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HomeMy WebLinkAboutSubsoil Study for Foundation Design 06.19.2015( G~tech HEPWORTH-PAWLAK GEOTECHNICAL SUBSOIL STUDY I h:p 1\ 111 li l'.111 I 1k ( ;, . .,c,dmic.il, lnL 5l\!l1 l •••mtj R,1,d I i-t ( i 1,·nn<11.J ..:pin:,;-. Cnl11r.11l1 •Ii I ('l'I l'li •m 97L1 .).fi-79~S f-1\ 9;l'·'J..;:;.~1H ,·1u 11I hpi.uf"hr ~,··•red1.,· .. 111 FOR FOUNDATION DESIGN PROPOSED RESIDENCE LOT 42, SPRING RIDGE RESERVE PUD, PHASE 3 HIDDEN VALLEY DRIVE, GARFIELD COUNTY, COLORADO JOB NO. 115 243A JUNE 19, 2015 PREPARED FOR: PALOMINO DESIGN BUILD ATTN: JACK PALOMINO 919 PALMER A VENUE GLENWOOD SPRINGS, COLORADO 81601 (jack pa lom in o55 @£m ail.com ) P,1rkcr "303 -841-7119 • C o lor,1duSprinµs 7l9-6H-5562 • Siln:rrhnrn~ 970-468-\9S9 TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY ............................................................................ -1- PROPOSED CONSTRUCTION .................................................................................... -1 p SITE CONDITIONS ....................................................................................................... -1 - FIELD EXPLORATION ................................................................................................. -2 - SUBSURFACE CONDITIONS ...................................................................................... -2 - FOUNDATION BEARING CONDITIONS .................................................................. -3 - DESIGNRECOMlvlENDATIONS ................................................................................ -3 • FOUNDATIONS ........................................................................................................ -3 - FOUNDATION AND RETAINING WALLS ........................................................... -4 - FLOOR SLABS .......................................................................................................... - 5 - ~T))ERDRAIN SYSTEM .......................................................................................... - 6 - SURFACE DRAINAGE ....................................... ~ .................................................... - 6 - LII\.1ITATIONS ............................................................................................................... -7 - FIGURE I -LOCATION OF EXPLORATORY BORINGS FIGURE 2 -LOGS OF EXPLORATORY BORINGS FIGURE 3 ·LEGEND AND NOTES FIGURES 4 AND 5 -SWEIL-CONSOLIDATION TEST RESULTS TABLE 1-SUMMARY OF LABORATORY TEST RESULTS J.ob No . 115 243A . . PURPOSE AND SCOPE. OF STUDY This report presents the results of a subsoil study for a proposed residence to be located on Lot 42, Springridge Reserve PUD, Phase 3, Hidden Valley Drive, Garfield County, Colorado. The project site is shown on Figure l. 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 Jack Palomino dated May 22, 2015. A field exploration program consisting of exploratory borings was conducted to obtain information on the subsurface conditions. Samples of the subsoils and bedrock 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 wi.ll be a two story wood frame structure over a basement The basement and attached garage floors will be slab-on-grade. Grading for the structure is assumed to have cut depths up to 10 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. SITE CONDITIONS At the time of our field exploration the lot was vacant and vegetated with grass and weeds. The eastern side of the lot slopes moderately steep down to the west, adjacent to Job No. 115 243A -2- Hidden Valley Drive. The building area was gently rolling with a moderate slope down to the north at grades of 4 to 7%. FIELD EXPLORATION The field exploration for the project was conducted on May 29, 2015. Two exploratory bori ngs 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. The borings were logged by a representative of Hepworth-Pawlak Geotechnical, Inc. Samples of the subsoils were taken with a 2 inch l.D. spoon sampler. The sampler was driven into the subsoils at various depths with blows from a 140 pound h~mmer falling 30 inches. This test is similar to the standard penetration test described by ASTM Method D-J 586. The penetration resistance values are an indication of Lhe relative density or consistency of the subsoils and hardness of the bedrock. Depths at which the samples were taken and the penetration resistance values are shown on the Logs of Exploratory Borings, Figure 2. T he samples were re turned to our laboratory for review by the project engineer and testing. SUBSURFACE CONDITIONS Graphic logs of t he subsurface conditions encountered at the site are shown on Figure 2. The subsoils consist of about 1 to 2 feet of topsoil overlying 14 to 15 feet of sti ff. sandy silty clay and up to 15 feet of medium dense, clayey sand. Sandstone bedrock was encountered at depths of 17 feet and 30Vl feet in the borings. Drilling in the hard bedrock with auger equipment was difficult and drilling refusal was encountered in t he deposit. Laboratory testing performed on samples obtained from the borings included natural moisture content, dry density and percent finer than sand size gradation analyses. Results of swell-consolidation testing performed on relatively undisturbed drive samples, presented on Figures 4 and 5, indicate the sandy silty clays have low to moderate Job No. 115 243 A -3 - compressibility under loading and wetting. 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 clay soils are somewhat variable with respect to density and compressibility potential. The clay soils can support lightly loaded spread footings with a low bearing pressure and settlement potential particularly if the bearing soils become wet. The amount of settlement would depend on the depth and extent of wetting and could be l to 2 inches. If the settlement risk for bearing on the clays is not acceptable, the underlying bedrock is hard and could support a pier or pile foundation with low settlement potential. If a deep foundation is proposed, we should be contacted for additional recommendations. DESIGN RECOMMENDATIONS FOUNDATIONS Considering the subsurface conditions encountered in the exploratory borings and the nature of the proposed construction, spread footings bearing on the natural clay soils can be used for the building support with some settlement risk. The design and construction criteria presented below should be observed for a spread footing foundation system. l) Footings placed on the undisturbed natural clay soils should be designed for an allowable bearing pressure of 1,500 psf. Based on experience, we expect settlement of footings designed and constructed as discussed in this section will be about 1 inch or less. If the bearing soils become wet, there could be additional settlement on the order of L inch. 2) The footings should have a minimum width of 20 inches for continuous walls and 2 feet for isolated pads. JobNo.115243A ~ -4- 3) Exlerior 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 wal ls should be reinforced top and bottom to span local anomalies such as by assuming an unsupported lengtµ of at least 14 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) All existing fill, topsoil and any loose or disturbed soils should be removed and the footing bearing level extended down to the relatively firm natural soils. The exposed soils in footing area s hould then be moistened to near optimum 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 soHs . Cantilevered retaining structures which are separate from the residence and can be expec ted to deflect sufficiently to mobilize the full active earth pressure condition should 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. All foundation and retaining structures shou1d be designed for appropriate hydrostatic and surcharge pressures such as adjacent footings . traffic. construction materials and equ ipment. 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 Job No. I 15 243A -5- 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 ~% of the maximum standard Proctor density at near optimum moisture content. 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 wan. 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 0.30. 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 Lhe 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 , are suitable to support lightly loaded slab- on-grade construction. 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 Job No. 115 243A -6- aggregate with at least 50% retained on the No. 4 sieve and less than 2% passing the No. 200sieve. 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, 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 l foot below lowest adjacent finish grade and sloped at a min imum l % 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 !h feet deep. An impervious membrane. such as 20 mil PVC should be provided below the drain gravel in a trough shape and attached to the foundation wall with masttc to prevent wetting of the bearfog soils. SURFACE DRAINAGE The following drainage precautjons should be observed during construction and maintained at all times after the residence has been completed: 1) Inundation of the fo~ndation excavations and underslab areas should be avoided during con~truction. Job No. 115 243A -7 - 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 IO 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 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 backfiII. 5) Landscaping which requires regular heavy irrigation should be located at least I 0 feet from foundation walls. Consideration should be given to use of x.eriscape to reduce the potential for wetting of soils below the building caused by irrigation. LIMITATIONS This study bas been conducted in accordance with generally accepted geotechnicaI 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 l, 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. Job No. l 15 243A -8 - This report has been prepared for the exclusive use by our client for design purposes. We are not responsible for tec hnica l interpretati ons 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 tes ring of structural fill by a representative of the geotechnical engineer. Respectfully Submitted, HEPWORTH-PAWLAK GEOTECHNICAL, INC. !M1tf,tt1,J 5 ~ti) Andrew R . Spickert Rev iewed by: ARS/ljf Joh No. 11 5 2·BA APPROXIMATE SCALE 1· = 50' r;p.r.l 6465 'Ob.efJ ' \ \ \ \ \ \ ~-\ \ I \ \ I \ \ I I fl r// / I I / PROPOSED j/ >/f I / RESIDENCE I / / (/ / I / / / I / I I I I \ LOT41 f ', LOT43 I I I I \ I \ ' I \ I 6'?6'o I I \ -----........ \ ....... -\ ........ _____ I -~-t --- -------~---------6480--- HIDDEN VALLEY DRIVE 115 243A LOCATION OF EXPLORATORY BORINGS ~ H worth-Pawlak Ceatedlnlcal Figure 1 0 5 10 15 Qj if • c: 0 ~ > Q) u:; 20 25 30 35 40 115 243A BOR ING 1 ELEV.= 6465 13/12 WC = 6.8 OD:: 112 1 1/12 WC:::8.9 DD= 100 ·200 = 82 -, 10/12 we ... 10.s DD "" 118 1B/12 20/12 39/6,50/3 BOAING2 ELEV.= 6462' 10/12 WC:: 10.1 OD= 112 13/12 WC= 14.4 OD= 11 1 B/12 WC= 133 00 = 115 -200 ... 74 24/12 50/0 Note: Explanation of symbols is shown on Figure 3. ~ LOGS OF EXPLORATORY BORINGS HEPWDR'nfoPAWl..AJ( G~CHNICAI. 0 5 10 15 Qj Q) ~ I c 0 ~ Q) 20 m 25 30 35 40 Figure 2 LEGEND : ~ TOPSOIL; clay, sandy, silty, moist, brown, organic. D CLAY (CL); sandy. silty, medium stiff to very stiff, slightly moist to moist. brown to red brown. ~ SAND (SC); clayey, medium dense. slightly moist. red brown . ~~~; SANDSTONE BEDROCK; hard to very hard, slightly moist, red. Maroon Formation . p Relatively undisturbed drive sample; 2-inch 1.0. California liner sample. Drive sample blow count; Indicates that 18 blows of a 140 pound hammer fall ing 30 inches were 18/12 required to drive the California sampler 12 inches . T Practical auger drilling refusal. NOTES: 1. Exploratory borings were drilled on May 29, 2015 with 4-lnch diameter continuous flight power auger. 2 . Locations of exploratory borings were measured approximately by pacing from features shown on the site plan provided. 3. Elevations of exploratory borings were obtained by interpolation between contours shown on the site plan provided. 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 transitions may be gradual. 6. No free water was encountered in the borings at the time of drilling . Fluctuation In water level may occur with time . 7. Laboratory Testing Results; WC = Water Content (%) DD = Dry Density (pc~ -200 = Percent passing No. 200 sieve 115 243A ~ H EPWCRTI+PAWl..AIC GEOTIECl-INICAI. LEGEND AND NOTES Figure 3 Moisture Content = 6 .8 percent Dry Density = 112 pcf Samp le or: Sandy Silty Clay ~ From: Bor ing 1 at 2 Feet 5 0 'ii) en """""' ·o [ 1 q,-) E 0 ~ / Compression 0 (_ ' upon c wetting 0 2 ·u; 1"'1 c: ro a. in 3 '\ 4 i'h 5 6 0.1 1.0 · 10 100 APPLIED PRESSURE -ksf Moisture Content = 10.9 percent Dry Density = 118 pcf Sample of: Sandy Silly Clay ~ From : Boring 1 at 1 o Feet c 0 1 '(ii en ~ a. E 0 -- 0 .Jtr i-...., No movement (.) ' --..... "1 upon c: 1 I>-... wetting ·~ ~[) a ~ 2 3 I I 0 .1 1.0 10 100 APPLIED PRESSURE ~ ksf 115 243A ~ SWELL-CONSOLIDATION TEST RESULTS Figure 4 HEPWORTJ+PA~ GEOTE:CHNI ~ Moisture Content = 14.4 percent Oiy Density = 111 pcf Sample of: Sandy Silty Clay From : Boring 2 at 5 Feet * c 0 1 'iii c tO ~ I 0 sion c ---<"" Comprei 0 r-.... i-.. upon ·c;; I°"'--"""r--wetting "' ~ 1 I-. J a. E ""' 0 0 2 '\ 3 \. \ 4 \ \ 5 \ ') 6 0 .1 1.0 10 100 APPLIED PRESSURE -ksf 115 243A omecr. SWELL-CONSOLIDATION TEST RESULTS Figure 5 HEPWORTl+PAWLA!C GEOTIECtfHICAI. HEPWORTH-PAWLAK GEOTECHNICAL, INC. TABLE 1 Job No. 115 243A SUMMARY OF LABORATORY TEST RESULTS SAMPLE LOCATION NATURAL NATURAL GRADATION ATTERBERG LIMITS UNCONFINED PERCENT MOISTURE DRY GRAVEL SAND PASSING LIQUID PLASTIC COMPRESSIVE SOIL OR BORING DEPTH CONTENT DENSITY (%) (%) N0.200 LIMIT INDEX STRENGTH BEDROCK TYPE lftl 1%) SIEVE (%) 1%) tPSF\ I Pen 1 2 6.8 112 Sandy Silty Clay ' 5 8.9 100 82 Sandy Silty Clay 10 10.9 118 Sandy Silty Clay 2 2 10 .1 112 Sandy Silty Clay 5 14.4 111 Sandy Silty Clay 10 13 .3 115 74 Sandy Silty Clay ·.