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HomeMy WebLinkAboutSoils Report 10.03.2014HEPWORTH-PAWLAK GEOTECHNICAL f It 'u.„rti1-I`,luIlk GcnicLiH1ic:11, Inc. it'_'l� l;,iunly IC id 17-E (.It1Ylh,hhl 17111 < C,.1<+rd,Iu,,qi60] 1070,tt li-7tlyti FAN: 970-9.41.S4i I11t1I: h1'err.5I4'140IlciI11 SUBSOIL STUDY FOR FOUNDATION DESIGN PROPOSED RESIDENCE LOT 292, RIVER BEND WAY GARFIELD COUNTY, COLORADO JOB NO. 113 47111 OCTOBER 3, 2014 PREPARED FOR: ASPEN SIGNATURE HOMES OF IRONBRIDGE, LLC ATTN: LLWYD ECCLESTONE P.O. BOX 7628 ASPEN, COLORADO 81612 lcrclestone n plftltfl.nct 1',tr1.3. 303-841-7119 • Col+Ir,3tl() Spring 714-633-5562 • SiIvertlumnc )7C -4(i -19,59 TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY _ 1 - PROPOSED CONSTRUCTION - 1 SITE CONDITIONS 2 GEOLOGY -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 - SURFACE DRAINAGE _ 8 - LIMITATIONS _ 9 - FIGURE 1 - LOCATION OF EXPLORATORY BORING FIGURE 2 - LOG OF EXPLORATORY BORING 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 292, 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 proposal for geotechnical engineering services to Aspen Signature Homes of Ironbridge, LLC dated October 2, 2014. We previously performed a preliminary geotechnical study for this area of the Ironbridge Phase 2 Subdivision development and presented our findings in a report dated February 28, 2014, Job No. 113 471A. The current study is an update of the information presented in our preliminary subsoil study report pertaining to the proposed Lot 292 building development plan. An exploratory boring was drilled on the Iot 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 above a basement in the living area. The garage floor slab will be close to the main building floor level. The 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. Job No. -1-13 471H Gtech -2 - If building loadings, Iocation or grading plans change significantly from those described above, we should be notified to re-evaluate the recommendations contained in this report. SITE CONDITIONS The lot is located on.a gently sloping alluvial fan along the uphill, western side of River Bend Way. The ground surface has been graded relatively flat by placing fill in the front part of the lot during the subdivision development with about 2 feet of elevation difference across the building area. Vegetation consists mainly of weeds. The underground utilities to the lot are complete and the lot is essentially unchanged since its original grading in 2006-2007. Lots 291 and 293 located to the south and north, respectively, are vacant. GEOLOGY The geologic conditions were described in our previous report conducted for planning and preliminary design of the overall subdivision development dated October 29, 1997, Job No. 197 327. The surficial soils on the lot mainly consist of sandy silt debris fan deposits overlying gravel terrace alluvium of the Roaring Fork River. The river alluvium is mainly a clast-supported deposit of rounded gravel, cobbles and boulders up to about 3 feet in size in a silty sand matrix which extends down to depths on the order of 25 to 30 feet below ground surface and overlies siltstone/claystone bedrock in the area of Lot 292. The underlying bedrock consists of the Eagle Valley Evaporite which contains gypsum and is generally associated with scattered sinkhole development in the Roaring Fork River valley. A sinkhole was identified near the northeast comer of the Phase 2 development area to the east of River Bend Way about 250 to 300 feet northeast of Lot 292. The sinkhole was backfilled during construction of the subdivision infrastructure. Voids have not been encountered in borings drilled into the bedrock near Lot 292 and the potential for subsidence due to dissolution of the evaporite throughout the service life of the residence, in our opinion, is low, but the owner of the lot should be aware of the sinkhole potential and the risk of future subsidence. Job No. 113 47111 sartech -4 - soil, presented on Figure 4, indicate low compressibility under existing low moisture condition and light loading and a low collapse potential (settlement under constant load) when wetted. The sample showed moderate compressibility under additional loading after wetting. No free water was encountered in the boring at the time drilling and the subsoils were moist. FOUNDATION BEARING CONDITIONS The upper silt (debris fan) soils typically have low bearing capacity and low to moderate settlement potential under loading when wetted. Foundations that extend down to the dense, river gravel alluvium (such as with piers or piles) would have moderate bearing capacity and low settlement risk. Spread footings placed on the natural soils at basement level or on compacted fill can be used for building support with a potential for differential settlement, mainly if the debris fan soils are wetted. The shallow garage level footings will have about twice the settlement potential as footings bearing at basement level due to the greater debris fan soil depth, and mitigation by soil compaction is recommended to reduce the differential settlement potential. DESIGN RECOMMENDATIONS FOUNDATIONS Considering the subsurface conditions encountered in the exploratory boring and the nature of the proposed construction, we recommend the building be founded with spread footings bearing on the natural subsoils at basement level or compacted fill at garage level. The design and construction cri`eria presented below should be observed for a spread footing foundation system. .job No. 1 1 3 471H Gtech -5- 1) Footings placed on the undisturbed natural soils or compacted fill should be designed for an allowable bearing pressure of 1,000 psf. Based on experience, we expect initial settlement of footings designed and constructed as discussed in this section will be about ] inch or less. In order to limit additional differential settlement in the event of subsurface wetting to on the order of 1 inch, we recommend the garage level footings be placed on at least 4 feet of replaced and compacted, onsite debris fan soils. 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 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 vegetation and loose disturbed soils should be removed and the footing bearing level extended down to the firm natural soils. The exposed soils in footing areas should then be moistened and compacted. Onsite soil fill placed below footing bearing level should be compacted to at Ieast 98% of standard Proctor density within 2 percentage points of optimum. The compacted fill should extend laterally beyond the footing edge a distance at least h the fill depth below the footing. 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 Iateral Job No. 113 4711.1 -7 - 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 and compacted fill are suitable to support lightly loaded slab -on - grade construction. The upper silt soils have variable settlement potential when wetted under load and there could be some post -construction slab movement if the subgrade soils become wet. 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. An underdrain should not be provided around shallow slab -on -grade foundations (such as garage areas). Job No. 113 471H 8 Where installed around basement areas, 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, sump and pump or drywell based in the underlying river gravel deposit. Free draining granular material used in the underdrain system should contain less than 2% passing the No. 200 sieve, Iess 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. In silt soil bearing areas, an impervious membrane, such as a 30 mil PVC Iiner, should be placed in a trough shape below the drain gravel and attached to the foundation wall with mastic to prevent wetting of the bearing soils. SURFACE DRAINAGE Providing proper perimeter surface grading and drainage will be critical in the satisfactory performance of the building. The following drainage precautions should be observed during construction and maintained at all times after the building 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 10% for at least 5 feet away from the building in unpaved areas and a minimum slope of 2'/z inches in the first 10 feet in paved areas. Free -draining basement wail backfill should be covered with filter fabric and capped with at least 2 feet of the on-site, fine grained soils to reduce surface water infiltration. Job No. 113471H Gtech -9- 4) Roof gutters should be provided with downspouts that discharge at least 5 feet beyond the foundation and preferably into subsurface solid drain pipe to suitable discharge. Surface swales should have a minimum grade of 4%. 5) Landscaping which requires regular heavy irrigation, such as sod, should be located at least 10 feet from foundation walls. Consideration should be given to use of xeriscape to help prevent subsurface wetting 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 Iocation 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. 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 lob No. 11347IH HP ech - 10 - 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: Daniel E. Hardin, P.E. SLP/ksw )4„E_ cc: Silich Construction — David Guthrie (deuthrielcisilicliconstruction.cam) Silich Construction — John Silich (iohn(lsilichconstruction.coin) Silich Construction — Jodi Thimsen (jodi)silichconstruction.com) Job No. 113 47111 APPROXIMATE SCALE =20' root R ar root emmemcBORING 6 \, • LOT 292 II CV = I I IICV f 1� .I� L 1+o _ 111 113 471H GeStech HEPWORTH-PAWLAK GEOTECHNICAL i 1 1 LOCATION OF EXPLORATORY BORING FIGURE 1 T d w 5945 5940 — 5935 5930 5925 5915 113471H LOT 292 BORING 6 ELEV. = 5941' MAIN LEVEL FINISH FLOOR = 5942' 19/12 9/12 WC=7.7 DD=104 / -200=79 / 8/12 WC=7.8 DD=105 / -200=44 I6112 . BASEMENT LEVEL FINISH FLOOR = 5932' Note: Explanation of symbols is shown on Figure 3. Gtech HEPWORTKPAWLAK GEOTECHNICAL LOG OF EXPLORATORY BORING 5945 5940 — 5935 5930 5925 5915 DEPTH - FEET FIGURE 2 LEGEND; pFILL; mixed clayey silt, sand and gravel with cobbles, medium dense, slightly moist to moist, constructed mainly in 2006. 7 L b 19/12 SILT (ML); sandy to very sandy, slightly clayey, gravel layers, loose to medium dense/stiff, moist, light brown to brown, slightly calcareous. GRAVEL AND COBBLES (GM -GP); slightly silty, sandy, probable boulders, dense, moist, brown, rounded river 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 0-1586. Drive sample blow count; indicates that 19 blows of a 140 pound hammer falling 30 inches were required to drive the California or SPT sampler 12 inches. NOTES: 1. Exploratory boring was drilled between December 24, 2013 and January 2, 2014 with 4 -inch diameter continuous flight power auger. 2. Location of exploratory boring was measured approximately by pacing from features shown on the site plan provided. 3. Elevation of the exploratory boring was obtained by interpolation between contours shown on the site plan provided. 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 logs 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 (pct) -200 = Percent passing No. 200 sieve 113 471H Gtech HFPWr RTH PAW! AK r;F9TFCANIC:AI s. LEGEND AND NOTES FIGURE 3 COMPRESSION (% ) 0 1 2 3 4 5 0.1 113471H Moisture Content = 7.7 percent Dry Density - 104 pcf Sample of: Sandy Clayey Silt From: Boring 6 at 5 Feet 10 10 APPLIED PRESSURE (ksf ) Ge Ptech H EPWORTH-PAW LAK GEOTEC H NICAL Compre'sion upon wetting SWELL -CONSOLIDATION TEST RESULTS 100 FIGURE 4 V z J a 0 z 0 W 1- 0 0CD r Q Et ,.-10 mQm Qta-J a IL I 0 Ce 0 < a 2 w m z to Job No.113 47111 Y TEST RESULTS