54 Extensometers

54.1 General


Extensometers are used for measuring changes of distance between two or more measuring points located along a measuring line in, for example, embankments, slopes, piles, diaphragm walls and tunnelling.

In ground, the measuring points should typically be installed in boreholes and the measuring line should coincide with the axis of the borehole. At each measuring point the movements of the medium (e.g. soil, rock and concrete structures) being investigated should be transferred to the measuring point by anchors. The anchors, which are fixed to the measuring points, should make good contact with the medium being investigated. Two types of extensometer are commonly used, magnetic probe extensometers and rod extensometers.

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

54.2 Probe extensometers


Magnetic probe extensometers consist of a hollow stiff non-magnetic central flush access pipe (typically of 33 mm diameter), through which a magnetic reed switch is passed. Each measuring point (commonly known as a "spider magnet" because of its appearance) consists of a plastic doughnut containing magnets and has legs (typically three or six) made from sprung steel (see Figure 25). The spider magnets are threaded over the access pipe and positioned where the movements are to be measured. When a magnetic reed switch is lowered into the access pipe the magnetic field generated by a spider magnet causes a reed inside the switch to close, forming an electric circuit and a buzzer is activated.

The open diameter of a spider magnet's legs should be wide enough to make contact with the surrounding ground when opened and the borehole should be filled with a soft, deformable cement-bentonite grout. The position of the spider should be measured, relative to the top of the access pipe using a measuring tape connected to the reed switch. The absolute location of the spider magnets should be evaluated by either positioning a datum magnet, glued to the access casing at a depth below where the movements are expected to cease (i.e. in stable ground) or more reliably by precise levelling the top of the access casing at the same time as the measurements are made.

NOTE The main problem with magnetic probe extensometers is that it can be very difficult to position the spider magnets accurately. The sprung legs are frequently held back during insertion using a rubber band and when a spider reaches the correct depth the rubber band is cut or removed and the legs unfold. Their sprung nature causes them to dig into the surrounding ground and this can be unreliable, particularly in stiff ground. Moreover, it is not unusual for the rubber band to release before the spider reaches the required depth or for users to, therefore, find it difficult to position the spiders accurately.

To avoid this problem, spiders with three rather than six legs can be used and inserted over the access casing with the legs pointing upwards. The spider is pushed to the required location using an insertion pipe/tool that also fits over the access pipe. If the insertion tool and the spider are connected using a bayonet fixture the spider can be pulled when the required location is reached causing the legs to dig into the ground and the bayonet can be released to remove the insertion tool. In stiff unsupported ground the legs of the spider scrape along the edges of the borehole during insertion. In soft ground it might be necessary to use a drilling support casing and the spider can be pushed to the bottom of the drilling casing when the latter is close to the required location and then pushed beyond the bottom of the casing before releasing the bayonet connector.

Some larger spiders are made for insertion over flush inclinometer casings, but if they are used, care should be observed to use an appropriate grout because to function correctly the spiders need a soft, deformable grout, whereas the inclinometer might need a stiffer grout (depending on the ground conditions).

Figure 25 Magnetic probe extensometer system


1 Measuring tape 6 Cement-bentonite grout
2 Reed switch 7 Stable ground
3 Unstable ground 8 Datum magnet (glued to access pipe)
4 Six-legged spider magnet
5 Flush access pipe (greased) 9 Sealed bottom cap

54.3 Rod extensometers


Rod extensometers consist of anchors, connecting rods and a measuring head. Each anchor is positioned at a measuring point and joined to a connecting rod, running to the measuring head, which is normally located at the ground surface (see Figure 26). At the measuring head is a means of measuring the position and change in position of the connecting rods and hence the anchors. This can be electric displacement transducers connected to the measuring head or else the position of the rods can be measured using a portable dial gauge. The latter can only be used for discrete measurements whereas displacement transducers might be connected to a data logger for automatic and remote monitoring solutions.

The anchors might be a grouted type, whereby there is no direct contact between the anchor and the ground and conformance is maintained through the use of a suitable grout. Alternatively the anchors might have hydraulically activated prongs that can be extended into the ground.

At least one of the anchors should be located where no displacement is expected because it can then be used as a reference for the displacements of the remaining anchors. If it is not possible to locate an anchor in stable ground, the elevation of the reference head should be independently measured using a precise level each time a measurement is made. If grouted anchors are used, the grout should be deformable because if it is too stiff the whole extensometer might move as a solid element, the movements of the anchors might be restricted and the movements at the measuring points might be underestimated.

NOTE 1 Remote monitoring is not practical if the reference head has to be precisely levelled.

NOTE 2 Connecting rods are typically made from stainless steel or fibreglass and are surrounded by a plastic sleeve to isolate them from the grout that is used to fill the borehole after the anchors and connecting rods have been installed. Stainless steel rods come in discrete, relatively short sections (typically 2 m to 3 m) with screw threads on each end and are joined together during installation, the final, uppermost length being adjustable to suite the required location of the respective anchor. Fibreglass rods come in fixed continuous lengths, which are coiled for transporting and while they can be installed in one step, it is important that the anchors are positioned accurately because fibreglass rods are impossible to lengthen and difficult to shorten after they have been installed. The choice of material for the rods (stainless steel or fibreglass) is influenced by the skill of the installer to position the anchors accurately and the thermal stability of the installation.

Figure 26 Rod extensometer system


1 Displacement transducer for automatic and remote readings 6 Rod (steel or fibre-glass) with protective pipe cover
2 Protective cover 7 Borehole
3 Dial gauge for manual readings 8 Grouted anchor
4 Measuring head 9 Hydraulic anchor
5 Guide bracket for rods 10 Soft deformable grout