The URL can be used to link to this page

Home My WebLink AboutSoils Report 02.23.2006G&tech HEPWORTH- PAWLAK GEOTECHNICAL Ilepworth-PawIal: Geotechnical, Inc. 5020 County Road 154 Glenwood Springs, Colorado 81601 Phone: 970-945.7988 Fax: 970-945-8454 email: lipgen@hpgenrech.com SUBSOIL STUDY FOR FOUNDATION DESIGN PROPOSED RESIDENCE LOT H-22, THE HOMESTEAD AT ASPEN GLEN HORSESHOE LANE GARFIELD COUNTY, COLORADO JOB NO. 106 0128 FEBRUARY 23, 2006 PREPARED FOR: JAMES AND ANN KENNEY 5949 SHERRY LANE, SUITE 960 DALLAS, TEXAS 75225 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 SUBSIDENCE POTENTIAL FIELD EXPLORATION -' SUBSURFACE CONDITIONS - 3 FOUNDATION BEARING CONDITIONS - 4 - DESIGN RECOMMENDATIONS - 4 FOUNDATIONS -4- _ 5 - ` 5 - FOUNDATION AND RETAINING WALLS FLOOR SLABS - 6 - UNDERDRAIN SYSTEM 8 - SURFACE DRAINAGE LIMITATIONS - 8 - REFERENCES -10- FIGURE 1 - LOCATION OF EXPLORATORY BORINGS FIGURE 2 - LOGS OF EXPLORATORY BORINGS FIGURE 3 - LEGEND AND NOTES FIGURE 4 - SWELL -CONSOLIDATION TEST RESULTS FIGURE 5 - GRADATION TEST RESULTS PURPOSE AND SCOPE OF STUDY This report presents the results of a subsoil study for a proposed residence to be located on Lot H-22, The Homestead at Aspen Glen, Horseshoe Lane, 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 James and Ann Kenney dated January 9, 2006. Chen -Northern, Inc. previously conducted a preliminary geotechnical engineering study for development of Aspen Glen and geotechnical engineering study for preliminary plat design (Chen -Northern, 1991 and 1993). 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, compressibility or swell and other engineering characteristics. The results of the field exploration and laboratory testing were analyzed to develop reconunendations 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 proposed residence will be a two story wood frame structure over a partial basement level and crawlspace. The attached garage and basement floors 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 report. Job No, 106 0128 Gbh - SITE CONDITIONS The site was vacant and covered with about 1 to 11/= feet of snow at the time of our field exploration. There appears to be some minor fill on the lot from overlot grading as part of the subdivision development. The ground surface is relatively flat with about 2 to 4 feet of elevation difference across the building area. There is a moderately steep slope on the northeast side of the lot down to existing golf course ponds. Vegetation consists of grass and weeds. 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 by Chen -Northern (1991 and 1993), several broad subsidence areas and smaller size sinkholes were mapped scattered throughout the Aspen Glen development. These sinkholes were primarily Iocated on the east side of the Roaring Fork River and appear similar to others associated with the Eagle Valley Evaporite in areas of the Roaring Fork River valley. 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 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 H-22 throughout the service life of the proposed residence, in our opinion, is low; however, the owner should be made aware of the potential for sinkhole development. If further investigation of possible cavities in the bedrock below the site is desired, we should be contacted. Job No 106 0128 G leg -tech -3 - FIELD EXPLORATION The field exploration for the project was conducted on January 24, 2006. 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. Samples of the subsoils were taken with 1% inch and 2 inch I.D. spoon samplers. The samplers were 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 consist of about'/_ to 1% feet of granular fill and 7 to 8 feet of stiff to very stiff sandy silty clay overlying relatively dense, slightly silty to silty sandy gravel with cobbles and possible boulders to the drilled depths of 11'/ to 13'% feet. Drilling in the dense granular soils with auger equipment was difficult due to the cobbles and boulders and drilling refusal was encountered in the deposit. Laboratory testing performed on samples obtained from the borings included natural moisture content, density and gradation analyses. Results of swell -consolidation testing performed on relatively undisturbed drive samples of the clay soils, presented on Figure 4, indicate low compressibility under existing moisture conditions and light loading and a minor to low expansion potential when wetted under a constant light surcharge. Results of gradation analyses performed on a small diameter drive sample (minus 1'/ inch fraction) of the coarse granular subsoils are shown on Figure 5. Job No. 106 0128 GgEtech 4 No free water was encountered in the borings at the time of drilling or when checked 9 days later and the subsoils were slightly moist to moist. FOUNDATION BEARING CONDITIONS The upper clay soils appear to possess an expansion when wetted which could result in movement of footings bearing on the clay soils if they become wetted. Surface runoff, landscape irrigation, and utility leakage are possible sources of water which could cause wetting. A lower risk alternative would be to place the foundation entirely on the underlying gravels or compacted structural fill. The subgrade should be observed for bearing conditions and settlement/heave potential at the time of construction. 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 subsoils below the fill or on compacted structural fill. The design and construction criteria presented below should be observed for a spread footing foundation system. 1) Footings placed on the undisturbed natural subsoils soils should be designed for an allowable bearing pressure of 1,500 psf Footings bearing entirely on the natural gravel soils or compacted structural fill can be designed for an allowable bearing pressure of 3,000 psf. Based on experience, we expect settlement of footings designed and constructed as discussed in this section will be about 1 inch or less. There could be some additional movement for footings bearing on the clay soils if they become wetted. The movement would be differential between footings bearing on the clay soils and footings bearing on the gravel soils. Job No. 106 0128 -5- 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 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) All existing fill, topsoil and any Ioose or disturbed soils should be removed and the footing bearing level extended down to the natural soils. Where the clay soils are removed to reduce the settlement potential, the design bearing level can be re-established with compacted structural fill. The fill should be a granular material approved by the geoteclmical engineer compacted to at least 100% of standard Proctor density at a moisture content near optimum. The fill should extend laterally beyond the footing a distance at least equal to the depth of fill below the footing. 6) A representative of the geoteclmical 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 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 50 pcf for backfill consisting of the on-site soils. Backfill should not contain vegetation, topsoil or oversized rock. lob No 106 0128 Gg5teCh -6 - 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. 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 0.35 for the clay soils and 0.45 for the gravels. Passive pressure of compacted backfill against the sides of the footings can be calculated using an equivalent fluid unit weight of 350 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 compacted to at least 95% of the maximum standard Proctor density at a moisture content near optimum. FLOOR SLABS The natural soils, exclusive of topsoil, can be used to support lightly loaded slab -on -grade construction. The clay soils possess an expansion potential which could result in slab movement and distress if the bearing soils become wetted. The risk of slab heave can be Job No. 106 0128 -7 - reduced by removing the clay soils and placing at least 3 feet of compacted structural fill, such as road base, below the 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 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 or imported granular material 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 and where clay soils are present that local perched groundwater can develop during times of heavy precipitation or seasonal runoff. Frozen ground during spring runoff can 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' feet deep. Job No. 106 0128 Gelgt+e:eh -8 - 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 6 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 finer graded soils to reduce surface water infiltration. 4) Roof downspouts and drains should discharge well beyond the limits of all backfill. 5) Irrigation sprinkler heads and landscaping which requires regular heavy irrigation, such as sod, 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 (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 Job No. 106 0128 GCh -9 - 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. Jordy Z. Adamson, Jr., P.E. Reviewed by: Steven L. Pawlak, P.E. JZA/ksw cc: John Muir Architects, Inc. -- Attn: Kristin Mule Job No. 106 0128 -10 - 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. 106 0128 GecPtech APPROXIMATE SCALE 1'=30' 1002 HORSESHOE LANE LOTH21 / 1000 1002 / 1 r /BORING 2 LOT H22 PROPOSED RESIDENCE BORING 1 1 1000 LOT H23 998 996 { 1 994 ti 994 996 GOLF COURSE 996 994 Elevation - Feet 1000 995 990 985 BORING 1 ELEV.= 999' 13/12 13/12 WC=9.0 DD=110 15/6,30/6 BORING 2 ELEV.= 1000' 13/12 18/12 WC= 72 D0=107 25/6,25/3 WC= 1,3 +4=67 -200=11 1000 995 990 985 980 980 Note: Explanation of symbols is shown on Figure 3. Elevation - Feet 106 0128 H EPWOI71 WAWLAK Gl gTL'CHHI GAL LOGS OF EXPLORATORY BORINGS Figure 2 LEGEND: KI FILL; sandy clayey gravel with cobbles, firm, moist, brown. CLAY (CL); silty, sandy, stiff to very stiff, slightly moist, reddish brown, low plasticity. GRAVEL (GP -GM); slightly silty to silty, sandy, with cobbles and possible boulders, dense, slightly moist, brown. Relatively undisturbed drive sample; 2 -inch I.D. California liner sample. Drive sample; standard penetration test (SPT), 1 3/8 inch I.D. split spoon sample, ASTM D-1566. —7 h 13/12 Drive sample blow count; indicates that 13 blows of a 140 pound hammer falling 30 inches were required to drive the California or SPT sampler 12 inches. T —s Practical drilling refusal. Where shown above bottom of log, indicates that multiple attempts were made to advance the boring. Depth at which boring had caved when measured on February 2, 2006. NOTES: 1. Exploratory borings were drilled on January 24, 2006 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 and checked by instrument level. 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 Togs 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 9 days later. Fluctuation in water level may occur with time. 7. Laboratory Testing Results: WC = Water Content (%) DD = Dry Density (pcf) +4 = Percent retained on the No. 4 sieve -200 = Percent passing No. 200 sieve 106 0128 1-# HFPWO177'6-PAW K G CaTEC1iNIC L. LEGEND AND NOTES Figure 3 0 Moisture Content = 9.0 percent Dry Density = 110 pcf Sample of: Sandy Silty Clay From: Boring 1 at 5 Feet t 1 — S------...%\ UE 2 I Expansion upon wetting c 0 Compression - Expansion % 1 0 1 2 0.1 1.0 10 APPLIED PRESSURE - ksf 100 Moisture Content = 7.2 percent Dry Density = 107 pcf Sample of: Sandy Silty Clay From: Boring 2 at 5 Feet -SCI Expansion upon wetting Nc r 0.1 1.0 10 APPLIED PRESSURE - ksf 100 CENT R s . 1 r HYDROME1ER ANALYSIS f fq TIME READINGS U S STANDARD SERIES 0 24 MIN 15 MIN 60MIN19MIN 4 MIN, 1 MIN. #200 #100 #50 #30 #16 #B SIEVE ANALYSIS 10 20 30 40 50 60 70 BO 90 #4 CLEAR SQUARE OPENINGS I 3/8" 3/4' 11/2' 3' 5'6' 8' 100 ' 1• 1 1. r r 1 1 _L - 't 90 80 70 60 50 40 30 20 10 100 Q 3 001 .002 .005 .009 019 .037 074 150 .300 600 1 18 2 36 4,75 9.5 19.0 37.5 76.2 152 203 12,5 127 DIAMETER OF PARTICLES .N MILLIMETERS CLAY TO 5' GRAVEL 67 % LIQUID LIMIT % SAMPLE OF: Slightly Silty Sandy Gravel SAND SAND 22 % GRAVEL rim I CZARSE COMES SILT AND CLAY 11 % PLASTICITY INDEX % FROM: Boring 2 at 10 Feet