A tunnel is to be driven that will be provided with a reinforced concrete lining for permanent

support. The tunnel is to be $9$ m wide, $5$ m in height and will be $50$ m below the ground surface.

The tunnel will be horizontal and will run from roughly southwest to northeast, bearing $35,$

through hard rock. Unconfined compression tests on intact cores of this rock indicate UCS$=100$

to $120$ Mpa. The average density of the rock is $2900$ kg$/$m$3$

$.$ The pore pressure $($actually water

pressure in the fractures of course$)$ measured at the depth and vicinity of the tunnel is

approximately $300$ kPa.

A representative core of the rock is shown below. There are three significant joint sets identified

from the mapping.

Set Strike Dip Aperture Spacing Joint surface condition and filling

A $4530<1$mm $0\mathrm{.}5$ m moderately rough, no filling $($slight weathering and staining$)$

B $120152-3$ mm $2$ m smooth, weathered, some gouge

C $185402-3$ mm $1\mathrm{.}5$ m smooth, weathered, some gouge

a$)$ Determine the Rock Mass Rating. How soon after excavation must the reinforced lining be

placed?

b$)$ Estimate the GSI. Clearly state any assumptions. If $3=0$ immediately after

construction, comment on the likelihood of failure using the Hoek$-$Brown failure criterion.