Doctor of Philosophy
School of Civil, Mining and Environmental Engineering
Cao, Chen, Bolt profile configuration and load transfer capacity optimisation, Doctor of Philosophy thesis, School of Civil, Mining and Environmental Engineering, University of Wollongong, 2012. http://ro.uow.edu.au/theses/3772
Rapid advances in rock bolting technology over the past four decades have firmly established the usage of rock bolts as the primary rock reinforcement system in underground mine support design.
Experimental studies have confirmed that bolt surface profile plays an important role on load transfer of fully grouted rockbolting systems. However, there are no related theories nor mechanical models to explain these experimental observations and to identify the role of bolt profile in the load transfer mechanisms. In traditional rockbolting mechanism studies, the shear resistance caused by the bolt profile reacting with surrounding materials is termed “mechanical interlock” of the system. The effect of the mechanical interlock is integrated into proposed load transfer models by various manners, such as zigzag dilation, supposed shear stress-displacement behaviour, or supposed shear stress-strain behaviour. The interaction between the bolt rib profile and surrounding mediums, which is the origin of mechanical interlock, is ignored in all analytical approaches.
This research work provides a fundamental understanding of the role of the bolt profile and its influence on rock bolting failure. In addition, the research outcomes provide theoretical support for achieving optimum bolt design in engineering applications.
A series of experimental studies were undertaken to identify the interactions between the bolt profile and the resin under axial or lateral loading. Accordingly, the failure modes of the resin around a bolt profile have been classified into two categories: “parallel shear failure” and “dilational slip failure”. Parallel shear failure of the resin is characterised by a cylinder failure surface, which just passes through the tips of the bolt profiles. It occurs if the bolt has closely spaced profiles, or is confined by high radial stiffness materials. Dilational slip failure is characterised by lodged resin in front of the bolt profile forming a conical shaped failure surface. It occurs in lower confinement stiffness (soft rock) or when rib spacing is large.
An achievement of this research work was to formulate dilational slip failure of rock bolting. The governing equation for this kind of failure mode has been derived as:
F = π(R2 - r2) [cos φ/sin(i) • cos(i+φ) • c + tan(i+φ)/sin(i) • p]
Where F is bolt axial load when failure occurs, R and r are bolt rib geometrical parameters, c and φ are the mechanical properties of the grouting material, i is dilational slip face angle and p is radial pressure. Once the dilational slip face angle, i, is acknowledged, the influence of bolt profile geometry on rock bolting failure can be analytically evaluated.
The domain of dilational slip face angle, which is different to bolt rib face angle, can be estimated by derived equation:
sin(φ+2i) ≥ R2-r2/RL cosφ + sinφ
Where L is the rib geometric parameter. In addition, the most vulnerable surface of dilational slip failure has also been predicted. Consequently, the influence of the bolt profile in load transfer mechanism of rockbolting system can be predicted using the governing equation of dilational slip failure mode.
Furthermore, the direct parallel shear failure of rock bolting has been investigated using a stress analysis method. The stress field within the resin annulus introduced by the axial load of the bolt has been formulated based on the half space theory. That is, parallel shear failure will occur if:
T0 = F/π(G2-G1tanφ)
T0 = the initial shear resistance of the failure surface;
G1 = (sinθ - sin2θ)[m(π - θ - γ) + ksinθ + ycosθ] + (cos2θ + sin22θ)γsinθ +ksin2θ(2sinθsin2θ - 1) + 2sin2θ(kcosθ - km + m lnm - γsinθ)
G2 = γsinθ(sinθ - sin2θcos2θ) + cos2θ[m(π - θ - γ) + ksinθ + γcosθ] + ksinθcos22θ - sin2θcosθ(kcosθ - km + m lnm - γsinθ)
θ is bolt rib face angle
k, m and γ are geometric parameters of the bolt profile
Parametric studies of the bolt profile have also been undertaken. Results show that smaller rib face angle or smaller profile height to length ratio bolts are favourable to transfer load radially. Hence, they should be used in hard rock environments. Large rib face angle with higher rib height to length ratio bolts will transfer the major part of axial load into the resin annulus at a direction parallel to the bolt axis. They should be used in soft rock conditions.