Foundation Specifications 0154

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Slurry Displacement Piles

of the effect of filter cake on the ability of the pile to transfer load through skin friction has

not been completed, current Office of Structure Construction policy defines excessively thick

filter cake as a filter cake that has formed in a drilled hole where mineral slurry has been

continuously agitated or recirculated in excess of 24 hours or a filter cake that has formed

in a drilled hole where mineral slurry has been unagitated in excess of 4 hours. Due to the

fact that each site is different, some engineering judgement should be applied before

implementing this policy. There are other indicators that can be used to assist the Engineer

in making a judgement on the amount of filter cake present on the sides and bottom of the

drilled hole. One indicator is the level of mineral slurry in the drilled hole. If the mineral

slurry level is difficult to maintain at the required level in the drilled hole, this is an

indicator that the mineral slurry is continuously being driven into the ground formation

through the sides of the drilled hole. This means that filter cake build-up is continuing and

it is likely that the thickness of the filter cake is excessive. However, if the mineral slurry

level is stable in the drilled hole, this is an indicator that the mineral slurry has clogged up

the ground formation on the sides of the drilled hole. This means that the filter cake build-

up would have ceased and it is likely that the thickness of the filter cake is not excessive.

Removal of excessively thick filter cake is accomplished by slightly overboring the full

length of the drilled hole.

The contract specifications require that mineral slurries be mixed and fully hydrated in

mixing tanks prior to placement in the drilled hole. Mixing and hydration of mineral

slurries usually requires several hours. One way to determine that the mineral slurry is

thoroughly hydrated is to take Marsh funnel viscosity tests at different time intervals. In

general, mineral slurries will achieve their highest viscosity value when they have fully

hydrated. Once the viscosity test values have stabilized at their highest level, the mineral

slurry can be assumed to be fully mixed and fully hydrated.

The physical properties of the mineral slurry should be carefully monitored while the

mineral slurry is in the drilled hole. The mineral slurry’s density, sand content, and viscosity

should be tested and the values maintained within the limits stated in the contract specifi-

cations to prevent excessive suspended materials and to keep the filter cake thickness on the

sides of the drilled hole to a minimum. The mineral slurry’s pH should be tested and

maintained within the limits stated in the contract specifications to prevent flocculation or

destabilization. It should be noted by the Engineer that it will usually take the Contractor

some time to get the mineral slurry’s properties within the limits stated in the contract

specifications. The important factor is to verify that the mineral slurry’s properties are

within the limits stated in the contract specifications prior to concrete placement.