Precise levelling may be required in certain instances in construction such as in deformation monitoring, the provision of precise height control for large engineering projects such as long-span bridges, dams and hydroelectric schemes and in mining subsidence measurements. For example, a dam that has been in place for many years is unlikely to be moving.
However, should the dam fail the results would be catastrophic for those on the downstream side. Being under the pressure of water when full, the dam may be liable to distortion. The behaviour of the dam must therefore be monitored. One way of monitoring any vertical movement along the dam is by levelling. Since early warning of small movement is required, and since conclusions about movement must be made with statistical confidence, the levelling must be very precise.
There is more to precise levelling than precise levels. High quality equipment is very important, but so is the method by which it is used. Indeed the two components of precise levelling are precise equipment and precise procedures. Precise levelling uses the same principles as ordinary levelling but with:
(1) Higher quality instruments and more accurate staves
(2) More rigorous observing techniques
(3) Restricted climatic and environmental conditions
(4) Refined booking and reduction
(5) Least squares adjustment for a levelling net
Precise invar staff:
The precise levelling staff has its graduations precisely marked (and checked by laser interferometry) on invar strips, which are attached to wooden or aluminium frames. The strip is rigidly fixed to the base of the staff and held in position by a spring-loaded tensioning device at the top. This arrangement provides support for the invar strip without restraining it in any way. Usually there are two scales on each staff, offset from each other by a fixed amount (Figure). The staff is placed upon a change plate at intermediate stations.
A conventional levelling change plate is small and light and is designed to give a firm platform for the staff on soft ground. Precise levelling should only ever take place on firm ground and the precise levelling change plate is designed to be unmoving on a hard surface. It is therefore heavy. The feet are rounded so that they do not slowly sink or heave when placed on tarmac. The top is smooth, round and polished. The change plate in Figure 3.34 is made from a solid piece of steel and weighs about 10 kilograms.
For the most precise work, two staffs are used; in which case they should be carefully matched in every detail. A circular bubble built into the staff is essential to ensure verticality during observation. The staff should be supported by means of steadying poles or handles.
(1) The staff should have its circular bubble tested at frequent intervals using a plumb-bob.
(2) Warping of the staff can be detected by stretching a fine wire from end to end.
(3) Graduation and zero error can be counteracted by regular calibration.
(4) For the highest accuracy a field thermometer should measure the temperature of the strip in order to apply scale corrections.
The instruments used should be precise levels of the highest accuracy. They should provide high-quality resolution with high magnification (×40) and be capable of being adjusted to remove any significant collimation. This may be achieved with a highly sensitive tubular bubble with a large radius of curvature that gives a greater horizontal bubble movement per angle of tilt. In the case of the automatic level a highly refined compensator would be necessary. In either case a parallel plate micrometer, fitted in front of the object lens, would be used to obtain submillimetre resolution on the staff.
The instrument’s cross-hairs may be as shown in Figure . The distance that the staff is away from the instrument will affect which side of the cross-hairs is to be used. If the staff is far away use the normal horizontal hair, right-hand side of diagram. If the staff is close, the mark on the staff will appear too large to be bisected accurately. By comparing the two white wedges formed by the sloping cross-hairs and the mark on the staff, see the arrows in Figure 3.35(b) where the mark on the staff is not correctly aligned and Figure where it is, a more precise setting of the micrometer can be made.