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 |
normally 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 the excavation and for N = 6 the loss of stability of tunnel face occurs. The 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.