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HomeMy WebLinkAboutSubsoil StudyGtech HEPWORTH - PAWLAI< GEOTECHNICAL November 9, 2011 Peter Robson 73 Corral Drive Carbondale, Colorado 81623 I IL in, jt'_'t' I :nlnll Int ;1J l 1 a. ltill 11 i, i mus, Ltl1 '17hI0.Sjr I F:I 970-14-1-5-i-45-1 k 113,111: hNei. i'!PI11J3i'Ok.•L il.Corn Job No.111 348A Subject: Subsoil Study for Foundation Design, Proposed Residence, Lot E-8, Aspen Equestrian Estates, Equestrian Way, Garfield County, Colorado Dear Mr. Robson: 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 proposal for geotechnical engineering services to you dated October 24, 2011. The data obtained and our recommendations based on the proposed construction and subsurface conditions encountered are presented in this report. Proposed Construction: The proposed residence will be a 2 story wood frame structure with an attached garage located on the site as shown on Figure 1. Ground floor will be slab -on -grade. Cut depths are expected to range between about 2 to 4 feet. Foundation loadings for this type of construction are assumed to be relatively light and typical of the proposed 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 is a vacant meadow vegetated with grass and weeds. The site is relatively flat with a slight slope down to the south. There is an irrigation ditch just east of the east side of the site. Subsidence Potential: The site is underlain by Pennsylvania Age Eagle Valley Evaporite bedrock. The evaporite contains gypsum deposits. 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 in the Roaring Fork River valley to the north and east of the Catherine Store area. Sinkholes were not observed in the immediate area of the subject lot. The exploratory pits were shallow, for foundation design only. No evidence of underground voids were encountered. Based on our present knowledge of the site, it cannot be said for certain that sinkholes will not develop. In our opinion, the risk of ground subsidence at Lot E-8 C/Z� copy Parker 7119 • t_01( ir;• 71 - 5(1 0 Silo rrhl-Irnc -2 - throughout the service life of the residence is low and similar to other lots in the area but the owner should be aware of the potential for sinkhole development. 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 1 to 11/2 feet of topsoil, consist of nil to 2'/2 feet of medium stiff to loose, silty sand and clay overlying medium dense slightly silty sandy gravel with cobbles down to the maximum depth explored, 5 feet. Unconfined compressive strength testing by pocket penetrometer indicated a low strength of 300 psf. Free water was observed in Pits 1 and 2 at about 3 to 3'/2 feet deep at the time of excavation. The upper soils were moist to very moist. Our experience in the area is that shallow groundwater conditions are typical and are related to irrigation of nearby properties. Foundation Recommendations: Considering the subsoil conditions encountered m the exploratory pits and the nature of the proposed construction, we recommend spread footings placed on the undisturbed natural gravel soil designed for an allowable soil bearing pressure of 2,000 psf for support of the proposed residence. Footings should be a minimum width of 16 inches for continuous walls and 2 feet for columns. Loose and disturbed soils and existing clay and sand soils encountered at the foundation bearing level within the excavation should be removed and the footing bearing level extended down to the undisturbed natural gravel soils_ 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 10 feet. Foundation walls acting as retaining structures (if any) should be designed to resist a lateral earth pressure based on an equivalent fluid unit weight of at least 50 pcf for the on-site predominantly granular soil as backfill. Dewatering of the excavation will likely be required and can probably be accomplished by pumping from shallow sump pits adjacent to the excavation. If the excavation is made prior to irrigation season, it is possible that dewatering can be avoided. Crawlspaces and basement levels are not recommended at this site due to the seasonal relatively high groundwater. Floor Slabs: The natural on-site gravel soils, exclusive of topsoil and sand and clay soils, are suitable to support lightly loaded slab -on -grade construction. The existing topsoil and sand and clay soils should be removed from the building area and slab subgrade re-established with compacted structural fill. The structural fill should consist of the on-site gravel or an imported gravel such as road base or screened rock. 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 slabs. This material should Job No.111 348A Gag -tech -3 - 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 or imported gravel soils devoid of vegetation, topsoil and oversized rock. Underdrain System: Based on the proposed shallow foundation and slab -on -grade lower floor, an underdrain system is not needed. Surface Drainage: The following drainage precautions should be observed during construction and maintained at all times after the residence has been completed: 1) 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. Exterior backfill can consist of the onsite soils in landscape areas. Fill placed under exterior patio areas should consist of the on-site gravel or a suitable imported gravel soil devoid of vegetation, topsoil or oversized rocks. 2) 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 6 inches in the first 10 feet in unpaved areas and a minimum slope of 3 inches in the first 10 feet in pavement and walkway areas. 3) Roof downspouts and drains should discharge well beyond the limits of all backfill. 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 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 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 ofour information. As the project evolves, we should provide continued consultation and field services during construction to review and monitor the implementation ofour recommendations, and to Job No.111 348A HStech -4 - 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 let us know. Respectfully Submitted, HEPWORTH - PAWLAK GEOTECHNICAL, INC. 1/ rJ .� Daniel E. Hardin, P.I1 ilk. 2 • sS10NA1 ekC Rev. by: SLP DEH/ksw attachments Figure 1 -- Location of Exploratory Pits Figure 2 — Logs of Exploratory Pits Job No.111 348A PIT 2 ■ LOT E7 APPROXIMATE SCALE 1"=40' PIT i PROPOSED RESIDENCE LOT E8 111 348A H Hepworth—Pawlak Geotechnical LOCATION OF EXPLORATORY PITS Figure 1 0 0 PIT 1 5 p Yo•; L� 10 LEGEND: rte/ _J PIT 2 J PIT 3 1•:•,'Q UC=300 TOPSOIL; clay, silty, sandy, organics, roots, soft, moist, dark brown. SAND AND CLAY (SC -CL); silty, less clayey with depth, medium stiff to loose, moist to very moist, brown. GRAVEL (GM -GP); sandy, slightly silty, with cobbles, medium dense, moist to wet with depth, brown, sub -rounded rocks. 2" Diameter hand driven liner sample. Disturbed bulk sample. Free water level in pit at time of excavating. 0 rJ— 10 — NOTES: 1. Exploratory pits were excavated on October 28, 2011 with a mini -excavator. 2. Locations of exploratory pits were measured approximately by pacing from building corners staked by client. 3. Elevations of exploratory pits were not measured and the logs of exploratory pits are drawn to depth. 4. The exploratory pit locations 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 materia! types and transitions may be gradual. 6. Water level readings shown on the logs were made at the time and under the conditions indicated. Fluctuations in water level may occur with time. 7. Laboratory Testing Results: UC – Unconfined Compressive Strength (psf) 111 348A HEPWORTH-PAWLAK GEOTECHNICAL LOGS OF EXPLORATORY PITS I Figure 2