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HomeMy WebLinkAboutBLD2000-00512 Geotechnical Report • P RE L IM I NARY RE CONNA I S SANCE GEOT E CHN I CAL REPORT • I NT RODUCT ION z,. Northwestern Territories, Inc. conducted a preliminary investigation into the geotechnical characteristics of the subject properties in August of 1993 . At the time of the report, Mr. Larry Mayes, the prospective buyer, wished to further assess the geologic hazards associated with the property. The property concerned is located on a bluff facing the west side of Hood Canal about 1 . 75 miles southwest of the old South Point ferry landing. The property offers a spectacular view to the southwest across Hood Canal and a wonderful wilderness environment and privacy with relatively easy access to developed areas. The bluff itself is geologically active and slides are present. Large scale sliding has occurred in the region and especially to the southwest between the subject property and Thorndyke Bay. This report outlines the situation as it was observed in August. Field work was conducted at the site on August 12, 1993 . On that date, the undersigned and a staff assistant traversed the property and examined the general features of geologic and engineering interest. Several hours of work were done to identify the bounds - of the property and examine the bluff formation materials and map the site conditions. GEOLOGIC SETTING Sc HISTORY The concerned properties are part of a glaciofluvial deposit that is principally advance outwash of the Vashon glaciation. The formation is generally composed of fluvial (river deposited) gravels and sands that were laid down by meltwater outwash rivers that preceded the advancing glaciers during the Vashon glaciation. According to geologic mapping of the area, the property is capped by Vashon till, an unsorted mixture of rock and debris that accumulated as the glacier wasted away on the site. The top areas of the bluff are relatively flat lying and they are likely to be the un-eroded till surface left by glaciation about 20, 000 years ago. In the lower parts of the formation, a distinct 25-foot thick horizon of fine silt and some clay is present which acts as a barrier to percolating groundwater. Thorndyke Lake lies about a mile inland from the bluff in the vicinity of the subject properties . The lake is a captured drainage basin with no effective outlet other than percolation and evaporation. Local evidence suggests that surface water from the Geotechnical Report - Thorndyke Bay Short Plat Division 4 Pagel percolates down to the silt formations and mounds there before • ing laterally to the east and discharging in numerous springs and seeps . These springs erode fine sediments and reduce effective stress in the saturated zone adding greatly to the instability of the bluff in the lower areas . In 1978 the Washington State Department of Ecology surveyed the coastlines of the State and included the coast below the subject properties in their studies. (Please refer to Figure 1 . ) In the results of their study, the "Coastal Zone Atlas of Washington" , the Department of Ecology identified all of Parcel 1 and the lowest portion of Parcel 3 as "Geologically Unstable. " Approximately 13 years ago, major slides occurred in the lower portion of the bluff in the region south and east of the subject properties . These slides dumped huge quantities of earth materials into Hood Canal and destroyed an access road a few hundred feet southwest of the proposed building site on Lot #3 of the Thorndyke Bay No. 4 Short Plat. A 1983 resolution by Jefferson County placed the lower portion of Parcel 1 under a building moratorium that continues today. GEOT E CHN I CA L CHARACTERIZATION OP' SITE The subject property slopes slightly toward Hood Canal in the northerly portions at relatively flat grades in the portion noted as the "upper site" on Figure 2 . About 600 feet south of the northerly property line of Parcel 3, the grade begins to slope and then breaks abruptly into a nearly vertical bluff of moderately compact and dry sand and gravels. Below the bluff line, the "lower site" dips steeply to the west toward Hood Canal . The Upper Site In the upper portion above the top of the bluff there was no evidence of motion of the ground, sliding or instability. Soils appear to be unsorted till and large angular clasts of granite rock from glaciation were present. Trees in the upper part of the property ranging in age of up to perhaps 100 years showed little sign of the sway or skewness that characterize the trees in the lower areas where soils are in motion. Till soils in the upper part are likely to be very good in bearing capacity due to their compactness and the granular nature of the soil materials. About 100 feet north of the top of the bluff, a large Red Maple tree marks the site where a home may be constructed by a prospective buyer. The area surrounding the site is now covered with forest litter and brush and many evergreen trees which participate in the relative stability of the upper site. Geotechnic3l Report - Thoradyke Bay Short ?lat Division 4 ?ace 2 The Lower Site All of Parcel 1 and the lowest (southerly) 100 feet of Parcel 3 comprise the lower site. Below the top of the bluff a vertical escarpment shows an exposure of dry gravels and sands of fluvial (river laid) origin the Vashon glaciation. This bluff is slowly eroding under the effects of wind (deflation) as fine sands and silt are carried out of the sun-dried face of the escarpment. As fines are stripped from the scarp, gravels and sands have raveled out leading to a loose talus deposit of gravel and sand about 50 feet vertically below the proposed home site. These materials stand at an angle of about 42 degrees from the horizontal . Below these talus deposits, the slope is slightly flatter but local sliding has given rise to a series of alternating ridges and flatter areas. These distinct structures whose tops are sloped toward the bluff, are slump slides that are in active, but very slow, motion. Trees in this zone are swayed and skewed showing that the earth beneath them is mobile. Several large trees exhibit a skewness suggesting that the rate of movement is no more than approximately one or two inches a year. In the vicinity there are several large flatter areas that appear to be the tops of larger slump slide blocks. A test pit and a large red maple tree stand on one of these slumps along the axis of the old road on the lower site. The slump slide terrain continues until groundwater is encountered about 140 feet above the level of Hood Canal . About 70 feet above the water lies the top of a 25-foot thick silt horizon probably formed by ice damming of the glacial meltwaters . The top of the impervious silt member demarks a spring line. Springs and seeps of considerable discharge were evident at this elevation in the subject property and further to the west and east. Because of the occurrence of groundwater, all the earth materials above the silt formation were saturated. Wet soil conditions extend at least 25- feet vertically above the impervious layer. The discharging groundwater tends to float the earth materials, thus reducing the friction between the soil particles. The additional weight of saturated sands and silts and the constant removal of fine material by the water (piping) adds to the instability of this portion of the bluff which is critically unstable and subject to local sliding. Much of the lower area above the damp zone is actively creeping down the grade at very slow but significant rates. Most of the slump slides in the mid-slope areas appear to be up to 30 feet in width. Some slides are large enough to carry living mature trees slowly down the slope . The motion of the ground in this part of the lower area is a slow but steady transfer of earth toward the upper part of the saturated zone where sudden and unpredictable slides occur . Figure 2 depicts a slide surface that is very likely to occur on the property in the future. This has been denoted as the "most probable slide" surface. Geotechnical Report - Thorndyke Bay Short Plat Division 4 Page 3 idence from historic sliding suggests that larger scale sliding in the lower site is possible, but less probable than the smaller scale slides. Figure 2 illustrates an imaginary slide on the scale of other local occurrences . The slide is denoted as the primary larger slide on the figure. It is important to note that the large landslides of 13 years ago were roughly this size. Fortunately, none were large enough to pose an immediate threat to any of the upper parcels in the Thorndyke Short Plats . SUITABILITY FOR HOUSING Sc SITE DEVELOPMENT The Lower Site The lower site, including all of Parcel 1, is actively moving and the parcel is not suitable for construction of anv permanent structures or roads . Sudden local slides may occur and the construction of roads to the lower site will aggravate the sliding and instability. No road in the lower site can be considered permanent. Trails may be built in accordance with the recommendations below. There is a small chance that a larger primary slide, such as the one illustrated on Figure 2, may occur within the life of structures proposed. The probability of such an occurrence cannot be determined accurately without extensive geotechnical work beyond the scope of this preliminary study. The local occurrence of similar slides gives reason for caution. The Upper Site The upper site is relatively stable because of the lay of the land and the nature of the formation underlying it. The compact sands and gravels that underlie the upper site are high in shear strength and due to the absence of groundwater seeps, they should remain relatively stable. Two erosional mechanisms may affect the upper site and must be considered in building plans . First, the active transport of earth materials by the creep and minor slides from the bluff just below the upper site will continue. Most of these slides are likely to be small and the slide cusps should be little more than two or three feet in width at the top of the bluff. The average rate of recession ( lateral upland movement) of the scarp just below the proposed building site is most likely to be only a few inches per year over long periods of time . The exact rate of recession of this bluff is impossible to predict with accuracy. Nevertheless, during a 50 year period, a recession of 20 feet to 30 feet should be planned for. The second, and more improbable, erosional mechanism is larger secondary sliding resultingfrom large g primary slide failures of Geotechnical Report - Thorndyke Bay Short Plat Division 4 Page 4 lower site. If large-scale sliding were to occur in the lower , ite, the scarp below the prospective house could be lengthened and over-steepened. Judging from the scales of existing slides in the region, it seems possible that a 40 foot width failure of the scarp below the house could occur under these circumstances. Initial indications are that such a failure is unlikely. Due to the competence of the formation materials at the upper level, such slides would necessarily occur at a high angle, thus limiting the width of the "bite" that the largest slides might take out of the upper site. Evidence suggests that large scale earth slides are more common in the areas to the west where discharges of groundwater from Thorndyke Lake tend to create a larger saturated and unstable zones. Nevertheless, it should be recognized that there is a possibility that a 30 to 40 foot width of the upper site could be lost in a single larger slide event triggered by large slides below in the active area. Even larger slides are possible, but seem improbable at this level of investigation. CONCLUSIONS Although the lower site is markedly unstable, the upper site_ appears to offer a reasonably safe location for housing, provided that sufficient setbacks from the top of the bluff are observed. Once a bluff has moved to within 45 feet of a residence, the property can scarcely be sold. Over a 50 year period it would be best to plan for a bluff recession of 30 feet. It would also be prudent to allow and additional 40 feet of setback to provide a cushion against the large scale slide that might occur even if the occurrence is improbable. In summary, the property offers good value to the buyer who is willing to set his home back a prudent distance from the bluff top and tolerate a small risk that larger slides may occur. The occurrence of groundwater springs, while aggravating the sliding, means that water well development should be easy. RECOMMENDATION S The following recommendations stem from the preliminary work conducted by Northwestern Territories, Inc . 1 . Buildings should be set back a minimum of 115 feet from the top of the bluff below the proposed building site. ( Decks may extend further. ) The bluff top is the first notable break in grade beyond the proposed homesite. Additional setback should be considered for reduced risk. 2 . Maintain trees, brush and groundcover in the entire site inasmuch as possible. Trees in the view space should be topped or trimmed rather than removed. Leave ground covers in place. Geotechnical Report - Thorndyke Bay Short ?lat Division 4 Pace 5 ,. Route site and building drainage into dry wells set back from the bluff. Do not construct ponds on the property that might contribute to the groundwater discharges. 4 . Avoid construction of roads on the lower site. 5 . If trails must be built, use components that will move with the soils. Emphasize lightweight parts and plan for frequent maintenance, rather than trying to overcome problems with weight and strength. STANDARD DISCLOSURE STATEMENT This report is preliminary in nature and is based on an inspection of the existing site conditions. The site investigation was made without the benefit of detailed measurements, geotechnical materials tests, borings or detailed investigation of the subsurface conditions. The work was done to the standard of care and in accordance with the practice of the engineering profession but unforeseen and unknown subsurface conditions cannot be completely understood by rapid surveys or examination of surface conditions. Reduction in risk and better reliability of conclusions and recommendations can be obtained by more detailed investigations of the site that include borings, accurate measurements and laboratory testing of materials. The Engineer encourages the user who wishes a higher degree of reliability of investigation results to obtain such services. - Unless construction plans are reviewed and construction inspected by the Engineer, no warranty is made concerning the conformity of the finished construction to the conclusions and recommendations of this report. The observations, interpretations and conclusions herein are based on the generally accepted principles of engineering and geology. This warranty is in lieu of all others, either express or implied. This report is intended for the sole use of Mr. Larry Mayes and may be used by others only with his express permission. j Steve S . Luxton4 PE Professional Engineer fc: Mayes .Rpt dir: Eng/Rpt Geotechnical Report - Thorndyke Bay Short Plat Division 4 PaGe 3