Alison McQuillan – University of New South Wales Sydney & Rocscience Inc. Ismet Canbulat – University of New South Wales Sydney Joung Oh – University of New South Wales Sydney Steven Gale – Thiess Thamer Yacoub – Rocscience Inc.

In this case study, Slide3 was used to determine the likely behaviour of an open cut coal mine excavated slope. As the slope’s actual performance was known, this was an ideal case study to test the reliability of the slope stability analysis methodology in Slide3.

This case is sourced from an open cut coal mine in Queensland, Australia. The excavated slope (i.e. highwall) under review was excavated using a dragline for the main overburden, and then truck and shovel for coal removal. The highwall under review was pre-split to a design of 65 degrees and consisted of a light-coloured sandstone upper band approximately 10 to 15 m thick followed by an interbedded sandstone and siltstone horizon down to the target coal seam. The coal was excavated 12 hours prior to the highwall failure. The geological model indicated no major structure within the failed area; however, seismic lines have located large faulting (approximately 20 to 50 m in displacement) east of the failure area (approximately 80 m away). Pre-failure dimensions and conditions are illustrated in Figures 1 and 2.

Pre-failure, as built slope geometry (acquired from Maptek (2017) i-site scans, three days prior to the failure event) and surfaces of significant units (i.e. coal roof and floors exported from the geological model) were input into the Slide3 model. Anisotropic material strengths were assigned, with weaker strengths assigned in the directions

CASE STUDY: Comparing Slide3 models to actual slope failure in an open cut coal mine

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Figure 1. Case study: Pre-failure highwall geometry (highwall height approximately 53 m)

Figure 2. Case study: Pre-failure slope conditions

of orthogonal jointing previously measured in the area. Material strengths used in Slide3 modelling are typical of those applied in the Bowen Basin, where the case study is located, as shown in Table 1.

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MATERIAL UNIT WEIGHT (KN/M3) COHESION (KPA) FRICTION ANGLE (º)

Fresh Coal Measure Rock 24 110 30

Fresh Coal 15 35 30

Joint 15 2 12

Jointed Coal Measure Rock Joint set 1: Dip = 81°; Dip Direction = 132°Joint set 2: Dip = 74°; Dip Direction = 49°

Table 1. Case study: Modelled material strengths

Groundwater conditions were not well defined by operations prior to the failure event. The site had significant rainfall associated with regional weather event Tropical Cyclone Debbi approximately two months before the slope failure. The site had no groundwater monitoring stations located within the vicinity of the failed slope and therefore could not quantify any build-up of pore pressure behind the slope as a result of this rain event. However, no evidence of water seepage out of the face was present in the days leading up to slope failure. Highwall conditions were therefore initially modelled as dry, understanding the FOS calculated would be over-estimated if increased pore

pressures were present behind the excavated slope face.

Slide3 model settings were as follows:

• Slip surface = Ellipsoid

• Search method = Cuckoo Search with Surface Altering Optimization

• Analysis methods = Janbu.

Without applying slope search limits, the critical failure surface as calculated by Slide3 is presented in Figure 3.

Figure 3. Case study: Slide3 model output displaying the location of the critical (lowest FOS) failure surface and contours of Base Normal Stress (kPa)

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Figure 4. Case study: Post-failure slope condi-tions

Figure 5. Case study: Comparison of Slide3 predicted critical failure surface location (red polygon) vs actual failure location (greyscale). Slide3 predicted the critical failure surface within approximately 35 m of the actual slope failure.

Actual failed conditions are shown in Figure 4.

By comparing the location of the calculated critical failure surface, Figure 3, with actual slope failure conditions, there is a good correlation between the Slide3 predicted critical area and where failure occurred, Figure 5.

AcknowledgementsThe case study in this paper was extracted from an upcoming publication titled ‘Geotechnical Review of an Open Cut Coal Mine Slope using 3D Limit Equilibrium Modelling and New Empirical Run Out Prediction Charts’ which has been accepted for presentation at the Slope Stability Symposium in Seville, Spain, April 2018. The research presented in this paper is funded by the Australian Coal Association Research Program (ACARP).

References1. Cheng, Y., Yip, C. 2007, Three-dimensional asymmetrical

slope stability analysis extension of Bishop’s, Janbu’s and Morgenstern-Price’s techniques, J. Geotech. Geoenvironment, 133 (12), 1544-1555.

2. Maptek Pty Ltd. 2017, http://www.maptek.com/products/i-site/i-site_studio.html. Online reference.

3. McQuillan, A. 2015-, New Methodology for Estimating the Likelihood and Consequence of Excavated Slope Failure in an Unaltered Sedimentary Deposit, PhD Thesis – To be Submitted, University of New South Wales, Sydney.

4. Rocscience Inc. 2016, Slide Version 7.0 - 2D Limit Equilibrium Slope Stability Analysis. www.rocscience.com, Toronto, Ontario, Canada.

5. Rocscience Inc. 2017, Slide3 Version 1.0 - 3D Limit Equilibrium Slope Stability Analysis. www.rocscience.com, Toronto, Ontario, Canada.