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HomeMy WebLinkAboutGeotech Report 502311021Steve S. Luxton, Geotechnical Engineer 672 West Anderson Road Sequim, WA, 98382 (360) 797-1901; ssluxton@yahoo.coin Mrs. Michelle Duncan July 19, 2021 41 Fergy Lane Sequim, WA 98382 Subject: Geotechnical Assessment Report Regarding Jefferson County Parcel # 502311021 Dear Mrs. Duncan and Others Concerned, During the Spring of 2021, we inspected Jefferson County Parcel # 502311021 and the surrounding area and completed a geotechnical assessment of the parcel. The subject 4.7-acre parcel lies on the north side of Highway 101 in Jefferson County, Washington, within Section 31 of Township 25, North, and Range 2, West. The mostly -forested parcel is near the southerly tip of Jefferson County along the west side of Hood Canal near Fulton Creek. The site includes a 75- foot high escarpment that falls toward Hood Canal on the east. The slope, with some sections rising at 50 degrees from the horizontal, was cleared of trees last year. Some initial stabilization of soil on the slope was completed prior to this assessment. The photograph below shows the crest of that slope and the recently graded and leveled "platform" area are shown. Although there is currently no plan for a single-family residence on this leveled area, the site is in use for RV parking. Thus, setbacks, slope stabilization, and other precautions have been recommended for this site. GRADED PLATFORM AT SUBJECT PARCEL WITH STEEP SLOPE BEYOND FIR TREE 1 VIEW OF EXISTING SHED AND AMENITIES AT SITE WITH RV PARKED ON THE LEFT 1.0 PURPOSE AND SCOPE OF THE ASSESSMENT WORK The purposes of this geotechnical assessment are: to provide geotechnical guidance to the development of the site, to provide an assessment of the stability of the slope at the parcel which has been designated as having a moderate landslide hazard, and to define appropriate setbacks and buffers. The assessment work includes a review of proposed septic system components and their safety from earth movement and observations on the control of runoff and drainage of the site. Clearing and re -grading of this site occurred before this geotechnical evaluation. During the past year runoff from local convergence eroded the disturbed exposed slope and the platform above it leading to silt -laden runoff. The accumulation of erosional debris and slope movements threatened an existing drainfield at the toe of the slope. Initial soil stabilization including hydroseeding, silt fencing, installation of wattles and placing woody debris in an erosional gulch were completed earlier. This assessment provides guidelines for further stabilization by replanting the slope with native plants and trees. At the request of the owners, some assistance on site planning and observations to assist on the wastewater system design was gladly provided during the assessment. 2.0 GEOGRAPHY AND GEOLOGY OF THE SITE The site slopes down to the south with a steep escarpment rising about 75 feet from a flatter area that adjoins Highway 101. That lower area contains an existing wastewater drainfield that serves a house on the beach below. Most of the development activity is focused on the upper "platform", a nearly level re -graded area of about three-quarters of an acre whose southerly margin is the rim of the slope as shown in the photograph above. The owners park 2 an RV at the site, and they have built a shed, and other site amenities as shown above. The northerly half of the site is heavily wooded with a more gentle 10% to 20% slope leading to higher ground on the north. Geologic mapping by the State of Washington Geologic Survey indicates that the parcel lies within a regional deposit of undifferentiated Pleistocene ( Ice Age ) terrace and glacial outwash deposits. These glacial terrace deposits included gravel, sand, silt and some clay that was spewed from alpine glaciers and the Hood Canal Glaciers during the Vashon Stade of the Pleistocene some 14,000 years ago. This zone is shaded green and denoted as "Qguc(2)" on the geologic map below. The nearby "Qao" geologic unit, with a gray color, is outwash from alpine glaciers of the Olympic Mountains and the ancestral Fulton Creek. In modern times this creek has created a delta deposit in Hood Canal. The Pleistocene -age gravelly outwash deposit from Fulton Creek was examined at the northeast corner of the parcel on the east side of the gravel road leading up to the subject property. Close examination of soil exposures and test pits on the site indicated the widespread presence of gravelly silt loam soil with an underlying deposit of moderately stiff silt with slight amounts of clay. Test pits and exposures along a deep drainage -way and ditch below the slope show gravelly silt giving way to moderately stiff silt and clayey silts at depths of a few feet. Some sections of this exposure showed varved silts with dropstones suggesting deposition from floating ice within glacial lakes that formed at times along Hood Canal. Water well drillers' logs and the well log of a neighbor's well reported dozens of feet of clay- like material underlain by basalt rock that is found at increasingly shallower depths at higher elevations above the site. The soil pinches out and it gives way to basalt bedrock at the surface on the slopes above this site. 5 Qao Qls �� Qao Qa Qao , l � Z D 200 400 ft NEEEEL� A PORTION OF GEOLOGIC MAP "HOLLY" SHOWING THE VICINITY OF THIS PARCEL P 1 3 2021 f A geotechnical report from the adjoining property on the west suggested the presence of glacial till. A light colored and moderately stiff deposit of diamict was noted at the lowest level of the parcel in the bottom of the ditch. This material was closely examined and it appears to be a glacial drift deposit. No glacial till exposures were found on this property nor were any seen within the parcel on the west during our visits to the site. Nearby basalt rock outcrops along Highway 101 and well logs indicate that the region is underlain by basalt bedrock of the Crescent Formation. A well log from an adjoining parcel on the north, above the site, shows the occurrence of a thin mantle of silty gravel underlain by basalt bedrock at a depth 38 feet. A second well log of the parcel on the west shows basalt bedrock at a depth of 96 feet lying beneath a clay -like material. It is likely that the Crescent Basalt unit extends beneath the entire region. Soil mapping by the USDA's Soil Conservation Service indicates that the surface soil at the site is Hoodsport Gravelly Loam Soil and very gravelly loam soil. This soil type was confirmed at the site. 3.0 CONTROL OF STORMWATER AND RUNOFF AT THE SITE On the northeast, Fulton Creek has deposited a large modern delta deposit along Hood Canal that is now a favorite location for clam diggers. A similar but much older delta was deposited on the east when the water in Hood Canal was at a higher level during the most recent Ice Age. The remnants of the gravelly delta deposit touches the southeast corner of this parcel and its flat surface is visible on the LIDAR image below above the letters "101". Topographic maps show a small intermittent drainage originating on the slopes above and west of the site. This drainage crosses the subject parcel above the recently regraded platform area shown in the photographs above. In September of 2020 Quadra Engineering provided a Stormwater Pollution Prevention Plan for the site after the disturbance of the site's soil to reduce silt -laden runoff and slope erosion. The stormwater plan called for immediate measures to stem soil loss and capture silt eroding from the parcel. The work also provided some stabilization of soil from the slope that was slumping onto the drainfield easement at the toe of the slope. This initial stabilization project included grass seeding, wattles, mulching, and woody debris check dams in an erosional gully that formed on the slope. A good ground cover of grasses and budding maple tree stumps were seen at the site during our visits during the spring and summer of 2021. Additional work was done to provide a drainage route for water that, in the past, crossed the site to join a larger stream running along the east side of the site. The earth of the platform area was shaped to provide a fall to a catch basin with a drainage pipe leading down the slope. This catch basin lies at the position shown on the sketch map in the appendix of this 4 13 2o21 1 report. The intent of the installation was to prevent the future erosion of the slope by routing, this runoff through a drainage pipe to the toe of the slope. The rushing flow of water through a drainage pipe creates a very large "pull -apart" tension on this drainage pipe when it is flowing full. In addition, the downslope motion of the slope's soil beneath the pipe adds to the tensile forces acting on the pipe. For these reasons, the drainage pipe on the slope should be inspected regularly each winter and spring. If separation at the pipe joints is observed, the pipeline may be repaired before extensive erosional damage occurs. Observations of the recently -graded catchment area around the catch basin is also recommended to make sure that the ground slopes around the catch basin are steep enough to capture all of the runoff. This is especially important during prolonged heavy storms when runoff is very high. The catch basin intake and grating must be kept clear of debris so that all the runoff may enter without creating a backwater or ponded condition on the platform. Ponding of stormwater runoff on the platform area is undesirable because it leads to soil saturation and it can trigger deeper slope instability at this location. These points were discussed on site with the owners. 4.0 ASSESSMENT OF GEOL7OGIC HAZARD AT THE SITE IIrAU - F r -%K, Jh �.�I, JW all LIDAR IMAGE OF THE SUBJECT PARCEL AND THE VICINITY 4.1 The Potential For Landsliding And Slumping At This Escarpment 1 i' -i_ r� 200 400 ft„ Field examination of the escarpment beyond the building platform showed a variable slope with the steepest section near the crest where it falls at approximately 50 degrees from the 61 7 s.3 2o F.. 1 horizontal. The slope moderates on the lower sections so that the overall gradient is about 50% or 50 feet of rise for every 100 feet of horizontal distance. The LIDAR image above shows the detailed shape of earth in the vicinity of this property. A drainage has carved an erosional gulch across the lower and eastern corner of the parcel and it continues up onto the adjoining parcel. On the LIDAR image, the 75-foot-high escarpment is clearly seen in the southerly portion of this parcel. That slope has the steepest aspect on the westerly half of the parcel and it is less severe on the eastern third as shown on the image above. Field observations indicate that there is a zone of 110 feet where landsliding and slumping are a concern. That zone is shown on the map attached in the appendix and it has been staked at the site. Some jagged disruptions of the earth on and above the slope are visible on the parcel to the west. These features have a northwest -southeast lineation that suggests "benching" type of slope failures. Near the toe of the slope on the west there is a knob of earth that appears to be slide debris or a slump block. Field inspection of these slumps show that they are about 30 feet in width. The escarpment beyond the regraded platform on this site is believed to consist of silty gravel underlain and interbedded with silt and clayey silt. The fine-grain texture of the soil matrix here provides only moderate strength to the slope. During prolonged rainfall strength of this soil type may be substantially reduced. Topographic maps indicate that a small intermittent drainage crosses the site. Due to drainage improvements, water is now better controlled at this parcel. Nevertheless, the presence and collection of runoff on the platform area and the wet upland area adds to the likelihood that slumps or landslides might someday occur here. _ t s iF • � f r / r � 3 J. A} II ��• IMAGE FROM 1979 SLOPE STABILITY MAP OF JEFFERSON COUNTY - R.J. CARSON FP 1 13 2021 rrT.- The map above was prepared in 1976 by geologist R. J. Caron. It depicts the relative slope stability in Jefferson County. The red area on the map a few hundred feet west of the Duncan parcel was rated as "Class 5" indicating "recent active downslope movement' according to the author. Carson generally described CLASS 5 slopes as "Areas of recently active, rapid downslope movement, probably within the past 50 years. "This gulch appeared to be erosional rather than the result of a large rotational slide or slump, but there is no doubt that the steep slopes in this region are active. Seismic motion of the soil here could also help trigger slumping of the escarpment especially if it were to occur in combination with high rainfall periods and soil saturation. This site is close to the Seattle Fault and the Whidbey Island Fault that have in the past been epicenters of seismic events. The much -discussed subducting Juan de Fuca Plate and the related Cascadia Fault are positioned in the Pacific Ocean about 70 miles offshore of the Olympic Peninsula. In our view, the LIDAR evidence, historic observations of local instability and the potential for nearby sources of earthquakes indicate that the probability of earth motion at this property is large enough to warrant a generous setback from the bluffs rim. Efforts to improve the stability of the slope by the nurturing of native plants and trees on the slope and buffer are also worthwhile here. 4.2 Conclusions and Recommendations On Slope Stability As a result of all the observations above, the following are recommended: 4.2.1 Recreation Vehicles and Residential Structure Setback Provide eighty (80) feet of setback from the rim of the slope to any permanent residential building foundation that may be built on this parcel in the future. Similarly, provide eighty (80) feet of setback from the bluffs rim for any recreation vehicle or trailer that may be occupied in the late fall and winter when sliding is more probable. This setback line has been staked at the site as shown in the photograph below. Allow a 30-foot setback from any future structures to the rim of the slope on the east side of the platform area. This area is now used as an entry driveway. The existing shed building on the site is over 150 feet from the rim of the slope and it is not likely to be affected by slope instability here. FA 1 ' I .)FP 13 2021 4.2.2 Steep Slope Hazard Buffer Provide for and re-establish a 30-foot undisturbed vegetated buffer measured back from the rim of the bluff toward the north (that is, toward the existing shed on the parcel.) This buffer should extend through the 110-foot zone where the escarpment is distinct. This vegetative buffer, if well planned and attended, can be an attractive feature of the parcel. Do not install pressurized irrigation pipes or risers on his lot due to the danger of a pipe break and undetected leakage that might affect the slope and lead to sliding. Additionally, do not dump grass clippings or yard waste onto the slope. Doing so can reduce slope stability and lead to sliding. 4.2.3 Inspect and Maintain Drainage Structures Frequently inspect and clear the drainage inlet and drainage grate at the catch basin. Keep the inlet free of debris. At least annually, inspect the drainage pipe on the slope for possible separation at pipe joints and repair it as needed. Pot VIEW TO THE NORTHWEST SHOWING STAKED SLOPE SETBACK LINE 4.3 Geologic Hazard of Wastewater System Components After some initial delays, we were recently able to obtain a view of the site plan for the wastewater system. Two approximate sketch versions of the draft wastewater site plan were examined and reviewed at the site. What appears to be the positions for the proposed septic drainfield and reserve area are well away from the steep slope here in a wooded area north of the existing cottage. That area has a low to moderate slope and the gentle slope above is anchored by a mature forest. The designers intend to place the septic tank near the existing shed on the platform area. This position set back over 150 feet from the escarpment and it is a safe location. (See the photograph above.) 0 P 13 5.0 PERMANENT IMPROVEMENT TO SLOPE STABILIZATION BY PLANTINGS SOUTH -FACING SLOPE AT THE SUBJECT PARCEL - AFTER EROSION CONTROL WORK The escarpment in the photo above shows the conditions as seen in late May of 2021. A grass soil cover had emerged and silt arresting measures were completed prior to our inspections. 5.1 Slope Stabilization Using Vegetation - Recommended Plantings The following trees and plants were seen to be established and growing well at or near this site. Thus, these trees and plants are likely to survive and spread on this south -facing slope. The quantities below are preliminary estimates and subject to the field judgement of the landscaper at the time of the planting work. TYPE OF TREE NUMBER OF SAPLINGS REQUIRED Big Leaf Maple 15 Red Alder FILL IN AS NEEDED Douglas Fir 10 Western Red Cedar 10 TYPE OF SHRUB NUMBER OF PLANTS REQUIRED Indian Plum 10 Evergreen Huckleberry 4 Western Sword Fern 30 0 13 2021 Ocean Spray 5 Snowberry 5 Flowering Red Currant 5 Tall Oregon Grape 10 5.2 Suggestions For Slope Plantings Himalaya blackberry, an invasive and ugly nuisance, has invaded the slope and nearby disturbed area and it should be cut to the ground and poisoned repeatedly to suppress its growth. Dry soil conditions should be expected in summer and early fall due to its exposure to the morning sun. Thus November and late October are the best time for planting of these trees and shrubs due to the availability of moisture during the root development. The late Fall planting gives the tree saplings and shrubs the longest possible time for root development before the onset of next summer's heat and soil dryness. The lower section of the slope can be planted in Red Cedar and Big Leaf Maple. A ground cover of sword ferns may be planted along with Indian Plum to begin to reproduce the understory of the original slope. Sword ferns will do well in holding the soil here if some shade is available. Additional sword ferns may be planted over a period of years as more shade becomes available. The steeper upper section of the escarpment may do well with sword ferns, snowberry, Indian Plum and Flowering Red Currant in addition to some slower growing Western Red Cedar saplings. The Department of Ecology's publication "Slope Stabilization and Erosion Control Using Vegetation" includes detailed suggestions for plantings. This publication on soil stabilization is online at: https://apps.ecology.wa.gov/publications/documents/9330.pdf 6.0 LIMITATIONS AND ACKNOWLEDGEMENTS This report and its conclusions are the opinions of the undersigned Geotechnical Engineer. The undersigned Geotechnical Engineer warrants that the investigations and research underlying this report were conscientiously performed in accordance with the professional practice of geotechnical engineering and the principles of geologic science. No other warranty, either express or implied, is provided herewith. The author wishes to thank Trent Adams PE, a professional geotechnical engineer and a graduate geology student, for assistance with research and field inspections related to this report. 10 Thank you for calling on us for this geotechnical site assessment. Sincerely, 4.5 tZl wAsyr� �0 -�y Steve S. Luxton, MSc. PE Senior Geotechnical EngineerAL SEP 13 2021 �r 11 r Appendix — To the July 19, 2021 Geotechnical Assessment Jefferson County Parcel 502311021 12 /--I. STEVE LUXTON GEOTECHNICAL ENGINEERING Sheet No. Calc. No. (360) 797 1901 ssluxton@yahoo.com Rev. No. io Job No.-_`o�IZ/ Job GgoF -� — By Date 7-�5 �✓ Client M- ZWNCI*-Al Subject � � � '�23<ra2i Chk'd.7A4 Date IEW OF 5t:T9-ecw LiAtE Kv Pwm, s,,,z7 //D Elop 9p' 5gTBRCh' LJN I C� - �d C.�4rcF1 y�i�lCX . �z aC�ll.�, f}F1'ROX E 80-FOOT SETBACK FROM STEEP SLOPE AT DUNCAN PROPERTY JEFFERSON COUNTY PARCEL NUMBER 502311021 F-t32(7/e»