A new approach in determining the load transfer mechanism in fully grouted bolts

Rock bolts are used as temporary and permanent support systems in tunnelling and mining operations. In surface mining they are used for slope stability operations and in underground workings to develop roadway, sink shafts, and stoping operations. Rock bolting technology has developed rapidly over the past three decades due to a better understanding of load transfer mechanisms and advances made in the bolt system technology. Bolts are placed into discontinuous rock to prevent movement between the discontinuity planes, depending on the direction of installation and nature of the discontinuity surfaces. Rock bolting can increase the tension and shear properties of the rock mass. Nowadays, the application of rock bolts for ground reinforcement and stabilisation is worldwide, but its effectiveness depends on rock type, strata lithology, and encapsulation characteristics. Thus the bolt, rock interaction, particularly near the shear joints, and how a bolt reacts to surrounding conditions require continuous evaluation and research. Work provides an in depth study of the bolt, grout, concrete interaction during under axial and lateral loading. To better understand load transfer characterisation bolt shearing across joint and planes, this research programme consists of three parts. Accordingly, a series of experimental studies and field work was undertaken. A numerical technique was developed to obtain the stress and strain developed along the bolt and surrounding materials under axial and lateral loading. Finally, a field investigation programme was undertaken to obtain the load developed along different bolt profiles (another objective of this thesis). Bolt profiles were also investigated by laboratory studies. A double shearing system (DSS) was used to examine bolts shearing. Testing was undertaken in 20, 40, and 100 MPa strength concrete to simulate different rock strengths. Only three bolt types were used in axial loading tests and different thicknesses of resin were evaluated under axial and lateral loading. Tests subjected to lateral loading were undertaken in 0, 5, 10, 20, 50 and 80 kN pre-tension loads, which revealed that the strength of the concrete significantly affects the bolt – joint contribution. Also shear displacement was dramatically reduced when the strength of the concrete was increased. Pre-tension increases the shear resistance of the system. The profile of a rock bolt affects the shear performance and load transfer under axial and lateral loads. The 3-D FE code, ANSYS V. 9.1 was used. To investigate the load transfer and interaction between bolt, grout, and concrete under non-linear conditions, special element types for the materials and contact interfaces were introduced. The stress and strain built up along the materials under axial and lateral loads was examined. A laboratory study on shearing at the bolt, resin interface of fully grouted bolts was extended to field studies in Appin and Metropolitan Collieries in the Southern Coalfields of the Sydney Basin, NSW, Australia. Twelve instrumented bolts were installed at both mines. Both installation sites were in the heading of a retreating long wall mine. The field investigation revealed that the load transfer on a bolt is affected by horizontal in-situ stresses and profile of the bolt surface. It showed that bolt with higher ribs and wider spacing offered greater shear resistance at the bolt – resin interface, which agreed with the laboratory results.