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HomeMy WebLinkAboutSoils & Foundation Investigation 02.14.2008February 14, 2008 G. H. Daniels & Associates, Inc. 140 G. H. Daniels Boulevard Gypsum, CO 81637 Attention: Mr. Jody Daniels Subject: Soils and Foundation Investigation Lots 1 through 7 Daniels/Hasenberg Subdivision Garfield County, Colorado Project No. GS05135-120 CTL ITHOMPSON I N C O R P O R A T E D CTL 1 Thompson, Inc. performed a geologic evaluation and preliminary geotechnical investigation for the Daniels/Hasenberg Subdivision under this project number, dated January 11, 2008. You asked if sufficient information was obtained to provide design -level criteria for the planned residential construction. In our opinion, we performed sufficient subsurface sampling to provide design - level soils and foundation investigations for the seven (7) lots that comprise the subdivision. However, we should be called to observe subsurface conditions exposed in the completed foundation excavations for each lot to check that conditions are as anticipated and appropriate for the foundation systems recommended. This soils and foundation letter should be used in conjunction with our geologic evaluation and preliminary geotechnical investigation. Proposed Construction Building plans for the residences were not developed at the time of our investigation. If construction will differ significantly from the descriptions below, we should be informed so that we can adjust our recommendations and design criteria as necessary. We expect the proposed residences will be one or two- story, wood -frame buildings with attached garages. A basement and/or crawl space may be constructed below the buildings. Similar residences in the area are typically constructed with slab -on -grade floors in basement and garage areas. Maximum foundation excavation depths will likely be on the order of 7 to 9 feet if a basement is constructed. Foundation loads are expected to vary between 1,000 and 3,000 pounds per lineal foot of foundation wall with maximum interior column loads of 30 kips. Completed wall backfill depth may be slightly more than excavation depth as final grades are adjusted for drainage. Earthwork We anticipate excavations for the building foundations and utilities can be accomplished using conventional, heavy-duty excavation equipment. Excavation sides will need to be sloped or braced to meet local, state and federal safety 234 Center Drive 1 Glenwood Springs, Colorado 81601 Telephone: 970-945-2809 Fax: 970-945-7411 regulations. We believe the soils at this site will generally classify as Type B and Type C soils based on OSHA standards governing excavations. Temporary slopes deeper than 5 feet should be no steeper than 1 to 1 (horizontal to vertical) in Type B soils and 1.5 to 1 in Type C soils. Contractors should identify the soils encountered in the excavations and refer to OSHA standards to determine appropriate slopes. We do not anticipate excavations for foundations or utilities will encounter significant amounts of ground water. However, excavations should be sloped to a gravity discharge or to a temporary sump where water can be removed by pumping. The ground surrounding the excavations should be sloped as much as practical to direct runoff away from the excavations. Fill may be required to obtain subgrade elevations for the garage floor and exterior concrete flatwork. Areas which will receive fill should be stripped of vegetation, organic soils and debris. The on-site soils free of rocks larger than 3 inches in diameter, organic matter, and debris are suitable for use as fill below new construction. Fill below structures should be placed in loose lifts of 10 inches thick or less, moisture conditioned to within 2 percent of optimum moisture content, and compacted to at least 100 percent of standard Proctor (ASTM D 698) maximum dry density. Moisture content and density of fill should be checked by a representative of our firm during placement. Properly placed backfill adjacent to foundation wall exteriors is important to reduce infiltration of surface water and subsequent consolidation. Backfill placed adjacent to foundation wall exteriors should be free of organic matter, debris and rocks larger than 3 inches in diameter. Backfill should be moisture conditioned to within 2 percent of optimum moisture content and compacted to at least 95 percent of standard Proctor (ASTM D 698) maximum dry density. Foundation We recommend constructing the residences on footings constructed on the undisturbed, natural sand or gravel soil. Criteria for construction of footings is presented below. 1. Footing foundations should be supported entirely on the natural sands or gravels. Soils loosened during the excavation or forming process for the footings should be removed or the soils can be re - compacted prior to placing concrete. 2. Footings on the natural sands or gravels should be designed for a maximum soil bearing pressure of 4,000 psf. 3. Continuous wall footings should have a minimum width of at least 16 inches. Foundations for isolated columns should have minimum dimensions of 24 inches by 24 inches Larger sizes may be required, depending upon foundation loads. G. H. DANIELS & ASSOCIATES, INC. LOTS 1 THROUGH 7 DANIELS/HASENBERG SUBDIVISION PROJECT NO. GS05135 120 S:\G505135.000112013. Letters \GS05135120 L1.doc 2 4. Grade beams and foundation walls should be well reinforced, top and bottom. We recommend reinforcement sufficient to span an unsupported distance of at least 10 feet. Reinforcement should be designed by the structural engineer. 5. The soils under exterior footings should be protected from freezing. We recommend the bottom of footings be constructed at a depth of at least 36 inches below finished exterior grades for frost protection. The Garfield County building department should be consulted to verify the required depth. Floor Systems and Slab -on -Grade Slab -on -grade floors will likely be constructed in garage and some lower level living areas of the residences. Based on our laboratory test data and our experience, we judge slab -on -grade construction can be supported by the undisturbed, natural sand or gravel or densely compacted structural fill with the on-site gravel free of deleterious material and rock larger than 3 inches in diameter or Class 6 aggregate base course with low risk of differential movement and associated damage. Slabs should be separated from exterior walls and interior bearing members with slip joints which allow free vertical movement of the slabs. Underslab plumbing should be pressure tested for leaks before the slabs are constructed. Plumbing and utilities which pass through slabs should be isolated from the slabs with sleeves and provided with flexible couplings to slab supported appliances. Exterior patio and porch flatwork should be isolated from the residence. These slabs should be well -reinforced to function as independent units. Frequent control joints should be provided, in accordance with American Concrete Institute (ACI) recommendations, to reduce problems associated with shrinkage and curling. The 2003 International Building Code (IBC) or 2003 International Residential Code (IRC) may require a vapor retarder be placed between the base course or subgrade soils and the concrete slab -on -grade floors. The merits of installation of a vapor retarder below floor slabs and PT slabs depend on the sensitivity of floor coverings and building to moisture. A properly installed vapor retarder (10 mil minimum) is more beneficial below concrete slab -on -grade floors where floor coverings, painted floor surfaces or products stored on the floor will be sensitive to moisture. The vapor retarder is most effective when concrete is placed directly on top of it. A sand or gravel leveling course should not be placed between the vapor retarder and the floor slab. The placement of concrete on the vapor retarder may increase the risk of shrinkage cracking and curling. Use of concrete with reduced shrinkage characteristics including minimized water content, maximized coarse aggregate content, and reasonably low slump will reduce the risk of shrinkage cracking and curling. Considerations and recommendations for the installation of vapor retarders below concrete slabs are outlined in Section 3.2.3 of the 2003 report of American Concrete Institute (ACI) Committee 302, "Guide for Concrete Floor and Slab Construction (ACI 302.R-96)". G. H. DANIELS & ASSOCIATES, INC. LOTS 1 THROUGH 7 DANIELS/HASENBERG SUBDIVISION PROJECT NO. GS05135 120 5:IG505135.000112013. Letters\G505135120 L1.doc 3 Below -Grade Construction Foundation walls which extend below -grade should be designed for lateral earth pressures where backfill is not present to about the same extent on both sides of the wall. Many factors affect the values of the design lateral earth pressure. These factors include, but are not limited to, the type, compaction, slope and drainage of the backfill, and the rigidity of the wall against rotation and deflection. For a very rigid wall where negligible or very little deflection will occur, an "at -rest" lateral earth pressure should be used in design. For walls which can deflect or rotate 0.5 to 1 percent of wall height (depending upon the backfill types), lower "active" lateral earth pressures are appropriate. Our experience indicates that typical basement walls can deflect or rotate slightly under normal design Toads, and that this deflection results in satisfactory wall performance. Thus, the earth pressures on the walls will likely be between the "active" and "at -rest" conditions. If the on-site soils are used as backfill, we recommend design of below - grade walls using an equivalent fluid density of at least 45 pcf for this site. This equivalent density does not include allowances for sloping backfill, surcharges or hydrostatic pressures. The recommended equivalent density assumes deflection; some minor cracking of walls may occur. If very little wall deflection is desired, a higher equivalent fluid density may be appropriate for design. Our recent experience indicates most basement and below -grade walls designed with 45 pcf to 50 pcf equivalent fluid density have performed satisfactorily. Backfill should be placed and compacted in accordance with the recommendations outlined in the Earthwork section Subsurface Drainage Water from rain, snow melt and surface irrigation of landscaping frequently flows through relatively permeable backfill placed adjacent to a residence and collects on the surface of relatively undisturbed soils at the bottom of the excavation. This can cause wetting of foundation soils after construction. To mitigate these concerns, we recommend provision of a foundation drain around the below -grade areas of the residences. The drain should consist of a 4 -inch diameter, slotted pipe encased in free draining gravel. The drain should lead to a positive gravity outfall, or to a sump pit where water can be removed by pumping. Typical foundation drain details are presented on Figures 1 and 2. Surface Drainage Surface drainage is critical to the performance of foundations, floor slabs and concrete flatwork. We recommend the following precautions be observed during construction and maintained at all times after the residence is completed: 1. The ground surface surrounding the exterior of the residence should be sloped to drain away from the residence in all directions. We G. H. DANIELS & ASSOCIATES, INC. LOTS 1 THROUGH 7 DANIELS/HASENBERG SUBDIVISION PROJECT NO. G505135 120 59G505135.000112013. Letters10505135120 1.1.doo 4 recommend providing a slope of at least 12 inches in the first 10 feet around the residence, where possible. In no case should the slope be less than 6 inches in the first 5 feet. 2. Backfill around the exterior of foundation walls should be placed in maximum 10 inch thick loose lifts, moisture conditioned to within 2 percent of optimum moisture content and compacted to at least 95 percent of standard Proctor (ASTM D 698) maximum dry density. 3. The residence should be provided with roof gutters and downspouts. Roof downspouts and drains should discharge well beyond the limits of all backfill. Splash blocks and downspout extensions should be provided at all discharge points. 4. Landscaping should be carefully designed to minimize irrigation. Plants used near foundation walls should be limited to those with low moisture requirements; irrigated grass should not be located within 5 feet of the foundation. Sprinklers should not discharge within 5 feet of the foundation and should be directed away from the building. 5. Impervious plastic membranes should not be used to cover the ground surface immediately surrounding the residence. These membranes tend to trap moisture and prevent normal evaporation from occurring. Geotextile fabrics can be used to control weed growth and allow some evaporation to occur. Limitations This investigation was conducted in a manner consistent with that level of care and skill ordinarily exercised by geotechnical engineers currently practicing under similar conditions in the locality of this project. We should observe condition in excavations to verify that soils are appropriate for support of the foundation system as designed. A representative of our office should check the moisture and density of fill soils. No warranty, express or implied, is made. Very Truly Yours CTL 1 THOMPSON, INC. Reviewed by: Edward R. White, E.I. John Mechling, P.E. Staff Engineer Branch Manager ERW:JM:cd (7 copies sent) G. H. DANIELS & ASSOCIATES, INC. LOTS 1 THROUGH 7 DANIELS/HASENBERG SUBDIVISION PROJECT NO. 6505135120 S:1GS05135.000112013. Letters1G805135120 L1.doc 5 SLOPE PER OSHA SLOPE PER REPORT BACKFILL (COMPOSNION AND COMPACTION PER REPORT) BELOW GRADE WALL NOTE: DRAIN SHOULD BE AT LEAST 2 INCHES BELOW BOTTOM OF FOOTING AT THE HIGHEST POINT AND SLOPE DOWNWARD TO A POSITNE GRAVITY OUTLET OR TO A SUMP WHERE WATER CAN BE REMOVED BY PUMPING. ENCASE PIPE IN WASHED CONCRETE AGGREGATE (ASTM C33, NO. 57 OR NO. 67) EXTEND GRAVEL TO AT LEAST 1/2 HEIGHT OF FOOTING. COVER GRAVEL WITH FILTER FABRIC. REINFORCING STEEL PER STRUCTURAL DRAWINGS PROVIDE POSITIVE SUP JOINT BETWEEN SLAB AND WALL. FLOOR SLAB MINIMUM 8" MINIMUM OR BEYOND 1:1 SLOPE FROM BOTTOM OF FOOTING. (WHICHEVER IS GREATER) 4—INCH DIAMETER PERFORATED DRAIN PIPE. THE PIPE SHOULD BE LAID IN A TRENCH WITH A MINIMUM SLOPE OF 0.5 PERCENT. Project No. GS05135-120 FOOTING OR PAD PROVIDE POLYETHYLENE SHEETING GLUED TO FOUNDATION WALL TO REDUCE MOISTURE PENETRATION Exterior Foundation Wall Drain Fig. 1 Ltr. 2/14/08 SLOPE PER OSHA SLOPE PER REPORT K - BACKFILL (COMPOSITION MID COMPACTION PER REPORT) BELOW GRADE WALL PROVIDE POLYETHYLENE SHEETING GLUED TO FOUNDATION WALL TO REDUCE MOISTURE PENETRATION COVER GRAVEL WITH FILTER FABRIC ................ :rr}- b— NOTE: DRAIN SHOULD BE AT LEAST 2 INCHES BELOW BOTTOM OF FOOTING AT THE HIGHEST POINT AND SLOPE DOWNWARD TO A POSITIVE GRAVITY OUTLET OR TO A SUMP WHERE WATER CAN BE REMOVED BY PUMPING. STRUCTURALLY SUPPORTEDJr FLOOR SYSTEM A - CRAWL SPACE REINFORCING STEEL PER STRUCTURAL DRAWINGS. FOOTING OR PAD ENCASE PIPE IN WASHED -' "J orr 2" MINIMUM i CONCRETE AGGREGATE (ASTM C33, NO. 57 OR NO. 67). EXTEND GRAVEL TO AT LEAST —4r MINIMUM — BOTTOM OF 1/2 HEIGHT OF FOOTING. OR BEYOND 1:1 EXCAVATION SLOPE FROM BOTTOM OF FOOTING. (WHICHEVER IS GREATER) 4 -INCH DIAMETER PERFORATED DRAIN PIPE. THE PIPE SHOULD BE LAID IN A TRENCH WITH A SLOPE RANGE BETWEEN 1/8 INCH AND 1/4 INCH DROP PER FOOT OF DRAIN. Exterior Foundation Wall Drain Project No. GS05135-120 Fig. 2 Ltr. 2/14/08