Defining the Upper Viscosity Limit for Mineral Slurries Used in Drilled Shaft Construction
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Defining the Upper Viscosity Limit for Mineral Slurries Used in Drilled Shaft Construction

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      Final Report
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      Drilled shaft construction often requires the use of drill slurry to maintain borehole stability during excavation and concreting. Florida Department of Transportation (FDOT) specifications require that the mineral slurry used for all primary structures must stay within viscosity limits of 30 and 50 sec/qt. This study addressed the rationale for defining the uppermost limit. The lower limit was previously investigated. Two types of tests were undertaken to assess the effect of the upper viscosity limit on shaft performance. These tests, included: rebar pullout tests and side shear tests where the presence of slurry may impede the structural and geotechnical capacities, respectively. The results of these tests concluded that the presence of any bentonite slurry at the time of concreting reduces the rebar bond. Reductions ranged from 25 to 70% for slurry viscosity of 30 to 90 sec/qt, respectively. Similar tests with polymer slurry showed smaller reductions. The effect on side shear was evaluated at both model and full scales. Model tests conducted over a wide range of viscosity showed only modest reductions in side shear for shafts cast with 50 sec/qt slurry when compared to shafts cast in 40 sec/qt slurry. However, shafts cast with 90 sec/qt slurry showed a marked increase in side shear. Full-scale tests were conducted with both mineral and polymer slurry where four different slurries were used (average viscosities were: 40 sec/qt and 74 sec/qt bentonite as well as 50 sec/qt and 131 sec/qt polymer). Those tests showed use of higher viscosity bentonite or polymer slurry had no adverse effects on side shear (again compared to 40 sec/qt). In fact, increases in the unit side shear resistance of 19%, 12%, and 13% were recorded for 74 sec/qt bentonite, 50 sec/qt and 131 sec/qt polymer, respectively. Finally, physical inspection of shafts with congested cages (6 in. clear space) cast in bentonite slurry indicated that there may exist potential durability issues as pathways/creases of trapped bentonite formed at all rebar locations that extended from the rebar out to the soil/concrete interface. This was most pronounced for slurry with viscosity greater than 40 sec/qt.
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