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Effect of Polymer Slurry Stabilization on Drilled Shaft Side Shear Over Time

Slurry supported construction of drilled shafts requires highly engineered fluid made from mineral clay or polymer powder mixed with water. In the state of Florida the present open excavation time limit / specification is directed at the use of mineral slurry that forms a layer of clay along the excavation walls and thereby can affect the as-built concrete / soil interface bond. As use of polymer slurry is relatively new in the state, there is no definitive specification that addresses the slurry exposure time of open excavations supported by polymer slurry. The primary objective of this study was to quantify the effects on side shear (if any) from prolonged open excavation times where polymer slurry is present. To this end, both small and large scale field evaluations of shafts constructed with polymer slurry were performed. Bentonite supported shafts were also cast and tested as means for comparison. Small scale field tests involved casting 32, 1/10th scale shafts constructed with four different slurry products including bentonite and three different commercially available polymer products. Exposure times were varied from 0 to 96 hours. Results of the small scale tests showed a clear reduction in capacity over time for bentonite constructed shafts where up to a 50% reduction was observed after 96 hrs of exposure. but no affects were noted for polymer constructed shafts. On average, polymer shafts exhibited 26% higher side shear than bentonite shafts. Full scale field testing entailed casting 5, 2ft diameters shafts with exposure times of 2 and 48 hours with both bentonite and polymer slurry. In this series of tests no reduction in capacity was noted with prolonged slurry exposure times. However, like the small scale testing, polymer shafts performed better (15%) in side shear resistance than bentonite shafts. Finally, automated slurry testing performed at several sites indicated that polymer slurry viscosity was often lower than manufacturer recommendations throughout the length of the shaft despite meeting the minimum acceptance criteria at the bottom of the shaft where testing is always performed. The results of these tests showed that side wall stability was drastically reduced as the polymer slurry viscosity fell below 40 sec/qt. Where slurry was mixed in the casing, large variations in slurry viscosity was noted. Given these findings three recommendations were made: (1) slurry should be tested at bottom, middle and top of the shaft regardless of shaft length, (2) all slurry should be premixed and meet specifications prior to being introduction, and (3) polymer slurry should always target a minimum viscosity of 50sec/qt. This minimum slurry viscosity would not necessarily be suitable for all soil conditions but falling below it presents opportunity for sidewall collapse.

Language

  • English

Media Info

  • Media Type: Digital/other
  • Edition: Final Report
  • Features: Figures; Photos; References; Tables;
  • Pagination: 204p

Subject/Index Terms

Filing Info

  • Accession Number: 01686013
  • Record Type: Publication
  • Contract Numbers: BDV25 977-19
  • Files: NTL, TRIS, ATRI, STATEDOT
  • Created Date: Nov 20 2018 10:21AM