Drilled shafts are cast-in-place concrete, deep foundation elements that require high levels of quality control to ensure the borehole does not become unstable either during excavation or during concreting. Bentonite slurry is a popular choice among state DOT officials nationwide to maintain borehole stability as it has a long history with reasonable load carrying performance. However, specifications developed to replicate successful shaft construction are largely based on empirical data. Further, as slurry construction is a blind process, the final as-built shaft is rarely visually inspected and much of the perceived concrete flow and slurry interaction with rebar and the soil interface are largely unverified. This thesis presents the wide range of nationwide specifications for slurry viscosities (upper and lower) and notes that in only one case out of a hundred (50 states with an upper and lower viscosity limit) is there a rational basis for setting the limit. To this end, the objective of this thesis was to provide compelling evidence to support or dispute present upper viscosity limits. The study was part of a larger scope to show the effects of high viscosity slurry on concrete / soil interface and rebar bond. However, this thesis addresses only the latter via large scale testing to show concrete flow patterns, the build-up of bentonite slurry on rebar, and the degradation of rebar pull-out capacity as a function of bentonite slurry viscosity. Pull-out test results from 126 specimens, comprised of No. 8 rebar embedded in 42in diameter shafts, showed that rebar bond degraded as much as 70%#37; and more when in the presence of bentonite slurry that conformed to most state viscosity specifications (40 to 90 sec/qt). Visual inspection which is rarely possible on drilled shafts showed convincingly that the concrete that flowed through the cage to form the cover concrete does not fully encapsulate the rebar. In most cases a void/crease was formed reflecting the cage grid and which would provide a pathway from the soil pore water directly to the reinforcing steel. While present specifications nationwide dictate bentonite slurry ranges from a minimum of 28 to a maximum of 60 sec/qt, the study findings indicate that only viscosity levels of 30 sec/qt and below are reasonable from both a bond and durability stand point. As pure water has a viscosity of 26 sec/qt, this leaves only a very slight window of acceptability which is unlikely to provide sufficient lateral borehole stability.

In recent years drilled shafts have become the preferred foundation method for marine bridges. Typically, the drilled shaft is selected over traditional driven piles due to soil strata encountered, construction economy, increased lateral stiffness requirements, and/or vibration control considerations. The most critical component in drilled shaft construction is borehole stabilization. Wall sloughing or groundwater inclusion can have devastating effects on the strength of the finished shaft however recent research has shown that the materials, more specifically the slurry, used to accomplish stabilization may be having a negative impact on the durability of the finished product. This thesis investigates the durability of drilled shaft specimens as it relates to the slurry type and viscosity. Electrochemical corrosion potential test results from 23 lab cast specimens showed that the shafts cast using bentonite slurry were 54% more likely to exhibit corrosion potential crossing the ASTM threshold of -350mV. The laboratory setting allowed for visual inspection of each shaft. This inspection showed reflective quilting on all bentonite cast shafts, this quilting was visible to a lesser degree on select polymer cast shafts and not present on shafts cast in water. This creasing appears to be directly related to the slurry used and the resulting decrease in durability. While current construction practice favors the use of bentonite slurry, the study indicates that both polymer slurry and the casing method are more advantageous from a durability standpoint.

Drilled shaft construction often requires the use of drill slurry to maintain borehole stability during excavation and concreting. While drill slurry may be composed of fluids ranging from air to petroleum, drilled shaft construction typically makes use of water based drilling fluids. Although clean water may be utilized as a drilling fluid, a premixed slurry consisting of water, minerals, and/or polymers is more commonly used. Florida Department of Transportation (FDOT) specifications require the use of mineral slurry for all primary structures. The slurry resists the intrusion of groundwater, slows the outward migration of drilling fluid from the excavation, and aids in the removal of suspended soil cuttings. The mechanisms by which mineral slurries work are quite different from those of polymer slurries. Due to these differences, it is unclear whether a mineral based slurry, which has been fortified with polymers by manufacturers or enhanced through the addition of polymers in the field, behaves more like a mineral slurry rather than polymer slurry. This thesis provides an overview of the methods used to measure physical slurry parameters of interest. These parameters include density, viscosity, pH, sand content, and filtration control. Methods employed to describe the slurry parameters include tools and instrumentation commonly used in both field and laboratory settings.

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.