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Home My WebLink AboutSubsoil Study for Foundation Design~tech HEPWORTH-PAWLAK GEOTECHNICAL April 21, 2006 Chris and Arny Mays 0024 Southard Station Carbondale, Colorado 81623 J-lcpwurrh-Pawl,11.: Gcotcchna:al, Im:. 5020 C nunty Ro.1LI 154 GlcnwooJ Srrings, Colorado 81601 Ph cint>: 970.945 . 7988 F,1x : 970-945-8454 email. hri:cn@hpi:cotcch.com Job No .106 0316 Subject: Subsoil Study for Foundation Design, Proposed Residence, Lot 85, Filing 7, Elk Springs Subdivision, Garfield County, Colorado Dear Mr. and Mrs. Mays: As requested, Hepworth-Pawlak Geotechnical, Inc. performed a subsoil study for design of foundations at the subject site:. The study was conducted in accordance with our agreement for geotechnical engineering services to you dated April 7, 2006. The data obtained and our recommendations based on the proposed construction and subsurface conditions encountered are presented in this report. Hepworth-Pawlak Geotechnical, Inc. previously performed a preliminary geotechnical study for Filings 6 through 9, Elk Springs (fonnerly Los Amigos Ranch PUD) and reported our findings on February 14, 1997, Job No. 197 617. Proposed Construction: Plans for the proposed residence are conceptual and the report was prepared for purchase of the property. Typical construction in the area consists of one to two story wood frame structures over a basement, crawlspace or slab-on-grade. The building envelope is shown on Figure 1. Cut depths would be expected to range between about 2 to 8 feet. Foundation loadings for this type of construction are assumed to be relatively light and typical of the assumed 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 site was vacant of structures and free of snow at the time of our field work. Vegetation consists of grass and weeds with sage brush. The general topography consists of a rolling upland mesa with a gentle to moderate slope down to the Parkl!r 303-841-7119 • Colorado Springs 719-633-5562 • Silverthorne 970-468-1989 -2 - south in the building envelope. Elevation difference the building envelope is about 12 feet. Scattered basalt cobbles and boulders were observed on the ground surface. Subsurface Conditions: The subsurface conditions at the site were evaluated by excavating three exploratory pits at the approximate locations shown on Figure 1. The logs of the pits are presented on Figure 2. The subsoils encountered, below about one foot of topsoil, consist of silty sandy clay overlying basalt cobbles and boulders in a silt and sand matrix. Results of swell-consolidation testing perfonned on relatively undisturbed drive samples, presented on Figures 3 and 4, indicate low compressibility under existing moisture conditions and light loading. The soils showed nil to minor movement upon wetting. All the samples indicted moderate compression after wetting under additional loading. 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 1.500 psf for support of the proposed residence. The subsoils soils tend to compress after wetting and there could be some post-construction foundation settlement. Footings should be a minimum width of 16 inches for continuous walls and 2 feet for columns. Utility trenches and deep cut areas may require rock excavating techniques such as chipping or blasting. 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. Voids created from boulder removal at footing grade should be filled with a structural material such as concrete or road base compacted to 95 percent standard Procotor density at a moisture content near optimum. 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 reinforced top and bottom to span local anomalies such as by assuming an unsupported length of at least 12 feet. Foundation walls acting as retaining structures should be designed to resist a lateral -earth pressure based on an eguivalent fluid unit weight of at least 50 pcf for the on-site soil as backfill. Job No .106 0316 - 3 - Floor Slabs: The natural on-site soils, exclusive of topsoil, are suitabJe 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 alJow unrestrained vertical movement. Floor slab control joints sbouJd 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 aggregate with less than 50% passing 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, 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 1 foot 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 Jess 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 Y2 feet deep. Surface Drainage: The following drainage precautions should be observed during construction and maintained at all times after the residence has been completed: Job No .106 0316 - 4 - I) Inundation of the foundation excavations and underslab areas should be avoided during construction. Drying could increase the expansion potential of the soils. 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. Free-draining wall backfill 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 I 2 inches in the first 10 feet in unpaved areas and a minimum slope of 3 inches in the first I 0 feet in pavement and walkway areas. A swale may be needed uphill to direct surface runoff around the residence. 4) Roof downspouts and drains should discharge well beyond the limits of all backfill. 5) Landscaping which requires regular heavy irrigation and lawn sprinkler heads should be located at least 10 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 geotecbnical 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 pits excavated at the locations indicated on Figure 1 and to the depths shown on Figure 2, 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 pits and variations in the subsurface conditions may not become evident until excavation is performed. If Job No.106 0316 -5 - conditions encountered during construction appear different from 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 responsible for technical interpretations by others of our infonnation. 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 testing of structural fill by a representative of the geotechnical engineer. If you have any questions or if we may be of further assistance, please Jet us know. Respectfully Submitted, HEPWORTH -PAWLAK GEOTECHNICAL, INC. ~--Louis E. Eller Reviewed by: LEE!kmtn attachments Job No .106 0316 COMMON AREA / LOT85 /\ / \ / \ / \ / \ \ \ I PIT2 \) // • PIT3 . / I / \ BENCH MARK: GROUND AT BUILDING ENVELOPE CORNER ; ELEV. = 100.0', ASSUMED . \ PIT1 // \ '/ v -.... ELK SPRINGS DRIVE / / / / / APPROXIMATE SCALE 1· = 100' LOT87 106 0316 LOCATION OF EXPLORATORY PITS Figure 1 I .c a ~ 0 5 10 LEGEND: PIT 1 ELEV.= 91 .5' -.;;..;&,,,;ol WC=26.9 00=87 -200=65 PIT2 ELEV.= 99.5' WC•t 1.5 00=97 -200m92 TOPSOIL; sandy silt and clay, organic, firm, moist. dark brown. PIT 3 ELEV.= 95.5' WC •9 .5 00-102 WCst4.7 ·l/X'J--.,r---< D0• 107 -200=66 CLAY (Cl); sandy, silty. medium stiff. moist. reddish brown, porous . 0 5 10 BASALT COBBLES AND BOULDERS (GM); in a sandy silt matrix, dense , slightly moist to moist , light brown to white, calcareous . 2· Diameter hand driven liner sample. Disturbed bulk sample. T Practical digging refusal with backhoe. NOTES: 1. Exploratory pits were excavated on April 10. 2006 with a Cat 420D backhoe. 2. Locations of exploratory pits were measured approximately by pacing from features shown on the site plan provided. 3. Elevations of exploratory pits were measured by instrument level and refer to the Bench Mark shown on Figure 1 . The pit logs are drawn to depth. 4. The exploratory pit 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 pit logs represent the approximate boundaries between material types and transitions may be gradual. 6. No free water was encountered in the pits at the time of excavating . 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 ' .c ii ~ 106 0316 LOGS OF EXPLORATORY PITS Figure 2 Moisture Content = 26.9 percent Dry Density = 87 pcf Sample of : Sandy Silt Matrix From: Pit 1 at 5 Feet 0 "P 1 -i.- "#. ~ ~ i---_Compression c: 0 '-... upon ·a; 2 wetting I/) C1> ~" .... a. E 0 ,, 0 3 4 0.1 1.0 10 100 APPLIED PRESSURE -ksf Moisture Content = 11 .5 percent -ae. 1 Dry Density = 97 pcf c Sample of: Slightly Sandy Silty Clay .Q "' From: Pit 2 at 6)'2 Feet c aJ 0 a. Uj :~ ' c 0 1 'Ci) (/) \ I\, C1> .... a. E 2 0 0 Expansion \ upon 3 wetting 4 \ I) 5 0.1 1.0 10 100 APPLIED PRESSURE -ksf 106 0316 c&'~ch HEPWOA'ntPAWUJC GEO"IWCHNICAL SWELL-CONSOLIDATION TEST RESULTS Figure 3 Moisture Content = 9.5 percent Dry Density = 102 pct Sample of : Slightly Sandy Silty Clay From: Pit 3 at 3 Feet 0 ~~ 1...-~ ii.-- -ae. , i"'--- c: '\ ..... "-"" ,., ... No movement 0 '(ij ' upon en 2 wetting Q) a. ' ~ \ u 3 b 4 0.1 1.0 10 100 APPLIED PRESSURE -ksf 106 0316 Oe~ch H1EPW0..,,..PAWUK Gll!X111ilC,_CAL SWELL-CONSOLIDATION TEST RESULTS Figure 4 HEPWORTH-PAWLAK GEOTECHNICAL, INC. TABLE 1 Job No. 106 0316 SUMMARY OF LABORATORY TEST RESULTS SAMPLE LOCATION NATURAL NATURAL GRADATION PERCENT ATIERBERG LIMITS UNCONFINED MOISTURE DRY GRAVEL SAND PASSING LIQUID PLASTIC COMPRESSIVE SOIL OR PIT DEPTH CONTENT DENSnY NO. 200 LIMIT INDEX STRENGTH BEDROCK TYPE (%) (%) SIEVE (ft) {o/o) CocO (%) (%1 (PSFJ 1 5 26 .9 87 85 Sandy Silt Matrix 2 6 1/2 11.5 97 92 Slightly Sandy Silty Clay 3 3 9.5 102 Slightly Sandy Silty Clay 6 14.7 107 66 Sill and Sand Matrix