Monitoring of tunnel support and ground performance is now fundamental to successful underground construction, especially when ‘observational methods’ are used, and allows the parameters for successful tunnelling to be extended. The means of monitoring some ground conditions and movements have been around for many years. These used to be dominated by manual measurement, traditional surveying techniques and borehole surveying. All are time consuming and demand skilled measurement collection. The main difference today is that electronic, often digital, methods have come to the fore. Together with the availability of remote monitoring, these represent facilities for increased accuracy, speed of collection, and sometimes automatic warning of conditions with potential for concern.
Rock Mechanics Technology’s Dr Dave Bigby believes that the company’s range of instrumentation has a major role to play in observational design methodology. The range includes strain gauged rockbolts, sonic extensometers and rockbolt ‘telltales’. The latter make the semi-skilled detection of ground separation a simple matter in that movements greater than 25mm are revealed by the position of a 3-colour indicator at the end of the borehole.
This is attached to anchors just inside the hole and at the back of the hole within the predicted stable zone. Dual height indicators show both differential and total movement in the hole. A programme of ‘telltales’ can be used to reveal any movements in primary tunnel support before application of the final lining.
Extensometers
More accurate determination of strain, especially in rockbolted zones, is provided by borehole extensometers, the latest version of which is the sonic extensometer. Strain in fully encapsulated rockbolts can be measured using strain gauges along their length, which can be orientated to reveal bending and shear as well as loading on the longitudinal axis.
Sonic extensometers have proved to be a major operational improvement on traditional types. Each can have up to 20 anchors in a probe up to 7.5m long and measure to a sensitivity of 0.025mm. They operate by a torsional pulse generated when a current pulse passes through a ferromagnetic probe, part of which is in a magnetic field. The pulse travels at the speed of sound, giving a delay between the current pulse emission and the return of the mechanical pulse proportional to the distance travelled. Geokon developed the original form with an intrinsically safe version developed by Soil Instruments. The latter supplies a wide range of devices, including strain gauged rockbolts and the Sentinel integrity monitoring rockbolt.
Solexperts manufactures ground displacement instruments developed by the Swiss Federal Institute of Technology. These include sliding micrometers and deformeters, and the Trivec. The sliding micrometer is a portable probe which precisely measures continuous strain distribution along a borehole. The sliding deformeter costs less but is a less accurate version. The Trivec gives high precision measurement of displacement of three vectors along a vertical line.
Solexperts has also developed theodolite motion control (for Leica & Zeiss automatic levels or total stations) which can be integrated into its GeoMonitor data acquisition systems for automatic settlement monitoring. Automatic control of a Leica TCA 1103 was employed to aid correction of settlement in the Adler Tunnel for Swiss Federal Railways. Level measurements with extensometers indicate settlements of up to 100mm/month caused by erosion of salt at a depth of 150m.
A 150m long cut+cover section of the tunnel had to be uncovered again in May 1999 and lifted with hydraulic jacks to the design alignment. This allowed the rail tracks to be laid within design tolerances. Ninety-two jacks were installed and monitored with displacement transducers. The Leica total station and Solexperts’ Geomonitor was employed to monitor the position of the lifted tunnel section relative to the rest, giving 3-D data rather than simple levels. Grouting restored the tunnel foundation, but monitoring continued with remote control of the site set-up via modem.
Vibrating wire
Perhaps the most universally used type of instrument is the vibrating wire (VW) gauge. In tunnelling, strain gauges of this type can be mounted on supports like steel arches or embedded in a pattern concrete to monitor localised loading when linked to a data acquisition system. Most Geokon instruments employ the vibrating wire principle, which has a frequency output capable of transmission over long (>2000m) cables without significant degradation. Geokon’s Tony Simmonds makes the point that VW sensors have to be properly constructed to achieve their potential for long-term stability and so be suited to long-term measurements in adverse environments.
“It is relatively easy to build a vibrating wire sensor, but not so easy to build one that has long-term stability,” he says, “as many of our imitators in India and South East Asia are beginning to discover. Unfortunately, many contractors in these parts of the world are only concerned with cost, and they seem to be quite happy to use locally made products which look identical to ours but that are sold at less than half our price.”
The Geokon range includes strain gauges (including weldable types); convergence meters; extensometers; piezometers; pressure cells and transducers; settlement sensors; inclinometers; data loggers; and stress meters. The company is a major supplier to the Boston CA/T Project and to other tunnelling projects in Los Angeles, Seattle, Hong Kong, Seoul, Istanbul and Singapore.
On the CA/T Project, Geokon worked with the design team, which included Shannon & Wilson, Bechtel, Parsons Brinckerhoff and Gannett Fleming, to design a special instrumentation programme. This was used to monitor the structural integrity of the concrete lining of two Red Line metro tunnels at Fort Point Crossing during construction of the immersed tube highway tunnel. A major feature was a liquid level system, designed to give the best combination of accuracy and rapid reading in a compact mounting on the edge of the tunnel.
Ground improvement
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“Monitoring tunnel support and ground performance is now fundamental to successful underground construction, especially when ‘observational’ methods are used” |
Instrumentation and monitoring are now considered essential in most cases of ground improvement, especially in grouting programmes, where successful monitoring ensures that work is carried out in the right place to the optimum degree. Jean Lutz specialises in the design and manufacture of instrumentation for site investigation, grouting, piling and anchoring. All relevant drill rig performance data are collected, with data on grout pump performance. The system can also control rig mast positioning in 3-D and automatic lifting of placers for concreting and jet grouting.
Botte Sade Fondations used Jean Lutz instrumentation for grouting and jet grouting procedures from the surface to recover tunnelling work on the Rennes Metro in France. The specialist contractor used a CL88 drilling parameter recorder to detect decompressed zones requiring treatment when a TBM, 30m down, had been halted by decomposed schists. Jean Lutz’ latest development, the CinauT, was used to control tube-à-manchette grouting from up to eight pumps for selective ground treatment. Another, specially adapted, CL88 measured all jet grouting parameters. The procedure has now been adopted for over 400 jet columns.
For similar applications, Atlas Copco Craelius has developed the new Logac 4111 grout recorder to work with the Logac 4000 computer-based logging system for sampling and storing data during grouting work. The simply operated system now features logging and registration of grouting time, flow, volume and pressure in real time. The systems can be used on site either as a stand alone unit or integrated into the Unigrout or other grouting system.
Acquisition & management
Major instrumentation suppliers also offer data acquisition and processing systems that can be set for automatic operation on several channels. Systems can warn of the exceeding of pre-set threshold values and the alarm settings can be optimised to eliminate spurious readings. Alarms can be transmitted by various means, including modem, fax, pager, telephones, sirens and lights.
Considering the vast amount of data available from automatic systems, it is important that they can be handled to highlight important values without burdening the project engineers with reams of paper. Solexperts’ data visualisation software (DAVIS) provides a visual representation of data collected from multiple instruments and sensors. It can also plot data from a complex site for evaluation with ‘point-and-click’ operation. A recent software addition is GeoACE for the evaluation of total station and digital level data. This is said to provide more reliable displacement results than other geodetic net calculation packages. 3-D displacement is calculated on-line with automatic temperature and pressure compensation.
The Vismon-NT data acquisition system, developed by the Norwegian Geotechnical Institute, bridges the gap between manually operated laboratory systems for dynamic data acquisition and typical long-term monitoring systems. It can handle well reduced data and large volumes of raw data equally well to represent the dynamic behaviour of a structure being monitored.
Protection
It is common to view urban tunnel instrumentation in terms of protecting existing underground structures from new tunnelling, but the reverse may also be true. The Transport Research Laboratory (TRL) in the UK has recently been working on monitoring vibration from piling operations at Tower Pier on the River Thames to provide assurance about structures adjacent to Tower Subway near the Tower of London. Of particular concern has been a Victorian cast iron lined tunnel now used as a utilities duct.
TRL senior engineer David Hiller explained that, by using geophones installed on the lining, TRL could monitor ppv to give a signal voltage proportional to the vibration velocity. A low threshold value is set to trigger recording for a set period (producing a permanent printout in the resident engineer’s office) and a higher threshold value triggers an alarm. Data are collected via a 16-channel Cambridge Electronic Design logger, 15 channels of which can receive signals from three instruments at each geophone covering the three vector axes. Hiller explained that geophones provide smaller ranges of vibration coverage but tend to be more economical and rugged than accelerometers.
Another service tunnel was the subject of an investigation by non-destructive testing specialist the Rock Solid Group (RSG), a geotechnical, geophysical and R&D consultancy at Sydney Airport in Australia. Using ground probing radar (GPR), and electromagnetic methods, RSG located the foundations of the service tunnel so that any impact from the excavations for the New Southern Railway could be determined. The existence and depth of steel piles supporting the service tunnel also had to be verified to ensure that the rail tunnel would not intercept them.
In Brisbane, RSG, in JV with the city council, used wave impedance and seismic resonance, developed in house, to determine why earthquake repairs had not worked in sealing a syphon tunnel. The survey discovered additional areas of damage behind the lining at different locations.
