Construction related difficulties with drilled shaft foundations and recommendations for mitigation.

摘要

In the 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 with little corresponding flow through the cage.; Field testing on full scale drilled shafts was then conducted in order to corroborate the findings in the lab. Using a down-hole camera and 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.; Finally, lab testing in the Frustum Confining Vessel revealed perhaps the most significant finding of this research. Using the temporary casing method of construction, when the slump of the concrete was allowed to drop to 4 inches or less prior to pulling the casing, the unit skin friction developed by the shaft was reduced drastically. At slumps of 3.5 inches or less, most shafts were no longer constructable, and those that were developed nearly zero unit skin friction.