Loading...
HomeMy WebLinkAbout998500064 Geotech AssessmentRepOrt f - Geotechnical Engineering-Services Proposed CreekSide II Residential Development Port Ludlow, Washington January 15, 1997 Ground water seepage was observed in test pit 2 of our 1995 study within the residual basalt at depths of 4 and 7 feet. We also observed ground water seepage in test pits 9, 11, 14, 15, and 16 of our 1995 study at depths ranging from 3~h to 11 feet. Ground water seepage was encountered in fractured basalt in test pit 2 of our 1992 study at 4.5 feet below the ground ._.~ surface. We expect that seasonally perched ground water conditions are likely to exist in the fractured zone. in other areas of the site during wet weather conditions and/or during the winter season. CONCLUSIONS AND RECOMMFNDATION$ ---~ GENERAL' Based on our observations of surface and sUbsurface conditions, it is our opinion that the . .,J 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 setback recommendations have been developed. __J · Some of the on-site soils are moisture sensitive and grading will be more economical if performed during dry weather conditions. 3 · Grading may include fills on slopes. All fill should be properly keyed into the slopes and drained, as appropriate. ' --7 · The majority of the shallow basalt bedrock is highly fractured and we anticipate it can be excavated by ripping. Excavations and cuts 'into unweathered basalt'could require the use of hoe-rams or controlled blasting, and may generate large cobbles and boulders that are not suitable for structural fill. · Shallow foundations founded on native materials, bedrock or structural fill may' be used for '  support of structures. · Where mixed subgrade materials (such as bedrock/native soil) 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 bedrock or in "~'~ fractured/weathered bedrock zones. Site development should include drainage facilities as appropriate to intercept ground water seepage. --'2 LANDSLIDE HAZARD A copy of the Geologically Hazardous Areas Section of the Jefferson County Critical Areas -~ is attached as Appendix A. Jefferson County defines landSlide hazard areas as: Ordinance · 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. G ¢ o E n g i n e e r s 4 File No. 2378-040-T03/011597 · 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 site, except as noted below. However, we believe that surficial soils on steeper slopes creep the will be vulnerable to 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 direct it away from areas. Recommendations for fill construction, drainage and erosion protection are slope presented in greater detail in following sections of this report. We did not observe streams on or adjacent to the site which undercut site slopes except near the southeast end of the north ridge area. Some evidence of surficial soil movement or creep was observed on the steep slopes in this portion of the site. The site is located in an area mapped by the SCS (Soil Conservation Service) as having limitations to construction of dwellings with basements which range from slight to severe depending on the soil type and slope. Site soils are included in the Olete and Everett series in the Soil Survey of Jefferson County. The soil survey describes the limitations to dwellings with basements of the Olete soils as slight to moderate for slopes ranging from 0 to 30 percent. The soil survey describes the limitations to dwellings with basemems of the Everett soils as slight for slopes ranging from 0 to 8 percent, as moderate for slopes ranging from 8 to 15 percent, and as for slopes greater than 15 percent. severe Portions of the site meet the Jefferson County criteria for landslide hazard areas due to the SCS classification. However, based on our site exploration and experience on similar sites it is our opinion that landslide hazards are not a limiting factor for this development provided the setbacks recommended below are maintained and our recommendations presented in other portions of this report are followed. The recommended setbacks are dependent on house locations and cannot be plotted on a plan until house locations are established. SETBACKS Ridge Areas: We recOmmend a minimum horizontal distance of 8 feet be maintained between the bottom outside edge of foundations and the face of slopes steeper than 30 percent, as illustrated in Figure 4. For slopes steeper than 50 percent, we recommend the foundation setback be 12 feet. Upland Areas: No landslide hazard areas were identified within this portion of the site. G ¢ o E n g i n e e r s 5 File No. 2378-040-T03/011597 If a lot straddles the approximate boundary between ridge and upland areas the ridge area setbacks should be applied. ,,. EROSION HAZARD ..j 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 moderate depending on slope. Site soils are included in the Olete and Everett series in the Soil Survey of Jefferson County. The soil survey describes the erosion hazard of the Olete soils as slight to moderate for slopes ranging from 0 to 30 percent. The soil survey describes the erosion hazard of the Everett soils as slight to moderate for slopes ranging from 0 to 30 percent and as moderate for slopes ranging from 30 to 50 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 runoff onto cut or fill slopes, natural slopes or other erosion-sensitive shaped to avoid directing areas. Temporary ground 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. However, on-site dispersal of stormwater from individual lots may be appropriate as discussed in the Drainage section. Long term erosion control will require that the vegetative cover on the slopes be maintained. Bare ground areas should be vegetated, as necessary. Erosion resistant plant species include: · shrubs such as' oregon grape, service berry, and salal. Woody · Grass mixtures including: rye; fescue, bent, and clover. · Other deep rooted site tolerant vegetation. File No. 2378-040-T03/011597 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. Heavy ripping should be expected for excavations in the basalt bedrock, especially where the bedrock is less weathered. Heavy ripping may necessitate using a Caterpillar D9L dOzer with a single shank ripper, or comparable equipment. Blasting may also be required if unweathered, unfractured bedrock is encountered. 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 grading 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 cleared and we expect that only nominal stripping of vegetation will be required in these areas. However, 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. 13 e o E n g i n e e r s 7 File No. 2378-040-.T031011597 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 using hand probing. Soft areas noted during proofrolling or probing should be overexcavated and replaced with structural fill as outlined below. We recommend that a GeoEngineers representative, or other qualified geotechnical engineer, 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 MDD (maximum dry density) determined in accordance with ASTM D-1557. Where foundations, slabs or pavement will be founded directly on native material, we recommend that the subgrade soil be compacted'to at least 95 percent of MDD. Surficial materials over most 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 will probably be necessary to overexcavate and replace native materials with compacted structural fill containing less than 5 percent fines. Beneath building and pavement areas, we recommend minimum overexcavation depths of 2 feet. 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 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. O · o E n g i n e e r s 8 File No. 2378-040-T03/011597 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 on-site soil be considered for use as During dry weather construction, nonorganic may structural fill provided it is at a suitable moisture content when placed and can be compacted as recommended. If the material is wet when excavated, it will be difficult or impossible to compact. The soil will require aeration and drying out prior to placement as structural fill. During wet weather construction, material with no more than about 5 percent fines should be workable. In general the site soils are not suitable for use as structural fill during wet weather construction. Fill Placement on Slopes 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 as up fill it is brought 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 and rock 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 used to construct the keyway. In general, keyways should be at least 10 feet equipment wide or about 1 th 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 existing slopes into 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. 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, straw mulch, 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. G e o E n g i n e e r s 9 File No. 2378-040-T03/011597 ¸3' 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. When the grading plan has been comPleted, can locations for subdrains generally be predetermined by GeoEngineers prior to construction, based on our understanding of subsurface conditions. We recommend anticipating that additional subdrains will be required during grading to intercept ground water seeps that are encountered. A contingency for additional subdrains should be included in the project budget, Fill Settlement Postconstruction settlement of fill will depend on the type and compaction of the fill, the thickness of the fill, and subgrade conditions. We estimate that structural fills may experience settlement· in the range of 1/2 to 1 inch for every 10 feet of new fill thickness. Because fill thickness may vary beneath each individual lot, settlement of the fill may occur differentially. 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. For Planning purposes, cuts in unweathered bedrock may be inclined at 1 to 1. Where the bedrock is fractured, weathered, jointed, or otherwise unstable, flatter cut inclinations or stabilization techniques may be required. It is likely that cuts in unweathered bedrock will require the use of heavy ripping equipment and/or controlled blasting. G e o E n g i n e e r s 10 File No. 2378-O40-T03/011597 FOUNDATION SUPPORT We recommend that residential structures be supported on conventional spread footings founded on medium dense to dense native soil, basalt bedroCk, 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. Shallow Spread Footings We recommend that all· footing elements be embedded a minimum of 18 inches below 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 using an allowable bearing of 1,000 psf applied to dead and live loads.~ We estimate that postconstruction settlement of structures supported uniformly on basalt bedrock should be less than about 1/4 inch, with negligible differential settlement. Postconstruction settlement of structures supported on medium dense to dense native soil or on structural fill may range from about 1/2 to 1 inch. Maximum differential settlements of structures on native or fill soils should be less than 1/2 inch, measured along 25 feet of continuous wall footing or between adjacent comparably loaded isolated footings. We expect that essentially as the loads are aPplied. settlements will occur 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, at contacts between dissimilar materials within cuts, and especially where fills or native soils abut the basalt bedrock. Where contacts between dissimilar materials are exposed pad or lam 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. Alternatively, where dissimilar subgrade materials occur, it may be more economical to extend footings so that the .entire foundation is supported on bedrock. G e o E n g i n e · r s 11 File No. 23784)40-T03/011597 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 protect bottoms to the Subgrades from disturbance. We recommend that all footing excavations be observed by a rePresentative from our firm immediately prior to mud mat or crushed rock placement, or reinforcing steel and structural concrete placement to confirm 'that the bearing surface has been prepared in a manner consistent with our recommendations and that the subsurface conditions are as expected. 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, with negligible sand or silt, and preferably should be crushed material. A vapor barrier should be placed beneath the slab. In areas where ground water is near the surface, we recommend that underdrainage be provided toCollect 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 SYSTEMS Cantilevered Concrete Gravity Walls Portions of the building stem walls may serve as retaining walls, particularly if daylight basement construction is used. Other cantilevered concrete gravity walls may be used at the site for grade transitions. Retaining walls that are allowed to yield during backfilling should be designed for lateral pressures based on an equivalent fluid density of 35-pcf (pounds per cubic foot) if the ground surface behind the wall is level for a distance of two times the wall height. This value applies to fill behind the walls that is placed and compacted as recommended above, with the exception that fill within a distance equal to the wall height behind the wall should be compacted to a maximum of 92 percent of the MDD. Care must be' taken by the contractor to avoid overcompaction. Alternative design values can be presented for sloping backfills once the grading plans have been determined. '-~ G e o E . g i n e e r s 12 File No. 2378-040-T03/011597 The recommended equivalent fluid density assumes a free-draining condition behind the wall. This can be accomplished by placing a 12- to 18-inch-wide Zone of sand and gravel containing less than 5 percent fines against the wall. The uppermost 1 foot should be backfilled with more silty soil to form a plug against surface water intrusion. A 4- or 6-inch-diameter, heavy-walled perforated pipe drain should be installed within the free-draining material at the base of the wall. The pipe should be laid with a minimum slope of one percent to a suitable discharge point. The pipe installation should include a cleanout riser with cover located at the upper end of the pipe run. : The value for soil bearing presented for the foundation design is applicable to retaining wall design. Resistance to lateral loads will be developed both through friction on the base of the footing and passive resistance on the sides of the footing. Friction resistance between the concrete and native soils or compacted structural fill may be computed using a coefficient of friction of 0.35 applied to the vertical dead load forces. We recommend passive resistance be computed using an equivalent fluid density of 300 pcf applied over the embedment depth of the wall and footing. The above coefficient of friction and passive equivalent fluid density values include a factor of safety of about 1.5. We recommend that we be provided the opportunity to review our recommendations if wall heights exceed about 8 feet or if retaining walls with sloping backfill will be constructed for this project. If free-standing retaining walls will be necessary in fill sections to accommodate grade transitions, we suggest that design alternatives that may include the use of geosynthetics be evaluated. 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 behind 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. Other Retaining Options A variety of retaining systems may be used where the recommended maximum cut and fill slope inclinations will not accomplish the necessary grade transitions. Specific design criteria G e o E n g i n e e r s 13 File No. 2378-040-T03/011597 - should be developed for each wall after its height and location are determined. Mechanically -- stabilized walls (fill embankments with geosynthetic reinforcement and facing materials), soldier - pile walls, soil nailed walls and others may be cost effective depending upon the circumstances. 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 into subdrains in fills and roadways. Roof drain water should not be discharged to footing drains. However, use of separate dispersal trenches on individual lots may be appropriate for disposal of roof drainage and should be evaluated on a case by case basis. '- 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 a good deterrent to prevent water collection in 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 greater than 5 feet in height to intercept surface runoff' and to prevent it from flowing.in an uncontrolled manner across the slopes. Surface water should not be discharged over the undisturbed slopes outside the grading areas. PAVEMENT DESIGN AND SUBGRADE PREPARATION Roadway subgrades should be prepared as described previously for general earthwork. The upper 24 inches of roadway subgrade should have a density of at least 95 percent of the MDD. In areas where soft or disturbed soils are encountered, subgrade preparation should consist of overexcavating the unsuitable soils down to firm unyielding material, or about 2 feet maximum, whichever is less. If soft soils are present at 2 feet below design subgrade elevation, we recommend placing -] a woven geotextile fabric such as Mirafi 500X (or similar material approved by the geotechnical --- engineer) on the subgrade and covering the geotextile with at least 12 inches of gravel or rock '-1 spalls. ~ ~ Our recommended pavement section is for the proposed arterial roads and cul-de-sacs within the development. We anticipate that these roads will be subjected to primarily automobile traffic "1 with occasional trucks. The recommended pavement section is 2 inches of Class B asphalt "-' concrete underlain by a 2-inch layer of crushed top course material. The top course is underlain ] .~ by 6 inches of gravel base compacted to 95 percent of the maximum dry density as'determined -'~? by ASTM D-1557. Calculations supporting the proposed pavement section are included in !_~: · Appendix B. I 1 ~?i ~:i';~i~:'::. a eoEngineers 14 File No. 23784)40-T03/011597 EROSION AND SEDIMENTATION CONTROL Temporary erosion protection should be used and maintained during construction to protect slope surfaces, adjacent areas and receiving waters. Erosion control measures should include proper channeling of surface water runoff, and liberal use of straw bales or geotextile filters, as appropriate. Grading should be accomplished to avoid concentration of runoff onto fill areas, cut or fill slopes, natural slopes or other erosion-sensitive areas. We recommend that disturbance to slopes outside the immediate work areas be minimized. Removal of vegetation and forest duff should be limited. Some sloughing and ravelling of cut slopes and natural slopes that are disturbed should be expected. Graded areas should be shaped to divert water away from slope areas. Surface runoff should be prevented from flowing into excavations by using berms, drainage ditches, swales or other appropriate methods. As temporary erosion protection, we recommend the use of straw, jute matting, visqueen sheeting or other forms of ground cover on all areas disturbed by the construction. Permanent erosion protection should be provided by reestablishing vegetation. Slope surfaces should be restored so that surface runoff does not become channeled. 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 hydroseeded. LIMITATIONS We have prepared this report for use by Pope Resources and members of the project team involved in the Creekside Village Division II residential development. 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. If 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. ...~_ G e o E n g i n · ¢ r s 15 File No.~378-O40-T03/011597 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 consistent with those indicated by the explorations; to provide recommendations are 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 conditions, express or implied, should be understood. 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. Garry H. Squires ~ Senior Geotechnical Engineer , EXP,RES 10/23/q'~ Gary W. Henderson Principal GHS:GWH:vc Document ID: 2378040R.R Attachments Six copies submitted O · o E n g i n · e r s 16 File No. 2378-040-T03/011597 --] Foundation . . . ,~. · " · . . -. .. · . · · · · . Minimum Setbock (See Notes) Dense N~tive Soil or Properly Compocted Structural Fill ] -'] NOTES' ~ 1. Setback shall be at least 8' fOr slopes greater than 30%. -~ ~ 2. Setback shall be o[ least 12' for slopes greater than 50%. FOUNDATION DETAIL - Geo Engineers III II __. ..