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Home My WebLink AboutSubsoil Study for Foundation Design 10.18.2021l*rf u',çiffi:'*:ßHf ''.'Ê;o**' Ån Employce Owned Compcny 5020 County Road 154 Glenwood Springs, CO 81601 phone: (970) 945-7988 fax: (970) 945-8454 email: kaglenwood@kumarusa.com www.kumarusa.com Office Locations: Denver Q'IQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado October 18,2021 Woodstone,Inc. Attn: Alan Short 263 Lewis Lane Basalt, Colorado 81621 alan@.woodstoneinc.net RECEIVËD {ìAt.tFlr-Lü c0!lryTY Ï*li' *, i' Ñ tii n F v F Lo P ÌüE llÏ Pro.iect No.21-7-698 Subject:Subsoil Study for Foundation Design, Proposed Shop/ADU, 3004 Mountain Springs Road, Southwest of Glenwood Springs, Garfield County, Colorado Dear Mr. Short: As requested, Kumar & Associates, Inc. performed a subsoil study for design of foundations at the subject site. The study was conducted in accordance with oul agreement for geotechnical engineering services to Woodstone, Inc. dated August25,202l. The data obtained and our recommendations based on the proposed construction and subsurface conditions encountered are presented in this rePort. Proposed Construction: The proposed shop will be 45 feet by 28 feet is plan size with a slab- on-grade floor. The ADU will be a second story, 14 feet by 45 feet in plan size. The building will be located southwest of the existing residence. Cut depths are expected to range between about 3 to 5 feet below existing ground surface. Foundation loadings for this type of construction are assumed to be relatively light and typical of the proposed type of construction. If building conditions or foundation loadings are significantly different from those described above, we should be notified to re-evaluate the recommendations presented in this report. Site Conditions: The building site had been cleared of vegetation and topsoil and had been excavated in a sloping cut about 3 to 5 feet below natural grade. The soils exposed in the excavation consisted of sandy silty clay with basalt rocks up to small boulder size. Vegetation surrounding the building area consisted of oak brush, grass and weeds with aspen and blue spruce trees located near the existing residence, northeast of the shop area. Subsurface Conditions: The subsurface conditions at the site were evaluated by excavating two exploratory pits off the northeast corner (Pit I ) and southwest corner (Pit 2) of the proposed shop building. The logs of the pits are presented on Figure 1. The subsoils encountered, below about two feet of topsoil inPit2, consisted of 2to 4 feet of stiff, sandy silty clay with rock fragments a overlying very stiff sandy clay with scattered basalt cobbles. Results of swell-consolidation testing performed on relatively undisturbed samples of the sandy clay, presented on Figures 2 and 3, indicate low compressibility under existing moisture conditions and light loading and a low to moderate expansion potential when wetted. Atterberg Limits testing indicates the clay soils are medium plastic. No free water was observed in the pits at the time of excavation and the soils were slightly moist to moist. Foundation Recommendations: Considering the subsoil conditions encountered in the exploratory pits and the nature of the proposed construction, we recommend spread footings placed on the undisturbed natural soil designed for an allowable soil bearing pressure of 3,500 psf and a minimum dead load 1,000 psf for support of the proposed shop/ADlJ. As an alternative to designing for a minimum dead load pressure, the footings could be designed for an allowable bearing pressure of 3,500 psf and be placed on at least 2 feet of structural fill such as 3/o-inchroad base compacted to at least 98% of the maximum standard Proctor density. The clay soils at this site tend to swell after wetting and there could be some post-construction foundation movement whether the footings are placed on the natural soils or compacted structural fill. Footings should be a minimum width of 16 inches for continuous walls and2 feet for columns. Loose and disturbed soils encountered at the foundation bearing level within the excavation should be removed and the footing bearing level extended down to the undisturbed natural soils. Exterior footings should be provided with adequate cover above their bearing elevations for frost protection. Placement of footings at least 36 inches below the exterior grade is typically used in this area. Continuous foundation walls 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 should be designed to resist alateral earth pressure based on an equivalent fluid unit weight of at least 55 pcf for the on-site soil as backfill. Floor Slabs: The natural on-site clay soils can be expensive if wetted. We recommend that at least2 feet of non-expansive soil, such as3/q-inch road base be placed below slabs-on-grade. 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. Structural fîll material placed below slabs should consist of minus l-inch aggregate with less than 50o/o passing the No. 4 sieve and less than 12%o passing the No. 200 sieve. All fill materials for support of floor slabs should be compacted to at Ieastg5o/o of maximum standard Proctor density at a moisture content near optimum. Kumar & Associates, lnc. @ Project No. 21.7-698 -J Underdrain System: An underdrain system should not be needed for the proposed slab-on- grade structure. If required, 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 backfìll 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 lo/oto a suitable gravity outlet. Free-draining granular material used in the underdrain system should contain less than 2o/o passingthe 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 IYzfeet deep' An impervious membrane such as 20 mil PVC should be placed beneath the drain gravel in a trough shape and attached to the foundation wall with mastic to prevent wetting of the bearing soils. Surface Drainage: The following drainage precautions should be observed during construction and maintained at all times after the shop/ADU building has been completed: l) Inundation ofthe foundation excavations and underslab areas should be avoided during construction. Drying could increase the expansion potential of the clay soils. Z) Exterior backhll should be adjusted to near optimum moisture and compacted to at least 95o/o of the maximum standard Proctor density in pavement and slab areas and to at least 90Yo of the maximum standard Proctor density in landscape areas. Free-draining wall backfill (if any) should be capped with about 2 feet of the on- site, finer graded soils to reduce surface water infiltration. 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 l2 inches in the first 10 feet in unpaved areas and a minimum slope of 3 inches in the first 10 feet in pavement and walkway areas. A swale may be needed uphill to direct surface runoff around the shop/ADU. 4) Roof downspouts and drains should discharge well beyond the limits of all backfill. 5) Landscaping which requires regular heavy iruigation should be located at least 5 feet from the building. Consideration should be given to the use of xeriscape to limit potential wetting of soils below the foundation caused by irrigation. Limitations: This study has been conducted in accordance with generally accepted geotechnical engineering principles and practices in this area atthis time. We make no warranty either Kumar & Associates, lnc. @ Project No, 21-7-698 4 express or irnplied. The conclusions and recommendations subrnitted in this report are based upon the data obtained from the exploratory pits excavated at the locations indicated and to the depths shown on Figure 1, the proposed type of construction, and our experience in the area. Our services do not include determining the presence, prevortion 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 pits and variations in the subsurface conditions may not become evident until excavation is performed. If conditions encountered during construction appear different frorn those described in this report, we should'be notified at once so 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 respeinsible 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 veri$i that the recommendations have been appropriately interpreted. Signifîcant design changes may require additional analysis or modifications to the recommendations presented herein. '!Ve recommend on-site observation of excavations and foundation bearing strata and testing of structural fill by a representative of the geotechnicai engineer. If you have any questions or if we may be of further assistance, please let us know. Respectfully Subrnitted, Ku's¡¡an S¿ .Àsscei¿ttes, ãne. Daniel E. Rev. by: SLP DEH/kac attactunents Figure I * Logs of Exploratory Pits Figures 2 and 3 - Swell-Consolidation Test Results Table 1 - Summary of Laboratory Test Results I rift+;i :- ¿ K¡:nrar & Ássoöiateg, lnÇ. "'Proje*t No. ¿1"7"698 ? PIT 1 NORTHEAST PIT 2 SOUTHEAST 0 t 0 t-LJ L¡l LL I-t-. o_t¡lo Ft¡l Lrl LL I-t- o_L!o WC= 1 9.1 DD= 1 04 WC= 1 2.9 DD= 1 08 WC= 1 4.5 DD=112 -2OO=73 LL=35 Pl=15 Ã 10 10 ND TOPSOIL; HEAVY ROOT ZONE, ORGANIC SANDY SILTY CLAY, SOFT, MoIST' DARK BRoWN. CLAY ( MOIST, CL); SILTY, SANDY, WITH SMALL GRAVEL_SIZE ROCK FRAGMENTS, STIFF' SLIGHTLY BROWN. N. I atot (cH); SANDY, VERY STIFF, WITH SCATTERED BASALT COBBLES SLIGHTLY MolST, I \ enowN ro GRAY.NI HAND DRIVE SAMPLE. NOTES 1. THE EXPLORATORY PITS WERE EXCAVATED WITH A BACKHOE ON AUGUST 26' 2021' 2. THE EXPLORATORY PITS WERE LOCATED BY CLIENT. 3. THE ELEVATIONS OF THE EXPLORATORY PITS WERE NOT MEASURED AND THE LOGS OF THE EXPLORATORY PITS ARE PLOTTED TO DEPTH. THE PITS WERE DUG AT APPROXIMATE PROPOSED FOOTING GRADE. 4. THE EXPLORATORY PIT LOCATIONS SHOULD BE CONSIDERED ACCURATE ONLY TO THE DEGREE IMPLIED BY THE METHOD USED' 5. THE LINES BETWEEN MATERIALS SHOWN ON THE EXPLORATORY PIT LOGS REPRESENT THE APPROXIMATE BOUNDARIES BETWEEN MATERIAL TYPES AND THE TRANSITIONS MAY BE GRADUAL' 6. GROUNDWATER WAS NOT ENCOUNTERED IN THE PITS AT THE TIME OF EXCAVATION. 7. LABORATORY TEST RESULTS: WC = WATER CONTENT (%) (ASTM D 2216); DD = DRY DENSITY (PCt) (NSTV D 2216); _2OO= PERCENTAGE PÀSSING NO. 2OO SIEVE (ASTM D 11AO); LL = LIQUID LIMIT (ASTM D 4318); Pl = PLASTICITY INDEX (ASTM D 4318). F Fig. 1LOGS OF TXPLORATORY PITSKumar & Associates21 -7 -698 E SAMPLE OF: Sondy Cloy with Grovel FROM:Pit1@3' WC = 19.1 %, DD = 104 pcf EXPANSION UNDER CONSTANT PRESSURE UPON WETTING 2 àq JJ t¡.j =Ø I zoÉ ôfottlzoo 0 -1 -2 1,0 APPLIED - KSF 10 100 x .JJ Ld =ln I zotr ô =oØz.oo 0 -1 -2 -3 PRESSURE -10 SAMPLE OF: Sondy CloY FROM:Pit1@5' WC = 1 2,9 %, DD = 108 pcf ! EXPANSION UNDER CONSTANT PRESSURE UPON WETTING of D-454€. t€stsd, fhâ without the 1.0 100 Fig. 2SWELL_CONSOLIDATION TESI RTSULTSKumar & Associates21 -7 -698 I p ì SAMPLE OF: SondY CloY wilh Grovel FROM¡Pit29-2' WC = 1 4.3 %, DD = 112 pc'f -ZQO = 73 %, LL = 35, Pl = 15 EXPANSION UNDER CONSTANT PRESSURE UPON WETTING l 4 7 ñ JJ L¡J =ln 2 0zIt-- Õ =o U}z.o(J -l -2 -3 -4 1.0 ED - KSF 10 100 Fig. 3SWTLL_CONSOLIDATION TEST RTSULTSKumar & Associates21 -7 -698 ¡ lGr\iffi fi',ffifffi1i'iÍü*"':r TABLE 1 SUMMARY OF LABORATORY TEST RESULTS ATTERAEf LIMITS SOIL TYPELIQUID LIMIT rol"ì PLASTIC INDEX f%ì losl) UNCOÑFINED COMPRESSIVE STRENGTH GRAVEL v"t SAND t:/"1 PERCENT PASSING NO. 200 stEVE lftì DEPTH t%t NATURAL MOISTURE CONTENT NATURAL DRY DENSITY loc0 PIT Sandy Clay with GravelI319. I 104 Sandy Clay512.9 108 Sandy Clay with Gravel7335151t222r4.3