INTRO: Innovative management and a commitment to research have halved the cost of hauling iron ore to Port Hedland in Western Australia in the last five years. A key lever has been less variable loading that sees axleloads now nudging 37 tonnes, with 40 tonnes in prospect

BYLINE: Mike Moynan, Alex Cowin, Graeme Offereins and Graham Tew*

BYLINE: *Mike Moynan is Railroad Technical Superintendent at BHP Iron Ore, Alex Cowin is Rolling Stock Maintenance and Graeme Offereins is Traction Maintenance Superintendent. Graham Tew is Manager, Rail Research, at BHP Melbourne Research Laboratories

BHP Iron Ore has long been recognised as one of the world’s leading heavy haul railway operators, thanks to a continual push towards lower transport costs through a combination of innovative management and technical solutions. The nominal axleload to which ore cars are loaded is a major factor, and we are now progressing towards the 37·5 tonne (41200lb per wheel) barrier.

The railway operated by BHP-IO (formerly Mt Newman Mining) was constructed in 1969 to move iron ore from Mt Whaleback to the unloading facility at Port Hedland. The original main line was 425 km of single line track with passing sidings at about 30 km intervals. Trains of 144 cars typically operating at 30 tonnes axleloads soon saw this track carrying 30 million gross tonnes (MGT) a year.

Current operations involve Locotrol consists of 220 to 240 cars at axleloads up to 37 tonnes moving around 100 MGT a year using, in the main, the original rolling stock. To provide increased capacity, approximately 250 new cars have been added to the fleet.

Significant changes have occurred over the years, including several new mine sites with their own rail loading facilities located on branches off the original main line. It is from one of these sites, Yandicoogina, that the highest axleloads are currently being delivered. The benefits have been recognised, so axleloads are now being raised at the other mine sites.

Rising axleload trend

Fig 1 shows the trends in mean axleloads despatched from the two main ore sources at Whaleback and Yandi since July 1994. Approximately 80% of the ore delivered to Port Hedland comes from these loading points in approximately equal proportions. As may be seen, the mean axleload originating at Yandi has reached 36·5 tonnes this year.

Prior to 1994, BHP-IO had run been running 32·5 tonne axleloads, thanks to a study by BHP Research in 1979 which justified a rise from 30 tonnes1. Whilst this initial study identified 35 tonne axleloads as feasible and cost effective, caution dictated a staged approach.

The assessment driving this first change was built around theoretical relationships allied to detailed knowledge of past performance. It included major costs associated with rail (defects, replacement, grinding), surfacing, ballast, sleepers and subgrade as well as wheel wear, coupler fatigue, bearing fatigue, inspection and fuel consumption. While many areas of uncertainty existed, the benefits of increasing axleload far outweighed costs.

In 1987 a further axleload assessment was carried out. The methodology used was similar, but now information gained from the previous axleload increase was available. This study showed that 37 tonnes was economically viable, the only significant impediment being bridge capacity.

Impediments to higher axleloads

Prior to increasing axleloads, several fundamental calculations need to be carried out to see what the physical barriers are. At BHP-IO two such limits were bridges and ore car suspension. Both formed the basis of extensive R&D programmes.

Two types of steel bridge are found on the BHP-IO line: 10·4m spans at various skew angles, and 7m spans. Both are simply supported and have a ballast tray/box construction. In total there are 37 bridges ranging from 3 to 20 spans. They were designed to carry 32 tonne axles.

Bridge capacity evaluations using standard procedures clearly indicated that 40 tonne axleloads would not be possible on these bridges without breaching current codes of practice.

Major parameters are axleload distribution, the dynamic amplification factors used and load distribution through the bridge superstructure. Dynamic amplification factors are significantly influenced by track condition at the approaches and on the bridges as well as the vehicle suspension characteristics.

A modified bridge capacity evaluation procedure was developed which derived the major parameters within the BHP-IO operations. Several bridges were instrumented and the dynamic amplification factors and the stress reduction due to the distribution of loads were derived for current operating conditions. These studies revealed that the dynamic amplification factors derived were much lower than those specified by current codes of practice (AREA, BS5400, etc).

Based on these results, the bridge capacity evaluations carried out permitted operation of 37·5 tonne axleloads on the 7m spans and 40 tonnes on the 10·4m spans. Also, 40 tonnes could be operated if the axleload variability was reduced to