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HomeMy WebLinkAboutSoils Report 11.30.2015cec!tech HEPWORTH-PAWLAK GEOTECHNICAL Htpwurth Nub!: Gentechnic.al, Int. 5020 County R0.11,1154 Glcnuixxl Springs, Colorant 81601 PItnnc• 970.945.7983 F. . 970-945-8454 cm eil hrgec 4 hpgenttxh coal SUBSOIL STUDY FOR FOUNDATION DESIGN PROPOSED RESIDENCE LOT 33, HERON CROSSING AT IRONBRIDGE RIVER BEND WAY GARFIELD COUNTY, COLORADO JOB NO. 115 533A NOVEMBER 30, 2015 PREPARED FOR: RM CONSTRUCTION ATTN: BLAKE PILAND 151 CLUBHOUSE DRIVE NEW CASTLE, COLORADO 81647 blake@buildwithrm.com Parker 303-84I.7119 • Colorado Springs 719-633-5562 • Si[verthome 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 FLOOR SLABS - 5 - UNDERDRAIN SYSTEM - 6 - SURFACE DRAINAGE - 6 - LIMITATIONS - 7 - FIGURE 1 - LOCATION OF EXPLORATORY BORINGS FIGURE 2 - LOGS OF EXPLORATORY BORINGS FIGURE 3 - LEGEND AND NOTES FIGURE 4 - SWELL -CONSOLIDATION TEST RESULTS TABLE 1- SUMMARY OF LABORATORY 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 33, Heron Crossing at Ironbridge, River Bend Way, 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 agreement for geotechnical engineering services to RM Construction dated November 4, 2015. We previously conducted a preliminary subsoil study in the Heron Crossing at Ironbridge development area and presented our findings in a report dated February 28, 2014, Job No. 113 471A. 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 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 single story, wood frame structure Iocated as shown on Figure 1. Ground floor will be structural over crawlspace in the living area and slab -on - grade in the garage. Grading for the structure is assumed to be relatively minor with cut depths between about 3 to 5 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 115 533A Gegritech SITE CONDITIONS The lot was vacant at the time of our field exploration. The ground surface had been stripped of topsoil and the subdivision grading was in progress. River Bend Way is asphalt paved. The ground surface slopes gently down to the northeast with about 2 feet or less elevation difference across the building footprint. SUBSIDENCE POTENTIAL Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the Ironbridge development. These rocks are a sequence of gypsiferous shale, fine-grained sandstone and siltstone with some massive beds of gypsum and limestone. 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. Several sinkholes were observed during geologic assessments conducted for the Ironbridge development. These sinkholes appeared similar to others associated with the Eagle Valley Evaporite in areas of the Roaring Fork River valley. A sinkhole opened in the cart storage parking lot in January 2005 and irregular bedrock conditions have been identified in the affordable housing site located roughly 500 to 1,000 feet south of the current development area. 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 33 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. 115 533A -3 - FIELD EXPLORATION The field exploration for the project was conducted on November 24, 2015. 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'/s 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 8 to 11 feet of sandy clay and silt overlying dense, slightly silty sandy gravel and cobbles with boulders. Drilling in the coarse 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 and density and finer than sand size gradation analyses. Results of swell -consolidation testing performed on relatively undisturbed drive samples of the clay and silt soils, presented on Figure 4, indicate low to moderate compressibility under Job No 115 533A Gtech -4 - conditions of loading and wetting. The samples showed a low expansion or compression when wetted under light loading. The laboratory testing is summarized in Table 1. No free water was encountered in the borings at the time of drilling and the subsoils were slightly moist. FOUNDATION BEARING CONDITIONS The upper clay and silt soils have low bearing capacity and low to moderate compressibility mainly when wetted. Shallow spread footings placed on the natural clay and silt soils can be used with a risk of settlement as described below. The footing bearing level should be at least 3 feet below existing ground surface so there is no more than 7 feet of compressible soils below the bearing level. Use of a deep foundation placed on the underlying dense gravel and cobbles soils could be used to achieve a low settlement risk. DESIGN RECOMMENDATIONS FOUNDATIONS Considering the subsurface conditions encountered in the exploratory borings and the nature of the proposed construction, the building be founded with spread footings bearing on the natural clay and silt soils with a settlement risk. If a deep foundation is desired, we should be contacted for supplemental recommendations. 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 1,500 psf. Based on experience, we expect initial settlement of footings designed and constructed as discussed in this section will be about y to 1 inch or less. Job No. 115 533A Gestech -5 - Additional differential settlement up to about 1 inch could occur if the bearing soils are wetted. 2) The footings should have a minimum width of 20 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 a lateral earth pressure corresponding to an equivalent fluid unit weight of at least 55 pcf for the onsite clay and silt soil as backfill. 5) Any existing fill, topsoil and loose or disturbed soils should be removed in the footing areas. The exposed soils in footing area should then be moistened and compacted. The soils should be protected from frost and concrete should not be placed on frozen soils. 6) A representative of the geotechnical engineer should observe all footing excavations prior to concrete placement to evaluate bearing conditions. FLOOR SLABS The natural on-site soils, exclusive of topsoil, can be used to support lightly loaded slab - on -grade construction with a settlement risk if the bearing 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. Job No. 115 533A G tech -6 - 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 clay and silt soils devoid of vegetation and topsoil. UNDERDRAIN SYSTEM It is our understanding that the finished floor elevation at the lowest level will be at or above the surrounding grade. Therefore, a foundation drain system is not required. 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 and wall drain system. An underdrain should not be provided around the crawlspace to help limit surface water infiltration to the bearing soils. If the finished floor elevation of the proposed structure has a basement level, we should be contacted to provide recommendations for an underdrain system. All earth retaining structures should be properly drained. SURFACE DRAINAGE Providing proper surface grading and drainage is very important to the satisfactory performance of the building. 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 115 533A - -7 - 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. 4) Roof gutters should be provided with downspouts and drains that discharge well beyond the limits of all backfill. 5) Landscaping which requires regular heavy irrigation such as sod should be located al least 10 feet from foundations. 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 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 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 Job No. 1t5533A -8 - 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. Steven L. Pawlak, P.E. Reviewed by: Danie E. Hardin, P.E. SLP/ksw Job No 115 533A RIVER BEND WAY LOT 32 115 533A GARAGE F.F. 98.5 APPROXIMATE SCALE i' 20' BORING 1 • PROPOSED FIESIDENCE F F. = 99 92' Nib BORING 2 • LOT 33 1 H Hepworth—Pawlak Geotschnkol LOCATION OF EXPLORATORY BORINGS LOT 34 Figure 1 0 5 10 15 20 BORING 1 BORING 2 16/12 7/12 WC=4 5 � 00=94 ' •200 91 14/12 19/12 WC= C=1Q 6 DD= 92 62/12 23/12 c;„. 0 Liz 55/12 Note: Explanation of symbols is shown on Figure 3. 0 5 10— 15 20 m iv m 115 533A Hepworth—PaM1ak Geatechnical LOGS OF EXPLORATORY BORINGS Figure 2 LEGEND: ---7 0 14/12 T CLAY AND SILT (CL -ML); slightly sandy to sandy, slightly moist, brown, slightly porous. GRAVEL AND COBBLES (GM -GP); slightly silty, sandy, boulders, dense, slightly moist, brown, rounded rock. 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-1586. Drive sample blow count; indicates that 14 blows of a 140 pound hammer falling 30 inches were required to drive the California or SPT sampler 12 inches. Practical drilling refusal, NOTES: 1. Exploratory borings were drilled on November 24, 2015 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 not measured and the logs of exploratory borings 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. B. 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: WC = Water Content (%) DD = Dry Density (pcf) -200 = Percent passing No. 200 sieve 115 533A HEPIMORT*PAWLAK G6aTE.cHNICAL LEGEND AND NOTES Figure 3 Compression - Expansion % Compression % 1 0 1 2 3 4 0 1 2 3 4 5 6 7 0.1 Moisture Content 10.6 percent Dry Density 92 pct Sample of: Silty Sandy Clay From: Boring 1 at 5 Feet Expansion upon wetting 9 0.1 1.0 10 APPLIED PRESSURE - ksf 100 0 Moisture Content 4.5 percent Dry Density = 94 pct Sample of: Slightly Sandy Silt From: Boring 2 at 2 Y2 Feet \\\\ L Compression upon wetting 1.0 10 APPLIED PRESSURE - ksf 100 115 533A I-1 Hepworth—Pawlak Geoteehnlcd SWELL -CONSOLIDATION TEST RESULTS Figure 4 Job No. 115 533A z J a V_ Z = U w I- o O I.J C„7 Y 5 71W J 'CC 9- fa-- z 1 - cc D O. W = SUMMARY OF LABORATORY TEST RESULTS SOIL DR BEDROCK TYPE Silty Sandy Clay 11 Slightly Sandy Silt 11 UNCONFINED COMPRESSIVE STRENGTH Inn I ATTERBERG LIMITS z 7-- ... a. 2 a c ie 5 g PERCENT PASSING NO. 200 SIEVE .--1 ON -f- O a a a a to D x x N > g k NATURAL DRY DENSITY Ipcn el et CA NATURAL MOISTURE CONTENT I%I O in 4 SAMPLE LOCATION o — N BORING ...i N