Saturday, November 17, 2012

Cased Shafts, Drilling Fluid Shafts, Pressure-Grouted Shafts.

Cased shafts.  A cased shaft is made by inserting a shell or casing into almost any type of bored hole that requires stabilization before placing concrete.  Boreholes are caused where  soil is weak and loose, and loss of ground into the excavation is significant.  The bottom of the casing should be pushed several inches into an impervious stratum to seal the hole and allow removal of the drilling fluid prior to completion of the excavation and concrete placement.  If an impervious stratum does not exist to push the casing into, the concrete can be placed by tremie to displace the drilling fluid.

Drilling fluid shafts.  Shafts can be installed in wet sands  using drilling fluid, with or without casing.  This procedure  of installing drilled shafts can be used as an alternative  to the uncased and cased shafts discussed

Pressure-grouted shafts.  A special type of nondisplacement deep foundation is the uncased auger-placed
grout  shaft.  This shaft is constructed by advancing  a continuous-flight, hollow-stem auger to the required depth and filling the hole bored by the concrete grout under pressure as the auger is withdrawn.  Careful inspection is required during installation,  and shaft continuity should be verified by  a combination  of load tests and nondestructive testing as described later.

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