Inclinometers

The water filled casing is lighter than grout backfill, so the uplift force acting on the bottom cap of the casing tends to push the casing out of the borehole.

We refer to this as an orientation error, however, it is probably not a problem at all. In fact, you may find it to be an advantage. At 10 degrees, your A axis readings will capture 98.4% of the magnitude of the total movement, and movement seen in the B axis can be used to confirm that the A movement is real. 

This drawing shows a right triangle with sides A, B, and R, which represent the A-axis value, the B-axis value, and the resultant R, which represents the actual magnitude and direction of movement. The angle is the angle of misalignment (10 degrees in this drawing). The ratio between A and R is equal to the cosine of the angle. The cosine function works slowly, so even at 25 degrees of misalignment, the A-axis value contains 90% of R. The ratio between B and R is equal to the sine of the angle. The sine function works more rapidly. With a 10 degree misalignment of the A-axis, as shown in the drawing, the B-axis value will contain about 17% of the resultant. Thus any displacements seen in A should also be seen in B

The heavy-gauge PVC pipe could work if it survives the hoisting and insertion process. I don’t think the softer modulus of the PVC is an issue at all. The PVC pipe will be totally encased in the reinforced concrete. Both the PVC pipe and a grouted-in ABS inclinometer pipe inside will move as the wall moves, they will be slaved to the concrete as weak members.

The main issue is how well the PVC is attached to the cage to survive bending during hoisting and the uplift force acting at the bottom cap of the pipe in the fluid concrete. (Please calculate the forces to find out what you are dealing with, tying wire may not be sufficient.)

A steel pipe would be stronger and heavier in this situation and would be preferred by most contractors. Again, I don’t think the lateral stiffness of the steel pipe is an issue in reflecting correct displacements. Should it act as a stiff member, the displacements would be distributed over a slightly greater interval of depth, usually insignificant since the vertical instrument resolution is 0.5 meter.

This is an easier installation if the wall concrete is tremied in after the pipe is inserted with the steel cage and the pipe is filled of bentonite slurry or water.

One technique is to weld a pipe to the sheet pile. Plug the end of the pipe and drive it in with the pile. Drill out the plug, boring 10 or 20 feet into the soil below the bottom of the wall. Then install your inclinometer casing inside the pipe. Grout it in. With the bottom of the casing in stable ground, you will be able to detect movement at the base of the wall in addition to monitoring the profile of the wall (the pipe is unlikely to add significant strength to the wall).

 The hole should be drilled as close to vertical as possible. Verticality is limited by the capabilities of your drill rig, the crew, and the local geology. However, one to two degrees from vertical are typical values used in specifications. Accuracy specifications from Slope Indicator assume that the inclination is less than 3 degrees.

Vertical boreholes eliminate one type of systematic error, called rotation error, that can occur if inclinometer probes are interchanged or if the mechanical alignment of a single probe changes over time AND the borehole has significant tilt.

Yes. Ordinarily, the torque (twist) strength of the QC coupling is satisfactory without any supplemental action. However, if the drillers suspect that they will have problems removing the drill casing or if the borehole is very deep, you could consider placing a steel band around the casing, covering the white button. This low-profile band provides much additional torque strength and is quick and easy to install.

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.

It is not possible to predict the yield of the mix design because we don’t recommend a fixed quantity of bentonite. The quantity of bentonite used in the mix will depend on the pH and mineral content of the water used, the moisture content of the cement and bentonite, and the amount and type of byproduct within the locally sourced bentonite powder.

It is possible to calculate a gross estimate of the yield using 3.15 as the specific gravity of cement and 2.35 as the specific gravity of bentonite, but it must be considered a gross estimate as the expansion rate of the bentonite will not be the 10X volume change when it is mixed with fresh water. It will be a variable due to the redox reaction of the cement and water changing the properties of the water. This also increases the difficulty in calculating the yield ahead of time.

The best method of determining the yield is to mix a test batch using the actual materials that will be used on the project, although it should be noted that there will still be some differences between the test and the production grout, as the moisture content of the cement and bentonite will likely be different on test day versus on production day.

Try this quick method first. It should should work with most systems.

1. Download and install the most recent software: Check our downloads page for DigiPro2.
2. Download and run the USB Driver Installer: CDM212364_Setup.zip.
3. Connect the DataMate II and switch on. Windows should recognize the device.
4. If you have problems, please look at the DataMate Communcations FAQ.

Multi-Part SetupThis multipart setup should not be necessary for most systems. .Part 1: Installing the Digitilt DataMate USB Driver

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