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Home My WebLink AboutBLD2000-00805 Geotechnical Report Mr. Alan Hoshor SHANNON iWILSON,INC. November 2, 2000 Page 5 Once a large slide has occurred, the bank reaches a new, more stable configuration, and the soils gain strength with time. In our opinion, it is likely that the big slide that caused the current bench topography is relatively stable, and we did not find any signs indicating that this condition will change. A failure along the slickensided clay zone seen above the bench would daylight above the bench and is not affected by our previous comments regarding stabilization with time. We did not find any indication that these slickensided materials are particularly weak, carry a particularly large amount of water, or have moved in the past. In contrast, the deposit that caused a recent slope failure 1,000 feet or so to the southeast of your lot is-perhaps 20 feet thick, carries p p a significant amount of water year round, is very broken up, and has obviously moved historically. Based on our observations, it is our opinion that a failure along this zone is unlikely in the foreseeable future. There are relatively few exposures, which means we may not have seen the most serious geologic features. We did not see any indication of sliding in the headscarp, and we feel we have a fairly good understanding of the mechanisms involved in the failure that has occurred. Based on this, it is our opinion that we have defined and described above the most serious geologic implications to the stability of your site. RECOMMENDATIONS REGARDING STABILITY OF THE SLOPES From the above discussion, it is obvious that this site is not without some risk. In our opinion, the near-vertical bluff will slough back over time but should not be a serious concern except to the hot tub. It is very likely that you will have some warning about instability in that area if you continue to observe the condition of the near-vertical bluff along the beach. Cracks will probably occur in the bluff well in advance of a failure, but when the bluff fails it will probably v be rapid. The performance of the upper slope is more difficult to predict. Based on our experience with slopes and geologic conditions similar to those at your property and a limited engineering analysis at this site, a line drawn on a 2 horizontal to 1 vertical (2H:1V) slope from the bottom of thepotential slideplane to the ground surface will define the point behind which the next slide .21-1-09195-001.L 1/wp/lkd 21-1-09195-001 Mr. Alan Hoshor SHANNON&WILSON,INC. November 2, 2000 Page 6 will not affect a structure. The known slide plane is at the top of the near-vertical bluff. Drawing a 2H:1 V line from that point reaches about the top of the upper steep slope in either cross-section shown. From this, we conclude that there is very little risk(not zero risk) of a failure on that plane endangering either your garage or your house if the house is set back near the road. We did not find any indication that the slickensided clays found above the bench had been involved in a previous slide or were about to cause instability. From the conditions observed, we did not see any indication that there were other unobserved materials that had caused failures in the past or were about to cause instability in the future. However, this geology, on this hillside, does carry such a risk. A 2H:1 V line drawn from the bottom of the observed layer to the ground surface appears from our measurements to extend back to the house pad or beyond. If a slide were to occur, it would probably give warning if you were to continue to observe for cracks on the relatively flat slope below the house, but it could be rapid with virtually no warning. From a practical standpoint, there is nothing you can do to increase the stability of the slope,particularly under seismic conditions. We were asked to give a probability of a failure, which would impact the proposed house location. In our opinion, there may be a 10 percent chance of having a failure that would involve the slope in front of the house in the next hundred years. This translates to a 50-50 split in 500 years or 1 chance in 1,000 next year. There is a much lower probability that the slide would extend back to the house itself. It is easy to give this kind of probability because we did not completely eliminate the chance of a failure tomorrow, but it does indicate that we feel there is a very low probability of a failure that would encompass the house in your lifetime. f RECOMMENDATIONS REGARDING THE HOUSE FOUNDATIONS The house pad appears to be on glacially overridden sands and gravels. They are very dense and excellent foundation materials. The house should be founded below the fill and in undisturbed glacially overridden soils or on well-compacted clean sand and gravel placed on the undisturbed • glacially overridden soils. # 21-1-09195-00111/wp/lkd 21-1-09195-001 Mr. Alan Hoshor SHANNON&WILSON,INC. November 2, 2000 Page 7 wyd The house should be placed as near the road as practical to get it as far as practical from the zone that could be impacted by a slope failure. There is a general rule when working above high banks and steep slopes that water should be controlled as much as possible. In our opinion, this site is so massive and the potential for a damaging landslide is generated so deeply, that any attempt to control minor amounts of water would be useless. Water should not be allowed to collect along Lane DeChantal and infiltrate in a localized area, but downspouts and drainfields should not affect the overall stability. CLOSURE Our study was limited to evaluating the stability of the steep slopes below the house and garage and general recommendations regarding the foundations for the house. The study was limited to a review of the literature, a geologic reconnaissance of the exposures, and relatively crude surface measurements. The analyses, conclusions, and recommendations presented in this report are based on the observed site conditions as they existed at the time of our site visit. It is not possible to fully define the geologic conditions at the site based on our limited explorations and observations. The work was done in accordance with generally accepted engineering practice in this area at this time. No other warranty is made, either expressed or implied. Our study did not include any environmental assessment, or any evaluation regarding the presence or absence of wetlands, or the presence or absence of hazardous or toxic materials in the soil, surface water, groundwater, or air, on or below or around this site, or for the evaluation or disposal of contaminated soils or groundwater. It is possible that these are issues, particularly near the existing structure. The house will be above a steep slope. We believe that instability on the slope has a very low probability of affecting the house or garage in the foreseeable future. However, as with any site located on or near a slope, there is always a risk of instability that the present and future owners must be prepared to accept. We have prepared an Appendix, "Important Information About Your Geotechnical Report," to assist you in the use of the report. f .-` 21-I-09195-001.LI/wp/lkd 21-1-09195-001 Mr. Alan Hoshor SHANNON&WILSON,INC. November 2, 2000 Page 8 We have appreciated this opportunity to be of service. If you have any questions,please contact Bill Laprade at (206) 695-6891. Sincerely, SHANNON & WILSON,INC. I 1 .POS:C. As f�GERTIFIEp o ,o 1cc.�� t 0 OREGON r,i ', L? WILLIAM T .,"ACE - > /if I1 E:P"- ,L //7Z/Oo ► s. / 11978 ,.,....7t'm2C O�� r r r0 CIS 5C i / �. O (� SJONAL �FR/NG GEO\. f EXPIRES 9/3 r William T. Laprade, C.E.G. Thomas C. Kinney, Ph.D., P.E. Vice President Vice President TCK:WTL/lkd Enclosures: Figure 1 — Surface Profiles Figure 2—Typical Section Appendix—Important Information About Your Geotechnical Report I , , x.; ay u'1 ' 21-1-09195-001.L1/wp/lkd 21-1-09195-001