage control devices by the use of berms or swales. Drainage from the north gully may need to be directed to the drainage control system as well if it appears that too much water from this source is saturating the bench. 8. Drainage control devices should be maintained in good working order and inspected at least once a year. 4 9. The Soil Survey mentions the possibility of shallow saturated soil during the wet season. Thus it would be beneficial to allow for wetness under the home in_ the building design. One measure would be the use of footing drains. 10. Silt fences or other sediment control devices may be needed during construction such that sedimentation to adjacent properties does not significantly exceed predevelopment conditions. 11.An engineered drainage and erosion control plan should be developed for this property to address items 6 thru 10 above. NTI can accomplish this if desired. Based on the findings, recommendations and limitations of this report: There has been a shallow slide on the upper fill slope, however we anticipate that there will be minimal landslide hazard to the proposed house based upon the recommendations of this report. 2. The proposal and/or the landslide hazard area could be modified so that hazards are mitigated. 3. The proposal would not increase surface water discharge or sedimentation to adjacent properties beyond predevelopment conditions. 4. The proposa!-would not decrease slope stability on adjacent properties. 5. The proposal would be stable under normal geologic conditions. For further information please review the three online publications published by the Washington State Department of Ecology (DOE) entitled: "Slope Stabilization and Erosion Control Using Vegetation", "Vegetation Management: A Guide for Puget Sound Bluff Property Owners" and "Surface Water and Groundwater on Coastal Bluffs". These publications are riow out of prin# but can be obtained from the DOE website at: http://www.ecy.wa.gov/biblio/sea.html under the 1993 and 1994 year heading. The DOE website-also contains much more useful information regarding slope stability and site development; this reference is highly recommended. Limitations This report has been prepared for the exclusive use of our client in conjunction with the above referenced project. The report has not been prepared for use by others or for other locations. It may be used by others only with the expressed written permission of the Engineer. Within the limits of scope, schedule and budget, this report was prepared in general accordance with accepted professional engineering and geological principles and practices in this or similar localities at the time the report was prepared. No other 5 warranty, expressed or implied, is made ~as to the conclusions and professional advice included in this report. The.observations, conclusions and recommendations presented in this report were -based on our visual observations of the subject property at the time of our site visit; no laboratory tests were-performed. -Soil and geologic conditions can vary significantly between test holes and/or surface outcrops. If there is a substantial lapse of time, conditions at the site have changed or appear different than those described in this report, we should be contacted and retained to evaluate the changed conditions and make modifications to our report if necessary. Sincerely, NORTHWESTERN TERRITORIES, INC. ~-K~~ Robert A. 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