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翻译一篇文章The subsurface conditions for a proposed six-story $94M medical facility in
Knoxville, Tennessee, consisted of a thick clay layer overlying an irregular weathered
limestone bedrock surface. Because the design loads were significant and because of
the sinkhole risk associated with the underlying limestone bedrock, the initial design
considered the use of drilled shafts bearing on rock. While drilled shafts represented
a robust technical solution, construction of drilled shafts in karst can be expensive
and time-consuming. In an effort to save both time and money, the authors and other
members of the project design team developed a combination ground improvement
program to allow support of the medical facility on conventional shallow foundations.
The combination ground improvement program consisted of (1) cap grouting the rock
surface to significantly reduce the sinkhole risk, and (2) construction of Geopier®
elements to reinforce and stiffen the soils immediately below the planned shallow
foundations. For development and refinement of the combination ground
improvement program, extensive subsurface characterization was performed using
cone penetration testing (CPT) at every column location. The CPT results were used
to develop three dimensional models of the subsurface conditions including pertinent
karst features. The cost savings provided by the combination ground improvement
program is estimated to be in excess of $1M.
Rock-bearing drilled shafts have historically been the foundation of choice for
major buildings and other structures constructed in the limestone and dolostone
geology of East Tennessee. In the region, drilled shafts are typically advanced to the
top of rock with a soil auger and fully cased. Rock augers, core barrels, and other
tools are used to advance the shaft excavation into rock. Until very recently, the
accepted engineering observation practice has been to lower an engineer into the
excavation to observe the prepared rock surface prior to placement of the reinforcing
steel and concrete. Due to safety concerns, alternative methods, such as downhole
video, have been employed to confirm the rock subgrade conditions. Albeit
expensive, drilled shafts represent a robust foundation choice for overcoming
challenging karst conditions.