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HomeMy WebLinkAbout502072027 Geotech Assessment J G c o En gin eef:s . v_ . Geo ' Englneers December 7, 2000 Mr. Bill Wolenski 47524 288th Ave SE Enumclaw, Washington 98022 Report Geotechnical Engineering Services Landslide/Erosion Hazard Evaluation Jefferson County, Washington File No. 8774-001-00 INTRODUCTION AND SCOPE We are pleased to present this report for geotechnical engineering services regarding the construction of a single-family dwelling and rockery in tax lots 26 and 27 of the Huckleberry Heights subdivision. The Huckleberry Heights subdivision is located three miles from the intersection of Duckabush Road and US HWY 101 in Jefferson County, Washington. The approximate site location of the property is shown on the Vicinity Map. Our understanding of the project is based on topographic maps and a well log provided by you. Based on the above information, we understand that you recently installed a well and septic system and you are applying for a building permit from Jefferson County. The purpose of our services is to explore the subsurface conditions at the site as a basis for satisfying the anticipated requirements of Jefferson County Interim Critical Areas Ordinance and to provide recommendations for a proposed rockery. Specifically, our scope of services for this project includes the following: 1. Conduct a geologic reconnaissance of the site. 2. Evaluate the steep slope and landslide hazards per Jefferson County Critical Areas Ordinance. 3. Provide recommendations concerning appropriate buffer distances from landslide hazard areas. 4. Provide recommendations for bearing capacity of shallow spread footing foundations. 5. Provide recommendations for the design of a rockery located at the site. 6. Provide recommendations for site drainage. 7. Prepare a written report containing our observations, conclusions, and recommendations along with the supporting data. GeoEngineers, Inc. 1101 Fawcett Ave., Suite 200 Tacoma, WA 98402 Telephone (253) 383-4940 Fax (253) 383-4923 ~x~v. geoengineers.com '~: ~!~, '~ ~ '~ Mr. Bill Wolenski December 7, 2000 Page 2 SITE CONDITIONS SURFACE CONDITIONS The site is located on a southwest-facing slope in a valley along the Duckabush River. The property is approximately square in shape and encompasses approximately 10 acres. Access to the property is from a narrow, private gravel road off Duckabush Road. Large tracts of undeveloped land are to the north, south, and west of the property. One single-family residence exists to the east. A building pad has been cleared near the approximate center of the property. The site of the building relative to surrounding topography and site features is shown on the Site Plan, Figure 2. We understand that a rockery is to be constructed on the northwest side of the building pad as shown on Figure 2, about project Elevation 105. The toe of the uphill slope appears to have been regraded (from the contours shown on Figure 2) where the rockery will be located. At the time of our visit, we observed a recently constructed well, septic tank and drainfield located southeast and northwest of the building pad." Based on the topographic map, the slope upgradient of the planned residence is around 40 percent. The slope downgradient of the planned residence is about 26 percent. Vegetation on all slopes and the Surrounding properties consist of second growth conifer and deciduous trees with a dense understory of grass, salal, and blackberries. The building pad had been recently revegatated with grass for erosion control. Straw bales have been placed downslope and on contour of the recently graveled road for erosion protection. During our site visit, we did not observe any evidence of active erosion or seeps. SUBSURFACE EXPLORATIONS Subsurface conditions at the site were explored with several hand dug observation holes in the slope above the building pad, on the building pad and two open test pits (excavated by others prior to our visit for drainfield evaluation). The approximate location of the test pits are shown on Figure 2. The soils were classified in the field in general accordance with the system described in Figure 3. The test pit logs are shown on Figure 4. In addition, the owner provided a well log of the on-site well, completed on May 5, 2000. From the hand excavated holes, representative soil samples were brought back to our laboratory where they were re-examined to determine the general engineering characteristics, consistency, and to refine field classification. Classification procedures were performed in general accordance with ASTM D-2488. SUBSURFACE CONDITIONS Subsurface conditions consist of colluvium, overlying recessional outwash, which in turn overlies bedrock. The colluviurn, observed in both test pits and the slope above the building pad from the ground surface to a depth of about 2 ft, consist of a medium dense silty sand with gravel. Underlying the colluvium at the above described locations, we observed a medium dense gravelly sand. G e o E n g i n e e r s File No. 8774-001-00-3130 Mr. Bill Wolenski December 7, 2000 Page 3 This material was interpreted to be recessional outwash, which are similar to soils mapped nearby (Grove Series, a gravelly sandy loam), as described in the Soil Survey of Jefferson County (1975). Cady, et al. (1972) also mapped similar soils, as described above, overlying basalt bedrock in the vicinity of the project area. The well log for the onsite well shows basalt bedrock at a depth of 14 ft below the ground surface. No ground water seepage was encountered in any of the near surface explorations. The water well report describes a static water level about 192 ft below the ground surface (194 ft below top of well minus about 2 feet of stickup). We expect ground water levels to fluctuate as a result of precipitation, seasons, and other factors. It is likely that local zones of perched ground water occur on top of the bedrock. CONCLUSIONS AND RECOMMENDATIONS GENERAL Based on our explorations and evaluation, it is our opinion that the site is suitable for the proposed residence. The new residence may be supported on .shallow spread footings founded on undisturbed native soils or structural fill extending to undisturbed native soil. Surface topography suggests that portions of the site may have fill or disturbed native soil. near the proposed housing structure. If during excavation for structural areas (i.e. foundation elements, slabs, etc.), loose soil is encountered, we recommend that it be removed and recompacted as described in the "Structural Fill" section of this report. Based on our explorations and observations, the slopes consist primarily of a fairly rapidly draining soil over basalt bedrock. In addition, evidence of deep-seated slope instability or erosion was not observed at the time of our site visit. The following paragraphs describe an evaluation of the critical areas, followed by specific evaluations and recommendations for erosion and sedimentation control, site drainage, building setback distance, foundation bearing capacity, structural fill, and rockery design. CRITICAL AREAS EVALUATION Landslide Hazard Areas Jefferson County, Critical Areas ordinance defines a landslide hazard area as: · Areas containing potentially subject to mass movement due to a combination of geologic, topographic and hydrologic factors including: 1. Areas of historic failures or potentially unstable slopes such as: (1) areas described and mapped as having severe or very severe building limitations for dwellings without basements within the United States Department of Agriculture/Soil Conservation Service (USDA/SCS) Soil Survey of Jefferson County Area, Washington, (2) areas described and mapped as recent or old landslides or slopes of unstable materials within the Ecology Coastal Atlas of Jefferson County, and (3) areas described and mapped as areas of poor G e o E n g i n e e r s File No. 8774-001-00-3130 Mr. Bill Wolenski December 7, 2000 Page 4 natural stability, former landslides and recent landslides by the Washington State Department of Natural Resources, Division of Geology and Earth Resources. 2. Areas potentially unstable as a result of rapid stream incision, stream bank erosion, or undercutting by wave action. 3. Areas with any indication of earth movement, such as: (1) rockslides, (2) earthflows, (3) mudflows, and (4) landslides. Based on our observations, the USDA/SCS soil type at the site is the Grove Series (Go), defined by the Soil Survey as very gravelly loamy sand. The Grove Series is further classified by the Soil Survey based on percent slope. For slopes between 15 to 30 percent, the soil is given the designation (GOD). For slopes between 30 to 50 percent, the soil given the designation (GoE). Therefore, upslope of the proposed residence, the soil is classified as (GoE), because the slope is about 40 percent. Downslope of the proposed residence, the soil is classified as (GOD), because the slope is about 26 percent. In the Soil Survey (Table 9: Engineering Interpretations of the Soils) these soils (GOD, GoE) are listed as having a severe building limitation for dwellings without basements. Based on the Jefferson County Critical Areas Ordinance, the site is classified as a landslide hazard area. The Coastal Zone Atlas (Atlas) of Washington, Volume 11, Jefferson County (1978) does not map the location of the proposed residence. As described in the subsurface conditions section of this report, the native soils in the sloped portion of the site consist of native colluvium overlying glacial recessional outwash, which overlies bedrock. The Atlas does mention that typically glacial recessional outwash is stable up to the angle of repose and that the angle of repose for outwash sand and gravel generally is 30 to 39 degrees. The steepest portion of the site (upgradient of the house) has an angle (40 percent) equal to about 23 degrees, which is significantly below the angle of repose. During the time of our visit we saw no evidence of deep-seated slope stability. Based on our calculations and observations it is our opinion that the slopes are stable in their present configuration with regard to deep-seated slope stability. Erosion Hazard Areas Erosion hazard areas are defined by Jefferson County Critical Areas Ordinance as: · Areas containing soils or soil complexes described and mapped within the United States Department of Agriculture/Soil Conservation Service (USDA/SCS) Soil Survey of Jefferson County as having a severe or very severe erosion hazard potential. The near surface soils on the site are interpreted to be the Grove Series, a gravelly sandy loam, as described above. This soil series is described as slight to moderate to erosion hazard depending on slope. Recall that upslope of the proposed residence, the slope is about 40 percent and downslope of the proposed residence, the slope is about 26 percent. For slopes between 15 to G e o E n g i n e e r s File No. 8774-001-00-3130 Mr. Bill Wolenski December 7, 2000 Page 5 30 percent, the hazard of erosion is slight to moderate. For slopes between 30 to 50 percent, the hazard of water erosion is moderate. Based on the Jefferson County Critical Areas Ordinance, the site is not classified as an erosion hazard area. In addition, no evidence of active erosion was observed on the site. However, it is our experience that soils of this type are susceptible to erosion should the surface water become concentrated. Therefore, we recommend that the owner and contractor follow the provided recommendations presented in the "Erosion and Sedimentation Control" portion of this report are followed. EROSION AND SEDIMENTATION CONTROL Exposed ground surfaces will be subject to erosion during wet weather.. Removal of natural vegetation should be minimized and limited to active construction areas. Temporary and permanent erosion control measures should be installed and maintained during construction or as soon as practical thereafter to limit the additional influx of water to exposed areas: This can be accomplished by silt fence and berms as needed downslope of exposed area to intercept and direct surface runoff. Native soils exposed during construction should be protected~ Ground cover such as straw, jute matting, erosion control blankets, gravel or other forms should be considered until permanent erosion control measures are developed. SITE DRAINAGE Runoff can be expected to increase as site development increases the amount of impervious surface at the site. Accordingly, we recommend that stormwater be diverted away from the face of the slope and directed to an appropriate collection point. To reduce the possibility of water ponding and infiltrating near the foundations, exterior grades should be sloped to promote surface drainage away from the building. Downspout runoff should be collected and conveyed by tightline to an appropriate disposal system. Surface water should not be discharged on' or near slopes. BUILDING SETBACK As previously discussed, the site soils are free draining and generally stable relative to deep- seated failure. Based on our understanding of the project, subsurface explorations and our geological reconnaissance, it is our opinion that a buffer area is not required for this project, provided that the owner/contractor read and follow the recommendations of this report. However, we understand the Jefferson County Critical Areas Ordinance (Section 9.507) under no circumstance allows the buffer distance (measured perpendicular from the top, toe or edge of the landslide hazard area) to be reduced to less than 10 feet. The 10-foot buffer requirement can be achieved by extending the footing on the downslope side of the proposed site and moving the proposed rockery further up the slope. We recommend that the rockery be located at least 10 feet from the house to provide access for maintenance. G e o E n g i n e e r s File No. 8774-001-00-3130 Mr. Bill Wolenski December 7, 2000 Page 6 FOUNDATION SUPPORT We recommend that spread and strip footings be founded on the medium dense native soils or on properly compacted structural fill that extends to these soils. The soils at the base of the footing excavations should be disturbed as little as possible. All loose, soft or otherwise disturbed material should be removed or recompacted, as necessary. All footing elements should be embedded a minimum of 18 inches below the lowest adjacent external grade. We recommend a minimum width of 2 feet for isolated footings and at least 16 inches for continuous wall footings. For subgrades prepared as previously indicated, footings can be designed using an allowable soil bearing pressure of 2,000 pounds per square foot (psf) for combined dead and long-term live loads, exclusive of the weight of the footing and any overlying backfill. This value may be increased by one-third for transient loads such as those induced by seismic events or wind loading. We estimate that settlements of footings designed and constructed as recommended will be less than 1-inch, for the anticipated load conditions, with differential settlements between comparably loaded footings of ½- inch or less. Most of the settlements should occur essentially as loads are being applied. If foundation subgrades become disturbed during construction, settlements larger than predicted could occur. STRUCTURAL FILL GENERAL All fill placed beneath structures should be placed and compacted as structural fill. Structural fill material should be free of debris, significant organic materials, and rock fragments larger than about 6 inches. The workability of material for use as structural fill will depend on the gradation and moisture content of the soil. As the amount of fines increases, soil becomes increasingly more sensitive to small changes in moisture content and adequate compaction becomes more difficult or impossible to achieve. If any structural fill is required, it should be placed and compacted as described below. No fill should be placed on the steep slopes located in the western margin of the site, Figure 1. All fills should be placed in thin lifts so that uniform compaction can be achieved. Typically, loose lift thickness should be limited to about 12 inches. All structural fill should be compacted to at least 95 percent of the maximum dry density (MDD) based on the ASTM D-1557 (modified Proctor) testing procedure. SUITABILITY OF ON-SITE MATERIALS AS FILL During dry weather, any on-site soil that does not contain significant amounts of organic material may be considered for use as structural fill, provided it is at a suitable moisture content when placed and can be compacted as recommended. The near surface colluvium soils contain a moderate amount of fines and will be moisture sensitive. This material will likely not be suitable for use as fill under wet weather conditions. G e o E n g i n e e r s FileNo. 8774-001-00-3130 Mr. Bill Wolenski December 7, 2000 Page 7 ROCKERY WALLS Based on our knowledge of the project, we understand that a rockery is to be constructed northwest of proposed house on contour at about project Elevation 105 feet. At this time we do not know the vertical or horizontal dimensions of the rockery. Therefore, to help you determine the above described dimensions and factors that are needed to design a rockery, we have provided for your use a "Rockery Guide". The Rockery Guide is located at the end of this report as Attachment A. LIMITATIONS We have prepared this report for use by Mr. Bill Wolenski and members of the design team specifically for the property shown on the Vicinity Map and Site Plan. Our report, findings, and recommendations are not intended for use by other parties and are not applicable to other sites. In addition, our report should not be construed as a warranty of the subsurface conditions. Our recommendations are intended to improve the overall stability of the site and to reduce the potential for future damage related to earth movements, drainage, or erosion. However, all construction near slopes involves risk, only part of which can be mitigated through qualified engineering and construction practices. Favorable performance of structures in the short term does not imply a certainty of long term performance, especially under conditions of adverse weather or seismic activity. Our recommendations are not intended to direct the contractor's methods, techniques, sequences, or procedures, except as specifically described in our report for consideration in design. The contractor is solely responsible for job site safety and for managing construction operations to minimize risks to adjacent property owners. If there are any changes in the loads, grades, locations, configurations, or type of facilities to be constructed the conclusions and recommendations presented in this revised report may not be fully applicable. If such changes are made, we should be given the opportunity to review our recommendations and provide written modifications or verifications, as appropriate. G e o E n g i n e e r s File No. 8774-001-00-3130 Mr. Bill Wolenski December 7, 2000 Page 8 We trust this provides the information you require at this time. We appreciate the opportunity to be of service to you on this project. Please contact us should you have any questions concerning our findings or recommendations, or should you require additional information. Yours very truly, GeoEngineers, Inc. Ken W. Gill Staff Geological Engineer Gary V~. Henderson, P.E. PrincitSal KWG:GWH:dj Document ID: 877400IR.Doc Attachments G e o E n g i n e e r s File No. 8774-001-00-3130 SEAL ROCK + CAMP UN I~ON W ! LDCAT 2400 SCALE IN FEET 4800 Reference: This map reproduced with permission granted by THOMAS BROTHERS MAPS. This map is copyrighted by THOMAS BROTHERS MAPS. It is unlawful to copy or reproduce all or any part thereof, whether for personal use or resale, without permission. '00' Geo Engmeers VICINITY MAP FIGURE I SOIL CLASSIFICATION SYSTEM GROUP MAJOR DIVISIONS SYMBOL GROUP NAME GW WELL-GRADED GRAVEL, FINE TO COARSE GRAVEL GRAVEL CLEAN GRAVEL COARSE GP POORLY-GRADED GRAVEL GRAINED More Than 50% GM SILTY GRAVEL SOILS of Coarse Fraction GRAVEL Retained WITH FINES on No. 4 Sieve GC CLAYEY GRAVEL SW WELL-GRADED SAND, FINE TO COARSE SAND SAND CLEAN SAND More Than 50% SP POORLY-GRADED SAND Retained on More Than 50% No. 200 Sieve SM SILTY SAND of Coarse Fraction SAND Passes WITH FINES SC CLAYEY SAND No. 4 Sieve ML SILT FINE SILT AND CLAY INORGANIC GRAINED CL CLAY SOILS Liquid Limit ORGANIC OL ORGANIC SILT, ORGANIC CLAY Less Than 50 MH SILT OF HIGH PLASTICITY, ELASTIC SILT More Than 50% SILT AND CLAY INORGANIC Passes CH CLAY OF HIGH PLASTICITY, FAT CLAY No. 200 Sieve Liquid Limit ORGANIC OH ORGANIC CLAY, ORGANIC SILT 50 or More HIGHLY ORGANIC SOILS PT PEAT NOTES: SOIL MOSTURE MODIFIERS: 1. Field classification is based on visual examination of soil in Dry - Absence of moisture, dusty, dry to the touch general accordance with ASTM D2488-90. Moist - Damp, but no visible water 2. Soil classification using laboratory tests is in general accordance with ASTM D2487-90. Wet - Visible free water or saturated, usually soil is obtained from below 3. Descriptions of soil density or consistency are based on water table Interpretation of blow count data, visual appearance of soils, and/or test data. ~-'~-~ SOIL CLASSIFICATION SYSTEM Geo Engineers Project Job Number Location Bill Wolenski 8774-001-00 Jefferson County LOG OF TEST PIT 01 Date Excavated: ! I/]6/00' Logged by: KWG Equipment: Back]aoe Surface Elevation (fi): Not Measured z _ ~ . .o -~ ~o o~ ~, Other Tests u_ ~ ipti ~" ~--z~o~~-~' :~ o~>'E Material Descr on ~- ~_~ ~v" And ~--z -~ _~ { Notes _ ?..].: SM Brown silty fine to medium sand with a trace of gravel (loose to 0 .i'1~.].', medium dense, moist) (colluvium) · '..'.' SP Gray gravelly fine to medium sand (medium dense, moist) · .: - - -X (recessional) /-- _ *Test pit completed at a depth of 4.0 feet prior to our arrival. - -5 No ground water seepage observed. No caving observed. _ 10- - -- -- 20- -- -- -20 Notes: lbo depths of the test pit logs are based on an average of measurements across the test pit and should bo considered accurate to 0.5 foot. LOG OF TEST PIT 02 Date Excavated: ! ]/16/00' Logged by: Equipment: Backhoe Surface Elevation (ft): Not Measured z ~) .j ,_-~ E LU - gc ~ Other Tests ,, e ~- o (5.o -- '- ipti o ~ ~- '~ And z ;~ ~g~' =~. ~>,E Material Descr on ,-~ g~ >~ ,_- ~ ~ ~ -~ Notes c~0 ~. __,__~ _'. _ medium dense, moist) (codlin, turn) ..'-i"~ 'i" $? Gray gravely fine to medium sand (medium dense, moist) - - -N (recessional) /- *Test pit completed at a depth of 4.0 feet prior to our arriYal. - No 8zound water seepage observed. No caving observed. _ 10- - - -10 -- 15- -- - -15 -- 20- - -- --20 -- -- Notes: The depths of the test pit logs are based on an average of measurements across the test pit and should be considered accurate to 0.5 foot. Geo F, ngineers LOG OF TEST PIT FIGURE 4 .. ' N. ? .. / / f . . ATTACHMENT A ROCKERY GUIDE INTRODUCTION Rockeries generally act as a gravity wall to resist lateral load. Important elements of a rockery are: 1) its size, weight and shape, 2) friction developed between individual rocks (internal friction), 3) friction between the base layer of rocks and the underlying ground, 4) passive resistance to sliding developed by soil or pavement in front of the rockery, and 5) lateral load acting on or resisted by the rockery. Internal friction is very difficult to quantify and is, in part, dependent on the rock strength at the contact and, to a large degree, on the skill and judgement of the builder. Internal friction can change over time, due to weathering of the rock and from rockery movement. Rockeries typically experience a "settling in" during and for some time after construction. Also, many rockeries are subject to additional lateral load that causes additional movement due to wetting of the retained soil or other factors that reduce the strength of the soil. For poorly constructed or marginal rockeries, movement can result in loss of internal friction and a rockery failure. HEIGHT Unless specifically designed for given situations, the maximum rockery heights should be as follows: 1. No more than 6 feet when retaining compacted fill with a 2 to 1 (horizontal to vertical) slope above the rockery. 2. No more than 8 feet when retaining compacted fill with level ground above the rockery. 3. No more than 10 feet when used as a facing for stable cuts in undisturbed native soil or reinforced earth fill with a 2 to 1 slope above the rockery. No more than 12 feet when used as a facing for stable cuts in undisturbed native soil or reinforced earth fill with level ground above the rockery. Where surcharge loads act on the rockery, the maximum height must be reduced. Where the required height is greater than the maximum height, a stepped rockery or other types of retaining structures should be evaluated. 0 Se ROCK Rock must be sound, unweathered ledge rock from an established source that has demonstrated that it produces suitable rock. The rock shall be free of fractures, clay seams and evidence of weathering. G e o E n g i n e e r s A-I File No: 8774-001-00-3130 A- The most important dimension is the horizontal thickness of the rock measured perpendicular to the retained soil. The minimum thickness varies with the height of the rockery and lateral loads to be carded by the rockery. The rockery can be tapered in thickness providing that the thickness equals or exceeds the following: Minimum Height Height Thickness for Fill for Facing 44 inches 8 feet 12 feet 38 inches 6 feet 9 feet 32 inches 5 feet 7 feet 24 inches 4 feet 5 feet CONSTRUCTION GUIDE 1. The individual constructing the rockery should be an experienced and skillful craftsman in rockery construction. 2. The contractor should use sufficient space so that he can select among a number of rocks for each space in the rockery to be filled. Rocks which have spaces which do not match the spaces offered by the previous course of rock should be rejected. Rock must be angular, tabular, or semi-rectangular shaped; any rocks of basically rounded form shall not be used. 3. The first course of rocks must be placed on fh'm, unyielding soil. There must be full contact between the rock and soil which may require shaping of the ground surface or slamming or dropping the rocks into place so that the soil foundation conforms to the rock face bearing on it. As an alternative, it is satisfactory to use lean concrete in which to seat the first course of rocks or to use 3/4-inch minus crushed rock into which the foundation rocks are seated. The bottom of the first course of rock should b e a minimum of 1.5 feet below the lowest adjacent grade. 4. The rockery face shall slope toward the bank being protected at not steeper than 1 (horizontal) to 6 (vertical), but not flatter than 1H to 3V. 5. The rocks should be placed so that there are no continuous joint planes in the vertical or lateral direction. Each rock must bear solidly on two or more rocks below it and so there is no sign of instability such as "rocking" or "tipping" of individual boulders. The rocks should fit so not open spaces or voids larger than 6 inches exist. Rocks should be placed so that there is some bearing between flat rock faces, rather than on points. Horizontal or nearly horizontal joints should slope downward into the material protected (away from the rockery face). 6. Spalls should be used behind the rockery rocks to block spaces and, where necessary, to wedge between rocks and to lock them together. This should also serve to prevent washing of backfill material through the rockery. G e o E n g i n e e r s A-2 File No: 8774-001-00-3130 7. Backfill between the rockery and the adjacent soil face should be a minimum of 1 foot wide and consist of washed and screened crushed rock ranging from 3/4-inch minimum to 4-inch maximum gradation with the majority about 11/2 inches in particle size. The backfill zone must be filled and thoroughly tamped as each course of boulders is placed. 8. If there is seepage or a potential for infiltration of surface water into the backfill zone, special drainage measures or sediment filtration may be required. If these conditions occur, a soils engineer should be contacted for further advice. 9. Surface drainage above the rockery should be diverted or collected and carded in closed conduits to a point below the rockery. Rockery construction is an art and depends largely on the skill of the builder. Although rockeries can offer significant lateral restraint, they are partially indeterminate and they present unusual risk relative to other retaining structures. Even when the foundation and retained material are satisfactory and the rockery materials and construction appear satisfactory, there is some risk of movement and failure. G e o E n g i n e e r s A-3 File No: 8774-001-00-3130