Q-slope and SSAM applied to excavated coal mine slopes

The Q-slope classification system is used to assess the stability of excavated rock slopes and provide an indication of long-term stable, reinforcement-free slope angles. Q-slope is based on over 500 rock slope case studies from mines, road and rail cuttings hosted in igneous, sedimentary and metamorphic rocks around the world. Q-slope can be applied for slopes ranging from less than 5 m to more than 250 m in height in both civil and mining environments. This paper describes the application of Q-slope classification system to 38 failed and intact slopes from Australian open cut coal mines. It further describes the relationship between Q-slope and Slope Stability Assessment Methodology (SSAM) ratings for stable slopes based on the available case studies.


Specification
In this study, 38 failed and stable slope cases from Australian open cut coal mines have been assessed using the Q-slope classification system. A relationship is given for stable, quasi-stable and failed slopes based on Q-slope rating. Further, a relationship is defined between Q-slope and SSAM (Slope Stability Assessment Methodology), two empirical classification systems for predicting slope stability.
Q-slope ratings are calculated using Eq. (1) [1] . (1) where RQD = rock quality designation [4] ; Jn = joint set number; Jr = joint roughness number; Ja = joint alteration number; Jwice = environmental and geological condition number, and SRFslope = the maximum strength reduction factor for weathering, low strength and/or faulted zones that may adversely affect slope stability. SSAM ratings are determined by selecting the slope conditions most applicable from the list of critical parameters defined in Table 1 [3] . Fig. 1 displays slope performance and Q-slope rating for 531 slope cases (384 intact, 8 quasi-stable and 139 failed cases).
On the applicability of Q-slope to predict excavated coal mine slope performance, Fig. 1 illustrates a good correlation between Q-slope's prediction of stability, Eq. (2) [2] .
where β = the steepest slope angle not requiring reinforcement or support.
Q-slope and SSAM ratings, Eq. (3) , calculated for the same open cut coal mine slopes, were compared to determine any correlation between Q-slope and SSAM, Fig. 2 .
where SSAM = overall SSAM rating as calculated from conditions selected in Table 1 . Fig. 2 indicates two correlations between Q-slope and the overall SSAM rating for stable slopes. Eq. (4) is a line-of-best fit indicating the main boundary between stable and failed slopes. For Qslope values equal to, or exceeding 2, the upper bound Eq. (5) , may also be used to estimate SSAM. However, Eq. (5) should be cautiously used given it is based on relatively limited data.

Fig. 2.
Relationship between Q-slope and the overall SSAM rating for 54 slope cases.
An example application of Q-slope and SSAM to an excavated coal mine slope is also included. The example case study slope was 46 m high and had an as-built slope geometry of 66 °This case study, shown in Fig. 3 , represents a single geotechnical domain (i.e. zone of expected similar ground behaviour), exhibiting both an intact section of slope (highlighted in the green) and a failed section of Fig. 3. Case study -an intact section of slope is highlighted by the green polygon and a failed section of slope is highlighted by the red polygon. slope (highlighted by the red). Both highlighted sections are bounded by two intersecting sub-vertical discontinuities projected to form a wedge [3] .
An overall SSAM rating of 112 was estimated, which corresponds to a LOF of 85% as per a summation of the following conditions: