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HomeMy WebLinkAboutSuboil Study 11.26.2014Ge"kol 1-f o9o'f tech HEPWORTH-PAWLAK GEOTECHNICAL SUBSOIL STUDY FOR FOUNDATION DESIGN PROPOSED RESIDENCE LOT F 7, ASPEN GLEN GARFIELD COUNTY, COLORADO JOB NO. 114 437A NOVEMBER 26, 2014 PREPARED FOR: WOODBRIDGE MORTGAGE INVESTMENT FUND 2, LLC ATTN: RICK SALVATO 22 CENTER STREET, FRONT SUITE FREEHOLD, NEW JERSEY 07728 (risalvQuaol.con Parker 303-841-7119 0 Colorado Springs 719-6-33-5562 0 Silverthorne 970-468-1989 TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY..........................................................................- 1 - PROPOSED CONSTRUCTION.................................................................................. - 1 - SITECONDITIONS....................................................................................................- 2- SUBSIDENCE POTENTIAL.......................................................................................- 2- FIELDEXPLORATION..............................................................................................- 3- SUBSURFACE CONDITIONS..............................._....................................................- 3- DESIGN RECOMMENDATIONS...............................................................................- 4 - FOUNDATIONS......................................................................................................- 4- FOUNDATION AND RETAINING WALLS..........................................................: 5- FLOORSLABS.................................................................................................:.....- 6 - UNDERDRAINSYSTEM........................................................................................- 6- SITEGRADING......................................................................................................- 7- SURFACEDRAINAGE..........................................................................................- 8- LIMITATIONS...........................................................................................................: 8- REFERENCES.............................................................................................................- 9- FIGURE I - LOCATION OF EXPLORATORY BORINGS FIGURE 2 - LOGS OF EXPLORATORY BORINGS FIGURE 3 - LEGEND AND NOTES FIGURE 4 - GRADATION TEST RESULTS PURPOSE AND SCOPE OF STUDY This report presents the results of a subsoil study for a proposed residence to be located at Lot F 7, Aspen Glen Subdivision, Garfield County, 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 proposal for geotechnical engineering services to Woodbridge Mortgage Investment Fund 2, LLC dated October 2, 2014. Chen -Northern, Inc. (1991 and 1993) previously conducted preliminary geotechnical engineering studies for the development and preliminary plat design. 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 laboratory to determine their classification 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 Building plans for the proposed residence are conceptual. Typical residences in the area are one and two story wood frame above a basement or crawlspace with an attached garage. Ground floors are typically slab -on -grade. Grading for the proposed 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 assumed 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 437A Ge Ptech -2- SITE CONDITIONS The vacant property is located at the cul-de-sac of River Park Lane. Vegetation consists of sparse grass and weeds. Topography at the site consists of relatively flat upper and lower terraces separated with by a steep slope. The terraces have a slight slope down to the northwest. Elevation difference between the terraces is about 15 feet. A wetland area and drainage are located beyond the site to the northwest. The upper terrace appears to have been graded with minor cuts during subdivision development and gravel and cobbles are exposed on the ground surface. A golf course fairway is located along the northern property line. SUBSIDENCE POTENTIAL Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the Aspen Glen development. These rocks are a sequence of gypsiferous shale, fine-grained sandstone/siltstone and limestone with some massive beds of gypsum. There is a possibility that massive gypsum deposits associated with the Eagle Valley Evaporite underlie portions of the lot. Dissolution of the gypsum under certain conditions can cause sinkholes to develop and can produce areas of localized subsidence. During previous studies in the area, several broad subsidence areas and smaller size sinkhole areas were observed scattered throughout the Aspen Glen development, predominantly on the east side of the Roaring Fork River (Chen -Northern, Inc., 1993). These sinkholes appear similar to others associated with the Eagle Valley Evaporite in areas of the Roaring Fork River valley. Lot F 7 is located just outside of one of the broad subsidence areas mapped by Chen - Northern. Signs of active ground movements have not been observed in the subsidence area. The nearest sinkhole was mapped about 900 feet to the northeast of Lot F 7. Sinkholes were not observed in the immediate area of the subject lot. No evidence of cavities was encountered in the subsurface materials; however, the exploratory borings Job No. 114 437A GeMech -3- were relatively shallow, for foundation design only. Based on our present knowledge of the subsurface conditions at the site, it cannot be said for certain that sinkholes will not develop. The risk of future ground subsidence on Lot F 7 throughout the service life of the proposed residence, in our opinion, is low but the site should not be considered totally risk free. If further investigation of possible cavities in the bedrock below the site is desired, we should be contacted. FIELD EXPLORATION The field exploration for the project was conducted on October 7, 2014. Two 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 truck -mounted CME-45B drill rig. The borings were logged by a representative of Hepworth-Pawlak Geotechnical, Inc. A previous boring (B-5-93) was drilled near the front of the lot by Chen -Northern, Inc. Samples of the subsoils were taken with a 1% inch I.D. spoon sampler. The sampler was driven into the subsoils 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. 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 consist of silty sandy gravel with cobbles and boulders. Results of gradation analyses performed on a small diameter drive sample (minus 11/z inch fraction) of the coarse granular subsoils are shown on Figure 4. Drilling in the dense Job No. 114 437A GecPtech granular soils with auger equipment was difficult due to the cobbles and boulders and drilling refusal was encountered in the deposit at relatively shallow depths. No free water was encountered in the borings at the time of drilling and the subsoils were slightly moist to moist. Chen -Northern, Inc. Boring B-5-93 indicates the granular soils extend down to about 60 feet and are underlain by Eagle Valley Evaporite Bedrock. 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 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 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 36 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. 5) All topsoil and any loose or disturbed soils should be removed and the footing bearing level extended down to the relatively dense natural Job No. 114 437A C,CPtE--Ch -5- granular 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 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 45 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 40 pcf for backfill consisting of the on -site soils. 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 near optimum. Backfill 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. Job No. 114 437A G'(�0'1'teCh I Me 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 0.50. Passive pressure of compacted backfill against the sides of the footings can be calculated using an equivalent fluid unit weight of 400 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. 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% 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 vegetation, topsoil and oversized rock. UNDERDRAIN SYSTEM Although free water was not encountered during our exploration, it has been our experience in the area that local perched groundwater can develop during times of heavy Job No. 114 437A Gc5&' ech -7- 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 1 %2 feet deep. SITE GRADING The risk of construction -induced slope instability at the site appears low provided cut and fill depths are limited. We assume the cut depths for the basement level (if any) will not exceed one level, about 10 to 12 feet. Fills should be limited to about 8 to 10 feet deep 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. 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. This office should review site grading plans for the project prior to construction. Job No. 114 437A GeH<Pitech 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. 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 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. 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 (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 Job No. 114 437A Mech 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. Louis E. Eller Reviewed by: Daniel E. Hardin, P.E. LEE/ksw REFERENCES Chen -Northern, Inc., 1991, Preliminary Geotechnical Engineering Study, Proposed Aspen Glen Development, Garfield County, Colorado, prepared for Aspen Glen Company, dated December 20, 1991, Job No. 4 112 92. Chen -Northern, Inc., 1993, Geotechnical Engineering Study for Preliminary Plat Design, Aspen Glen Development, Garfield County, Colorado, prepared for Aspen Glen Company, dated May 28, 1993, Job No. 4 112 92. Job No. 114 437A CagUPb� WETLANDS APPROXIMATE SCALE 1"=50' 114 437A 1 1 LOT F7 000* '00 ` Q ok BORING 2 1 BENCH MARK: GROUND AT PROPERTY 1 CORNER; ELEV. = 100.0', ASSUMED. BORING 1 LOT F 6 1 i i 1, B-5-93 O RIVER PARK LANE LOT F 8 LOCATION OF EXPLORATORY BORINGS I Figure 1 BORING 1 BORING 2 ELEV.= 102.3 ELEV.= 99.5' 105 105 o�: 100 100 b 21,50/3 a) LL +4=54 LL -200-11 ' o g a' roe 50/6 co w 95 95 w 90 90 Note: Explanation of symbols is shown on Figure 3. 1—I 114 437A LOGS OF EXPLORATORY BORINGS Figure 2 He worth—Pawlak Geatechnical LEGEND: 0o GRAVEL, COBBLES AND BOULDERS (GP -GM); sandy, silty, dense, slightly moist, brown, subrounded rocks. e Drive sample; standard penetration test (SPT), 1 3/8 inch I.D. split spoon sample, ASTM D-1586. 39/12 Drive sample blow count; indicates that 39 blows of a 140 pound hammer falling 30 inches were required to drive the SPT sampler 12 inches. TPractical drilling refusal. NOTES 1. Exploratory borings were drilled on October 7, 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 measured by instrument level and refer to the Bench Mark shown on Figure 1. 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. Fluctuation in water level may occur with time. 7. Laboratory Testing Results: +4 = Percent retained on the No. 4 sieve -200 = Percent passing No. 200 sieve 114 437A I I LEGEND AND NOTES Figure 3 Hepworth—Pawlak Geotechnical