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HomeMy WebLinkAboutSoils Study for Foundation Design 05.29.2015GtBc1h HEPWORTH- PAW LAI4 GEOTECHNICAL 1LI,,,„viiiI'li#II.,v I -{ ulv ; SUBSOIL STUDY FOR FOUNDATION DESIGN PROPOSED RESIDENCE LOT 74,1171 RIVER BEND WAY GARFIELD COUNTY, COLORADO JOB NO. 113 471E SEPTEMBER 22, 2014 REVISED MAY 29, 2015 PREPARED FOR: ASPEN SIGNATURE HOMES OF IRONBRIDGE, LLC ATTN: LLWYD ECCLESTONE P.O. BOX 7628 ASPEN, COLORADO 81612 Iccrlesione.a phIlhfl.+tet P.trk' i 303-N41-7119 e Celt ratio Sr illo 719-631-5562 5ilo erthurnc 970.46ti-1919 TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY - I - 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 - NONSTRUCTURAL FLOOR SLABS 7 - UNDERDRAIN SYSTEM 7 - SURFACE DRAINAGE _ 7 - LIMITATIONS - 8 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 74, 1171 River Bend Way, Garfield County, Colorado. The project site is shown on Figure I . 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. 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 74 building foundation design, dated September 28, 2007, Job No. 107 0486. An exploratory boring was drilled an 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 post - tensioned (P/T) slab foundation. The P/T slab will include garage floor and patio/porch slabs which will be close to the main building floor level. Grading for the structure is assumed to be relatively minor with cut and fill depths between about 2 to 4 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. 113 471 Gtech -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. The ground surface has been graded relatively flat with shallow cuts and fills during the subdivision development and slopes down to the east with a few feet of elevation difference across the building area. Vegetation consists of sparse grass and weeds. A natural dry drainage channel is located about 300 feet to the south o f the lot. The underground utilities to the lot are complete and tlhe lot is essentially unchanged since its original grading in 2006-2007. Lot 73 located to the north and Lot 75 located to the south are each occupied with a two story residence. 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 74. 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 600 feet southeast of Lot 74. The sinkhole was excavated and backfilled during construction of the roadway. Voids have not been encountered in borings drilled into the bedrock near Lot 74 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. 113471 G.VStecin -3 - 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 -55 drill rig. The boxing 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 laboratory for review by the project engineer and testing. SUBSURFACE CONDITIONS A graphic log of the subsoil profile encountered at the boring is shown on Figure 2. The subsoils consist of about 3 feet of compacted sand, silt and gravel mixed fill and 8 feet of loose to medium dense, sandy silt (debris fan deposits) overlying dense, silty sandy gravel with cobbles at a depth of 11 feet down to the drilled depth of 12 feet. Drilling in the coarse granular soils with auger equipment was difficult due to the cobbles and drilling refusal was encountered in the deposit. Based on subsurface exploration conducted on adjacent lots, the total depth of debris fan deposit is interpolated as 18 to 20 feet and overlying dense river gravel alluvium. The existing fill material was placed during the subdivision development and typically monitored during the overall construction for compaction by Il:epworth-Pawlak Geotechnical. 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 soil, presented on Figure 4, indicate Iow compressibility under existing low moisture Job No. 113 47] Ge""�tech -4 - condition and light loading and a moderate collapse potential (settlement under constant load) when wetted. The sample showed moderately high 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 (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) interpolated at a depth of about 18 to 20 feet would have moderate bearing capacity and low settlement risk. A shallow foundation placed on compacted filI can be used for building support with a potential for differential settlement, mainly if the debris fan soils are wetted. The compacted fill placed below the P/T slab foundation to a certain depth is recommended to reduce the differential settlement potential. If a deep foundation is proposed for a low settlement risk, we should be contacted to provide additional recommendations. 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 a P/T slab foundation placed on at least 4 feet of compacted fill to help limit settlement potential. The design and construction criteria presented below should be ohscrved for a P/T slab foundation system 1) A post -tensioned slab placed on at least 4 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 to be about 1 inch or less_ Job No. 113 471 G mach -5 - Additional diferential 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 placement and observe all trench excavations prior to concrete placement for 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 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 Jab No. 113 471 -6 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 Ieast 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. 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 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. Job No, 1 l3 471 cc tech 7 NONSTRUCTURAL FLOOR SLABS The natural on-site soils and compacted fill can be used to support lightly loaded, nonstructural 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, nonstructural 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. 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 an -site soils devoid of vegetation, topsoil and oversized rock. UNDERDRAIN SYSTEM It is our understanding 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 also create a perched condition. We recommend below -grade construction (if any), such as retaining walls and basement areas, be protected from wetting and hydrostatic pressure buildup by an underdrain system. If the finished floor elevation of the proposed structure has a floor level below the surrounding grade, we should be contacted to provide recommendations for an underdrain system. All earth retaining structures should be properly drained. 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; Job No. 113471 Gtech -8- 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 1 0% for at Ieast 10 feet away from the building in unpaved areas and a minimum slope of 2%i inches in the first 10 feet in paved areas. Free -draining retaining wall backfill (if constructed) 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. 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 arca 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 MOI3C, then a professional in this special field of practice should be consulted. Our findings include interpolation and extrapolation of the subsurface Job No. [13471 C- C•tech -9 - 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 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 cc: Silich Homes Jodi Thimsen (iii �i siliclthtrrt�es,ccfm Job No 113 471 /'ROBERTSON DITCH (PIPED) 1 LOT 75 (DEVELOPED) 1 I� / 1 113 471 E 1 LOT 74 APPROXIMATE SCALE 1"=30' / s BORING 1 998 • (2007) / PROPOSED RESIDENCE F.F.= 5950' ti 1 1 1-4 Hepworth—Powlok Geotechnleoi _6.946 RIVER BEND WAY LOT 73 (DEVELOPED) LOCATION OF EXPLORATORY BORINGS Figure 1 Elevation - Feet - 5950 5950 -- 14/9 LOT 74 BORING 1 (2007) ELEV. = 5948 FINISH FLOOR ELEV. = 5950' - 5945 DD= 103 5945 -200 =21 13/12 - 5940 5935 113471E 7/12 WC=5.8 DD=94 NOTE: Explanation of symbols is shown on Figure 3. I-1 Hepworth--Pcwlak GeotechnIcal LOG OF EXPLORATORY BORING 5940 5935 FIGURE 2 Elevation - Feet LEGEND: R X 7 o o. Q. FILL; mixed clay, silt and sand, scattered gravel, medium dense, slightly moist, brown. SILT (ML); sandy, scattered gravel, 'nose to medium dense, sightly moist ,I'.ght brown. GRAVEL (GM); silty, sandy, cobbles, medium dense ,slightly moist, brown, subangular rock. Relative'y undisturbed drive sample; 2 -inch I, D. California liner sample. Drive sample: standard penetration test (SP1), 1 3/8 inch I.D. split spoon sample, ASTM 1586. 7/12 Drive sample blow count; indicates that 7 blows of a 140 pound hammer fat'ing 30 inches were required to drive the California or SP1sampler 12 inches. Practical drilling refusal. T --> Caved depth when checked on July 23, 2007. NOTES: 1. The exp.oratory boring was dril ed on July 20, 2007 with a 4 -inch diameter continuous flight power auger. 2. The exploratory boring location was measLred approximately by pacing from features shown on the site plan provided (2007). 3. The exploratory boring elevation was interpolated from the contours shown on the plan provided (2015). 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 (pct) -200 = Percent passng No. 200 sieve 113 471E H Hepworth—Pawfak Geotechnical LEGEND AND NOTES • • FIGURE 3 Compression % 0 1 2 3 4 5 6 7 8 0.1 1.0 10 100 APPLIED PRESSURE - ksf 113 471E 1-1 Hepworth—Pawlait Geotechnical SWELL -CONSOLIDATION TEST RESULTS FIGURE 4 Moisture Content = 5.8 percent Dry Density = 94 pcf Sample of: Sandy Silt From: Boring 1 at 9 Feet, Lot 74 Compression upon wetting ' rl--------- ) / 1 1 1 0.1 1.0 10 100 APPLIED PRESSURE - ksf 113 471E 1-1 Hepworth—Pawlait Geotechnical SWELL -CONSOLIDATION TEST RESULTS FIGURE 4 Job No. 113 471E