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CLIPEX |
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| Background | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
The construction of the
first shaft and the two galleries (URL and Test Drift) in the 1980’s
has shown the feasibility of underground excavation and construction.
Due to the pioneering nature of these works, this construction was performed
in a fully manual way. An actual repository would require mechanised excavation
techniques, allowing excavation at the higher excavation rate, at a lower
cost when large lengths are considered. The extension of the underground
research facility offered the opportunity to assess the performance of
such an improved underground construction technique – the first
deep (220 m) application of the wedge-block technique. A higher excavation
rate would also result in a reduced perturbation of the host rock and
hence, a reduced extent of the excavation damaged zone (EDZ). As the connecting
gallery has been excavated from the second shaft, a unique opportunity
was given to monitor the hydro-mechanical parameters ahead of the face
of the connecting gallery. This measurement programme was complemented
with model predictions to assess our capability to predict the hydromechanic
response of the host rock to the gallery excavation and construction.
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| Objective of the experiment | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
The objective of the
overall CLIPEX project is to assess the hydromechanical response
of the Boom Clay host rock to the gallery excavation and construction,
where a novel technique is applied. This is accomplished by an extensive
measurement programme, complemented with model predictions. The resulting
data should give us a better view on the (improved) performance of the
new technique, and on the performance of the current models describing
the hydro-mechanical response of the Boom Clay host rock to gallery excavation.CLIPEX
has concentrated on operational aspects (gallery construction and its
short-term influence on the host rock), rather than long-term safety aspects. |
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| Description of the experiment | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Design: |
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Geotechnical sensors were installed in two major zones in the Boom Clay: eight instrumented boreholes installed from the front of the Test Drift, and two instrumented boreholes installed from the second shaft. In addition, three rings of the lining of the connecting gallery were constructed with instrumented segments. Test Drift Front From the shotcreted front, four subzones were instrumented in May 1998 with pressure and displacement transducers. Each subzone contained two 30 m-deep boreholes: one for pressure measurements (blue on the figure), and one borehole to monitor displacements (purple). The first zone “A” was located in the axis of the connecting gallery. An extensometer was installed in borehole A1. In the borehole, six anchors were installed at a depth between 20 and 30 m. Each anchor, from the geotextile packer type, was connected with a displacement transducer located in the measurement head at the front.
The pore water pressures at six locations between 20 and 30 m were measured through filters integrated in the borehole casing of the pressure borehole A2, through the so-called multipiezometer. At three depths in this borehole, the total pressure was measured through miniature total pressure sensors which were also integrated in this borehole casing. At each depth, three sensors were installed.
The second zone “B” was located slightly inclined above the connecting gallery. The vertical deformation towards the gallery was to be monitored by an inclinometer in borehole B1. This inclinometer consisted of a chain of 10 3 m-long segments, each equipped with an electrolevel sensor, and was installed permanently in a plastic (ABS) inclinometer casing. The pore water and total pressures were measured through borehole B2, which had a design similar to A2.
The third zone “C” was located more inclined above the connecting gallery, and contained also an inclinometer in borehole C1, and a multipiezometer with integrated total pressure sensors in borehole C2. After completion of the excavation and lining of the gallery, the filters have been reconnected to the pressure transmitters and they continue to provide us with pore pressure measurements around the connecting gallery.The fourth zone “D” was located in the horizontal plane, towards the west (right when looking at the front of the Test Drift) with respect to the connecting gallery axis. The expected movement towards the gallery was measured through an deflectometer in borehole D1. This deflectometer consisted of a chain of 11 three m-long segments, connected to each other by angle transducers, and installed in a permanent way in the borehole casing. Borehole D2 was equipped with a multipiezometer with integrated total pressure sensors. As for C2, the filters of D2 have been reconnected to the pressure transmitters after completion of the excavation and lining of the gallery.
Instrumentation installed from the second shaft From the bottom part of the second shaft, two horizontal boreholes have been drilled and instrumented a few metres above the connection gallery (zone “E”). Borehole E1 is 30 m long and contains an in-place inclinometer with 15 2 m-long segments, equipped with electro-level sensors. Due to an unsufficient strength of the casing, we had to redrill this borehole, and equip it with a double casing (outer steel casing in addition to the inclinometer casing).Borehole E2 is 21 m deep and contains seven filters to measure the pore water pressure. Both instruments are still working.Some other instruments were also installed near the bottom of the second shaft, such as cells measuring the total pressure on the lining of the second shaft. This instrumentation is discussed in the part on the second shaft, but they showed also interesting results related to the construction of the connecting gallery.
Instrumented lining segments To monitor the pressure build-up on the lining of the connecting gallery, three rings (nos. 15, 30 and 50) were constructed with instrumented segments. All ten segments of these rings (except the key segments) were equipped with vibrating wire strain gauges cast in the concrete. In addition to the global strain, the configuration also allowed to measure intrados and extrados strains separately, so that bending of the segments could also be monitored.
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| Protocol/explanation : | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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| Instrumentation : | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| cf. supra (“description of the instrument”) | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Status/timing/planning : | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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| Associated works : | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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| Results of the experiment : | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
A lot of measurement data were obtained, and were compared with the model predictions. The performance of the sensors depended on the type of instrument. The porewater pressure measurements were very succesful, and showed the different stages during the excavation (pressure increase, pressure drop, up to suction, and finally atmospheric pressure). The total pressure sensors showed values similar to the porewater pressure, and therefore we could not derive any meaningful value for the effective pressure. Probably the measurement technique (integration of total pressure sensors in borehole casing) is not optimal, although the actual sensors performed quite well. The displacement measurements from the Test Drift front gave rather qualitative indications, as they were not completely adapted to the host rock environment. Inclinometer and deflectometer casing, as well as extensometer anchors and connection rods, should be adapted to the specific clay conditions.
The inclinometer E1, where we had improved the casing, gave very satisfying results, showing very clearly the clay subsidence due to the excavation of the mounting chamber and the connecting gallery. Also the piezometer E2 provided us with very valuable results.
The strain gauges in the lining segments are all still giving reliable strain data. They showed the stress rapid build-up of stress on the lining (within a few weeks), and also gave indications on bending of these segments, which seems to be related to the ring geometry. In order to include the creep and shrinkage behaviour of the concrete, an independent strain – stress characterisation (over one year) has been running.
When we compare the measurements with the model predictions, it appears that the current models underestimate the effects of the excavation. We are currently checking if newer models can give a better agreement with the observations.
Figure 11 Comparing the observed data with the model predictions shows a large underestimation of the excavation impact on the pore water pressures in the Boom Clay |
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| Conclusions : | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
The instrumentation programme has given a wealth of information on the behaviour of the Boom Clay around the excavation. The experience gained with the instrumentation will allow us to improve future instrumentation programmes (such as the PRACLAY tests). The actual measurements have also indicated the validity of the current models, and have pointed out the main issues to improve more these models :
Implications of the results
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| Bibliography : | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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