WHY CASING FLOATS IN GROUT
Grout backfill has a higher density than water-filled casing. During installation, when the grout is still a fluid, it exerts an uplift force on the bottom cap of the casing. This uplift force is greater than the down force exerted by water-filled casing. The net uplift can be calculated as the density of grout minus the density of water filled casing x depth of casing x area of bottom cap. Lateral forces do not contribute to uplift.
The density of water-filled casing is about 62.4 pounds per cubic foot or 1000 kg per cubic meter. The density of ABS casing and the density of water are nearly the same. The density of bentonite-cement grout is about 80 pounds per cubic foot or 1280 kg per cubic meter. The area of the bottom cap of 2.75" (70mm) inclinometer casing is 0.0412 square feet or 3.8485E-3 square meters. Thus if the bottom cap is 100 feet deep, the uplift pressure on the 2.75 inch bottom cap will be about 72.5 pounds or 32.3 kg
THE WORST WAY TO COUNTER BUOYANCY
Unfortunately, the easiest way to keep the casing in place - holding the casing down from the top - is also the worst way. The uplift force acts on the bottom of the casing, so if the casing is held in place from the top, the casing goes into compression. When the casing goes into compression, it tends to snake from side to side in the borehole. This problem is particularly severe in large diameter boreholes and in deep installations, where the uplift force is largest and where portions of the borehole may be enlarged. Snaked casing increases the potential for:
- Kinked or Separated Casing: The combination of compressive force and eccentric loading (due to snaking in enlarged diameter boreholes) can produce strong bending moments in the casing. In some severe cases, this bending moment has caused snap-together joints to fail where the glue-and-rivet joints do not.
- Depth Errors: In snaked casing, slight changes or errors in the positioning of the probe will produce reading errors. The larger the curvature, the larger the error. For example, if the change of inclination between adjacent reading increments is two degrees, and the probe is positioned 25 mm from the correct depth, the resulting error in displacement would be 1 mm.
BETTER WAYS TO COUNTER BUOYANCY
- Suspend a steel pipe or drill rods inside the casing. For the best results, suspend the pipe just an inch or so off the bottom cap of the casing.This ensures that the steel pipe remains straight and avoids resting the full weight of the pipe on the bottom cap. As the casing rises to meet the pipe, the down force of the pipe is activated to keep the casing in place. The main drawback to this method is that you must use the drill rig to suspend the pipe or you must return with the drill rig to retrieve the pipe. A variation is to rest the pipe on the bottom cap, which you must reinforce.
- Pre-install an anchor at the bottom of the casing. Simple prong anchors or packer types have been used. Slope Indicator has produced several prototypes of such anchors. Different soils may require different types of anchor.
Pre-attach a weight to the bottom of the casing. This method requires a weight, a safety line to prevent casing from sinking, a borehole drilled deeper to accommodate the weight, and calculation of the uplift force. It is best used in shallow boreholes.
- Grout the borehole in stages. The uplift force of grout varies with the height of the grout column. If the column is short, the uplift force is low and the casing can be held in place by its own weight or with very little down-force applied from the top. When the grout sets, the bottom cap is isolated from the column of grout and there is no surface for the uplift force to act on. No more than two or three meters to 3 m need to be grouted in the first stage. If the normal bentonite/cement grout is used it needs to set for at least 12 hours before second-stage grouting. Avoid use of a quick-set grout, since the heat of hydration could melt and deform the plastic. The two stages can be placed via an outside tremie pipe. Alternatively the first stage can be placed before lowering the casing, provided that all is done efficiently so that there is no chance of the grout setting prematurely. If grout valve method is used, first stage grouting can be done through the valve, and then the valve is abandoned. An outside tremie pipe for the second stage is lowered with the casing, with its bottom at the level planned for the top of the first stage. After first stage grouting, the excess is flushed out via the tremie pipe, and then this is raised until the first stage has set.
- Fill the casing with drilling fluid that is heavier than the grout. This is a sure and easy method, but requires disposal of the drilling fluid.