Shotcrete Lining Research Articles

Principles and methods of rock support for rockburst control

This paper presents the principles of rock support for rockburst control and three rockburst support systems used in deep metal mines. Before the principles of rock support are presented, rock fracture related to strain burst is first discussed with the help of photos taken on site, and the energy sources and transformations during bursting are illustrated through conceptual models. Surface parallel extension fracture usually occurs in the ejected and surrounding rocks in a strain burst event, while the ejected rock in a fault-slip rockburst is often already pre-fractured before the event. There must be excessive release energy available for rock ejection. The excessive release energy comes from both the ejected rock itself and the surrounding rock. To prevent rock ejection in a rockburst, the support system must be able to dissipate the excessive release energy. All support devices in a support system for rockburst control must be able to dissipate energy, be firmly linked, and be compatible in deformability. A support system for rockburst control comprises surface-retaining devices and yield rockbolts as well as yield cablebolts when needed. Laying mesh on the top of shotcrete liner is a good practice to enhance the surface-retaining capacity of the support system. Energy-absorbing yield rockbolts dissipate energy either by stretching of the bolt shank or by sliding of the inner anchor in the borehole. Mesh, mesh strap and shotcrete are the surface-retaining devices widely used in the current rock support systems. The three types of rock support used for rockburst control at present are soft support system using Split Set bolts, hybrid support system using rebar and two-point anchored yield bolts, and entirely yieldable support system using strong yield bolts.

Influence of shotcrete accelerators on the hydration of cement pastes and their impact on sulfate resistance

Alkali-free, aluminium-based accelerators are often used for shotcrete linings in tunnels and mines, where external sulfate attack can occur. Therefore, it is essential to know, if the influence of the accelerators on the cement hydrate assemblage negatively impacts sulfate resistance. This study focuses on the effect of aluminium-based accelerators on cement hydration and the consequences for external sulfate attack. Aluminum sulfate-based accelerators cause rapid setting due to the very early formation of ettringite and accelerate alite hydration. At late ages, significant more AFm phases are formed compared to the reference without accelerator, which during storage in sodium sulfate solution react to ettringite. A significantly higher volume increase due to the formation of additional ettringite during sulfate exposure was calculated by thermodynamic modelling for the accelerated paste compared to the reference. Alkaline accelerators based on sodium aluminate form mainly amorphous calcium (sulfo-) aluminate hydrates at very early age, while ettringite seems to be destabilized. The ettringite quantities formed during storage in sodium sulfate solution are significantly higher than for the reference and the paste with the alkali-free accelerator. This finding compares well to experiments on concrete specimens, where in the mixture with the alkaline accelerator deleterious expansion was observed.

Three-Dimensional Numerical Analyses of Perpendicular Tunnel Intersections

This paper investigates the effect of constructing a junction tunnel, intersecting an existing main tunnel at a normal angle, via parametric 3D Finite Element (3D-FE) analyses. The 3D interaction between the tunnels significantly modifies the stress state of the primary support and the surrounding rockmass at the intersection area, compared to that of the single tunnel (quasi plane strain problem), thus making 3D-FE analyses necessary for the realistic design of the primary support at the junction area. The numerical investigation includes deep, circular intersecting tunnels with the junction tunnel excavated, after the main tunnel, via a conventional (non-TBM) method and supported with shotcrete lining. The parametric analyses are performed for a wide range of junction tunnel’s diameter, overburden height, in-situ horizontal stress ratio, strength and deformability of the surrounding rockmass. They focus on calculating the axial forces acting on the primary support at the intersection area before, during and after the construction of the junction tunnel. In addition, they identify the extent of the zone influenced by the tunnel interaction. The results of the analyses indicate that the construction of the junction tunnel causes significant additional compressive loading at the springline of the main tunnel. In contrast, the crown/invert of the opening of the main tunnel is subjected to either compressive loading or unloading (often reaching tensile loading). The results of the analyses are presented in normalized design charts of the axial forces, versus key geomaterial and geometry parameters to facilitate preliminary estimations of primary support requirements at tunnel junctions.

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.

An analytical design method for ductile support structures in squeezing tunnels

Ductile linings have been proved to be highly effective for tunnelling in heavy squeezing grounds. But there still has not been a well-established design method for them. In this paper, an investigation on an analytical design method for ductile tunnel linings is performed. Firstly, a solution in closed form for ground response of a circular tunnel within Burgers viscoelastic rocks is derived, accounting for the displacement release effect. Then based on the principle of equivalent deformation, the mechanical model of segmental shotcrete linings with yielding elements is established using the homogenization approach. Analytical prediction for behaviour of ductile tunnel linings is provided. Furthermore, the proposed design method for ductile tunnel linings is applied in Saint Martin La Porte access tunnel and the analytical prediction is in good agreement with field monitoring data. Finally, a parametric investigation on the influence of yielding elements on performance of ductile tunnel linings is conducted. Results show that the length of yielding elements poses a great influence on linings. It is feasible and effective to increase the length of yielding elements to obtain the pressure within the bearing capacity of linings. However, yield stress of yielding elements does not significantly affect the performance of the lining. It is suggested to apply yielding elements with relatively higher yield stress in linings for higher stability.

Verification of failure mechanisms and design philosophy for a bolt-anchored and fibre-reinforced shotcrete lining

Falling or sliding of loose blocks is one of the most common failure modes in a rock tunnel. For tunnels in hard and jointed rock, fibre-reinforced shotcrete (sprayed concrete) in combination with rock bolts is one of the most commonly used supports to prevent such failures. The structural behaviour, and especially the failure, of this type of rock support, is complex and involves several failure mechanisms; such as cracking of the shotcrete and interface failure along the shotcrete-rock, bolt-grout and rock-grout interface. Therefore, rock supports are normally designed using analytical solutions based on the independent failure modes. However, these failure modes are derived based on experimental testing and the assumption that no interaction between the failure modes occur. This assumption has not been verified. Therefore, this paper presents a numerical model capable of simulating the failure of a bolt-anchored and fibre-reinforced shotcrete lining. The model includes bond failure between shotcrete and rock, cracking of the shotcrete and pull-out failure of rock bolts. The structural behaviour for each failure mode and the complete structure have been verified against experiments from the literature. This shows that the model is capable of simulating the different phases of failure, and show good agreement with results from full-scale experimental tests from the literature. Furthermore, results from the numerical simulation confirms that the design of the shotcrete lining can be based on individual failure mechanisms. Moreover, it was shown that a design based on the residual strength of the fibre-reinforced shotcrete is conservative compared to a design based on the bond strength.

High-density polyurethane rigid foam usability as liner support material in rock engineering

To eliminate the problems like brittle material characteristics, low crack resistivity, low tensile strength values, late curing time and rebound problems of shotcrete liners, high density polyurethane rigid foams (PRF) were investigated as an alternative liner support material in this experimental and numerical study. Its rapid polymerization makes the PRF liners be able to supply support pressure in very early times in comparison with the shotcrete liners. Some fundamental tests were carried out to determine some support performance properties of a high-density PRF product with the foam density of 180 kg/m3. Additionally, a pilot PRF liner spraying application was practiced in an underground mine to investigate whether its application is advantageous in terms of eliminating the problems of shotcrete applications. In the field study, PRF liners were comparatively investigated with the shotcrete liner sprayed in the mine. Within the numerical part of this study, a series of finite element analyses were performed to investigate support performances of the PRF liners. According to the results of both experimental and numerical analyses, high density PRF liners were found to have proper load bearing capacities as a new liner support material, and the PRF liners were assessed to be a strong candidate to be popular in rock engineering. The findings from this study are promotive for new investigations to better understand support properties of the PRF liners for improving effectiveness of their systematic use.

Evaluation of the safety factors of shotcrete linings during the creep stage

Sprayed concrete linings (SCLs) used in tunnels generally develop secondary deformations over time, even in the presence of constant stress levels within the lining. These deformations influence the loading process on the lining and therefore also the stress levels within the support structure. The behaviour of a SCL in a tunnel was investigated under different operating conditions in order to evaluate the effect of secondary deformations on the evolution of stability conditions (safety margins with respect to the possible concrete failure) over time, after completion of the tunnel construction. A parametric analysis was performed to study eight different types of tunnels, with variable geometry and rock quality, and eight different types of sprayed concrete. The 64 cases of the parametric analysis covered the vast range of variability of influential parameters and yielded useful information concerning the effects of secondary deformations over time on the static behaviour of the lining and on the safety factor with reference to the possible failure of the sprayed concrete.

Nonlinear Time-Dependent Analysis of the Load-Bearing Capacity of a Single Permanent Shotcrete Lining at the Brenner Base Tunnel

The Brenner Base Tunnel (BBT), a 64 km long railway tunnel between Austria and Italy, is under construction at the time of writing. The complete tunnel system of the BBT will comprise railway tunnels as well as exploration, emergency, ventilation and access tunnels. The latter are tunnels with generally lower requirements on the serviceability, watertightness, durability, surface quality or availability. In some cases, a single permanent shotcrete lining can fulfill the reduced requirements and offers a very economic tunnel design. Realistic assessment of the load-bearing capacity of shotcrete linings is particularly required for early and large deformations in deep tunnels. This is the motivation for a more sophisticated approach to the back-analysis of a permanent shotcrete lining of a single lined rescue tunnel of the BBT. Thus, a detailed finite element model is developed for analyzing the bearing capacity of the permanent shotcrete lining on the basis of an advanced constitutive model considering the time-dependent and nonlinear material behavior of shotcrete. The material model represents the time-dependent evolution of material properties due to hydration of shotcrete, hardening and softening material behavior, the latter resulting in cracking or crushing of shotcrete, creep and shrinkage. In the finite element model, the time-dependent construction process of the drill-and-blast method for tunnels driven by partial face excavation is considered. The material parameters for the shotcrete model were calibrated based on an experimental program, performed with specimens sampled directly on the construction site of the BBT.

The Analysis on Seepage Field of Grouted and Shotcrete Lined Underwater Tunnel

Groundwater control in underwater tunnels by drilling and blasting method is generally carried out by grouting ring, shotcrete lining (primary lining), and concrete lining. The permeable grouting ring and shotcrete lining have an important impact on seepage field. However, the currently published research models of related results are for homogeneous and isotropic single-layer unlined tunnels, ignoring the important effects of tunnel grouting circles and primary lining. If the conclusions of the relevant literature are directly used to guide the tunnel design, large errors may occur. Therefore, on the basis of previous studies, this article extends the tunnel seepage research model and incorporates the tunnel grouting ring and primary lining into the research model. The research model is more in line with actual working conditions. Based on the principle of mirror method, the seepage field of a drainage tunnel in an infinite aquifer is superposed with that of a water supply tunnel in an infinite aquifer, and the analytical solution to the seepage field of an grouted and shotcrete lined underwater tunnel in a semi-infinite aquifer is obtained, which is further verified by numerical analysis and experiment. In addition, the influence of grouting ring and primary lining parameters on seepage field is discussed by using partial differential analysis. The results show that the seepage flow of tunnel can be significantly alleviated by either reducing the permeability coefficient of grouting ring and primary lining or increasing the thickness of grouting ring and primary lining, but the water pressure of grouting ring and primary lining will increase.

Time-dependent material behaviour of shotcrete – New empirical model for the strength development and basic experimental investigations

In conventional tunnelling, the shotcrete lining is loaded at an early stage, so consideration of the time dependency of material properties during construction is vital. An important property of shotcrete concerning rock deformation is its early strength, which can influence the development of rock pressure and support resistance. The mechanical properties of the shotcrete change continuously during the hydration process. Material models exist to describe the mechanical behaviour, but these were mostly validated with dry shotcrete or older shotcrete mix designs. Continual improvements in admixture technology have improved the performance of shotcrete.Therefore, strength developments of a test series and practical projects are evaluated and a shotcrete test to determine the time-dependent material properties is carried out with a reference mix design. An empirical prediction model, which serves as a qualitative estimation of shotcrete strength processes, is designed based on the results of the evaluated strength developments. Based on the experimental investigations, a first basis for in-depth investigations of the time-dependent material behaviour of shotcrete is created.

Shotcrete performance-loss due to seepage and temperature coupling in cold-region tunnels

Road and railway tunnels in cold regions are often affected by water leakages and freezing temperatures, which increase the risk of severe damage in shotcrete linings. Within this context, two environmental conditions are investigated in this paper with reference to a low-alkali-based shotcrete; i.e., the coupling of seepage and temperature (named the “open system”) and the temperature uncoupled with seepage (named the “closed system”). Based on the physical and mechanical tests performed on two shotcrete slabs, the previously mentioned environmental conditions are analyzed with reference to concrete mechanical properties, strain characteristics and pore structure. The results are summarized as follows: (1) The stiffness and compression strength of shotcrete are weakened due to water continuously seeping through the shotcrete during the freezing and thawing process. (2) The residual tensile strain of the open system is significantly higher than that of the closed system. (3) The seepage and temperature coupling favors the deterioration of micro-pore structure, which has adverse effects on the mechanical performance of shotcrete matrix.

Performance of segmental and shotcrete linings in shallow tunnels crossing a transverse strike-slip faulting

Abstract In this paper, three-dimensional numerical modeling is performed to study the effects of strike-slip fault movement on the performance of shotcrete and segmental linings in shallow tunnels that transversely cross the fault. For this purpose, a parametric study is conducted on lining thickness, soil geo-mechanical properties, tunnel depth, and fault dip angle to assess their influence on the tunnel movements and deformation. A comparison between the segmental and the shotcrete tunnels is made to highlight their performance. The results show that the greater strike-slip fault dip angle increases the separation of the lining segments. As well, after faulting, the maximum tunnel displacement in denser soils is greater than in loose soils.

Mechanical properties of steel and polymer fiber reinforced concrete

AbstractThe present study aimed to investigate the influence of a number of fiber parameters including fiber type, content and hybridization on strength and ductility of polymer fiber reinforced concrete (PFRC) and steel fiber reinforced concrete (SFRC) used mostly in tunneling practices as the primary shotcrete lining. Numerous cylindrical and prismatic beams were casted and undergone various tests in which main previously mentioned fiber traits varied. It was understood that SFRC excels at every mechanical feature in comparison to PFRC; however, such transcendence found predominant in compressive strength but marginal in flexural and tensile strength. Despite being classified under different compressive strength classes (SFRC in the upper and PFRC in the lower class) according to EFNARC, both FRC types fell under a similar flexural class (at 4% of fiber fraction); a result possibly in debt to excellent bonding properties and more slender polymer fibers. Tensile strength of PFRC was measured lower than SFRC. Augmentation of fiber content positively affected mechanical characteristics of FRC at most cases. Hybridization of different fibers at a specific range of fiber mixing proportions was observed to have advantageous impacts on ductility and strength of a more corrosive resistant and cost efficient hybrid fiber reinforced concrete (HFRC).

Progressive deformation and mechanical behaviour of intersecting tunnels in soft ground

Stress concentrations and large ground deformations are sources of major concern during the construction of tunnel intersections. Such problems are even more critical in urban areas with buildings affected by the induced progressive ground movements. Therefore, particular considerations have to be taken into account for such projects. This paper deals with the effects of the underground openings on primary tunnels (tunnel intersection) using detailed three-dimensional finite-element models on a case study – Hakim tunnel project, Tehran, Iran. The effects of different stages of construction of intersections on stress distribution and ground deformation are evaluated. Moreover, the effective trailing distance of different faces of the tunnel from the centre of the intersection and the effect of tunnel drilling on internal forces of the shotcrete lining are discussed. The results showed that the values of the surface settlement remain almost unchanged at a distance approximately twice the equivalent diameter of the child tunnel from the intersection centreline. Also, the increase in the circumferential bending moments near the intersection is non-significant compared with the changes in the longitudinal bending moment. It was found that the circumferential and longitudinal bending moments next to the intersection can increase by up to 250 and 350%, respectively.

Chloride diffusion of shotcrete lining structure subjected to nitric acid, salt–frost degradation, and bending stress in marine environment

The chloride diffusion of shotcrete lining at the middle and passageway of submarine tunnels subjected to chloride attack, combined action of NxOy from automobile exhaust and chloride attack, salt–frost degradation, and surrounding rock stress was investigated using 3.5%NaCl and HNO3+3.5%NaCl solution immersion method, accelerated salt–freezing-thawing method, and spring four-point bending loads method, respectively. Chloride content in shotcrete was measured using solid–liquid extraction and potential method. Meanwhile, the influence of chloride diffusion on nitric acid attack, salt–frost deterioration, bending stress, and shotcrete mixture parameters were analyzed. Nitric acid attack, salt–frost degradation and bending stress accelerated chloride migrate into shotcrete. When immersed in NaCl and HNO3+NaCl solution, shotcrete with low water–binder ratio and high steel fiber content had lower chloride content. With the increase in fly ash replacement, the chloride content decreased initially and then increased rapidly. Under salt–frost degradation, the shotcrete with low water–binder ratio and fly ash replacement had better chloride diffusion resistance. The distribution of chloride content in concrete conformed to Fick's second law. The chloride diffusion model was established based on the temperature of environmental and humidity of concrete, exposure time to solution, mixture parameters of shotcrete, nitric acid attack, salt–frost degradation, and bending stress.

Direct and Distributed Strain Measurements Inside a Shotcrete Lining: Concept and Realisation

This paper introduces the successful implementation of a fibre-optic sensing system for direct and distributed strain measurement within the shotcrete lining of a conventional tunnel drive. The shotcrete lining of the top-heading and the invert are equipped with two layers (rock side and cavity side) of fibre-optic sensing cables installed in circumferential and longitudinal direction. All cables are measured autonomously for several weeks to capture the strain evolution inside the lining from the day of the construction to a posterior, well-hardened state. An additional follow-up measurement is conducted 2 months afterwards. The measurements enable an assessment of the strain distribution inside the lining with a spatial resolution in the range of some centimetres and a measurement resolution of up to 1 µm/m. Besides the conventional monitoring targets used for displacement recordings, measurement equipment like strain gages and pressure cells are also installed in the cross-section under investigation. Back-calculated strain from absolute displacements and the readings from the strain gages show good agreement with the results of the conducted fibre-optic measurements and verify the suitability of the used system.

A numerical model to assess the creep of shotcrete linings

The behaviour of sprayed concrete (SC) tunnel linings was analysed considering the secondary deformation effects over time. Considering the behaviour of SC support under the loads applied by the rock mass and the interaction between the lining and the rock mass, a detailed analysis of the stress and deformation was performed using the convergence–confinement method (CCM) and the hyperstatic reaction method (HRM). In order to develop a procedure for the correct design of a shotcrete lining, an analysis was required that combined the CCM and the HRM. To take into account the creep phenomenon, the Voigt–Kelvin model was used to model the shotcrete; this model employs two springs and a viscous damper to reproduce the actual behaviour physically. Some useful findings were obtained on the trend of the safety factors of shotcrete linings over time.

Assessment of the Safety Factor Evolution of the Shotcrete Lining for Different Curing Ages

The behavior of the shotcrete linings during the tunnel construction is complex due to the variability of its mechanical characteristics during the curing time. After the installation of the support structure, the lining is loaded along with the excavation face. A new calculation procedure involving two analytical methods, i.e. the convergence-confinement method and hyperstatic reaction method, have been developed. By means of these two methods, it is possible to assess the evolution of the stress state in the lining, and therefore, also of the safety factor with respect to the failure in compression of the shotcrete. Due to the analysis of the safety factor evolution over time, it is possible to correctly design the lining, to choose the type of sprayed concrete and to define the maximum admissible advance rate of the excavation face, in order not to critically load the lining. In the following paper, after having shown the definition of the safety factor, a parametric analysis is performed, in order to investigate the evolution of the safety factor of the lining for two different rock types, three different shotcrete types and two tunnel advance rates have been considered.

Utilization of crumb rubber and FBC-based ternary binder in shotcrete lining

This paper describes the early-age properties of shotcrete modified by i) up to 12.5% by volume crumb rubber as a replacement for aggregates and ii) substitution of ordinary Portland cement (OPC) by a fluidized bed combustion-based ternary binder (FBC-TB) up to 80% of mass. For the tests conducted, an increase in the crumb rubber volumetric fraction of the mixture always led to a decrease in uniaxial compressive strength. Replacing OPC with FBC-TB up to 20% enhanced the development of the early-age compressive strengths; however, a larger substitution resulted in the reduction in the concrete’s final compressive strength. In a full-size experimental mock-up test, the highest-performing mixture—in terms of acceptable mechanical properties and a reasonable amount of reused by-products—was subjected to full deployment in a concrete batching plant. Two batches (6 m3 in total) of concrete with crumb rubber and FBC-TB replacement were sprayed onto a full-scale model of a tunnel for the mock-up test. Evaluation of shotcrete performance included an analysis of the development of compressive strength and a comparison of results with criteria from the relevant code of practice, the New Austrian Tunnelling Method. Based on laboratory experiments, optimization, and in-situ measurements, the mock-up tests proved shotcrete containing crumb rubber and FBC-TC is applicable for full-scale use.