Tunnel Squeezing Deformation Control and the Use of Yielding Elements in Shotcrete Linings: A Review

Shotcrete lining shows high resistance but extremely low deformability. The utilization of yielding elements in shotcrete lining, which leads to the so-called ductile lining, provides a good solution to cope with tunnel squeezing deformations. Although ductile lining exhibits great advantages regarding tunnel squeezing deformation control, little information has been comprehensively and systematically available for its mechanism and design. This is a review paper for the purpose of summarizing the development history and discussing the state of the art of ductile lining. It begins by providing a brief introduction of ductile lining and an explanation of the importance of studying this issue. A following summary of supporting mechanism and benefits of ductile lining used in tunnels excavated in squeezing ground conditions is provided. Then, it summarizes the four main types of yielding elements applied in shotcrete lining and introduces their basic structures and mechanical performances. The influences of parameters of yielding elements on the supporting effect are discussed and the design methods for ductile lining are reviewed as well. Furthermore, recommendations for further research in ductile lining are proposed. Finally, a brief summary is presented.

A simplified approach to reliability evaluation of deep rock tunnel deformation using First-Order Reliability Method and Monte Carlo simulations

abstract: The Monte Carlo simulation (MCS) and First-Order Reliability Method (FORM) provide a reliability analysis in axisymmetric deep tunnels driven in elastoplastic rocks. The Convergence-Confinement method (CV-CF) and Mohr-Coulomb (M-C) criterion are used to model the mechanical interaction between the shotcrete lining and ground through deterministic parameters and random variables. Numerical models synchronize tunnel analytical models and reliability methods, whereas the limit state functions control the failure probability in both ground plastic zone and shotcrete lining. The results showed that a low dispersion of random variables affects the plastic zone's reliability analysis in unsupported tunnels. Moreover, the support pressure generates a significant reduction in the plastic zone's failure, whereas the increase of shotcrete thickness results in great reduction of the lining collapse probability.

A 3DEC Numerical Analysis of the Interaction Between an Uneven Rock Surface and Shotcrete Lining

Rock tunnels excavated using drilling and blasting technique in jointed rock masses often have a very uneven and rough excavation surface. Experience from previous studies shows that the unevenness of a rock surface has a large impact on the support effect of shotcrete lining. However, clear conclusions regarding the effect of 2D and 3D uneven surfaces were not obtained due to limited studies in the literature. The numerical analyses reported in this paper were made to investigate the influence of the surface unevenness of a circular tunnel opening on the support effect of shotcrete using a 3D numerical code (3DEC). The models were first calibrated with the help of observations and measured data obtained from physical model tests. The influential factors were investigated further in this numerical study after calibration had been achieved. The numerical analyses show that, in general, the unevenness of a tunnel surface produces negative support effects due to stress concentrations in recesses (compressive) and at apexes (tensile) after excavation. However, shotcrete sprayed on a doubly waved uneven surface has better support effect compared to shotcrete sprayed on a simply waved tunnel surface. The development of shear strength (specifically frictional strength) on the uneven interface between the shotcrete and the rock contributes to this effect, in the condition where bonding of the shotcrete does not work effectively. The interface is a crucial element when the interaction between the rock and shotcrete is to be simulated. When an entire tunnel surface is covered by shotcrete with high modulus, more failures will occur in the shotcrete especially when rock surface is uneven. Based on the numerical model cases examined, some recommendations on how to incorporate tunnel surface conditions (2D or 3D unevenness) in the design of a shotcrete lining are given.

Three-dimensional Numerical Investigation of the Interaction Between Twin Tunnels

The construction of twin tunnels is an obligatory guideline and a prevailing practice in either conventional or mechanized tunneling. Nevertheless, most of the design methods for calculating the tunnel loads focus on single tunnels, thus, neglecting the potential interaction between neighboring tunnels. The effect of such interaction can be significant, especially for closely-spaced twin tunnels. In this context, this paper investigates via parametric 3D Finite Element (3D-FE) analyses the interaction between deep, parallel-twin, circular and non-circular tunnels excavated with a conventional (non-TBM) method and supported with shotcrete lining. The numerical investigation focuses on the axial forces acting on the primary support of the tunnels by examining the effect of a wide range of geometrical (pillar width, overburden height, tunnel diameter and section (shape), lagging distance), geotechnical (strength and deformability of the surrounding rockmass, horizontal stress ratio), structural (thickness and deformability of the shotcrete lining) and construction parameters (full- or partial- face excavation and support of the tunnels). The results of the analyses indicate that the construction of the subsequent tunnel influences the loads of the precedent. The stress state of the single tunnel is used as the reference for the quantification of the interaction effect. The output is presented in normalized design charts of the quantified interaction effect on the axial forces, versus key geomaterial and geometry parameters to facilitate preliminary estimations of primary support requirements for twin tunnels.

Prospects of Shotcrete Shaft Lining for Underground Sustainable Mining

Investment in underground mining is coming down all over the world, but deep reserves are available for extraction. Conventional sinking and lining of mine shaft projets or pits for winding men and materials from deep underground mines are getting very costly and being deferred. We found Polish technology of Shaft sinking with monolithic mining very efficient in Sudamdih, Monidih and Satgram projects in India. Russian collaboration for Jhanjra for shaft sinking and mine construction for large mine has been successful. Underground production is declining world over, while remaining reserves are at greater depths. Designing and coding original model programs by collecting actual field data to run the programs to determine cost benefit at different depths with standard diameters has been done. Most companies are avoiding deep mining projects, because of exorbitant cost of shaft sinking. As per experience of the researcher, pre-split blasting and shotcrete lining can be much faster and cheaper. So model programs have been designed and run with practical field cost and technical data, as exemplified in this paper. Future of deep pit mining will be assured with successful adoption of the method in projects, especially through hard rock strata

LABORATORY TESTS ON THE STRENGTHENING OF WET-MIX SHOTCRETE LINING WITH THE USE OF NANOMATERIALS

Adding nanomaterials to concrete extends the size range of constituent particles well into nano-scale dimensions, which could help the compacting of particles in cement-based materials. Regarding the differences between shotcrete and cast concrete, in this study, the properties of shotcrete with nano and micro-silica and nano-clay were experimentally studied. The micro and nanomaterials have been added at different percentages (6%, 9%, and 12%) to a shotcrete paste. The comparison was based on the uniaxial compressive strength, flexural strength, tensile strength, and porosity tests of different specimens. The results indicated that the maximum increase in compressive, flexural, and tensile strengths of shotcrete was related to 12% nano-SiO2, while for 12% nano-clay, all of these strengths were decreased at 28 days. The optimum percentage for shotcrete substitution by nano-clay was established to be 6%. Also, the application of the nanomaterials led to a decrease in the water absorption and porosity of shotcrete. Eventually, the results revealed that the improvement of mechanical properties by the introduction of the nanomaterials in shotcrete could be satisfactory.

Study on support characteristic curve of primary support structures in underground excavation considering bond-slip behavior

Steel reinforced shotcrete lining (SRSL) support is the primary structure to maintain the stability and mobilize the self-bearing capacity of surrounding rock. However, the structural design of SRSL in underground excavation still relies on experience-based method and lack of quantitative mechanical analysis. This paper aims to propose a modified analytical model of support characteristic curve (SCC) that represents the mechanical behavior of SRSL structures in underground construction, through which the interface bond-slip behavior between steel arch and shotcrete layer is taken into consideration. Four-point bending test of SRSL composite beam was carried out to study the bearing mechanism and failure performance. Test results show that the shotcrete-steel interface is prone to slip failure which significantly reduces the overall strength of SRSL. The laboratory test is complemented by non-liner finite element parametric studies considering the bond-slip properties to clarify the design principles and to obtain the flexural stiffness of tunnel primary lining structures. Based on above studies, the simplified formulas for the SCC of SRSL is constructed. The research results provide a theoretical basis for the design and application of SRSL structure in related projects.