FAQ » Inclinometer
Do you have any instructions for installing horizontal casing? I'm planning to embed the casing in a foundation slab.
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. 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.
- 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.
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.
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?
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.
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.
We've have prepared a tech note on recommended grout mixes .
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.
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
We want to install an inclinometer with our sheet-pile walls. Do you have recommendations for how to do this?
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.
We're adding casing to the top of our inclinometer installation. How will this affect our readings?
This question is answered in the DMM FAQ