Presently the survey of absolute displacements of targets fixed at the tunnel wall is the state of the art in performance monitoring of tunnels. Monitoring data are used to assess the stabilization process of the tunnel, more recently also for the short term prediction of the ground conditions ahead of the face.
The displacements do not substantially vary in rock mass conditions with nearly constant properties and influencing factors. On the other hand, changes in the rock mass structure or properties result in changes in the displacement characteristics.
For nearly constant conditions, the displacement trends show minor fluctuations within a certain normal range, due to minor variations in the rock mass properties. Deviations from this range are clear indicators for changing conditions ahead of the face or outside of the tunnel. To identify such trend deviations the normal range of the trend lines becomes crucial. Geostatistical methods allow an automatic identification of trends along the tunnel. Using data from completed tunnel projects a “reference trend table” can be established. By comparing actual observed trend characteristics to this reference table changing ground conditions ahead of the face can be identified and hence, the change in the displacement characteristics and magnitudes can be predicted.
The uncertainties in the geological conditions and ground parameters require an observational approach for safe and economical tunnel construction. Several conditions must be fulfilled for a successful application of the observational method (Peck 1969, Schubert 2008). One of the requirements is the assessment of possible behaviors and the establishment of their acceptable limits during design. This includes the identification of potential failure modes, as well as the determination of deformation characteristics and magnitudes. As the ground in general is all but homogeneous, continuous, and isotropic, simple homogeneous models usually do not provide enough insight to establish a realistic “normal behavior” for structured and heterogeneous ground conditions. It is also unrealistic to think that sophisticated numerical models can be used for an entire project during the design. A reasonable way to produce expected realistic ground behaviors is to first use simplified models to determine the range of expected displacements, and then modify the results with the help of expert knowledge.
During construction the measurement results contain all influences of the ground structure, stresses, and interaction between ground and support. The previously established characteristic behaviors for certain conditions are compared to the monitoring results. In case of agreement it can be established that the observed behavior is “normal”. Deviations from the expected behavior can have various reasons. One may be that the behavior during design was not assessed correctly. In this case, a refinement of the model is required. Another reason for behavior deviating from the expected can be a change in the ground conditions ahead of the face. It is meanwhile well known that trends of displacement vector orientations can be used to predict changing ground conditions ahead of the face (Schubert & Budil 1995, Steindorfer 1997, Jeon et al. 2005).