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Recommended Values of Parameters for Volume Loss Analysis

Data needed for the determination of subsidence trough using the volume loss method:

Coefficient to calculate inflection point k

Soil or rock

k

cohesionless soil

0.3

normaly consolidated clay

0.5

overconsolidated clay

0.6 - 0.7

clay slate

0.6 - 0.8

quartzite

0.8 - 0.9

Percentage of volume loss VL

Technology

VL

TBM

0.5 - 1

Sequential excavation method

0.8 - 1.5

Several relationships were also derived to determine the value of lost volume VL based on stability ratio N defined by Broms and Bennermarkem:

where:

σv

-

overall stress along excavation axis

σt

-

excavation lining resistance (if lining is installed)

Su

-

undrained stiffness of clay

For N < 2 the soil/rock in the vicinity of excavation is assumed elastic and stable. For local plastic zones begin to develop in the vicinity of excavation, for a large plastic zone develops around excavation and for N = 6  the loss of stability of tunnel face occurs. Figure shows the dependence of stability ration and lost volume VL.

Literature:

Broms, B.B., Bennemark, H., 1967. Stability of clay at vertical openings. ASCE, Journal of Soil Mechanics and Foundation Engineering Division, SMI 93, 71-94.