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Inclinometer Casing FAQ


Question: Do you have any instructions for installing horizontal casing? I'm planning to embed the casing in a foundation slab.

Answer: The manual for the horizontal probe provides some instructions for installing horizontal casing in soil. Most of these instructions will apply to your application as well. For example,

  • The traversing probe requires 70 or 85 mm (2.75 or 3.34") casing.
  • One set of grooves must be vertical. This is essential. Check for this during installation.
  • Telescoping sections are not recommended or needed.
  • Casing should be flat and straight. It's ok to build in an inclination for drainage.
  • A pull cable is required to pull the probe to the far end. Install the cable as you assemble the casing.

For your application, also consider this:

  • Support the casing well, at least every 5 feet and under couplings. Tie the casing to the supports so that it does not float upwards when the concrete is poured. Wrap the joints with tape to make sure grout does not enter.
  • Remember that the casing is plastic and weaker than steel. Avoid pouring concrete directly onto the casing.
  • The casing is plastic and will start deforming if temperatures exceed 82° C ( 190° F). If the ambient temperature is above 26° C (80° F), you may need to fill the casing with cool water at the same time as you are pouring concrete. Make sure that the casing is supported by the concrete before filling it. There are other ways to avoid high temperatures as well, such as icing the concrete.

Question: Is sand backfill as good as grout backfill?

Answer: No. Sand backfill has two negatives: it settles, and it washes away.

One west coast department of transportation has studied this. According to them, sand backfill settles for about a month after installation, unless it is well tamped. Unfortunately, it is very difficult to tamp effectively in a small diameter borehole. This settlement means that a good initial reading cannot be obtained for a month after installation. If you obtain the reading earlier, you are likely to detect only movement of the casing within the borehole, rather than movement of the ground.

In ground that has faults sand is sometimes used because grout leaks out the fault. Over time, however, the sand will wash away. Then the casing will move within the borehole and all readings are suspect.

Finally, if there are gaps in backfill (and this goes for grout as well as sand), you'll find that it takes longer for the inclinometer probe to stabilize. Evidently, movement of the probe in the casing can set up some vibrations.


Question: How does inclinometer casing behave in soft soil? Does it deform sufficiently to detect the shear zone or does its stiffness simply distribute the movement?

Answer: Erik Mikkelsen says:

I have heard this concern voiced from time to time, but the evidence from measurements in slopes and landslides suggest that it should not be a big concern, particularly in soft (loose) soil.

In soft soils, shear displacements are more distributed than in harder soils. For example, the shear zone in a basalt landslide complex was measured to be 15 inches thick using special inclinometer procedures. On the south Oregon coast (Arizona Inn landslide) in mélange, the shear zone is on the order of 2 to 3 feet thick after a major rupture and repair work. Measurements at the toe of an embankment in soft silts and clays next to the Puyallup River in Tacoma indicated a shear zone of 4 to 6 feet.  

Based on the experience above, I think that within the measurable range of the inclinometer system, the stiffness of the inclinometer casing will not appreciably redistribute displacements. A significant rupture occurs most likely on a plane or a thinner zone, but that is the beyond the range that the inclinometer can measure. (The casing simply closes and does not allow passage of the probe).

Inclinometer casing made from ABS is relatively weak compared to the ground and is quite ductile. It does not normally crack due to excessive deformation, it cold flows. It would be difficult to come up with a "softer" material that would meet production, installation and tracking requirements.

There are two other significant factors in soft soils. Drilling of the borehole can alter the conditions around the installation. Loose soil can densify and cave, changing the natural conditions.  The stiffness of the backfill also has an effect, since it typically occupies 75% of the borehole cross-sectional area.  Grout stiffness is probably a greater issue than the ABS casing stiffness. Not much information is available on design and characteristics of soft cement-bentonite grout as far as I know. In soft soil the "instrument disturbance factor" can be significant.

Finally, even if there is some redistribution of displacements due to grout and casing stiffness and borehole disturbance, the inclinometer will measure the correct total amount, but over a longer interval of the borehole.  Also, the true profile of the casing in the shear zone cannot be defined by ordinary methods. Special techniques using 3-inch reading intervals and special calculation methods must be used.


Question: In the winter, water in my inclinometer casing tends to freeze, and I can't obtain a survey. What can I do about this?

Answer: There are many ways to address this issue. Our current recommendation is to fill the casing with a water + propylene glycol solution. Propylene glycol is a non-toxic antifreeze fluid. Although this solution should be harmless, always wash off the probe and cable after use.

Here is a table showing the freezing point for various solutions. The table is provided by engineeringtoolbox.com.

Freezing Point - Propylene Glycol and Water Solutions
% Solution by mass 0 10 20 30 40 50 60
Temperature F 32 26 18 7 -8 -29 -55
C 0 -3 -8 -14 -22 -34 -48

Question: Should I used large, medium, or small diameter casing?

Answer: Casing is designed to deform with movement of the adjacent ground or structure. The useful life of the casing ends when casing curvature forces the inclinometer probe out of the grooves in the casing. Continued movement of the ground pinches or shears the casing, and eventually prevents passage of the inclinometer probe.

Large diameter casing (85 mm, 3.34 inch) is suitable for landslides and long term monitoring. It is also approprate for monitoring multiple shear zones or very narrow shear zones. Large diameter casing is required when the horizontal probe is used.

Medium diameter casing (70 mm, 2.75 inch) is suitable for construction projects. It can also be used for slope stability monitoring when only a moderate degree of deformation is expected.

Small diameter casing (48 mm, 1.9 inch) is suitable for applications where small deformations are distributed over broad zones. It is generally not installed in soils.

Read more about casing curvature and tracking.


Question: What's the best grout mix for inclinometers?

Answer: We've have prepared a tech note on recommended grout mixes .


Question: Is a bentonite grout, such as Volclay, suitable for backfilling around inclinometer casing?

Answer: It is important to have some cement in the mixture for dimensional stability. Bentonite alone will never set and its volume varies with moisture levels.


Question: Why does casing tend to float in grout?

Answer: 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. Read our technote on Countering casing buoyancy .


Question: My casing was not oriented correctly during installation. The A-grooves are misaligned about 10 degrees from the actual direction of movement. Is this a problem?

Answer: 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.

orientationThis 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


Question: We're adding casing to the top of our inclinometer installation. How will this affect our readings?

Answer: This question is answered in the DMM FAQ .


Question: We want to install an inclinometer to monitor deformation within our cast-in-place, reinforced concrete retaining wall. We intend to tie a heavy-gauge PVC pipe to the reinforcing cage. Then when the wall is cast, we will drill about 6 meters below the bottom of the pipe to ensure that our inclinometer is anchored in stable ground. Then we will install inclinometer casing inside the PVC pipe and grout it in. Will the heavy PVC pipe degrade our inclinometer readings?

Answer: 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.


Question: We want to install an inclinometer with our sheet-pile walls. Do you have recommendations for how to do this?

Answer: 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).


Question: For best results with my inclinometers, must my boreholes be very close to vertical?

Answer: 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.


Question: I want to add torque strength to the QC coupling. Is there a way to do that?

QC casing joint with steel strapAnswer: 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.

 


Question: Do you have any advice for installing casing in deep boreholes?

Answer: 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. See our grout-mixes page. 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.

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