Volume loss is one of the most important risks when boring a tunnel. This is particularly true when a tunnel is being constructed in soft soils. The risk of excessive volume loss, if materialised can lead to large consequences such as damage in buildings on the surface.
I believe it can be obtained as follows:
- In saturated clays, the volume of the measured settlement trough (2.5 * maximum settlement * i ; assuming Peck's Gaussian distribution of the settlement trough) may be considered representative of the volume loss occurring during the undained excavation of the tunnel.
- In sand /drained tunnel excavation, the dilatancy affects the equality of the volume of the settlement trough and the volume loss at the tunnel so the radial deformation towards the tunnel may be obtained using inclinometers and extensometers located closely to the tunnel. The volume loss may be obtained by summing the measured radial deformations along the tunnel perimeter.
Generally, for each type of tunneling and ground condition, representative volume loss values are reported in literature. In Cairo, Egypt, the ground loss was about 0.25% of the tunnel volume when the hydroshield TBMs were used in the Nile alluviums in the Greater Cairo Metro project - Lines 2 & 3.
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