Shotcrete for Underground Support X

This paper describes the activities of the International Tunnelling Association (ITA) Working Group 12 on Shotcrete Use. The Working Group has carried out tasks with the purpose of promoting information exchange between National Groups and planning, coordinating and releasing the results of testing programs in order to advance the state-of-the-art of shotcrete for underground support. The intention of the Working Group is to further the idea that the rational use of shotcrete improves possibilities for design and construction of new underground works.

While shotcrete technology has been generally developed over the last millennium, the evolution of mining and tunneling methods has placed unique demands on the materials, means and methods that comprise current concepts of shotcrete systems for ground support. The industry has struggled to keep the specifications, test methods and guides (standards) relevant to the new concepts and applications for shotcrete underground. This discussion outlines the standards available currently to the industry, comment on those under development, and proposes new directions for the community writing documents to assure owners that high quality shotcrete ground support can be produced in a consistent, safe and economical manner.

Recent years have seen a remarkable increase in the trend for building tunnels with larger cross section and in the efforts to implement construction with higher safety and greater consideration to environment. Against this backdrop, shotcrete for tunneling is subject to requirements for advanced performance in a wide range of areas, not only mechanical properties such as high strength and high toughness, but also operational performance, labor safety and health, environmental performance and durability. In order to respond to these requirements, the Japan Society of Civil Engineers formulated the Recommendation for Shotcreting (Draft) Part "Tunnels by NATM" in July 2005. The Recommendation has the following characteristics: it is structured as a set of standard stipulations driven by performance verification, clarifies the association between the design stage and the construction planning/execution stage. And it reflects the important impact that operations implemented in the construction stage have on shotcrete quality.

Shotcrete, produced via the dry-mix or the wet-mix processes, is fast becoming a material of choice for repair, rehabilitation, slope stabilization and ground support. With substantial advances both in the materials technology and high yield shotcreting pumps, shotcrete is fast shunning its image as a weak, inefficient material of low quality. The purpose of this paper is to summarize the research carried out at the University of British Columbia in the last decade or so. The topics covered include process optimization, mechanical properties and durability. Fiber reinforcement has been a topic of particular interest and this is covered in a greater detail. The paper also highlights novel areas of applications.

This paper presents some of the most interesting outcomes of over a decade of shotcrete research at the Department of Civil Engineering, Laval University (Quebec City, Canada). While focusing mainly on dry-mix shotcrete, the paper also covers experimental work done on fresh and hardened properties of wet-mix shotcrete. Topics presented include rheological interpretation of the influence of fresh properties of shotcrete on placement and performance, effects of certain mixture design parameters (presence of accelerator, use of air-entraining admixture, etc.) on strength and durability, and effect of reinforcement encapsulation on performance. Key experimental observations, typical results, and recommendations are presented. Illustrations from actual field practices as well as ongoing research are provided.

The concept of Bayesian statistics has been applied here to analyse aspects of monitoring the thickness of shotcrete linings. This paper presents a theoretical model describing the relationship between the minimum mean thickness stipulated in the design and the estimated mean thickness of the shotcrete in place. It is shown that prior information as well as, new information systematically can be taken into account in the estimation of the population parameters of shotcrete linings, such as mean values and standard deviation. A performance criterion is introduced that opens up for a transparent and systematic approach to monitor the actual shotcrete thickness in place as well as for means to adapt the mean thickness of the shotcrete in place towards the requirements of the design.

Shelters and some drifts in mines should be designed for rock burst induced by detonations. Absorption of released momentum is studied experimentally for both steel fibre and mesh reinforced shotcrete as well as rock bolts anchoring the shotcrete. Even if the shotcrete exhibits a bonding failure primarily, it cannot absorb a significant momentum because the failure mechanism is local. A rock bolt can absorb the momentum if it plasticizes over a reasonable length. A bolt design is demonstrated which gives the bolt such properties.

From a view point of durability improvement of shotcrete for NATM construction under Acid Sulfate Soils conditions, applicability of using shotcrete made with portland blast-furnace slag cement was investigated. The test results showed that even under low temperature conditions the shotcrete made with portland blast-furnace slag cement can ensure the desired early aged strength as shotcrete for NATM by increasing cement content. Moreover it was found that the shotcrete made with portland blast-furnace slag cement has higher sulfuric acid resistance than shotcrete made with normal portland cement. From these results it was indicated that the shotcrete made with portland blast-furnace slag cement has a practical use for NATM in the Acid Sulfate Soils conditions.

Centrally loaded round concrete panels reinforced with various combinations of fibers and welded wire meshes (WWM) were tested under impact loading. Two strength levels (50 MPa and 120 MPa) of the concrete matrix were studied. Both steel and synthetic fibers, at volume concentrations of 0.5% or 1.0%, were used. The impact strengths and fracture energies of the concrete panels were determined, as well as the strain-rate sensitivity of the various mixtures. Quite different behaviour was observed under impact loading, compared to that under static loading. It was concluded that a hybrid reinforcement system was more effective than any single type of reinforcement in mitigating the brittleness of the concrete.

Shotcrete with high early strength, over 2 MPa at the age of 10 minutes, has been developed for enhancement of rock support, improvement of safety, increase of excavation speed, and decrease of total cost in tunneling. The high early strength shotcrete is achieved by use of a calcium-aluminate based powder accelerator and calcium-sulfate based powder additive. The calcium-aluminate based powder accelerator is hydro-reactive and exhibits self-hardening properties on reacting with water. Ettringite (3CaO-Al2O3-3CaSO4-32H2O) is observed by reaction of the accelerator and calcium-sulfate based powder additive, together with cement and water. Formation of Ettringite is thought to enhance the strength development in the early stages of hydration in shotcrete. Shotcrete experiments in a model tunnel resulted in very high strength from early to later ages, 4.0 MPa after 10 minutes, 19.3 MPa after 1 day, and 58.8 MPa after 28 days. Application of the high early strength shotcrete can decrease the required thickness of shotcrete, exclude the need for steel ribs in some applications, shorten the excavation period, and decrease the total tunneling cost.

Aluminium sulphate-based shotcrete set accelerators (also called activators) are known to influence the rate of early-age hydration in shotcrete. This is the primary motivation for their widespread use throughout the mining and tunnelling industries. However, these admixtures are also known to change the degree of compaction that can be achieved with a given shotcrete mix and they can alter the age-dependent post-crack performance of Fibre Reinforced Shotcrete (FRS). The present investigation was undertaken to assess the magnitude and character of these changes in performance as the dosage rate of set accelerator is raised to higher than normal rates.

This study describes an investigation of the post-cracking performance of fibre-reinforced-concrete round determinate panels under large deflections, i.e., after the panels are severely cracked. By measuring the rotation and sliding of the panel segments, an innovative test setup was incorporated into a modified version of ASTM C1550 to measure crack widths. Four types of fibres (one deformed steel fibre and three different synthetic fibres) were incorporated into different concrete matrices to validate the proposed test method. It was found that the test method provides reasonable accuracy for crack width measurement, and that the residual load vs. crack width opening can provide a good indication of the crack resistance of the fibres.

The importance of shotcrete anisotropy has not been properly taken into consideration for the design of underground works. The spraying process creates strata, perpendicular to the direction of spraying, which induce anisotropy on both strength and deformability. In addition to that, properties are usually measured by testing cores obtained perpendicular to the stratification, whereas the major principal stresses are parallel to that direction. This paper presents the results of uniaxial compression tests carried out on specimens cored from a fiber-reinforced wet mix shotcrete used for final lining of the Morro Doce Tunnel, one of the tunnels constructed for the West section of the São Paulo Ring Road. Strength and elastic constants were determined on specimens oriented both parallel and perpendicular to the strata. Differences in the post-peak behavior were encountered, indicating different degrees of ductility.

In the study presented here, the authors illustrate that the properties of steel fiber reinforced shotcrete are a function of the manufacturing process. The paper examines the causes for the higher penetration resistance in dry-mix shotcrete, while highlighting the differences in composition and rebound during the fresh state. A strong influence of the spray process is witnessed on the hardened properties under both compression and flexure. For an identical fiber content, the toughness of shotcrete is noticeably different from that of the corresponding cast concrete. It is seen that the optimal post-crack performance is achieved via the wet mix technique.

The importance of adequate curing to the hydration of cement and development of mechanical strength and durability in concrete is well known. Most measures of concrete matrix performance suffer if good curing practice is not maintained over the first few days or weeks after placement. However, the consequences of inadequate curing in concrete diminish with distance from a drying surface because of the protection provided by the surface layer to underlying material. Shotcrete is widely used to construct tunnel linings and swimming pools, and stabilise slopes and excavated ground. Since the fibres within Fibre Reinforced Shotcrete (FRS) are normally well distributed throughout the material it is valid to question how significant the effects of inadequate curing are on the post-crack performance of FRS. In this investigation, panels representing Fibre Reinforced Shotcrete have been exposed to various early-age storage conditions to assess the influence of curing conditions and mix parameters on post-crack performance.

The use of monofilament macro-synthetic fibres in wet-mix shotcrete applications has grown significantly worldwide since their introduction in the late 90's. As opposed to the stiffer steel fibres, which have to be used at relatively short lengths (30mm) to reduce line blockage, the more flexible macro-synthetic fibres can typically be used, in well proportioned wet-mix shotcrete mixtures, at lengths ranging from 50 to 100mm, without significantly reducing the pumpability and shootability of the mixture. Other reported benefits of macro-synthetic fibres over steel fibres in wet-mix shotcrete include reduced equipment wear and fibre rebound, and increased shotcrete layer built-up thickness. However, attempts to date to incorporate macro-synthetic fibres in dry-mix shotcrete have failed for numerous technical reasons. The problems encountered range from the inability to uniformly mix the fibres at the dry-batching plant using a standard mixing sequence, to fibres causing conduit blockage of the batching dispensing unit, fibres causing blockage of the dry material exiting from the bulk bags, fibres collecting and blocking the shotcrete gun and lines, very high fibre rebound and consequently, very poor in-situ performance. This paper presents the details and results of a systematic testing program that was conducted to identify the key parameters affecting the performance of monofilament macro-synthetic fibres in dry-mix shotcrete. Based on the results obtained from this study, modifications were made to the geometrical characteristics of a specific fibre type to eliminate the problems observed and enable the production of high quality macro-synthetic fibre reinforced dry-mix shotcrete for commercial use.

High performance fibre reinforced shotcrete (HPFRS) is a concrete type differing from other shotcrete types in its higher strengths and in at least one other improved property. In this paper a description is given of the use of HPFRS for the construction of a test section for the primary support of the St. Kuzam Tunnel under construction in Croatia. The test section had a total length of 24 m and was the first application of HPFRS in Croatia. Also, the results of preliminary tests on HPFRS are presented. The test results obtained from the constructed test section illustrate that HPFRS exhibits better properties than conventional fibre reinforced shotcrete which is normally used for the construction of tunnel primary support. In control testing of HPFRS, non-destructive test methods were also employed, with special attention being brought to the importance of their use in tunnelling.

This paper discusses the important characteristics of steel and polymer fibre reinforced shotcrete when used for ground support. It goes on to discuss how the properties of the fibres themselves affect the shape of load versus deflection graphs derived using the main laboratory test methods for evaluating fibre reinforced shotcrete performance. It then discusses the relevance of these differing load/deflection characteristics to actual ground support applications.

The Gotthard Base tunnel (2 x 57 km) is the world's longest tunnel under construction with a total length of 153 km of tunnels, including all the additional structures of the project. All tunnels are supported with shotcrete either from the Back Up of the four TBM's or with spraying mobiles during excavation by drilling and blasting. The focus of the paper is rather the Know How transfer of detailed shotcrete technology and shotcrete experience, than description of the general tunnel project. The paper presents the point of view of the contractor at the Gotthard Base Tunnel, Faido section, the Joint Venture TAT. The following items are discussed in detail: (1) The general behaviour of shotcrete under squeezing rock conditions and consequences and proposals for future projects; (2) Limits of tunnelling support with shotcrete, with steel fibre reinforced shotcrete and methods used under the enormous squeezing rock conditions; (3) Prequalification and specifications for permanent shotcrete quality with alkali-free accelerators; (4) Adaptation of the obtained experience from the prequalification to the real site conditions at the beginning of the project; (5) Quality control and statistics of the site test results; and (6) Detailed analysis of the adhesive properties between the individual shotcrete layers. To investigate and analyse the adhesive properties in detail, various parameters were taken into account: age of the substrate shotcrete, zones under squeezing rock conditions in comparison to zones with stable hard rock conditions, and different methods of cleaning the substrate before spraying the next layer.

The Beacon Hill Tunnel Station is currently under construction as part of the Sound Transit Central Link Light Rail Line in Seattle, Washington. With an excavated depth roughly 49 m (160 ft) below ground, the station is the deepest to be constructed in soft ground in North America. Station excavation is being completed using the Sequential Excavation Method (SEM) with shotcrete used for both the initial excavation support lining and for portions of the final structural lining. This paper presents the general design basis used for construction contract bidding and provides a review of the current excavation status with specific emphasis on lessons learned from field application of both dry-mix Steel Fiber Reinforced Shotcrete. (SFRS) and conventional wet-mix shotcrete used for initial excavation support of the tunnel.

The Seymour Capilano Water Filtration Project includes major underground works comprising twin, 3.8 m diameter, 7.2 km bored tunnels, the 180 m deep Seymour Shaft as the main construction shaft, and the two 275 m deep Capilano raisebore shafts. Construction started in the fall of 2004 with the excavation of the 12 m diameter, 180 m deep, Seymour Shaft by conventional shaft sinking methods through 30 m of dense glacial deposits followed by 150 m of granitic bedrock. An alternative shaft support system consisting of lattice girders, mesh and dry mix shotcrete was implemented by the Tunnel Contractor for sinking through the dense glacial deposits. Shaft support for the bedrock portion followed the base design comprising pattern rock bolts, mesh and dry mix shotcrete. The Tunnel Contractor took advantage of a local batch plant to provide batched shotcrete mix during the day shift and used a manufactured shotcrete product during the night shifts. Following the completion of the main shaft a large shaftbase chamber has been excavated and supported using a similar support system. The dry mix shotcrete was thoroughly evaluated through quality control and quality assurance testing during construction and performed well to meet the project requirements to provide stable and safe shaft and shaftbase excavations to allow for the completion of subsequent underground works.

The George Massey Tunnel is an immersed concrete tube tunnel under the South Arm of the Fraser River near Vancouver, BC, Canada. It was constructed over 45 years ago to take while traffic on Hwy 99 the main highway connecting Vancouver and the 15 interstate highway to Seattle and beyond. This 629 m long immersed tunnel carries four lanes of traffic in two split running tunnels, each 8.2 m wide and 5.2 m high. There are fresh air ducts 2.3 m wide by 5.4 m high on each side of the sunken box tubes. Structural analysis found that the tunnel was seismically deficient by current seismic standards and was vulnerable to breaching in the event of an earthquake of sufficient magnitude. The British Columbia Ministry of Transportation dictated that the tunnel be retrofitted to with-stand a nearby non-subduction earthquake of a magnitude of 7.0 and a distant subduction earthquake of magnitude 8.2. This paper describes the reinforced shotcrete seismic retrofit of the air ducts component of the immersed tube tunnel. It was determined that the form and pour method of reinforced concrete retrofit, while suitable for use in seismic strengthening of the floor of the air ducts and roadways, was not suitable for use in the air duct soffits (because of a requirement for full bond between the prepared duct soffit and repair material). It was thus decided to carry out the retrofit using reinforced shotcrete, with the reinforced shotcrete being applied in three successive 100 mm thick layers. Details of preparation of the concrete air duct soffit for shotcrete application, the shotcrete mixture design, batching and supply, as well as the shotcrete application, finishing and curing processes used are described. The results of quality control tests for compressive strength, boiled absorption and volume of permeable voids, as well as bond tests and finished shotcrete tolerance measurements are also presented. At the time of writing this paper work on the east air duct was nearing completion and work had yet to start on the west air duct. This paper demonstrates that reinforced shotcrete is a viable means of seismically strengthening an underwater tunnel under challenging working conditions.

Following a number of notable traffic tunnel fires in Europe that have led to loss in human life, severe structural damage and extended periods of loss in service due to repair, the methods of structural fire protection and tunnel user safety have been investigated by numerous agencies in Europe in recent years. Both passive and active fire protection systems are seen as necessary elements of a tunnel fire safety system in a holistic approach to effectively combat the devastating effects of fires. In light of ongoing studies and research a great number of systems are entering the industry. It is apparent that worldwide there is a lack of understanding of the role of these systems employed in tunnels, which this paper attempts to rectify. With respect to fire protection of structural concrete tunnel linings, the paper establishes the roles of passive and active systems, then examines the protection performance of fibre modified concrete and spray applied thermal barriers. Based on the performance of each system, and taking into consideration various risk factors, the paper suggests appropriate passive systems for existing and new build tunnel structures. The views expressed in this document are those based on currently known issues and requirements in the tunnel fire protection industry and are the views of Degussa Construction Chemicals. The authors are aware that further research studies are being conducted in both fibre-modified concrete and thermal barriers.

Wet-mix shotcrete continues to evolve in the North American underground mining industry on a variety of different fronts. State-of-the art shotcrete mix designs and the latest in chemical additives are combined in a holistic approach to suit custom requirements. Logistical challenges are being overcome through sophisticated delivery systems to accommodate the mine layout. Robotic equipment and application techniques are enhancing both the rate and quality of in-situ shotcrete placement. Combined, these factors make wet-mix shotcrete a safe, economical and efficient choice as a component of a modern ground control program, both outside of and within mining development cycles. This paper discusses the latest advancements in wet-mix shotcrete in mining applications including a specific treatment of earlier re-entry into shotcreted headings.

In 2003, the challenge of delivering a wet-mix shotcrete to the underground workings of the Laronde mine, at 2,150m (7,050 ft) below surface was met through careful mix design. Sending the mix through the vertical piping system in the shaft, literally put the mix into a free-fall situation, attaining speeds of 315 km/hr over a distance of 2 km. The friction encountered during the fall, raised the temperature of the mix by 10 degrees Celsius, altering its properties. Segregation and shootability were also concerns. An SCC wet-mix shotcrete recipe was developed to overcome these extreme conditions. Between the completion of testing, and January 2006, Laronde Mine has shot more than 35,000 m³ (46,000 yds³) of the SCC wet-mix shotcrete and is currently expanding their shooting capacity.

Historical shotcreting practice at Xstrata Copper's Mount Isa Copper Operations (MICO) has been for the rehabilitation of old drives and reinforcement at the drawpoints of stopes. The Northern 3500 (N3500) project is being developed in highly deformable, talcose ground with relatively high stress conditions, initiating the move to `in-cycle' shotcreting. Criteria for shotcrete being applied during the development cycle include sufficient early age strength to allow the installation of further ground support (resin bolts and welded mesh) without compromising long term strength and therefore maintaining the required development rate. Accelerators were identified as a key factor in the determination of both early age and long term strength and were tested initially on the surface followed by underground trials. Data from penetrometer and beam testing (early age strength) and core testing from panels (long term strength) were used to optimise shotcrete performance in the development cycle.

This paper presents an overview of in-cycle shotcrete application in development activity at the DOZ Mine at Pt. Freeport, Indonesia. Information is provided regarding the development of the in-cycle shotcrete sequence as well as the development of the split-set sequence. An economic analysis is provided for the in-cycle shotcrete application. It is demonstrated that in-cycle shotcrete allows the mine to safely, economically and quickly achieve the necessary ground support and maximize advance development rates, while minimizing cost.

The pumping and shooting of high performance wet-mix shotcrete usually involves a certain amount of compromise. On the one hand, engineers design a mixture with high workability for ease of transport through the hose, and on the other hand, they strive for a mixture that is relatively stiff, adhesive, and cohesive to achieve good adhesion and build-up on vertical or overhead shooting surfaces. Although a decade of developments in set accelerating admixtures and dosing equipment have greatly simplified the application of wet-mix shotcrete in underground environments, only a few fundamental or practical studies have been made on the pumpability of concrete. This paper presents some of the most recent research on the understanding of the key parameters affecting concrete mobility and stability under pressure, i.e. pumpability. Taking into account the mechanics of life-size pumping equipment, complete pressure profiles and pump cylinder fill-rates, along with rheological and tribological properties, are used to predict the mobility and pumpability of a given concrete mixture. A model that accounts for fluid properties and friction is also presented. Experimental results used to validate the model allow explanation of behavioral, variations between the different concrete mixtures.

This paper presents the background and lessons learned from a 23-year old shotcrete repair of two corrosion-damaged concrete bridges. The two CPR Railroad Overpasses (northbound and southbound structures) at Cayley were exposed to highway de-icing salts and had developed severe curb corrosion and spalling in 1981 at age 21 years. The curbs were extensively repaired in the summer of 1982 with site-batched latex-modified dry-mix shotcrete. The work consisted of chipping to remove 220.6 lineal metres (91.3% of total length) of corrosion damaged curb face to an average depth of 150 mm. The scope of work left behind a significant amount of adjacent chloride contaminated non-repaired curb concrete. The repairs are in good condition and remain in service in 2005. No spalling of the non-repaired curb concrete, as might be expected due to the ring anode effect after leaving adjacent concrete un-repaired, has occurred. The repairs have been monitored for corrosion with inspection and copper sulfate electrode (CSE, ASTM C-876) testing on a five year cycle since 1983. The CSE results show that the post-repair readings on both structures declined progressively following repair in 1982 until approximately 1995, when they reached their minimum values and began to increase. The most recent tests in 2004 showed a northbound bridge curb average of –297 mV, which is 78 mV lower than when repaired in 1982. The average 2004 reading on the southbound bridge curbs was –290 mV, still 65 mV lower than in 1982. The repaired curbs are in sound structural condition with no visible damage or debonding, few cracks and little staining after 23 years of service. The test data suggests that the repairs will provide significant additional service life, and a total service life of 35 years is expected. The repair details included the use of galvanized welded wire mesh, manual chipping for concrete removal, followed by sandblasting, shotcreting into forms along the longitudinal edges for trueness. The repair perimeter was not saw cut. The paper presents current condition data and discusses how various repair features contributed to it.

There is a right place and perhaps a wrong place for using dry-mix shotcrete. The right place is usually a remote location where getting wet-mix shotcrete to a jobsite is either not practical, cost effective or both. Also, there are times when scheduling of ready-mix concrete supply conflicts with the project schedule, such as during night shifts or when product is needed at a moment's notice. These situations happen frequently, particularly in some of the faster growing areas in North America. However, it is a known fact that wet-mix shotcrete is a preferred method of shotcreting if for no other reason than that there is less dust to have to contend with and less rebound- which are two good reasons. There have been advances in shotcrete materials supply, particularly in the past six or seven years by some innovative shotcrete contractors, equipment manufacturers and suppliers. They have developed methods of mixing pre-bagged or pre-mixed materials on a jobsite or bringing in material in bulk bags or "super sacks", with all of the dry ingredients that are used in conventional ready mix shotcrete mixtures. This is sometimes accomplished with specialized equipment where dry-mix is transformed to wet-mix shotcrete while adding water and concrete admixtures at the jobsite. Alternatively powdered admixtures are commonly incorporated into the dry "preblended" shotcrete products. Those in the shotcrete industry who have seen and used the dry-to-wet process for years say that this is nothing new, and indeed it is not. However, the vast majority of the construction industry as a whole, including tunneling and shotcrete contractors have neither seen nor used this dry-to-wet shotcrete supply method. Some of the contractors who have successfully used this method are not eager to share their expertise with their competition, and rightly so. The rate at which the shotcrete industry grows and is accepted by designers, engineers and architects has a direct relationship to their knowledge of shotcrete techniques which include proper design methodology along with meaningful shotcrete specifications. Many designers are eager to know the most cost effective and efficient ways to place quality shotcrete, but their knowledge of the equipment and methods of shotcrete supply and construction can be limited. There is no doubt that the ready-mix concrete/shotcrete supplier for wet-mix shotcrete has been, and will continue to be, an important part of growing the shotcrete business and the acceptance of the shotcreting method for placing concrete.

The paper gives a short presentation about the reasons for mechanization of shotcrete application and how this development ties in with trends in the industry, going from wet-mix to dry-mix shotcrete and from mesh reinforcement to fiber reinforcement. These trends and the increasing use of shotcrete for construction of final permanent linings is another reason to use robotic application, to control the quality variation within a given project. Computerized aid is now available both for control of accelerator dosage and for operation of the nozzle manipulator. The main elements of such a system are described using the MEYCO Logica system as a basis. The available modes of operation and the benefits offered are presented along with some practical examples. The expected future developments of the system are described as well. For users who have suitable basic conditions and needs, the extra investment in computer control is both economically favorable as well as it offering quality advantages with a reduced demand for operator proficiency.