Civil Engineering Reference
In-Depth Information
ands lining measures. By comparing the results of measurements with those of the design
calculations, conclusions can be reached about the suitability of the calculation procedure
used, and comparative calculations can also be performed to correct the rock mass param-
eters used as a basis for calculation. Calculations based on the improved rock mechanical
parameters can then be used to check whether the support is sufficiently dimensioned
or additional measures may be necessary. Measurements of stress and displacement are
generally essential in the construction of rock cavities because they offer the only cur-
rently available method of checking the results of the investigation of structural stability
performed at the design stage and thus making a reliable statement about structural safety.
The planning of a measurement programme has to consider the preliminary investigations
in geological engineering and rock mechanics, the design calculations and the construc-
tion process. The type of instruments, construction schedule and any resulting obstruction
of construction also have to be considered. It is generally true that the measurement pro-
gramme has to be planned for each project and there can be no standardisation.
The extent of the measurement programme and the layout of the monitoring cross-sections
are determined by the requirement that the measured data should deliver geotechnical
parameters that can be regarded as representative and can be extrapolated for larger areas.
In fault zones with an unfavourable relation to the tunnel and when impairment of the
structural safety of existing buildings is a risk, additional targeted measurements are un-
dertaken. Advance probing should also be carried out to enable better estimation of the
geology along the tunnel alignment.
Instrumentation should be installed as early as possible to enable the observation and
evaluation of deformation tendencies caused by tunnelling from the start. If this is not the
case, the geological conditions can be interpreted incorrectly and the overall behaviour of
the rock mass would not be recorded, particularly in case of very high initial deformation.
The measurement frequency should be appropriate for the rate of deformation or loading.
Monitoring should continue after the completion of excavation until the deformation rate
has finally tended to zero. The measured data is then interpreted in combination with the
design calculations and supplementary investigations.
In tunnels driven by shield machines, the monitoring requirements are different due to
the immediate stiffness of a segment lining and the high degree of mechanisation. These
special requirements are described in more detail in Section 4.2.4 of this chapter.
4.2
Measurement programme
4.2.1 General
Monitoring of the displacement and force parameters of the rock mass and the support car-
ried out during the driving of a tunnel have the following essential objectives:
- To obtain information about the actual behaviour of the rock mass under the specific
excavation and support methods being used.
- Early detection of unforeseen deformation of the rock mass in combination with the
support.
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