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Home My WebLink AboutObservation of Excavation 05.21.14, ~tech HEPWORTH-PAWLAI< GEOTECHNICAL May 21, 2014 Giard Homes Attn: Roger Giard 1431 Airport Road Rifle, Colorado 81650 ruia1 d a rofnet I kp11 •rth l' n1I1!, <.. ,,,,1., l.111, ii !11, ~l,~l' l ,,, 111• It •• 1.I Ii~ l rl, 1111 1 I"-Ill'.!• l (.,,. .~ • ·''"''! ''"'lh.' "il1 ·1~ 7'''' ~I\ 1;\1 q, ,J';J Ill oi (•J' .• if q ,l I , Ii I I Job No. 114 013A Subject: Observation of Excavation, Proposed Detached Garage, Anderson Residence, 1876 County Road 236, West of Rifle, Colorado Dear Roger: As requested, a representative of Hepworth-Pawlak Geotechnical observed the excavation at the subject site on May 6, 7 and 19, 2014 to evaluate the soils exposed for foundation support. The findings of our observations and recommendations for the foundation design are presented in this report. We previously conducted a subsoil study for design of foundations at the site and presented our findings in a report dated January 31, 2014, Job No. 114 013A. The proposed construction is similar to that discussed in our previous report and will consist of a detached garage located to the east of the existing residence. The garage will be a two story wood frame structure construction with a slab-on-grade ground floor. Spread footings bearing on the natural soils designed for an allowable bearing pressure of 1,200 psf were recommended for foundation support with a risk of settlement. At the time of our May 6 site visit, the foundation excavation had been cut in one level from approximately 2 to 4 feet below the adjacent ground surface. The soils exposed in the bottom of the excavation consisted of medium dense silty clayey sand. An area approximately 15 feet in length in the middle portion of the south wan line footing trench had existing fill soils and asphalt pieces exposed at subgradc. The results of swell- consolidation testing performed on two of the natural soils obtained from the bottom of the excavation, shown on Figure I, indicate the soils have low compressibility at existing moisture contents and low compressibility when wetted under constant light load. The samples showed moderate compressibility when loaded after wetting. No free water was encountered in the excavation and the soils were slightly moist to moist. J Giard Homes May 21, 2014 Page2 As requested, we met with you et the site on May 7, 2014 to discuss the existing fill conditions observed in the excavation. We recommended removal of the existing fill to expose the natural on-site soils and extending the foundation ievel bearing down to the natural soils or replacement of the over-excavated areas with properly placed and compacted granular structural fill. We discussed a minimum 2 feet of select granular structural fi.11 below the footings would reduce the settlement risk. We were contacted on May 19 and informed that the existing fill and asphalt pieces had· been removed from the footing (and floor slab) area and the excavation over-excavation was about 2 to 5 feet below proposed footing grade, and that the over-excavated areas had design bearing elevation re-established with compacted COOT Class 2 granular material. The existing fill in the garage slab area was also reportedly removed and replaced with similar compacted granular material. Photos were provided to us that indicated that free water was present in portions of the excavation during the over~excavation process. The water, which apparently reportedly originated from perched areas under asphalt pieces in the garage slab, area had dissipated prior to fill placement. It was requested that we visit the site to observe the completed excavation and backfill. On May 19, 2014 we observed that Class 2 type granular backfill material (silty sandy gravel) had been placed in the footing areas . The top of fill in the footing areas was approximately 1 to 4 feet below the existing site grade. The foundation areas were probed with a metal hand probe and were firm to hard in consistency. The fill appeared well compacted. The natural site soils exposed in the excavation are consistent with those previously encountered on the site and suitable for support of spread footings designed for the recommended allowable bearing pressure of 1,200 psf. The COOT Class 2 ~low the footings will act to reduce the risk of foundation settlement which is primarily if the bearing soils become wetted. Prior to the footing construction, any loose disturbed soils in the footing areas should be removed or re-compacted. Any water seepage or run-off into the excavation should be collected and diverted away ftom the footing areas and any softened soils removed before concrete placement. Other recommendations presented in · our previous report which are applicable should also be observed. The recommendations submitted in this letter are based on our observation of the soils exposed within the foundation excavation , construction methods reported by the Job No. 114 Ol3A , Giard Homes May 21, 2014 Page 3 contractor 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, HEPWORTH -PAWLAK GEOTECHNICAL, fNC. James A. Parker, P.E ., P.G. Rev . by: DAY JAP/ksw attachment Figure I -Swell -Consolidation Test Results Joh No. 114 01 3A ~tech , Moisture Content = 12.5 percent Dry Density = 120 pcf Sample of: Silty Clayey Sand From: Bottom of East Side of Excavation 0 -------.... \ 1 1\'" ' -~ \ ~ ~ -z 2 ~ 0 '"'-...... ....... Compression Ci) --en ~ upon w 3 ) wetting cc Q_ :::? 0 u 4 5 0.1 1.0 10 100 APPLIED PRESSURE ( ksf) Moisture Content = 12.3 percent Dry Density = 117 pct Sample of: Silty Clayey Sand From: Bottom of South Side of Excavation 0 -----:--;--. I"'--. 1 -... -"" \ 'ii. -z 2 0 ...... ~ ~ Compression en l--upon en ,_ w 3 ).... wetting a: a.. ' :? ~) 0 (.) 4 5 0 .1 1.0 10 100 APPLIED PRESSURE ( ksf) 114 013A ~ech SWELL-CONSOLIDATION TEST RESULTS FIGURE 1 HEPWORTKPAWLAK GEOTECHNICAL