Stresses in the shotcrete lining and rock anchors may be reduced significantly by delaying subsequent layers of shotcrete (except initial layers), but no later than the stand-up time. Instrumentation for the measurements of stress and deformation in the roof and the walls of a cavern or in tunnels is a must to ensure a safe support system. Instrumentation would also provide feedback for improvements in the designs of such future projects. Location of instrumentation should be judiciously selected depending upon the weak zones, rock mass quality, and intersection of openings.

Example 12.1

Two parallel road tunnels are constructed for six lanes in basalt. The tunnels are D-shaped with diameter (B) of approximately 16 m and with 2 m high side walls with clear spacing of 20 m. The maximum overburden (H) is 165 m. The rock mass parameters are RMR = 73, Q = 10, J_{a} = 1.0, J_{r} = 3.0, and J_{w} = 1.0 (minor seepage from side walls). The construction engineers want a rapid rate of tunneling and life of the support system should be 100 years. The UCS of SFRS is 30 MPa and its flexural strength is 3.7 MPa.

The short-term support pressure in the roof may be assessed by following correlation (Eq. 6.6) for the arch opening, given by Goel and Jethwa (1991):

${\text{p}}_{\text{roof}}=\frac{\text{7.5}{\text{B}}^{0.1}\phantom{\rule{0.25em}{0ex}}{\text{H}}^{0.5}-\text{RMR}}{20\phantom{\rule{0.25em}{0ex}}\text{RMR}}=\frac{7.5{\times 16}^{0.1}{\times 165}^{0.5}-73}{20\times 73}=0.037\phantom{\rule{0.25em}{0ex}}\text{MPa}$

The ultimate support pressure is read by the chart (Figure 8.2) of Barton et al. (1974) as follows (the dotted line is observed to be more reliable than correlation).

${\text{p}}_{\text{roof}}\phantom{\rule{0.25em}{0ex}}=\phantom{\rule{0.25em}{0ex}}0.9\times 1\times 1{\text{kg}/\text{cm}}^{2}\phantom{\rule{0.25em}{0ex}}\text{or}\phantom{\rule{0.25em}{0ex}}0.09\phantom{\rule{0.25em}{0ex}}\text{MPa}$

(The rock mass is in non-squeezing ground condition (H < 350 Q^{1/3}) and so f′ = 1.0. The overburden is less than 320 m, so f = 1.0.)

It is proposed to provide the SFRS (and no rock bolts for faster rate of tunneling). The SFRS thickness (t_{fsc}) is given by the following correlation (using Eq. 12.10):

$\begin{array}{cc}{\text{t}}_{\text{fsc}}& \frac{0.6\phantom{\rule{0.25em}{0ex}}{\text{B p}}_{\text{roof}}}{2\phantom{\rule{0.25em}{0ex}}{\text{q}}_{\text{fsc}}}\phantom{\rule{0.25em}{0ex}}=\frac{0.6\times 1600\times 0.09}{2\times 5.5}\phantom{\rule{0.25em}{0ex}}=8\phantom{\rule{0.25em}{0ex}}\text{cm}\\ & \phantom{\rule{0.25em}{0ex}}\phantom{\rule{0.25em}{0ex}}\phantom{\rule{0.25em}{0ex}}\phantom{\rule{0.25em}{0ex}}\phantom{\rule{0.25em}{0ex}}\phantom{\rule{0.25em}{0ex}}\phantom{\rule{0.25em}{0ex}}=16\phantom{\rule{0.25em}{0ex}}\text{cm}\phantom{\rule{0.25em}{0ex}}(\text{near portals})\end{array}$

The tensile strength of SFRS is considered to be about one-tenth of its UCS, so its shear strength (q_{sc}) will be approximately 2 × 30/10 = 6.0 MPa, but we will say 5.5 MPa (uniaxial tensile strength is generally less than its flexural strength). Past experience reflects the same information.