Foundation Specifications 0141

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Slurry Displacement Piles

CALTRANS FOUNDATION MANUAL

also take place. Filtration results in the formation of a filter cake along the sides and

bottom of the drilled hole. Figures 9-1(b) and 9-1(c) show the process of filtration and the

cleaning of the soil cuttings from the drilling slurry. If the drilling slurry is cleaned such

that it’s physical properties are within the specified limits for the particular type of drilling

slurry, the bottom of the drilled hole is cleaned of any settled materials using a cleanout

bucket (Figure 9-1(d)). Since the action of the cleanout bucket may cause soil cuttings to

recontaminate the drilling slurry, cleaning the bottom of the drilled hole and the drilling

slurry may take several iterations. Additional cleanings of settled materials from the bottom

of the drilled hole may be performed with a cleanout bucket, pumps, or an airlift. After the

final cleaning has been accomplished and prior to concrete placement, the drilling slurry is

retested to make sure its properties are within the specified limits. Once the drilling slurry is

ready, the rebar cage may be placed. Concrete is then placed, either by a rigid tremie tube or

by a rigid pump tube delivery system. Concrete is placed through the tube(s), starting at the

bottom of the drilled hole (Figure 9-1(e)). The tip of the rigid delivery tube is maintained

at least 10 feet below the rising head of concrete. As concrete is placed, the displaced drilling

slurry is pumped away from the hole and prepared for reuse or disposal. Concrete place-

ment continues until the head of concrete rises to the top of the pile and is then wasted

until all traces of settled material or drilling slurry contamination in the concrete are no

Principles of Slurry Usage

One of the ways drilling slurries function is by what is known as the “positive effective

stress” principle (Figure 9-2). Essentially, this means that the drilling slurry produces stress

on the sides of the drilled hole due to fluid pressure applied by differential head. This

induced stress is produced by maintaining the level of the drilling slurry as high as possible

(usually at least 5 feet) above the groundwater level in the drilled hole. In cases where the

groundwater level is very close to the ground surface, use of a surface casing may be

necessary to ensure positive effective stress is developed on the sides of the drilled hole.

Another way drilling slurries function is by the “filtration” principle. When drilling slurry is

applying fluid pressure to the sides of the drilled hole, some of the drilling slurry and soil

cuttings bonded to the drilling slurry may be forced into the ground formation. When this

material enters the formation, particles of the drilling slurry may be trapped or “filtered” by

the individual soil grains of the formation. This results in the development of filter cakes,

referred to as “mudcakes” if a mineral slurry is used, or “gelcakes” if a polymer slurry is