YIELDABLE GROUTED STUDS

Overview of the Support Requirements for Tunnels

The rock surrounding an excavation subjected to stress, fractures into slabs sub-parallel to the tunnel outline. The fractures extend into the surrounding rock to varying depths, depending on such factors as the magnitude and orientation of the stress and the strength of the rock. Dilation occurs during the fracturing process and more significantly, as a result of subsequent stress increases brought about by subsequent mining in the vicinity. The amount of dilation can exceed 500 mm and occurs either in a quasi-static manner or rapidly under the impulse of strong seismic ground motions. Conventional end-anchored or grouted support tendons become less effective or fail completely when subjected to deformations of 60 mm or more.

The rock surrounding an excavation can be accelerated to velocities in excess of 3.0 m per second during a seismic event. In order for the support to be effective, it must be able to absorb the kinetic energy imparted to the rock mass by the seismic event, without failing. Support tendons spaced at 1 per m2 must have the capacity to absorb at least 25 kJ of seismic energy. This is equivalent to the kinetic energy in a 2 m thick block in the sidewall, or a 1 m thick slab in the hangingwall, accelerated to a velocity of 3.0 m per second. The work done by conventional tendons prior to failure is of the order of 4 to 6 kJ.

The Cone Bolt

The Cone Bolt is a yielding tendon capable of providing effective support in areas prone to seismic events or high stress changes. Damage to tunnels normally resulting from, or associated with, rockbursts and large quasi-static rock deformations is consequently reduced.

Construction

The end of the tendon remote from the collar of the hole, has a conical enlargement forged into it. The opposite end is either threaded like a conventional end-anchored tendon, or may have a 'Shepherd's Crook' eye to facilitate cable lacing. The entire length of the tendon is coated with a debonding agent. The short leg of the 'Shepherd's Crook' is left uncoated to prevent unravelling of the eye under load.

Both types of tendon are used in conjunction with a suitable bearing plate and in the case of the threaded tendon, a nut and hemispherical seat is also used.

Principle of Operation

The forces generated by the dilating rock are transferred to the tendon via the bearing plate. As this force reaches the yielding point of the tendon, the cone is drawn through the surrounding grout. In so doing, work is done and energy absorbed from the surrounding rock.

Once all of the energy has been absorbed, a state of equilibrium is achieved and the tendon stops yielding. Any subsequent build up of force in the tendon, will result in the yielding process being reinitiated.

Performance

• Yields for 0.5 m or more at displacement rates ranging from very low to > 7 m per second whilst still maintaining support loads of up to 200 kN.
• Readily integrates with other elements of the support system thus requiring minimum training of personnel.
• As easy to install as conventional end-anchored or grouted tendons.
• Not highly sensitive to grout strength, will perform effectively in grouts with a uniaxial compressive strength ranging from 25 to 60 MPa.
• Easily meets the criteria of being able to absorb 25 kJ of energy, the 16 mm Cone Bolt will absorb 40 kJ of energy over a yield length of 500 mm and the 22 mm unit, up to 100 kJ without failing.

Types and Sizes Available

Copyright © 2008 Duraset
Manufacturer of Rock Bolts, Studs & Mechanical Anchors