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The Southern Link is a major road tunnel project, on the southern outskirts of Stockholm. It covers a distance of about 6 kilometres, of which 4.5 run through rock tunnel. The project is estimated to cost approximately SEK 7 billion. This project includes tunnels with imposing dimensions constructed by a drill and blast method through hard rock. Steel Fibre Reinforced Shotcrete (SFRS) and bolts were used as primary rock support. The performance requirements for the shotcrete were very stringent with respect to strength, durability, and environmental considerations. In order to find a mix design that could satisfy these requirements, several preconstruction trials were carried out. To date (October 2001) approximately 95% of the shotcrete work has successfully been completed. Due to blas ting, shotcreting could not take place closer than 50 metres from the working face. To determine the effect of peak particle velocities arising from the blasting, a test programme was carried out. Results from these tests showed that blastinginduced vibrations decline very quickly with distance from the face.

A criteria for how close, in time and distance, to young shotcrete blasting can take place would be an important tool in planning for safe and economical tunnelling projects. As a first step, field tests with shotcrete at ages of 1 to 25 hours, sprayed on tunnel walls, have been conducted in a Swedish mine. The shotcreted areas were subjected to vibrations from explosive charges detonated inside the rock. The acceleration measurements showed that the shotcrete had withstood high particle velocity vibrations without being seriously damaged. Drumminess of shotcrete appeared, indicating that the major failure mechanism was sudden loss of adhesion to the rock. Elastic wave propagation theory has been implemented numerically to further study the failure mechanism. Due to the changes in shotcrete-rock impedance ratio as the shotcrete cures, the stresses at the interface between shotcrete and rock experience a maximum during a short period within the first 24 hours of shotcrete age.

High Performance Polymer fibres for reinforcing shotcrete are a relatively new development in concrete reinforcing fibre technology. This paper traces the development of these fibres types, and presents a review of fibre properties. The testing methods developed to characterize their behavior in shotcrete are discussed. The performance of various High Performance Polymer fibre types, shapes and lengths in shotcrete are discussed relative to the testing methodology employed to describe them. Recent case studies of applications of wet-process shotcrete for ground support are presented and application and specification issues, based on the performance of these fibres, are presented.

To increase early compressive strength in wet mixed shotcrete a new two component chemical system has been investigated. This involves the combination of a ‘dormant’ accelerator, blended with a set consistency control admixture, and a commercial shotcrete accelerator. The effect of the ‘dormant’ accelerator starts once it is mixed with the shotcrete accelerator. An approximate 100% increase in one hour compressive strength values have been measured in small scale laboratory experiments. Two to three times higher compressive strengths have been measured after three hours, compared to concrete activated only by the conventional shotcrete accelerator. The effect on one day strength was insignificant. The new chemical admixture is a proprietary blend of phosphonates, carbonates, alkanol-amines and hydroxy carboxylates.

All cement-based materials are brittle and show a poor response to loads applied in tension. These micro-fracturing materials, therefore, also portray a highly strain-rate sensitive behavior and demonstrate an inferior resistance to fatigue and impulsively applied loads. In shotcrete applications, these limitations are particularly worrisome since inadequate sub-base rigidity may often require a high level of material deformability, toughness and capability to absorb energy. One highly effective solution is that of fiber reinforcement that can dramatically enhance material toughness in shotcrete. This keynote will demonstrates how shotcrete is different from conventional concrete, highlight the leading trends in fiber-reinforced shotcrete, discuss the challenges and elaborate on the opportunities that lay ahead.

Rock bolts are commonly used in conjunction with Fibre Reinforced Shotcrete (FRS) linings to stabilise ground around excavations. A steel cover plate is often placed over the protruding end of a rock bolt to dissipate load from the bolt into the FRS lining. The process of drilling and installing rock bolts takes place after the lining is sprayed, so a hole must be made through the lining to insert the bolt. To examine the effect of this on lining behaviour, an investigation has been carried out to determine the influence of a central hole on the punching capacity of a FRC panel. Grout or concrete is commonly placed between the cover plate and lining to distribute the load more evenly, so the effect of this aspect of rock bolting practice on punching behaviour has also been examined.

The Rossio railway tunnel was built in 1890, about 2 600m long, crossing several formations involving limestones, basalts and also sandy and clayey formations. Recently a new tunnel from the Lisbon Metro was executed above and normal to the railway tunnel with a short distance between the lower part of the metro tunnel and the railway one. A reinforcement support system was designed in the Rossio tunnel in order to assure its stability. The reinforced system comprised the use of grouted bolts and shotcrete. A monitoring system was implemented in order to observe the structural effects induced by the construction of the Metro tunnel in the Rossio tunnel. In this communication the construction procedure is analysed as well as the geomechanical tests performed that involved in situ and laboratory tests and the monitoring tests. For the interpretation of the obtained information a complex 3D model was built using the software FLAC3D, permitting the simulation of the sequential excavation scheme used and non-linear behaviour of the rock and soil formations, and the different support systems used. A simplified seismic was also performed in order to evaluate the influence of an earthquake in both underground structures. Some significant results and final considerations are presented.

Limited knowledge on safe vibration levels near newly sprayed concrete (shotcrete) often leads to over-conservative limits in underground construction and tunnelling, with additional costs and plan ...

Costanera Norte is an urban expressway concessionary in Santiago de Chile connecting its Western and Eastern sides. To enhance this connectivity several measures were necessary, including four new tunnels. The most singular one is ‘Tunnel Kennedy’, that has a length of 1,166 m with two sections (4 and 5 lanes), 17.45m and 20.95m wide, and 11.8m height. The tunnel was excavated in alluvial gravels, partially under the water table. The main challenges were: its section over 250m2, the ground conditions (requiring SEM), its urban environment and low overburden (10 to 16m). The support consists in a 3cm sealing, HEB steel arches spaced 1.0m and 27cm shotcrete. Systematically buttress and occasional canopy tubes have been used. A concrete lining (20cm to 40 cm, depending on the water column), was added, and a third layer 10cm with polypropylene fibers for fire protection. The excavation took place from October 2014 to February, 2017.

In this paper, the composite lining total safety factor design method based on load structure model was established, and the influence of different section shapes on safety was studied. The conclusions are as follows. (1) The representative value of surrounding rock pressure based on the elastoplastic theory is used as the design load, which is suitable for safety and economy. (2) The load structure models established in this paper can calculate the safety factors of shotcrete layer, bolt-surrounding rock bearing arch, anchor bolt and secondary lining, and can obtain the total safety factors of tunnel construction and operation period. (3) The height-span ratio can significantly affect the safety factor. (4) The influence of tunnel depth on surrounding rock pressure and safety factor is relatively large. It is suggested that the corresponding supporting parameters should be adopted according to different burial depths, so as to improve the economy.

The major concern over large underground silo-shaped caverns, consisted of domes and body shafts, is to select the proper excavation method because they are excavated sequentially by subdividing the large cross section considering safety, constructability and stability of working face. In this study, the excavation method and behavior characteristics of a large dome-shaped cavern, which is the upper part of an underground silo, were evaluated by 3-D numerical analyses in which each excavation sequence was simulated along with 3-D geometry. Bench type and pie type were finally proposed as possible excavation methods and compared each other in terms of construction sequence, support system, constructability, and stability, etc. The results showed that the pie type excavation sequence is more applicable than that of the bench type excavation. In the pie type excavation sequence less stress concentration was observed compared to the bench type excavation sequence. The orders of tunnel displacement and shotcrete stress were also comparatively small. In addition, extensive 3-D numerical analyses were performed considering full excavation modeling for the dome and body shaft of a silo. The results showed that construction with the proposed excavation method was safe and feasible.

Shotcrete is widely used as rock support and can support the load from blocks either by bonding to the rock or by bending between rock bolts. By introducing fibres, the ductility of the shotcrete w ...

The realistic assessment of the load-bearing capacity of shotcrete linings becomes necessary a) for early and large deformations in deep tunnels, b) for single lined permanent shotcrete supports and c) for the assessment of the load-bearing capacity of shotcrete structures subsequently stressed by nearby tunnel excavations. The nowadays used linear-elastic models of primary linings using a hypothetical shotcrete stiffness are not accurate enough. This contribution presents a more detailed determination of the load-bearing capacity of the primary shotcrete lining. It captures the time-dependence of the loading history and of the shotcrete properties. The capability of this approach is shown by the back analysis of a deep tunnel section with an advanced shotcrete material model. The material parameters of the young shotcrete have been calibrated based on uniaxial tests on specimens gained during construction. The back analysis presented here combines the advanced material model with a new testing approach.

The present paper presents the results of an experimental campaign aimed at investigating the mechanical performances of different FRC shotcrete mixes by means of tests on square (EN 14488-5) and circular panels (ASTM C1550-12). In particular, two different concrete mixes were tested, they featured the same aggregates, but one of the two mixes contained silica fumes a replacement of a fraction of cement. Different dosages of macro synthetic fibres (3 to 8.5 kg/m3) were used in each of the two mixes as well as a single dosage of steel fibres (30 kg/m3). Both sprayed and cast panels were considered. In addition to these panels cast notched prisms were tested in three-point bending. The paper discusses the mechanical behaviour of the various specimens, analysing the influence of concrete mix and fibre dosage, and presents correlations among different tests.

The hydropower industry has unfortunately experienced several tunnel failures over recent years. The risks and challenges for the successful design and construction of hydropower tunnels are much more elevated in comparison to other types of tunnels as hydropower tunnels are subjected to dynamic operating conditions that are not always fully known prior to operation. Many hydropower tunnels have been designed and constructed as unlined tunnels incorporating shotcrete lining only in limited areas based on technical assessments performed during and at the later stages of construction. The technical assessment can however be subject to limited access, limited data collection from mapping, concealed instability of TBM excavated tunnels, and the misunderstanding of dynamic hydraulic operations. The main risks associated with the design and construction of hydropower tunnels along with causes of recent collapses are presented and explained along with suggestions for improved industry practice.

VK: T20703 A thermally induced shotcrete spalling in-situ experiment will be carried out during 2013 in the ONKALO rock characterization facility on the west coast of Finland. In the technical areas of the planned spent nuclear fuel repository there will be shotcreted rock spaces which will be subjected to thermally induced deformations and stresses. In this heating experiment, we are attempting to establish the failure strength of shotcrete on a pre-stressed rock surface, when the stress state in both the rock and shotcrete are increased by heating. The in-situ strength can be used to evaluate the safety of the shotcreted structures. The interaction between the shotcrete and the rock mass will be observed as the rock approaches its strength limit. Posiva’s Olkiluoto Spalling Experiment niche is ideal for such an experiment due to the amount of supplementary data from preceding experiments. The area is already built, instrumented and contains running experiments. The test arrangement is described and a prediction of theexpected results is provided. Similar research has been carried out using fire-induced thermal actions, but this is the first test to use heaters in the surrounding rock mass in the sub-boiling thermal range.