Durabar® is an engineered yieldable tendon capable of providing effective support in areas with swelling ground and areas prone to seismic activity. A risk assessment done by SRK Consulting indicates that financial and safety risks should decrease one hundred fold when using yieldable Durabar® instead of rebar as support under similar circumstances.
- High ability to dissipate kinetic energy
- Yields at constant load
- High rates of energy dissipation
- Exceptional displacement range
- Very stiff in static conditions
- Least vulnerable to shear failure
- Debonding coating inhibits corrosion
- Easy to install
- No special training needed for existing crews
WHY THE NEED FOR YIELDABLE ARTIFICIAL REINFORCING OR SUPPORT FOR TUNNELS?
Durabar® YIELDABLE TENDONS SA PATENT 94/2177
Mine-induced seismicity and/or energy in the perimeter rock of deep underground openings can lead to severe rockburst damage.
Around an excavation, the rock is subjected to natural and mining induced stresses. These stresses will result in the rock tending to fracture into slabs sub-parallel to the walls of the tunnel. The depth and severity of these fractures are site and rock type specific, depending on factors such as the magnitude and direction of the stress in the rock, amount of fracturing caused by blasting operations and geological weaknesses such as dykes and slips.
Dilation will occur due to the fracturing process. Additional bulking of up to 550mm may occur, either dynamically due to rock bursts, or statically in squeezing ground conditions. Conventional bolt systems normally do not have the ability to accommodate such large movements, and bolts are likely to fail.
Ortlepp (1989) defines the fundamental requirement of a support as follows: “to keep the blocks or fragments in position, without itself developing such high internal stresses as to be destroyed in the process.”
During a seismic event, the peak particle velocity may be in excess of 3m/sec. For a support system in such a situation to be effective, it must be able to absorb the kinetic energy without failing. The movement must be opposed as strongly as possible without the resisting force in the bolt exceeding its strength. In the Durabar® the resisting force is equal to the yield strength of the steel – a value which is very consistent because the bar is an engineered product subject to strict quality control at all stages of manufacture.
In order to contain a moderately severe rockburst, present understanding is that a support tendon in a one metre square pattern should absorb 20kJ of seismic energy without failing. The work done by a 16mm rebar tendon prior to failing is in the order of 3kJ. A 16mm 2.2m Durabar® can effectively dissipate 45kJ over a 600mm displacement.
PRINCIPLES OF OPERATION
As the forces bearing on the Durabar® reach its designed yield load, it starts to slip, with smooth load/deformation characteristics, through the wave path created in the grout by the shape of the Durabar®. The mechanical work done on the steel as it moves through the wave path is the energy dissipation mechanism, leaving the grout surrounding the Durabar® unscathed. In the event of a subsequent force build-up, the yielding will continue along the same path. The performance of Durabar® is thus mainly dependent on the properties of the steel and requires only that the strength of the grout exceeds a minimum value.
The performance characteristics listed below are for 16mm-diameter 450MPa steel, and should be compared to similar systems.
- Capable of yielding for 600mm or more during quasi-static and dynamic load rates
- Easily integrated with other known support systems, thus requiring minimum training
- Robust sensitivity to grout strength
- Installation procedure similar to that of normal Shepherd Crooks
- Exceeds the criterion of 20kJ/m2 for rockburst containment. A 16mm 2.2m Durabar® will effectively yield over a length of 600mm, resulting in 45kJ of energy dissipated over the life of the tendon.