Subgrade reaction modulus of rock masses under the load of single and multiple footings

ABSTRACT

Subgrade reaction modulus is an essential parameter in the structural design of footings. Determination of this parameter is difficult, especially in rock masses containing an irregular distribution of discontinuities. In this paper, the subgrade reaction modulus of rock masses obeying Hoek-Brown failure criterion was investigated. It was assumed that the rock masses are under the pressure of a single isolated footing and also two adjacent strip footings. The subgrade reaction modulus was determined from the pressure-settlement curve of the rock masses. This study’s results were presented in two forms, which include the subgrade reduction factor and the interference coefficient. The obtained results show that the subgrade reaction modulus of rock masses may be smaller than 50% of intact rocks. Likewise, based on the distance between the multiple footings, the subgrade reaction modulus may be in the range of 0.7 to 1.3 times the subgrade reaction modulus in the case of a single isolated footing.

KEYWORDS:

• Hoek-Brown
• interference coefficient
• multiple footing
• rock foundation
• subgrade reaction modulus

Notations

The following symbols were used in this paper.

SRM:	=	Subgrade reaction modulus;
ks:	=	SRM of the intact rock;
kj:	=	SRM of the jointed rock;
ks(mass):	=	SRM of the rock mass obeying the Hoek-Brown failure criterion;
Jf:	=	Joint reduction factor;
Jmass:	=	Subgrade reduction factor;
Scr:	=	Critical spacing;
S:	=	Distance between two footings;
α:	=	Interference coefficient;
ks(int):	=	SRM of the rock mass subjected to the load of a footing in the presence of the adjacent footing (obtained from the pressure-settlement curve);
ks(iso):	=	SRM of the rock mass subjected to the load of a single isolated footing (obtained from the pressure-settlement curve);
Sd:	=	Spacing of the joints;
Id:	=	Inclination of the joints;
ϕj:	=	Friction angle of the joints;
cj:	=	Cohesion of the joints;
Kn:	=	Normal stiffness of the joints;
Ks:	=	Shear stiffness of the joints;
γ:	=	Unit weight of the rock mass

Disclosure statement

No potential conflict of interest was reported by the author(s).