1 INTRODUCTION
The engineering practice shows that the disturbance of tunnel excavation
changes the stress state of the surrounding rock and causes the
deformation of the surrounding rock. If the surrounding rock deformation
is not controlled by constructing a support structure, it will often be
difficult to stabilize the surrounding rock of the tunnel.1 In fact, after the continuous accumulation of
engineering experience, theoretical study, and experimental research,
the modern tunnel support theory has reached a consensus, that is, the
tunnel is a structure that integrates load, material, and structure.2 The surrounding rock of the tunnel is the primary
source of the load borne by the support structure, and it also shows a
certain bearing capacity. In the process of diagenesis, there are great
differences in the physical fields such as stress, groundwater, and
temperature, which lead to different properties of the rock and the
soil, and the mechanical response will significantly vary after being
disturbed by tunnel excavation. Developing the corresponding excavation
methods and support plans for different types of the surrounding rock is
considered an effective and controllable measure to control the
deformation of the surrounding rock and ensure the stability of the
surrounding rock of the tunnel.
The supporting mechanism of the support structure, the interaction
relationship between the surrounding rock and the support structure, the
selection of the timing of support, and the supporting effect of the
support structure under different surrounding rocks have been the focus
of researchers around the world, 3-8 among them the
action mechanism of the support structure is considered to be
particularly complex. The bolt is considered a very effective support
system in tunnel engineering. Although bolts are widely used, the
supporting mechanism of bolts has not been clearly identified. There are
many kinds of bolts for tunnels, among them, the most commonly used are
systematic rock bolts and steel pipes. Because of their grouting
function, steel pipes are often used to support or treat weakâbroken
surrounding rocks. Sometimes when the tunnel excavation has a great
impact on the shallow surface, steel pipes are often used for grouting
the surface or the slope. The systematic rock bolts are mostly used for
the initial support of tunnels. However, the existing support theories
mostly investigate the support effect of systematic rock bolts9-11 and rarely use the support mechanism of steel
pipes. Investigating the action mechanism of systematic rock bolts and
steel pipes is considered the basis of understanding their supporting
effect on surrounding rocks. Yanyi Yang 12,13 studied
the anchoring effect and the mechanism of systematic rock bolts on the
layered rock mass and proposed the constitutive relationship of the
anchored layered rock mass. He also pointed out that the bolt induces a
toughening and crack arrest effect on rock fractures, which thus reduces
the damage of the rock mass and improves the strength of surrounding
rocks. Yang et al 14,15 used the anchored layered rock
mass as the equivalent continuous medium and derived the constitutive
equation of the anchored layered rock mass. According to the analysis of
the formula, it is believed that the reinforcement effect of bolts on
the jointed rock mass primarily improves its shear strength and the
deformation capacity of joints. Zhang et al 16believed that the anchored rock bolts work together with the surrounding
rocks, and the bolt and the fractured rock mass reinforced by the bolt
can be considered as a damaged cylindrical bolt element, which along
with the surrounding damaged rock mass forms an anchoring damaged rock
mass element. During calculation, the stiffness of the cylindrical bolt
element is added to the stiffness matrix of the corresponding damaged
rock mass element in order to reflect the confinement effect of the bolt
on the deformation of the surrounding rock. Yang et al17 studied the deformation process of the anchoring
body of a layered rock by using laboratory experiments and concluded
that the anchoring effect of bolts on the layered rock mass primarily
improves the post-peak bearing capacity of the anchoring body and the
toughness of the anchoring layer and induces little influence on the
pre-peak mechanical properties. Wang et al 18 believed
that the existence and changes in the seepage pressure and fractures
change the damage characteristics of the rock mass, and its stiffness
decreases during the deformation process, whereas the bolts can improve
the mechanical and damage characteristics of the rock mass; this
improves its stiffness. They proposed the solution by using the concept
of added stiffness to reflect the anchoring effect of the bolt on the
fractured rock mass. Wei Zhang and Quansheng Liu 19-21have reported a comprehensive and systematic summary of the existing
theories of rock bolt reinforcement in the jointed rock mass and pointed
out that the study of the anchoring mechanism of the deep fractured rock
mass should collectively consider all factors, such as the effect of the
actual engineering application, the behaviors of the anchored rock mass,
the effect of anchoring components, etc. Zhang et al22,23 studied the mechanical characteristics and
failure features of the bolt-supported jointed rock mass by using the
bolt reinforcement tests on the rock mass with pre-existing flaws in the
laboratory. In order to solve the problem of the anchoring mechanism of
the rock and the soil, international scholars have also carried out a
lot of research work, primarily by applying the following two aspects:22,24-32 the load transfer mechanism of bolts and the
anchoring effect of bolts, by using methods such as laboratory
simulation, theoretical analysis, in-situ field test, etc., and the
research results have enriched and deepened the understanding of the
reinforcement mechanism of bolts on the rock and soil structure.
Therefore, similar simulation materials were used to form anchored
specimens with pre-existing holes. Then, laboratory uniaxial compression
tests and CT scan were carried out to study the strength, deformation
characteristics, and crack propagation laws. The numerical simulation
method is used for the coupled stress-damage simulation of fracturing in
the rock. By the analysis of different supporting characteristics, in
this paper, we investigate the supporting mechanism of support
structures on the layered and weak rock tunnel, which provides a
theoretical basis for field application.