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Home My WebLink AboutSubsoil Study for Foundation Designr~lht~~r~ HEPWORTH-PAWLAK GEOTECHNICAL SUBSOIL STUDY Hqmnnh-P;1wh1k Gt'('l!L hn1 11, In 5020Cnunrr RoaJ 15-f Glen"')\""\ SrrinJ.!~, c,,\m J,, •\fiO! Phnnc: 9jQ.94;.i938 far 9i0-'N5·84H cm:iil· hpi.:co@hp~c111cd1.wm FOR FOUNDATION DESIGN PROPOSED RESIDENCE LOT 62, SPRINGRIDGE RESERVE HIDDEN VALLEY DRIVE GARFIELD COUNTY, COLORADO JOB N0.116 040A MARCH 27, 2016 PREPARED FOR: KEITH WI'ITENBERG 114 DEER PARK COURT GLENWOOD SPRINGS, COLORADO CSha alS09@1·ahoo.com ) Parker 303-841-7 I l 9 • Colorado Springs 719-633-5562 • Silverthorne 970-468-1989 TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY ............................................................................ - 1 - PROPOSED CONSTR.UCTION .............................•........•..•..........•...•....•....••••••••.•........ - 1 - SITE CONDITTONS •.........•.............•...•..••..•.••.•......•..•.•.....••...............••...•.••....•..•..•...•.... - 2 - FIELD EXPLORATION ................................................................................•..............•. -2 - SUBSURFACE CONDITIONS ...........•.....................•.......•.....•...........•.........•..•.....•..•..•. - 2 - FOUNDATION BEARING CONDITIONS .................................................................. -3 - DESIGN RECOMMENDATIONS ..........•..................................................................... -4 - FOUN'DATIONS ....................................•......•..............••.....•............•......................... - 4 - aooR SLABS ............................................................................................................... -s - UNDERDRA.IN SYSTEM ............................................•...............•.•.......•................... -6 - S'UR.FACE. D'RAil'lAOE ............................................................................................... - 6 - L™IT A TIO NS .................................................................................................................. -1 ... FIGURE 1 -LOCATION OF EXPLORATORY BORINGS FIGURE2-LOGSOFEXPLORATORYBOR1NOS FIGURE 3 ·LEGEND AND NOTES FIGURES 4, SAND 6 -SWELL-CONSOLIDATION TF.sT RESULTS TABLE I-SUMMARY OF LABORATORY TEST REStn..TS PURPOSE AND SCOPE OF STUDY This report presents the results of a subsoil study for a proposed residence to be located at Lot 62, Springridge Reserve, Hidden Valley 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 agreement for geotechnical engineering services to you dated February 29, 2016. Hepworth-Pawlak Gcotechnical, Inc. previously conducted a preliminary geotechnical study for the Springridge Reserve Subdivision and presented our findings in a report dated June 22, 2004, Job No . 101 126. A field exploration program consisting of exploratory borings was conducted to obtain infonnation on the subsurface conditions. Samples of the subsoils obtained during the field exploration were tested in the laboratory to detennine 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 sununarizcs 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 one and two story wood frame structure over a crawlspace with an attached garage. Garage floor will be slab-on-grade. Grading for the structure is assumed to be relatively minor with cut depths between about 3 to 4 feeL We assume relatively light foundation loadings, typical of the proposed type of construction. U 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 . lob No. 116 040A -2- SITE CONDITIONS The vacant Jot is vegetated with tall grass and weeds. The ground surf nee is relatively flat with a slight slope down to the west at a grade of about 3 percent. A storm water detention area is located just west of the site in open space. FIELD EXPLORATION The field exploration for the project was conducted on March 17, 2016. 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. The borings were logged by a representative of Hepworth-Pawlak Geotechnical, Inc. Samples of the subsoils were taken with 1% inch 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 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 consist of about 3 to 6 inches of topsoil overlying sandy silty clay. Possible weathered sandstone was encountered in Boring I at 28 feet. Laboratory testing performed on samples obtained from the borings included natural moisture content, density, Atterberg limits and percent finer than sand size gradation Job No. 116 040A -3- analyses. Results of swell-consolidation testing performed on relatively undisturbed drive samples, presented on Figures 4, 5 and 6, indicate low compressibility under existing moisture conditions and light loading with a low collapse potential (settlement under constant load) when wetted. The samples had moderate to high compressibility under increased loading after 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 sandy silty clay soils encountered at typical shallow foundation depth tend to settle when they become wetted. A shaUow foundation placed on the upper sandy silty clay soils will have a risk of settlement if the soils become wetted and care should be taken in the surface and subsurface drainage around the house to prevent the soils from becoming wet. It will be critical to the long term performance of the structure that the recommendations for surface drainage and subsurface drainage contained in this report be followed. The amount of settlement, if the bearing soils become wet, will mainly be related to the depth and extent of subsurface wetting. We expect that initial settlements will be less than 1 inch. If wetting of the shallow soils occurs, additional settlements will be related to the depth and extent of the wetting and l to 2 inches of additional settlement could occur. Settlement in the event of subsurface wetting will likely cause building distress and mitigation methods such as deep compaction, a deep foundation such as piles or piers or a heavily reinforced mat foundation (on the order of 2 feet thick) should be used to support the proposed house. If a deep foundation or mat foundation is desired, we should be contacted to provide further design recommendations. Job No. 116 040A -4- 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 compacted structural fill with a risk of settlement, mainly if the underlying soils become wetted, and provided the risk is acceptable to the owner. Control of surf ace and subsurface runoff will be critical to the long-tenn pcrfonnance of a shallow spread footing foundation system. The footing areas should be sub-excavated down 3 feet below the proposed footing grade and the excavated soil replaced with compacted structural fill of restricted penneability such as the on-site clay soils. The structural fill should be compacted to 98 percent standard Proctor density at a moisture content near optimum. The design and construction criteria presented below should be observed for a spread footing foundation system. I) Footings placed on a minimum 3 feet of compacted structural fill should be designed for an allowable bearing pressure of 1,500 psf. Based on experience, we expect initial settlement off ootings designed and constructed as discussed in this section will be about I inch or less . Additional settlement of about l inch could occur if the deeper sandy silty clay soils below the structural fill become wetted. 2) The footings should have a minimum width of 18 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 14 feet. The foundation should be configured in a "box like" shape to Job No. 116 040A -5- help resist differential movements. Foundation walls acting as retaining structures should also be designed to resist an equivalent fluid lateral earth pressure of 50 pcf. 5) The topsoil and any loose or disturbed soils should be removed below the building area. The exposed soils in footing areas after sub-excavation to 3 feet below design footing grades should then be moistened and compacted. The structural fill should extend laterally beyond the footing edges equal to about ~ the fill depth below the footing. 6) A representative of the geotechnical engineer should evaluate the structural fill as it is placed for compaction and observe all footing excavations prior to concrete placement to evaluate bearing conditions. FLOOR SLABS The natural on-site soils, exclusive of topsoil, are suitable to support lightly loaded slab- on-grade construction with a risk of settlement if the soils get wet. 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 the garage slab . 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 can consist of the on-site soils devoid of vegetation and topsoil. Job No. 116 040A -6- 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 and crawlspace 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 or sump and pump . 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 I Y.z feet 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 obseivcd 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. Job No. 116 040A -7- 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 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 wans . Consideration should be given to use of xeriscape to reduce the potential for wetting of soils below the building caused by irrigation. LIMITATIONS This study has been conducted in accordance with generaUy 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 exploratocy 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 infonnation. As the Job No. 116 040A -8- 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 geotechnica1 engineer. Respectfully Submitted, HEPWORTH -PAWLAK GEOTECHNICAL. INC. JobNo.116040A OPEN SPACE 6387 \ \ \ I I \ \ \ \ \ \ I I I I l ' I I I I l I I I 116040A LOT68 I I I I I I \ \ I I \ } I I APPROXIMATE SCALE 1· -40' LOT69 LOCATION OF EXPLORATORY BORINGS Figure 1 0 5 10 15 i LL ' .c a. ID CJ 20 25 30 35 116 040A BORING 1 11/12 12/12 WC -=75 00 ~93 11112 WC •94 00 .. 102 20/12 13/12 WC-104 -200-62 LL=21 Pl=4 24/6,50/5 BOAING2 12/12 11/12 wc .. 1s 00•96 15/12 WC •109 00=>119 .200 .. 73 10/12 wc .. 97 00 ... 111 29/12 Note Explanation of symbols is shown on Figure 3. LOGS OF EXPLORATORY BORINGS \ 0 5 10 15 ~ I .r: 20 ! 25 30 35 Figure 2 LEGENO : D CLAY (CL); sandy, silty, medium stiff to very stiff, slightly moisl to moist ,reddish brown. ~ !::·· SANDSTONE BEDROCK: hard , moist, red . .. . jot• •• ~ p Relatively undisturbed drive sample; 2-inch 1.0. Caliromia liner sample. ~ Drive sample: standard penetration lest (SPT), 1 3/8inch1.0. split spoon sample, ASTM 0-1586. 20112 Drive sample blow count; indicates that 20 blows of a 140 pound hammer falling 30 inches were required to drive the CalUomla or SPT sampler 12 inches . NOTES: 1 . Explocatary borings wE!fe drilled on March 17, 2016 with 4-inch diameter contiruous Hight 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 not measured and the logs ol exploratOl)' borings are drawn to depth. 4. The exploratory boring locations 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 (pcf} -200 = Percent passing No . 200 sieve LL = Liquid Limil (%) Pl • Plasticity Index (%) ~ch HllDworth-Pawlak O.Olec:hnlcal 116 040A LEGEND AND NOTES Figure 3 Moisture Content "" 7.5 percent Dry Densily = 93 pcf Sample of: Silty Sandy Clay From: Boring 1 at 5 Feet 0 -Hl) 1 ,_ Compression --,_. I/ .upon ~ / v l.-i-L-i.. wetting ~ 2 i- 8 .(ii ~ 3 11 l 8 \ (.) 4 5 \ \ 6 7 \ ' 8 ~, ' 9 \ ~) 10 0 1 1.0 10 100 APPLIED PRESSURE · ksl 116 040A ~ Heawarth-Pawlak Geotec:hnlcol SWELL-CONSOLIDATION TEST RESULTS Figure 4 Moisture Content = 9.4 percent Dry Density -102 pct Sample of: Sandy Siity Clay From: Boring 1 al 1 O Feet 0 ~M J '-- 1 ~ 14"' t-' ;> Compression c: c:::: ....... upon 0 1n 2 wetting ell a. I\ ~ ~ 3 4 ~~ 0 .1 1.0 10 100 APPLIED PRESSURE -ksf 0 Moisture Content ~ 7.6 percent 1"111 Ory Density ... 96 pct 1 Sample of: Sandy Silty Clay ~ From: Boring 2 at 5 Feet 2 I --I'-.. \ I""--I> IC----3 \ ~ I'---Compression ~ ..... I'.., upon ~ 4 'wetting c \ 0 ·0 ti) CD a. 5 E \ 0 u 6 ' I 7 \ \ a \ 9 •:'I 0 .1 1.0 APPLIED PRESSURE -ksf 10 100 116 040A cCMech Heaworth-Pa.lllk Geot.c:hn1col SWELL-CONSOLIDATION TEST RESULTS Figure 5 Moisture Conlent .;::; 9.7 percent Ory Density .., 111 pct Sample of: Sandy Silty Clay From: Boring 2 al 15 Feet 0 -,.... ,_ r--1-o l t..-' ~ 1 ,,~ _J' a ~ t' Compression upon ·u; wetting "' 2 ' CD ... \ ~ s \ CJ 3 ~D 4 0.1 1.0 10 100 APPLIED PRESSURE· ksl 116 040A ~ SWELL-CONSOLIDATION TEST RESULTS Figure 6 Heoworth-Pnlolc GeotllChnlc:d HEPWORTH-PAWLAK GEOTECHNICAL, INC. TABLE1 Job No. 116 040A 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 lYPE SIEVE lfl) (%) (DCO (%} (%) (PSFl l 5 7.5 93 Silty Sandy Clay 10 9.4 102 Sandy Silty Clay 20 10.4 62 21 4 Sandy Silt and Clay 2 5 7.6 96 Sandy Silty C lay 10 10.9 119 73 Sandy Silty Clay 15 9.7 ll l Sandy Silty Clay