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HomeMy WebLinkAbout501041038 Geotech Assessment (2005) r_'f ...:..- t ""- / 1. ~ -.t . GEOl<CHNICAL ENGlNEEmNG STUDY ~ ~ ~y ~: :e ~ Proposed Residence and Guest House Lot 4 - Thousand Trails Donald Road Jefferson County, Washington J~FFrHcO!j COUNTY . ('IF!:'T, C~ (nl',1, 1~:!'I~.iY DfYELOr- r,'ENT ~ ,. This report presents the findings and recommendations of our geotechnical engineering study for the property in unincorporated Jefferson County. We were provided with a topographic survey showing the existing topography, and the locations of the existing and proposed residences. Northwestern Territories, Inc. prepared this survey, which is dated January 11, 2005. Based on the survey and conversations with Steve Hoedemaker of Bosworth Hoedemaker, we understand that the existing residence will be demolished and replaced with a new, larger residence west of the existing gravel driveway. The new residence will have a partial basement on its southern side that will daylight onto the southern yard. This structure is shown to be approximately 15 feet from the west slope and over 100 feet away from the south slope. A detached guest house will be constructed on a flat bench located to the east of the existing gravel driveway. Preliminary drawings and finish floor elevations for the proposed buildings as well as grading information were not developed as of the writing of this report. No development on the adjacent steep slopes is expected. If the scope of the project changes from what we have described above, we should be provided with revised plans in order to determine if modifications to the recommendations and conclusions of this report are warranted. SITE CONDITIONS SURFACE The Vicinity Map. Plate 1, illustrates the general location of the site in unincorporated Jefferson County. The subject property is located on the southern tip of the Tonados Peninsula at the terminus of Donald Road. Access to the property is by way of a private, unimproved driveway that splits in two directions onto the two flat proposed building areas. A one-story, L-shaped log house is located on a flat bench to the west of the existing driveway. The western portion of this house is supported on small pier pads. The eastern portion of this house is supported on continuous footings that showed some visible signs of foundation settlement. The proposed building areas are level and separated by a 10-foot-high, moderate to steep slope. The southern yard of the existing residence consists of two level tiers separated by a 10-foot-high moderate slope, then the ground slopes at a gentle to moderate rate towards a steep slope on the southern portion of the property. Bordering the development area to the west and south are steep slopes. The western slope is approximately 40 to 50 feet high, and the southern slope is approximately 30 feet high. Both of these slopes incline down towards the Hood Canal. Based on our observations and conversations with Steve Hoedemaker, it appears that numerous slides have occurred on these steep slopes, on and adjacent to the site. The toe of the southern slope adjacent to the Hood Canal shoreline appears to have been undercut, with some evidence of recessional near-surface sliding. The observed evidence of recent slides appears to indicate that they have involved relatively shallow movement. rather than deep-seated failures. The most active slope is the steeper, southwest- facing one. GEOTECH CONSULTANTS, INC. 1 ~.. !-. " } ....... .. Driscoll October 13. 2005 IN 05289 Page 2 The native soils on this property as classified as PY, "Pre-Vashon Stratified Sediments", by the Geologic Map of East-Central Jefferson County, Washington (Birdseye, 1976). The steep slopes to the west and south of the existing residence are designated as an Erosion Hazard Area and Landslide Hazard Area by the Jefferson County Code Title 18. Article VI-G. SUBSURFACE The subsurface conditions were explored by drilling three borings at the approximate locations shown on the Site Exploration Plan, Plate 2. Our exploration program was based on the proposed construction, anticipated subsurface conditions and those encountered during exploration, and the scope of work outlined in our proposal. The borings were drilled on September 12, 2005 using a track-mounted, hollow-stem auger drill. Samples were taken at 5-foot intervals with a standard penetration sampler. This split-spoon sampler, which has a 2-inch outside diameter, is driven into the soil with a 140-pound hammer falling 30 inches. The number of blows required to advance the sampler a given distance is an indication of the soil density or consistency. A geotechnical engineer from our staff observed the drilling process, logged the test borings, and obtained representative samples of the soil encountered. The Test Boring Logs are attached as Plates 3 through 5. Soil Conditions All borings encountered similar subsurface condijions. Approximately 4 feet of loose soils (possible fill) was encountered directly below the surface in Boring 1. Medium-dense silty sand was encountered underlying the loose soils in Boring 1, and below the surface in the other two borings. Dense silt, silty sand, and sand were encountered within 5 to 15 feet below existing grade; these soils became very dense with depth. The very dense soils were encountered to the maximum explored depth of the borings. We saw no indications of disturbed or fractured soils within the borings. Although our explorations did not encounter cobbles or boulders. they are often found in soils that have been deposited by glaciers or fast-moving water. Groundwater Conditions No groundwater seepage was observed in the borings. The borings were left open for only a short time periOd and were conducted following a relatively dry summer. It should be noted that groundwater levels vary seasonally with rainfall and other factors. We anticipate that perched groundwater could be found atop the dense silty sand and sandy silt layers. The stratification lines on the logs represent the approximate boundaries between soil types at the exploration locations. The actual transition between soil types may be gradual, and subsurface conditions can vary between exploration locations. The logs provide specific subsurface information only at the locations tested. If a transijion in soil type occurred between samples in the borings, the depth of the transition was interpreted. The relative densities and moisture descriptions indicated on the boring logs are interpretive descriptions ba nditi observed during drilling. [5)nl' - IE I W IE [R) lm MAY 2 4 2006 lW GEOTECH CONSULTANTS. INC. JfFm',;(p COUNTY m:r.T D? ('n~.", ,..,'."" ",;r'\TLliH~ENT t - ! " :\ ~ ,J Driscoll October 13, 2005 i5) IE It: Ifi'18,~ ~\ lnl\ MAY 2 4 ~2:~: J \ lVJ CONCLUSIONS AND RECOMMENDATIONS L____~~ \ ....~F..c.H ~Ol. C(,ur-'"ry ":~ ~ r~r~i"_!r- ___~~,._=-~_[-lr~~. GENERAL THIS SECTION CONTAINS A SUMMARY OF OUR STUDY AND FINDINGS FOR THE PURPOSES OF A GENERAL OVERVIEW ONL Y. MORE SPECIFIC RECOMMENDA TlONS AND CONCLUSIONS ARE CONTAINED IN THE REMAINDER OF THIS REPORT. ANY PARTY REL YING ON THIS REPORT SHOULD READ THE ENTIRE DOCUMENT. The borings conducted for this study encountered medium-dense soils within 4 feet of existing grade that then became dense to very dense. It is our opinion that the proposed residence and guest house can be supported on conventional foundations bearing on the medium-dense to dense native soils. Due to the loose, near-surface nature of the native soils and the close proximity of the proposed residence to the adjacent slopes, we recommend that the footings closest to the top of the slopes be founded on the dense native soils encountered at least 5 feet below existing grade. This will require that the western and southern erimeter foundation of the ro osed residence be desiQned as a cantilever foundation wall, unless the western portion 0 the proposed residence will contain a basement. From conversations with Steve Hoedemaker, we understand that the eastern portion of the proposed residence will contain a basement. The structural design should include details for a cantilever wall height varyinQ from 5 to 8 feet. The plan provided to us shows a setback of up to 20 feet from the top of the adjacent steep slopes, with a portion of the new residence situated within 5 feet of the north slope. Section 18.15.275 of the Jefferson County Code states that a standard buffer of 30 feet from the toP. toe, and edges of landslide areas is required. This buffer can be reduced to a minimum of 15 feet if it can be shown that the proposed project and landslide hazard area will be "adequately protected", and that the project cannot meet the required setback. If the recommendations in this report are followed, it is our opinion that the proposed location of the proposed residence will be adequate and that the cons ion of the osed residence will not adversely affect the overall stability of the eXlsflii(] slopes, However, due to in' t r uired b e n ou e . n of t e proposed residence mav need to be shifted sliohtly to the east to satisfy these criteria~ ~ reslClence should not be shifted closer to the south slope, which appears to be more active as a result of wave undercutting. The recommendations of this report are intended to protect the structures from damage in the event of future slope movement. However, a periOdic loss of ground at the top of the slopes should be expected, due to ongoing natural processes of weathering and wave undercutting. It appears that the toe of the southern slope adjacent to the Hood Canal shoreline is periodically undercut, with evidence of recessional near-surface sliding. These shallow landslides will likely continue unless a bulkhead is constructed. However, it is our opinion that the construction of a bulkhead is not necessary to protect the proposed buildings. The steep slopes should not be disturbed. Soil and debris must not be placed on, or near, the steep slopes. ater from do ns outs and im ervious surfaces uld . !lined to the base of the steep slopes in an above-ground pipe anchored to the slope's surface" The on-site silty soils are sensitive to moisture. Thus, it may be necessary to protect bearing surfaces with a la er of crushed rock to protect the sub rades from disturbance curing penods of wet weather. The silty sands can be reused as structural I I ey are placed in dry weather and are at, or near,. their optimum moisture content. These soils can also be reused as wall backfill if a minimum 12-inch thickness of free-draining soil or washed rock is placed between the fill and the GEOTECH CONSULTANTS, INC. , ." rr\\lE(C[E~~IE~ ,0)\ IN 05289 U lr~\ MAY 2 4 ~6t\lr 4 . '\ I ____~ foundation wall. However, the on-site silts should not be reused as stru1tl!.r,!I}~II.beneath b'e~rlrig 'EN: elements because of their high moisture contents and low compacted strengths.':'Theon-slte.sands-- can be reused as structural fill or wall backfill if they are placed in dry weather and are at. or near, their optimum moisture content. Driscoll October 13, 2005 , .... The erosion control measures needed during the site development will depend heavily on the weather conditions that are encountered. We anticipate that a silt fence will be needed around the downslope sides of any cleared areas. Rocked construction access roads should be provided to reduce the amount of soil or mud carried off the property by trucks and equipment. Wherever possible, these roads should follow the alignment of planned pavements, and trucks should not be allowed to drive off of the rock-covered areas. Cut slopes and soil stockpiles should be covered with plastic during wet weather. Following rough grading, it may be necessary to mulch or hydroseed bare areas that will not be immediately covered with landscaping or an impervious surface. As with any project, additional erosion control measures may be required depending on conditions encountered during construction. The drainage and/or waterproofing recommendations presented in this report are intended only to prevent active seepage from flowing through concrete walls or slabs. Even in the absence of active seepage into and beneath structures, water vapor can migrate through walls, slabs, and floors from the surrounding soil, and can even be transmitted from slabs and foundation walls due to the concrete curing process. Water vapor also results from occupant uses, such as cooking and bathing. Excessive water vapor trapped within structures can result in a variety of undesirable conditions, including, but not limited to, moisture problems with flooring systems, excessively moist air within occupied areas, and the growth of molds, fungi, and other biological organisms that may be harmful to the health of the occupants. The designer or architect must consider the potential vapor sources and likely occupant uses, and provide sufficient ventilation, either passive or mechanical, to prevent a build up of excessive water vapor within the planned structure. Geotech Consultants, Inc. should be allowed to review the final development plans to verify that the recommendations presented in this report are adequately addressed in the design. Such a plan review would be additional work beyond the current scope of work for this study, and it may include revisions to our recommendations to accommodate site, development, and geotechnical constraints that become more evident during the review process. We recommend including this report, in its entirety, in the project contract documents. This report should also be provided to any future property owners so they will be aware of our findings and recommendations. SEISMIC CONSIDERA nONS In accordance with Table 1615.1.1 of the 2003 International Building Code (IBC), the site soil profile within 100 feet of the ground surface is best represented by Soil Profile Type C (Very Dense Soil and Soft Rock). The site soils are not susceptible to seismic liquefaction because of their dense nature. SLOPE STABIUTY ANAL YSIS In order to evaluate the potential for a future failure of the 40-foot-high, 60-percent slope to the west of the existing residence, we completed a slope stability analysis using the PCSTABl5M GEOTECH CONSULTANTS, INC. , '. ~ Driscoll October 13. 2005 IN 05289 Page 5 computer program. This program was developed by Purdue University. Using a Modified Bishop's analysis, we analyzed the slope's stability both under static and earthquake conditions. A potential peak ground acceleration of 0.30g was included to account for a M7.0 earthquake. Of primary concem from a geotechnical engineering standpoint is the potential for a failure at the interface between the near-surface soils and the denser native soils. Our analysis yielded a static safety factor of 1.3 and a dynamic safety factor of 1.2 for such a failure surface. The safety factor against shallower slope movement is less, as exhibited by the shallow landslides that have affected the slopes in the past. Hence our design recommendations for the minimum buffer and extended foundation walls account for the potential of a slope failure near the interface between the two soil units. CONVENTIONAL FOUNDA TIONS The proposed structures can be supported on conventional continuous and spread footings bearing on undisturbed, medium-dense to dense native soil. The General section should be reviewed for additional considerations related to the westemmost and southemmost footings of the main residence. We recommend that continuous and individual spread footings have minimum widths of 12 and 16 inches, respectively. Exterior footings should also generally be bottomed at least 18 inches below the lowest adjacent finish ground surface for protection against frost and erosion. The local building codes should be reviewed to determine if different footing widths or embedment depths are required. Footing subgrades must be cleaned of loose or disturbed soil prior to pouring concrete. Depending upon site and equipment constraints. this may require removing the disturbed soil by hand. An allowable bearing pressure of 3,000 pounds per square foot (psf) is appropriate for footings supported on competent native soil. A one-third increase in this design bearing pressure may be used when considering short-term wind or seismic loads. For the above design criteria. it is anticipated that the total post-construction settlement of footings founded on competent native soil will be less than one inch. Lateral loads due to wind or seismic forces may be resisted by friction between the foundation and the bearing soil, or by passive earth pressure acting on the vertical, embedded portions of the foundation. For the latter condition, the foundation must be either poured directly against relatively level, undisturbed soil or be surrounded by level structural fill. We recommend using the following ultimate values for the foundation's resistance to lateral loading: -~I Coefficient of F rietion Passive Earth Pressure 0.45 350 pef Where: (I) pet is pounds per cubic foot. and (II) passive earth pressure is computed using the equivalent fluid density. 15l1E1C IE D H ~ liUi MAY 24 2006 l~J l_____ "r err :-f COUNTY l""1':....,.- r' r - ~,( f.C -[NT GEOTECH CONSULTANTS, INC. '. 'lit , .... Driscoll October 13, 2005 IN 05289 Page 6 If the ground in front of a foundation is loose or sloping, the passive earth pressure given above will not be appropriate. We recommend maintaining a safety factor of at least 1.5 for the foundation's resistance to lateral loading, when using the above ultimate values. PERMANENT FOUNDATION AND RETAINING WALLS Retaining walls backfilled on only one side should be designed to resist the lateral earth pressures imposed by the soil they retain. The following recommended parameters are for walls that restrain level backfill: ~~~R\'IE~ Active Earth Pressure . 35 pet Passive Earth Pressure 350 pcf Coefficient of Friction 0.45 Soil Unit Weight 140 pet [D) [E (G ~ ~ W lE rm\ l.mIMAY 24 2o:lllJ.J: I I__~! i G q, '~r r'jUi~"'"Y. I r~>'r I: r 1--\,.(. <N~' I ---" -------- Where: (I) pet Is pounds per cubic foot, and (Ii) active and passive earth pressures are computed using the equivalent fluid pressures. * For a restrained wall that cannot deflect at least 0.002 times Its height, a uniform lateral pressure equal to 10 pst times the height of the wall should be added to the above active equivalent fluid pressure. The values given above are to be used to design permanent foundation and retaining walls only. The passive pressure given is appropriate for the depth of level structural fill placed in front of a retaining or foundation wall only. The values for friction and passive resistance are ultimate values and do not inclUde a safety factor. We recommend a safety factor of at least 1.5 for overtuming and sliding, when using the above values to design the walls. Restrained wall soil parameters should be utilized for a distance of 1.5 times the wall height from comers or bends in the walls. This is intended to reduce the amount of cracking that can occur where a wall is restrained by a comer. The design values given above do not include the effects of any hydrostatic pressures behind the walls and assume that no surcharges, such as those caused by slopes, vehicles, or adjacent foundations will be exerted on the walls. If these conditions exist, those pressures should be added to the above lateral soil pressures. Where sloping backfill is desired behind the walls, we will need to be given the wall dimensions and the slope of the backfill in order to provide the appropriate design earth pressures. The surcharge due to traffic loads behind a wall can typically be accounted for by adding a uniform pressure equal to 2 feet multiplied by the above active fluid density. Heavy construction equipment should not be operated behind retaining and foundation walls within a distance equal to the height of a wall, unless the walls are designed for the additional lateral pressures resulting from the equipment. The wall design criteria assume that the backfill will be well-compacted in lifts no thicker than 12 inches. The compaction of backfill near the walls should be accomplished with hand-operated equipment to prevent the walls from being overloaded by the higher soil forces that occur during compaction. GEOTECH CONSULTANTS. INC. , . A Driscoll October 13, 2005 ~!E(clE~.~ [i\1 I "jl MAY 2 4 2006 L \ I L __-.-J I .f "iU'., 'Y 'I .::, \'~ Backfill placed behind retaining or foundation walls should be c6arse, free-:"drai~ing _" . structural fill containing no organics. This backfill should contain no more than 5 percent silt or clay particles and have no gravel greater than 4 inches in diameter. The percentage of particles passing the NO.4 sieve should be between 25 and 70 percent. The on-site silt is not acceptable for wall backfill. If the native sand is used as backfill, a drainage composite similar to Miradrain 6000 should be placed against the backfilled retaining walls. The drainage composites should be hydraulically connected to the foundation drain system. Free-draining backfill or gravel should be used for the entire width of the backfill where seepage is encountered. For increased protection, drainage composites should be placed along cut slope faces. and the walls should be backfilled entirely with free-draining soil. The later section entitled Drainage Considerations should also be reviewed for recommendations related to subsurface drainage behind foundation and retaining walls. RetaininQ Wall Backfill and Water1JroofinQ The purpose of these backfill requirements is to ensure that the design criteria for a retaining wall are not exceeded because of a build-up of hydrostatic pressure behind the wall. The top 12 to 18 inches of the backfill should consist of a compacted, relatively impermeable soil or topsoil, or the surface should be paved. The ground surface must also slope away from backfilled walls to reduce the potential for surface water to percolate into the backfill. The section entitled General Earthwork and Structural Fill contains recommendations regarding the placement and cOl'1Jpaction of structural fill behind retaining and foundation walls. The above recommendations are not intended to waterproof below-grade walls, or to prevent the formation of mold, mildew or fungi in interior spaces. Over time, the performance of subsurface drainage systems can degrade, subsurface groundwater flow patterns can change, and utilities can break or develop leaks. Therefore, waterproofinp should be rovided where future see a e throu h the walls is not acce table. This typically includes limiting cold-joints and wall penetra ions, and using bentonite panels or membranes on the outside of the walls. There are a variety of different waterproofing materials and systems, which should be installed by an experienced contractor familiar with the anticipated construction and subsurface conditions. Applying a thin coat of asphalt emulsion to the outside face of a wall is not considered waterproofing, and will only help to reduce moisture generated from water vapor or capillary action from seeping through the concrete. As with any project, adequate ventilation of basement and crawl space areas is important to prevent a build up of water vapor that is commonly transmitted through concrete walls from the surrounding soil. even when seepage is not present. This is appropriate even when waterproofing is applied to the outside of foundation and retaining walls. We recommend that you contact a specialty consultant if detailed recommendations or specifications related to waterproofing design, or minimizing the potential for infestations of mold and mildew are desired. SLABS-ON-GRADE The building floors can be constructed as ,slabs-on-gradE1, atop existing non-organic soils, or on structural fill. The subgrade soil must be in a firm, non-yielding condition at the time of slab construction or underslab fill placement. Any soft areas encountered should be excavated and replaced with select, imported structural fill. GEOTECH CONSULTANTS. INC. " ~ ~ IE <<; [E ~ 'w IE ~D I IN 052 nll MAY 2 4 2005ag ~_ : L_____._._____ \ Even where the exposed soils appear dry. water vapor will tend to naturflIlY,migr<ite upward t~rough", I the soil to the new constructed space above it. All interior slabs-an-grade must be underlain by a- capillary break or drainage layer consisting of a minimum 4-inch thickness of gravel or crushed rock that has a fines content (percent passing the No. 200 sieve) of less than 3 percent and a sand content (percent passing the NO.4 sieve) of no more than 10 percent. As noted by the American Concrete Institute (ACI) in the Guides for Concrete Floor and Slab Structures, proper moisture protection is desirable immediately below any on-grade slab that will be covered by tile, wood, carpet, impermeable floor coverings, or any moisture-sensitive equipment or products. ACI also notes that vapor retarders, such as 6-mil plastic sheetinQ, are tvpically us~. A vapor retarder is defined as a material with a permeance of less than 0.3 US perms per square foot (pst) per hour, as determined by ASTM E 96. It is possible that concrete admixtures may meet this specification, although the manufacturers of the admixtures should be consulted. Where plastic sheeting is used under slabs, joints should overlap by at least 6 inches and be sealed with adhesive tape. The sheeting should extend to the foundation walls for maximum vapor protection. If no potential for vapor passage through the slab is desired, a vapor barrier should be used. A vapor barrier, as defined by ACI, is a product with a water transmission rate of 0.00 perms per square foot per hour when tested in accordance with ASTM E 96. Reinforced membranes having sealed overlaps can meet this requirement. Driscoll October 13, 2005 In the recent past, ACI (Section 4.1.5) recommended that a minimum of 4 inches of well-graded compactable granular material, such as a 5/8 inch minus crushed rock pavement base, should be placed over the vapor retarder or barrier for protection of the retarder or barrier and as a "blotter" to aid in the curing of the concrete slab. Sand was not recommended by ACI for this purpose. However, the use of material over the vapor retarder is controversial as noted in current ACI literature because of the potential that the protection/blotter material can become wet between the time of its placement and the installation of the slab. If the material is wet prior to slab placement, which is always pOSSible in the Puget Sound area, it could cause vapor transmission to occur up through the slab in the future, essentially destroying the purpose of the vapor barrier/retarder. Therefore, if there is a potential that the protection/blotter material will become wet before the slab is installed, ACI now recommends that no protection/blotter material be used. However, ACI then recommends that, because there is a potential for slab cure due to the loss of the blotter material, joint spacing in the slab be reduced, a low shrinkage concrete mixture be used, and "other measures" (steel reinforcing, etc.) be used. ASTM E-1643-98 "Standard Practice for Installation of Water Vapor Retarders Used in Contact with Earth or Granular Fill Under Concrete Slabs" generally agrees with the recent AClliterature. We recommend that the contractor, the project materials engineer, and the owner discuss these issues and review recent AClliterature and ASTM E-1643 for installation guidelines and guidance on the use of the protection/blotter material. Our opinion is that with impervious surfaces that all means should be undertaken to reduce water vapor transmission. EXCAVATIONS AND SLOPES Excavation slopes should not exceed the limits specified in local, state. and national government safety regulations. Temporary cuts to a depth of about 4 feet may be attempted vertically in unsaturated soil. if there are no indications of slope instability. However. vertical cuts should not be made near property boundaries, or eXisting utilities and structures. Based upon Washington Administrative Code (WAC) 296, Part N, the soil at the subject site would generally be classified as Type B. Therefore, temporary cut slopes greater than 4 feet in height should not be excavated at GEOTECH CONSULTANTS, INC. i I '. < 1. ~:~ , I~ ~ rr: [f1 ~ ..( Pag ~ )11 Ilnll MAY 2 4 2006 I.~. an inclination steeper than 1:1 (Horizontal:Vertical), extending continuousILb~~;-e:'~J~~7Df~.,fP d j the bottom of a cut. l.::::. ' err.' " Fer); 'le'!T The above-recommended temporary slope inclination is based on the conditions exposed in our explorations, and on what has been successful at other sites with similar soil conditions. It is possible that variations in soil and groundwater conditions will require modifications to the inclination at which temporary slopes can stand. Temporary cuts are those that will remain unsupported for a relatively short duration to allow for the construction of foundations, retaining walls, or utilities. Temporary cut slopes should be protected with plastic sheeting during wet weather. It is also important that surface water be directed away from temporary slope cuts. The cut slopes should also be backfilled or retained as soon as possible to reduce the potential for instability. Please note that loose soil can cave suddenly and without warning. Excavation, foundation, and utility contractors should be made especially aware of this potential danger. These recommendations may need to be modified if the area near the potential cuts has been disturbed in the past by utility installation, or if settlement-sensitive utilities are located nearby. Driscoll October 13, 2005 All permanent cuts into native soil should be inclined no steeper than 2.5:1 (H:V). Water should not be allowed to flow uncontrolled over the top of any temporary or permanent slope. All permanently exposed slopes should be seeded with an appropriate species of vegetation to reduce erosion and improve the stability of the surficial layer of soil. . Any disturbance to the existing slope outside of the building limits may reduce the stability of the slope. Damage to the existing vegetation and ground should be minimized, and any disturbed areas should be revegetated as soon as possible. Soil from the excavation should not be placed n the slope, and this may require the oft-site disposal of any surplus soil. DRAINAGE CONSIDERA TIONS We recommend that foundation drains be installed at the base of all foundation and earth-retainin..Q. walls, This includes installing a drain at the base of the extended west and south foundations of the main house. These drains should be surrounded by at least 6 inches of 1-inch-minus, washed ock and en wra ed n-woven eotextile filter fabric (Mirafi 140N, Supac 4NP, or Similar ma erial). At its highest point, a perforated pipe invert should be at least 6 inches below the bottom of a slab floor or the level of a crawl space, and it should be sloped for drainage. All roof and surface water drains must be kept separate from the foundation drain system. A typical drain detail is attached to this report as Plate 7. For the best long-term performance, perforated PVC pipe is recommended for all subsurface drains. Drainage inside the building's footprint should also be provided if the excavation encounters significant seepage. We can provide recommendations for interior drains, should they become necessary, during excavation and foundation construction. As a minimum. a vapor retarder, as defined in the Slabs-On-Grade section, should be provided in any crawl space area to limit the transmission of water vapor from the underlying soils. Also, an outlet drain is recommended for all crawl spaces to prevent a build up of any water that may bypass the footing drains. No groundwater was observed during our field work. If seepage is encountered in an excavation, it should be drained from the site by directing it through drainage ditches, perforated pipe, or French GEOTECH CONSULTANTS, INC. , .~ ... t Driscoll October 13, 2005 IN 05289 Page 10 drains. or by pumping it from sumps interconnected by shallow connector trenches at the bottom of the excavation. The excavation and site should be graded so that surface water is directed off the site and away from the tops of slopes. Water should not be allowed to stand in any area where foundations, slabs, or pavements are to be constructed. Final site grading in areas adjacent to a building should slope away at least 2 percent. except where the area is paved. Surface drains should be provided where necessary to prevent ponding of water behind foundation or retaining walls. GENERAL EARTHWORK AND STRUCTURAL FILL All building and pavement areas should be stripped of surface vegetation, topsoil, organic soil, and other deleterious material. It is important that existing foundations be removed before site development. The stripped or removed materials should not be mixed with any materials to be used as structural fill, but they could be used in non-structural areas, such as landscape beds. Structural fill is defined as any fill, including utility backfill, placed under. or close to. a building, behind permanent retaining or foundation walls, or in other areas where the underlying soil needs to support loads. All structural fill should be placed in horizontal lifts with a moisture content at, or near, the optimum moisture content. The optimum moisture content is that moisture content that results in the greatest compacted dry density. The moisture content of fill is very important and must be closely controlled during the filling and compaction process. The allowable thickness of the fill lift will depend on the material type selected, the compaction . equipment used, and the number of passes made to compact the lift. The loose lift thickness should not exceed 12 inches. We recommend testing the fill as it is placed. If the fill is not sufficiently compacted. it can be recompacted before another lift is placed. This eliminates the need to remove the fill to achieve the required compaction. The following table presents recommended relative compactions for structural fill: 1_()(\rl()'\()I~~\~ I'L \( I:\lr'\ r I CO\II' \( no'\ I Beneath slabs or walkwa s Filled slopes and behind retainin walls 95% 90% Where: Minimum Relative Compaction. is the ratio, expressed in percentages, of the compacted dry density to the maximum dry density, as detennined in accordance with ASTM Test Designation 0 1557.91 (Modified Proctor). The General section should be reviewed for considerations related to the reuse of on-site soils. Structural fill that will be placed in wet weather should consist of a coarse, granular soil with a silt or clay content of no more than 5 percent. The percentage of particles passing the No. 200 sieve should be measured from that portion of soil passing the three-quarter-inch sieve. ID) IE!: IEn \'Q IE ~\ lnr~y: 4 ,: \~] GEOTECH CONSULTANTS, INC. rr." ~' (" '_(oj'!ENT , .r o [E [: [E ~ J~ I ;a;Z~\\ !II . ~~:~ r J The conclusions and recommendations contained in this report are based on site conditions-as- they existed at the time of our exploration and assume that the soil and groundwater conditions encountered in the borings are representative of subsurface conditions on the site. If the subsurface conditions encountered during construction are significantly different from those observed in our explorations, we should be advised at once so that we can review these conditions and reconsider our recommendations where necessary. Unanticipated soil conditions are commonly encountered on construction sites and cannot be fully anticipated by merely taking soil samples in borings. Subsurface conditions can also vary between exploration locations. Such unexpected conditions frequently require making additional expenditures to attain a properly constructed project. It is recommended that the owner consider providing a contingency fund to accommodate such potential extra costs and risks. This is a standard recommendation for all projects. i'.. r Driscoll October 13, 2005 LIMITA TIONS The recommendations presented in this report are directed toward the protection of only the proposed structures from damage due to Slope movement. Predicting the future behavior of steep slopes and the potential effects of development on their stability is an inexact and imperfect science that is currently based mostly on the past behavior of slopes with similar characteristics. Landslides and soil movement can occur on steep slopes before, during, or after the development of property. The owner must ultimately accept the possibility that some slope movement could occur, resulting in possible loss of ground or damage to the facilities around the proposed residences. This report has been prepared' for the exclusive use of William L. Driscoll, Bill and Lisa Hoffman, and their representatives, for specific application to this project and site. Our recommendations and conclusions are based on observed site materials and selective laboratory testing. Our conclusions and recommendations are professional opinions derived in accordance with current standards of practice within the scope of our services and within budget and time constraints. No warranty is expressed or implied. The scope of our services does not include services related to construction safety precautions, and 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. Our services also do not include assessing or minimizing the potential for biological hazards, such as mold, bacteria, mildew and fungi in either the existing or proposed site development. ADDITIONAL SERVICES Geotech Consultants, Inc. should be retained to provide geotechnical consultation, testing, and observation services during construction. This is to confirm that subsurface conditions are consistent with those indicated by our exploration, to evaluate whether earthwork and foundation construction activities comply with the general intent of the recommendations presented in this report, and to provide suggestions for design changes in the event subsurface conditions differ from those anticipated prior to the start of construction. However. our work would not include the supervision or direction of the actual work of the contractor and its employees or agents. Also. jOb and site safety, and dimensional measurements. will be the responsibility of the contractor. During the construction phase, we will provide geotechnical observation and testing services when requested by you or your representatives. Please be aware that we can only document site work GEOTECH CONSULTANTS, INC. ~ L Driscoll '. .' [ October 13, 2005 IN 05289 Page 12 we actually observe. It is still the responsibility of your contractor or on-site construction team to verify that our recommendations are being followed, whether we are present at the site or not. The following plates are attached to complete this report: Plate 1 Vicinity Map Plate 2 Site Exploration Plan Plates 3 - 5 Boring Logs Plate 6 Grain-Size Analyses Plate 7 Typical Footing Drain Detail We appreciate the opportunity to be of service on this project. If you have any questions. or if we may be of further service, please do not hesitate to contact us. Respectfully submitted, GEOTECH CONSULTANTS, INC. ~~ Gerry D. Bautista. Jr. Geotechnical Engineer GDB/MRM: alt GEOTECH CONSULTANTS, INC. rBl Ii: aa 1 \H ~\ ll'(~ 1006 IUJ] JEJFr 'c.:0 I COU!'Jry ~~~I 0'- (:i'( ~~0r ,~ENT l,... . ~1 . , . I lE tIfT W IE Inl1 MAY 24 2006 t , I I _______ \ !'r:.t' ,-;-.7 ~~. H ., ! , \ ',,' I '. > ~ ~PI\ltST,_: Sletr,. .n,_... ..). -:> <"", lt/C'~ ~<I,Q QI..... Ii'" ~ t-_. '" I .! ! s.., R. :i <:Oyle~= 0._... ,. """'q; ,...E1kRd.en .;- "'" FOx Rd- !o. 0..... 3: "4' . ...: ~ '.- 'l:L T C:!lii ,...... l M.~~ .~ (j en E'lIIcn..fl~- , -'-'. I f "'__,) \C .._'-'~ "~ \.,., ;c.. ':, . "', . 0.... ~i',;. " . \ '. , ;. / ./ c; ~~Rd, EnII Rd . -vr...---........ SmilIII,JI . ._,~ fJ. 02~ M~.com.lnc:.; C20Q!5 ~~ , . ..... >- VICINITY MAP Lot 4 - Thousand Trails Donald Road Jefferson County, Washington I JOb No' I Date' I 05289 Oct:. 2005 I Plate: 1 , GEOTECH CONSULTANTS, INC. . , ....;. J IE ~ f[ ~ IE \\f~l\ \ /,1--___________________ / ~, , ,'-'~------ " " " ------...... '... ,',' - ............ ...., " " ,...... -...-------... ..... ...... ,,' /' ;",/' '......... ......, ................. " " ... ...... '....... "', "" .....' .....' '......... -', .. " , ....... '... .. " ,..' ", ,'-.......... ............. ............. '.... /' /' ...' ".........-..................', ",,'" .. ...... ............. ',', // ,/ ,/ --,,/ ------ ---, \, "'--, , " " I / ............. ... ...... ... ,.....,\ .,,' 1/,' "",,- '... \, .......,\ " ,',' -...... '\, ", " I ... "" ",,/ /' I "'.. .... " 'f ""''''' .. ,',' l "''''... '...... " ,'" I ','............. .. " "I . ... ... ........ 390 / " /' l " '.... ... " / ....' ,--------.............. ".... ......... ............... I ,/ /", '''', ............. '''-, '... .' " \,,~...... '...... "',, ~ ...... "... " , / ...............-........... ... " ," -...... "''-... " 400 ,'/ / ,- ........ "''''...', ~,..' , " ......" "\, ,,- l " ........._... ',\ ," B 1 / l \ \... " .- ~/ / " \ 410 / 0 ..,' " \', \ ' .....--.......", ... /' "....... '''''''' ...,..." ,I' '\ ... 420 / ..J " \ /',/ , , , I.. '... /' /' .. 430 " . " , " , "" ,l ,,' " / / / , / ,....\ ' / I ,/ , L_," //' ,/ ....-...---..,' MAY "ENT 1 c' ""'-1 r" (~. - ...-----------... ~' ......-- ---- ,460 ~~~ ---~...-- -...-... " ,...... " ...-' ,~ ......' ~, " ...-' ----------------...-----...... ...-...---__...... 470 " " " ",-----------------------......'-------------------------,...-'-----------------~,' ~~ , GEOTECH CONSULTANTS, INe. SITE EXPLORATION PLAN Lot 4 - Thousand Trails Donald Road Jefferson County, Washington I Plate, z/ A ,.. I Job No, 05289 I Date, Oct. 2005 r No Scale '- 0'" BORING 1 ...~~ ,$!'" ~o l,.~ <tIO' ';<J'~ ~'o .o~~ ~~ ,,-c'" ,\0' 'cp <S~ Description Grass over FILL? Brown, silty SAND. fine- to medium-grained, moist, loose (FILL?) \. )I ..,' 5 10 25 40 '-~ , 50/6" 5 I 1111111 . ::::: :: Redlbrown, gravelly, silty SAND, fine- to medium-grained, mOist, dense SOlS" 1 I::: i i :: - blows may be overstated due to gravel in sampler 47 50/6" : Brown. gravelly, silty SAND, fine- to medium-grained, moist, very dense 50/6" 4 1 - becomes more gravelly 8M , " , " , " , " , " , " , " , " , " , " , " , " , " , " , " , " , " II II II III II I'" - brown, medium-grained sand lenses fD) [E rr: [E n~ lE ~1J MAY 2 4 2006 .' l 'Fr '.( COUN"( J rEq C~ [1"- ~'r, L.01 ,:; ,'H SOlS" 6 1 w::::::,' Brown, gravelly SAND, medium-grained, moist, very dense 50/1" 7 I::::i:: :~: 1: ~. . ,..': ';':;,: ....... 50/3" ..... 8 I mmmmw - no gravel; becomes fine-grained . Test boring was tenninated at 40.75 feet on September 12, 2005. . No groundwater seepage was encountered during drilling. BORING LOG Lot 4 - Thousand Trails Donald Road Jefferson Coun ,Washin ton IJObl Date: I Logged by: I Plate: . 05289 .. Oct 2005 .. GOB. 3 GEOTECH CONSULTANTS, INC. I ;,>- 0.... ....e,' ,;)J~ ~o ';"e, -$0' ~~ -<9,0 db' <,<i:Q G'? ,\0' 'i <;0' \)~ " ~... ",.I 5 10 15 20 25 ~" , BORING 2 Description 35 ; I ; ; ; I r Grass over 1111111 : : : : : :: Orangelbrown, gravelly, silty SAND, fine- to medium-grained, damp, dense "!IIII 1111111 1111111 III"" , : 8M 1 Ii:::::: - becomes moist with fine-grained sand lenses, less gravels 111"11 IIIIII1 IIIIII1 r"1111 111,.11 1111111 33 Brown, sandy SILT, with fine-grained sand lenses, non-plastic, very moist, 2 I dense ML 63 3 1.,;::11:111:1 Brown SAND, fine- to medium-grained, moist, very dense . . :;:::::;;:;::::: 5014" 4 I i.'!~;.}! - becomes gravelly ;:~~:Wji~!im ;~: ~:; n; n ~ :: : : : :: Brown, gravelly, silty SAND, fine- to medium-grained, moist, very dense 5012" 51111'''' IIIII11 Itlllll : 8M , , 1111'" 11111/1 1111111 1111111 50/3" 6 II I r , . I I . - * Test boring was terminated at 30.25 feet on September 12, 2005. * No groundwater seepage was encountered during drilling. IDJHIf!1H ~I Ilr(",y ~ ~J .1 \ f-fr" JIJI'iTY L~r."i ~ .r.i'_ - ," t..Li f:~EN. GEOTECH CONSULTANTS, INe. BORING LOG Lot 4 - Thousand Trails Donald Road Jefferson Coun ,Washin ton I Job I Date: I Logged by: I Plate: 05289 , Oct. 2005 " GOB , 4 ;r 0" "q,~ ,j'" <10 :",q, ~O' ';<J\.q,<(J\.0 ~~ ~~ GC;; ",0' ~ <:,0' \)':3 t, ....... ,/ 5 10 15 25 30 35 40 '-~ , BORING 3 Description :;::;;: Grass over ::::::: Orange/brown, gravelly, silty SAND, fine- to medium-grained, damp, : : : : :: medium-dense . 111111 "'l' 24 1 I 8M '"11 IIIII 11111 11111 IIIII III1I 11111 "111 - becomes moist 27 >':>;},: Orange/brown SAND, fine-grained, moist, medium-dense 2 IIII'~~:I!I- becomes gray ...... ~:~:::,::; :i:::; .... 30 31 38 41 42 51 :i:I:1 ~ 8M 55 50/5" 7 I ML .. - 65 Brown/gray, sandy SILT, with fine-grained sand lenses, non-plastic, moist, dense ML - becomes very moist Brown/gray SAND, with silt lenses, fine-grained, moist, dense Brown/gray. sandy SILT, with fine-grained sand lenses, non-plastic, moist, very dense IQ)rE<<:~~W~ ~ lnl[uAY 24~] i v" ,_ I ,~~' GEOTECH CONSULTANTS, INe. 8' * Test boring was terminated at 41.5 feet on September 12,2005. * No roundwater seepa e was encountered durin drillin . BORING LOG Lot 4 . Thousand trails Donald Road Jefferson Coun ,Washin ton ~ Job 05289 Plate: 5 DBte: Oct. 2005 f, ... '- -' .' 1~ . ~-OO 1. o. o. \ /'\. I., "- "- "- ~ 0- ~ ~r--. r-.... II '\ I"'-.. '- :'\ 'w. - Sieve Opening (mm.) ~ ~ ~ ! " <; ! o " 8 IS o o 100 1 90 eo 70 eo l ~ 50 a -: . . <0 l!. JO '" 10 0 Sieve Opening (US Nnd.rd) ---Bomgf,f5fMt.8~moIIdJ.n ---Bomg1,3Sfeet.3.9'l1ornolsture I [D) [E (G f] WlE fR\\ lntl"y 24 1o~lU) 1 .,,, ,_'v [.p.'. c (r.' r .ul .ENT ~l" , GEOTECH CONSULTANTS, INC. GRAIN SIZE ANALYSES Lot 4 - Thousand Trails Donald Road Jefferson Coun . Washin ton I Job I Date: I I Plate: 6 05289 Oct 2005 .. , ~... >- (.-.:-." ( . Slope backfill away from foundation. Provide surface drains where necessary. Washed Rock (7/8" min. size) Backfill (See text for requirements) Tightline Roof Drain (Do not connect to footing drain) Nonwoven Geotextile Filter Fabric Vapor Retarder/Barrier and Capillary Break/Drainage Layer (Refer to Report text) 4" Perforated Hard PVC Pipe (Invert at least 6 inches below slab or crawl space. Slope to drain to appropriate outfall. Place holes downward.) ,0) [E ~ lE -TI W lE IR\. I lnll'AY 2 4 ~lUJ \ ... L,.,,; .'",. '!Ty ~ T C r u,> :ENT NOTES: (1) In crawl spaces, provide an outlet drain to prevent buildup of water that bypasses the perimeter footing drains. (2) Refer to report text for additional drainage, waterproofing, and slab considerations. ~~ ) . - r FOOTING DRAIN DETAIL Lot 4 - Thousand Trails Donald Road Jefferson Coun ,Washin ton I Job I Date: I Scale: I Plate: 05289 Oct. 2005 Not to Scale 7 I GEOTECH CONSULTANTS, INC.