Numerical study on tunnel damage subject to blast-induced shock wave in jointed rock masses
Introduction
An explosion, such as accidental explosion, drill and blast excavation or weapon attack, has large effect to adjacent underground structures. Generally, the effect includes overpressure, thermal effects, energized projectiles (fragments, debris, and missiles), ground shock, and caterings (Ronald et al., 2010). Especially, the ground shocks are of great interests to engineers concerning the design of underground and surface structures. Kutter et al. (1988) noted that the direct loading by a shock wave created due to explosion is the principal mechanism to cause damage to underground structures. Therefore, research on effects of blast-induced shock wave to underground tunnel damage is both significant and practical (Zhao et al., 1999).
Studies have shown that the peak particle velocity (PPV) is the most representative parameter to describe the ground motion and tunnel response (Dowding, 1984). Extensive studies on damage of non-supported underground tunnel in terms of definition of tunnel damage and the threshold values of PPVs have been performed (Persson, 1997, Li and Huang, 1994, Hendron, 1977, Coates, 1981, Kartuzov et al., 1975, Oriad, 1972, Phillips et al., 1992, Siskind, 1997). For damage of rock tunnel with support, studies carried out by Stjern and Myrvang (1998) and Ortlepp and Stacey (1998) have shown that PPVs up to 1 m/s will not cause any measurable damage to the tunnel. For lined tunnels, Dowding (1984) suggested that the threshold value of PPV was roughly double that for unlined tunnels.
However, most of the definitions are not well defined and various terms, describing of damage, often have significant differences in definition and practical meaning (Zhou, 2011). Little considerations of effects of discontinuities on tunnel damage is included in these experiment-based studies. Generally, rock mass contains various discontinuities such as bedding planes, foliation, faults, and joints. The behavior (deformation characteristics, stress development, etc.) of rock mass around the tunnel is mainly controlled by the spatial and mechanical properties of the discontinuities (Tülin, 2009).
Model tests, analytical methods, and numerical methods are generally employed to evaluate the behavior of rock mass around tunnel in jointed rock masses. Few model tests were performed in jointed rock masses because of the limitation in joints setting. Analytical methods, considering block behavior in tunneling, are mainly based on the block theory (Goodman and Shi, 1985). Although the capability of the analysis is increased with improvement of the block theory with respect to stress conditions, decrease of forces acting on the key block (Brady and Brown, 2004) and use of sophisticated joint models (Pötsch, 2002), they are not applicable to cases where ground shows stress induced failure or more complicated problems are involved. Compared with theoretical and experimental studies, numerical modeling provides a convenient, economical approach to study underground explosions, especially for complicated cases where experiments are difficult and expensive to conduct and theoretical solutions are impossible to derive (Zhu et al., 2011).
In this study, numerical modeling on the damage of existing circular tunnel subject to blast-induced shock wave in jointed rock mass was performed with DEM-based code UDEC. The disturbed zones around tunnel and PPVs at tunnel surface were employed to analysis the damage of tunnel. The aim of this study is to evaluate the effects of joint spatial and mechanical properties, initial stress of rock mass, magnitude of shock wave amplitude, and bolt supports to damage of tunnel.
Section snippets
UDEC model
In this study, before performing UDEC modeling, an AUTODYN-2D modeling was carried out firstly to generate blast-induced shock waves, which would be applied as the velocity boundary conditions in UDEC model. The shock waves were generated by detonation of high explosive TNT. Fig. 1 shows the configuration of AUTODYN-2D model, the width and height of this model are both 30 m. The radius of TNT material r, as shown in Eq. (1), depends on the scaled distance (SD) (Zhou, 2011) with assumption that
Joint orientation
In this numerical modeling on effects of joint orientation on damage of existing circular tunnel subject to blast-induced shock wave, two intersected persistent joint sets are included in the model. One joint set is fixed in horizontal direction, while the orientation of the other joint set varies between 15° and 90° from horizontal direction with an interval of 15°. Thus, the expression of “joint dip angle” adopted in the following only represents the orientation of second joint set. The
Tunnel depth
The depth of tunnel is assumed to be 25, 50, 100, 200 m, respectively, and the lateral initial stress coefficient is fixed to be 0.8. The joint spacing is 0.5 m for both two joint sets, while the joint dip angle and joint normal stiffness kn (kn = 2ks) are equal to 60° and 50 GPa, respectively.
Fig. 11, Fig. 12 show the disturbed zones around tunnel and PPVs at tunnel surface, respectively, in terms of different tunnel depth, where the scaled distance is assumed to be 2.5 m/kg1/3 after 20,000
Effect of scaled distance on the tunnel damage
This numerical modeling is performed to investigate the damage of existing circular tunnel subject to blast-induced shock wave with different scaled distances. The joint spacing, joint dip angle and joint normal stiffness kn (kn = 2ks) are equal to be 0.5 m, 60° and 50 GPa, respectively. The tunnel depth and lateral initial stress coefficient employed in this numerical modeling are assumed to be 25 m and 0.8, respectively.
Fig. 17 shows the disturbed zones around circular tunnel in terms of different
Numerical modeling on the damage of existing circular tunnel with bolts support
The support for tunnel could restrict tunnel from damage. In this numerical modeling, the tunnel will be strengthened by bolts with different bolt length and bolt number. To facilitate the comparison, the parameters adopted in this section, such as joint dip angle, joint spacing, joint stiffness, tunnel overburden and lateral initial stress coefficient, are the same as those used in Section 5, except that the scaled distance is fixed to be 0.75 m/kg1/3. The parameters of bolts are listed in
Discussion and conclusions
In this numerical study, the damage of tunnel is mainly caused by the slipping and opening of rock joints. This is consistent with field observations that initial tunnel damage is a result of falling loose rocks, rather than damage of rock material created by the shock wave (Zhou, 2011).
It is extremely important to have a common understanding of the definition of dynamic damage before any meaningful discussion of tunnel damage can be made. Unfortunately, there currently exist no established
Acknowledgments
Deng XF receives financial support from the China Scholarship Council and the Chinese Central University Fundamental Research Funds (SWJTU11ZT33) for the work performed at EPFL Switzerland and NTU Singapore.
References (31)
- et al.
Effects of multiple parallel fractures on apparent wave attenuation in rock masses
Int. J. Rock. Mech. Min. Sci.
(2000) - et al.
Characteristics of surface ground motions induced by blasts in jointed rock mass
Soil Dyn. Earthq. Eng.
(2001) - et al.
Performance of tunnel support underground large deformation static and dynamic loading
Tunn. Undergr. Sp. Tech.
(1998) - et al.
Improvement of rock properties by bolting in the plastic zone around a tunnel: a numerical study
Int. J. Rock Mech. Min. Sci. Geomech. Abstr.
(1993) - et al.
The disturbed zone around tunnels in jointed rock masses
Int. J. Rock Mech. Min. Sci.
(1997) - et al.
The influence of blasting on grouted rock bolts
Tunn. Undergr. Sp. Tech.
(1998) - et al.
Rock dynamics research related to cavern development for ammunition storage
Tunn. Undergr. Sp. Tech.
(1999) - et al.
Validation study of the distinct lattice spring model (DLSM) on P-wave propagation across multiple parallel joints
Comput. Geotech.
(2011) - AUTODYN, 2005. Revision 4.3 Century...
- et al.
Rock Mechanics for Underground Mining
(2004)