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HomeMy WebLinkAboutB14-0418_103 Rockledge Road Subsoil Study_1413299940.pdf 11.1.4 Hepworth-Pawlak Geotechnical,Inc. 5020 County Road 154 Glenwood Springs,Colorado 81601 Phone: 970-945-7988 HEPWORTH-PAWLAK GEOTECHNICAL Fax:970-945-8454 cinail: hpgeo'�_hpgeotech.c,mi SUBSOIL STUDY FOR FOUNDATION DESIGN PROPOSED RESIDENCE LOT 4, BLOCK 7, VAIL VILLAGE FIRST FILING 103 ROCKLEDGE ROAD VAIL, COLORADO JOB NO. 114 198A JUNE 20, 2014 PREPARED FOR: MI WEBB ARCHITECTS ATTN: KYLE WEBB 710 WEST LIONSHEAD CIRCLE, UNIT A VAIL, COLORADO 81657 kvle(i)khvVebb.coni • Parker 303-841-7119 • Colorado Springs 719-633-5562 • Silverthorne 970-468-1989 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 - FLOORSLABS - 6 - UNDERDRAIN SYSTEM - 7 - SITE GRADING - 8 - SURFACE DRAINAGE - 8 - LIMITATIONS - 9 - REFERENCES - 10 - 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. 114 198A Gtech PURPOSE AND SCOPE OF STUDY This report presents the results of a subsoil study for a proposed residence to be located on Lot 4, Block 7, Vail Village First Filing, 103 Rockledge Road, 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 geotechnical engineering services to KH Webb Architects dated May 23, 2014. A field exploration program consisting of exploratory borings was conducted to obtain information on the subsurface conditions. Samples of the subsoils and bedrock 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 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 The residence will be a two story wood frame structure over a walkout basement level daylighting to the northeast and located on the lot as shown on Figure 1. Ground floors will be slab-on-grade. Grading for the structure is assumed to be relatively minor with cut depths between about 4 to 15 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 report. Job No. 114 198A Gtech - 2 - SITE CONDITIONS The lot is vacant and the ground surface appears mostly natural. The terrain is moderately steep to steeply sloping down to the northeast at grades from about 25 to 35% becoming steeper on the order of 40 to 50% in the lower part of the lot. Elevation difference across the proposed residence is about 28 to 30 feet and elevation difference across the lot is about 45 feet. Vegetation consists of grass and weeds with aspen and pine trees. There are existing residences on the adjacent lots to the east, north and west. GEOLOGIC CONDITIONS The site is not located within potential geologic hazard areas for rockfall, debris flow and avalanche according to the Town of Vail mapping. (Town of Vail 2000a, 2000b, and 2000c). The soils appear to consist primarily of colluvial and/or residual soil deposits. The underlying bedrock is the Mintum Formation. The bedding dip of the rock is typically moderate down to the northwest. FIELD EXPLORATION The field exploration for the project was conducted on May 28, 2014. Three exploratory borings were drilled at the 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. Access to the northern portion of the proposed building was not possible due to the steep terrain. The borings were logged by a representative of Hepworth-Pawlak Geotechnical, Inc. Samples of the subsoils and bedrock were taken with 1%inch and 2 inch I.D. spoon samplers. The samplers were driven into the subsoils and bedrock at various 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 and hardness of the Job No. 114 198A Gtech - 3 - bedrock. 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 encountered, below about 1 to 2 feet of organic topsoil, consisted of stiff, sandy to very sandy silty clay underlain at depths from about 4 to 6 feet by limestone bedrock. The limestone bedrock contained some shale layers or zones and was medium hard to very hard with depth. Drilling in the bedrock with auger equipment was difficult due to its hardness with depth and drilling refusal was encountered in the deposit. The silty clay soils contained some organics. Laboratory testing performed on samples obtained from the borings 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 of the soil and weathered bedrock,presented on Figures 4 and 5, indicate the silty clay is moderately compressible and the limestone bedrock is slightly compressible under conditions of loading and wetting. The laboratory testing is summarized in Table 1. No free water was encountered in the borings at the time of drilling or when checked 2 days later. The subsoils were moist to very moist and the bedrock was generally moist. FOUNDATION BEARING CONDITIONS The silty clay soils possess low bearing capacity and in general moderate settlement potential. The underlying bedrock possesses relatively high bearing capacity and low settlement potential. Spread footings are feasible for foundation support of the residence. At assumed excavation depths, we expect the subgrade soils will transition from the silty clay soils in the shallow cut areas to bedrock in the deeper cut area. For these bearing Job No. 114 198A G,egtech - 4- conditions, there is some risk of differential settlement due to variable bearing conditions and compressible nature of the silty clay soils. To reduce the risk of differential settlement, we recommend the footings bear entirely on the bedrock. This may require subexcavation below design footing bearing elevation in areas. 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 entirely on undisturbed bedrock. The design and construction criteria presented below should be observed for a spread footing foundation system. 1) Footings placed entirely on the undisturbed bedrock can be designed for an allowable bearing pressure of 4,000 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 10 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. 114 198A Gtech - 5 - 5) The topsoil, silty clay soils and any loose disturbed materials should be removed and the footing bearing level extended down to undisturbed bedrock. 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 about 15 feet high 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 soils and well broken bedrock. 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 soils and well broken bedrock. The backfill should not contain topsoil or plus 6 inch size rock fragments. If retaining walls taller than 15 feet are proposed, we should be contacted for additional 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 (SPD) at a moisture content near optimum. Backfill placed in Job No. 114 198A GecPtech - 6 - pavement and walkway areas should be compacted to at least 95% SPD. 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. Use of a select granular imported material such as base course and increasing compaction to 98% SPD could be done to reduce the 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 pc£ 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 a suitable granular material compacted to at least 95% SPD 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 slab subgrade conditions will probably vary from soil to bedrock in areas. Providing 2 feet of structural fill below the floor slabs in the soil subgrade areas is recommended to reduce the potential for differential settlement and distress to the floor slab. 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 Job No. 114 198A Gertech - 7 - 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 immediately 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%passing the No. 200 sieve. The under slab gravel should be connected to the perimeter underdrain with interior lateral subdrains. 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 and well broken bedrock devoid of topsoil and oversized rocks. The 2 feet of structural fill below floor slab areas discussed above should consist of suitable granular import soils such as road base. UNDERDRAIN SYSTEM Although free water was not encountered during our exploration, it has been our experience in mountainous areas and where bedrock is shallow that local perched groundwater can develop during limes of heavy precipitation or seasonal runoff. Frozen ground during spring runoff can also create a perched condition. We recommend below- grade 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. 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 11/2 feet deep and extend to above any seepage encountered in the adjacent cut slope face, and be covered with filter fabric such as Mirafi 140N. Job No. 114 198A C rt@Ch - 8 - SITE GRADING The risk of construction-induced slope instability at the site appears low provided the building is located above the steep slope as planned and cut and fill depths are limited. We assume the cut depths for the basement level will not exceed about 15 feet. Fills should be limited to about 8 to 10 feet deep, especially at the downhill side of the residence where the slope steepens. 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 into the portions of the hillside exceeding 20% grade. Excavation in deeper cut areas will be difficult due to the very hard and likely cemented rock. Permanent unretained cut and fill slopes should be graded at 2 horizontal to 1 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 conducted to determine if the seepage will adversely affect the cut stability. We should review the grading plans prior to construction. SURFACE DRAINAGE 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. Job No. 114 198A Ge Ptech - 9 - 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 fabric and 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 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. Our services do not include determining the presence, prevention or possibility of mold or other biological contaminants (MOB C) 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 Job No. 114 198A G& tech - 10 - 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 GER 'ICAL, INC. ,�•40 ► Ree, DTZ i . / + "lM David A. Young, P.E. .�. 216 Reviewed by: % t� ' c 2. "11.11111� ,4t,‘,t44 Steven L. Pawlak, P.E. DAY/ksw REFERENCES Town of Vail, 2000a. Official Rockfall 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, 2000b. Official Debris Flow 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, 2000c. Official Avalanche Hazard Map, Town of Vail. Prepared by the Town of Vail, Vail, Colorado (Adopted by the Town Council on October 17, 2000). Job No. 114 198A C- Stech APPROXIMATE SCALE 1" = 30' (2 2 8250 '70 N 8260\ l N. .N. ` ` \ -. / ' \/ N N N. \ \ \ J 8 />09I N \N '20 \ BORING 1 N \. �N I PROPOSED RESIDENCE LOT 3A 103 ROCKLEDGE ROAD \ \ LOT z \ LOT4 \ N\ i -- \ I/ N\ BORING 2 -- N iti co N 8260 IT — _ _ J • / j BORING 3 -- _ — 2 I/ — / / N2> 2,217 I — — o — o��°°�°SOP° J . H 114 198A @Ch LOCATION OF EXPLORATORY BORINGS Figure 1 Hepworth—Pawlak Geotechnical BORING 1 BORING 2 BORING 3 ELEV.= 8260' ELEV.= 8263' ELEV.= 8279' 0 ,--., .....• ,--., 0 , ^/ ^/ / - N ~ - - / 17/12 4 / / — 5 / WC=30.0 wr 5 44/12 — DD=90 / r 16/12 -200=66 I— ."_I WC=18.9 / WC=18.7 _ DD=112 DD=109 r LL -200=70 — u_ r �' .c — i 60/1 ':ti a O _ 10 14/12 10 0 — ` � WC=20.6 15 15 Note: Explanation of symbols is shown on Figure 3. 114198A G�it�tlpg��,�_� �7GV�CrI LOGS OF EXPLORATORY BORINGS Figure 2 Hepworth—Pawlak Geotechnical LEGEND: ^/ TOPSOIL; organic sandy silt and clay, very moist, dark brown. N n/ CLAY (CL); silty, sandy to very sandy, stiff, moist to very moist, brown, low plasticity, some organics. T LIMESTONE BEDROCK; with interbedded shale layers, medium hard to very hard with depth, moist, grey to dark grey. Minturn Formation. 11 Relatively undisturbed drive sample; 2-inch I.D. California liner sample. IDrive sample; standard penetration test (SPT), 1 3/8 inch I.D. split spoon sample,ASTM D-1586. 17/1Drive sample blow count; indicates that 17 blows of a 140 pound hammer falling 30 inches were 2 required to drive the California or SPT sampler 12 inches. TPractical drilling refusal in cemented rock. NOTES: 1. Exploratory borings were drilled on May 28, 2014 with 4-inch diameter continuous flight power auger. 2. Locations of exploratory borings were measured approximately by pacing from features shown on the site plan provided. 3. Elevations of exploratory borings were obtained by interpolation between contours shown on the site plan provided. Boring logs are drawn to depth. 4. The exploratory boring locations and elevations should be considered accurate only to the degree implied by the method used. 5. The lines between materials shown on the exploratory boring logs represent the approximate boundaries between material types and transitions may be gradual. 6. No free water was encountered in the borings at the time of drilling or when checked 2 days later. Fluctuation in water level may occur with time. 7. Laboratory Testing Results: WC = Water Content(%) DD = Dry Density (pcf) -200 = Percent passing No. 200 sieve 114 198A G Hp C'@C'3t@Cr'1 LEGEND AND NOTES Figure 3 HEPWORTH.PAWLAK GEOTECHNICAL Moisture Content = 30.0 percent Dry Density = 90 pcf Sample of: Sandy Silty Clay with Organics From: Boring 1 at 4 Feet 0 1 • Nomovement up � upon 0) 2 wetting 0) N a 3 • U 4 • 0.1 1.0 10 100 APPLIED PRESSURE- ksf Moisture Content = 18.9 percent Dry Density = 112 pcf Sample of: Limestone Bedrock From: Boring 2 at 5 Feet 0 C 0 O n 1 a 0 U 2 No movement upon wetting 0.1 1.0 10 100 APPLIED PRESSURE- ksf 114 198A Glilaigtech SWELL-CONSOLIDATION TEST RESULTS Figure 4 HEPWORTH-PAWLAK GEOTECHNICAL Moisture Content = 18.7 percent Dry Density = 109 pcf Sample of: Very Sandy Silty Clay From: Boring 3 at 5 Feet 0 • 0 1 • o Compression _N upon 2 wetting a E • 0 U 3 • 4 0.1 1.0 10 100 APPLIED PRESSURE-ksf H 114 198A CI'1 SWELL-CONSOLIDATION TEST RESULTS Figure 5 HEPWORTH•PAWLAK GEOTECHNICAL CO < \ \ § U §± © acs in G § » a » = O \ d CO 2 ( \ ƒ \ � / ) \ j ›- z 0 (/ \ / § \ re ( )z * y - § § § • - § 0 \ / j \\\� $ R 0 0 0 / Q o, 0 = — ƒ & ) : \ U k & LU � ® 0 2 § 7w o m N 2 ) \ k \ N / [ ( z .7J- un 20 in w g zca / - N m