Safe Support of Underground Hard Rock Excavations with Macrosynthetic Fibre Reinforced Shotcrete. E.S. Bernard.

Transcription

1 Safe Support of Underground Hard Rock Excavations with Macrosynthetic Fibre Reinforced Shotcrete E.S. Bernard

2 Safety in Underground Excavation Fibre Reinforced Shotcrete has revolutionised ground control in underground excavations over the last 20 years, Economic advantages have been the primary driver of the adoption of fibre reinforced shotcrete internationally. However, safety is an important responsibility that contractors, mine owners, and infrastructure operators must carry both during excavation and subsequent operation of underground spaces. For numerous reasons, macro-synthetic fibres offer the best available means of facilititing safe underground excavation both in the short and long term.

3 Safety in Underground Excavation Safety in Western Australian mines has been carefully studied for over 70 years. The records indicate a steady decline over the decades in the number of deaths as measures have been introduced to improve safety. Most of the improvement has been due to changing attitudes, but better technology and processes have also contributed.

4 Safety in Underground Excavation The late 1980s saw a spike in the deaths due to rock falls, and this prompted a review of ground control requirements. The review lead to the publication of MOSHAB requirements for shotcrete to be used in all excavations over 3.5 m high. This requirement has led to the almost universal adoption of shotcrete in underground mining in Australia and a subsequent fall in mortality to very low levels. Deaths and injuries due to rock falls

5 Safety in Underground Excavation Australian experience has shown: Compared to using steel mesh, shotcrete avoids the problems of exposing workers to danger during installation and shaking out of accumulated rocks, Use of mesh with shotcrete has proven to be uneconomic due to wasteful shadowing, shaking and fall-out during spraying, and poor post-crack performance, As a result, fibres have come to be the dominant form of reinforcement for shotcrete.

6 Safety in Underground Excavation Use of Fibre Reinforced Shotcrete is not an infallible means of stabilising ground, Rock bolts do most of the work in controlling stability, so correct design of bolting remains essential to effective control of ground regardless of the use of shotcrete However, shotcrete reduces the number of bolts required and improves overall stability, especially in the long term.

7 Early-age Behaviour Early application of shotcrete immediately after scaling is important to limit unravelling of the ground Securing keystones in place prevents the underlying ground from loosening Sensible scaling using either manual or robotic means is important to effective control of ground at early-age When shotcrete is used, accidents are most common during spraying and during early-age curing up to about 12 hours age. Procedures have been developed to reduce the risk to workers during this time.

8 Early-age Behaviour Accepted Practice for Safe In-cycle Spraying After blasting and mucking out, scale down the surface of the freshly excavated rock to bring down loose rocks and zones of fractured material using scaling bars, jumbos, or hydroscaling. Wash the surface of the rock to eliminate dust and debris that can compromise bond to the concrete, and ensure sufficient water is present at the surface so that moisture is not absorbed from the concrete immediately after spraying. Spray the FRS onto the rock surface, taking care to avoid spraying set accelerator or any other material such as hydraulic oil onto the rock before the concrete. Apply the concrete by first filling crevices and then covering the surface in a layer of uniform thickness. After spraying is completed, do not permit any workers or machines to enter the area under the freshly sprayed shotcrete until a prescribed waiting period has elapsed.

9 Early-age Behaviour Accepted Practice for Safe In-cycle Spraying When drilling of holes for the bolts commences, only the arms of the drilling jumbo are permitted under the freshly sprayed shotcrete because the ground is still considered unsafe until bolts have been fully installed. If the cover plates to each bolt are depressed into the surface of the concrete as each bolt is stressed up, or the concrete is washed away by the drilling water, then the concrete is too weak (or green ) for drilling to take place and the jumbo operator should stop and wait until the concrete has hardened further. During the period before all the bolts have been installed and stressed, the arms of the jumbo should be retracted to a safe area of ground under fully installed bolts if manual labour is required to place new bolts or drills in the jumbo arm.

10 Early-age Behaviour Accepted Practice for Safe In-cycle Spraying Workers must not venture under the freshly sprayed ground to place bolts or any other piece of equipment in the jumbo arm before all the bolts have been installed and stressed. Failure to comply with these requirements has caused the majority of deaths due to rock fall in Australian mines in recent years. Contractors have been compelled to apply these strict rules through the imposition of heavy fines and production shutdowns for failures. The principal question remaining is: how long shall the jumbo operator wait until drilling can start after spraying? The answer depends on the ground conditions, thickness of shotcrete, and rate of strength gain.

11 Early-age Behaviour Load capacity Delamination Start of delamination Time after spraying At early-ages essentially all failures are by the punching shear mode and macro fibres do NOT improve capacity. However, micro-synthetic fibres can improve shear resistance over the first few hours.

12 Early-age Behaviour Fall-outs of slabs of concrete with or without rock attached are the primary risk to worker safety at early ages. Experience has shown that fallouts of shotcrete alone are unlikely after the concrete has attained 0.2 MPa compressive strength. Engineering tests and calculations have shown that fall-outs of rock are unlikely for most ground conditions once the concrete has reached 1.0 MPa. Robotic machine operation is acceptable once the concrete has reached 0.6 MPa. Shotcrete thickness must be at least 50 mm.

13 Medium-term Behaviour Once the shotcrete has hardened, the stability of the ground will depend on the adequacy of lining performance. For hard-rock excavations, performance is measured as energy absorption in ASTM C1550. The rock bolts are the primary means of securing the ground, shotcrete is assumed to stabilise ground between the bolts. Type of Support Low deformation Moderate ground support High-level ground support Minimum Toughness* 280 Joules 360 Joules 450 Joules * Energy at 40 mm in ASTM C1550 ASTM C1550 Round Panel Test.

14 Medium-term Behaviour For thick shotcrete and FRC linings, performance is estimated using ASTM C1609 for tensile capacity.

15 Medium-term Behaviour The problem with beam tests like ASTM C1609 and EN14651 is that the Coefficient of Variation in performance is about 25% This poor repeatability makes beam tests unsuited for Quality Control mm diameter round panel test currently under development at ASTM exhibits a population COV of only 10%, which is half that of beams. This excellent repeatability improves the economy of FRC use and allows conventional statistics to be used in FRS design. For 150+ mm thick linings, performance is expressed as a residual strength just like for beams.

16 Long-term Performance In the long term, performance and safety depend on the reliable maintenance of FRS lining performance. The issues of importance include: Concrete Integrity Structural Ductility Embrittlement Corrosion Creep Embrittlement is an insidious and dangerous process whereby the ductility of a lining steadily falls with time, thus the ability of the lining to re-distribute stress falls below that assumed in common design methods.

17 Long-term Performance Concrete Integrity Shotcrete is a highly durable material in most environments, and especially in underground environments free from salt spray. Even in the most severe mining environments in Australia, the concrete matrix has performed well. The only major threats to the shotcrete matrix are freeze-thaw action combined with salt, and microbial breakdown of steel FRS in sub-sea tunnels as described by Hagelia in Norway. This problem is caused by the steel fibres and thus macro-synthetic fibres are now prescribed in sub-sea tunnels. Good matrix performance does not protect the reinforcement once the matrix cracks!

18 Long-term Performance Structural Ductility mm specimens Load (kn) Deflection (mm) Tests on large- scale macro- synthe1c FRS linings (2.4 m in diameter) show that in- plane compression combined with toughness provided by fibres leads to high capacity even for severely cracked linings. Thus, cracks in macro- synthe1c FRS linings indicate that the design is economical!

19 Long-term Performance Structural Ductility Tests of lining capacity in mines indicate that restraint provided by bolts plays a major role in controlling lining failure and that strain- hardening behaviour can occur even in a macro- synthe1c FRS displaying strain- sogening in panel tests.

20 Cast macro-synthetic FRC lining When shotcrete and cast-in-place FRC are used in a complete ring, the in-plane compression in the lining will combine with the toughness of the FRC to produce substantial ductility.

21 Long-term Performance Embrittlement Most people assume that when a FRS mix satisfies performance at 28 days, that this performance will be retained thereafter. This is NOT necessarily true when steel fibres are used. Post-crack ductility steadily falls for steel fibres in concrete due to a change in the mode of failure that occurs with aging.

22 Long-term Performance The performance of steel FRS may sometimes look good at 28 days, but the ductility evident in young shotcrete can largely disappear after one year presenting major problems in earthquake-prone areas.

23 Long-term Performance The performance loss that occurs in steel FRS happens in both cracked and uncracked concrete because it is related to the increased hardness of the concrete that happens with aging and bond development between the steel and concrete. Macro-synthetic fibres do not suffer embrittlement because the mode of failure does not change with age. Thus shotcrete reinforced with macro-synthetic fibres retains performance with age. The serious performance loss that can occur in steel FRS and FRC makes steel fibres unsuited for use in seismically active areas. This is one of the principal reasons steel fibres have disappeared from mining.

24 Long-term Performance Embrittlement also occurs in cast FRC that includes steel fibres. This mix incorporated 40 kg/m 3 of Dramix RC65/60 fibres and showed a dramatic fall in residual strength at 3 mm by only 1 year age.

25 Long-term Performance Dramatic performance loss also occurs in cast steel FRC due to the process of embrittlement. This mix exhibited a strength of 60 MPa at 28 days and by 1 year had lost 50% of performance at 3 mm in ASTM C1609 beams. In contrast, macro-synthetic fibres do not suffer embrittlement and are therefore much better suited for use in seismically active areas regardless of whether cast or sprayed concrete is used.

26 Long-term Performance Corrosion Cracks allow water, air, and salts access to the fibres If the fibres are made of steel, they will rust very quickly In percolating environments, cracks as small as 0.2 mm will lead to rapid corrosion and complete loss of structural continuity in less than 6 months Macro-synthetic fibres do not suffer any corrosion under any circumstance

27 Long-term Performance Creep Creep involves the steady increase in deformation of a structure under constant load. Creep of shotcrete matrix is high compared to normal concrete. This is considered good because it reduces the likelihood of cracks forming. Cracked steel mesh-reinforced shotcrete creeps more in bending than uncracked shotcrete. Steel FRS creeps very little across the cracks. Cracked macro-synthetic FRS creeps more than steel mesh, but only under high loads. Low performance synthetic fibres creep much more than high performance synthetic fibres.

28 Long-term Performance Creep It is important to note that creep deformation does not necessarily compromise safety because the load resistance is retained as the deformation increases. Creep deformation can enhance ductility under some circumstances. Only if creep rupture occurs is safety affected. Evidence indicates that high performance macro-synthetic FRS exhibits creep rupture at 60% of static capacity. Steel FRS exhibits creep rupture at 75% of static capacity in tests by Kusterle in Austria.

29 Safety in Underground Excavation Summary Fibre Reinforced Shotcrete has revolutionised safety in underground excavation Provided safe operating procedures are enforced, death and injury from rock falls should be close to zero for underground workers Macro-synthetic fibres have out-performed all alternatives when unbiased engineering assessment is applied to ground control using shotcrete In the long term, macro-synthetic fibres are the only form of reinforcement that both retains capacity as the concrete ages and remains completely free of corrosion. These features make macro-synthetic FRS the safest available means of stabilising ground both in the short and long term.