Deep boreholes are more than 50 meters (150 ft) deep. We cannot provide exact instructions, since there are different variables at each site: different geology, different drill equipment, and different expertise of the installers. However, we can suggest some guidelines:
Be Prepared
The engineer at the job site should be aware that deep installations require extra calculations and planning. For example: the engineer must calculate all the pressures and forces that will act against a successful installation and plan for stage grouting.
Type of Casing
When possible, use casing with glue-and-rivet couplings. Slope Indicator’s Standard casing is the best for this application because it has flush joints and is easy to seal. Slope Indicator’s QC casing, with its snap-together joints, can also be used, but it does not have the torsion strength of Standard casing. The torsion strength of the QC joints can be improved with straps, as shown in the photo above. Alternatively, pop rivets in the joint can add torsion strength, but the rivets must be sealed carefully to prevent entry of grout. Note that experienced installers have used un-reinforced QC casing sucessfully to depths of 230 m (700 ft). However, we recommend the reinforcement for most installers.
Grouting
A bentonite-cement grout is the preferred material for backfilling boreholes of any depth. It is particularly important for deep boreholes, since they may otherwise provide an unwanted connection of aquifers at different elevations. Grouting is done from the bottom to the top of the casing. In deep holes, the pressure of the grout can cause even water-filled casing to collapse.
Stage grouting is necessary to limit these pressures. The first stage is normally limited to about 30m or 100 feet. A tremie pipe – or plastic tubing – should be installed with the casing. In some cases, the first stage can be grouted via a grout valve installed in the bottom of the casing. In this case, the grout pipe is lowered through the casing to mate with the valve at the bottom. For deep depths, however, an external pipe is best. It can be left in place or withdrawn.
Before the grouting operation starts, it is a good idea to check the casing with a dummy probe to make sure that none of the joints have failed or pulled apart during installation. If everything is OK, proceed with the grouting.
Use a bentonite-cement grout mix. Adjust the strength of the grout by mixing cement and water first. This allows you to control the strength of the grout. Bentonite is then added as required. Always collect a sample of the grout in a paper cup. You can check it in 24 hours and also do lab tests, etc.
Other Standard Practices
Establish the proper alignment of the casing grooves. One set of grooves should be parallel with the expected direction of movement. (downhill or toward the excavation, in most cases).
Maintain this alignment, checking it each time another length of casing is added. Also check that the joint is made properly and not forced together incorrectly. The company name & casing size are printed along the length of the casing over one of the groves. So, by watching the printing, the position of the internal groves is known.
Avoid realigning the casing orientation after the casing has been placed in the borehole. With deep holes, twisting at the top is unlikely to affect the bottom, so the casing will become spiraled rather than realigned.
In water-filled holes, it will be necessary to fill the casing with water to allow it to be pushed downhole. In dry holes, avoid filling with water because the pressure can blow out the joints from inside. In holes that are partially filled with water, start adding water to the casing when the bottom of the casing reaches the water.