Engineering Rock Mechanics

388 Design and analysis of underground excavations

ttt

Pillar characteristic (determined by uniaxial loading under

plane strain)

p(MPa) 5.0 8.0 10.0 11.1 11.4 10.9 10.0 8.7 3.0 0.5

eEx lo3 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 7.0 12.0

Country rock characteristic

(a) traction ~\\~~\\ free slot

(a) lMPa normal stress over pillar contact area

(determined from BEM analysis)

1: 8!,::;2 E-3 m

I? La;;

-02 E-3 m

Solution : plot p-E, characteristics for pillar &

country rock. Operating point is at the

intersection of the curves.

+-I

Pillar

Initial state of strain is -

Plane strain :

eZO = - [ p,- yp,] = 0.5 E-3

Final state of stress is

and the resulting strain is

1-2

E 1 -v

5 + APB

Country rock

The BEM results tell us that

6, = (66.52-3.602~~) E-3

Substituting into @

eZ = (1 1 .59-0.6003pB) E-3

This is the country rock characteristic.

Figure 20.29 The ground response curve concept illustrated through the analysis

of a tabular excavation with and without a natural supporting pillar.