53 Inclinometers

53.1 General


Inclinometers are used for measuring displacements across a line in, for example, embankments, slopes, piles, diaphragm walls and tunnelling. Each inclinometer consists of a tube (known as the inclinometer access casing) with four orthogonal internal grooves, a probe (known as the inclinometer probe) and a readout device (see Figure 24). Inclinometers can be used in vertical, horizontal and inclined boreholes. Only vertical installations are described in detail in this British Standard. Information about horizontal and inclined inclinometers, together with other methods of measuring settlement and heave (e.g. hydrostatic profilers) can be found in Dunnicliff, 1988 [112], ICE, 2012 [114] and BS EN ISO 18674.

Inclinometer installations and measurements should conform to BS EN ISO 18674.

The probe and the readout device should be connected together using a graduated cable. The graduation markers may also act as hangers for supporting the inclinometer probe. The inclinometer probe should normally have two sprung wheel sets, one at either end, which are used to locate the probe along the grooved tube using the cable. At regular and known locations inside the inclinometer access casing, the inclination of the casing should be measured using the inclinometer probe. The measured inclinations should be used to calculate the shape of the inclinometer casing; displacements of the casing should be calculated by subtracting the initial shape of the casing from the current shape of the casing.

Figure 24 Probe inclinometer system
Probe inclinometer system


1 Graduated signal cable 5 Transducer body
2 Transducer readout 6 Cement-bentonite grout
3 Cable gate 7 Grooved inclinometer casing
4 Sprung wheel set 8 Sealed bottom cap

53.2 Inclinometer access casing and installation procedure


Inclinometer access casings are most often formed of ABS (acrylonitrile butadiene styrene), but aluminium casings are also available. Individual sections of casing are usually 3 m long and flush jointed with an O-ring seal to prevent ingress of grout or water. Couplers are available and can be used where sections of casing have no O-ring joint or where a length of casing has been cut. The couplers are either glued or riveted to the sections.

The joints should be sealed to prevent ingress of water or grout; if rivets are used, the coupler should be covered with a waxed tape. A sealed bottom cap should be used and the top of the casing should be covered during installation. Spiralling of the internal grooves should be quantified over the whole length of the installed casing before any readings are taken. If spiralling of the grooves is greater than ±5° over the length of the casing this should be taken into account when calculating the resultant inclinations.

Inclinometer casings should be installed inside boreholes or, in the case of piles and diaphragm walls, installed inside temporary void formers placed inside the structure until concreting is complete. The annulus between the inclinometer access casing and the ground or temporary void former should be filled with a cement-bentonite grout. Typical grout mixtures for stiff and soft ground are shown in Table 35 (see Mikkelsen, 2002 [127]). The cement should be added to the water first because the strength and stiffness of the grout are determined by the initial water/cement ratio. Bentonite is only added to restrict "bleed". Sufficient bentonite should be added to provide a creamy mix, which is pumpable. The amount of bentonite required to provide this depends on the ambient temperature and the acidity of the water. Care should be taken to prevent the grout from entering inside the inclinometer access casing during installation.

Where significant grout losses occur (e.g. in heavily fractured or voided ground and where the surrounding ground has a high permeability) it might not be feasible to satisfactorily fill the annulus between the access casing and the ground with grout. In these circumstances the annulus may be filled with a granular material (e.g. pea gravel). Care should be taken to ensure satisfactory backfilling and restraint are provided. Consideration should be given to drilling larger diameter boreholes so that there is sufficient annular space to prevent bridging of the gravel.

Table 35 Typical cement-bentonite grout mixes for inclinometers and extensometers
Applications Medium to high strength soils Soft soils and extensometers
Materials Volume/mass Ratio by weight Volume/mass Ratio by weight
Water 60 L –2,5 50 L 6,6
Ordinary Portland cement 25 kg –1,0 8 kg 1,0
Bentonite 7 kg –0,3 3 kg 0,4
Notes The 28 day compressive strength of this mix is about 350 kPa and the Young's modulus is about 69 MPa. The 28 day compressive strength of this mix is about 28 kPa.
NOTE Adapted from Mikkelsen, 2002 [127].

The weight of the inclinometer access casing is much less than the equivalent upward force exerted on the bottom of the casing by the grout, which means that the casing quickly becomes buoyant and starts to float in the grout. This should be countered by filling the inclinometer access casing with water as it is installed or placing a heavy rod (e.g. an SPT rod) inside the casing. If, however, the annulus between the casing and the ground is filled to the top (i.e. the surface) with grout, the weight of a water-filled tube is unlikely to be sufficient to counter the uplift force and in such circumstances either an anchorage system should be used at the bottom of the casing or the annulus should be filled in two or more stages. Once grout has set around the bottom of the inclinometer access casing and it is effectively isolated from the effect of the pressure generated by the grout that is added to the annulus, it ought to be possible to fill the annulus to the top without the casing rising due to buoyancy. Weight should never be applied to the top of the inclinometer access casing to counter the effect of the buoyancy, as this causes the casing to buckle and leads to a reduction in the quality of the resulting measurements.

Each inclinometer access casing should be installed with one pair of grooves orientated in the direction of the expected movement (e.g. up/down a slope and towards the excavation for a diaphragm wall). Orientation of the casing should be done before any grout is placed inside the borehole or void former. The casing should never be rotated inside the borehole or void former after the grout has been added to the annulus as this might cause the casing to become spiralled or misaligned. If none of the grooves are aligned with the direction of likely movement, the casing should be left in the same orientation and the measurements should be resolved to determine the displacements.

For near vertical installations, the inclination of the borehole or void former should be no more than ±5° to the vertical in all directions. The bottom of the inclinometer access casing should be located below where any expected lateral displacement occurs and this condition should be satisfied over a length of casing equivalent to at least five measurements (e.g. 2,5 m in the case of a 0,5 m inclinometer probe). If this is not possible, the absolute position of the top of the inclinometer access casing should be determined by surveying.

The grout should be allowed to set before any measurements are taken inside the inclinometer access casing. Typically this can take a few days but it is dependent on the mix used and the temperature.

Where inclinometer access casings are installed inside horizontal boreholes the bottom of the casing is in direct contact with the ground. The space above the casing should be filled with a cement-bentonite grout. Temporary, expandable packers should be used to retain the grout in the borehole until it has set. When the access casing is placed in a shallow trench (e.g. for measuring settlements beneath an embankment) a geotextile and a shallow layer of fine sand should be used between the access casing and the ground.

BS 5930:2015 Code of practice for ground investigations