The URL can be used to link to this page

Home My WebLink AboutSubsoil Study for Foundation Design~ec HEPWORTH-PAWLAK GEOTECHNICAL SUBSOil.. STUDY H..:p"or1h-Puwl.1k G.:n1t•dm1c:il. In, 5020 C.1111\IV !\,l,\d I;.; Gllnw'""' s'r11ng•. C11l11rad11 t\l(-01 Phone 9jl).o.i;. i9~S F.n: ?i0-9-15-1:14 54 1.m 111 hri:co'C:hri.:1.· .. ti:i:h.c.1111 FOR FOUNDATION D~IGN PROPOSED RESIDENCE LOT 13, PINYON ~A GARFIELD COUNTY, COLORADO JOB NO. 115 477 A OCTOBER 30, 2015 PREPARED FOR: URIEL MELLIN 3706 RED BLUFF LANE GLENWOOD SPRINGS, COLORADO 81601 (uriel.mellin@hotmaiJ.com ) Parker 303 -841·7119 • ColoraJu Springs 719 -633 -5562 • Sdv crrhornc 9 70-468-1989 TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY ............................................................................ - I - PROPOSED CONSTRUCTION .................................................................................... -l - SITE CONDITIONS ............................................................................................... ,.. ... , ........ -I - SUBSIDENCE POTENTIAL ......................................................................................... -2 - FIELD EXPLORATION ................................................................................................. -2 - SUBSURFACE CONDmONS ...................................................................................... -3 - FOUNDATION BEARING CONDITIONS .................................................................. - 3 - DESIGN RECOMMENDATIONS ................................................................................ -4 - FOUNDATIONS ........................................................................................................ -4 - FOUNDATION AND RET AINlNG WALLS ........................................................... -6 - FLOOR SLABS .......................................................................................................... -7 - UNDERDRAIN SYSTEM .......................................................................................... -7 - SURFACE DRAINAGE ............................................................................................. -8 - Lll\lllT ATIONS ................................. , ......... , ........................................................................ -9 ... FIGURE 1 -LOCATION OF EXPLORATORY BORING FIGURE 2 -LOG OF EXPLORATORY BORING FIGURE 3 -LEGEND AND NOTES FIGURES 4 AND 5 -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 located at Lot 13, Pinyon Mesa, 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 October2, 2015. An exploratory boring was drilled 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 enginc:ering considerations based on the proposed construction and the subsurface conditions encountered. PROPOSED CONSTRUCTION The proposed residence will be a two story structure above a full basement level. Basement and attached garage floors will be slab-on-grade. Grading for the structure is assumed to be relatively minor with cut depths between about 3 to 9 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 The vacant lot is located on the north side of Sage Meadow Road. Vegetation consists of sage brush, grass and weeds. Vegetation in the front part of site was removed during Job No I IS 477A -2- subdivision development. The ground surface is relatively flat with a gentle slope down to the west. SUBSIDENCE POTENTIAL Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the Pinyan Mesa development. These rocks are a sequence of gypsiferous shale, fine-grained sandstone und siltstone with some massive beds of gypsum and limestone. There is a possibility that massive gypsum deposits associated with the Eagle Valley Evaporite underlie portions of the lot. Dissolution of the gypsum under certain conditions can cause sinkholes to develop and can produce areas of localized subsidence. During previous work in the area, sinkholes have been observed scattered throughout the lower Roaring Fork River valley. Sinkholes were not observed in the inunediate area of the subject lot. No evidence of cavities was encountered in the subsurface materials; however, the exploratory boring was relatively shallow. for foundation design only. Based on our present knowledge of the subsurface conditions at the site, it cannot be said for certain that sinkholes will not develop. The risk of future ground subsidence on Lot 13 throughout the service life of the proposed residence, in our opinion, is low; however, the owner should be made aware of the potential for sinkhole development. If further investigation of possible cavities in the bedrock below the site is desired, we should be contacted. FIELD EXPLORATION The field exploration for the project was conducted on October 9, 2015. One exploratory boring was drilled at the location shown on Figure I to evaluate the subsurface conditions. The boring was advanced with 4 inch diameter continuous flight augers powered by a truck-mounted CME-45B drill rig. The boring was logged by a representative of Hepworth-Pawlak Geotechnical, Inc. JobNo llS477A - 3 - Samples of the subsoils were taken with 1 :V. 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 and hardness of the bedrock. 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 subsurface conditions encountered at the site is shown on Figure 2. The subsoils encountered, below a thin root zone and topsoil, consist of about 40 feet of stiff, sandy silty clay with scattered gravel underlain by hard claystone bedrock to the drilled depth of 51 feet. Laboratory testing performed on samples obtained from the boring included natural moisture content, 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 low collapse potential (settlement under constant load) to moderate expansion potential when wetted and were moderately compressible under increased loading after wetting. The laboratory testing is summarized in Table 1. No free water was encountered in the boring at the time of drilling and the subsoils were slightly moist. FOUNDATION BEARING CONDmONS The sandy silty clay soils encountered at proposed shallow foundation depth of the garage tend to settle when they become wetted. The lower compressibility potential of the underlying very stiff, sandy clay soils may also impact a shallow foundation if deep wetting were to occur but the risk appears low. A shallow foundation placed on the upper soils will have a high risk of settlement if the soils become wetted and care should be Job No 115 477A -4- 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 seulements of 2 to 3 inches 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 extending down roughly 45 feet below existing ground surface) or a heavily reinforced mat foundation, on the order of 2 feet thick, and designed by the structural engineer 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. DESIGN RECOMMENDATIONS FOUNDATIONS Considering the subsurface conditions encountered in the exploratory boring and the nature of the proposed construction, the building can be founded with spread footings bearing on compacted structural fill at the garage level and on the natural soils at the basement level with a risk of settlement, mainly if the bearing soils become wetted, and provided the risk is acceptable to the owner. Control of surface and subsurface runoff will be critical to the long-term performance of a shallow spread footing foundation system. The garage footing areas should be sub-excavated down about 8 feet below existing ground surface and replaced compacted back to design bearing level but to a depth of at least 5 feet below footing bearing level. The design and construction criteria presented below should be observed for a spread footing foundation system. Job No 115 477A 1) Footings placed on a minimum 5 feet of compacted structural fill of the garage and on the natural soil at basement level of the residence should be designed for an allowable bearing pressure of 1,200 psf. Based on experience, we expect initial settlement of footings designed and constructed as discussed in this section will be about 1 inch or Jess. Additional settlement of about 1 inch could occur if deep wetting below the bearing level was to occur. A Y> increase in the allowable bearing pressure can be taken for toe pressure of eccentricaJly loaded footings. 2) The footings should have a minimum width of 24 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 help resist differential movements. Foundation walls acting as retaining structures should also be designed to resist lateral earth pressures as discussed in the ti Foundation and Retaining Walls" section of this report. 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 design grades should then be moistened and compacted. Structural fill should consist of low permeable soil (such as the on-site silty sandy clay soils) compacted to at least 98% standard Proctor density within 2% of optimum moisture content. The structural fill should extend laterally beyond the footing edges equal to about 'A 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. Job No. I 15 477A -6- 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 fine-grained soils. Cantilevered retaining structures which are separate from the residence and can be expected to deflect sufficiently to mobiJize the full active earth pressure condition should be designed for n lateral earth pressure computed on the basis of an equivalent fluid unit weight of nt least 45 pcf for backfill consisting of the on-site fine-grained soils. 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 waJls . 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. Passive pressure of compacted backfill against the sides of the footings can be calculated using an equivalent Ouid unit Job No . llS 477A -7- weight of 325 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 with settlement risk similar to that described above for foundations in the event of wetting of the subgrade soils. To reduce the effects of some differential movement. floor slabs should be separated from all bearing walls and columns with expansion joints which aJlow 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 interior slabs to limit capillary moisture rise. 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 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 and basement areas, be protected from wetting and hydrostatic pressure buildup by an Job No . I IS 477A -8 - underdrain system. An underdrain should not be placed around shallow footing depth structures such as the garage area (and crawlspace, if provided). The drains should consist of drainpipe placed in the bottom of the wall backfill surrounded above the inven 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 an interior sump of solid casing. 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~ feet deep. An impervious membrane such as a 20 mil PVC liner 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 It will be critical to the building performance to keep the bearing soils dry. The following drainage precautions should be observed during construction and maintained at all times after the residence has been completed: I) 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 Qlld 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 at least 2 feet of the on-site soils to reduce surface water infiltration. Job No. I lS 4nA -9- 4) Roof downspouts and drains should discharge well beyond the limits of all backfill. Natural vegetation lined drainage swales should have a minimum slope of3%. 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. LIMITATIONS This study has been conducted in accordance with generally accepted gcotechnical engineering principles and practices in this area at this time. We make no warranty either express or implied. The conclusions and reco1IU11Cndations submitted in this repon are based upon the data obtained from the exploratory boring drilled at the location 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 boring 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 repon hns 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 lob No I IS 4nA -10- bearing strata and testing of structural fill by a representative of the gcotcchnicnl engineer. Respectfully Submitted. HEPWORTH -PAWLAK GEOTECHNICAL, INC. Reviewed by: LEE/ksw Job No. JJ 54n A APPROXIMATE SCALE 1· 30' I ii) c ~ -------- I I I I r-------, I I I I LOT13 I I I LOT12 I LOT14 I I I BORING 1 I I I • I I L _______ J -- SAGE MEADOW ROAD 115 477A ~ LOCATION OF EXPLORATORY BORING Figure 1 Heoworth-Pciwtak GeatlChnlcal BORING1 0 0 3Blt2 WC 42 DO 103 14/12 10 22/12 WC 73 10 00 103 27/12 WC 63 00 102 ·200 89 20 20 34/12/12 WC 73 I ~ DO 114 68/12 u.. I ' we 11 :5 :5 ! OD 122 0. Q) 30 ·200 92 30 0 31/12 WC 9.6 OD 117 40 3516,50/3/12 40 50 50/4 50 NOTE: Explanalion of symbols is shown on Figure 3. 115 477A LOG OF EXPLORATORY BORING Figure 2 LEGEND: 0 CLAY (CL); sandy, silty, upper few feet slightly gravelly, very stiff, 0 CLAYSTONE: hard, slightly moist, gray. p Relatively undisturbed drive sample; 2-inch l.D. California liner sample. ~ Drive sample; standard penetration test (SPn, 1 3/8inch1.0. split spoon sample, ASTM-1586. Drive sample blow count; indicates that 38 blows of a 140 pound hammer falling 30 inches were 38/12 required to drive the California or SPT sampler 12 inches. NOTES: 1. The exploratory boring was drilled on October 9, 2015 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 not measured and the log of exploratory boring is drawn to deplh. 4. The exploratory boring location 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. Fluctualion in water level may occur with time. 7. Laboratory Testing Results WC Water Content (%) DD Dry Dens ty (pcf} -200 Percent passing No 200 sieve 115 477A LEGEND AND NOTES Figure 3 Moisture Content 4.2 percent Dry Density 103 pcf 0 Sample of: Slightly Gravelly Sandy Silty Clay 1'111 From : Boring 1 at 2 Feet 1 --t--I~ c b 'i'. 2 c: ~ r--t---. Compression .Q ,_ C/l lo.. ~ .... i-upon K 3 t\. wetting ~ \ 4 \. \ 5 \ \ 6 I ~ 0.1 1.0 10 100 APPLIED PRESSURE· ksf Moisture Content ~ 7.3 percent Ory Density 103 pcf Sample of: Sandy S·lty Clay From: Boring 1 at 1 O Feet ~ -~ 1 Cll ! 0 ' --~ -c: ~-..... ~ ·i liJ' ~ 1 ~ ~ "'-~ ~ 2 Expansion upon wetting 0 .1 1.0 10 100 APPLIED PRESSURE· ksf 115 477A c$''1iech H~AWLNC Gmn:cttMCM. SWELL-CONSOLIDATION TEST RESULTS Figure 4 . . Mof sture Content -7.3 percent Ory Density 114 per 3 Sample or: Sandy Siity Clay From: Boring 1 at 20 Feet 2 ' "-----"' °;J?. ~ 5 1 ·u; "' ") ~ fij a. ~ ~ 0 I \ ' c ~, 0 ·u; II) 1 ! Ex pans bn ~b c9 upon 2 wetting 0.1 1.0 10 100 APPLIED PRESSURE • ksr Moisture Content -9.6 percent Ory Density 117 per Sample or: Sandy Srlty Clay From: Boring 1 at 30 Feet ~ 0 c -r--!~~~ 0 lo-. ·u; ...... ...... i 1 ........ a. ""' ~ in " • c 0 2 ' .in ~ \ 10 a. E 3 0 u Expansion upon wetting 0.1 1.0 10 100 APPLIED PRESSURE -ksf 115 477A ~ Hmworth-Pawlak Geotechnlcal . SWELL-CONSOLIDATION TEST RESULTS Figure 5 HEPWORTH-PAWLAK GEOTECHNICAL, INC. TABLE1 JobNo.115477A SUMMARY OF LABORATORY TEST RESULTS SAMPLE LOCATION NATURAL NATURAL GRADATION ATTERBERG LIMITS UNCONFINED PERCENT MOISTURE DRY GRAVEL SANO PASSING LIQUID PLASTIC COMPRESSIVE SOIL OR BORING DEPTH CONTENT DENSITY N0.200 LIMIT INDEX STRENGTH BEDROCK TYPE (%) (%) nu (%) (DCn SIEVE (%) (%) tPSFl I 2 4.2 103 Slightly Gravelly Sandy Silty Clay 10 7.3 103 Sandy Silty Clay 15 6.3 102 89 Sandy Silty Clay 20 7.3 114 Sandy Silty Clay 25 7.7 122 92 Slightly Sandy Silty Clay 30 9.6 11 7 Sandy Silty Clay