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HomeMy WebLinkAboutObservation of Backhoe Pits 11.02.2020cod f•Nritontrtenii9i .iu20 1 t•ls� i ii iiv5ei i i _••; (i1,11wilvgj F)t4.+e:C ,.. tc,7i;j'. • • November 2, 2020 Carla Ostberg 33 Four Wheel Drive Road Carbondale, Colorado 81623 Project No.20-7-659 Subject: Observation of Backhoe Pits, Proposed Residence and ADU/Pole Barn, 129 Cains Lane, Lot 10, Pinyon Peaks Subdivision, Garfield County, Colorado Dear Carla: As requested, Kumar & Associates, Inc. observed several backhoe pits for design of foundations at the subject site. The study was conducted in accordance with our agreement for geotechnical engineering services to you dated October 29, 2020. 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 two-story wood frame structure over a crawlspace. The attached garage will have a slab -on -grade floor. The ADU will be one story over crawlspace. The pole barn will be one story with gravel floor. Cut depths are expected to range between about 2 to 6 feet. We understand that 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 vacant and slopes down to the north at 10% to 15% grade. Vegetation consists of scattered to moderately dense juniper and pinon trees with an understory of sagebrush, grass and weeds. The house and ADU/pole barn corners were staked in the field. The lot is accessed from Cains Lane, located along the south side of the property. Subsurface Conditions: The subsurface conditions at the site were evaluated by excavating four exploratory pits, two in the proposed house area and two in the ADU/pole barn area. The subsoils encountered, below about 8 to 10 inches of topsoil, consist of about 1 foot of sandy silty clay with basalt rocks overlying basalt rocks up to large boulder size in a sandy clayey silt matrix starting at 11/2 to 2 feet deep down to digging refusal at the bottom of the pits at 4 to 41/2 feet -2 - deep. The clay soils overlying the basalt rocks were slightly deeper in the pole barn area. 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 clay or basalt rock soil designed for an allowable soil bearing pressure of 1,500 psf for support of the proposed residence and ADU/pole barn. The clayey portion of the soils tends 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. 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. 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 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 soil as backfill. Floor Slabs: The natural on-site soils, exclusive of topsoil, are suitable 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 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 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 also create a 3 perched condition. We recommend below -grade construction, such as retaining walls, deep crawlspace and basement areas, be protected from wetting and hydrostatic pressure buildup by an underdrain system. The drains should consist of rigid 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 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 11/2 feet deep. Surface Drainage: The following drainage precautions should be observed during construction and maintained at all times after the residence and ADU/pole barn have 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. 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 12 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. 4) 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 site, 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 Kumar & §-.'-.' 20-7-t.59 -4 - 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 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 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, Daniel E. Hardin, P DEH/kac Kuno? & Asscf i3!es, Inc- i'rul4L3 .u.