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Home My WebLink About602242009 Geotech Assessment 20 March 2006 r I i Bradley-Noble Geotechnical Services!' A Division of The Bradley Group, Inc. PO Box 12267, Olympia WA 98508-2267 ' Phone 360,357,7883 . FAX 360-867.9307' [r ; 'l . \iIAy~: 3 1 2006 Ii ti " , L_ i ou. ,\ I P ., ; '~''"''i'' .7 i -_.~ -------~_.__\ .; ',;"'! Kim & Chris Lowe 16053 SE 63'" Street Bellevue, Washington 98006 Subject: , Evaluation of slope stability in front of your cabin and means of providing long-term protection to aid in ensuring soil support for your cabin at 161 Wa Wa Point, Jefferson County Tax Parcel Number 602242009. Dear Mr. & Mrs. Lowe: As requested, I met with you at the your cabin site above in order to observe site soil conditions and the recent mass wasting event that has 'occurred on the east-facing slope fronting onto Hood Canal that is posing a long-term risk to soil support of your cabin. This report will present our field observations, discussion of probable cause of the slope failure, and means of providing protection to your cabin. Three soil units are found cropping out in the east-facing slope and toe area. The native soils in this area are mapped in the Coastal Zone Atlas for Jefferson County, Map JE 13. The upland area and low bluff are mapped as Qvt I, Vashon lodgement till. These soils are associated with the Vashon lobe of the Cordilleran glacier as it moved into the Puget Sound Lowland during the Fraser glaciation in late Wisconsinan time. Lodgement till is a compact mixture of boulders, cobbles, pebbles, silt and clay. This is material that the glacier has not carried forward to its actual terminus but is material carried forward at the base and lodged under the margins and may be called a lodge moraine. These are areas of undulating surfaces associated with osars. Osars are characterized as long, narrow, sharp ridges of gravel and sand with some associated boulders. In this area, it is common for I to 5 feet of ablation till to form the surface of the osars. At the base of the bluff, we find Qb, beach sand. After examination of the area of recent failure, we expect that this material is fill material that had been dozed over the slope by the former owner. This material is recent fill or Af, artificial fill. From our field discussion, we understand that the recent mass wasting event occurred after wave and long-shore currents removed soil from the toe area of the bluff. This removal of soil oversteepened the slope comprised of fill, and the fill material has slumped. The bluff in front of your cabin averages about 23 feet in vertical relief from toe to top. A tension crack has formed about 15 feet east of the edge of the deck. This 06030601 Page I of 4 ~ ~~ I, , , MAR "--'''i , I, I 3 I ?'10" :i I, I I ...U U -I. I ,'~i.,i;:n,:::'", I , '~-"~-~~2,.:':-' :! 06030601 Page 2 of 4 l! ';, , ' ~~___~ I -"':"'! tension crack is caused by soil movement in the bluff face. We observed another tension crack forming about 3 feet west of the main tension crack. At this time, this tension crack in not as clearly visible, but soil has moved down about one millimeter on the slope side, This crack parallels the main tension crack. We expect that continued soil movement will occur on the slope. With the cabin within the I: I line as measured up from the toe of the bluff, the cabin is in a zone such that it is at risk of damage due to loss of soil support. Enclosed are digital photographs we obtained during our site visit. In photos I & 2 we show the slope on the north and south sides of the cabin areas. The pad to construct the cabin was constructed by excavation into the low ridge to create a level building pad, We noted that the access road to the site has been raised above existing ground, probably by placement of fill material from the site excavation to minimize the grade. The size of the Douglas firs growing on the slope suggest that this pad was probably excavated about 30 to 40 years ago. We also expect that excavated material was dozed over the natural marine bluff slope. This material was placed as sidecast fill material and not keyed or compacted to the slope, thus creating a wedge of loose fill soils. We expect that movement and slumping of these fill soils has occurred in the past, before your purchase of the property, In photographs 3 & 4, we show fifteen feet from the edge of deck to the major tension crack to the east. In photograph 4, we show the secondary tension crack forming about three feet west of the main tension crack. Photographs 5 & 6 show the main tension crack and the dropping of the ground surface down to the east. Photograph 8 shows the limited area between the cabin and top of slope. Photographs 9, 10, and 12 show the major recent slide to the south of the cabin. Note that the soils lack stratification which suggests fill. Also, we understand that you did not encounter resistance to driving of the seven foot metal stakes into the slope to anchor the logs that you have placed to help hold the material. This suggest that either the failure plane is deep and you are driving into loose slough soils or that the material is loose fill. Photograph 14 shows mass wasting removing soil support on the north side of the cabin. These mass wasting events are removing soil support, with toe material being eroded away by longshore currents resulting in a oversteepened slope that is failing. This failure and downslope movement of the soil mass is the cause of the tension cracks developing in the upland area. Photographs II & 13 show loss of soil in the toe area by marine erosional processes. We would also like to point out in photograph 7 the leaning out towards the water of the vegetation growing on the slope in front of your cabin. This can also be seen in photograph 8. To the north of this site, in an area we consider to be undisturbed, we I " j :\ Ii ,I \l If! " , I ,~I. 3 1 2006 'L" , 1 " C--"'7' J- I', I:' 1'- I "-,'1' IT' ';. [" ',' . 'c-.n' '--- ."j ,., ,.~-- '___:~~::_~~'~~"':'_l 06030601 Page 3 of 4 I,",:' r ~ L..__c. find the vegetation growing vertically. Again, this supports our opinion that the slide mass is sidecast fill material and not natural soils. The risk to your cabin is that continued loss of soils will result in headward retreat of the bluff and removal of soil support causing damage and the risk of eventual collapse of the cabin. With the limited area that is level, moving the cabin back to the west would be difficult. We do not know where the drain field and well are located on this property which would affect the ability of the cabin to be moved. We understand that the width of this lot along the water is about 200 feet. About 120 feet in the northern portion would need to be stabilized to ensure long-term soil support. We do not expect that slope stabilization using "soft" means by natural vegetation would be a long-term solution. Since the soils are a fill section onto a slope, they are not stable. To protect the toe area from future marine erosion, construction of a rockery type of erosion control wall would be the best approach from a geotechnical perspective. This would provide toe protection. Continued sloughing and slumping of the soils above the wall would still occur. Stabilization above the rockery by use of planting and low terraces is possible. For plant selection to stabilize the slope, we suggest that you contact the Conservation District for Jefferson County. They can assist you with plant selection and soft means of slope protection. Even with the construction of a rockery to protect the toe from soil loss by marine processes, there will be some additional soil movement in the slope, At this time, the mass wasting event has loosened these soils and it will take some time for the soil mass to restablize. To aid in improvement to stability, all surface storm water from roofs should be collected into tightlines with this water conveyed to the toe of slope for disposal. Concentrated surface flows of storm water can create areas of localized saturation and induce accelerated surface soil erosion or mass wasting events. We noted in our observations of the beach area to the north of your property that the native soil profile was exposed. These are compact overconsolidated soils typical of osars. These soils are standing steeply with apparent stability and only slow erosion by marine processes occurring, The soil exposure shows stratification which is lacking in the soil exposure in the slide mass scarp. To the south of your property at the beach level, we find the Crescent Formation basalts cropping out. The osars at this site then appears to have been deposited unconformably onto the bedrock unit as this unit is resistant to erosion by the glacial ice and may be in fact the reaction of the sediment load to the bedrock unit at the base of the ice sheet. We did not observe springs or seeps in the area of the failure, Springs and seeps in this area are seasonal due to the limited water shed area on Wa Wa point. Seasonal springs and seeps are expected to develop at the contact of permeable soils with cemented 06030601 Page 4 of 4 f" , I I, , , soils or at the contact of the Vashon series with the bedrock unit. that ground water was a cause of the failure at this site. i : "I ,i-!' q j MAR 31 2006 ii," J.. t~ II ,-';: """:'--7' " .. J .' " ,,' L':-..._.. "'" ;:, ,.;_.~."_:~'~~:~; We do not consider Based on our field observations and interpretation of site geology and development history, it is our opinion that construction of a rockery type of bulkhead at the toe of the slope in front of the cabin will be of benefit to provide long-term protection to the cabin,' Construction of the rockery will require excavation into the loose slide mass. There is a risk of additional mass wasting occurring during the excavation to set the rockery wall. The excavation must be in conformance with Chapter 296-155 WAC, Construction Work, Part N, Excavation, Trenching and Shoring of the Department of Labor and industries. Sloping or shoring must use a soil type C for control. Once toe support is reestablished, you should expect some additional soil movement until stability returns to the loose slide mass, Even with toe support provided, some additional soil movement will occur. We do expect that this movement will not adversely affect foundation support. We noted during our site visit that the cabin is supported on posts. To resist seismic loads, we do recommend that you install a standard foundation using spread footings for the support of perimeter walls and either posts supported on isolated concrete piers or strip footings for interior loads. During a major seismic event, you are presently at risk of the cabin moving off the piers with risk of extensive damage or possible collapse. If you have any questions, or if we may be of additional assistance to you in obtaining permits for protection to the toe area, please contact us at our Olympia office. Cordially, BRADLEY. NOBLE GEOTECHNCIAL SERVICES H~" David C. Strong, L.E.G. Enclosures: Site Photographs DAVID c. STRON<:; , " I \~,d.r~ I 2 ,I , " ') 20'0, !! .i.JI .] I r.. - II ~.- - -.--.- ~ ." " r : ", " r I , 'r ~- ------- -.1 {"I ,~\ :~ 3 J ., I 2006 ll. -_~__.-.J ~Ul :"1 \' _~T; \/!..!fl HF:{ 'I " (cor I I '~__.~. Ji --- j :;;nUi 7 g , \ ,. , 'I " .'1:' reef :t. ! i j __,-'..c- _.~_:.;~j "I 10 r I I /I /2 , 'I " 2001 :,/ I i I i 'r'l , 'j " if , ';"n'{- ;.L'),