Most larger instrumentation suppliers and ground engineering laboratories offer stress measurement services. In tunnelling, these are used mainly for planning excavation support in deep locations and the orientation of major excavations. Suppliers see soft ground and shallow locations as two areas for potential growth in stress management techniques.
Stress measurement for underground structures is a speciality of Rock Mechanics Technology (RMT), which generally uses the overcoring technique with the CSIRO Hollow Inclusion Cell for which it is UK distributor. This gives accurate 3-D stress tensor measurement from a single overcore in hard and soft rock at depths of 20-1500m.
Solexperts is a leading practitioner of stress measurement by hydrofracture and hydrojacking. Dr Shosei Serata of Serata Geomechanics offers a digital stress measurement technique for installation in deep boreholes. It gives instantaneous in-situ measurements automatically and has developed that used in the Japanese earthquake prediction program. Serata has now miniaturised it for one-man portable operation. It is based on the single-fracture method of borehole measurement and is said to be particularly applicable to pre-fractured, complex ground since it obviates the need for theories of elasticity.
In civil engineering tunnelling, it can be used to validate numerical models for construction and support design. Other advantages claimed over overcoring and hydrofracturing methods include easy operation for both shallow and deep boreholes using computerised data acquisition; real-time results displayed graphically on site; direct interface with finite element modelling software; and the possibility of using old boreholes with minor grouting or reaming.
It is important to plan instrumentation and monitoring. Data may be required on a long-term basis or immediately. There should be some expectation of the location and extent of likely movement, which will aid in the most appropriate location of instrumentation. Sometimes, only a swift check is necessary, which might lead to more accurate instrumentation or to the alleviation of problems.
For example, the new handheld acoustic energy meter (AEM) from Rock Mechanics Technology can be used to check for voids behind support linings or for nearby ground separation. The AEM comprises an integral geophone and readout unit and is powered by a rechargeable battery. It measures the reverberation decay rate when a structure is struck with a hammer. The alphanumeric LCD readout is supplemented by green, amber and red LEDs for operation according to pre-set alarm levels.
Another application requiring a fairly rapid response is to check the performance of the lining immediately behind the face. With large TBM back-up assemblies, this is often not an easy task, since the necessary points cannot be readily observed because of obstructions. A procedure developed by VMT uses inclinometers mounted on each segment of a lining ring.
