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HomeMy WebLinkAboutSubsoil Study 04.21.2014Gtech I IEPWORTHI-PAWLAK GEOTECHNICAL 11.'➢ 411,F111 1'.!'•4:11 t i" dl hoi. II. I1 5k.\:0 'l • II1C4 I;. 1.1 I "� 4 I1 ll;t1�1�.1 �l cn1_ I I 1 L: - 111,1 1'It. Iry t70..11 1. lilt„ fI11 . SUBSOIL STUDY FOR FOUNDATION DESIGN PROPOSED RESIDENCE LOT 247, RIVER BEND WAY GARFIELD COUNTY, COLORADO JOB NO. 113 471B APRIL 21, 2014 PREPARED FOR: ASPEN SIGNATURE HOMES OF IRONBRIDGE, LLC ATTN: LLWYD ECCLESTONE P.O. BOX 7628 ASPEN, COLORADO 81612 Porlwr IM -841.7119 • CoIor;'ILlI priI1 719-611-5562 • Si'Iverihome 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 247, River Bend Way, Garfield County, Colorado. The project site is shown an 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 Blue Herron Management, LLC dated November 26, 2013. 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 May 31, 2005, Job No. 105 115-4. The current study is an update of our previous subsoil study report conducted for the Lot 247 building foundation design, dated September 28, 2007, Job No. 107 0486. An exploratory boring was 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 2 -story, wood frame structure above a basement in the living area. The garage floor and patio/porch slabs will be close to the main building floor Level. The basement and garage floors and patio/porch will be slab -on -grade. Grading for the structure is assumed to be relatively minor with cut depths between about 3 to 12 feet. We assume relatively light foundation loadings, typical of the proposed type of construction. Job Na 113 471B Gtech -2 - 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 lot is located on the eastern side of River Bend Way just to the north of the Mountain Cottages part of Phase 2. The underground utilities to the lot are complete. Minor overlot grading during subdivision development consists of shallow fills. The 17th Hole of the golf course borders the east side of the lot. The ground surface is relatively flat and slopes gently down to the east. Vegetation consists of sparse grass and weeds. The lot is essentially unchanged since its original grading in 2006-2007. Lot 246, Iocated to the south, is occupied with a two story residence and Lot 248 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 debris fan deposits overlying gravel terrace alluvium of the Roaring Fork River. The alluvium is predominantly a clast-supported deposit of rounded gravel, cobbles and boulders up to about 3 feet in size in a silty sand matrix which extended down to depths of about 25 to 30 feet below ground surface and overlies siltstone/claystone bedrock in the area of Lot 247. 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. An apparent sinkhole was observed along the south side of River Bend Way and River Bank Way intersection about 450 feet northeast of Lot 247. The sinkhole was excavated and backfilled during construction of the roadway. Voids have not been encountered in borings drilled into the bedrock near Lot 247 and the potential for subsidence due to dissolution of the evaporite throughout the service life of the residence, Job No. 113471B -3 - in our opinion, is low, but the owners of the lot should be aware of the sinkhole potential and the risk of future subsidence. FIELD EXPLORATION The field exploration for the project was conducted on July 20, 2007. An 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 -45 drill rig. The boring was logged by a representative of Hepworth-Pawlak Geotechnical, Inc. Samples of the subsoils were taken with 1-3/8 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 Log of Exploratory Boring, Figure 2. The samples were returned to our Iaboratory for review by the project engineer and testing. SUBSURFACE CONDITIONS A graphic log of the subsoil profile encountered in the boring is shown on Figure 2. The subsoils consist of about 2 feet of compacted sand, silt and gravel mixed fill and 141/2 feet of stiff, sandy silt and clay (debris fan deposits) overlying dense, slightly silty sandy gravel, cobbles and boulders (terrace alluvium) at a depth of 16'/2 feet down to the drilled depth of 20 feet. Drilling in the dense terrace alluvium with auger equipment was difficult due to the cobbles and boulders and drilling refusal was encountered in the deposit. The existing fill material was placed during the subdivision development and monitored during the overall construction for compaction by Hepworth-Pawlak Geotechnical. Job No. ]13471B Gegtech -4. - Laboratory testing performed on samples obtained from the boring included natural moisture content and density and finer than sand size gradation analyses. Results of swell -consolidation testing performed on a relatively undisturbed drive sample of the silt and clay 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 in 2007 and the subsoils • were slightly moist. FOUNDATION BEARING CONDITIONS The upper silt and clay (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 terrace 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 the basement level footings 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 or compacted structural fill. The design and construction criteria presented below should be observed for a spread footing foundation system. Job No, 113471B 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 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 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 Iocal 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) Any vegetation and Ioose disturbed soils should be removed and the footing bearing level extended down to the firm natural soils or compacted fill. 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 edge a distance at least 'A 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 IateralIy 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. 113 47113 GecPtech -6 - 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 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 footings placed on fine-grained soils and 0.50 for footings placed on gravel soils. Passive pressure of compacted backfill against the sides of the footings can be 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 Job IVa. 113 471B HPtech -7 - 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 and clay 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 and shallow crawispace areas). Job No, 1 I 3 471B Gate' ch -8 - Where installed aroundbasement 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 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'/2 feet deep. In silt and clay 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 and preferably 10 feet away from the building in unpaved areas and a minimum slope of 21/2 inches in the first 10 feet in paved areas. Free -draining basement wall 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. 113 471B 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 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. Ow- 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 Job No. 113 471B Gc itech -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 y,„0„ cc: Silich Construction — Dave Ockers (docker(vsilicltconstruction.conl) Silich Construction — John Silich (johri(siliclleonstruction.coni) Job No. 113417IH Gtech Brt: 5971,-0• FG: 59T0,-4 Vora.* �+ nwl nr r1 a aur s.. ........ 4-0.11.474 .1. Fitt 5971'-5' 1'G: 5911.-0' oBORIN r ..... M1 o L� r M1 i— --- 1 1 -, I I a f„ i e I i 44• , y—� II APPROXIMATE SCALE 1" = 25' 0 11 0 0 m 113 471B GeZtech I IEPWORTH-PAWLAK GEOTECHNICAL LOCATION OF EXPLORATORY BORING FIGURE 1 0 - 5 — 10 20 BORING 1 LOT 247 9/12 12/12 W0=8.9 DD=99 14/12 18/12 WC=5.9 DD=104 -200=89 2816,10/2 0 5 10 15 20 . 25 25 NOTE: Explanation of symbols is shown on Figure 3. v u_ a N 0 113471B GecPtech HEPWORTH-PAWLAK GKOTECHNICAL LOG OF EXPLORATORY BORING Figure 2 LEGEND: 2 FILL; mixed clay, silt and sand with gravel, medium dense, slightly moist, brown. SILT AND CLAY (ML -CL); slightly sandy to sandy, stiff, slightly moist, light brown, slightly calcareous. GRAVEL AND COBBLES (GM -GP); slightly silty, sandy, probable 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 ID, split spoon sample, ASTM -1586. 9/12 Drive sample blow count; indicates that 9 blows of a 140 pound hammer falling 30 inches were required to drive the California or SPT sampler 12 inches. T Practical drilling refusal. --> Depth at which boring had caved when checked on July 23 , 2007. NOTES: 1. The exploratory boring was drilled on July 20, 2007 with a 4 -inch diameter continuous flight power auger. 2. The exploratory boring location was measured approximately by pacing from features shown on the site plan provided. 3. The exploratory boring elevation was not measured and the log is drawn to depth. 4. The exploratory boring location 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 or when checked 3 days later. 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 113 471B G tech HEPWORTH•PAWLAK GEOTECHNICAL LEGEND AND NOTES Figure 3 Compression % -IN. W N -,. 0 Moisture Content = 8.9 percent Dry Density = 99 pef Sample of: Sandy Silt and Clay From: Boring 1 at 5 Feet, Lot 247 r rt Compression upon wetting 1 r r 0.1 1.0 10 100 APPLIED PRESSURE - ksf 113 471 B H Gtech H EPWORTH-PAWLAK GEOT4CHNICA4 SWELL -CONSOLIDATION TEST RESULTS Figure 4 HEPWORTH-PAWLAK GEOTECHNICAL, INC. TABLE 1 SUMMARY OF LABORATORY TEST RESULTS Job No. 113 471B Lot 247 SAMPLE LOCATION NATURAL MOISTURE CONTENT (%) NATURAL DRY DENSITY (pcf) GRADATION PERCENT PASSING NO. 200 SIEVE ATTERBERG LIMITS UNCONFINED COMPRESSIVE STRENGTH (PSF) SOIL OR BEDROCK TYPE BORING DEPTH (ft) GRAVEL (%) SAND (%) LIQUID LIMIT (%) PLASTIC INDEX (%) 1 5 8.9 99 Sandy silt & clay 15 5.9 104 89 Sandy silt & clay I i