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HomeMy WebLinkAboutObservation of Excavation 07.27.2018Geotechnical Engineering 1 Engineering Geology Materials Testing 1 Environmental 5020 County Road 154 Glenwood Springs, CO 81601 Phone: (970) 945-7988 Fax: (970) 945-8454 Email: hpkglenwood@kumarusa.com Office Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, Summit County, Colorado July 27, 2018 Chris and Marianne Stallings 201 Vista Drive Silt, Colorado 81652 north m endow dri ve @ pia i l.com Project No. 17-7-870.01 Subject: Observation of Excavation, Proposed Residence, Lot 5, Sun Meadows Estates, North Meadow Drive, Garfield County, Colorado Dear Chris and Marianne: As requested, a representative of H-P/Kumar observed the excavation at the subject site on July 17, 2018 to evaluate the soils exposed for foundation support. The findings of our observations and recommendations are presented in this report. The services are supplemental to our agreement for professional services to you dated December 13, 2017. We previously conducted a subsoil study for design of foundations at the site and presented our findings in a report dated January 30, 2018, Project No. 17-7-870. The proposed construction has changed some since that report and now consist of a single story wood frame structure over a basement level with an attached garage at the main level. Ground floors will slab -on -grade. Spread footings bearing on the natural soils sized using an allowable bearing pressure of 1,500 psf have been designed for foundation support of the building with some risk of settlement. At the time of our site visit, the foundation excavation which was essentially complete had been cut in two levels from around 21/2 to 81/2 feet below the adjacent ground surface. There was minor cut in the garage floor slab -on -grade area. The soils exposed in the bottom of the foundation excavation consisted of generally medium dense, intermixed sand and silt with scattered gravel. There were several inches of loose dry soils in the bottom of the excavation over the firmer undisturbed soils. Results of swell -consolidation testing performed on samples taken from the site, shown on Figure 1, indicate the soils are moderately compressible under conditions of loading and wetting and exhibit a moderate collapse potential when wetted under a constant 1,000 psf surcharge. The sample may have been partly disturbed due to the sampling process and rock content. No free water was encountered in the excavation and the soils were slightly moist. The soil conditions exposed in the excavation are consistent with those previously encountered on the site and suitable for support of spread footings designed for the Chris and Marianne Stallings July 30, 2018 Page 2 recommended allowable bearing pressure of 1,500 psf. The risk of settlement is primarily if the bearing soils were to become wetted and precautions should be taken to prevent wetting. Prior to the footing construction, all loose disturbed soils should be removed and the subgrade moistened and compacted. Other recommendations presented in our previous report which are applicable should also be observed. Additionally, a perimeter foundation drain around the basement level should be provided. The drain 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 least 1 foot below lowest adjacent finish grade and sloped at a minimum 1% to a suitable gravity outlet or to a sump with a pump. If PVC drain pipe is used (which we recommend) a pipe slope of 1/2% can be used. The drain gravel backfill should be at least 11 feet deep and be covered by filter fabric. The drain system should include at least 4 inches of drain gravel below the basement floor slab. 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. An impervious membrane, such as 20 or 30 mil PVC should be provided below the drain gravel in a trough shape and attached to the foundation wall with mastic to prevent wetting of the bearing soils. The recommendations submitted in this letter are based on our observation of the soils exposed within the foundation excavation and the previous subsurface exploration at the site. Variations in the subsurface conditions below the excavation could increase the risk of foundation movement. We should be advised of any variations encountered in the excavation conditions for possible changes to recommendations contained in this letter. If you have any questions or need further assistance, please call our office. Sincerely, H -P KU.MAI Robert L. Duran, E. 1. Reviewed by. David A. Young, P. E� ` 6.0.."H�4�� RLD/kac e,C;Vf._t5 attachment Figure 1 - Swell -Consolidation Test Results Project No. 17-7-870.01 EN J 1 cy co oo c\I (%) 113MS — NOI1V0110S1100 a 0) SWELL -CONSOLIDATION TEST RESULTS cr) co r••••••• • Yayla—ivitatf.01,414,Novr,,,, paporm uns '5 0,, •,....Ft', pecnicua 'uoi,onJawip 1 .