Casing Installation

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.

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.

Notes

• It is not really practical to try to match the strength of the grout to the strength of the soil, since the properties of grout and soil are so different. The cement-water ratio controls the strength of the grout. To decrease the strength of the grout, add more water.
• Drillers are accustomed to mixing water and bentonite first, but this will not allow you to control the water-cement ratio. Mix water and cement first. Then add bentonite. There is no particular amount of bentonite that you must add. The thickness of the grout varies with water, temperature, and agitation, so the amount of bentonite required will vary.
• The quantity of the third ingredient to be added must be adjusted to obtain a suitable consistency. A watery mix is NOT acceptable. A mix that is too thick cannot be pumped. The Marsh funnel number of the liquid grout should be about 55 seconds +/- a few seconds.
• If you mix cement and water first, the amount of bentonite has to be adjusted. The amount is rarely ever equal to exactly 25 lbs. The final quantity also varies with the type of bentonite used, the method of mixing and the pH of the water.
• If you mix bentonite and water first, the mix usually gets so thick with one bag of cement that it cannot be pumped. There is also a high risk of a flash-set. The cement content must usually be lowered. That results in a higher w/c ratio, lower strength and probably a higher permeability.
• If the mix is left too watery, not only do you get shrinkage, but segregation occurs: cement on the bottom, then bentonite and water on top (bleed). This is not acceptable.

• Suspend a steel pipe or drill rods inside the casing. For the best results, suspend the pipe just an inch or so off the bottom cap of the casing.This ensures that the steel pipe remains straight and avoids resting the full weight of the pipe on the bottom cap. As the casing rises to meet the pipe, the down force of the pipe is activated to keep the casing in place. The main drawback to this method is that you must use the drill rig to suspend the pipe or you must return with the drill rig to retrieve the pipe. A variation is to rest the pipe on the bottom cap, which you must reinforce.
• Pre-install an anchor at the bottom of the casing. Simple prong anchors or packer types have been used. Slope Indicator has produced several prototypes of such anchors. Different soils may require different types of anchor.
  Pre-attach a weight to the bottom of the casing. This method requires a weight, a safety line to prevent casing from sinking, a borehole drilled deeper to accommodate the weight, and calculation of the uplift force. It is best used in shallow boreholes.
• Grout the borehole in stages. The uplift force of grout varies with the height of the grout column. If the column is short, the uplift force is low and the casing can be held in place by its own weight or with very little down-force applied from the top. When the grout sets, the bottom cap is isolated from the column of grout and there is no surface for the uplift force to act on. No more than two or three meters to 3 m need to be grouted in the first stage. If the normal bentonite/cement grout is used it needs to set for at least 12 hours before second-stage grouting. Avoid use of a quick-set grout, since the heat of hydration could melt and deform the plastic. The two stages can be placed via an outside tremie pipe. Alternatively the first stage can be placed before lowering the casing, provided that all is done efficiently so that there is no chance of the grout setting prematurely. If grout valve method is used, first stage grouting can be done through the valve, and then the valve is abandoned. An outside tremie pipe for the second stage is lowered with the casing, with its bottom at the level planned for the top of the first stage. After first stage grouting, the excess is flushed out via the tremie pipe, and then this is raised until the first stage has set.
• Fill the casing with drilling fluid that is heavier than the grout. This is a sure and easy method, but requires disposal of the drilling fluid.

Unfortunately, the easiest way to keep the casing in place – holding the casing down from the top – is also the worst way. The uplift force acts on the bottom of the casing, so if the casing is held in place from the top, the casing goes into compression. When the casing goes into compression, it tends to snake from side to side in the borehole. This problem is particularly severe in large diameter boreholes and in deep installations, where the uplift force is largest and where portions of the borehole may be enlarged. Snaked casing increases the potential for:

• Kinked or Separated Casing: The combination of compressive force and eccentric loading (due to snaking in enlarged diameter boreholes) can produce strong bending moments in the casing. In some severe cases, this bending moment has caused snap-together joints to fail where the glue-and-rivet joints do not.
• Depth Errors: In snaked casing, slight changes or errors in the positioning of the probe will produce reading errors. The larger the curvature, the larger the error. For example, if the change of inclination between adjacent reading increments is two degrees, and the probe is positioned 25 mm from the correct depth, the resulting error in displacement would be 1 mm.

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.

To prevent the water from freezing we recommend filling 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.

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 appropriate 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. 

 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.

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