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HomeMy WebLinkAbout821211003 Geotech AssessmentReport J Geot~echnical Engineering Services Subsurface 'Investigation _ Proposed Teal Lake Village Division.II pOrt Ludlow, Washington March 28, 1995 For Pope Resources G eo Engineers :fie No. 2378-034-T03/032295 Geo Englneers March 28, 1995 Pope Resources P.O. Box 1780 Poulsbo, Washington 98370 Attention: Ms. Linda Mueller We are pleased to submit four copies of our "Report, Geotechnical Engine. ering Services, Subsurface Investigation, Proposed Teal Lake Village Division II, Port Ludlow, Washington for Pope Resources.'.' We appreciate the opportunity to be of service to Pope Resources. Please contact us if you have questions regarding this project or if we can provide additional services. Yours very truly, GeoEngineers, Inc. Gary W. Henderson Principal SLF:GWH:vc Document ID: 2378034R.R File No. 2378-034-T03 CC: Pope Resources 781 Walker Way Port Ludlow, Washington 98365 Attn: Mr. Ray Welch Printed on recycled paper. CONTENTS Pa,qe No. INTRODUCTION .................................................. 2 SCOPE OF SERVICES .............................................. 2 SITE CONDITIONS ................................................ 2 SURFACE CONDITIONS 2 SUBSURFACE EXPLORATIONS 3 SITE GEOLOGY 3 SUBSURFACE CONDITIONS 4 CONCLUSIONS AND RECOMMENDATIONS ............................... 4 GENERAL 4 LANDSLIDE HAZARD 5 SETBACKS 6 EROSION HAZARD 6 EROSION CONTROL 6 SEISMIC VULNERABILITY 7 EARTHWORK 7 General 7 Clearing and Site Preparation 7 Subgrade Preparation 8 Structural Fill 8 Suitability of On-Site Materials for Fill 9 Fill Placement on Slopes 9 Fill Slopes 9 Fill Drainage 1 0 Cut Slopes 10 Temporary Cut Slopes 10 Permanent Slopes 1 1 Utility Trenches 1 1 FOUNDATION SUPPORT 1 1 General 1 1 Foundation Design 1 2 Lateral Load Resistance 12 Foundation Settlement 1 3 FLOOR SLAB SUPPORT 1 3 RETAINING and SUBGRADE WALLS 1 3 Design Parameters 13 Backdrainage 14 Construction Considerations 1 5 Rockeries 1 5 DRAINAGE 1 5 PAVEMENT DESIGN AND SUBGRADE PREPARATION 1 5 LIMITATIONS ................................................... 1 6 G e '6 E n g i n e e r s i File No. 2378-034-T03/32895 CONTENTS (continued) FIGURES Vicinity Map/Site Plan Foundation Detail Soil Classification System Test Pit Logs APPENDICES Appendix A - Jefferson County Critical Areas Ordinance Fi.qure No. 1 2 3 4...8 Pa,qe No. A-1 G e o E n g i n e e r s ii File No. 2378-034-T03/32895 REPORT GEOTECHNICAL ENGINEERING SERVICES SUBSURFACE INVESTIGATION PROPOSED TEAL LAKE VILLAGE DIVISION II PORT LUDLOW, WASHINGTON INTRODUCTION This report presents the results of our geotechnical engineering services for the proposed Teal Lake Village Division II residential subdivision to be developed south of Port Ludlow, Washington. The site is located along Teal Lake Road, in the central portion of Section 21, Township 28 North, Range I East, Willamette Meridian, as shown on the Vicinity Map and Site Plan, Figure 1. We understand that the proposed development will include 54 single-fa/nily residences located as shown on the Site Plan, Figure 1. SCOPE OF SERVICES The purpose of our services was to explore subsurface soil and ground water conditions at the site as a basis for providing geotechnical recommendations and design criteria for the proposed development. Our specific scope of services included the following: 1. Excavate a series of backhoe test pits at the site to explore subsurface soil and ground water conditions. 2. Evaluate pertinent physical and engineering characteristics of the soils at the site. 3. Provide recommendations for site preparation and earthwork including stripping requirements, hillside grading, evaluation of on-site soils for use as fill and import fill, and compaction criteria. 4. Provide recommendations for building setbacks in steep slope areas in accordance with Jefferson County Critical Areas Ordinance No. 05-0509-94. 5. Provide recommendations for foundation and slab support of the proposed structures including allowable bearing values and estimates of settlement. 6. Provide recommendations for site drainage, as appropriate. 7. Provide recommendations for pavement design including subgrade preparation. 8. Prepare a report containing our findings along with our conclusions and recommendations. SITE CONDITIONS SURFACE CONDITIONS The proposed subdivision will be located in an upland area southwest of Teal Lake Village Division I and southeast of the proposed Springwood Development, as shown on the Site Plan. Lots I through 36 are located on the east side of Teal Lake Road. This portion of the site 2 File No. 2378-034-7'03/032895 GeoEngineers consists primarily of a north-south trending ridge. Elevations range from about 310 to 480 feet above mean sea level (MSL). The crest of the ridge slopes up toward the south. Side slopes on the eastern side of the ridge range up to about 60 percent with occasional steeper areas. Side slopes on the western side of the ridge range up to about 30 percent with occasional steeper areas. The majority of this portion of the site is vegetated with mature second growth Douglas fir and cedar trees with a dense understory of brush. Areas which have been cleared more recently are vegetated with alder trees, brush, and grass. Lots 37 through 54 are located on the west side of Teal Lake Road. Elevations range from about 225 to 460 feet above MSL. This portion of the site slopes down toward the west with an average slope of about 10 percent. Seasonally intermittent streams have developed drainage courses in the southwestern portion of the site. Slopes along the drainage courses range up to about 80 percent. The majority of this portion of the site is vegetated with mature second growth Douglas fir and alder trees with a dense understory of brush. Soil and construction debris has been stockpiled on portions of lots 40 through 42, 51 and 52. Soils exposed in a borrow pit located to the southwest of the site consist of dense fmc to medium sand. The face of the borrow pit is on the order of 100 feet in height. No groundwater seepage or springs were observed at the time of our site visit. Seasonal intermittent streams flow in drainage courses on the site. SUBSURFACE EXPLORATIONS Subsurface Conditions at the site were explored on February 20, 21, and 24, 1995 by excavating 13 test pits at the approximate locations shown on the Site Plan. The test pits were excavated to depths ranging from 9 to 12~h feet below the ground surface using a John Deere 310C rubber-tired backhoe. The locations of the test pits were established in the field by taping or pacing from existing features, and should be considered approximate. A representative from our firm continuously monitored the excavations and kept a detailed log of the soil and ground water conditions encountered. Soils were visually classified in general accordance with the system described on Figure 3. The logs of our explorations are attached as Figures 4 through o SITE GEOLOGY Our interpretation of the site geology is based on our review of published information in our library, our site reconnaissance including the borrow pit excavation to the southwest of the site, and subsurface explorations on this site and adjacent sites. In general, the site soils consist of a cap of glacial till. Glacial till consists of a mixture of clay to boulder-sized soil particles which was deposited by the ice and consolidated to a very dense condition. The glacial till is underlain by Vashon advance outwash deposits. Advance outwash deposits consist of sands and gravels deposited by melt water streams in front of advancing glaciers. These soils were also consolidated by the overlying ice. To the west of the site in the Springwood Development the Fac No. 2378-034-T03/032895 contact between Pre-Vashon clayey silt and overlying Vashon advance outwash was observed. The site soils are generally glacially consolidated and therefore very stiff to dense. SUBSURFACE CONDITIONS Based on our subsurface explorations the contact between the overlying glacial till and the underlying advance outwash appears to occur approximately along the 375 foot contour line. In general soils encountered below the 375 foot contour line (to the west) consist of advance outwash deposits, and soils encotintered above the 375 foot contour line (to the east) consist of glacial till deposits. Glacial till was encountered in test pits 3, 4, and 7 along the upper portion of the ridge on the east side of Teal Lake Road, and in test pit 2 and 9 tO the west of Teal Lake Road. The till extended to the depths explored, 9lA to 12 feet, except in test pit 5 in which the till is underlain at a depth of 7 feet by advance outwash deposits. The upper 1~,~ to 3 feet of the till has weathered to a medium dense condition. The ground surface is covered with 6 .to 12 inches of forest duff, local areas are overlain with fill or soils deposited through erosional processes. Advance outwash deposits were encountered in test pits 12 and 13 located along the eastern edge of the site at the toe of the slope, in test pits 5, 6, 8, and 11 located in the north-central portion of the site, and in test pits 1, 2, and 10 located in the western portion of the site. As discussed above, advance outwash deposits were also observed in a borrow pit to the southwest of the site. Fill was encountered in test pit 1 located on Lot 52. Fill consisting of a mixture of silty soil and construction debris appears to extend over portions of Lots 40 through 42, 51, and 52. The depth of fill appears to vary from a few feet to about 20 feet in the stockpile areas. Ground water levels are expected to vary seasonally. Perched groundwater was observed at the contact between the weathered and unweathered till. CONCLUSIONS AND RECOMMENDATIONS GENERAL Based on our observations of surface and subsurface conditions, it is our opinion that the site is generally suitable for the proposed development. Specific grading plans for the site have not yet been developed, however, we have addressed general geotechnical considerations for the project as follows: · Slopes at the site appear to be stable under existing conditions. General s~tback recommendations have been developed. · Some of the on-site soils are moisture sensitive and it is our opinion that earthwork and grading will be more economical if performed during dry weather conditions. 4 File No. 2378-034-T03/032895 G ¢ o Ert g inee rs Fill has been placed on portions of the site. This material does not appear to have been compacted, and is of variable quality. This material will be unsuitable for support of foundations or pavements, and should be removed from these areas. Grading may include fill placement on slopes. All fill should be properly keyed into the slopes and drained, as appropriate. Shallow foundations founded on dense native materials or properly compacted structural fill may be used for support of structures. Where mixed subgrade materials occur at footing or floor grades, overexcavation and replacement with structural fill may be required or alternative footing designs should be considered. Ground water may be seasonally perched immediately above the unweathered glacial till. Site development should include drainage facilities as appropriate to intercept ground water seepage. LANDSLIDE HAZARD A copy of the Geologically Hazardous Areas Section of the Jefferson County Critical Areas Ordinance is attached as Appendix A. Jefferson County defines landslide hazard areas as: Areas of historic failures, including areas of unstable slopes and old and recent landslides. Areas potentially unstable as a result of rapid stream incision, stream bank erosion, or undercutting by wave action. · 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 Soil Survey for Jefferson County. No evidence of landsliding or slope instability was observed on the s;_~.. However, we believe that surficial soils on the steeper slopes will be vulnerable to creep and/or sloughing if they are disturbed during construction, or if development of the top of the ridge increases or concentrates surface drainage or ground water seepage. Fills on or near slopes should be placed on properly proofrolled and compacted subgrade material, and should be keyed and drained as recommended below. Graded areas and fill slopes should be revegetated to reduce erosion potential. We recommend that a surface water drainage system be developed for. the subdivision' to collect drainage from impermeable surfaces and yard areas, and directed it away from slope areas. Recommendations for fill construction, drainage and erosion protection are presented in greater detail in following sections of this report. The site is located in an area mapped by the Soil Conservati. on Service (SCS) as having limitations to construction of dwellings with basements which range from moderate to severe depending on the soil type and slope. Site soils are included in the Cassolary and Sinclair series in the Soil Survey of Jefferson County. The soil survey describes the limitations to dwellings without basements of the Cassolary soils as moderate for slopes ranging from 0 to 15 percent and as severe for slopes greater than 15 percent. The soil survey describes the limitations to 5 File No. 2378-034-T03/032895 GeoEngincers dwellings with basements of the Sinclair soils as moderate for slopes ranging from 0 to 15 percent and as severe for slopes greater than 15 percent. Neither rapid stream incision nor stream bank erosion was observed in the seasonaIly intermittent stream beds which flow across portions of the site at the time of our site visit. Portions of the site meet the Jefferson County criteria for landslide hazard areas due to the SCS classification. However, based on our site explorations and experience on similar sites it is our opinion that landslide hazards are not a limiting factor for this development provided that our recommendations for building setbacks and site development are followed. SETBACKS In our opinion, a minimum horizontal setback Of 8 feet should be maintained between foundations and the face of slopes between 15 and 30 percent and greater than 10 feet in vertical height on this portion of the site. For slopes between 30 and 50 percent we recommend the foundation setback be increased to 12 feet, and 20 feet for slopes steeper than 50 percent. For clarity, an illustration of the recommended setback has been included as Figure 2, EROSION HAZARD Jefferson County defines erosion hazard areas as those areas that are classified as having severe or very severe erosion potential by the SCS. The site is located in an area mapped by the SCS as having erosion hazards which range from slight to severe depending on slope. Site soils are included in the Cassolary and Sinclair series in the Soil Survey of Jefferson County. The soil survey describes the erosion hazard of the Cassolary soils as slight to moderate for slopes ranging from 0 to 15 percent and as moderate for slopes of 15 to 30 percent. The soil survey describes the erosion hazard of the Sinclair soils as slight to moderate for slopes ranging from 0 to '15 percent and as moderate to severe for slopes ranging from 15 to 30 percent. EROSION CONTROL It is our opinion that the potential erosion hazard of the site is not a limiting factor for the proposed development. The proposed development will be located primarily in the more gently sloping portions of the site. 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 and protect potential receiving waters. Erosion control measures should include but not be limited to berms and swales with check dams to channel surface water runoff, ground cover/protection in exposed areas and silt fences. Removal of natural vegetation should be minimized and limited to the active construction areas, and reestablishment of vegetation should be undertaken as soon as possible. Graded areas should be shaped to avoid directing runoff onto cut or fill slopes, natural slopes or other erosion-sensitive areas. Temporary ground 6 File No. 2378-034-T03/032895 G¢oEngin¢¢rs cover/protection such as jute matting, excelsior matting, wood chips or clear plastic sheeting should be used until permanent erosion protection is established. We recommend that graded or disturbed slopes be tracked in-place with the equipment running perpendicular to the slope contours so that the track grouser marks provide a texture to help resist erosion. Thereafter, all disturbed areas should be revegetated. We recommend that no loose fill be placed on the slopes and that no water be directed toward or discharged on the slope areas. Tightlines should be used to direct storm or other surface water across slope areas. Long-term erosion control will require that the vegetative cover on the slopes be maintained. Any bare ground areas should be vegetated, as necessary. Erosion resistant plant species include: · Woody shrubs such as: Oregon grape, service berry, and salal. · Grass mixtures including: rye, fescue, bent, and clover. · Other deep-rooted site-tolerant vegetation. SEISMIC VULNERABILITY In our opinion, the site does not contain seismic hazards areas as defined by Jefferson. County criteria. The Puget Sound region is a seismically active area; all sites within this region can be expected to experience some damage in the event of a significant seismic event. Certain factors:can result in increased probability or degree of damage at a particular site. We did not encounter conditions which in our opinion place this site at risk of unusual damage in the event of a significant seismic event. Specifically, potentially liquefiable soils, loose sands and silty sands below the water table, were not encountered on the site. EARTHWORK General We expect that the majority of the grading can be accomplished with conventional heavy earthmoving equipment. Surficial soils at the site generally contain high amounts of silt, and are therefore sensitive to disturbance when they become excessively wet. Operation of heavy equipment at the site under wet conditions can be expected to result in considerable disturbance to the exposed subgrade soils. During wet weather construction, it will probably be necessary to provide temporary haul roads consisting of quarry spalls, crushed rock or pit run sand and gravel. We recommend that earthwork be undertaken during periods of dry weather, if feasible, to minimize grading costs. Clearing and Site Preparation The work area should be cleared of all surface and subsurface debris including underbrush, tree stumps, roots and organic-laden soils. Portions of the project area have previously been 7 File No. 2378-034-T03/032895 Gco E n gin¢¢ r s cleared. Our observations indicate that the upper 1/2 to 1 foot of soil has been previously disturbed. Stripping or recompaction of the soils to these depths may be required where previous site activities have softened surficial soils and/or mixed organic debris into the soil. If the clearing operations cause excessive disturbance, additional stripping depths may be necessary. Disturbance to a greater depth can also be expected if site preparation work is done during periods of wet weather. The organic laden strippings can be stockpiled and used later for landscaping purposes or be spread over disturbed areas following completion of grading. If spread out, the organic strippings should be in a layer less that 1 foot thick, and should not be placed on slopes. Materials which cannot be used for landscaping or protection of disturbed areas should be removed from the project site and wasted. Subgrade Preparation Following stripping, the' exposed subgrade should be evaluated prior to placing structural fill, pavement materials, or constructing foundations. During dry weather, subgrade evaluation should consist of proofrolling with heavy rubber-tired construction equipment. During wet weather, subgrade evaluation should be accomplished by hand probing. Any soft areas noted during proofrolling or probing should be overexcavated and replaced with structural fill as outlined below. We recommend that a GeoEngineers representative be present during proofrolling and/or probing to evaluate exposed subgrade soils. Prior to placement of structural fill, the exposed subgrade should be uniformly compacted to :at least 90 percent of maximum dry density (MDD) determined in accordance with ASTM D-1557. Where foundations, slabs or pavement are to be founded directly on native material, we recommend that the subgrade soil be compacted to at least 95 percent of MDD. Surficial materials over portions of the site contain enough fines (material passing the No. 200 sieve) that compaction of subgrade will be difficult, if not impossible, to achieve during periods of wet weather. If grading takes place during the wet winter months, it may be necessary to overexcavate and replace native materials with compacted structural fill containing less than 5 percent fines beneath building and pavement areas. Where underlying subgrades are excessively wet, it may be necessary to stabilize the subgrade with a layer of quarry spalls, clean gravel, or by placing a layer of geotextile fabric (such as Mirafi 500x) between the subgrade and structural fill. Structural Fill All fill in embankments and beneath structures or pavements should be placed as structural fill. Structural fill material should be free of debris, 'organic contaminants and rock fragments larger than 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 (material passing the No. 200 sieve) increases, soil becomes increasingly more sensitive to small changes in moisture content and adequate compaction becomes more difficult or impossible to achieve. If fill material is 8 File No. 2378-034-T03/032895 G e o E ng ine e r s imported to the site for wet weather construction, we recommend that it be a sand and gravel mixture, such as high quality pit run, with less than 5 percent fines. All structural fill should be compacted in horizontal lifts to at least 90 percent of the MDD per ASTM D-1557. The uppermost 24 inches of subgrade soils below structures, slabs-on-grade and pavements should be compacted to at least 95 percent of the MDD. We recommend that the fill prism supporting footings, defined by a plane extending down from the edges of the footing at 1 to 1 (horizontal to vertical) to native ground, be compacted to at least 95 percent of MDD. The lift size used during placement and compaction will depend on the moisture and gradation characteristics of the soil and the type of equipment being used. If necessary, the material should be moisture conditioned to near-optimum moisture content prior to compaction. During fill and backfill placement, sufficient testing of in-place density should be performed to verify that adequate compaction is being achieved. Suitability of On-Site Materials for Fill During dry weather construction, any nonorganic on-site soil and rock may be considered for use as structural fill provided it is at a suitable moisture content when placed and can be compacted as recommended. If the material is too wet when excavated, it will require aeration and drying prior to placement as structural fill. Fill Placement on Slopes : All fill placed on slopes steeper than 5 to 1 (horizontal to vertical) should be benched into the slope face and include keyways and subdrains. Bench excavations should be level and extend into the slope face until a vertical step of about 3 feet is constructed. The excavated materials may be pushed out and compacted into the structural fill as it is brought up if adequate compaction can be achieved. Keyways should be located below fill embankment toe areas where new fills meet existing hillside slopes. Additional keyways may be necessary depending on the extent of the proposed fill and the quality of the soil underlying the embankment. Keyways should be embedded at least 2 feet into stable material in the toe area. The width of the keyway will depend on several factors, such as the vertical height of the fill above the keyway and the size of the equipment used to construct the keyway. In general, keyways should be at least 10 feet wide or about 1~.6 times the width of the equipment used for grading or compaction. Fill Slopes Permanent fill slopes should be constructed at inclinations of 2 to 1 (horizontal to vertical) or flatter, and should be blended into existing slopes with smooth transitions.. To reduce postconstruction sloughing and ravelling, we recommend that fill slopes be overbuilt where possible and subsequently cut back to expose well compacted fill. Retaining structures should be used where cut and fill slopes 2 to 1 or flatter cannot be achieved. 9 File No. 2378-034-T03/032895 GeoEnginecrs To minimize erosion, newly constructed slopes should be hydroseeded as soon as practical. Until the vegetation is established, some sloughing and ravelling of the slopes should be expected. Erosion control measures such as temporary covering with clear plastic sheeting, revegetation fabric or jute matting should be used to protect these slopes until vegetation is established. We also recommend that graded areas above slopes be shaped to direct surface water away from the slope face. Fill Drainage Subdrains should be installed at the rear of each keyway and at other locations beneath fill embankments where ground water seepage is encountered during grading. The subdrains can be installed concurrently with fill placement, or in trenches excavated after filling, where the trench depth would not exceed about 4 feet. The drains should consist of a free-draining sand and gravel drainage material, placed in a trench about 2 feet wide, fully encapsulated within a suitable nonwoven, geotextile filter fabric, such as Mirafi 140N (or similar material). The drainage material should extend the full height of the rear keyway wall. Where subdrains are used to intercept ground water seepage at locations other than at keyways, the drainage material should be at least 3 feet high. A heavy-wall (SDR-35 or heavier) perforated pipe should be installed near the bottom of each subdrain and bedded in drainage material. Pipes should have minimum slopes of 1 percent and should drain to suitable collector and discharge points. All subdrain lines should include cleanout risers. We recommend that the cleanout risers be covered with tamper-proof locking caps. Discharge pipes should be covered with heavy galvanized wire mesh to prevent rodent access. Cut Slopes Permanent cut slopes in soils should be inclined at 2 to 1 (horizontal to vertical) or flatter, or should be retained with a properly designed retaining structure. Cut slopes should be hydroseeded shortly after completion of grading to prevent erosion. Temporary erosion protection may be necessary as discussed above for newly constructed fill slopes. Temporary Cut Slopes Temporary cut slopes are anticipated for construction of underground utilities. All temporary cut slopes and shoring must comply with the provisions of Title 296 WAC, Part N, "Excavation, Trenching and Shoring." The co.ntractor performing the work must have the primary responsibility for protection of workmen and adjacent improvements, deciding whether to use shoring, and for establishing the safe inclination for open-cut slopes. Temporary unsupported cut slopes more than 4 feet high may be inclined at iH:IV (horizontal to vertical) maximum steepness within native till or structural fill. Flatter slopes may be necessary if seepage is present on the cut face. Some sloughing and ravelling of the cut slopes G ¢ o E n g i n e e r s 10 File No. 2378-034-T03/032895 should be expected. Temporary covering with heavy plastic sheeting should be used to protect these slopes during periods of wet weather. Permanent Slopes We recommend that any permanent fill slopes be constructed no steeper than 2H: IV. To achieve uniform compaction, we recommend that fill slopes be overbuilt slightly and subsequently cut back to expose well compacted fill. To minimize erosion, newly constructed slopes should be planted or hydroseeded shortly after completion of grading. Until the vegetation is established, some sloughing and ravelling of the slopes should be expected. These may require localized repairs and reseeding. Temporary covering, such as clear heavy plastic sheeting, jute fabric, loose straw or excelsior matting could be used to protect the slopes during periods of rainfall. Utility Trenches Trench excavation, pipe bedding, and trench backfilling should be completed using the general procedures described in WSDOT Standard Specifications, Section 7-17, or other suitable procedures specified by the project civil engineer. · Utility pipes should be bedded in sand and smooth rounded gravel, such as specified in WSDOT Standard Specifications, Section 9-03.15. Additionally, we recommend that the pipe be covered with bedding material to at least one foot above the pipe. This bedding material should be lightly tamped into place. Backfill placed above the bedding material shall consist of structural fill quality material as discussed above. Utility trench backfill can be placed in lifts of 12 inches or less (loose thickness) below a depth of 5 feet from finish grade. Within 5 feet of finish grade, backfill should be placed in lifts of 8 inches or less (loose thickness) such that adequate compaction can be achieved throughout the lift. Each lift must be compacted prior to placing the subsequent lift. Prior to compaction, the backfill should be moisture conditioned to near optimum moisture content, if necessary. The backfill should be compacted in accordance with the criteria discussed above. FOUNDATION SUPPORT General We recommend that residential structures be supported on conventional spread footings founded on medium dense to dense native soil, or structural fill, prepared as recommended in the EARTHWORK section of this report. Shallow spread footings designed and constructed as described below may be used where minimum setback distances can be achieved on moderate slopes. G e o E n g i n e e r s 11 File No. 2378-034-T03/032895 Foundation Design We recommend that all footing elements be embedded a minimum of 18 inches below the lowest adjacent finished grade. Where footings are placed on sloping ground, the horizontal distance from the bottom of the footing to the ground surface should not be less than 8 feet. We recommend a minimum width of 2 feet for isolated footings and at least 16 inches for continuous wall footings. Deeper footing embedment may be required where minimum building setbacks cannot be achieved, 'and we recommend that design criteria for footings located on or near slopes be evaluated by a representative from our firm on a site-specific basis. Footings founded as described above can be designed using an allowable soil bearing pressure of 2,500 psf (pounds per square foot) 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 loadings. Where a crawlspace is used, footing pads for floor support may be cast on the ground, providing that the ground is firm and level. These pads should be designed usi.ng an allowable bearing of 1,000 psf applied to dead and live loads. Structures constructed across mixed subgrade conditions could experience distress because of differential performance of the subgrade materials. This is a concern at the contact between cuts and fills and at contacts between dissimilar materials within cuts. Where contacts between dissimilar materials are exposed at pad or footing grade, we recommend that the subgrade beneath the structure be overexcavated at least 1 foot below design grade, and the overexcavation backfilled with structural fill compacted to at least 95 percent of the MDD. The limits of the overexcavation and structural fill placement should extend at least 1 foot outside of the building footprint or footing area. Loose or disturbed subgrade soils in footing excavations may result in increased settlement. The native soils are susceptible to disturbance if allowed to become wet. If footings are constructed during wet weather, concrete should be placed as soon as possible after the footings are excavated. It also may be appropriate to place a lean concrete "mud mat" or a layer of crushed rock in footing excavation bottoms to protect the subgrades from disturbance. We recommend that all completed footing excavations be observed by a representative of our firm prior to reinforcing steel and structural concrete placement. Our representative will confirm that the bearing surface has been prepared in a manner consistent with our recommendations and that the subsurface conditions are as expected. Lateral Load Resistance Lateral loads can be resisted by a combination of friction between the footing and the supporting soil, and by the passive lateral resistance of the soil surrounding the embedded portions of the footings. A coefficient of friction between concrete and soil of 0.35 and a passive lateral resistance corresponding to an equivalent fluid density of 300 pcf (pounds per cubic foot) may be used for design. The friction coefficient and passive lateral resistance are allowable values, and incorporate factors of safety of approximately 1.5. G e o E n g i n ¢ ¢ r's 12 File No. 2378-034-T03/032895 If soils adjacent to footings are disturbed during construction, the disturbed soils must be recompacted, otherwise the lateral passive resistance value must be reduced. Foundation Settlement We estimate that the postconstruction settlement of shallow footings supported on native till or on structural fill may range from about ~A to IA inch. Maximum differential settlement should be less than lA inch, measured along 25 feet of continuous wall footing. We expect that settlements for these conditions will tend to occur rapidly after the loads are applied. Immediately prior to placing concrete, all debris and soil slough that accumulated in the footings during forming and steel placement must be removed. Debris or loose soils not removed from the footing excavations will result in increased settlement. FLOOR SLAB SUPPORT Floor slabs may be supported on-grade provided that the subgrade soils are prepared as previously recommended. Any areas disturbed by construction activities should be recompacted before proceeding with slab construction. We recommend that slabs-on-grade be constructed on a gravel layer to provide uniform support and to act as a capillary break. The gravel layer should consist of at least 4 inches of clean fine gravel or crushed rock, with negligible sand or silt. A vapor barrier should be placed over the gravel layer. We recommend that the vapor barrier be covered With 2 inches of sand to protect it during construction and to aid in curing of the slab concrete. This sand should not be allowed to become wet prior to casting the slab concrete, otherwise curing of the concrete may be adversely affected. In areas where ground water is near the surface, we recommend that underdrainage be provided tQ collect and discharge ground water from below the slabs. This can be accomplished by thickening the gravel layer below the slabs to 6 inches, and installing a 4-inch-diameter perforated collector pipe in a shallow trench placed below the gravel layer. The collector pipe should be oriented along the center, long axis of the structure. The trench should measure about 1 foot wide by 1 foot deep and should be backfilled with clean gravel. The collector pipe should be sloped to drain and discharge into the storm water collection system to convey the water off site. This pipe should also incorporate a cleanout. RETAINING and SUBGRADE WALLS Design Parameters We recommend that retaining and subgrade walls be designed using an active lateral earth pressure corresponding to an equivalent fluid density of 35 pcf. This lateral earth pressure is for a wall with level backfill. For walls with backfill sloping up at 2H: iV, the design lateral earth pressure should be increased to 55 pcf. If vehicles can approach the wall to within IA the height of the wall, a traffic surcharge should be added to the wall pressure. For car parking areas, the traffic surcharge can be 13 File No. 2378-034-T03/032895 G¢oEngineer$ approximated by the equivalent weight of an additional 1 foot of soil backfill behind the wall. For delivery truck parking areas and access driveway areas, the traffic surcharge can be approximated by the equivalent weight of an additional 2 feet of soil backfill behind the wall. These recommendations are based on the assumption that any retaining walls at this project will be provided with backdrainage and will be unrestrained against slight top rotation. If the walls will be restrained, higher pressures will be appropriate. Walls are assumed to be restrained if top movement during backfilling is less than H/1000, where H is the wall height. The values for soil bearing, frictional resistance and passive resistance presented above for foundation design are applicable to retaining wall design. Backdrainage The retaining walls could be exposed to water from ground or surface water sources, or from landscape watering. As the proposed structures will likely utilize the retaim'ng wall as basement walls, we recommend that the buried portions of the walls be waterproofed. To reduce the potential for hydrostatic water pressure buildup behind the retaining walls, we recommend that the walls be provided with backdrainage. Backdrainage can be achieved by using free- draining material or prefabricated drainage panel products, with perforated pipes to discharge the collected water. Free-draining material should consist of sand and gravel containing less than 3 percent fines. The draining material should be 2 feet wide and should extend from the base of the wall to within 1 foot of the ground surface. The free-draining material should be covered with 1 foot of less permeable material, such as the on-site silty sand. Prefabricated drainage panel products, such as Mirafi Miradrain 6000 (or similar material), consist of a geotextile filter fabric bonded to a molded plastic drainage element. The drainage panel is placed directly behind the wail, and should extend from the base of the wall to about 1 foot from finished grade. The panel should be covered with 1 foot of less permeable material, such as the on-site silty sand. Wall backdrains should include a perforated pipe with a minimum diameter of 6-inches. We recommend using either heavy-wall solid pipe or rigid corrugated polyethylene pipe. We recommend against using flexible tubing for wall backdrain pipe. The pipe should be installed with about 3 inches of drainage material below the pipe, or the drainage panel geotextile filter fabric should extend from the panel to wrap around the pipe. The pipes should be laid with minimum slopes of one percent and discharge to appropriate disposal points to convey the water away from the retaining walls. The pipe installations should include cleanout risers located at the upper end of each pipe run. We recommend that the cleanouts be provided with tamper-proof locking caps, completed within flush mounted utility boxes. We recommend that roof downspouts not discharge into.the perforated pipes providing wall backdrainage. 14 File No. 2378-034-T03/032895 G¢o Engincers Construction Considerations Care should be taken by the contractor during backfilling to avoid overstressing the retaining walls. Backfill placed within about 5 feet of the walls should be compacted with hand- operated or small self-propelled equipment. Heavy compactors or other heavy construction equipment should not be used within about 5 feet of the walls. Rockeries Rockeries may be planned in areas with grade transitions. Rockeries essentially serve as protection against erosion and minor sloughing along existing stable slopes and provide little "retaining" support. Rockeries are best suited for use along stable slopes cut in competent soils. When a rockery is constructed along the face of a fill embankment, adequate compaction of the fill behind the rockery is critical for long-term stability; the fill should be compacted to at least 95 percent of the MDD, and the fill height should be limited to about 4 feet. Any surcharge conditions above a rockery or seepage conditions within the fill embankment be. hind a rockery can lead to distress or failure of a rockery-faced slope. The potential need for maintenance of rockeries should be recognized. We recommend that rockeries be constructed in accordance with the most current edition of "The Association of Rockery Contractors Standard Rockery Construction Guidelines." For planning purposes, we recommend that all rockeries be limited to a maximum height of 8 feet. DRAINAGE All ground surfaces, pavements and sidewalks should slope away from structures. Surface water runoff should be controlled by a system of curbs, berms, drainage swales, and/or catch basins, and conveyed off-site through a storm water collection system. Surface water should not be discharged over slopes or into subdrains. Roof drains should be tightlined to discharge into the storm water collection system or to an appropriate outlet structure. Roof drain water should not be discharged to footing drains. Footing, wall and underslab drainage systems may be needed depending on final design grades and localized ground water conditions. Footing drains with an invert elevation at the base of the footing are generally effective to limit water seepage into crawlspaces. The crawlspace should not be excavated deeper than the invert of the footing drains, or additional areal drains will need to be provided. Permanent drainage systems should be installed at the top and/or bottom of cut and fill slopes to intercept surface runoff and to prevent it from flowing in an uncontrolled manner across the slopes. PAVEMENT DESIGN AND SUBGRADE PREPARATION Parking area and access drive pavement subgrades should be prepared as described previously in the EARTHWORK section of this report. We recommend the pavement in areas 15 File No. 2378-034-I'03/032895 GeoEngineers to be used exclusively by automobiles consist of 2 inches of Class B asphalt concrete over 4 inches of crushed surfacing base course. For pavement in access roads and truck parking areas, we recommend providing 3 inches of asphalt concrete over 6 inches of crushed surfacing base course. The base course should be compacted to at least 95 percent of the MDD. The crushed base course should comply with Washington Department of Transportation Standard Specifications for Road, Bridge and Municipal Construction, 1994, Section 9-03.9(3) "Base Course." The asphalt concrete materials and procedures should comply with specifications in that document for Class B Asphalt Concrete Pavement. LIMITATIONS We have prepared this report for use by Pope Resources and members of the project team involved in the Teal Lake Village Division II. The data and report should be provided to prospective contractors for bidding or estimating purposes; but our report, conclusions and interpretations should not be construed as a warranty of the subsurface conditions. Our scope 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. The project was in the design development stage at the time this report was prepared. We expect that further consultation regarding specific design elements will be necessary. If there are any changes in the grades, location, configuration or type of construction planned, the conclusions and recommendations presented in this report might not be fully applicable. I[ such changes are made, we should be given the opportunity to review our conclusions and recommendations and to provide written modification or verification, as appropriate. When the design is finalized, we recommend that we be given the opportunity to review those.portions of the specifications and drawings that relate to geotechnical considerations to see that our recommendations have been interpreted and implemented as intended. There are possible variations in subsurface conditions between the locations of the explorations and also with time. Some contingency for unanticipated conditions should be included in the project budget and schedule. We recommend that sufficient monitoring, testing and consultation be provided by our firm during construction to confirm that the conditions encountered are consistent with those indicated by the explorations; to provide recommendations for design changes should the conditions revealed during the work differ from those anticipated; and to evaluate whether or not earthwork and foundation installation activities comply with the contract plans and specifications. Within the limitations of scope, schedule and budget, our services have been executed in accordance with generally accepted practices in this area at the time the report was prepared. No other warranty, express or implied, should be understood. 16 Fire No. 2378-034-T03/032895 GeoEngineers We appreciate the opportunity of working 'with you on this project. If you have any questions or need further assistance, please call. Yours very truly, GeoEngineers, Inc. MaS]~ Thomas V. Geotechnical Engineer Gary W. Henderson Principal TVM:GWH:vc Document ID: 2378034R.R Attachments Four copies submitted CC: Pope Resources 781 Walker Way Port Ludlow, Washington 98365 ^ttn: Mr. Ray Welch 17 File No. 2378-034-T03/032895 Geo Eng in e e rs