During a spate of recent tunnel collapses where NATM was employed as a construction method, the future of sprayed concrete itself seemed in doubt. But in a high-cost industry, the savings made possible by the use of sprayed concrete for both temporary and permanent support meant that the process could not be set aside so easily. Sprayed concrete lining can still be used in an engineering design to reduce costs without increasing risk. It is in the best interests of manufacturers to reduce the possibilities of error by standardising materials composition and materials use, so increasing designers’ confidence in shotcrete. This is important for quality assured permanent linings and will also help to provide early temporary support of the face area.
Costs secure method
Post-collapse reports in 1996 resulted in predictions that the costs of sprayed concrete methods in sensitive situations would soar. Since then, it is apparent that a more conservative approach to designing ‘on the hoof’ will not greatly affect the use of sprayed concrete. That said, there is a perceived need for improved permanent support and for rapid and effective temporary support of the face to guard against collapse. For the latter, mobile systems of sufficiently high capacity are required to supply the material when and where it is needed.
The UK tunnelling industry has been more sceptical than most about employing sprayed concrete for permanent lining. It has been usual to specify a membrane between the temporary and permanent lining which can negate the economies of shotcrete. Design engineers have not, so far, been confident enough to specify single-shell lining.
Work since 1994 by the UK Transport Research Laboratory TRL) on behalf of the Highways Agency has investigated the use of fibre reinforced shotcrete in a single pass to determine how tunnelling can be made more economic. The Single Pass Tunnel Lining (SPTL) method could then be considered for a wider range of schemes than NATM as adopted in the UK.
Recent UK use of high performance shotcrete lining has been mainly for temporary support, but the first true SPTL applications include small diameter utility tunnels and a 15m diameter, 15m deep storm relief shaft in clay. A Highways Agency Advice Note on SPTL is in its draft stage and is set to implement the TRL findings. TRL work on SPTL also forms part of a specification for tunnelling works currently with the Highways Agency for approval to form part of the Manual of Contract Documents for Highway Works.
Over a similar period there has been a detailed 41¼2 year study into shotcrete mixes and the effects that pumping and spraying processes have on the properties of both fresh and hardened sprayed concrete. The work was supported financially by the EC’s BRITE-EURAM programme and by the participants, which comprise: Mott MacDonald; Dragados; IMMG; Taywood Engineering; Sprayed Concrete; and the Imperial College of Science & Technology in London.
The study was performed to improve sprayed concrete design and construction practice for underground structures, especially in soft ground. Three of the key findings were: the importance of compatibility testing when using admixtures; potential inconsistencies in equipment for administering accelerators; and the importance of choosing the right aggregate. The work also showed that sprayed concrete with conventional mixes had a strength and durability similar to that of conventionally placed concrete.
The TRL, BRITE-EURAM and other studies show single-pass shotcrete lining to be a valid structure for project design life.
Additives
Although it is possible to apply sprayed concrete with only traditional ingredients, the inclusion of admixtures changes the properties of the mix to meet particular requirements. Chief among them is an accelerator to ensure rapid curing for surface adhesion. Other additives can lengthen periods of workability and can improve pumpability and final structural properties
Accelerators used to be associated with loss of shotcrete strength but recent developments have ensured that losses are minimal. Modern alkali-free accelerators, usually liquid for easier mixing, are claimed to improve strength as well as safety. Knut Garschol of MBT points out that these accelerators can achieve early strength ratings in the J2 and J3 ranges of the Austrian Guidelines for Shotcrete, as well as a good 28 day strength. "The typical early strength using, for example, Meyco SA160, can be given as 1MPa strength increase/h after application. Typical final strength is in the range 40-60MPa. Rather than a typical strength loss at 28 days of 20-30% compared to concrete without accelerator, the typical result with alkali-free accelerator would be 5% strength reduction." (Fig 1 and T&T International, Aug ’99, p53.)
The logistics of using wet mix shotcrete can be improved by employing a hydration control admixture, which stops normal hydration for periods of two hours to some days. This is particularly important in long tunnels, in mining and when there are other operational difficulties. Garschol says that normal curing of sprayed concrete is generally impractical: "An admixture added in the concrete mixer can take care of this and thus increase the bond strength, reduce cracking and improve other quality parameters." A major research programme at Innsbruck University has yielded good results. Other recent admixture technology affects the water-cement ratio of the mix.
Quality & plant
According to Garschol, poor equipment is one of the reasons why designers hesitate to consider sprayed concrete for permanent and durable tunnel linings. But by specifying the required performance criteria for the equipment, worn out and outdated systems can be excluded. "Today," he says, "the capacity of a piston pump for sprayed concrete must be up to about 20m3/h, but it must also function well down to 25% of this capacity. As far as possible, the concrete flow must be pulsation free."
Modern equipment also incorporates facilities for accurate accelerator dosing control linked to the capacity of the concrete pump.
CIFA‘s CSS Spritz System incorporates a choice of additive dosing systems. Traditional forms are Silicaflux for sodium silicates dosing in percentages of 7-20%; Dosaflux for one or more liquid additives at low percentages (0.5-7%); and Fluxair for powders at 1-10%. CIFA has developed Uniflux for the newer alkali-free additives, using a proportional peristaltic batcher. The system incorporates electronic controls for automatic dose setting, datalogging and monitoring.
Sprayed concrete pump manufacturer OCMER says that different rotor shapes and sizes can be used in its machines to obtain pulsation free flow at the nozzle for any kind of mix. The output range is 0.3-18m3/h. OCMER’s Bruno Messina claims that pneumatic delivery at low pressures ensures that the nozzle can be handled manually in spaces where a manipulator cannot be used or where a high quality of finish is required. The company’s OCM-060 machine has an effective maximum capacity of 18m3/h in wet, dry and semi-wet mixes. The sealing system can prevent leakage of even fine, dry mixes. Two hydraulic cylinders are provided to overturn the feed hopper.
The ASS consortium, currently working on the Sedrun intermediate excavations for the Gotthard Base Tunnel in Switzerland, is using a special OCM-060 to meet needs for pneumatic, dependable, sturdy equipment in limited space. All plant must pass through 2m diameter hoist holes in the shaft sinking platforms during every work cycle. The OCM-060 is used daily to apply wet mix shotcrete over rockbolts. The nozzle is handled manually, with the pump operating at 40-50% of its maximum capacity.
Mechanised spray application in larger tunnels increases output and improves control. This trend makes possible pre-programmed computerised control to ensure consistent coverage of the tunnel surfaces. Garschol says that another trend is to integrate all the necessary functions for sprayed concrete application into a single unit.
In large tunnels, application efficiency is improved by using spray booms. In Aliva‘s case, these are up to 17m high; the telescopic AL-307 arm is mounted on the AL-500 modular system. Automatic control always maintains the telescopic mount in the horizontal when spraying walls. A smaller arm, the AL-302, is available for spray heights of over 9m.
According to Sika’s Herzog and Galehr, manual nozzle handling is limited by reaction forces of the concrete spray at 8-9m3/h even without pulsation. With mechanised booms, dry mix shotcrete with moist aggregates can be used to increase production without resorting to wet mix, they say. By retrofitting high-performance spray pumps, the spray capacity of existing carriers has been increased to 10m3/h/nozzle. They claim that one remote-control spray boom has the same output as two hand-held nozzles. Mechanised boom shotcreting removes the operator from the most dangerous location and reduces the time taken for immediate support.
Referring in particular to construction of the Alte Burg Tunnel in Germany, Aliva’s Zaengerle warns that any shortfall in efficiency would necessitate a drastic reduction in output capacity from the designed 20m3/h, resulting in excessive accelerator dosage and the need for constant boom movements from side to side to spray thin layers.
CIFA comments that, with continuous development, problems arise in establishing a univocal general technique for shotcreting. The company asserts that tunnelling requires that various problems be faced in different projects, making it impossible to solve them all using only one type of machine. It recommends an optimal solution for each project. CIFA’s assemblies are mounted on a 4-wheel-drive vehicle with a double motor drive and diesel prime mover. The 30m3/min capacity pump has a double hydraulic setting for rapid valve changing to maintain concrete flow.
Robots
MBT has converted a modern robot system into a fully automatic, computer controlled working prototype unit. It features automatic control of spraying distance, nozzle angle, topographical mapping of the tunnel profile and control of concrete thickness. It is based on the proven Meyco Robojet with the addition of a new operator panel for computer control. Manual, semi-automatic and fully automatic control is possible.
A one-hand joystick – the SpaceMouse, relieves the operator of the need to assess boom movements. With semi-automatic operation, a scanner measures the distance to the tunnel surface over a pre-programmed pattern to an accuracy of +10mm. The operator uses the SpaceMouse to select the area to be scanned and sprayed, and the computer control calculates the correct position and angle for the nozzle based on the scanning data. The system avoids rebound and poor compaction locally. Actual concrete thickness and rebound can be calculated by scans before and after the operation. With this data, it is also possible to back calculate the correct spraying distance for the next operation.
In the fully automatic version, the required nozzle movements are input in advance. When the SpaceMouse is released, computer control takes over. All modes of operation use a dead-man button for safety.
Computer control allows the complete spraying process to be documented; soon this will be linked to a longitudinal position to provide a reference record of tunnel support.
Related Files
Accelerator Dosing Chart
