Factors Affecting Anomaly Formation in Drilled Shafts

Abstract

In instances where excavation around drilled shafts has been required (e.g., footings, foundation caps, etc.) imperfections or anomalous conditions have frequently been observed. In many instances this was thought to have been caused by the presence of the water table. This seemingly alarming trend prompted the Florida Department of Transportation to sponsor a research program aimed at revealing the mystery of this phenomenon. Upon reviewing the preliminary research findings this study defined factors that likely affected the occurrence of these conditions. Primary focus was directed at borehole cleanliness, concrete placement techniques, slump, clear spacing of rebar, aggregate size, and placement of concrete under a fluid head (such as drilling slurry). Laboratory testing in the Lateral Pressure Cell was conducted to investigate the relationship between lateral pressure development and slump during pour, coarse aggregate size, clear spacing of rebar, and fluid head in the borehole. The most interesting finding of this series of tests was that the rebar clear spacing to aggregate diameter ratio of 3 to 5, which is most often specified, leads to substantial build-up of material inside the cage before enough pressure is developed to push the shaft mix through the cage to the annular volume outside the cage. Field testing on full scale drilled shafts on numerous sites and over 40 data sets was then conducted in order to corroborate the findings in the lab. Using a down-hole camera and/or weighted tape measurements, head differentials between inner and outer cage material were found to be excessively large even when using common mixes and rebar spacing. This build-up was found to be closely related to the clear spacing of the rebar, aggregate size, and rate of concreting. A second series of laboratory testing in the Frustum Confining Vessel evaluated construction factors affecting finish shaft performance (e.g., casing extraction rate, slump, and slump loss). These tests found that when using the temporary casing method of construction, the unit skin friction developed by the shaft was drastically reduced when the slump of the concrete was allowed to drop below 5 in. prior to pulling the casing. At slumps of 3.5 in. or less, most shafts were damaged during casing extraction, and those that appeared fine developed nearly zero unit skin friction. Finally, a large scale concrete pour simulator was designed and fabricated to investigate the effect of slurry properties and sand content on accumulation (settling soil particles) at the bottom of the excavation and/or a rising concrete-slurry interface. Results showed that almost half of the suspended sand would fall out of suspension within 2 hr. If left undisturbed, the remaining sand stayed in suspension up to 12 hr. (the longest test run).