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Home My WebLink About1.09 Subsoil study for foundationHEPWORTH-PAWLAK GEOTECHNICAL Hepworth-Pawlak Geotechnical, Inc. 5020 County Road 154 Glenwood Springs, Colorado 81601 Phone: 970-945-7988 Fax: 970-945-8454 email: hpgeo@hpgeotech.com SUBSOIL STUDY FOR FOUNDATION DESIGN PROPOSED PRE -FABRICATED LOG HOME AND ATTACHED GARAGE 425 MOUNTAIN SHADOWS DRIVE GLENWOOD SPRINGS, COLORADO JOB NO. 114 333A SEPTEMBER 17, 2014 PREPARED FOR: ROBYN STARR 425 MOUNTAIN SHADOWS DRIVE GLENWOOD SPRINGS, COLORADO 81601 (robynstarr a,mindspring.com) 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 - SUBSIDENCE POTENTIAL - 2 - FIELD EXPLORATION - 3 _ SUBSURFACE CONDITIONS - 3 - FOUNDATION BEARING CONDITIONS - 4 - DESIGN RECOMMENDATIONS - 4 - FOUNDATIONS -4- FOUNDATION AND RETAINING WALLS - 5 - FLOOR SLABS - 7 - UNDERDRAIN SYSTEM _ 7 - SITE GRADING - g - SURFACE DRAINAGE - g - LIMITATIONS - 9 - FIGURE 1 - LOCATION OF EXPLORATORY BORING FIGURE 2 - LOG OF EXPLORATORY BORING FIGURE 3 - LEGEND AND NOTES FIGURES 4, 5 AND 6 SWELL -CONSOLIDATION TEST RESULTS TABLE 1- SUMMARY OF LABORATORY TEST RESULTS Job No. 114 333A Gtech PURPOSE AND SCOPE OF STUDY This report presents the results of a subsoil study for a proposed pre -fabricated log home with a site built, attached garage to be located at 425 Mountain Shadows Drive in Glenwood Springs, 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 Robyn Starr, dated August 15, 2014. A field exploration program consisting of an exploratory boring 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 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 proposed residence will be a prefabricated log home over a crawlspace with an attached site built, basement level garage with a slab -on -grade floor. Grading for the structure is assumed to vary from relatively minor to moderate with cuts up to about 12 feet in depth. 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 333A Gglitech -2 - SITE CONDITIONS The site is a developed residential lot on the north (uphill) side of Mountain Shadows Drive with an existing single family residence on the southwest portion of the lot. The proposed building area is located northeast and uphill of the existing single family residence. The existing residence is a two level structure with a walkout lower level and is constructed on a level graded area with a 6 to 8 foot cut on the north edge of the building area. The proposed building will be located on a relatively steep, south facing slope north of the driveway to the existing residence with an elevation difference of about 15 to 18 feet across the building footprint. Vegetation in the area of the proposed building includes grasses, weeds, small brush and trees. SUBSIDENCE POTENTIAL The geology of the project site is a younger, relatively inactive debris fan deposit originating from the uphill slope to the north. Bedrock of the Pennsylvanian age Eagle Valley Evaporite likely underlies the debris fan deposit and the project site. The evaporite rocks are a sequence ofgypsiferous shale, fine-grained sandstone and siltstone with some massive beds of gypsum and limestone. Evaporite bedrock was not encountered in our exploratory boring but there is a possibility that massive gypsum deposits associated with the Eagle Valley Evaporite could underlie portions of the property. No evidence of cavities was encountered in the subsurface materials of the exploratory boring. Based on our present knowledge of the subsurface conditions at the site, it cannot be said for certain that sinkholes will not develop. The overall risk of future ground subsidence on the project site, throughout the service life of the building, in our opinion is low, but the building 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. 114 333A G& tech -3 - FIELD EXPLORATION The field exploration for the project was conducted on September 5, 2014. One exploratory boring was drilled at the location shown on Figure 1 to evaluate the subsurface conditions. The boring was advanced with 4 inch diameter continuous flight augers powered by a truck -mounted CME -45B drill rig. The boring was logged by a representative of Hepworth-Pawlak Geotechnical, Inc. A trail was rough cut to the boring location by others for the drill rig access. Samples of the subsoils were taken with a 2 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 Log of Exploratory Boring, Figure 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 trail cut for our drill rig access to the proposed building area removed about 6 inches of silty sand topsoil. The subsoils encountered in our boring consisted of sandy clay and silt with gravel, scattered cobbles, and possible boulders with zones of rocky soils to the full depth of exploration of 21 feet. Laboratory testing performed on samples obtained from the borings included natural moisture content and density and finer than sand size (-200 screen) sieve analyses. Results of swell -consolidation testing performed on drive samples, presented on Figures 4 through 6, indicate relatively low compressibility under conditions of light loading at existing low moisture contents and moderate to high compressibility under light loading and wetting (hydro -compression), with moderate to high compressibility under additional Job No. 114 333A Gegtech -4 - loading after wetting. Due to the dry and gravelly nature of the test samples, disturbance of the samples probably occurred and likely exaggerated the amount of compressibility indicated. The laboratory testing is summarized in Table 1. No free water was encountered in the boring at the time of drilling and the subsoils were slightly moist. FOUNDATION BEARING CONDITIONS The sandy clay and silt soils encountered at the site possess moderate to high compressibility potential, mainly when wetted under loading. Spread footings bearing on the natural soils could experience excessive movement if the subsoils become wetted. Providing a minimum of 3 feet of properly compacted structural fill below the footings would help to mitigate but not eliminate the settlement risk. Minimizing the potential for wetting of the foundation soils will be critical to the satisfactory performance of the structure. The compressibility potential of the bearing soils should be further evaluated at the time of excavation for the foundation. Structural fill in foundation areas can consist of the on-site soils or a relatively well graded, imported granular soil such as road base. The structural fill should be moisture conditioned to near optimum moisture, placed in maximum 8 inch loose lifts and compacted to a minimum of98% of standard Proctor value. The structural fill should extend at least 2 feet beyond the perimeter of the footings. DESIGN RECOMMENDATIONS FOUNDATIONS Considering the subsurface conditions encountered in the exploratory boring and the nature of the proposed construction, we recommend the residence and garage be founded with spread footings bearing on properly compacted structural fill. Job No. 114 333A GecPtech -5 - The design and construction criteria presented below should be observed for a spread footing foundation system. 1) Footings placed on a minimum of 3 feet of properly compacted structural fill should be designed for an allowable bearing pressure of 1,500 psf with some risk of settlement. The risk of settlement is primarily if the bearing soils become wetted and care should be taken in design and construction to prevent wetting of the bearing soils. Based on experience, we expect initial settlement of footings designed and constructed as discussed in this section will be about 1 inch or less. Additional settlement of about 1 inch or more could occur if the underlying fine grained soils become wetted. 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 36 inches below exterior grade is typically used in this area. 4) Continuous foundation walls should be heavily reinforced top and bottom to span local anomalies such as by assuming an unsupported length of at least 14 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 to expose the natural soils prior to placement of structural fill. The exposed soils in footing area should then be moistened and compacted. 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 55 pcf Job No. 114 333A GeStech -6 - for backfill consisting of the on-site soils and at least 45 pcf for backfill consisting of imported granular materials. Cantilevered retaining structures which are separate from the structure 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 and at least 45 pcf for backfill consisting of imported granular materials. 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 95% 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 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 0.40. Passive pressure of compacted backfill against the sides of the footings can be calculated using an equivalent fluid unit weight of 360 pef. 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 Job No. 114 333A Gtech -7 - 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, can be used to support lightly loaded slab - on -grade construction. There is a risk of slab settlement similar to footings if the natural soils are wetted. 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 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 precipitation or seasonal runoff. Frozen ground during spring runoff can create a perched condition. We recommend below -grade construction, such as retaining walls and basement areas, be protected from wetting and hydrostatic pressure buildup by an underdrain system. A perimeter drain should not be provided around crawlspace areas (less throughout 5 feet deep) due to the potential for wetting of shallow footings. Job No. 114 333A GE Ptech -8 - 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 least 1 foot below lowest adjacent finish (basement floor) 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. An impervious membrane such as 30 mil PVC should be placed beneath the drain gravel in a trough shape and attached to the basement foundation wall with mastic to prevent wetting of the bearing soils. SITE GRADING The risk of construction -induced slope instability at the site appears low, provided the building is located as planned and cut and fill depths are limited. We assume the cut depths for the basement level will not exceed one level, about 10 to 12 feet. Fills should be limited to about 8 to 10 feet deep. 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. This office should review site grading plans for the project prior to construction. SURFACE DRAINAGE Proper grading and drainage around the building will be critical to keeping the bearing soils dry. 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 Job No. 114 333A Gtech -9 - 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 at least 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 and foundation areas. 5) Landscaping which requires regular heavy irrigation should be located at least 10 feet from foundation walls. Consideration should be given to use of xeriscape to reduce the potential for wetting of soils below the building caused by irrigation. 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 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 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 boring 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. Job No. 114 333A GecPtech -10 - 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. James A. Parker, P.E., P.G. Reviewed by: S?L,/ Steven L. Pawlak, P.E. JAP/ljg cc: Kurtz & Associates, Inc. - Brian Kurtz (kurtzengineer(4ahoo.com) Job No. 114 333A 1035 APPROXIMATE SCALE 1"=30' 1030 1035 / / / 7 / i /� / / / / / / 7 7 / /7 7 i 7 1030 7 i i —' i 7 7 nor?' / / 7 1005____7 �\�POO�SS)"<" / / PROPOSED RESIDENCE / / • BORING 1 / �_— / MO UNiA1N SHADOWS DRIVE 7 / 1025 1o20 105 Y 114 333A as"' A O :ir7%1 In ijroCyar r_ LOCATION OF EXPLORATORY BORING FIGURE 1 Depth - Feet 0 5 10 15 20 BORING 1 ELEV.= 1021' '• 56/12 WC=2.6 • :• DD=105 31/12 WC=5.9 DD=84 -200=63 14/6.40/3 r' 44/12 WC=4.8 .r: DD=102 ;:f 18/6,32/1 r:: I WC=4.4 DD=115 -200=44 0 5 10 15 20 25 25 NOTE: Explanation of symbols is shown on Figure 3. m LL a 0 0 114 333A Gtech HEPWORTH-PAWLAK GEOTECHNICAL LOG OF EXPLORATORY BORING FIGURE 2 LEGEND: CLAY AND SILT (CL -ML); sandy to very sandy, with gravel, scattered siltstone/sandstone fragments, cobbles and possible boulders, layers of rocky soils, very stiff to hard/medium dense, slightly moist, light brown. Relatively undisturbed drive sample; 2 -inch I.D. California liner sample. 56/12 Drive sample blow count; indicates that 56 blows of a 140 pound hammer falling 30 inches were required to drive the California sampler 12 inches. NOTES: 1. The exploratory boring was drilled on September 5, 2014 with a 4 -inch diameter continuous flight power auger. 2. Location of the exploratory boring 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. The boring log is drawn to depth. 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 (pcf) -200 = Percent passing No. 200 sieve 114 333A Ge Ptech HEPWORTH-PAWLAK GEOTECHNICAL, LEGEND AND NOTES FIGURE 3 Moisture Content = 2.6 percent Dry Density = 105 pcf Sample of: Sandy Clay and Silt with Gravel From. Boring 1 at 3 Feet 0 1 COMPRESSION (% ) Co co �I (n .A co N -' O 0 Compression upon wetting O • 12 13 • 0 1 1 0 10 100 APPLIED PRESSURE (ksf ) 114 333A (eHV"teCh HEPWORTH-PAWLAK GEOTECHNICAL SWELL -CONSOLIDATION TEST RESULTS FIGURE 4 COMPRESSION (% ) N op O) N O O co a) .A N O Moisture Content = 5.9 percent Dry Density = 84 pcf Sample of: Sandy Clay and Silt with Gravel From. Boring 1 at 5 Feet b Compression upon wetting �r • • • 0 1 1 0 10 100 APPLIED PRESSURE (ksf ) 114 333A Ge(!tech HEPWORTH-PAWLAK GEOTECHNICAL SWELL -CONSOLIDATION TEST RESULTS FIGURE 5 COMPRESSION (% ) (0 co v 6> 01 .A co N O —y O Moisture Content = 4.8 percent Dry Density = 102 pcf Sample of: Sandy Clay and Silt with Gravel From: Boring 1 at 15 Feet Compression upon wetting • • • 01 1 0 10 100 APPLIED PRESSURE (ksf ) 114 330A" rt ech HEPWORTH-PAWLAK GEOTECHNICAL SWELL -CONSOLIDATION TEST RESULTS FIGURE 6 Job No. 114 333A TECHNICAL, INC. F- J w F - w O 0 < J o J m m <F -J = 0 c 0 < 2 a 2 w w R J_ O tll Sandy Clay and Silt with Gravel 11 Sandy Clay and Silt with Gravel II Sandy Clay and Silt with Gravel 11 Silty Clayey Sand with Gravel 11 UNCONFINED COMPRESSIVE STRENGTH (PSF) PLASTIC INDEX (%) ATTERBE LIQUID LIMIT (%) PERCENT PASSING NO. 200 SIEVE m 44 a w GRAD GRAVEL (%) NATURAL DRY DENSITY (pcf) ten O y 84 N O .-- t(1 .--- .--t NATURAL MOISTURE CONTENT (%) 1O N Q\ vl W c- 71- 4 11 SAMPLE LOCATION DEPTH (ft) 3 5 SI 20 0 z K O in —1