Graduation Year

2016

Document Type

Thesis

Degree

M.S.C.E.

Degree Name

MS in Civil Engineering (M.S.C.E.)

Degree Granting Department

Civil and Environmental Engineering

Major Professor

Austin G. Mullins, Ph.D.

Committee Member

Rajan Sen, Ph.D.

Committee Member

Michael Stokes, Ph.D.

Keywords

Deep Foundation, Drilled Shafts, Exposure Time, Mineral Slurry, Polymer Slurry, Side Shear Resistance

Abstract

Drilled shafts are cylindrical deep foundations constructed by casting fluid concrete into an excavated hole. These elements rely largely on side shear resistance generated by their substantial diameters and lengths to effectively transfer loads. Therefore, an exceptional concrete to soil interface is essential for proper performance of these structures. The FDOT preferred stabilization fluid, bentonite slurry, has been proven to degrade this interface with increased exposure time due to filter cake formation. For this reason, slurry exposure time has been limited to 36 hours by FDOT. Alternately, polymer slurries do not form a filter cake but rather stabilize excavations through continuous soil infiltration and the associated cohesion that accompanies the slurry presence. As polymer slurry use is relatively new to the state of Florida, FDOT does not presently have clear specifications regarding polymer exposure time limits. Hence, this thesis presents the results of pullout tests performed on 1/10th scale drilled shafts constructed with both polymer and bentonite slurries and with varying exposure times.

To explore the effect of exposure time on side shear resistance, 24 - 4in diameter 8ft long shafts were constructed: 6 with bentonite and 18 with three different polymer products (6 each). After being exposed to the respective slurry for 0, 1, 2, 4, 8 or 24 hours, each excavated hole was concreted and tested after 7 days of curing. Pullout tests served as a direct measure of side shear resistance due to the absence of base resistance associated with compression testing. After testing, shafts were exhumed and sectioned for filter cake measurement.

As expected, shafts constructed using bentonite slurries immediately exhibited a decrease in capacity with increased exposure time. Between 0 and 24 hours a 34% reduction in capacity was witnessed. These reductions were attributed to filter cake thickness which increased with exposure time along with the reduction in effective shaft. After 4 hours of exposure, side shear resistance for the bentonite shafts approached an asymptote, likely defined by the strength of the filter cake formed.

The side shear resistance of all polymer shafts exceeded that of the 24hr bentonite control shaft by 20-50 percent depending on the polymer product used. For a given polymer product no capacity reduction was noted with respect to time. Inspection of exhumed shafts revealed the formation of a soil cake (region of stabilized soil stuck to the shaft) around the perimeter of the polymer shafts. Therefore, the side shear resistance of the polymer shafts was defined by the soil-to-soil interface surrounding the shaft and not by an intermediate filter cake.

The study resulted in the following findings: (1) open excavations using polymer slurry stabilization are not adversely affected by extended exposure time, (2) the effect of filter cake formation in bentonite supported excavations slows with time whereby no significant degradation in capacity was noted after 8 hours, (3) shafts constructed with polymer slurry performed better than those constructed with bentonite, and (4) as polymer slurry flow into the surrounding soil does not slow with time, more slurry volume is required (compared with bentonite) and slurry level must be continuously maintained.

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