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HomeMy WebLinkAboutSoils Report 01.15.2018--k-4<UM R Geotechnical Engineering I Engineering Geology Materials Testing 1 Environmental 5020 County Road 154 Glenwood Springs, CO 81601 Phone: (970) 945-7988 Fax: (970) 945-8454 Email: hpkglenwood@kumarusa.com Office Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, Summit County, Colorado SUBSOIL STUDY FOR FOUNDATION DESIGN PROPOSED RESIDENCE LOT 47, OAK MEADOWS FILING 4, PHASE 2 SILVER KING COURT GARFIELD COUNTY, COLORADO PROJECT NO. 17-7-840 JANUARY 15, 2018 PREPARED FOR: JULIE YOUNG P.O. BOX 3555 GLENWOOD SPRINGS, COLORADO 81602 (Iu lieyoung970@outtook.com) TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY - 1 - PROPOSED CONSTRUCTION - 1 SITE CONDITIONS - 1 - GEOLOGIC CONDITIONS 2 FIELD EXPLORATION - 2 - SUBSURFACE CONDITIONS - 2 FOUNDATION BEARING CONDITIONS - 3 - DESIGN RECOMMENDATIONS - 3 - FOUNDATIONS - 3 - FOUNDATION AND RETAINING WALLS _ 4 FLOOR SLABS - 5 _ UNDERDRAIN SYSTEM _ 6 - SITE GRADING _ 6 - SURFACE DRAINAGE _ 7 -- LIMITATIONS LIMITATIONS - 8 FIGURE 1 - LOCATION OF EXPLORATORY BORINGS FIGURE 2 - LOGS OF EXPLORATORY BORINGS FIGURE 3 - LEGEND AND NOTES FIGURES 4 AND 5 - SWELL -CONSOLIDATION TEST RESULTS TABLE 1- SUMMARY OF LABORATORY TEST RESULTS H-PtKUMAR Project No. 17-7-840 PURPOSE AND SCOPE OF STUDY This report presents the results of a subsoil study for a proposed residence to be located on Lot 47, Oak Meadows, Filing 4, Phase 2, Silver King Court, 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 agreement for geotechnical engineering services to Julie Young dated November 29, 2017. A field exploration program consisting of exploratory borings was conducted 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 At the time of our study, design plans for the residence were conceptual. In general, the residence will be located in the downhill, southeastern part of the lot roughly between the exploratory borings shown on Figure 1 and have a walkout lower level. We assume excavation for the building will have a maximum cut depth of one level, up to about 12 feet below the existing ground surface. For the purpose of our analysis, foundation loadings of the structure were assumed to be relatively light and 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 property was vacant and free of snow cover at the time of our field exploration. Vegetation consists of native grass and weeds. The site is located on a southeast facing hillside that slopes H-P%KUMAR Protect No. 17-7-840 -2 - at a grade of about 10 percent across most of the building area. The ground surface in the lower part of the lot next to Silver King Court was cut relatively steep for the roadway construction. GEOLOGIC CONDITIONS Lot 47 is located near the lower limit of a mapped, very large, dormant landslide complex. Hepworth-Pawlak Geotechnical, Inc. evaluated the overall stability of the landslide as part of the subdivision approval by Garfield County in 1999. The evaluation included depth to bedrock and depth to groundwater level, both being relatively deep. The conclusion was that the landslide complex was not near critical stability condition and moderate cut and fill depths made for subdivision infrastructure and individual lot development should not affect the overall stability of the landslide. FIELD EXPLORATION The field exploration for the project was conducted on December 1, 2017. Two exploratory borings were drilled at the locations shown on Figure 1 to evaluate the subsurface conditions. The borings were advanced with 4 -inch diameter continuous flight augers powered by a truck- mounted CME -45B drill rig and were logged by a representative of H-P/Kumar. 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 Logs of Exploratory Borings, Figure 2. The samples were returned to our laboratory for review by the project engineer and testing. SUBSURFACE CONDITIONS Graphic logs of the subsurface conditions encountered at the site are shown on Figure 2. The subsoils, below about 2 to 5 feet of topsoil, generally consist of very stiff sandy clay to medium dense clayey sandy gravel with basalt cobbles and probable boulders. In Boring 2, the soils transitioned to mostly clayey sandy gravel with basalt cobbles and probable boulders below a depth of about 20 feet. H-P%KUMAR Project No. 17-7-840 -3 - Laboratory testing performed on samples obtained from the borings included natural moisture content and density and finer than sand size gradation analyses. Results of swell -consolidation testing performed on relatively undisturbed drive samples of the more clay soils, presented on Figures 4 and 5, indicate low compressibility under existing moisture conditions and light loading with generally minor expansion potential when wetted. The laboratory testing is summarized in Table 1. No free water was encountered in the borings at the time of drilling and the subsoils were slightly moist to moist. FOUNDATION BEARING CONDITIONS The sandy clay soils and clayey sandy gravel and cobble soils encountered at typical shallow foundation depth have moderate bearing capacity and somewhat variable settlement/heave potential. A shallow foundation placed on the sandy clay and gravel soils should typically have a low risk of post -construction movement potential. It will be critical to the long term performance of the structure that the recommendations for surface grading and subsurface drainage contained in this report be followed. The amount of settlement/heave, if the bearing soils become wet, will mainly be related to the depth and extent of subsurface wetting. Sub - excavation of expansive clay layers could be needed to help limit the movement potential and should be further evaluated at the time of excavation. DESIGN RECOMMENDATIONS FOUNDATIONS Considering the subsurface conditions encountered in the exploratory borings and the nature of the proposed construction, the building can be founded with spread footings bearing on the natural sandy clay and gravel soils with a risk of movement. The more expansive clay soils should be sub -excavated and replaced with compacted structural fill. The design and construction criteria presented below should be observed for a spread footing foundation system. H-PKUMAR Project No. 17-7-840 -4- 1) Footings placed on the undisturbed natural soils or structural fill should be designed for an allowable bearing pressure of 2,500 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. Additional differential movement up to about 1 inch could occur depending on the depth and extent of future wetting of the subgrade soils. 2) The footings should have a minimum width of 16 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 heavily reinforced top and bottom to span local anomalies such as by assuming an unsupported length of at least 12 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 topsoil and any loose or disturbed soils should be removed and the footing bearing level extended down to the firm natural soils. Expansive clay soils should also be removed up to 3 feet below footing bearing level as needed and replaced with structural fill such as CDOT Class 6 base course compacted to at least 98% of standard Proctor density. The exposed soils in footing area should then be moistened and compacted. 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 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 H-PMKUMAR Project No, 17-7-840 -5 - 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. Backfill should not contain topsoil or rock larger than 6 inches. 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 slightly above 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 0.40. Passive pressure of compacted backfill against the sides of the footings can be calculated using an equivalent fluid unit weight of 350 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, exclusive of topsoil, can be used to support lightly loaded slab -on -grade construction. The clay soils could exhibit expansion potential and heave the floor slab if wetted H-Pk-KUMAR Project No. 17-7-840 -6 - and may need to be sub -excavated at least 2 feet and replaced with compacted structural fill. 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 should consist of predominantly granular soils devoid of vegetation, topsoil and oversized rock or CDOT Class 6 road base. UNDERDRAIN SYSTEM Although free water was not encountered during our exploration, it has been our experience in mountainous areas and where clay soils are present 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, crawlspace and basement areas, be protected from wetting and hydrostatic pressure buildup by an underdrain system. 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. 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. SITE GRADING The risk of construction -induced slope instability at the site appears low provided cut and fill depths are limited. We assume the cut depths for the basement level will not exceed one level, H-P*KUMAR Project No. 17-7-840 7 about 10 to 12 feet. Fills should be limited to about 8 to 10 feet deep. Embankment fills should be compacted to at least 95% of the maximum standard Proctor density near optimum moisture content. Prior to fill placement, the subgrade should be carefully prepared by removing all vegetation and topsoil and compacting to at least 95% of the maximum standard Proctor density. The fill should be benched into the portions of the hillside exceeding 20% grade. Permanent unretained cut and fill slopes should be graded at 2 horizontal to 1 vertical or flatter and protected against erosion by revegetation or other means. The risk of slope instability will be increased if seepage is encountered in cuts and flatter slopes may be necessary. If seepage is encountered in permanent cuts, an investigation should be conducted to determine if the seepage will adversely affect the cut stability. This office should review site grading plans for the project prior to construction. SURFACE DRAINAGE The following drainage precautions should be observed during construction and maintained at all times after the residence 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 12 inches in the first 10 feet in unpaved areas and a minimum slope of 3 inches in the first 10 feet in paved areas. Free -draining wall backfill should be covered with filter fabric and capped with about 2 feet of the on-site soils to reduce surface water infiltration. 4) Roof downspouts and drains should discharge well beyond the limits of all backfill. 5) Landscaping which requires regular heavy irrigation should be located at least 10 feet from foundation walls. Consideration should be given to use of xeriscape to reduce the potential for wetting of soils below the building caused by irrigation. H-PKUMAR Project No. 17-7-840 -8 - 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 borings drilled at the locations 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 borings 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, H-Pt-KUMAR Steven L. Pawlak, Reviewed by: Daniel E. Hardin, P.E. SLP/kac H-PkiKUMAR Project No. 17-7-840 ��� X3855 I ..,wap ASSS, I mod t r 11 101.03'R=211 27X X19'61'61 ' 07' U*111'61101 • ? .00' SEMACKANR1— __+n /DRAINAGE AND UTILITY \ / / EASEMENT \ r r /1IAP L=4226 l / .D 11'762TCES2r r i C=42.1B' d • r�4 �p \\ '� /.OT 47 � \ / r 20,026. •. ft. say • e04¢, \ r \ BORING 2• ff 0/7 f. ,rf 0.45 cres .OQ' SETBACK • • A • Fes/ \i\ f ff 15 0 15 30 APPROXIMATE SCALE -FEET 17-7-840 f 1 H-PtiKUMAR Silver King Cp6rt LOCATION OF EXPLORATORY BORINGS Fig. 1 — 6980 6975 — 6970 — 6965 — 6960 6955 L 6950 17-7-840 BORING 1 EL. 6976' 17/12 43/12 WC=11.2 -200=57 19/12 WC=7.5 DD=119 H-PtiKUMAR BORING 2 EL. 6980' . e . 15/12 • - WC=12.4 DD=114 %�] 15/12 ' WC=15.3 • o DD=113 -200=49 . 4 • 71 re] 17/12 /"' WC=14.9 DD=111 . • 7, • ? 47/12 1•.e V *. • I • e r" /▪ ' 36/12 LOGS OF EXPLORATORY BORINGS 6980 — 6975 6970-- 6965 6960 6955 - 6950 -- Fig. 2 LEGEND TOPSOIL; ORGANIC SANDY SILTY CLAY, SCATTERED GRAVEL, VERY STIFF, SLIGHTLY MOIST, DARK BROWN. CLAY AND GRAVEL (CL—GC); SANDY, COBBLES, POSSIBLE BOULDERS, VERY STIFF/MEDIUM DENSE, SLIGHTLY MOIST, MIXED BROWN, SANDSTONE AND BASALT ROCK, LOW TO MEDIUM PLASTICITY FINES. GRAVEL AND COBBLES (GC); CLAYEY, SANDY, POSSIBLE BOULDERS, DENSE, SLIGHTLY MOIST, MIXED BROWN, SANDSTONE AND BASALT ROCK. RELATIVELY UNDISTURBED DRIVE SAMPLE; 2—INCH I.D. CALIFORNIA LINER SAMPLE. 17/12 DRIVE SAMPLE BLOW COUNT. INDICATES THAT 17 BLOWS OF A 140—POUND HAMMER FALLING 30 INCHES WERE REQUIRED TO DRIVE THE CALIFORNIA SAMPLER 12 INCHES. APRACTICAL AUGER REFUSAL. WHERE SHOWN ABOVE BOTTOM OF BORING, INDICATES THAT MULTIPLE ATTEMPTS WHERE MADE TO ADVANCE THE HOLE. NOTES 1. THE EXPLORATORY BORINGS WERE DRILLED ON DECEMBER 1, 2017 WITH A 4—INCH DIAMETER CONTINUOUS FLIGHT POWER AUGER. 2. THE LOCATIONS OF THE EXPLORATORY BORINGS WERE MEASURED APPROXIMATELY BY PACING FROM FEATURES SHOWN ON THE SITE PLAN PROVIDED. 3. THE ELEVATIONS OF THE EXPLORATORY BORINGS WERE OBTAINED BY INTERPOLATION BETWEEN CONTOURS ON THE SITE PLAN PROVIDED AND THE RELATIVE ELEVATION CHECK BY LEVEL SURVEY. 4. THE EXPLORATORY BORING LOCATIONS AND ELEVATIONS 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 THE TRANSITIONS MAY BE GRADUAL. 6. GROUNDWATER WAS NOT ENCOUNTERED IN THE BORINGS AT THE TIME OF DRILLING. 7. LABORATORY TEST RESULTS: WC = WATER CONTENT (%) (ASTM D 2216); DD = DRY DENSITY (pcf) (ASTM D 2216); —200= PERCENTAGE PASSING NO. 200 SIEVE (ASTM D 1140). 17-7-840 H -P- KUMAR LEGEND AND NOTES Fig. 3 CONSOLIDATION - SWELL CONSOLIDATION - SWELL SAMPLE OF: Clayey Sand with Gravel FROM: Boring 1 ® 10' WC = 7.5 %, DD = 119 pcf 1.7 EXPANSION UNDER CONSTANT PRESSURE UPON WETTING E�1 II 1.0 APPLIED PRESSURE - KSF 10 SAMPLE OF: Sandy Clay FROM: Boring 2 ® 5' WC = 12.4 %, DD = 114 pcf TN. �W1 uv eppy lh m Il.a .tine not Le The t 7.1.1. *:thou. Pee app.. eppr W In Kamer end Mxieter, In. 500 Cenaeae,tcn I .Jnrpi inrteern,n In rmnkece *Ith MIL 0-.54ZI. 17-7-840 100 EXPANSION UNDER CONSTANT PRESSURE UPON WETTING 4.0 H-P1KUMAR APPLIED PRESSURE - KSF 10 100 SWELL -CONSOLIDATION TEST RESULTS Fig. 4 CONSOLIDATION - SWELL SAMPLE OF: Sandy Clay with Gravel FROM: Boring 2 © 15' WC = 14.9 %, DD = 111 pcf NO MOVEMENT UPON WETTING Sh... ton romans .aah enh to the .%nnott6.t•n. ene:eL.°o.n +uM, whnxi th..rm.n eenr..M or Kvnnr .rd Nexrel.e, u.0 0.41 Canw.tletN. Wily t.rmod to ettad..t. rdh AV.rst 0-4546. 17-7-840 1.0 APPLIED PRESSURE - KSP H -P- KUMAR SWELL -CONSOLIDATION TEST RESULTS Fig. 5 H -PKU MAR TABLE 1 SUMMARY OF LABORATORY TEST RESULTS Project No. 17-7-840 SAMPLE LOCATION BORING DEPTH NATURAL NATURAL j GRADATION MOISTURE DRY GRAVEL CONTENT I DENSITY (%) (pcf) 1 5 10 11.2 1 (%) SAND (%) PERCENT r ATTERBERG LIMITS PASSING : LIQUID NO.200 I LIMIT SIEVE (%) 57 PLASTIC INDEX (A) UNCONFINED COMPRESSIVE STRENGTH IPSF) SOIL TYPE 7.5 119 Sandy Clay with Gravel Clayey Sand with Gravel 2 5 12.4 114 Sandy Clay 10 15.3 113 49 15 14.9 111 Very Clayey Sand with Gravel Sandy Clay with Gravel