Dynamic rock bolt assembly

An underground excavation comprising:

• a tunnel extending into a rock mass;
• a borehole formed in the rock mass and extending from the tunnel through a rock face of the rock mass;
• and
• a rock bolt assembly including:
• a rock bolt body installed in the borehole and extending through the rock face of the rock mass;
• a face plate for transferring a load between the rock face and the rock bolt body, the rock bolt body extending through the face plate;
• a first nut threadably engaged with the rock bolt body and disposed behind the face plate, the first nut hindering an axially rearward movement of the face plate along the rock bolt body;
• and
• a second nut threadably engaged with the rock bolt body, the second nut being disposed behind the first nut and axially spaced apart from the first nut, the second nut hindering the axially rearward movement of the face plate along the rock bolt body after the face plate has moved axially rearwardly past the first nut to accommodate a movement of the rock face during a dynamic event in the rock mass.

A method of retaining a rock face in an underground excavation, the method comprising:

• retaining the rock face using a face plate that is retained by a first retainer attached to a rock bolt body extending through the rock face and into a rock mass from the underground excavation;
• during a dynamic rock mass event causing movement of the rock face into the underground excavation:
• absorbing energy from the rock mass by deformation of the face plate while providing a first resistance to rearward movement of the face plate with the first retainer;
• after deformation of the face plate, allowing the face plate to be pushed by the rock face axially rearwardly along the rock bolt body and past the first retainer;
• and
• after the face plate has been pushed rearwardly along the rock bolt body past the first retainer, providing a second resistance to rearward movement of the face plate with a second retainer attached to the rock bolt body and disposed behind the first retainer, the second resistance to rearward movement of the face plate provided by the second retainer being greater than the first resistance to rearward movement provided by the first retainer.