Groove (keyway) alignment
One set of grooves should be installed in the direction of the expected movement and typically this is referred to as the A axis.
If during installation the keyways become misaligned from the direction of movement this can be rectified as long as the grout hasn’t set. To realign the casing lift the casing up and down to lower the friction from the grout and carefully turn it.
NEVER TWIST THE CASING FROM THE TOP AS THIS WILL CAUSE TWISTING OF THE GROOVES.
Vertical reservation tube
Where casing is installed in diaphragm walls it is usual practice to install it inside what is often referred to as a reservation tube. This is typically a 150mm steel or plastic pipe which is first installed with the cage and then the inclinometer casing installed and grouted up. As the bottom of the inclinometer casing is usually required to be below the toe of the wall a plug is normally placed at the base of the reservation tube which can be drilled out, the borehole drilled to the required depth and the casing installed.
The first section is the most critical as the more accurately this is installed with the minimum amount of torsion the easier the subsequent sections will be.
Ensure that suitable mountings are provided if installed within reinforcement and ensure the sections are secured tightly to the mountings using cable ties, tie wire and duct tape. When laid in ground a good
Note 1: A pull cable is required to pull the probe to the far end and back and should be installed progressively with the casing.
Note 2: Where access is only available from one end a “dead end pulley”, together with pull through cable and access tubing will be required.
The QJ casing has one external rib over a groove which can be used to align each section and the XC has four external ribs over the grooves one of which can be marked for alignment.
Maximum depth of Installation
The maximum depth of installation is dependent on whether the borehole is dry, the groundwater level if present, density of the grout if pre-grouted and the experience of the installer. Where deep boreholes are to be installed care should be taken as regards pressures and forces that could be exerted on the casing.
- QJ – the maximum dry string length recommended is 100 metres. Placing a 50mm wide waterproof duct tape around each joint will increase the strength of the joint by approximately 50%.
- XC – the maximum dry string length recommended is 150 metres assuming the use of glue and rivets.
However we would always recommend using a rope attached to the bottom section to take the majority of the weight on installations greater than 60 metres in a dry hole. Where groundwater and/or grout exists then buoyancy (see below) would be more of an issue than the joint strength in tension.
Buoyancy is often experienced during installation due to groundwater or grout if a pre-grouting technique is used. It is therefore always recommended to fill the casing with water when groundwater exists.
Grout in a borehole has a much higher density than water-filled casing. During installation, liquid grout will exert an uplift force on the bottom of the casing. This uplift force is greater than the downward force exerted by the casing even if water-filled. The net uplift can be calculated as the density of grout minus the density of water in the casing x depth of casing x area of bottom.
The options to address buoyancy are as follows:
- Fix an appropriate anchor to the bottom of the casing
- Suspend steel pipe or drill rods inside the casing
- Pre-attach a weight to the bottom of the casing
- Grout the borehole in stages
It is standard practice to grout in inclinometer casing using a tremie method which can be done by the following:
- Tremie pipe installed alongside the casing
- Tremie pipe connected to a grout shoe on the bottom of the casing
- Tremie pipe installed to the base and the borehole pre-grouted before the casing is installed
For grout mixes please see the Grout section (hyperlink to section)
Spiral surveys are generally used when:
- It is suspected that the casing has become twisted during installation
- Where deep installations occur > 50 metres
- The project specification requires it
The spiral survey only needs to be taken once at the beginning and the data can be entered into Inclinalysis to provide corrections.
Dummy probe survey
Before placing an inclinometer probe into a borehole it is recommended that a “dummy probe” which is the same construction as a normal probe except there are no sensors inside by run up and down the casing to check that there are no problems which could result in an expensive probe getting stuck in the casing.