2. OPENINGS IN ROCK. Stress analysis differs for two rock groups: sound,
nonswelling rock that can sustain considerable tensile stresses, and
fractured blocky, seamy, squeezing, or swelling rock. For detailed
explanations of these rock groups, see Chapter 1.
a. Sound Rock. Determine stresses surrounding tunnels or openings in
intact, isotropic rock, such as crystalline igneous types, or homogeneous
sandstone and limestone, by elastic analyses. Use the methods of Reference
20, Design of Underground Openings in Competent Rock, by Obert, et al.
For these materials, stresses in rock surrounding spheroidal
cavities are lower than those for tunnels with the same cross section. Use
elastic analyses to determine the best arrangement of openings and pillars,
providing supports as required at locations of stress concentrations. For
initial estimates of roof pressure, Table 1 (Reference 21, Rock Tunneling
with Steel Supports, by Proctor and White) may be used.
b. Broken and Fractured Rock. Pressure on tunnels in chemically or
mechanically altered rock must be analyzed by approximate rules based on
experience. For details, see Reference 21.
c. Squeezing and Swelling Rocks. Squeezing rocks contain a
considerable amount of clay. The clay fraction may be from non-swelling
kaolinite group or from highly swelling montmorillonite group. These
rocks are preloaded clays and the squeezing is due to swelling. The
squeeze is intimately related to an increase in water content and a
decrease in shear strength.
LOADS ON UNDERGROUND OPENINGS IN ROCK.
a. Vertical Rock Load. Table 1 gives the height of rock above the
tunnel roof which must be supported by roof lining.
b. Horizontal Pressures. Determine the horizontal pressure P+a, on
tunnel sides by applying the surcharge of this vertical rock load to an
active failure wedge (see diagram in Table 1). Assume values of rock shear
strength (see Chapter 3 for a range of values) on the active wedge failure
plane, which allow for the fractured or broken character of the rock.
Evaluate the possibility of movement of an active failure plane that
coincides with weak strata or bedding intersecting the tunnel wall at an
c. Support Pressures as Determined From Rock Quality. As an alternate
Classification of Rock Masses for Tunnel Support, by Barton, et al., to
determine required support pressures as a function of rock mass quality "Q".
The analysis incorporates rock quality designation (RQD) and various joint
properties of the surrounding material, and is applicable for sound or
fractured rock. Results may be used directly for evaluating type of roof or
wall support required.