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HomeMy WebLinkAboutB15-0213_B15-0213 Open Pit report_1441922580.pdfSUBSOIL STUDY FOH FOUNDATION DESIGN PROPOSED GAU.AGE ADDITION TO COLLIS RESIDENCE LOT 6, VAIL VILLAGE \VEST, FILING 2 1755 \VEST GORE CIU:EK D1uv1~: VAIL, COLOUADO .JOB NO. 115 146A MAY 27, 2015 PREPARED FOR: .JOHN AND .JUDY COLLIS P. 0. BOX 3363 llAMILTON JIM PX BEHMUDA D ~©~~w~~ SEP 1 o 2015 ~ TOWN OF VAIL ___ _i I' TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY .......................................................................... -1 - PROPOSED CONSTRUCTION ................................................................................... -1 - SITE CONDITIONS .................................................................................................... -2 - GEOLOGIC CONDITIONS ........................................................................................ :-2 - FIELD EXPLORATION .............................................................................................. -2 - SUBSURFACE CONDITIONS ................................................................................... :-3 - FOUNDATION BEARING CONDITIONS ................................................................. -3 - DESIGN RECOMMENDATIONS ............................................................................... -4 - FOUNDATIONS ...................................................................................................... -4 - FOUNDATION AND RETAINING WALLS .......................................................... :-5 - FLOOR SLABS ........................................................................................................ -6 - UNDERDRAIN SYSTEM ....................................................................................... :-7 - SURF ACE DRAINAGE .......................................................................................... ;-7 - LIMITATIONS ............................................................................................................ -8 - REFERENCES ............................................................................................................. -9 - FIGURE 1 -LOCATION OF EXPLORATORY BORING FIGURE 2 -LOG OF EXPLORATORY BORING FIGURE 3 -LEGEND AND NOTES FIGURE 4 -SWELL-CONSOLIDATION TEST RESULTS TABLE 1 -SUMMARY OF LABORATORY TEST RESULTS Job No. 115 146A ~tech PURPOSE AND SCOPE OF STUDY This report presents the results of a subsoil study for a proposed garage addition to the Collis residence located on Lot 6, Vail Village West, Filing 2, 1755 West Gore Creek Drive, Vail, Colorado. The project site is shown on Figure 1. The purpose of the study was to develbp recommendations for the foundation design. The study was conducted in accordance with our agreement for geotechnical engineering services to John and Judy Collis dated April 13, 2015. · A field exploration program consisting of an exploratory boring was conducted to obtain information on the subsurface ~onditions. Samples of the subsoils obtained during the field exploration were tested in the laboratory to determine their classification, compressibility or swell and other engineering characteristics. The results of the field exploration and laboratory testing were analyzed to develop recommendations for foundation types, depths and allowable pressures for the proposed building foundation. This report sununarizes the data obtained during this study and presents our conclusions, design recommendations and other geotechnical engineering considerations based on the proposed construction and the subsurface conditions encountered. PROPOSED CONSTRUCTION The garage addition will be attached to the east side of the existing 2 to 3 story residence as shown on Figure 1. The addition will be 2 story wood frame construction over a walkout basement level. Ground floor will be slab-on-grade. Grading for the structure is assumed to be relatively minor with cut depths between about 3 to 10 feet. We assume relatively light foundation loadings, typical of the proposed type of construction. If building loadings, location or grading plans change significantly from those described above, we should be notified to re-evaluate the recommendations contained in this repo1i. Job No. 115 146A ~tech -2- SITE CONDITIONS The lot is occupied by the existing residence as shown on Figure 1. The addition area has been graded with a retaining wall and fill up to about 10 feet deep for parking. The parking surface is concrete. The terrain below the retaining wall appears mostly natural and is strongly sloping down to Gore Creek which crosses the northwest part of the lot. Vegetation is primarily landscape plants with pine and spruce trees. There are several other retaining walls at the site. Elevation difference across the proposed addition area is about 1 0 feet. GEOLOGIC CONDITIONS There are no apparent geologic hazards at the lot per the town of Vail Geologic Hazards maps (Town of Vail, 2000a, Town of Vail, 2000b and Town of Vail, 2000c). FIELD EXPLORATION The field exploration for the project was conducted on May 13, 2015. One exploratory boring was drilled at the location shown on Figure 1 to evaluate the general subsurface conditions. The boring was advanced with 4 inch diameter continuous flight augers powered by a truck-mounted CME-45B drill 1ig. Access for additional borings was not possible to the retaining walls and sloping ten-a.in. The boring was logged by a· representative ofHepw01ih-Pawlak Geotechnical, Inc. Samples of the subsoils were taken with 1% inch and 2 inch I.D. spoon samplers. The samplers were driven into the subsoils at va1ious depths with blows from a 140 pound hammer falling 30 inches. This test is similar to the standard penetration test described by ASTM Method D-1586. The penetration resistance values are an indication of the relative density or consistency of the subsoils. Depths at which the samples were taken and the penetration resistance values are shown on the Log of Explorat01y Boring, Figure Job No. 115 !46A ~tech - 3 - 2. The samples were returned to our laboratory for review by the project engineer and testing. SUBSURFACE CONDITIONS A graphic log of the subsurface conditions encountered at the site is shown on Figure 2. The subsoils encountered, below about 6 inches of concrete pavement, consisted of generally medium dense, very clayey sand with gravel fill underlain at about 6Yi feet by medium dense, clayey silty sand and gravel with cobbles that extended down to the depth drilled of 16 feet. Drilling in the granular soils with auger equipment was difficult due to the cobbles and possible boulders and drilling refusal was encountered in the deposit. Laboratory testing performed on samples obtained from the bo1ing included natural moisture content and density, and percent finer than sand size gradation analyses. Results of swell-consolidation testing performed on relatively undisturbed drive samples, presented on Figure 4, indicate low to moderate compressibility under conditions of loading and wetting. The samples could have been paiily disturbed due to the rock content. The laboratory testing is summarized in Table 1. No free water was encountered in the bo1ing at the time of drilling and the subsoils were slightly moist to moist. FOUNDATION BEARING CONDITIONS The natural granular soils possess generally moderate capacity and low to moderate settlement potential. Spread footings bearing on the natural soils should be suitable for foundation support of the addition with some risk of settlement. The settlement may be differential with respect to the existing structure which should be considered in the design. Job No. I 15 146A ~tech -4- Care should be taken not to undennine the existing building foundation, which we assume to be supported on shallow spread footings. Undermining of the foundation can be avoided by providing a 1 horizontal to 1 vertical or flatter slope of natural soils below and beyond the existing footings. The existing foundation could also be underpinned to avoid undermining. DESIGN RECOMMENDATIONS FOUNDATIONS Considering the subsurface conditions encountered in the exploratory boring and the nature of the proposed construction, we recommend the building be founded with spread footings bearing on the natural granular soils. The design and construction criteria presented below should be observed for a spread footing foundation system. · 1) Footings placed on the undisturbed natural granular soils should be designed for an a11owable bearing pressure of2,000 psf. Based on experience, we expect settlement of footings designed and constructed as discussed in this section will be up to about 1 inch. 2) The footings should have a minimum width of 18 inches for continuous walls and 2 feet for isolated pads. 3) Exterior footings and footings beneath unheated areas should be provided with adequate soil cover above their bearing elevation for frost protection. Placement of foundations at least 48 inches below exterior grade is typically used in this area. 4) Continuous foundation walls should be well reinforced top and bottom to span local anomalies such as by assuming an unsupported length of at least 12 feet. Foundation walls acting as retaining structures should also be designed to resist lateral earth pressures as discussed in the "Foundation and Retaining Walls" section of this report. Job No. 115 146A ~tech -5 - 5) All existing fill, debris, topsoil and any loose or disturbed soils should be removed and the footing bearing level extende.d down to the firm natural granular soils. The exposed soils in footing area should then be moistened and compacted. 6) A representative of the geotechnicaJ engineer should observe all footing excavations prior to concrete placement to evaluate bearing conditions. FOUNDATION AND RETAINING WALLS Foundation walls and retaining structures which are laterally supported and can be expected to undergo only a slight amount of deflection should be designed for a lateral earth pressure computed on the basis of an equivalent fluid unit weight of at least 50 pcf for backfill consisting of the on-site granular soils. Cantilevered retaining structures which are separate from the main building and can be expected to deflect sufficiently to mobilize the full active earth pressure condition should be designed for a lateral earth pressure computed on the basis of an equivalent fluid unit weight of at least 45 pcf for backfill consisting of the on-site granular soils. The backfill should not contain debris, topsoil or oversized (plus 6 inch) rocks. All foundation and retaining structures should be designed for appropriate hydrostatic and surcharge pressures such as adjacent footings, traffic, constrnction materials and equipment. The pressures recommended above assume drained conditions behind the walls and a horizontal backfill surface. The buildup of water behind a wall or an upward sloping backfill surface will increase the lateral pressure imposed on a foundation wall or retaining strncture. An underdrain should be provided to prevent hydrostatic pressure buildup behind walls. Backfill should be placed in unifonn lifts and compacted to at least 90% of the maximum standard Proctor density at a moisture content near optimum. Backfill placed in pavement and walkway areas should be compacted to at least 95% of the maximum standard Proctor density. Care should be taken not to overcompact the backfill or use Job No. 115 146A ~tech - 6 - large equipment near the wall, since this could cause excessive lateral pressure on the wall. Some settlement of deep foundation wall backfill should be expected, even if the material is placed correctly, and could result in distress to facilities constructed on the backfill. Use of a select granular material such as road base and increasing compaction to at least 98% standard Proctor density could be done to reduce the backfill settlement potential. The lateral resistance of foundation or retaining wall footings will be a combination of the sliding resistance of the footing on the foundation materials and passive earth pressure against the side of the footing. Resistance to sliding at the bottoms of the footings can be calculated based on a coefficient of friction of 0.45. Passive pressure of compacted backfill against the sides of the footings can be calculated using an equivalent fluid unit weight of 375 pcf. The coefficient of friction and passive pressure values recommended above assume ultimate soil strength. Suitable factors of safety should be included in the design to limit the strain which will occur at the ultimate strength, particularly in the case of passive resistance. Fill placed against the sides of the footings to resist lateral loads should be compacted to at least 95% of the maximum standard Proctor density at a moisture content near optimum. FLOOR SLABS The natural on-site soils, exclusive of topsoil, are suitable to support lightly loaded slab- on-grade construction. The suitability of the existing fill to support slabs should be fmiher evaluated at the time of construction. To reduce the effects of some differential movement, floor slabs should be separated from all beaiing walls and columns with expansion joints which allow unrestrained vertical movement. Floor slab control joints should be used to reduce damage due to shrinkage cracking. The requirements for joint spacing and slab reinforcement should be established by the designer based on experience and the intended slab use. A minimum 4 inch layer of free-draining gravel should be placed beneath basement level slabs to facilitate drainage. This material should consist of Job No. 115 146A ~tech -7 - minus 2 inch aggregate with at least 50% retained on the No. 4 sieve and less than 2% passing the No. 200 sieve. All fill materials for support of floor slabs should be compacted to at least 95% of maximum standard Proctor density at a moisture content near optimum. Required fill can consist of the on-site granular soils devoid of debris, topsoil and oversized (plus 6 inch) rocks. UNDERDRAIN SYSTEM Although free water was not encountered during our exploration, it has been our experience in the area and where clayey soils are present that local perched groundwater can develop during times of heavy precipitation or seasonal runoff. Frozen ground during spring runoff can also create a perched ·condition. We recommend below-grade constrnction, such as retaining walls, crawlspace and basement areas, be protected from wetting and hydrostatic pressure buildup by an underdrain system. The drains should consist of drainpipe placed in the bottom of the wall backfill surrounded above the invert level with free-draining granular material. The drain should be placed at each level of excavation and at least 1 foot below lowest adjacent finish grade and sloped at a minimum 1 % to a suitable gravity outlet. Free-draining granular material used in the underdrain system should contain less than 2% passing the No. 200 sieve, less than 50% passing the No. 4 sieve and have a maximum size of2 inches. The drain gravel should be covered with filter fabric and be at least 1 Yi feet deep. SURF ACE DRAINAGE The following drainage precautions should be observed dming construction and maintained at all times after the addition has been completed: 1) Inundation of the foundation excavations and underslab areas should be avoided during construction. Job No. 115 146A ~tech - 8 - 2) Exterior backfill should be adjusted to near optimum moisture and compacted to at least 95% of the maximum standard Proctor density in pavement and slab areas and to at least 90% of the maximum standard Proctor density in landscape areas. 3) The ground surface surrounding the exterior of the building should be sloped to drain away from the foundation in all directions. We recommend a minimum slope of 12 inches in the first 10 feet in unpaved areas and a minimum slope of 3 inches in the first 10 feet in paved areas. Free-draining wall backfill should be capped with filter fab1ic at least 2 feet of the on-site finer graded soils to reduce surface water infiltration. 4) Roof downspouts and drains should discharge well beyond the limits of all backfill. 5) Landscaping which requires regular heavy irrigation should be located at least 5 feet from foundation walls. LIMITATIONS This study has been conducted in accordance with generally accepted geotechnical engineering principles and practices in this area at this time. We make no waITanty either express or implied. The conclusions and recommendations submitted in this report are based upon the data obtained from the exploratory boring drilled at the location indicated on Figure 1, the proposed type of construction and our experience in the area. Our services do not include dete1mining the presence, prevention or possibility of mold or other biological contaminants (MOBC) developing in the future. If the client is concerned about MOBC, then a professional in this special field of practice should be consulted. Our findings include interpolation and extrapolation of the subsurface conditions identified at the exploratory boring and variations in the subsurface conditions may not become evident until excavation is perfonned. If conditions encountered during construction appear different from those described in this report, we should be notified so that re-evaluation of the recommendations may be made. Job No. 115 146A ~tech -9 - This report has been prepared for the exclusive use by our client for design purposes. We are not responsible for technical interpretations by others of our infonnation. As the project evolves, we should provide continued consultation and field services during construction to review and monitor the implementation of our recommendations, and to verify that the recommendations have been appropriately interpreted. Significant design changes may require additional analysis or modifications to the recommendations presented herein. We recommend on-site observation of excavations and foundation bearing strata and testing of structural fill by a representative of the geotechnical engineer. Respectfully Submitted, Stcwn L. Pawlak, P.E. DAY/ksw cc: R1\ Nelson and Associates -Ryan L1Vire ( : , I ' ) REFEHENCl•:S Town of Vail, 2000. Official Roclflall Hazard Map, Town of Vail, Prepared by the Town of Vail, Vail, Colorado (Adopted by the Town Council on October 17, 2000). Town of Vail, 2000. Official Debris Flmv Hazard A-fap, To-wn of' Vail. Prepared by the Town of Vail, Vail, Colorado (Adopted by the Town Council on October 17, 2000). Tovv n uf Vail, 2000. O;Jiciai Avalanche Haw rd 1vlap, J'own (~l Vail. Pn.:pared by the Town of Vail, Vail, Colorado (Adopted by the Town Council on October 17, 2000). -------------------------~------~· -------··--------~-·------·--·--------·------------ Job No. I J 5 !46A c~6tech LOT? 115 146A EXISTING DECK ------- EXISTING RESIDENCE 1755 W. GORE CREEK DRIVE LOT6 -- -------- APPROXIMATE SCALE 1" :::: 20' -.:.. .. -------7952 " ----'-.. \ -' /954 --7656 ....._ -.__ __,, ----- --">. & EXISTING <5'9 RETAINING WALL ~ LOCATION OF EXPLORATORY BORING Figure 1 He worth-Pawlak Geotechnlcal 7970 7965 ..... Q) Q) LL c 7960 0 ~ > Q) w 7955 7950 115 146A BORING 1 ELEV.= 7968' 20/12 13/12 WC=12.4 00=112 -200=40 45/12 50/5 WC=11.5 00=115 -200=34 NOTE: Explanation of symbols is shown on Figure 3. ~ LOG OF EXPLORATORY BORING He worth-Pawlak Geotechnfcal 7970 7965 ....... Q) (1) LL 7960 c 0 ~ > (1) w 7955 7950 Figure 2 LEGEND: CONCRETE PAVEMENT; about 6 inches thick. FILL: manplaced very clayey silty sand with gravel, medium dense, moist, brown. SAND AND GRAVEL (SM-GM): with cobbles, probable boulders, silty, medium dense to dense with depth, slightly moist to moist, red-brown, rocks are primarily subangular. Relatively undisturbed drive sample; 2-inch l.D. California liner sample. Drive sample; standard penetration test (SPT), 1 3/8 inch 1.0. spilt spoon sample, ASTM-1586. Drive sample blow count; indicates that 20 blows of a 140 pound hammer falling 30 inches were 20/12 required to drive the California or SPT sampler 12 inches. T Practical drilling refusal. NOTES: 1. The exploratory boring was drilled on May 13, 2015 with a 4-inch diameter continuous flight power auger. 2. The exploratory boring location was measured approximately by pacing from features shown on the site plan provided. 3. The exploratory boring elevation was obtained by interpolation between contours on the site plan provided. 4. The exploratory boring location and elevation should be considered accurate only to the degree implied by the method used. 5. The lines between materials shown on the exploratory boring log represent the approximate boundaries between material types and transitions may be gradual. 6. No free water was encountered in the boring at the time of drilling. Fluctuation in water level may occur with time. 7. Laboratory Testing Results: WC= Water Content(%) DD = Dry Density (pen -200 = Percent passing No. 200 sieve 115 146A ~ Hepworth-Powlak Geotechnlcol LEGEND AND NOTES Figure 3 Moisture Content = 12.4 percent Dry Density = 112 pcf Sample of: Very Clayey Silty Sand wih Gravel-Fill From: Boring 1 at 5 feet 1---i----r; 0 '#. I/ Compression c --( upon 0 I~ i..-._L-·u; i--wetting en 1 -<J) ..... '~ n. E 0 ~ 0 2 ' l\D 3 0.1 1.0 10 100 APPLIED PRESSURE -ksf Moisture Content = 11.5 percent Dry Density= 115 pcf Sample of: Clayey Silty Sand with Gravel From: Boring 1 at I 5 feet 0 ,.-. "---,-D 1 " / '#. ~ (_ Compression c upon 0 '(i) wetting (fJ 2 ~ ~ Q. E 0 0 3 \ 4 \ l:l 5 0.1 1.0 10 100 APPLIED PRESSURE -ksf 115 146A ~ SWELL-CONSOLIDATION TEST RESULTS Figure 4 Heoworth-Powlok Geotechniccl HEPWORTH-PAWLAK GEOTECHNICAL, INC. TABLE 1 Job No.115146A SUMMARY OF LABORATORY TEST RES UL TS SAMPLE LOCATION NATURAL NATURAL GRADATION ATTERBERG LIMITS UNCONFINED PERCENT MOISTURE DRY GRAVEL SAND PASSING LIQUID PLASTIC COMPRESSIVE SOIL OR BORING DEPTH CONTENT DENSITY (%) (%) N0.200 LIMIT INDEX STRENGTH BEDROCK TYPE SIEVE (ft) (%) (pct) (%) (%} (PSFl 1 5 12.4 112 40 Very Clayey Silty Sand with Gravel (Fill) 15 11.5 115 34 Clayey Silty Sand with Gravel