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HomeMy WebLinkAboutSoils Reportetch HEPWORTH—PAWLAK GEOTECHNICAL 14crtloitht•m•I•11• t ;L:44,2,111114—.11. I»L 50201 o,imti I:1,,,J 154 rib au•: ,i; 019-}i.7 9 SS rw,ul: lir 1c,..irlry,jlei.tr.I1., “in SUBSOIL STUDY FOR FOUNDATION DESIGN PROPOSED RESIDENCE LOT 71, RIVER BEND WAY GARFIELD COUNTY, COLORADO JOB NO. 113 471G OCTOBER 27, 2014 REVISED OCTOBER 31, 2014 PREPARED FOR: ASPEN SIGNATURE HOMES OF IRONBRIDGE, LLC ATTN: LLWYD ECCLESTONE P.O. BOX 7628 ASPEN, COLORADO 81612 leccicstonerrr jhlbft nef Parker 303-841-7119 • CttlulSpr•ir7,uti 119-633-'5562 • Silvcrtlitlrne 97CL468-1989 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 - STRUCTURAL SLAB FOUNDATION - 5 - DEEP FOUNDATION ALTERNATIVE - 4 - FOUNDATION AND RETAINING WALLS - 5 - FLOOR SLABS (NON-STRUCTURAL) - 7 - UNDERDRAIN SYSTEM - 8 - SURFACE DRAINAGE _ g - 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 71, 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 August 27, 2014. The current study is an update of our previous subsoil study report conducted for the Lot 71 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 supported on a structural slab foundation in both the living area and the garage. The garage and front porch slab grades will be close to the main building floor level. Grading for the structure is assumed to be relatively minor with cut depths between about 3 to 5 feet. We assume relatively light foundation Ioadings, 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. 113 471G -2 - SITE CONDITIONS The lot is located on a strongly sloping alluvial fan along the uphill, western side of River Bend Way. The Robertson Ditch (now buried) and easement borders the uphill side of the lot. A detention basin for surface runoffborders the north side of the lot and contained cattail and other marsh type vegetation at the time of this report preparation. The ground surface of the lot has been graded relatively flat with Iikely shallow fill placed in the central to eastern part during the subdivision development and is nearly level across the proposed building area. Vegetation consists of mainly weeds on the lot. The underground utilities to the lot are complete and the lot is essentially unchanged since its original grading in 2006-2007. Lot 72 located to the south 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 alluvial fan deposits with inter -bedded sandy and gravelly layers 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 35 to 40 feet below ground surface and overlies siltstone/claystone bedrock in the area of Lot 71. 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 900 feet southeast of Lot 71. The sinkhole was excavated and backfilled during construction of the roadway. A sinkhole occurred in the parking lot adjoining the golf cart storage tent in 2005, located about 700 feet to the northwest of Lot 71, which was backfilled and compacted. Both sinkholes have not shown signs of reactivation such as ground subsidence since their remediation. Subsurface voids have not been encountered in borings drilled into the bedrock near Lot 71 and the potential for subsidence due to dissolution of the evaporite throughout the Job No. 113 4710 Gegtech -3 - service Life of the residence, in our opinion, is low, but the owner of the Iot should be aware of the sinkhole potential and the risk of future subsidence. FIELD LB 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 -45B drill rig. The boring was logged by a representative of Hepworth-Pawlak Geotechnical, Inc. Samples of the subsoils were taken with 1% 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 -I 586. 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 the boring is shown on Figure 2. The subsoils consist of about 6 feet of silty sand and gravel overlying 7 feet of loose to medium dense, sandy silt and silty sand above 10 feet of medium stiff sandy silt and clay (debris fan deposits) overlying dense, slightly silty sandy gravel, cobbles and boulders (river alluvium) at a depth of 23 feet down to the drilled depth of 34 feet. Drilling in the dense river alluvium with auger equipment was difficult due to the cobbles and boulders and drilling refusal was encountered in the deposit. The upper few feet of the boring may be fill material placed during the subdivision development. 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 Job No. 113 471G Gatech -4 -- sandy silt 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 of drilling in 2007 and the subsoils were slightly moist to moist with depth. FOUNDATION BEARING CONDITIONS The upper sand, silt and clay (debris fan) soils typically have low bearing capacity and low to moderate settlement potential when wetted under loading and extend down about 20 feet below a shallow foundation such as spread footings. Considering the compressible nature of the debris fan soils and the potential for (or continued) wetting from the adjacent detention pond, spread footings could have a high risk of excessive settlement and are not recommended for the building foundation support. With a risk of differential settlement and minor distress, the building could be founded with a heavily reinforced structural (mat) slab or post -tensioned slab foundation bearing on at least 5 feet of compacted structural fill and is recommended for the building support. As an alternative, foundations that extend down to the dense, river gravel alluvium (such as piers or piles) could be used and would have moderate bearing capacity with low sett:ement and building distress risk. DESIGN RECOMMENDATIONS STRUCTURAL SLAB FOUNDATION The design and construction criteria presented below should be observed for a slab foundation system. 1) A structural slab or post -tensioned slab placed on at least 5 feet of compacted structural fill should be designed for an allowable bearing pressure of 1,000 psf. Post -tensioned slabs placed on structural fill should be designed for a wetted distance of 10 feet but at least half of the slab width whichever is more. Initial settlement of the foundation is estimated Job No. 1134710 Gatech -5 - to be about 1 inch or less. Additional differential settlement of about 1 to 2 inches is estimated if deep wetting of the debris fan soils were to occur. 2) The thickened sections of the slab for support of concentrated loads should have a minimum width of 20 inches. 3) The perimeter turn -down section of the slab (if used) should be provided with adequate soil cover above the bearing elevation for frost protection. Placement of foundations at least 36 inches below exterior grade is typically used in this area. If a frost protected foundation is used, the perimeter turn -down section should have at least 18 inches of soil cover. 4) The foundation should be constructed in a "box -like" configuration rather than with irregular extensions which can settle differentially to the main building area. The foundation walls, where provided, 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 (if any) should also be designed to resist lateral earth pressures as discussed in the "Foundation and Retaining Walls" section of this report. 5) The root zone and any loose or disturbed soils should be removed. Structural fill placed below the slab bearing level should be compacted to at least 98% of the maximum standard Proctor density within 2 percentage points of optimum moisture content and can consist of the onsite soils. 6) A representative of the geotechnical engineer should evaluate the compaction of the fill materials during its placement and observe all footing excavations prior to concrete placement for bearing conditions. DEEP FOUNDATION ALTERNATIVE Considering the compressibility potential of the debris fan soils encountered al. the site and the nature of the proposed construction, piles or piers that extend down into the underlying river gravel alluvium could be used for building support. Deep foundations can typically consist of drilled or pushed micro -piles or helical piers. We expect the piles or piers will be at least 20 feet long and penetrate the river gravel to achieve downward allowable pile load capacity on the order of 20 to 40 kips. Uplift capacity will depend on Job No. 113 4710 Gtech -8 - 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 (if provided), be protected from wetting and hydrostatic pressure buildup by an underdrain system. An underdrain should not be provided around shallow foundations (such as structural slabs and crawlspace areas if used). Where installed around basement areas (if constructed), 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 drywelI 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 feet deep. 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. JobNo. tI347IG Gec&tech - 10 - 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 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. S LP/ksw cc: Silich Construction , David Guthrie (dguthrie(i silichconstruction.crnn) Silich Construction - Jodi Thimsen (Jodi ri silichhomes.corn) Job No, 11347IG APPROXIMATE SCALE DETENTION POND • rBORING 1 LOT, 71 t 12°2 • rr 5951 Mf— ! maga r n ap�u�iq 1 a+c• 'tr� r+.�.. rp //jLOT2} ___ 1 r; 5951 */- 1 ! / i i 1 5 -� I I � • I _:- 1 Gtech r lt PWORTH PAWLAK GEOTECI IWICAL • 1 LOCATION OF EXPLORATORY BORING 1 FIGURE 1 Elevation - Feet - 5950 - 5945 - 5940 5935 - 5930 5925 - 5920 5915 LOT71 BORING ELEV. 5951' MAIN FLOOIR ELEV. 5951' INVERT LEVEL OF ADJ. DETENTION BASIN 354112 ' : .tl°:: 18112 Xti� WC==3.6 DD=99 {•• -200-18 g 18/12 WC -11.1 DD=107 4 "/ 5/12 r J 5935 5950 5945 5940 4 .p Q1 .b.• 4112 60/12 NOTE: Explanation of symbols is shown on Figure 3. 5930 5925 5920 5915 Elevation - Feet 113 471G G VStech HF.PWORm.PAW W K GEOTECHNICAL LOG OF EXPLORATORY BORING FIGURE 2 1 LEGEND: oPO' -71 2 54/12 T NOTES: SAND AND GRAVEL (SM -GM); silty, medium dense, slightly moist, brown, subangular rock. Poss b e compacted fill in upper part. SAND AND SILT (SM -ML); loose to medium dense, slightly moist, Tight brown, stratified. SILT AND CLAY (ML -CL); slightly sandy, medium stiff, moist, light brown. GRAVEL AND COBBLES (GM -GP); slightly silty, sandy, probable boulders, dense, brown, rounded rock. Relatively undisturbed drive sample; 2 -inch I.O. California liner sample. Drive sample; standard penetration test (SPT), 1 3/8 inch 1.0. split spoon sample, ASTM -1586. Drive sample blow count; indicates that 54 blows of a 140 pound hammer falling 30 Inches were required to drive the California or SPT sampler 12 inches. Practical drilling refusal, Caved depth when checked on July 23, 2007. 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 interpreted from the contours shown on the site plan provided by H.C.E. 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 or when checked 3 days later. Fluctuation in water level may occur with time. 7. Laboratory Testing Results: WC = Water Content (%) DD = Dry Density (pci) -200 = Percent passing No. 200 sieve 113 471G HEIMORTH.pgyyLAK GEOTECHNICAL LEGEND AND NOTES FIGURE 3 Compression 0 1 2 3 4 Moisture Content = 11.1 percent Dry Density = 107 pct Sample oft Very Sandy Silt with Gravel From: Boring 1 at 10 Feet, Lot 71 upon npression up wetting 0.1 1.0 10 100 APPLIED PRESSURE - ksf 113 471G G&ec.1 1 H EA W ORTI+PAW LAK GEOTECHNICAL SWELL -CONSOLIDATION TEST RESULTS FIGURE 4 Job No. 113 471G U (n z ~r J U) a w U Ice H z = w U h- } O w OAF Yw CO co < 1— a1-- a EL O ce cc O < w E = cn