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HomeMy WebLinkAboutBLD2001-00062 geo tech KA No. 102-01036 May , 01 • • Page30 No.20 71 soils will be subject to erosion. Erosion of the exposed soils will be most noticeable during periods of rainfall and may be controlled by the use of normal erosion control measures, i.e., silt fences, hay bales, mulching, control ditches or diversion trenching, and contour furrowing. By ordinance requirement a native vegetation buffer is required from the edge of all slopes that are • geologically hazardous areas. It is our opinion that a vegetation buffer having a minimum width of 30 feet should be maintained parallel to the top of the south facing slopes below the building site. This setback is shown on Figure 2. In addition to the maintenance of buffer we recommend that all disturbed areas be replanted as soon as possible after construction is complete. It is our opinion that the vegetation buffer may be comprised of native vegetation and properly designed and installed landscaping. However, the portion of the buffer comprised of native vegetation should not be reduced to less than 10 feet in width along the indicated top of the slope. The following site preparation and foundation design recommendations are provided to aid in minimizing potential erosion and landslide damage risks and should be incorporated into site planning, design and construction. RECOMMENDATIONS Site Preparation and Grading Water Related Concerns: Only minor storm water related problems are anticipated if site grading and preparation are undertaken during the normally drier portions of the year. If site work is undertaken during wet weather the near surface sands and silty soils may become over-saturated and temporarily unworkable. If the site work is undertaken during wet weather the contractor should be fully prepared to deal with possible elevated water levels in addition to other soil and water problems normally encountered in these materials during wet weather work including the filtering of runoff, as needed, to prevent the siltation of down slope areas. It should be anticipated that silt fences and other erosion control devices could be used to control sediment transport off the site. Depending upon the final site grades and weather conditions it is possible that areas of perched water or seeps may develop in some areas. In that we are unable to predict where or when this might occur we recommend that any development of springs or seeps be treated as a construction maintenance problem. The contractor should be prepared to deal with any water-related problems during construction. Water seepage can cause failure of the excavation walls and the contractor should be observant for possible cave-in or other hazardous conditions and provide shoring for all cuts and excavations in accordance with local, state, and federal regulations. 1 Krazan &Associates,Inc. Offices Serving The Western United States KA No. 102-01036 • May 30, 2001 Page No. 8 Development Recommendations: Under all buildings, pavements and fill areas, it is recommended that all ?' sod, organic soil, and debris is removed. Over most of the site it is anticipated that a stripping depth of 12 inches will be adequate. However, localized deeper stripping depths may be required to remove tree root balls. Stripped soils, contaminated with organics or debris, should be wasted off site or used in landscape areas. 4:4 Stripped soils free of organics and debris may be used as structural fill subject to the following considerations. Following stripping of the site and prior to the placement of any fill, the exposed subgrade should be proof rolled to a firm, unyielding condition using suitability sized equipment. Compaction of the stripped subgrade should be continued until field density tests in soil areas show that a minimum compaction of 95% of the maximum dry density, as determined by ASTM method D-1557, has been achieved in the top 12 inches of subgrade beneath all building, driveway, and parking areas. In areas of bedrock exposure the architect, engineer, geologist or other qualified person should inspect the subgrade to confirm adequacy and absence of excessive amounts of disturbed/loose material. Any soft or weaving areas disclosed during proof rolling should be excavated and replaced with compacted structural fill. With the exception of driveway side slopes, it is recommended that permanent cut slopes not exceed 2H: 1 V (50%). Fill slopes should not be steeper than 2H:1 V (50%) for fill placed in accordance with the requirements of appendix chapter 33 of the Uniform Building Code(1997 edition) or 3 to 4H:I V (33%to 25%) for uncontrolled fills of moderate quality material. In areas where steeper slopes are required, retaining structures should be provided. In areas where fills are to be made on slopes steeper than 5H:1V the subgrade should be benched and prepared in accordance with UBC (1997) requirements prior to fill placement. Benches should be cut at a maximum vertical height of 24 inches. It should be anticipated that, if steeply cut, the more granular near surface soils may be subject to caving, and sloughing will occur as the soils are exposed to drying. All temporary cuts and excavations should be sloped or shored in accordance with local, state and federal requirements. Areas which are to be filled to bring the building or pavement grades up to the desired elevations should be filled with compacted granular material free from roots, trash or other deleterious'materials. During wet weather most of the on site soils are not expected to be suitable for use as fill. These soils are sufficiently fined grained, such that with the addition of small quantities of water they become overly saturated and are difficult or impossible to compact to the desired density. As a result, we recommend Krazan& Associates,Inc. Offices Serving The Western United States • KA No. 102-01036 May 30,2001 Page No. 9 grading and preparation be undertaken and completed during dry weather. If grading in that all site we recommend that all excavated soilbe orpavement areas is necessary during wet weather, � building placed on slopes or under buildings or removed from the site and that materials used as structural fill (fill article size of 3 inches and not more gravel with a maximum p sandy ' pavements) consist of free draining than 5.0% fines, material passing a U.S. No. 200 sieve. All imported fill material should conform to the above recommendation regardless of the weather. approximately 8 inches in loose thickness, conditioned to a All structural fill should be placed in layers P acted to 95% of the maximum dry density as action, and comp _ moisture content suitable for compa ction, determined by ASTM D-1557. Field density tests should be made at a frequency adequate to assure that the required compaction is achieved. -u of ground water or storm runoff in the soils adjacent to the residence, the ! To preclude the possible build p laced, perforations down, is recommended that a four inch diameter perforated, rigid pipe be p outside of the building foundation at the footing subgrade elevation. All of the drainage syse should om the anyaccumulated water away bedded in a drainage sand and gravel and designed to carry structure to an aPPP ting drains ro riate discharge area. Roof drainage should not be connected o the lfcfoot d at least but mayuse the same outfall piping provided that the connection between a sys drain. water from backing up into the footing 10 feet down grade of the house and designed to prevent 1 All runoff from roofs, s and hard surfaced areas should be intercepted, co II llected llec to d and driveways, patio do disposed of away from structure s and site slopes, and discharged where the water will not effectorm Ovate '' Specific recommendations for and design of a s slope structures, slopes, walls or pro Perties. osay re aced by other consultants fully disp 1 s stem are beyond the scope of our services and should be eo tchnical perspective tight lining familiar with design and discharge requirements. However, from a g I of the collected water to an infil tration system located on the flatter slopes to the east of the building site, near the present shed,would be an acceptable means of disposal. Ealing-Si-Wig topof slope setback and foundation of structures we recommend that the following P For the sitingproposed building be maintained. From the indicated top of the south facing slobs k belowof the feet be maintained. embedmentsite shown of slope se site shown on Figure 2 we recommend that a minimum should bottom a minimum of 18 inches below the a����y, all footings on gentle slopes lowest adjacent exterior grade and 12 inches below the lowest interior grade. Footings on or within of 10 is a minim to es steeper than 15% should be designed so that the bottom of the footing 36 feet of s p Krazan&Associates,Inc. Offices Serving The Western United States KA No. 102-01036 • , May 30,2001 Page No. 10 inches below the lowest adjacent exterior finished grade and a minimum of 60 inches back of the finished soil slope face. , 4 For foundations bearing on the medium dense to dense in situ soils a preliminary allowable soil bearing 14. capacity of 1,800 psf may be used. Additional foundation design considerations should be in accordance with Uniform Building Code requirements, as modified by local codes and regulations, in effect at the time of construction, for structures within seismic zone 3 as defined by the Uniform Building Code (1997) or the UBC seismic zone in effect at the time of construction. It is recommended that all foundation excavations be inspected by Krazan & Associates, prior to placing concrete, to verify that the bearing surface has been properly cleaned,prepared and soil conditions are as anticipated. All foundation subgrade areas should be recompacted following excavation. Bearing surfaces should be firm and free of sloughed or water-softened soil. Preliminary Cast-In-Place Retaining and Subsurface Wall Recommendations The following earth pressures and preliminary design values are provided for cast-in-place retaining and subsurface walls up to ten feet in height. It is recommended that foundations for all retaining structures and subsurface walls be designed and constructed as previously described under the Foundation Design section of this report. Retaining and subsurface walls should be designed for an active equivalent fluid pressure of 35 pcf,if the top of the wall is allowed to deflect,assuming a horizontal ground surface behind the wall. If the top of the wall is restrained an equivalent fluid pressure of 50 pcf is recommended. Active or at rest pressures will need to be increased for sloping ground or surcharge loads behind the wall. Ultimate passive pressures for retaining structures,considering a horizontal ground surface,will be 150 pcf. Passive pressures will need to be reduced for a sloping ground condition in front of the wall. Additional resistance to sliding can be developed through base friction. A coefficient of friction between the footing • and soil of 0.32 should be used. An appropriate safety factor should be applied to the above resistive values when calculating resistive values. 1 J The above-recommended pressures do not include the effects of hydrostatic pressure on the wall as they assume a drained condition exists. The maintenance of a dewatered/drained condition behind all retaining 1 structures is required for the above values to be valid. The following drain system and backfill requirements are recommended. Krazan&Associates,Inc. Offices Serving The Western United States KA No. 102-01036 • May 30, 2001 Page No. 11 A longitudinal subdrain with a minimum diameter of 4 inches should be constructed at the footing elevation behind the walls. This drain should be constructed of a 4-inch diameter perforated pipe laid perforations down, bedded in an eighteen-inch envelope of free-draining sand and gravel. This system should be sloped to drain and the water disposed of in the storm drainage system. Clean-outs should be provided at bends and convenient intervals, so that the drainage system can be maintained in a well- functioning condition. Flexible plastic piping(such as corrugated ADS-type piping) should not be used • behind the wall. Roof and parking area drainage systems should not be connected to the wall subdrain system, but may utilize the same tight-line outfall well away from the wall. All wall backfill over the gravel envelope should consist of clean, free-draining, well-graded sand and gravel containing less than 2.0% fines (material passing an U.S. No. 200 sieve). This material should extend out from the rear wall face a minimum of eighteen inches. The free-draining backfill should be placed to the surface in paved areas or to within eighteen inches of the surface in non-paved areas. Backfill should be compacted as recommended above for fills. In non-paved areas, the final eighteen inches of backfill should consist of topsoil or native materials firmly tamped into place. • Construction Considerations As a preliminary guideline for temporary cuts less than 10 feet in height we recommend temporary slopes be made no steeper than 1.5H: 1V for the loose to medium dense silty soils and 1H: 1V in the dense soils. For temporary cut slopes over 10 feet in height we recommend temporary slopes no steeper than 2H:1 V for the full height of the cut. Temporary slopes or excavations should be benched as required by safety • regulations in effect at the time of construction. These temporary slope recommendations are for native soils and fill materials; flatter slopes may be required in wet weather or if soil conditions other than those • previously described are encountered. The contractor should be aware that slope height, slope inclination, and excavation depths (including utility trench excavations) should in no case exceed those specified in • local, state, or federal safety regulations; e.g., OSHA Health and Safety Standards for Excavations, 29 CFR Part 1926, or successor regulations. Such regulations are strictly enforced and, if not followed, the * owner, the contractor, or the earthwork or utility subcontractors could be liable for substantial penalties. The contractor should be made responsible for the stability of all excavations and slopes during j construction because he is continually on site and can observe the stability of the exposed soils. In 1 addition, the contractor should be prepared to shore any unstable slope area and provide shoring as a required by local, state, or federal laws or codes. 1 In no case should excavated soils be placed on the slope or stockpiled within the defined buffer or slope setback areas along the steep slopes or within 20 feet of the top of any other existing or excavated slope, Krazan&Associates, Inc. Offices Serving The Western United States • KA No. 102-01036 ; May 30, 2001 Page No. 12 rockery or retaining structure. Failure to comply with these guidelines may lead to destabilization of the slope. The site soils may be easily eroded by channelized water or sheet flow storm runoff. Therefore, it is recommended that all site preparation and excavation work be completed during the normally drier *Y` portion of the year. During periods of heavy rainfall, ditching should be used to divert water away from 1 stripped areas and visqueen should be used to cover the slopes and soil stockpiles to aid in preventing excessive surface erosion. This covering also aids in preventing infiltration of water into the unprotected soils. All disturbed soil areas and slopes should be replanted with fast-growing, deep-rooted grass, shrubs and other ground cover as soon after final grading as possible. If the vegetation is not fully established prior to the on set of wet weather, the slopes should be covered with visqueen to aid in preventing excessive erosion and water infiltration. It should be anticipated that there could be a number of additional site development or construction problems, particularly, if the earthwork has not been completed and the site properly protected at the onset of wet weather. It is recommended that the architect, structural engineer or their representative make periodic inspections of all excavations and slopes to provide early recognition and recommendations. REPORT LIMITATIONS This report has been prepared for the exclusive use of Zane and Pam Taylor and their agents for use in planning of the referenced development. The conclusions and recommendations in this report are based on our interpretation of site conditions as they presently exist, anticipated future construction activities, and the expectation that the exploratory efforts adequately define the subsurface conditions throughout the building site. The soil conditions described in this report and the conclusions and recommendations contained in this report are provided for this specific site only and should not be expanded for use on adjacent properties without additional exploration and review of those sites by our firm. The data and report should be provided to prospective contractors for their bidding or estimating purposes, but the report conclusions and interpretations should not be construed as a warranty of the subsurface conditions. There are possible variations in subsurface conditions. In the event that the scope or location of the project should change or subsurface conditions different from those encountered during this study be observed or suspected, we should be advised. At that time a review of the changed conditions will be made, and alternative or remedial recommendations given as required. Krazan &Associates,Inc. Offices Serving The Western United States