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 N∈ < 2,4 local plastic zones begin to develop in the vicinity of excavation, for N∈ < 4,6 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.

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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.