Year

2014

Degree Name

Doctor of Philosophy

Department

School of Civil, Mining and Environmental Engineering

Abstract

Rockbolts have become a popular technique for reinforcing rock mass all over the world as they are very effective and simple to install. Rockbolts are installed to reinforce fractured rock mass by resisting dilation and shear movement along the fractures. Several rockbolts can create a reinforced zone in fractured rock mass to improve the self-supporting capacity of rock. When the bolted rock mass deforms, load transfer occurs between the bolt and the rock. The grouted steel rebar is the most commonly used reinforcement system, using resins or cement grouts as an anchorage.

Better understanding of the bolt load transfer mechanism leads to optimisation of the bolt profile design that can significantly improve the performance of the rockbolt reinforcement system. Historically, analytical models predicting the load distribution along the bolt subjected to tensile loading were proposed however, there were limitations in the available theories, requiring improvements. This thesis uses a combination of approaches involving laboratory tests, together with analytical and numerical models to propose new understanding of the rockbolt load transfer mechanism, aiming to optimise bolt design for ground support and improve rock mass reinforcement.

In this thesis, laboratory push tests were conducted on 100 mm long rockbolt sections grouted in steel tubes with the objective of studying the factors influencing the load transfer capacity of rockbolts. Resin has a key role in activating the mechanical interlocking mechanism of bolt to resin or resin to borehole interface and hence, resin strength was studied.

Two new analytical models are proposed here. The first analytical model of fully grouted rockbolts loaded in tension uses the non-linear bond-slip relationship describing the mechanical interaction at the bolt-grout interface. Derived equations of the loaddisplacement profile, shear stress distribution at the bolt-resin interface and axial load distribution in the bolt are given here. These equations were subsequently validated with experimental results from both the laboratory and in situ studies.

The second analytical model considers the difference in behaviour between fully grouted bolts with and without the bolt free end slip when loaded in tension. This model validated by experimental data aims to predict the stress distribution of fully grouted bolts installed in weak strata where the bolt experiences free end slip. By taking the bolt free end slip into consideration, the shear and axial load distribution along the bolt can be accurately predicted.

Numerical studies using the Fast Lagrangian Analysis of Continua (FLAC) software were undertaken to further validate the bolt-grout bond-slip behaviour. A new approach of non-linear bond-slip relationship embedded into rockbolt elements was used to realistically model the bolt-resin load transfer and displacement profiles. This was achieved by writing the subroutine within FLAC using the programming language FISH. Fully grouted rockbolts subjected to tensile load with and without free end slip were modelled showing a close match with the experimental results in terms of loaddisplacement relationship, rockbolt axial force distribution and interfacial shear stress distribution.

A mine roadway reinforced by fully grouted rockbolts was modelled using FLAC. Interaction between the rockbolts and the rock mass was studied using a numerical modelling approach with the rockbolt elements modified by the FISH subroutine. Rockbolt properties are characterised by the bond-slip relationship of the bolt-grout interface or grout-rock interface, which can be determined by pull or push tests of short rockbolt sections. Numerous 100 mm long bolt sections were push tested and the results implemented in the FLAC model. The interaction between rockbolts and rock mass subject to various stress state, rock mass quality and the presence of rock fractures or joints, was studied and the results are reported here. This new methodology provides engineers with valuable tools to understand the axial behaviour of the rockbolts installed in the field.

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