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Home My WebLink AboutSubsoil Study for Foundation Designc..s!rtech HEPWORTH-PAWLAK GEOTECHNICAL SUBSOIL STUDY Hepworth-Pawlak Geocechnical, Inc. 5020 County Road 154 Glenwood Springs, Colorado 81601 Phone: 970-945-7988 Fax: 970 ·945-8454 email: hpgco@hpgeotcch.com FOR FOUNDATION DESIGN PROPOSED RESIDENCE LOT 4, UON'S RIDGE ESTATES NORTH OF HIGHWAY 82 AND EAST OF COUNTY ROAD 100 GARFIELD COUNTY, COLORADO JOB NO. 103 253 MAY14,2003 PREPARED FOR: NEIL AND NANCY LAATSCH 2935 CYPRESS POINT WADSWORTH, ILLINOIS 60083 Parker 303-841-7119 • Colorado Springs 719-633-5562 • Silverthorne 970-468-1989 TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY .......•........ , . . . . . . . . . . • . . • • . . • . . 1 PROPOSED CONSTRUCTION ......•....•...•.•...•.........•........• 1 SITE CONDITIONS . • . . . . . . . . . . . . . . . . . . . . . . . . • . . . . . . . . . . • . . . . . . . . • • . . 2 FIELD EXPLORATION .•...•..•........•.•......•..•....•.•....•....• 3 SUBSURFACE CONDmONS ................•..•.•......•••........•.. 3 FOUNDATION BEARING CONDITIONS ......•.•..••.........•...•.•... 4 DESIGN RECOMMENDATIONS . • . . • . . . . . . . . . . . • . • . . • . . . . . • . . . . . . • . . . . 5 FOUND A TIO NS • • • • • . . • • . . . • . . . • . . . . . . . . • • . . • . . • . • • . . • • • • • • • . • 5 FLOOR SIABS . . . . . . . . . . . . • . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 tJNDERDRAIN SYSTEM . . . . . . • . . • . . . . . . . . . . • . . . . . . . . . . . . . . . • • . . 6 SURFACE DRAINAGE ••.• : .•••.••••.•.••••..•.•...•••••••••..••• 7 PERCOLATION TESTIN'G .....•... : ..........•.......••..•••.•........ 8 LIMITATIONS • . . . • . . . . . . . . . . . • . • • . . . • . . . . . • . . . • . . . . . . . . . . . . . • • . . • • . 8 FIGURE 1 -LOCATION OF EXPLORATORY BORING AND PITS FIGURE 2 -LOGS OF EXPLORATORY BORING AND PITS FIGURE 3 ·LEGEND AND NOTES FIGURES 4 TO 7 -SWELL-CONSOLIDATION TEST RESULTS TABLE 1-SUMMARY OF LABORATORY TEST RESULTS TABLE 2 ·PERCOLATION TEST RESULTS Job#103 253 PURPOSE AND SCOPE OF STUDY This report presents the results of a subsoil study for a proposed residence to be located on Lot 4, Lion's Ridge Estates, north of Highway 82, east of County Road 100, 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 Neil and Nancy Laatsch dated April 22, 2003. After low density silt soils were identified in the backhoe pits, a boring was drilled to evaluate the deeper soils as authorized by Ken Garvi.k. A field exploration program consisting of exploratory pits and a boring 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 and other engineering characteristics. The results of the field exploration and l~boratory 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 subsoil conditions encountered. PROPOSED CONSTRUCTION The proposed residence will be a one story wood frame structure over a partial walkout basement and partial crawlspace. The basement floor will be slab-on-grade. Grading for the structure is assumed to be relatively minor with cut depths between about 3 to 8 feet. We assume relatively light foundation loadings, typical of the proposed type of construction. Job#103 253 -2- If building loadings, location or grading plans change significantly from those descnoed above, we sbquld be notified to re-evaluate the recommendations contained in this report. SITE CONDITIONS The site was vacant at the time of our field exploration. The building area slopes strongly down to the south-southwest at grades of 10 to 15 percent. A gully borders the proposerty on the west side. The site is vegetated with sage brush, cactus, weeds and grass. Scattered basalt cobbles were observed on the ground surface. SUBSIDENCE POTENTIAL Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the Llon's Ridge Estates. These rocks are a sequence of gypsiferous shale, fine-grained sandstone and 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 were observed scattered throughout the lower Roaring Fork Valley. These sinkholes appear similar to others associated with the Eagle Valley Evaporite in areas of the Eagle Valley. Sinkholes were not observed in the immediate area of the subject lot. No evidence of cavities was encountered in the subsurface materials; however, the exploratory pirs and boring were 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 4 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 Job #103253 --·--- -3- 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 April 24 and May 6, 2003. Three exploratory pits were excavated at the locations shown on Figure 1 to evaluate the subsurface conditions. The pits were dug with a rubber-tired backhoe. Due to the low density silt soils encountered in the pits, an exploratory boring was drilled to evaluate the deeper subsurface conditions. The boring was advanced with 4 inch diameter · continuous flight augers powered by a tr11ck-mounted CME-45B drill rig. The pits and boring were logged by a representative of Hepworth-Pawlak Geotechnical, Inc. Samples of the subsoils were taken from the pits with relatively undistUrbed and disturbed sampling methods. Samples of the subsoils were taken from the boring 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 Boring and Pits, 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 16 feet of loose to medium stiff sandy silt overlying medium dense, sandy silt and silty sand with scattered gravel down to the maximum depth drilled, 26 feet. The sandy silt contained roots and organics near the ground surface. Job#103 253 -4- Laboratory testing performed on samples obtained from the pils and boring included natural moisture content and density, and percenl finer than sand size gradation analyses. Results of swell ~con~olidation testing performed on relatively undisturbed liner samples, presented on Figures 4 to 7, indicate low compressibility under existing moisture conditions and light loading. The shallow samples from above 10 feet (Figures 4 to 6) showed low to moderate collapse potential (settlement under constant load) when wetted and high compressibility under increased loading after wetting. The deeper samples (Figure 7) showed low to minor collapse potential and moderate compressibility under increased loading after wetting. The laboratory testing is summarized in Table 1. No free water was encountered in the pits and boring at the time of excavation and drilling. The subsoils were slightly moist. FOUNDATION BEARING CONDITIONS The subsoils above about 10 feet at the site tend to settle when ~etted. There is a risk of potential settlement and distress to structures built on these soils in the event that the bearing soils become wet. Sources of wetting include concentrated roof runoff, poor surface drainage around lhe house, over irrigation and leaking utilities. The risk of wetting can be reduced by following the recommendations contained in this report. Spread footings placed on the upper natural soils can be used for building support with a risk of differential settlement and building distress. A relatively deep foundation placed on the underlying, higher density rocky sand soils could be used to reduce the settlement potential. If a deep foundation is considered, we should be contacted for additional analysis and recommendations. Job#103 253 C~I -5- DESIGN RECO:MMENDATIONS FOUNDATIONS Considering the subsoil conditions encountered in the exploratory pits and boring and the nature of the proposed construction, we recommend the building be founded with spread footings bearing on the narural subsoils. The design and construction criteria presented below should be observed for a spread footing foundation system. 1) Footings placed on the undisturbed natural 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. Additional settlement of 1 to 2 inches or more could occur if the bearing soils become wet. 2) The footings should have a minimum width of 20 inches for continuous walls and 2 feet for isolated pads. 3) Exterior footings and footings beneath unheated i!feas 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 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 aJso be designed to resist a lateral earth pressure corresponding to an equivalent fluid unit weight of 50 pcf. 5) The topsoil and any loose or disturbed soils should be removed and the footing bearing level extended down to natural subsoils. The exposed soils in footing area should then be moistened and compacted to at least 95% of standard Proctor density. Additional compaction to a depth of at Job#103 253 -------· ---------- -6- least 10 feet below existing ground surface could be performed to reduce the post-construction settlement potential. 6) . A representative of the geotechnical engineer should 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. The shallow subsoils have settle~ent potential when wetted. To reduce the effects of some differential movement, floor slabs shouJd be separated from all bearing walls and columns with expansion joints which allow unrestrained vertical movement. Floor slab control joints should be use~ 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 ~ith 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 ~hould 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 mountainous areas that local perched groundwater can develop during times of heavy precipitation or seasonal runoff. Frozen ground during spring runoff can also 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. Job #103 253 ---··-··---··---·----· -----··. -_____ .... ·-··-·--- -7- 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 pl3:ced at each level of excavation and at least 1 fool 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 d~p. 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 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 op~wn 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 fro.m the foundation in all directions. We recommend a minimum slope of 12 inches in the first 10 feet in unpaved areas and a minimwn slope of 3 inches in the first 10 feet in paved areas. Free-draining wall bacJr:fill sboul~ be capped with about 2 feet of the on-site soils to reduce surface water infiltration. 4) Job#103 253 Roof downspouts and drains should discharge well beyond the limits of all backfill. ---··-----· ·--- 5) Sprinkler heads and landscaping which requires regular heavy irrigation, such as sod, 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. PERCOIATIONTESTING Percolation tests were conducted on April 25, 2003 to evaluate the feastbility of an infiltration septic disposal system at the site. One profile pit and tluee percolation holes were dug at the locations shown on Figure 1. The test holes (nominal 12 inch diameter by 12 inch deep) were hand dug at the bottom of shallow backhoe pits and were soaked with water one day prior to testing. The soils exposed in the percolation holes are similar to those exposed in the Profile Pit shown on Figure 2 and consist of sandy silt. The percolation test results . are presented in Table 2. Based on the subsurface conditions encountered and the percolation test results we recommen~ that a civil engineer design the infiltration septic disposal system. 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 repon are based upon the data obtained from the exploratory pits excavated and boring drilled at th~ locat;ions indicated on Figure lt the proposed type of construction and our experience in the area. Our .f'mdings include interpolation and extrapolation of the subsurface conditions identified at the exploratory pits and boring and variations in ·the subsurface conditions may not became 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#103 253 -9- 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 e~olves, 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 scructural fill by a representative of the geotechni~l engineer. Respectively Submitted, HEPWORTII -PAWLAK GEOTECHNJCAL, Inc. Daniel E. Hardin, P.E. Reviewed by: Steven L. Pawlak, P .E. DEH/ksw Attachments cc: Garvik Consuuction -Attn: Ken Garvik White Horse Consulting • Attn: Larry Doble Job#103253 APPROXIMATE SCALE 1H = 40' LOT 5 PROPERTY BOUNDARY I I ( \ \ ?.J~ /Oj \ \ '\. / / \. \. ~ ', I I .....___ I BORING 1e \ PIT '1 ' UllUlY EASEMENT " " " " ',,_ 11 '-' 1i-__ . I -~---------------~ 103 253 HEPWORTH-PAWLAK GEOTECHNICAL, INC. LOCATION OF EXPLORATORY BORING AND PITS LOT 3 Figure 1 1000 995 990 -985 Cl : I s ;; ~ 980 t> w 975 970 965 103 253 BORING 1 ELEV.=995' 9/12 WC-3.3 00-118 -2oo-86 20/12 WC-3.0 -2oo-34 52/12 PIT 1 El.EV.•999' WC-5.7 00•92 -~88 PIT 2 ELEV.s982' WC-4.S 00•92 -:Z00-84 WC-3.4 00-90 -:Z00-83 Note: ExplonoUon of s)mbols is shown on Figure J. PROFILE PIT aEV.=962' 1000 995 990 985 980 975 970 965 HEPWORTH-PAWLAK GEOTECHNICAL, INC. LOGS OF EXPLORATORY BORING AND PITS Figure 2 -Cl Cl t&.. c 0 :;; ~ II ii] LEGEND: 39/12 NOTES: SILT (ML); slightly sandy to sandy, medium stiff, moist, light brown, upper shallow root zone. SILT AND SANO (ML-SM); sandy slit to sDty sand, with scattered sand to grovel size basalt fragments, medium dense, slightly moist, llght brown . 2" Diameter hand driven liner sample. Relatively undisturbed drive sample; 2-lnch 1.0. Collfomla llner sample. Drive sample; standard penetration test (SPT), 1 3/8 inch 1.0. split spoon sample. ASlM D-1586. Drive sample blow count; Indicates that 39 blows of a 140 pound hammer foiling 30 Inches were required to drive the Colifomio or SPT sampler 12 Inches. 1. Exploratory pits were excavated on April 24, 2003 with a Cot 416 B 4x4 backhoe. Exploratory Boring 1 was drilled on Moy 6, 2003 with a 4-inch diameter continuous flight power auger. 2. Locations of exploratory pits and Boring 1 were measured approximately by pacing from features shown on the site pion provided. 3. Elevations of exploratory pits and Boring 1 were obtained by Interpolation between contours on the site pion provided. 4. The exploratory pit and Boring 1 locations and elevations should be considered accurate only to the degree Implied by the method used. 5. The llne between materials shown on the Boring 1 log represents the approximate boundary between material types and the transition may be gradual. 6. No free water was encountered In the pits or Boring 1 at the time of excavating and drilling. Fluctuations in water level may occur with time. 7. Laboratory Testing Results: WC • Water Content ( " ) OD = Dry Density ( pcf ) -200 = Percent passing No. 200 sieve 103 253 HEPWORlH-PAWLAK GEOTECHNICAL, INC. LEGEND AND NOTES Figure 3 Moisture Content • 5.7 percent Ory Density = 92 pcf Sample of: Sandy Slit From: Pit 1 ot 8 Feet 0 ' 1 Compression .......---v ........ ---upon 2 -i....-wetting {... i..--- 3 4 \ \ 5 " c: ' 0 6 'ii ' IO ~ c. \ E 7 0 \ 0 8 \ 9 I I 10 \ ' 11 12 \ I~ 13 0.1 1.0 10 100 APPLIED PRESSURE -ksf 103 253 HEPWORTH-PAWLAK SWELL CONSOLIDATION TEST RESULTS Figure 4 GEOTECHNICAL. INC. Moisture Content .. 4.6 percent Ory Density = 92 pcf Sample of: Sandy Sitt From: Pit 2 at J Feet 0 ,, , Compression ..-.----7 ...... ... upon ..... wetting 2 -,_ L --- 3 ( ~ \ g 4 Ci \ tit C) ... \ c. 5 E 0 \ (.) 6 7 \ 1 8 \ ' 9 ~ ) 0.1 1.0 10 100 APPLIED PRESSURE -ksf 103 253 HEPWORTH-PAWLAK SWELL CONSOLIDATION TEST RESULTS Figure 5 GEOTECHNICAL, INC. Moisture Content =-3.4 percent Dry Density -90 pcf Sample of: Sandy Slit From: Pit 2 at 6 Feet 0 1 Compression .---v _,... upon .,_. /' i.---wetting 2 - Cl " 3 \ c \ 0 'ii .. 4 ., ... \ a. E 0 ~ 0 5 6 \ 7 \ ·\ 8 j \ 0 .1 1.0 10 100 APPLIED PRESSURE -ksf 103 253 HEPWORTH-PAWLAK SWELL CONSOLIDATION TEST RESULTS Figure 6 GEOTECHNICAL, INC. Moisture Content = 3.3 percent Dry Density -98 pcf Sample of: Sandy Silt From: Boring 1 ot 10 Feet 0 ~ ,, ~ 1 - I•~ -Compression ., upon f Cl. 2 wetting ~ "'< ~ 0 3 ~ 0.1 1.0 10 100 APPLIED PRESSURE -ksf Moisture Content • 4.6 percent Ory Density • 122 pcf Sample of: snty Sand From: Boring 1 at 25 Feet 0 - ~ --i-. -~ § 1 -... ' m ~ -;;::> Compression m u upon ... " Cl. 2 wetting s " () 3 \, 0.1 1.0 10 100 APPLE> PRESSURE -ksf 103 253 HEPWORTH-PAWLAK SWELL CON SOU DA TION TEST RESULTS Figure 7 GEOTECHNICAL, INC. HEPWORTH-PAWLAK GEOTECHNICAL, INC . TABLE 1 JOB NO. 103 253 SUMMARY OF LABORATORY TEST RESULTS SAMPLE LOCATION NATURAL NATURAL GRADATION PERCENT ATTERBERG UMIJS UNCONFINED PIT OEPTM MOISTURf DRY GRAVEL s.iuro PASSING LIQUID PLASTIC COMPRESSIVE SO!lOR ,, .. ., CONTENT DENSITY ('K) (%) NO.ZOO LIMIT INDEX STRENGTH BEDROCK TVPI! 1%1 (pd) SIEVE l'KI 1%1 (PSF) 1 8 5.7 92 88 sandy silt 2 3 4.6 92 84 sandy silt 6 3.4 90 83 sandy silt Boring 1 10 3.3 98 86 sandy silt 20 3.0 34 silty sand 25 4.6 122 46 silty sand HEPWORTH-PAWLAK GEOTECHNICAL, INC. TABLE 2 PERCOLATION TEST RESULTS JOB NO. 103 253 HOLE NO. HOLE DEPTH LENGTH OF WATER DEPTH WATER DEPTH DROP IN AVERAGE (INCHES) INTERVAL AT START OF AT END OF WATER PERCOLATION (MINI INTERVAL INTERVAL LEVEL RATE (INCHES I UNCHESI (INCHES) IMIN.nNCHI P·1 41 5 11 7 518 3 318 7 5/8 4 314 2 7/8 water eddlld 11 8 3 8 6 2 water added 11 314 8 112 3 1/4 8 1/2 6 112 2 water added 10 112 7 114 31/4 1114 4 518 2 518 4 5/8 2 518 2 2 P·2 43 5 11 9 2 9 61/8 2 7/8 water added 11 7/8 9 6/8 21/4 9 518 7 318 2114 water added 12 1/8 9 518 2 112 9 518 7 314 1 7/8 weter added 11 8 5/8 2 3/8 8 518 6 318 2 1/4 6 3/8 4 S/8 1 3/4 2 1/2 p.3 44 s 12 3/4 10 118 2 5/8 10 1/8 6 7/8 41/4 water added 12 7/8 10 2 7/8 10 6 3/4 3 1/4 weter added 13 1/2 10 3 1/2 10 7 1/4 2 3/4 water added 13 1/4 9 3/8 3 7/8 9 318 6 3/8 3 6 318 4 2 3/8 2 Note: Percolation test holes were hand dug in the bottom of backhoe pits and soaked on April 24. 2003. Percolation tests were conducted on April 25, 2003. The average percolation rates were based on the last two readings of each test.