## 3 Results and Discussion  
### Creep Behaviour of Coal  
The creep strain and increasing stress did not follow the monotonous incremental path expected. On the contrary, two rapid collapses at relatively low stress levels, 15.99 MPa and 11.02 MPa, occurred. Table 1 lists the calculated strain rates for the secondary stages of the creep tests. The results scatter considerably and no convincing correlation can be established between creep strain and stress level. Although the samples were drilled from the same block of coal, their creep behaviour varied significantly, indicating pronounced inhomogeneity between samples. Because the attempt to demonstrate a relationship between creep and stress failed, the samples were classified according to creep behaviour. The creep curves are divided into three types--Types 1, 2, and 3. On Type 1 curves, (Figure 4a), the axial strain (\(\epsilon_{\mathrm{a}}\)) clearly exhibits primary and secondary creep but the radial strain (\(\epsilon_{\mathrm{r}}\)) creeps at a slow but relatively steady rate for the entire duration of the experiment. Another important feature of this Type is that the volumetric strain (\(\epsilon_{\mathrm{v}}\)) is dominated by axial strain, reflecting the compression. This relationship can also be observed in the highly similar trends of volumetric strain rate (r\({}_{\mathrm{v}}\)) and axial strain rate (r\({}_{\mathrm{a}}\)) shown in Figure 5a or Figure 5c. Both rates increase from the initial point to a peak value, then reverse direction and gradually decline before reaching a stable stage (the stable stage corresponds to secondary creep). Figure 5b presents the change in radial strain rate (r\({}_{\mathrm{r}}\)) for Type 1 creep. It can be seen that r\({}_{\mathrm{r}}\) steeply decelerates (the increasingly  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Creep Stress (MPa)**} & **Axial Strain Rate** & **Radial Strain Rate** & **Volumetric Strain Rate** \\  & **(per hour)** & **(per hour)** & **(per hour)** \\ \hline
8.03 & \(1.59\times 10^{-4}\) & \(-1.08\times 10^{-5}\) & \(1.38\times 10^{-4}\) \\
8.98 & \(2.53\times 10^{-4}\) & \(-1.26\times 10^{-4}\) & \(1.04\times 10^{-6  
negative strain rates) from the initial dilatancy to a relatively stable stage. Compared to volumetric and axial strain rates, the radial strain rate is fairly low. The results from these tests have a moderate amount of scatter. It causes difficulties in identifying the transition point from the primary creep to the secondary creep. In order to facilitate further analysis, equations were fit to the \(\mathrm{r_{a}}\), \(\mathrm{r_{v}}\), and \(\mathrm{r_{r}}\) data, and these are presented below as Equations (1)-(3). \[\mathrm{r_{a}}=\Big{(}-3.3t^{6}+111.9t^{5}-1488.7t^{4}+9732.9t^{3}-31344t^{2} +39750.2t+2738\Big{)}\times 10^{-4}\Big{(}R^{2}=0.94\Big{)}, \tag{1}\]  
\[\mathrm{r_{v}}=\Big{(}-3.7t^{6}+124.3t^{5}-1654.4t^{4}+10817t^{3}-34908t^{2}+45 097.1t+241.2\Big{)}\times 10^{-4}\Big{(}R^{2}=0.94\Big{)}, \tag{2}\]  
\[\mathrm{r_{r}}=\Big{(}-0.5t^{6}+13.9t^{5}-154.2t^{4}+858.6t^{3}-2460.3t^{2}+33 02.5t+1659.1\Big{)}\ \times\ 10^{-4}\Big{(}R^{2}=0.55\Big{)}, \tag{3}\]  
where \(t\) denotes time. As described above, the axial deformation is the dominate mechanism of this type of creep process. Through examining the derivation of Equation (1), the primary creep and the secondary creep are identified and shown in Figure 4a. It can be observed that the primary creep takes up a large portion of the test duration and then transforms into the secondary creep. Another long duration of the secondary creep can also be expected. Type 1 creep occurs at pressures of 8.03, 8.98, 11.98, 13.00, 14.0,2 and 14.97 MPa. Type 2 creep, dilatancy caused by loading, dominates initial deformation (Figure 4b). Approximately 20 min after the load is first imposed, the increasing axial strain cannot be balanced by radial strain, thus the volumetric strain reverses direction to follow the trend of the axial strain. This marks the beginning of compaction.