Project Description
Section 310 of the “Channel Tunnel Rail Link (CTRL) is located along the Thames river on a marsh area with 10 to 12 meters of soft alluvium. A new bridge was to be built for the Tank Hill road to cross the new high speed tracks. The construction of the tracks and of the access embankment to the bridge were not possible without soil treatment works as unacceptable settlement would occur.
The technique proposed by DGI-Menard was the application of Controlled Modulus Columns (CMC). A CMC network was installed in the soft soil down to the stiff layer. The typical grid is 1.85 * 1.85 m2 and the CMC diameter is 36 cm for the tracks and 1.00 * 1.00 m2 to 1.60 * 1.60 m2 for the Tank Hill road embankment resulting in a loading per CMC of about 20 tons.
The specifications required an equivalent Young modulus of 25 MPa which would result in the following maximum allowable settlement:
- For the tracks: 1.5 cm,
- For the embankment: 2.5 cm during construction and 2.5 cm during design life.
A transition layer made of a 500 mm thick, well-compacted granular material was then installed on top of the CMC network followed by the ballast and tracks. This Soil Improvement results in a final platform able to bear the foreseen tracks together with the train live loading (up to 150 kN/lm of track) and ensuring total and differential settlement acceptable for the train.
Vertical displacements where computed thanks to a 3D model (see above plot).
To assess the model parameters an initial full scale kentledge test was carried out a group of 9 CMC with the transition layer. In the end, the obtained settlement under the 40 kPa loading was less than 10 mm at the top of the transition layer, and the measured equivalent Young modulus of the complex soil + inclusion was higher than 31 MPa (compared to the specified 25 MPa).