Additional considerations

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It is usually the case that our testing is one part only of the operations being carried out in a borehole, and we are operating as specialist sub-contractors to the Main Contractor. This page is concerned with making clear the separation between what we supply and what we need.

Self boring

This comes in three varieties:

A stand-alone drilling system requiring no additional equipment

There are not many circumstances where this is possible but it does happen. Usually it will be a green field site. The system consists of hydraulic rams to jack the probe, a small motor to rotate the inner drill string, and a water pump to provide circulating fluid. A portable hydraulic power pack and control panel distributes power to the various units.

One difficulty is that kentledge for the hydraulic rams is limited, so in practice suitable material will be of low to medium strength only. The SBP must drill every metre of the borehole so additional testing is not an option. An ample water source is required. Figure 1 shows a self boring, stand-alone system.

a self boring , stand-alone drilling system
Fig.1 Self boring, stand-alone system (click for full-size)

A stand-alone drilling system operating underneath a cable percussion rig

This is a common way of working, using all the special self boring drilling parts described above but working in conjunction with a cable percussion rig (shown in figure 2 below). The rig places a column of water well casing to a depth just above where the first test is required, hammering it in the last 0.5 metre. The SBP system couples to the top of the casing column and the skin friction on the casing is enough to allow self boring into most materials. An ample water source is required, not normally part of a cable percussion operation.

If the test spacing is more than 2 metres then the operation is usually 'one test and out'. The rig open-holes to the next test depth, carrying out additional testing if required.

There are some locations that only a reduced height cable percussion rig can access, so the combined system is versatile.

If the hole is left open for a long time then the tested zones begin to collapse so a reasonably quick operation is important.

A cable percussion drilling system
Fig.2 Cable percussion system (click for full-size)

A system for operating under a rotary rig

In this method we supply the pressuremeter, a special drill string and a purpose-built adapter for the rotary drill head. The probe is drilled as if it were a core barrel, but the adapter has a thrust bearing to separate down-thrust from rotation. Everything above the adapter spins, everything below is static and the probe enters the ground without being rotated.

Water needs to be supplied at appropriate flows. This means that the rig pump and water swivel must be in good order, because the SBP water path is a narrow annulus compared to normal drill rod. Air mist can be used but is more difficult and only suitable for relatively shallow holes.

This system allows core to be taken in the test intervals. In material with a tendency to collapse or in boreholes deeper than 40 metres it is the only appropriate option. Figure 3 shows the rotary rig system.

A rotary rig drilling system
Fig.3 Rotary rig system (click for full-size)

Pre-boring

For pre-boring the problems of getting the probe into the ground are the responsibility of the drilling contractor. The additional issues to be considered are these:

Size of borehole

It sometimes happens that the same size borehole is cored from surface to some considerable depth, and the High Pressure Dilatometer (HPD) must test layers in this borehole. Because the probe is a close fit to the nominal core size this can be risky. Any material falling down onto the probe can make it difficult to recover the device.

Wireline coring

We are often asked to consider adapting the equipment to work with a wireline coring system. The fit of the probe to the cavity has to be reasonably close for a successful test. It is not practical to test the cavity made by a wireline system with a probe small enough to pass through the wireline core bit. There are wireline systems able to core at two diameters but it is not advisable to use the wireline cable for lowering the pressuremeter. If the probe becomes trapped the wire cable will not be able to exert more than a nominal force to help pull it back. We therefore recommend lowering the probe on rods. These rods must has a diameter no greater than the diameter of the borehole less two times the diameter of the umbilical connecting the probe to the pressure source on the surface. This umbilical must be taped at intervals to the rod to prevent loops occurring.

Inflation method

The HPD can be inflated with oil or air. The decision about what method to use depends on circumstances. The best test is obtained with oil because it allows pressure to change without large temperature alteration. In good rock where certainty over tiny displacements is important this is an issue, especially where surface temperature is considerably different from the downhole state. However oil raises environmental issues. We use bio-degradeable transformer oil to minimise the risk.

Oil also gives a slower overall test, as time has to be allowed for oil to return to the surface. There are ways of speeding up the process but it means adding an additional umbilical to the system, making the lowering and raising procedure more complex and time consuming.

For speed and convenience air inflation is used in most circumstances. However oil is always used when calibrating the pressure capability of the probe on the surface because it is inherently safe in the event of a failure of any part.

Speeding up testing

The easiest way to accelerate the test rate is to reduce the number of lowering and raising events. We sometimes test a borehole that has been completely cored prior to our arrival. In such circumstances the probe is lowered to the deepest location first, then tests are carried out in reverse order to depth. Normally the deepest part will be the tightest fit because the core barrel has made the fewest passes.

Material with cavities

Limestone in particular can be prone to solution cavities. Testing in this material is frustrating because if the HPD membrane is not completely supported at all places then it will burst at pressures too low to give useful data. Where such testing is required we advise that the boreholes be cored in advance of our arrival. They should then be grouted up. Once we are on site the grouted holes can be re-cored, with the grout core available for inspection to prove the integrity of the cavity wall. The grout will be weak compared to the limestone so no reinforcement takes place.

Pre-boring with the 47mm RPM

The difficult with this device is that the diameter is smaller than the customary drill parts a drilling contractor can be expected to keep. The holes for the RPM need to be formed with drill bits and drill rods based on the AW size. Typically the RPM is used to target certain layers at significant depth, and in these circumstances there is no alternative but to make a large diameter borehole first, then drill a 51mm or similar diameter pocket out of the base of the larger hole. Provided the pocket for the RPM is not too long (no more than six metres) then we supply the necessary rods to take the probe and umbilical from its small hole into the larger hole. At this point we expect to couple to whatever drill rods are available via a suitable adapter.

For very shallow tests, within 5 metres of the surface, we can sometimes make the borehole ourselves using a powered hand auger.

Successful tests can also be made using the RPM to ream out an existing cone penetrometer (CPT) hole. This technique has been applied with some success in weak chalk, taking advantage of a hole made by a 102cm CPT.

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