Environmental Engineering Reference
In-Depth Information
FIGURE 6.48
Interbedded sandstones and shales in a 60-m-high cut in the Morro Pelado formation (late Permian) (Highway
BR 116, km 212, Santa Cecilia, Santa Catarina. Brazil). The montmorillonite in the shale expands, fracturing the
sandstone; as the shale decomposes, support of the sandstone is lost and blocks fall to the roadway. Shot-
creting the slope retards moisture changes in the shale.
JRC represents the joint roughness coefficient and ranges from 20 to 0.
20
for rough undulating joint surfaces, (tension joints, rough sheeting, and
rough bedding)
JRC
10
for smooth undulating joint surfaces (smooth sheeting, nonplanar folia-
tion, and ibedding)
5
for smooth, nearly planar joint surfaces (planar shear joints, planar folia-
tion, and bedding)
The JRC results from tests and observations and accounts for the average value of the
angle j of the asperities, which affects the shear and normal stresses acting on the failure
surface. Hoek and Bray (1977) suggest that Barton's equation is probably valid in the range
for
0.01 to 0.03, applicable to most rock slope stability problems. Ranges for fric-
tion angles for various joint conditions are given in Table 3.34. If movement has occurred
along the joint in the past, the residual strength may govern.
Case 3 : Strength parameters for joints are also given as joint stiffness k and joint stiffness
ratio k s / k n . Joint stiffness is the ratio of shear stress to shear displacement or the unit stiff-
ness along the joint. Joint normal stiffness k n is the ratio of normal stress to normal dis-
placement or the unit stiffness across the joint (Goodman et al., 1968).
σ N /JRC
Applications
Block behavior is the most important consideration in the foundation design of concrete grav-
ity or arch dams, and in rock-slope stability analysis. Rigorous analysis requires accurate
 
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