Shotcrete Lining Research Articles

Fiber-reinforced shotcrete lining for stabilizing rock blocks around underground cavities

Fiber-reinforced shotcrete is a high-performance material that presents some special characteristics, which can provide some suitable applications in the excavation of underground cavities. The presence of fibers induces an increase in the tensile strength, flexural strength and shear strength in the concrete, as well as allowing a ductile rather than brittle type of behavior. It can be used to create a lining of the underground cavity that allows the stabilization of rock blocks that show a tendency to slip or fall (from the side walls or from the crown area, respectively). In this work, some full-scale tests on the fiber-reinforced shotcrete lining are presented. From these tests, it was possible to measure the behavior of this material when it is loaded locally: it is the same type of action produced by the rock block when it is held back from falling or slipping. The results obtained have allowed to characterize this type of material from a mechanical point of view. A subsequent detailed analysis of the stability of rock blocks surrounding an underground cavity permitted to determine the static stabilizing contribution offered by the fiber-reinforced shotcrete lining, leading to the definition of the minimum thickness required, in relation to the type of block that is present (shape and size). It was possible to predict how a lining thickness of about 3.5 cm is able to stabilize (just 15 min after its spraying) rock blocks with an exposed surface area of up to 10 m2 and a distance of the internal vertex from the border of the cavity of up to 3 m

Study on creep performance of shotcrete lining for tunnels

Creep, as a long-term deformation characteristic of shotcrete, significantly influences the load-bearing performance and durability of tunnel support structures. This study conducted creep tests on shotcrete under varying stress levels. The results reveal that shotcrete exhibits linear creep response within the stress range of 30%–40%, while the critical stress range for nonlinear creep response occurs between 40% and 50%. Employing SEM-EDS and XRD analysis methods, the impact of accelerator on the microstructure evolution and creep mechanism of shotcrete is explored. The accelerator accelerates the hydration of C3S, promoting the generation of AFt and C–S–H gel, leading to rapid solidification of the matrix and enhanced early strength. With the consumption of SO42− ions, AFt gradually transforms into AFm, resulting in increased matrix porosity. This phenomenon contributes to the reduced later-stage strength and substantial creep deformation of shotcrete. Based on experimental outcomes, this study evaluates the applicability of existing creep calculation models to shotcrete.

Design and Analysis of Shotcrete Lining in Tunnels Under Squeezing Ground Conditions: A Case Study of Atal Tunnel

Design and Analysis of Shotcrete Lining in Tunnels Under Squeezing Ground Conditions: A Case Study of Atal Tunnel

Experimental study on the mechanical properties of FRP reinforced shotcrete lining of tunnel in extremely fractured surrounding rock

Tunnel excavation in the southwestern area of China encounters many difficulties caused by the large deformation of extremely fractured surrounding rock, which is the product of high plate tectonic stress. Improving the initial cracking strength and ductility of shotcrete layer is one of the most important means to reduce the frequency of support structure repair, which will improve tunnel construction efficiency and ensure personnel safety. In this case, this paper introduced fiber reinforced polymer (FRP) mesh to replace the steel mesh as reinforcement to optimize the support effect of shotcrete lining. Both large-scale model test and mechanical test are conducted to compare the reinforcement effect between FRP and steel mesh. Research results indicate that replacing steel mesh with FRP can increase the ultimate deformation capacity of tunnel lining by more than four times. The parametric analysis results indicate that increasing the lining thickness, the diameter of the FRP bars and the mesh density have different degrees of effect on improving the stiffness and strength of tunnel lining. The research results can provide reference for relevant projects.

Mitigation of tunnel support design risks for hydropower projects within high tectonic stress regimes – an empirical analysis

Abstract The study for the design of tunnels in similar geological settings, providing insights into potential challenges that may arise during excavation and offering strategies for mitigating risks in District Kalam on the Ushu River, Khyber Pakhtunkhwa, Pakistan. The methodology involved geological mapping, rock sampling, discontinuity surveys, and laboratory testing for empirical analysis of tunnel parameters at the Weir House, Powerhouse, and tunnel alignment locations. Empirical analysis of tunnel parameters using three rock classification systems, rock mass rating (RMR), rock quality tunneling index, and rock mass index (RMi). Based on the classification, the rock quality was found to be fair, indicating favorable rock properties. The Q-system rated the rock as poor to fair, suggesting low discontinuity intensity, medium rock strength, or medium deformation modulus. According to the RMi, rock was rated as medium to strong, indicating low discontinuity intensity, high rock strength, or low deformability. The support design for the tunnel is based on empirical analysis, it recommends support design for the tunnel reinforcement elements such as rock bolts, wire mesh, and shotcrete lining. Overall, the tunnel is stable and does not have complex structure and weak zones.

Cover Picture: Geomechanics and Tunnelling 2/2024

AbstractLining stress controllers (LSC) have been developed as special supporting measure for tunnelling in zones with stress‐induced failure involving large ground volumes and large deformations. The shotcrete lining is divided into several segments by longitudinal construction joints. The purpose of this segmentation is to absorb large deformations occurring during tunnel driving in weak ground. LSC elements are installed into these deformation joints. These elements have a defined workload during compression so the primary lining is not damaged (Source: DSI Underground).

Study on the influence law of cavities behind the shotcrete lining on the lattice girders

The field monitoring data showed that a small amount of main reinforcement bars of lattice girder at the arch of a tunnel were pulled, and the calculation showed that the initial support structure should be compressed. To find out the reason for the tension of the main reinforcement, the geological radar was used to detect the cavity in the sprayed concrete layer at the tension position. In order to clarify the tension mechanism of the main reinforcement and the influence of factors such as the position and size of the cavity on the main reinforcement, numerical simulations were carried out. The results show that the cavity causes the eccentric compression of the shotcrete layer, resulting in moment of the lattice girder and the change of the stress distribution of the main reinforcement. The main reinforcement experiences tensile stress when the cavity size surpasses 3 m × 0.2 m, reaching a tensile stress of 81 MPa at a cavity size of 6 m × 0.2 m. Notably, the cavity located at the foot of the arch is more likely to produce substantial tensile stress on the primary reinforcement compared to those at the arch crown and waist. The research results provide a theoretical basis for the interpretation and analysis of tunnel lattice girder monitoring data.

Evaluation on the Squeezing and Design of Support system in Headrace Tunnel of Middle Modi Hydroelectic Project

Tunneling through a weak rock possess a great challenge. These challenges depends upon type of rock, excavation method, stress and deformation behavior of the rock etc. Squeezing is one of the major problem which is likely to occur during excavation of tunnel through weak rock. A reliable prediction of the extent of squeezing is essential so that a strategy can be established regarding stabilizing measures and for optimizing the support well in advance. In this paper, Middle Modi Hydroelectric Project located in the Kaski District has been taken as the case study. In this project, huge squeezing problem occurred at about chainage 1+140m At this section deformation has been recorded well over 65cm Hence, this paper basically deals with squeezing analysis using different approaches. Rock types along the headrace tunnel alignment are sheared phyllite and fractured quartzite. Mostly, intercalation of phyllite and quartzite has been found in the squeezed section Rockmass quality found in the squeezed section is extremely poor to exceptionally poor. Four main methods have been used to evaluate the squeezing phenomenon viz empirical methods such as Singh (1992) and Goel (2000), semi-empirical such as Hoek and Marinos (2000), analytical method such as (Convergence Confinement Method, 2000) and mumerical program Phase2. The main factors that control the squeezing phenomenon are the rock mass parameters and rock stresses. The uniaxial unconfined compressive strength of intact rock has been back calculated from measured deformations using phase2 program and found to be in the range of 10 to 15Mpa in the squeezed section Deformation was calculated using CCM and Compared with phase2 result. Due to excessive deformation temporary supports were provided at several locations, steel ribs are buckled and shotcrete lining is also cracked. All these have to be removed before application of final lining Finally two different approaches have been studied using phase program to address the existing problems in squeezed section of headrace tunnel ie. design of support system considering either circular shape with final lining (shotcrete and steel rib) or reshaping existing D-shaped tunnel into horseshoe shaped and providing final concrete lining.

Static behavior of corrugated steel-shotcrete composite arches

A new type of corrugated steel-shotcrete composite (CSSC) arch is proposed, which possesses the advantages of both high load-bearing capacity and ease of construction. Thus it is particularly suitable for bridges, culverts, shafts, and other projects. In this study, the static behavior of the CSSC arch under three-point loading was investigated experimentally and numerically. Four specimens with different rise-span ratios (1/2, 1/8), different spans (3 m, 6 m), and different locations of shotcrete lining (shotcrete outer lining, shotcrete inner lining) were tested. The test results show that the load-bearing capacity and initial stiffness of the CSSC arch could reach 6.8 times and 13.9 times that of the corrugated steel (CS) arch, respectively. When the load was less than 0.4 times the load-bearing capacity of the specimens, CS and shotcrete worked together. After that, the composite effect between CS and shotcrete was weakened. The ductility of the shotcrete outer lining CSSC arch is better than that of the shotcrete inner lining CSSC arch. A finite element model (FEM) was developed to investigate the influences of key parameters on the load-bearing capacity, including the thickness of materials, the strength of materials, the rise-span ratios and spans. Furthermore, calculation methods were proposed to predict the axial compressive capacity of the CSSC arch.

Study on the time‐dependent interaction between surrounding rock and yielding supports in deep soft rock tunnels

AbstractAs yielding supports can deform without being damaged, the yielding principle has attracted significant attention for tunnelling in squeezing ground compared with the heavy support method. The deformation energy of surrounding rock can be managed using yielding supports that maintain the rock pressure within the bearing capacity of the supports. However, although the yielding principle has been often applied in practical scenarios, a design method for yielding supports has yet not been well developed. This study discusses interaction between the squeezing ground and yielding supports and investigates the influence of the time‐dependent behaviour of rock on the yielding supports. The yielding supports can be implemented by the movement of sliding joints of steel arches or by shortening the yielding elements inserted in shotcrete linings. The deforming process of yielding supports can be divided into three stages: elastic a, yielding, and elastic b. A three‐stage pressure‐displacement relationship of yielding supports is proposed and a method is provided for the calculation of the support stiffnesses of yielding supports in different stages. Furthermore, a unified design approach for the yielding supports is presented and the theoretical model is established after simplifying the asymmetry of problem. The analytical solution for tunnel displacement and support pressure in a tunnel supported by the yielding supports is obtained. The proposed design method for the yielding supports is successfully applied to the Saint Martin La Porte access tunnel, and the mean tunnel deformation in this tunnel is appropriately predicted by the analytical solution. Additionally, a parametric investigation is performed based on the analytical solution, and the effects of yielding path, yielding displacement, support thickness, and installation time of the yielding supports are discussed. Finally, certain highlights of the analytical model of yielding supports in this study are identified, compared with other researches, and recommendations for the design of the yielding supports are provided. The outcome of this research has potential applications in the preliminary design of deep tunnels in squeezing ground.

Influence of Tunnel Shape on the Shotcrete Lining Stress

Tunnel excavation is generally done using drill and blast method and the weak geology causes the over breaks of tunnel. The over breaks not only increases the cost and construction time but also it has influence on the stress on shotcrete lining. This paper presents the influence of shape of the tunnel on the shotcrete lining stress is studied using a numerical modelling with the case study of Kathmandu University Research Tunnel. The region of over breaks is taken as 0.15-0.30 m for weak rock mass [1].Two numerical models have been prepared and the obtain result shows that the stress on the shotcrete lining increases with increase in the over breaks and should be consider in the design of shotcrete lining if the shotcrete lining is to be used as final lining.

Reliability analysis of bolt‐supported tunnels regarded as homogenized structures

AbstractThis paper presents a reliability analysis of deep tunnels reinforced by fully grouted bolts using a homogenization approach. From the constitutive viewpoint, the anisotropic elastoplastic behavior of bolted rock mass is formulated in the context of the homogenization method. Reliability design analysis is undertaken considering the configurations of both radial and face bolts used separately or in association. The combination of rock bolting associated with classical shotcrete lining is also addressed. Restricting to deep tunnels and axisymmetry conditions, a simplified analytical approach followed by finite element numerical modeling are developed in the framework of the homogenization method. Reliability analyzes are performed using the Monte Carlo simulation (MCS) and the first‐order reliability method (FORM). The response surface method (RSM) provides analytical approximations to enable reliability analyses without time‐consuming computational models. The results show excellent correlation between the obtained reliability parameters, which emphasizes the relevancy of the adopted reliability methods. Structural failure probabilities below or close to 1.0% were obtained in all cases studied for the best structural tunnel conditions.

Investigating High-Strength Expanded Polystyrene (HS-EPS) as yielding support elements for tunnelling in squeezing ground conditions

Tunnelling conditions characterized by large and long-term deformations demand yielding support elements as a part of the shotcrete lining. The yielding support elements transform radial displacements into a tangential closure without compromising the lining system's support capacity and avoiding micro-cracking and overstressing of the shotcrete during the curing process. While products available on the market are satisfactory in performance, they are often heavy, difficult to handle on-site, and hence time-consuming for installation. A novel lightweight yielding support element made out of High-Strength Expanded Polystyrene (HS-EPS) is introduced to overcome these operational drawbacks. Unconfined compression tests on small- and large-scale samples were carried out to investigate the HS-EPS strength and deformation characteristics. Further, the combination of several HS-EPS panels with different stiffness properties allows the specific design of a yielding support element with optimum stress–strain behaviour for minor to severe squeezing conditions. Finally, deformation analyses with stress–strain curves and Digital Image Correlation(DIC) were carried out, and suggestions for the practical application on-site are given.

Strukturoptimierung von Spritzbetonschalen – Versagensanalysen von Spritzbetonschalen bei Betrachtung von Vortriebsoptimierungen

AbstractA framework with a robust design of the driving classes is fundamental for the tunnel design. During tunnel advance, the detailed design of the lining is depending on the documented geology and the observed system behaviour. To reduce costs, the excavation profile is optimized with the aim of reducing overbreaks and shotcrete masses. This achieved profile accuracy is beneficial for the load‐bearing capacity of the rock and the shotcrete lining. For temporary and permanent tunnel shotcrete linings, the excavation profile accuracy influences the stress distribution, the crack pattern and the expected deformations. In the course of this study, the effect of the excavation geometry on the load‐bearing capacity of the primary lining of drill and blast tunnels was examined. For this purpose, shotcrete linings, reinforced by steel meshes and fibre‐reinforced linings, were compared in optimized and non‐optimized scanned excavation cross‐sections. In each case, an ultimate load and failure analysis was carried out to assess shells in the hardened state by means of numerical modelling (ATENA). The aim is to determine the load‐bearing capacity of the shell structure in order to obtain a conclusion regarding its structural robustness. The interaction between the rock/soil soil and the structure as well as a time‐dependent material behaviour were not considered. It could be shown how an irregular overbreak or an inconsistent shell thickness has a negative effect on the load‐bearing capacity of the structure.

Failure Mechanisms of Fibre Reinforced Shotcrete: Numerical Simulations Considering Local Variations in Thickness and Bond Strength

Abstract Fibre-reinforced shotcrete is the most common support method for hard rock tunnels in the Nordic countries. The design of shotcrete is often based on empirical methods or simplified analytical equations, which neglect variations in mechanical properties and shotcrete thickness. Data collected from the field shows that significant variations in shotcrete thickness and bond strength should be expected during tunnel construction. However, how this affects the structural behaviour and capacity of the shotcrete lining is unknown. Moreover, the design philosophy for shotcrete assumes that the primary failure modes of shotcrete, i.e. bond and flexural failure, can be treated separately. This was derived based on observations of experiments in a laboratory environment. Therefore, the focus of a finalized doctoral project was to develop a numerical framework to simulate the structural behaviour of fibre-reinforced shotcrete in interaction with hard rock and rock bolts. The effect of variations in shotcrete thickness and bond strength was studied through numerical simulations to increase the understanding of its effect on the failure load of the lining. The results indicate that the most important parameter is the mean value of the shotcrete thickness and bond strength around a narrow perimeter of the block.’

Design And Construction Of Roma Street Station Cavern, Cross River Rail, Brisbane

The new Roma Street underground railway station in Brisbane is being constructed as part of Cross River Rail’s Tunnel, Stations and Development (TSD) package. The joint venture of CPB Contractors, BAM International Australia, Ghella and UGL (CBGU JV) is building the 5.9km long twin tunnels from the Southern Tunnel Portal near Dutton Park station, beneath the Brisbane River and CBD to the Northern Tunnel Portal in Spring Hill. The Cross River Rail project includes excavation and construction of four new underground stations. Roma Street station comprises a 280m long cavern, five smaller connecting tunnels (adits) and three shafts. The station cavern has an excavated span of up to 24.4m with approximately 15m rock cover. It has been excavated within the Neranleigh-Fernvale Group (NFG) rock mass, which comprises weakly metamorphosed sandstone (meta-greywacke and arenite), phyllite and subordinate quartzite and meta-basalt. The station lies within the regional Normanby Fault Zone, characterised by a major fault up to 20m wide comprising a combination of intact rock, rock breccia and clay gouge. The fault zone encountered during the station cavern excavation required heavier primary support and localised foundation treatment. The initial primary (temporary) support of the cavern and adits comprised rock bolts, cable bolts and a thin synthetic fibre-reinforced shotcrete lining. In some areas a passive shotcrete arch lining was required. Overlying piled footings from an existing busway overpass structure were within a metre of the adits’ excavated profile which necessitated a complex load transfer structure at the surface and verification of pile toe levels during tunnel construction. The cavern permanent lining typically comprises steel fibre-reinforced concrete in the crown, bar reinforced concrete for the sidewalls, and bar and steel fibre-reinforced concrete invert slabs. Bar reinforcement is used in the cavern crown where it intersects the adits. Ground loads for the permanent structure had to consider the influence of future developments. This paper presents some of the challenges of the primary support and permanent lining design of the station cavern and adits. It summarises the as-encountered ground conditions, aspects of the primary support and permanent lining design that were geotechnically challenging and the solutions developed to meet the project requirements.

Numerical Study on the Mechanical Behavior of Shotcrete Lining with Yielding Support in Large Deformation Tunnel

Numerical Study on the Mechanical Behavior of Shotcrete Lining with Yielding Support in Large Deformation Tunnel

Understanding the impact of adhesion on the mechanical behavior of shotcrete

ABSTRACT Shotcrete is widely used as part of ground support in underground excavations. Although adhesion is recognized as a critical mechanical property of shotcrete, there is no strong consensus from a design perspective on whether high or low adhesion is preferred. This is probably due to our relatively limited understanding of the role of adhesion on the field performance of shotcrete. Laboratory investigations can provide useful insights, but it is difficult to extrapolate to field conditions. Numerical models can complement laboratory investigations, but to develop confidence in their results, they must adequately reproduce the failure mechanism of shotcrete. This paper investigates how different adhesion properties control the mechanical behavior of shotcrete. Laboratory tests were used to calibrate a three-dimensional discrete element method model to capture the adhesion failure of a shotcrete liner. The developed model explicitly reproduced the adhesion failure mechanism, including the load redistribution, loading capacity, and displacement of the shotcrete liner. This was used as a basis for a parametric investigation of the impact of adhesion strength on the mechanical behavior of shotcrete. The numerical results indicated the significant implications in selecting adhesion properties of shotcrete and led to recommendations for adhesion capacity under different ground stress conditions.

EFFECT OF CURVATURE ON THE TENSILE STRENGTH OF FRCM

Fabric reinforced cementitious matrix (FRCM) systems are promising composite materials which are increasingly used for the strengthening of reinforced concrete components. The interaction of high performance, fine-grained concretes and noncorrosive, high strength textile reinforcements combines many advantages of shotcrete linings and externally bonded fiber-reinforced polymers (FRP). For this reason, especially the retrofitting of plane concrete structures with FRCM has already been investigated appropriately. However, the application of FRCM systems in edge regions and curved areas, such as the confinement of reinforced concrete columns or shear strengthening of beams, leads to a reduction of the systems' tensile strength. This reduction is mainly caused by additional transverse pressure, cracks in the concrete matrix, and possible pre-damage due to the bent textile. As most available prediction models use static reduction factors, the dependencies of these influences have not been considered sufficiently yet. This paper presents an experimental study regarding the effects of curvature on the tensile strength of FRCM-strengthening layers. Therefore, 93 specimens with varying radii, FRCM composite materials, and numbers of applied textile layers were tested in a newly developed setup. The test results show significant dependencies on the various parameters and contrast with the static reduction factors used so far.

EFFECT OF CURVATURE ON THE TENSILE STRENGTH OF FRCM

Fabric reinforced cementitious matrix (FRCM) systems are promising composite materials which are increasingly used for the strengthening of reinforced concrete components. The interaction of high performance, fine-grained concretes and noncorrosive, high strength textile reinforcements combines many advantages of shotcrete linings and externally bonded fiber-reinforced polymers (FRP). For this reason, especially the retrofitting of plane concrete structures with FRCM has already been investigated appropriately. However, the application of FRCM systems in edge regions and curved areas, such as the confinement of reinforced concrete columns or shear strengthening of beams, leads to a reduction of the systems' tensile strength. This reduction is mainly caused by additional transverse pressure, cracks in the concrete matrix, and possible pre-damage due to the bent textile. As most available prediction models use static reduction factors, the dependencies of these influences have not been considered sufficiently yet. This paper presents an experimental study regarding the effects of curvature on the tensile strength of FRCM-strengthening layers. Therefore, 93 specimens with varying radii, FRCM composite materials, and numbers of applied textile layers were tested in a newly developed setup. The test results show significant dependencies on the various parameters and contrast with the static reduction factors used so far.