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HomeMy WebLinkAboutSoils Reportr Gtech HEPWORTH-PAWLAK GEOTECHNICAL I Irl+u, rtlt 1'.ntl i1 ('1,u,t«hni.il, Int 7020 ( :t Ru,1,1 15.1 lilcnx,nnl `I•nm,+, (ul +r.i,l, 'l(tl1i 111nft• 970-945-79,S IrjE: 970 945' -15-,1 CM til SUBSOIL STUDY FOR FOUNDATION DESIGN PROPOSED RESIDENCE LOT 293, IRONBRIDGE SUBDIVISION 1387 RIVER BEND WAY GARFIELD COUNTY, COLORADO JOB NO. 113 471L APRIL 30, 2015 PREPARED FOR: ASPEN SIGNATURE HOMES OF IRONBRIDGE, LLC ATTN: LLWYD ECCLESTONE P.O. BOX 7628 ASPEN, COLORADO 81612 ( eelettOrker>^ [jI)1hll.nei) P+irker 303-841-7119 • Colorado Springs 719-633-5562 • SiIverrh+tree 970-468-1989 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 - 6 - FLOOR SLABS - 7 - UNDERDRAIN SYSTEM - 7 - SURFACE DRAINAGE - g LIMITATIONS - 9 - FIGURE I - LOCATION OF EXPLORATORY BORINGS FIGURE 2 - LOG OF EXPLORATORY BORING FIGURE 3 - LEGEND AND NOTES FIGURE 4 - SWELL -CONSOLIDATION TEST RESULTS PURPOSE AND SCOPE 01? STUDY This report presents the results of a subsoil study for a proposed residence to be located on Lot 293, Ironbridge Subdivision, 1387 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 April 27, 2015. 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. That study included the subsurface conditions information presented in our previous report dated September 14, 2005, Job No. 105 115 6 for the same Phase 2 subdivision development area. The current study is an update of the information presented in the two preliminary subsoil study reports pertaining to the proposed Lot 293 building development plan. An exploratory boring was previously drilled on the lot 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 Job No. 113 4711- Gmech -2 - be relatively minor with cut depths between about 5 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. SITE CONDITIONS The Iot 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 Iot during the subdivision development with about 1 foot of elevation difference across the building footprint. Vegetation consists mainly of weeds. The underground utilities to the lot are complete and the lot is essentially unchanged since its previous grading in 2006-2007. Lot 292 located to the south is under construction (with a basement level) and Lot 294 located to the north is 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 and clay 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 30 feet below ground surface and overlies siltstonelclaystone bedrock in the area of Lot 293. 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 corner of the Phase 2 development arca to the east of River Bend Way about 250 feet northeast of Lot 293. The sinkhole was backfilled during construction of the subdivision infrastructure. Voids have not been encountered in borings drilled into the bedrock near Lot 293 and the potential Job No. 113 471L -3 - for subsidence due to dissolution of the cvaporite 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. FIELD EXPLORATION The field exploration on the lot was conducted between July 6 and 8, 2005. Exploratory Boring 12 was drilled at the location shown on Figure I 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. 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 subsoil profile encountered in Boring 12 is shown on Figure 2. The subsoils encountered, below about % foot of topsoil, consist of medium dense/stiff, sandy clayey silt with scattered gravel (debris fan deposits) overlying dense, slightly silty sandy gravel, cobbles and boulders (river alluvium) at a depth of about 16 feet down to the drilled depth of 181/2 feet. Drilling in the dense river alluvium with auger equipment was difficult due to the cobbles and boulders and practical auger refusal was encountered in the deposit. Fill material was placed during the subdivision development in about 2006 and monitored during the overall construction for compaction by Hepworth-Pawlak Geotechnical. Based on the subsurface profile of the boring recently drilled on Lot 292, (Boring 6) the fill depth is expected to be about 3 feet on Lot 293. Joh No. 113 4711, G 'tech Laboratory testing performed on samples obtained from the boring included natural moisture content and density. Results of swell -consolidation testing performed on a relatively undisturbed drive sample of the upper clayey soil, presented on Figure 4, indicate low compressibility under existing low moisture condition and Iight loading and a low expansion potential when wetted. The debris fan soils typically encountered in this arca have a low collapse potential (settlement under constant load) when wetted and show moderate compressibility under additional loading after wetting. No free water was encountered in the boring at the time drilling and the subsoils were slightly 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. The expansion potential of the sample tested is not representative of the debris fan soils and has not been considered in our analysis. 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. Job No, 113 471 L GecPrtech 5 The design and construction criteria presented below should be observed for a spread footing foundation system. 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 1 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 5 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 Ievel 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 least 98% of standard Proctor density within 2 percentage points of optimum. The compacted fill should extend laterally beyond the footing edges a distance at least 1 the fill depth below the footing but at least 3 feet. 6) A representative of the geotechnical engineer should observe all footing excavations prior to concrete placement to evaluate bearing conditions. Job No. !13471L Gec!tech -b - 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 45 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 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 Iarge 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 footings placed on fine-grained soils. Passive pressure of compacted backfill against the sides of the footings can be Job No. 113 47IL -7 - calculated using an equivalent fluid unit weight of 300 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 and compacted fill are suitable to support lightly loaded slab -on - grade construction. The upper silt soils typically 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. AlI 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 - Jobl;o.113471L -8 - 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). Where installed around basement areas, the drains should consist of drainpipe placed in the bottom of the wail backfill surrounded above the invert level with free -draining granular material. The drain should be placed at each level of excavation and at least I 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, 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'/z feet deep. In silt soil bearing areas, an impervious membrane, such as a 30 mil PVC liner, 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 21/2 inches in the first I0 feet in paved areas. Free -draining basement wall backfill should be Job No. 113 471L Gtech -9 - covered with filter fabric and capped with at least 2 feet of the on-site, fine grained soils to reduce surface water infiltration. 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 Icast 10 feet from foundation walls. Consideration should be given to use of xeriscapc 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 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. 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 lob No, 113471L ATtech - 10 - 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. Steven L. Pawlak, P.E. Reviewed by: 1?) Daniel E. Hardin, P.E. SLP/ljf cc: Silich Construction — Hayden Horsford(hhorsIordOP.siIichconqruction.com) Silich Construction — John Silich (John @ siIichconstruction.cont) Silich Construction Jodi Thimsen (iodi@silichconstruction.com) Job No. 113 4711_ Ge&ech ORING 6 VArtila C.12.2,11 [012.101:1 I1C//Mt1OOi •Vil O nnito W Ler ptAll „3 Baia W« 113 471A Ge Ptech IaEPh. ORTI I PAWtAK GEOTE'_}+NIc A;. LOCATION OF EXPLORATORY BORINGS FIGURE 1 Elevation - Feet 5940 5935 5930 5925 5920 5915 BORING 12 Job No. 105 115 6 ELEV. = 5938' MAIN FLOOR ELEV. = 5941.5' BASEMENT FLOOR ELEV. - 5931.5' 33/12 WC -6.1 DD=117 ettr 43/12 14/12 Note: Explanation of symbols is shown on Figure 3. 5940 5935 5930 5925 5920 5915 Elevation - Feet 113 471L H HEPWORTH PAWLAK GEOTECHNICAL LOG OF EXPLORATORY BORING Figure 2 LEGEND: ® TOPSOIL; root zone, sandy silt, slightly moist, brown. SILT AND CLAY (ML -CL); slightly sandy to sandy, scattered gravel to gravelly, stiff to very stiff, s:ighlly most, mixed brown, slightly calcareous and porous, low plasticity. GRAVEL, COBBLES AND BOULDERS (GM -GP); slightly silty, dense, typically moist, brown, rounded rock. Relatively undisturbed drive sample; 2 -inch I.D. California liner sample. 39112 Drive sample blow count; indicates that 39 blows of a 140 pound hammer failing 30 inches were required to drive the California sampler 12 inches. TPractical drilling refusal. NOTES: 1. Exploratory boring was drilled between July 6 and 8, 2005 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 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. 7. Laboratory Testing Results: WC = Water Content (%) DD = Dry Density (pcf) 113 471L H H EPWORT* PAW+LAX GEOTECHNICAL LEGEND AND NOTES Figure 3 •0 0 , 0 0 cn 'j 1 0. 0 0 2 Moisture Content 6.1 percent Dry Density -- 117 pcf Sample of: Sandy Silty Clay From: Boring 12 at 4 Feet Expans upon wetting on 0.1 1.0 10 1 OO APPLIED PRESSURE - ksr 113471L I-1 HEPWOR H PAWLAK GEOTECHNICAL SWELL -CONSOLIDATION TEST RESULTS Figure 4