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HomeMy WebLinkAboutSoils Report 06.10.2015HEPWORTH-PAMAK GEOTECHNICAL 1 J t , J II 1 lid Il t iii It n km, d. IIIA twit 1r1�Jltil•�� 1'I Illi_.l "Iq 41.r -.I1001 11• IL "70 it -1 i, r'11ti IL III 11 151 - r � 1 1' . 111 11 veil SUBSOIL STUDY FOR FOUNDATION DESIGN PROPOSED RESIDENCE LOT 172, IRONBRIDGE 21 BENT GRASS DRIVE GARFIELD COUNTY, COLORADO JOB NO. 113 471M JUNE 10, 2015 PREPARED FOR: ASPEN SIGNATURE HOMES OF IRONBRIDGE, LLC ATTN: LLWYD ECCLESTONE P.O. BOX 7628 ASPEN, COLORADO 81612 lecclesioiierii pbpl<17.iiet C,rrkei 303-S41.7119 • C1lur.r.141S1prin 719-633-5562 • S1hirr1111mc 970-4&i-193.~9 TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY - I - PROPOSED CONSTRUCTION - I - 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 (NON-STRUCTURAL) - 6 - UNDERDRAIN SYSTEM - 7 - SURFACE DRAINAGE - 7 - LIMITATIONS - 8 - FIGURE I - 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 Iocated on Lot 172, Ironbridge, 21 Bent Grass Drive, 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 May 28, 2015. We previously performed a preliminary geotechnical study for this area of the subdivision development and presented our findings in a report dated May 31, 2005, Job No. 105 115-4. We also performed observation and testing of compaction during the infrastructure and lot grading construction in 2006-2007 and presented those findings in daily reports. The current study is an update of our previous subsoil study report conducted for foundation design on Lot 172 dated September 28, 2007, Job No. 107 486. 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 patio slab grades 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 3 feet. We assume relatively Iight foundation loadings, typical of the proposed type of construction. Job No. 113 4711s4 -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 uphill, southeastern side of Bent Grass Drive across the street to the southeast of the Mountain Cottages part of the subdivisions. The underground utilities to the lot and the roadway construction are complete. OverIot grading during subdivision development consists of relatively shallow cuts and fills. The 1691 Hole of the golf course borders the southeast side of the lot. The ground surface has been benched relatively flat and slopes gently down to the northeast. Vegetation consists of grass and weeds. 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 was encountered to depths of about 40 feet below ground surface in the area of Lot 172. The underlying bedrock consists of the Eagle Valley Evaporite which contains gypsum deposits and is generally associated with scattered sinkhole development in the Roaring Fork River valley. A sinkhole was observed prior to the subdivision development in Fairway 16 about 350 feet south of Lot 172. The sinkhole was backfilled during construction of the golf course. Voids have not been encountered in borings drilled into the bedrock near Lot 172 and the potential for subsidence due to dissolution of the Evaporite throughout the service Iife 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 471M Gaitect, -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 boring was logged by a representative of Hepworth-Pawlak Geotechnical, Inc. Samples of the subsoils were taken with a 2 inch 1,13. spoon sampler. The sampler was driven into the subsoils at various depths with blows from a l40 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 the boring is shown on Figure 2. The subsoils consist of about 4 feet of compacted silt, sand and clay mixed fill and 10Y2 feet of loose to medium dense, sandy silt (debris fan deposit) overlying dense, slightly silty sandy gravel, cobbles and boulders (gravel terrace alluvium) down to the drilled depth of 15 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. A similar soil profile was encountered on Lot 173, located immediately southwest of Lot 172, with about 4 feet greater sandy silt depth due to the lot being about 4 feet higher in elevation. The existing fill material was placed during the subdivision development and monitored during construction for compaction by Hepworth-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 low compressibility under existing low moisture condition and light loading and a minor collapse po`ential (settlement under constant lob No. 113 471M -4 - load) when wetted. The sample showed moderate compressibility under additional loading after wetting. The laboratory test results arc summarized in Table 1. No free water was encountered in the boring at the time drilling or when checked three day later 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 light loading when wetted. With a risk of differential settlement and minor distress, the building can be founded with a heavily reinforced structural (mat) slab or post -tensioned slab foundation bearing on at Least 4 feet of compacted structural fill and is recommended for the building support. As an alternative, foundations that extend down to the dense, gravel terrace alluvium (such as piers or piles) could be used and would have moderate bearing capacity with low settlement and building distress risk. DESIGN RECOMMENDATIONS FOUNDATIONS Considering the subsurface conditions encountered in the exploratory boring and the nature of the proposed construction, the building can be founded on a structural, slab foundation bearing on compacted structural fill. If a deep foundation is proposed, we should be contacted for additional recommendations. 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 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. Additional differential settlement of about 1 to 2 inches is estimated if deep wetting of the debris fan soils were to occur. Job No. 113 471M Ge ttech -5- 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. FOUNDATION AND RETAINING WALLS Foundation walls and retaining structures (if any) 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 an the basis of an equivalent fluid unit weight of at least 45 pcf for backfill consisting of the on-site soils. lob No. 113 471M Cie itech -6- AlI 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 unposed 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 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. 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. FLOOR SLABS (NON-STRUCTURAL) The natural on-site soils and compacted fill are suitable to support lightly loaded slab -on - grade construction (if used). The upper silt soils have variable settlement potential when wetted under load and there could be some post -construction slab movement if the Job No. 1E3471M -7- subgrade soils become wet. To reduce the effects of some differential movement, non- structural 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 relatively well graded sand and gravel, such as road base, should be placed beneath slabs for subgrade support. This material should consist of minus 2 -inch aggregate with at least 50% retained on the No. 4 sieve and less than 12% passing the No. 200 sieve. All fill materials for support of non-structural 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 It is our understanding the finished floor elevation at the lowest building level will be at or above the surrounding grade. Therefore, a foundation drain system is not recommended. 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 reconunend below -grade construction, such as retaining walls and basement areas, be protected from wetting and hydrostatic pressure buildup by an underdrain and wall drain system. If the proposed structure has a finish 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. 13 471M le tech -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 10% for at least 10 feet away from the building in unpaved areas and a minimum slope of 2% inches in the first 10 feet in paved areas. 4) Roof gutters should be provided with downspouts that discharge at least 5 feet beyond the foundation and preferably into subsurface, rigid solid PVC drain pipe sloped 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. 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 Job No. 113 47111 Gtech -9 - 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: -1.(X Darnel E. hrdin, .E. SLP/ksw cc: Silich Homes — Jodi Thimsen (iodi a. Ai. .corn) Job No. 113 471 NI APPROXIMATE SCALE 1"=30' LOT 173 (DEVELOPED) �1\ BENT GRASS DANE 1 - L \ — PATIO PATIO i 1 PROPOSED RESIDENCE 1 FFE = 5984.5' 1 • \ BORING 1 NN 1 9$1` 7- VACANT OPEN SPACE 7ilm. r r / r — � // � / i / ca '5„. --� I ir �r ,r -7 ,— , , �� 46) 59 GOLF COURSE 16th HOLE 113 471M Hepworth-Pawlak Geotechnical LOCATION OF EXPLORATORY BORING Figure 1 Elevation - Feet - 5990 - 5985 - 5980 - 5975 5970 113471M BORING 1 LOT 172 ELEV. = 5985.5' 24/12 8/12 WC=13.3 DD=108 4/12 WC=14.8 0U=103 -200=91 2116, 10/0 FINISH FLOOR ELEV.= 5984.5 NOTE: Explanation of symbols is shown on Figure 3. I-1 Hepworth—Pawlak Gaateehnical LOG OF EXPLORATORY BORING 5990 5985 5980 5975 5970 Figure 2 Elevation - Feet LEGEND: FILL; mixed clay, silt and sand with gravel and cobbles, medium dense, slightly moist, brown. ® SILT (ML); slightly sandy to sandy, loose to medium dense, slightly moist to moist, Tight brown. GRAVEL AND COBBLES (GM -GP); slightly silty, sandy, probable boulders, moist, brown, rounded rock. ebf I:1 24/12 T Relatively undisturbed drive sample; 2 -inch I.D. California liner sample. Drive sample blow count; indicates that 24 blows of a 140 pound hammer falling 30 inches were required to drive the California sampler 12 inches. Practical drilling refusal. ---,. Caved depth 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 obtained from contours lines shown on the site 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 (pcf) -200 = Percent passing No. 200 sieve 113 471M H Hepworth—Pawfak Geotechhtc'al LEGEND AND NOTES Figure 3 i Compression % 0 1 2 3 4 Moisture Content — 13.3 percent Dry Density = 108 pcf Sample of: Sandy Silt From: Boring 1 at 4 Feet, Lot 172 . Compression upon wetting 0.1 113 471M H 1.0 APPLIED PRESSURE - ksi 10 100 Hepworth—Pawlak Geolachnlcol SWELL -CONSOLIDATION TEST RESULTS Figure 4 Job No. 113 471M U F.12. z 11 5' J [1) Q U — 1— z tn 2 w U 1— H O uJ O • 1— Ca m < Cl. O 2 to GLS O < a 2 w Z U] SOIL OR BEDROCK TYPE Sandy Silt Slightly Sandy Silt 11 z 0 ,. z .� fij SAMPLE LOCATION 0 L7 Z 0 2