It is obvious there should be construction controls, not only after completion of specific work procedures but continuously, in parallel with the ongoing construction. Monitoring data can then be used not only as a base for the quality assessment of the construction work, but also for the direct control of the operations. For example, in the previously discussed case of drill-hole deviation measurements, it is obvious that modern directional drilling rigs are to be employed in the first instance. With these rigs, the inclination and azimuth of the drilling head are continuously monitored and adjusted as required. However, it should be realised that, up until now, such rigs have been significantly more costly than conventional drilling rigs and are not always suitable in geotechnical applications.
Real-time monitoring
A precondition for the direct control of any construction procedure is on-site, real-time monitoring. Such monitoring is the actual ‘hit’ in geotechnical instrumentation. Key geotechnical parameters are continuously monitored and processed by automatic data acquisition and evaluation procedures. Real-time monitoring not only helps to lower the risk of unforeseen events to a minimum – it also opens up the possibility for innovative construction procedures.
An example is the Soilfrac compensation grouting method developed by the Keller Company. This method is increasingly being used when tunnelling is carried out beneath settlement sensitive structures such as buildings, railroads, freeways or pipelines.
The construction of a four-lane freeway through the city of Bielefeld, Germany included tunnelling in highly weathered shales beneath a number of six-storey buildings. The roof of the 25m wide twin tunnel was just 4.5m away from the strip footings of the buildings. The cross sectional area of the tunnel was 220m². The maximum allowable settlement difference was specified by the structural engineer as Δsmax ≤1mm/m (≤1‰). Conformance to the specifications had to be documented for all tunnelling phases. Tunnel construction with such stringent settlement requirements is only possible with special construction measures.
Compensation grouting was carried out during various tunnelling phases, targeting the zone between the tunnel roof and the foundation of the buildings. Fine-tuning of the compensation grouting procedures was carried out in such way that no excessive settlement differences occurred during the tunnel excavation and no undue heave occurred because of excessive grouting pressure.
The success of this method depended on real-time monitoring of settlement and heave. In total, 76 electronic liquid level gauges were mounted in the cellars of the buildings. Each gauge was connected through tubing to an automatic level controller, which held the elevation of the liquid constant by means of a mini-pump, reservoir and an overflow unit. LVDT float sensors monitored the height of the liquid in each gauge. When settlement or heave occurred, the sensor detected an apparent change in the height of the liquid. In fact, the gauge and sensor had moved relative to the elevation of the liquid surface, which remained constant. The system was connected to a data logger and a PC for continuous monitoring and real-time updates of graphs and tables. The monitoring system was thus part of a closed loop feedback circle on construction operations.
The accuracy of the system is ± 0.3mm. This is better than the accuracy of alternative real-time settlement monitoring systems such as motorised digital levels and chains of 10 interconnected electrolevels. Table 1 summarises the advantages, disadvantages and limitations of the various real-time monitoring methods.
Integrated TBM control
One of the most complex controls of tunnelling procedures through geotechnical instrumentation is the Integrated TBM Control System, as proposed by Kaalberg and Hentschel (1997) and Doom et al (1999). This system was conceived to control the TBM operations in near-surface, inner-city tunnelling in soft ground. Reference is made to conditions in Amsterdam, Holland, characterised by an inhomogeneous ground structure, historic buildings founded on wooden piles and numerous cases of settlement damage occurring over past centuries.
The main characteristics of the system is the integration of geotechnical parameters into the control of the TBM. Until now, machine parameters were exclusively used for this purpose. However, this is insufficient for TBM operations in settlement sensitive environments such as in Amsterdam.
Numerous settlement measuring points are placed at the buildings (targets for motorised tachymeter and/or electrolevels), in the ground and on or near the piles (multiple-point borehole extensometers). Geotechnical monitoring data, together with the TBM machine data, make up the information base of the ‘reality’, which is continuously updated with the TBM operations. This base provides the input for a complex closed-loop control mechanism. The control signal acts on the TBM actuator for adjusting TBM shield forces and, in particular, adjustment of the contact grouting pressure within the shield’s specially designed tail to avoid detrimental settlements.
To be successful the integrated system depends on a control function that incorporates not only monitoring signals from the buildings, the ground and the piles, but also relevant soil and building parameters to produce a realistic model.
It remains to be seen whether the Integrated TBM Control System will be successful. For the time being, it marks a definite peak in efforts to achieve better control of construction procedures through geotechnical monitoring.
Conclusions
In continental Europe, the following trends can be identified in the use of geotechnical instrumentation in the control of tunnel construction procedures:
Related Files
Tunnelling beneath settlement-sensitive buildings in Bielefeld, Germany
Real-time monitoring system
Multipoint Liquid Level System
Function diagram of the Integrated TBM Control System.
Compensation grouting in a tunnel project
Geotechnical and geodetic monitoring points
