The Coefficient of Tunnel Broken Zone and Field Test

The coefficient of tunnel broken zone combined with the max thickness is used to descript the loosening zone distribution after excavation, which defined to the ratio of broken zone area to tunnel cross section. With 16 operation modes designed by Orthogonal test, coefficient of the broken zone can be obtained with numerical methods. The findings suggest that the order of each factor is: surrounding rock grades, ratio of depth to span, side pressure coefficient, buried depth, span length. Gonghe tunnel of Yu-Xiang highway is used to introduce the basic flow of tunnel broken zone coefficient test. Results show that the broken zone coefficient is closely related to geological conditions and excavation methods and the scope of broken zone can effectively reduce by selecting suitable ratio of depth to span and excavation method.


INTRODUCTION
Influencing factors of tunnel loosening zone include three aspects, geological conditions, section structures and disturbance influence.The thickness of loosening zone is used to determine the size of loosening zone; however this index cannot fully reflect the distribution of loosening zone in the surrounding rock.Tunnel broken zone coefficient, the ratio of loosening zone area to tunnel cross section is introduced to evaluate the loosening zone.Dong et al. (1991) analyzed the main influencing factors of loosening zone with physical simulation experiment; Xia (2009) studied the influence of depth (ground stress), mechanical parameters of rock (cohesion and internal friction angle) and ratio of depth to span; Sun (2006) analyzed the influencing of excavation cross-sectional area, height and span; Xiao et al. (2010) researched the influence of blasting on loosening zone based on the in-situ testing results.Five influencing factors (Liu and Song, 2003;Jing et al., 1999;Zhao et al., 2004), surrounding rock grades (Wang et al., 2007), depth, span length, ratio of depth to span (Chen et al., 2011) and side pressure coefficient are considered in this study, with 16 operation modes designed by orthogonal test (Fang and Changxin, 2001), thickness and coefficient of the loosen zone can be obtained by numerical methods (Wu, 2005).The importance degree of influencing factors is determined by Range analysis (Li et al., 2004).In-situ testing of Gonghe tunnel in Chongqing China is used to introduce the general methods and processes of loosening zone coefficient field test.
In this study, the coefficient of tunnel broken zone combined with the max thickness is used to descript the loosening zone distribution after excavation, which defined to the ratio of broken zone area to tunnel cross section.With 16 operation modes designed by Orthogonal test, coefficient of the broken zone can be obtained with numerical methods.The findings suggest that the order of each factor is: surrounding rock grades, ratio of depth to span, side pressure coefficient, buried depth, span length.Gonghe tunnel of Yu-Xiang highway is used to introduce the basic flow of tunnel broken zone coefficient test.Results show that the broken zone coefficient is closely related to geological conditions and excavation methods and the scope of broken zone can effectively reduce by selecting suitable ratio of depth to span and excavation method.

COEFFICIENT OF BROKEN ZONE
Coefficient of broken zone (Fs), the ratio of loosening zone area (Ss) to tunnel cross section (Sd).At the assumption of the excavation section is circular (Fig. 1), radius equal to r 1 , the surrounding rock is isotropic and homogeneous rock and the horizontal stress is equal is equal to vertical stress (the side pressure coefficient is 1); the radius of loosening zone is r 2 , thickness Rs = r 2 -r 1 and Fs is defined by Eq. ( 1) and (2):  ( ) Influencing factors of broken zone coefficient: Mechanical parameters of rock: Compressive strength of rock mass is significantly larger than the tensile or shear strength, we usually just consider the influence of rock compressive strength.Based on mohrcoulomb criterion, the compressive strength of rock Eq. ( 3) can be defined with cohesion and internal friction angle.Research shows that the thickness of loosening zone is decreases with the increase of cohesion or internal friction angle: Buried depth: Before excavation, surrounding rock is in equilibrium state under triaxial stresses.After excavation, if concentrated stress exceeds surrounding rock strength, surrounding rock will break and reach a new equilibrium state of triaxial stresses in the deep.Due to that, the loosening zone is developed with the increase of buried depth and decreases with big strength.When the concentrated stress is less than its strength, the surrounding rock is in stable elasto-plastic state and no loosening zone exists.

Construction of tunnel section:
The influence factors of tunnel section include excavation cross-sectional area, height and span, section shape.Study shows that the Loosening Zone increases with the excavation area; both sides of tunnel the loose areas will rapidly spread with the section height increases; with the excavation section span increases, the loosening zone become larger.As other section shape may cause unbalance distribution of stresses, three-centered circular are usually used.

Side pressure coefficient:
The side pressure coefficient (S 2 ) is the ratio of horizontal stress to vertical stress.Measurement data at home and abroad shows that, the    Numerical simulation: Use finite element software to simulation the 16 condition.The surrounding rock is isotropic and homogeneous, section shape is Threecentered circular (Fig. 3), the boundary of model is 5 times bigger than the span length of tunnel, failure     (JTG D70-2004).Table 3 shows orthogonal experimental operation modes.
Range analysis: By orthogonal experiments and range analysis, surrounding rock grades T is the most important factor, following by ratio of depth to span S 1 , side pressure coefficient S 2 , depth H and span length D. Table 4 shows the range analysis of broken zone coefficient.

PRINCIPLE AND PROCESS OF TEST
Basic principle of test: Ground-Penetrating Radar (GPR) is a geophysical method that uses radar pulses to image the subsurface.This nondestructive method uses electromagnetic radiation in the microwave band (UHF/VHF frequencies) of the radio spectrum and detects the reflected signals from subsurface structures.GPR uses high-frequency (usually polarized) radio waves and transmits into the ground.When the wave hits a buried object or a boundary with different  In the case that distances between transmitting and receiving antenna are much closed, the formula of depth of object is:

Fig. 2 :
Fig. 2: The distribution of broken zone with different side pressure coefficient (S 2 )

Fig. 3 :
Fig. 3: Mesh diagrams for finite element models criterion is DP.The mechanical parameters of rock are determined by the Code for Design of Road Tunnel (JTG D70-2004).Table 3 shows orthogonal experimental operation modes.

Fig. 4 :
Fig. 4: Test line of section dielectric constants, the receiving antenna records variations in the reflected return signal.The time of radio waves travel:

Table 1 :
Five factors and 4 level of test

Table 2 :
Physical and mechanical parameters of rock masses T: Surrounding rock grades; H: Buried depth; D: Span length; S 1 : Ratio of depth to span; S 2 : Side pressure coefficient

Table 3 :
Orthogonal experimental operation modes

Table 4 :
Range analysis of broken zone coefficient

Table 5 :
The section of measuring BRZ

Table 6 :
The max thickness of broken zone at each section