1

Abstract A well-maintained railway system is an essential requirement in the successful running of heavy haul coal railway networks. To minimise whole of life costs incurred in such a rail network, infrastructure assets must be effectively managed. Incurred costs clearly include the maintenance cost of failures and the resulting delays and cancellations. A well-functioning and healthy track foundation consisting of ballast, capping and subgrade layers may face gradual deterioration due to dynamic wheel loads, coal fouling, etc. Therefore, identification of ongoing issue(s) under or adjacent to the rail track is the primary duty of the geotechnical engineer. In this article, common failure types investigated in Queensland’s heavy haul coal rail network are presented followed by an explanation of associated signs and causes. A shear failure case study is also presented. Keywords: GPR, drainage, ballast pocket, mudhole, moisture pocket, shear failure. 1 Introduction The Central Queensland heavy haul coal railway is Australia’s largest export coal rail network. It transports mining products to nearby ports and power stations on approximately 2400 kilometres total length of track in four major rail corridors [1]. Table 1 presents the general specifications of those corridors and Figure 1 shows a bird’s eye view of the network. Aurizon Network provides, maintains and manages access to the whole network and its associated rail infrastructure. For inspection and maintenance purposes, Aurizon regularly undertakes a series of geotechnical tests including soil classification, consistency limits, and Proctor compaction and CBR tests on formation soil at specific locations of interest. These locations are primarily chosen based on results of the Ground Penetration Radar (GPR) survey and track geometry data. GPR surveys are carried out at least once a year on all corridors and track geometry data are collected several times a year.

Paper 0123456789 Classification of Common Geotechnical Failure Types Occurring in Queensland’s Heavy Haul Rail Network M. Mirzababaei1, J. Egwurube1, Y. Gyasi-Agyei1, D. Foun2 A. Hammond1, P. Keleher1 and D. Nissen3 1Centre for Railway Engineering (CRE) Central Queensland University, Rockhampton, Australia 2AURIZON, Brisbane, Queensland, Australia 3AURIZON, Rockhampton, Queensland, Australia

Civil-Comp Press, 2014 Proceedings of the Second International Conference on Railway Technology: Research, Development and Maintenance, J. Pombo, (Editor), Civil-Comp Press, Stirlingshire, Scotland.

2

Corridor Name

Service Target Length (km)

Type Major Destinations

Blackwater Bowen Basin coal region in Central Queensland

985 Bidirectional Stanwell Power Station Gladstone Power Station Port of Gladstone

Goonyella Bowen Basin coal region in Central Queensland

924 Bidirectional Hay Point port

Newlands Bowen Basin coal region in North Queensland

255 Single track Abbot Point port

Moura Dawson and Callide Valley coal in Central Queensland

235 Single track Gladstone Power Station Comalco Alumina Refinery Cement Australia Port of Gladstone

Table 1. Central Queensland heavy haul coal railway network corridors [1]

3

Figure 1. View of the Central Queensland rail network

An efficient railway network requiring minimal effort to maintain its regular

track geometry is indicative of a healthy subgrade geo-structure. Excessive track deflections may arise either from defects in track structure or instability of the underlying geo-structure. The ballast layer masks the behaviour of the capping layer

4

and subgrade geo-structure as it hides them from view. Therefore, the status of the track’s geo-structure is primarily judged by monitoring changes in the track geometry and/or any deflection in the rails [2]. Alarming symptoms can be adequately recognised well before failure of the track’s geo-structure and frequent maintenance can be organised to prevent speed restrictions, derailments or any other operational interruptions.

Instability of the track’s geo-structure can generally be attributed to one or more of the following: weak subgrade, inadequate capping layer, ballast breakdown, fouled ballast, inadequate thickness of the ballast layer, presence of expansive soils in the subgrade and poor drainage conditions. All these issues could result in subgrade shear failure, ballast pocket formation, mudhole formation or ballast heaving. Therefore, a proactive maintenance programme must be arranged based on the observed failure symptoms followed by a proper repair of the failures.

In the Central Queensland railway network, different types of failure have been observed to the date. Principal failure types can be categorised as follows:

a) Mudholes and pumping sleepers; b) Ballast pockets; and c) Shear failures.

2 Mudholes and pumping sleepers During a high rainfall season, mudholes may emerge because of insufficient drainage of ballast and/or subgrade. Blockage of the ballast drainage pathways for heavy haul coal railway may arise from external freight materials (i.e., coal fines), breakdown of ballast stones or fouling from subgrade material. This phenomenon potentially leads to reduction in load bearing capacity of the ballast layer due to particle lubrication. Moisture retained in fouled ballast can also weaken the capping layer and the underlying subgrade can similarly be weakened by a high water table [3]. At locations with sleepers not properly supported in the ballast, they can be displaced vertically under cyclic wheel loading and exert a pumping action. Mudholes can be formed under this situation at locations with fouled ballast and trapped water. This can result in the bottom surface of the sleeper being gradually worn away. This phenomenon is called "pumping sleepers" [2].

An effective action plan including ballast cleaning (i.e., ballast undercutting), drainage path restoration and considerations for prevention of further coal contamination must be prioritised in areas with mudholes. Drainage paths can be opened up through cleaning of contaminated ballast shoulders to slow the rate of growth of the problem. This is followed by the replacement of the ballast at the locations with emerging mudholes. It is also important to ensure that the water table is kept low so as not to impede drainage.

Central Queensland generally has a warm to hot climate. In the northern tropics, rainfall is mostly confined to the summer and 90% of the annual rainfall occurs between November and April which is in excess of 400mm [4]. Therefore, some of the failures may occur in summer.

5

Table 2 shows a few examples of reported mudhole failures along Central Queensland rail corridors and Figure 2 shows photographs of mudholes with slurry pushing ballast away. Where pumping sleepers occur, vibration of ballast stones around the sides/ends and on top of the sleepers against each other and against the sleeper causes rounding/abrasion of the stones. Therefore, a dust-like fine powder forms from abrasion breakdown of the ballast/sleepers. This phenomenon is called "rolling ballast". Where rolling ballast occurs, the surface of ballast stones turns a white-grey colour. Figure 3 presents an example of rolling ballast.

Location Kilometre Description Possible reason Blackwater 153.600 Isolated mudhole. This location has

had ballast renewed, but fresh ballast is very dusty and appears to have excessive small sized stones.

Poor drainage from ballast

Goonyella 7.900 to 7.940

Mudhole at dipped welded joint. Pumping slurry due to the heavily coal-fouled ballast of inadequate depth (i.e., 200mm).

Poor drainage from ballast

Goonyella 28.700 to 28.750

Mudholes evident around turnout due to the heavily coal-fouled ballast of inadequate depth and dipped welds.

Poor drainage from ballast

Table 2. Reports of mudholes along Central Queensland rail corridors (prior to 2003) [2]

3 Ballast pockets A thin capping layer or soft capping layer may cause formation of ballast pockets under overstressed loadings. A ballast pocket is the localised failure of the capping layer or subgrade directly resulting in a depression under one or both rails due to penetration of ballast into the underlying material and heaving of the ballast at the shoulder and/or the cess. Therefore, water draining from the ballast is trapped in the formed depression. The ballast pocket is highly destructive if the underlying soil is of expansive clay type. In the wet season, the ballast pocket could potentially lead to a shear failure if proactive maintenance is not scheduled immediately. Figure 4 and Table 3 show examples of reported ballast pocket failures along Central Queensland rail corridors.

6

Figure 2. Examples of mudholes and pumping sleepers in Central Queensland rail network

Figure 3 Example of rolling ballast observed in

Central Queensland rail (prior to 2003)

Pumping Sleepers Under Bitumen

7

Figure 4. Example of ballast pockets in

Central Queensland rail Location Kilometre Description Possible reason Blackwater 28.100

to 29.000

Low embankment with water ponding in trench either side is indicative of old cess building work. Ballast is moderately coal-fouled. Ballast pockets identified by GPR. Suggested to install trench drains and lime treatment.

Low strength capping layer

Newlands 143.140 Ballast pocket with progressive formation shear failure

Soft subgrade due to rainfall and fouled ballast

Table 3. Reports of ballast pockets along Central Queensland rail corridors (prior to 2003) [2]

8

4 Shear failure (case study) Subgrade shear failure is generally a gradual slip failure resulting from dynamic rail traffic loading. It is accelerated by the presence of trapped water in sub layers (i.e., ballast pockets) or moisture content changes in the soil from dry to wet. Figure 5 and Table 4 show examples of reported shear failures along Central Queensland rail corridors.

An embankment shear failure occurred on the Newlands corridor in mid-February 2013. The initial inspection carried out a week after the slip failure did not indicate any obvious track settlement at this location. Embankment repair was undertaken 2 weeks afterwards with the placement of a mixture of loose clay and small rocks (approximately 300mm) at a slope of 2H:1V for the full embankment height of 5m.

In early March 2013, track subsidence occurred at this location. This suggested the existence of a poor formation material under both rails further to the observed slope stability issue on the eastern side of this embankment. Therefore, the train speed at this location was restricted to 15km/h rather than the full line speed of 80km/h. Figure 6 shows the profile of the location before and after the shear failure. Dynamic Cone Penetration (DCP) testing indicated the top 1m consists of soft embankment with a CBR value of 2-3. However, the CBR value tended to increase to 10 from a depth of 1m to 2m. During the excavation, water flowed from a ballast pocket under the rail into the side of the trench. This location has now been repaired by renewing drainage paths and compaction of embankment [5].

Figure 5. Example of shear failure in Central Queensland rail

9

Location Kilometre Description Possible reason Moura 12.850 Progressive shear

failure under low leg of curve

Fouled ballast, inadequate ballast depth (180mm), poor drainage

Moura 142.580 and 142.620

Shear failures on tangent track

Inadequate ballast depth (150-200mm) on poor subgrade (measured CBR 1 to 3 in top 400mm) with high water table (water ponding outside the fence).

Blackwater 61.180 Progressive shear failure on both legs of curve to east of public open level crossing

Coal-fouled ballast, inadequate ballast depth (150mm) on poor subgrade

Table 4. Reports of shear failures along Central Queensland rail corridors (prior to 2003) [2]

Figure 6. Formation shear failure stages [5]

10

5 Conclusion The economic growth of Australia owes a great deal to the efficiency of its rail transportation infrastructure. The Central Queensland heavy haul coal railway network, as one of the largest Australian rail networks, transports in excess of 150 Million tonnes of mining product annually. As an efficient heavy haul coal railway, it must carry ever-increasing volumes of commodity with less speed restriction at greatest possible speed to nearby ports and power stations. Any interruption to network operations will incur a huge loss. To fulfil this task, a sound track system including healthy superstructure and substructure is essential. Track subgrade geo-structure plays a key role in the quality of the service. A few types of common geo-failures observed in the Central Queensland heavy haul railway were discussed including a recent formation shear failure case study. Frequent inspection and appropriate maintenance must be scheduled to prevent any failure in the network. Acknowledgement The team involved in carrying out this work is indebted and grateful to Aurizon who has generously supported this work. References [1] Aurizon Corporate Profile 2013, viewed 22 November 2013,

http://www.aurizon.com.au/aboutus/corporate-profile [2] McSweeney, T., Martin, M., Nissen, D. (2003). “Joint CRE/QR Review of QR

Coal Network”, Report No. CRE C 88 CRE-7/03. [3] Australian Rail Track Corporation Ltd (2006). “Track Drainage - Inspection

and Maintenance”, Engineering Practices Manual, Civil Engineering, RTS 3432.

[4] “Aurizon: Blackwater System Information Pack”, viewed 22 November 2013, http://www.aurizon.com.au/Downloads/Blackwater%20System%20Information%20Pack%20-%20Issue%205.1.pdf.