Page 375 - Engineering Rock Mass Classification_ Tunnelling, Foundations and Landslides
P. 375
330 Engineering Rock Mass Classification
Residual Joint Condition Factor
The residual joint surface condition factor JCr is calculated now from Eq. (26.16).
JrC ¼ JWr JrS ð26:16Þ
JAr
where JWr , JSr , and JAr are residual values of large-scale waviness, small-scale smooth-
ness, and joint alteration factor, respectively. The reduction of JWr and JSr is based on
the concept of mobilized joint roughness and the equations are given as
If JW < 1, JWr ¼ 1; Else JrW ¼ JW ð26:17Þ
2 2 ð26:18Þ
If JS < 0:75, JSr ¼ 0:75; Else JSr ¼ JS
2 2
There is no reduction in JA.
Residual GSI Value and Strength Parameters
The residual GSIr is a function of Vbr and JCr , which can be estimated using Eq. (26.5).
Fracturing and shearing do not weaken the intact rocks (even if they are broken into
smaller pieces) so the mechanical parameters (qc and mr) should be unchanged. There-
fore the generalized non-linear criterion for the residual strength of jointed rock masses
can be written as
s1 ¼ s3 þ s3 þ nr ð26:19Þ
qc mbr qc sr
where mbr, sr, and nr are the residual constants for the rock mass. These constants can be
determined from a residual GSIr (Cai et al., 2007).
GSIr À 100
mbr ¼ mrÁ exp 28 ð26:20Þ
ð26:21Þ
GSIr À 100
sr ¼ exp 9
nr ¼ 1 þ 1 =15 À ð26:22Þ
2 eÀGSIr eÀ20=3
6
Because the rock masses are in a damaged, residual state, D ¼ 0 is used for the residual
strength parameter calculation.
CLASSIFICATION OF SQUEEZING GROUND CONDITION
Hoek (2001) classified squeezing ground conditions based on tunnel strain (ua/a) or
the ratio between rock mass strength and in situ stress (gH), as shown in Figure 26.4.
In very severe squeezing ground (ua/a >5%), the tunnel face may exhibit plastic extru-
sion due to the failure of rock mass all around the tunnel and the face has to be stabilized.
For a rock mass strength (qcmass) of 1.5 MPa and in situ stress of 13.5 MPa (gH), the ratio
(qcmass/gH) ¼ 0.11. Figure 26.4 shows that this corresponds to a tunnel strain of approx-
imately 10% and very severe squeezing ground condition should be anticipated.
