Technical Note
Physical model test and numerical analysis on the behavior of stratified rock masses during underground excavation

https://doi.org/10.1016/j.ijrmms.2011.11.001Get rights and content

Introduction

Stratified rock masses, formed by preexisting original materials on the earth crust, are distributed broadly over land area. During the forming process of this kind of rock mass, differences in material composition, particle size, color and fabric result in rock stratification. Consequently, one set of dominant structure plane, called a bedding plane, is generated. In addition, it is common to encounter interstratified joints, caused by geotectonic movement, cutting through rock layers. The intersections of the two types of discontinuities form the so-called stratified rock masses.

Today, many underground constructions, such as tunnels for civil engineering projects and mining excavations, have to be constructed in stratified rock masses. The rock mass structure of a stratified rock mass plays a vital role on failure of an underground opening during excavation and in operation. In particular, it is known that discontinuities will affect the behavior of stratified rock masses and special considerations, e.g. rock bolting and/or heavy lining, have to be made when the stratified rock masses are not stable [1]. A general way to investigate the failure modes and deformations of underground excavations is to perform physical model tests and/or numerical simulations. Many studies have been conducted to investigate the mechanical behavior of jointed rock masses. Terzaghi [2] used the trap door test to study the transfer of stress during tunnel excavation. He has suggested a theory of stress calculation to predict the magnitude of a vertical load resulting from tunneling in the trap door test. Murayama [3] have studied the changes of stresses during tunnel excavation using frictional aluminum rods to simulate rock mass in the trap door test. Results of laboratory model studies on rock like materials [4], [5], [6], [7] have shown that many different failure modes are possible with jointed rock and that the internal distribution of stresses within the jointed rock mass can be highly complex. For the blocky structure cut by joints, key block theory has been suggested by Goodman and Shi [8] to determine the stability of rock blocks around underground excavations. Numerical simulations of jointed rock blocks based on the finite element method with Goodman's joint element [9] and the distinct element method [10], [11] have shown anisotropic, scale dependent mechanical behavior of jointed rock masses [12], [13], [14]. Jeon et al. [15] have examined the behavior of deformation of a tunnel in a jointed rock mass using a scaled model test. Wu et al. [16] have performed laboratory loading tests on inclined layered block systems, formed by assembling frictional aluminum blocks and rods of different dimensions, using the trap door test to study the mechanical behavior of the inclined layered blocks. They have shown the influence of the dip angle of the layered blocks on the stability of the system. They have also shown the effect of the dip angle of layered jointed rock blocks on the stress arching during excavations by the Discontinuous Deformation Analysis (DDA) method. Based on biaxial tests performed on physical models of rock with persistent and/or non-persistent joints, Kulatilake et al. [17] and Prudencio and Van Sint Jan [18] have investigated the strength and failure modes of jointed rock masses. Jiang et al. [19] have investigated the effect of joint geometry on deformational behavior of underground openings. A numerical code called RFPA has been used to study the influence of dip angle of layered joint systems on tunnel stability [20]. Some model tests and numerical results have shown that the stratified rock masses can exhibit complicated failure modes such as sliding or rotation of ‘virtual blocks’ [18], [21]. This failure mode cannot be explained by Terzaghi theory, which is applicable to continuous media or traditional key block theory that only investigates single blocks.

The most important feature of stratified rock masses is the anisotropic rock mass mechanical behavior resulting from occurrence of highly persistent bedding planes, covering a large area, and interstratified joints. In performing physical model tests and numerical analysis to simulate stratified rock mass mechanical behavior, one should consider the specific structure of the stratified rock mass and the excavation process to capture the effect of the orientation and mechanical properties of discontinuities on stability of the rock mass. To obtain realistic results for a stratified rock mass during underground excavations, rock blocks of different sizes representing the variable discontinuity structure of the rock mass should be tested at significant stress levels under different excavation conditions. With in situ tests, such an experimental program would be difficult, time-consuming and expensive [17]. Therefore in this research, at the laboratory level, as a physical model test, physical experiments on assemblages of aluminum blocks composed of weak portions of the model material interspaced with model joints are performed by the wax door test to study the mechanical behavior of stratified rock masses. To study the effects of discontinuities on the behavior of stratified rock masses numerically, the Universal Distinct Element Code (UDEC) [22] is applied. By comparing simulation results with experiments, the failure modes and mechanism of stratified rock masses are deduced.

This paper presents an approach, which incorporates the “virtual block” concept for stratified rock masses with the UDEC results, to improve the interpretation of stability around underground openings. This potentially represents a significant advance over earlier stratified rock engineering approaches, which did not consider the virtual block concept. To demonstrate the sensitivity of stratified rock mass structure on model test and numerical modeling results, the influence of the dip angle of discontinuities and mechanical properties of weak seams on stratified rock mass stability is explored in this research. Moreover, displacement distribution and stress arching are studied as well.

Section snippets

Test materials for the stratified rock mass

Ideally, the model material should have mechanical properties that could reproduce. In order to simulate the behavior of a stratified rock mass, aluminum blocks and rods of different dimensions are used in a wax door test as shown in Fig. 1. The model material used to simulate the weak seam of bedding planes is a mixture of talc, gesso and water, mixed in the proportion of 100/15/16.5 by weight. The underground excavations in the stratified rock mass are simulated by heating the wax door with

Numerical simulation of physical model test using UDEC

To provide an insight into the failure mode and mechanism of underground openings in stratified rock masses, UDEC [22], a discrete element method, is adopted to numerically simulate the aforementioned physical model problems and to analyze the stability of underground excavations. It can accommodate a large number of discontinuities and it permits the modeling system to undergo large geometrical changes through the use of the contact updating scheme [22], [26]. However, it is difficult in UDEC

Analysis of the results

Fig. 5 shows the failure mechanisms/modes of the physical and numerical models after excavation for Series-1 tests. As indicated in Fig. 5, it is clear that the failure modes obtained by UDEC coincide quite well with the physical model test results, and the structure of stratified rock mass plays a vital role on the failure modes during underground excavations. Note that tests do not allow observing the influence of dilatancy on the rock mass behavior in this paper. Fig. 7 shows the failure

Conclusions

The paper presented laboratory physical model tests and UDEC numerical simulations conducted for evaluating the effects of the structure of a stratified rock mass on its failure mode and mechanism around underground openings. The numerical results on failure modes were found to be agreeable to physical model test results. The paper showed how “PhotoInfor” technique can be used to monitor deformations around underground excavations of physical models. The physical model test results and

Acknowledgment

The study was sponsored by the State Key Laboratory for Geo-Mechanics and Deep Underground Engineering (No. SKLGDUEK0912) and supported by Kwang-hua program (2010), the national Natural Science Foundation of China (No. 41172249) and the Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT,IRT1029). These supports are greatly acknowledged and appreciated.

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