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Acceptable methods for overcoming buoyancy

Insert steel pipe inside the casing until the grout has set

This can be done when an external tremie pipe is used for grouting . Flushing must be carried out to ensure there is no grout inside.

This can also be done in conjunction with the gasket-type, one-way grout valve. In this case, the grout pipe is temporarily raised offthe gasket, the top ofthe casing temporarily held down and the casing flushed clean with water (with this method some grout will inevitably be released into the casing when the grout rods are pulled, hence flushing is essential). The grout pipe is then lowered to rest on the bottom cap to act as a counterweight.

The quick-connect type grout valve is more time consuming and risky to use, because the grout pipe and associated female part ofthe quick-connect must be removed from the casing (again temporarily holding the top ofthe casing down), and replaced with a pipe for weighting purposes, which may trip the check valve at the bottom. Lowering steel pipe into the casing without reinforcing the bottom cap is risky, as the plastic bottom cap is usually only 3mm thick and can easily break or be punched through by the pipe.

The main disadvantage ofthis method is that ifthe weight ofthe pipe is greater than that which can easily be lifted by two people, the drill rig needs to return to the location to pull it out, at least 12 hours after grouting.

Insert PVC pipe inside the casing until the grout has set

This is essentially the same as the first method, when an external tremie pipe is used for grouting , except that an adequate weight must be placed on the top of the PVC pipe until the grout has set.

If thick-wall PVC pipe with threaded couplings is selected, it can be reused. If thinner-wall PVC pipe is used, with cemented couplings (threaded couplings should not be used on the thinner-wall pipe, as this weakens the pipe too much), it can be reused by sawing off the couplings.

The pipe should be as large a diameter as possible, remembering ofcourse that it must fit inside the inclinometer casing, and that threaded couplings have a large outside diameter. The drill head can be used for weighting.

Alternatively, arrangements can be made to hold down the top of the PVC pipe by tying it with rope to weights, such as drill casing, on the ground at either side. The buoyancy force should be calculated first (see the fourth method).

Holes can be drilled near the bottom of the PVC pipe to allow for flushing without lifting the pipe offthe bottom cap.

Install a special anchor at the bottom of the casing

Simple prong anchors or packer types have been used. The main disadvantage is that no standard parts are available for this purpose, although some manufacturers will supply them to special order. Some users have made anchors to suit their situations. The design ofthe anchor must depend, ofcourse, on the properties ofthe material in which the casing is to be anchored.

Attach a weight to the bottom of the casing

If this method is used, the density of the grout must be predetermined, so that the necessary weight can be calculated.

Remember that the calculation must recognise that the weight is submerged in grout. There needs to be an adequate and special method for attaching the weight to the casing, which should ofcourse be tested ahead of time.

If connected to the bottom cap, this needs to be reinforced.

Use of this method is limited to relatively shallow installations, because of the magnitude of the weight.

The borehole will need to be drilled deep enough to accommodate the weight.

A disposable tag line, for example a 6mm manila rope, is usually required to lower the assembly.

Two - stage grouting

The first stage isolates the bottom cap from the potential buoyancy force. Note that the benefit of this method is hydraulic isolation, not the restraint against uplift caused by bonding between grout and the outside ofthe casing.

When planning for this method it is important to make sure that the sequence is compatible with the sequence ofwithdrawing any drill casing. A quick-set grout should not be used for the first stage because the heat ofhydration could melt and deform the plastic. No more than 2m to 3m needs to be grouted in the first stage. Ifnormal bentonite/cement grout is used it needs to set for at least 12 hours before second-stage grouting.

The two stages can be placed via an outside tremie pipe. Alternatively, the first stage can be placed before lowering the casing, provided that it is done efficiently so that there is no chance of the grout setting prematurely.

If either 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 ofthe 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.

Installations with telescopic couplings

When using telescopic couplings it is important to minimise compression and/or tension in the inclinometer casing string. For installations less than about 30m deep, the fourth of the above methods is preferable.

For deeper installations, the first or second methods should be used, using an external tremie pipe for grouting.

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