It is well know that any kind of underground supporting system
should be able to assist the ground in supporting itself by building a
ground structure. The grouted rock bolt becomes an integral part of
the ground, thereby restricting its deformability by internal strengthening.
The description of interaction behaviors between the rock bolt
and ground is very critical to the evaluation of the reinforcing effect.
Unfortunately, the interaction mechanism of rock bolt and ground is
very complicated, and there is few ways to evaluate the performance
of rock bolt quantitatively at present. The rock bolting design is still
in an empirical and semi-empirical state. During a tunneling construction,
the wall convergence is a readily recordable indicator of
overall response of the ground, and the so-called convergence–
confinement approach has been widely accepted in tunneling. In
the convergence–confinement approach, the rock bolt or shotcrete is
expected to adding an equivalent internal pressure to the tunnel wall
[1–7]. This equivalent internal pressure is originated from the axial
force in case of grouted rock bolt, and it is related with the
deformable behavior of rock mass [8]. Obviously, the equivalent
internal pressure of grouted rock bolt should be estimated from the
interaction behavior because the axial force distributes a nonuniform
shear stress along the whole length. In order to obtain the
distribution of axial force, field monitoring is a good way and
sometimes it is necessary to ensure a rock bolting design. However,
the monitoring results could only provide limited information at the
design stage, and it is more expensive either to some degree in-situ.
The rock bolt design in NATM tunneling is based on the classification
of the ground in Japan and other countries. The monitoring results
are also considered to be necessary especially for the excavation in
soft rock. The measured distribution of the axial force along a rock
bolt is also considered as an important factor to adjust the rock
bolting design during tunneling construction [9].
A lot of field monitoring works were carried out on the rock
bolts installed in various rock formations [10–14]. Freeman [10]
monitored both the loading process of the bolts and the distribution
of the stresses along the bolts, and proposed the concept of
“neutral point”, “pick-up length” and “anchor length” first. At the
neutral point, the displacement of the ground and the rock bolt is
consider as the same, and the induced shear stress at the interface