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HomeMy WebLinkAboutB17-0161_116204A (06-30-16) Subsoil Study signed_1499293440.pdf p Hepworth-Pawlak Geotechnical, Inc. Gtech 5020 County Road 154 Glenwood Springs, Colorado 81601 Phone: 970-945-7988 HEPWORTH-PAWLAK GEOTECHNICAL Fax: 970-945-8454 Email: hpgeo@hpgeatech.com SUBSOIL STUDY FOR FOUNDATION DESIGN PROPOSED RESIDENCE LOT 18, BLOCK 8, VAIL RIDGE 2655 DAVOS TRAIL VAIL, COLORADO JOB NO. 116 204A JUNE 30,2016 PREPARED FOR: SENTRY CONSTRUCTION ATTN: MIKE YOUNG P. O. BOX 480 EDWARDS, COLORADO 81632 mikeyoung2020em ac.com TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY - I - PROPOSED CONSTRUCTION - 1 - SITE CONDITIONS - 2 - FIELD EXPLORATION - 2 - SUBSURFACE CONDITIONS - 2 - FOUNDATION BEARING CONDITIONS - 3 - DESIGN RECOMMENDATIONS - 3 - FOUNDATIONS - 3 - FOUNDATION AND RETAINING WALLS - 4 - FLOOR SLABS - 6 - UNDERDRAIN SYSTEM _ 6 - SITE GRADING - 7 - SURFACE DRAINAGE - g - LIMITATIONS - g - FIGURE 1 - LOCATION OF EXPLORATORY BORINGS FIGURE 2 - LOGS OF EXPLORATORY BORINGS FIGURE 3 - LEGEND AND NOTES FIGURES 4 and 5 - SWELL-CONSOLIDATION TEST RESULTS TABLE 1- SUMMARY OF LABORATORY TEST RESULTS Job No, 1 I6 204A - Gagtech PURPOSE AND SCOPE OF STUDY This report presents the results of a subsoil study for a proposed residence to be located at Lot I8, BIock 8, Vail Ridge, 2655 Davos Trail, Vail, Colorado. The project site is shown on Figure 1. The purpose of the study was to develop recommendations for the foundation design. The study was conducted in accordance with our agreement for professional services to Sentry Construction dated May 23, 20I6. A field exploration program consisting of exploratory borings was conducted to obtain information on the subsurface conditions. Samples of the subsoils obtained during the field exploration were tested in the Iaboratory 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 summarizes 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 At the time of our study, design plans for the residence had not been developed. The building is assumed to be a two story wood frame structure over a walkout basement and located between the exploratory borings shown on Figure 1. Ground floors could be slab- on-grade or structural above crawlspace. Grading for the structure is assumed to be relatively extensive with cut depths between about 4 to 15 feet. We assume relatively light foundation loadings, typical of the proposed type of construction. When building location, grading and loading information have been developed, we should be notified to re-evaluate the recommendations presented in this report. Joh No. 116 204A �ech - 2 - SITE CONDITIONS The lot was vacant at the time of our field exploration. The site is situated on moderately steep,south facing hillside terrain on the north valley side. The slope has an average grade of about 35%with a steeper road cut at the bottom of the lot. Vegetation on the lot was dominated by grass and weeds with brush and a significant stand of aspen trees occupying the northeast portion of the lot. The adjacent lots are developed. FIELD EXPLORATION The field exploration for the project was conducted on June 8, 2016. Two exploratory borings were drilled at the approximate locations shown on Figure 1 to evaluate the subsurface conditions. The borings were advanced with 4 inch diameter continuous flight augers powered by a track-mounted CME 45 drill rig. The track rig was needed due to the relatively steep slope of the lot. The borings were logged by a representative of Hepworth-Pawlak Geotechnical, Inc. Samples of the subsoils were taken with I% inch and 2 inch I.D. spoon samplers. The samplers were driven into the subsoils at various depths with blows from a I40 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 Logs of Exploratory Borings, Figure 2. The samples were returned to our laboratory for review by the project engineer and testing. SUBSURFACE CONDITIONS Graphic logs of the subsurface conditions encountered at the site are shown on Figure 2. The subsoils consist of about 2 to 2!72 feet of organic topsoil overlying stiff to very stiff, silty sandy clay with scattered gravel and cobbles. A thin layer of gravelly clayey sand Job No, 116 204A - - -- — H - 3 - was encountered immediately below the topsoil in the upper Boring 1. Below the clay soils at Boring 1, dense clayey sand and gravel was encountered at a depth of about 44 feet. The sand and gravel soils were not encountered in the lower Boring 2. Laboratory testing performed on samples obtained from the borings included natural moisture content and density, percent finer than sand size gradation analyses and unconfined compressive strength. Results of swell-consolidation testing performed on relatively undisturbed drive samples of the clay soils, presented on Figures 4 and 5, indicate low to moderate compressibility under conditions of loading and wetting. The unconfined compressive strength tested indicates a stiff consistency. The laboratory testing is summarized in Table 1. No free water was encountered in the borings at the time of drilling and the subsoils were slightly moist to moist. FOUNDATION BEARING CONDITIONS The subsoils encountered at the lot mainly consist of stiff to very stiff, silty sandy clay with low to moderate bearing capacity and relatively low compressibility under light loading. Shallow spread footings placed on the natural soils can be used for building support with relatively low settlement potential. Due to the relatively extensive, expected excavation depth, care should be taken to maintain stability of the hillside including shoring and retaining of excavations as needed. DESIGN RECOMMENDATIONS FOUNDATIONS Considering the subsurface conditions encountered in the exploratory borings and the nature of the proposed construction, we recommend the building be founded with spread footings bearing on the natural soils. Job Na 116 204A Ggistech - 4 - 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 allowable bearing pressure of 2,500 psf. Based on experience, we expect settlement of footings designed and constructed as discussed in this section will be about 1 inch or less. 2) The footings should have a minimum width of 16 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 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. 5) The topsoil and any loose or disturbed soils should be removed and the footing bearing level extended down to the stiff natural soils. The exposed soils in footing area should then be moistened and compacted. If water seepage is encountered, the footing areas should be dewatered before concrete placement. 6) A representative of the geotechnical engineer should observe all footing excavations prior to concrete placement to evaluate bearing conditions. FOUNDATION AND RETAINING WALLS Foundation walls and retaining structures up to 15 feet in retained height 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 Job No. I!6204A -- _ G@Ot@ch - 5 - fluid unit weight of at least 55 pcf for backfill consisting of the on-site soils. Cantilevered retaining structures which are separate from the residence 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 soils. For walls taller than 15 feet, we should review our lateral earth pressure recommendations. All foundation and retaining structures should be designed for appropriate hydrostatic and surcharge pressures such as adjacent footings, traffic, construction 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 structure. An underdrain should be provided to prevent hydrostatic pressure buildup behind walls. Backfill should be placed in uniform lifts and compacted to at least 90% of the maximum standard Proctor density at a moisture content slightly above 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 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. The settlement potential can be reduced by use of a relatively well graded, imported granular soil and increasing compaction to at least 98% of standard Proctor density. 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 0.40. Passive pressure of compacted backfill against the sides of the footings can be calculated using an equivalent fluid unit Job No. 116 204A Gvstech - 6 - 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 Ioaded slab- on-grade construction. To reduce the effects of some differential movement, floor slabs should be separated from all bearing 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 minus 2 inch aggregate with at least 50% retained on the No. 4 sieve and less than 2°o 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 soils or imported granular soils devoid of vegetation, 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 clay soils are present that 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 Joh No, 116 204A - Gtech - 7 - construction, 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 and drainage mat placed on the foundation wall and connected to the underdrain gravel. 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 of 2 inches. The drain gravel backfill should be at least 1' feet deep and covered by filter fabric such as Mirafi 140N or I60N. SITE GRADING There is a risk of construction-induced slope instability at the site due to the relatively steep slope of the lot and the expected relatively extensive excavation depths for the building foundation. We assume the excavation side cuts for the basement level will be sloped back to a stable grade or retained with shoring as needed. Fills should be Iimited to about 8 to 10 feet deep and could need to be retained with walls on the downhill side of the building. Embankment fills should be compacted to at least 95% of the maximum standard Proctor density near optimum moisture content. Prior to fill placement, the subgrade should be carefully prepared by removing all vegetation and topsoil and compacting to at least 95% of the maximum standard Proctor density. The fill should be benched horizontally into the hillside slope. Permanent unretained cut and fill slopes should be graded at 2 horizontal to I vertical or flatter and protected against erosion by revegetation or other means. The risk of slope instability will be increased if seepage is encountered in cuts and flatter slopes may be necessary. If seepage is encountered in permanent cuts, an investigation should be Job No. I 1 G 204A Gecrtecti - 8 - conducted to determine if the seepage will adversely affect the cut stability. We should review site grading plans for the project prior to construction. SURFACE DRAINAGE Positive surface drainage is an important aspect of the project to prevent ‘\euin�g of the bearing materials. The following drainage precautions should be observed during construction and maintained at all times after the residence has been completed: 1) Inundation of the foundation excavations and underslab areas should be avoided during construction. 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 covered with filter fabric and capped with about 2 feet of the on-site 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 warranty either express or implied. The conclusions and recommendations submitted in this report are based upon the data obtained from the exploratory borings drilled at the locations indicated on Figure 1, the proposed type of construction and our experience in the area. Joh No. 116 204A — -- Gtech - 9 - Our services do not include determining 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 borings and variations in the subsurface conditions may not become evident until excavation is performed. 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. 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 information. 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, HEPWORTH - PAWLAK GEOTECHNICAL, INC. .11A— 0000Ilir A 4101 eiTi Steven L. Pawlak P.E. ;'k3 15222 i f s • r 77 '4 ; Reviewed by: • .� �•• + ;te f� °143 David A. Youi g, P. E. SLP+ksw Job No. 116 204A Gtech APPROXIMATE SCALE 1" = 20' 1 LOT 9 BOBO - 1 - - �BO-Kr- BORING 1 -LOT 17 0 / �p6pi LOT 19 LOT 18 i 8pfp • BORING 2 E - - i - f -' 1303°- 1 DAVOS TRAIL 116 204A ~ Ch LOCATION OF EXPLORATORY BORINGS I Figure 1 Hapworth—pawlok Geotechnical BORING 1 BORING 2 ELEV. 8045. ELEV.=8065. - o — „---= a r., .,, i 30/12 —7 — ✓ '• tc _J WC 146 - - 16/12 ' r DD 111 - ' r WC=146 , /] DD.-112 ' ' 10 14/12 10 -200=72 ' WC=-11.7 ' , UC-2250D13 ,114, _ 22/12 r — • '-I WC=13 .2 , 12/12 .___ • / DD 115 / WC=17.1 A ' DD-108 - /..., 23/12 r - ' WC 165 12/12 - - 20 DD 112 ,,D 20 - r - r , 14112 A - ti r 11112 � _ ' ' �i ✓ , — • ' 19/12 r _ B 30 ' .. WC=138 r 30 O ✓ DD-•118 r / .- / r — r r — / r — / , r — - '� 21/12 r - - 40 r �-' r 40 r ' — ' r — ✓ ' _ . TT” / — b ' / — 50/3 r — 50 �� 50 Note Explanation of symbols is shown on Figure 3. H 116 204A LOGS OF EXPLORATORY BORINGS Figure 2 I-IEPWORTH•PAWLAK GEOTECHNICAL LEGEND: ® TOPSOIL; sandy, silty, clayey,with organics, moist, dark brown. E] SAND (SC); gravelly, clayey, medium dense, slightly moist to moist, mixed brown. CLAY(CL); silty, sandy to very sandy, occasional gravel and cobbles, stiff, slightly moist to moist, brown, ' low plasticity. 5 SAND AND GRAVEL(SC-GC); clayey, cobbles, dense, slightly most to moist, mixed brown. 11 Relatively undisturbed drive sample; 2-inch I.D. California liner sample. ill Drive sample; standard penetration test (SPT), 1 3/8 inch i.D. split spoon sample, ASTM D-1586. 30/12 Drive sample blow count; indicates that 30 blows of a 140 pound hammer falling 30 inches were required to drive the California or SPT sampler 12 inches. NOTES: 1. Exploratory borings were drilled on June 8, 2016 with 4-inch diameter continuous flight power auger. 2. Locations of exploratory borings were measured approximately by pac ng from features shown on the site plan provided. 3. Elevations of exploratory borings were estimated from contours shown on the p'an provided.The logs of exploratory borings are drawn to depth. 4. The exploratory boring locations and elevations should be cons.dered accurate only to the degree implied by the method used. 5. The Ines between materials shown on the exploratory boring logs represent the approximate boundaries between materia:types and transitions may be gradual. 6. No free water was encountered ;n the borings at the time of drilling. Fluctuat on in water level may occur with time. 7. Laboratory Testing Results: WC Water Content(%) OD = Dry Density(pcf) -200 = Percent passing No.200 sieve UC = Unconfined (psf) 116 204A 11G6116tech LEGEND AND NOTES Figure 3 ` HEPWORTH-PAWLAK GEOTECHNICAL Moisture Content — 13.2 percent Dry Density = 115 pcf Sample of: Silty Sandy Clay From: Boring 1 at 13 Feet 0 1 M No movement 0 upon n 2 wetting 0 v 3 4 0.1 1.0 10 100 APPLIED PRESSURE-ksf Moisture Content— 16.5 percent Dry Density = 112 pcf Sample of: Silty Sandy Clay From: Boring 1 at 18 Feet 1 No movement upon a 2 wetting U7 • cu Q E 3 o • U 4 r 0.1 1.0 10 100 APPLIED PRESSURE- ksf 116 209A V�'3CJ1:@C "1 SWELL-CONSOLIDATION TEST RESULTS Figure 4 HEPWORTH•PAWLAK GEOTECHNICAL e o w 3 oQ U U U U U U U G in a m c C c G G c C o ez r r r r et cs C0 00 00 C/] CI] CID Z CO C/1 En En VD CD v] v] C7 [/] • W Zit- p LI W 0 i+ in oacvl CI'. 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