Sleepers are the superstructure component that distributes the load from the rails to the substructure. Sleepers are very influential towards the performance and safety of railway track (Taherinezhad, et al., 2013).
Various types of Railway sleepers are used in South Africa and around the world, these include; wooden sleepers, steel sleepers, concrete sleepers, and compound sleepers (limited). Whilst very little is known about compound sleepers (new product), wooden sleepers and steel sleepers are no longer very popular demand in South Africa. Most of the railway infrastructure in the country use Concrete sleepers as opposed to the traditionally used
MPHIL: Engineering Management Page 44 Timber/wooden sleepers. Tanherinezhad et al. argued that the decision to use steel and concrete sleepers around the world was premised on limited resources (timber) and the rapid development of railway (Taherinezhad, et al., 2013).
3.3.1 Failure mechanism of wooden sleepers
According to a survey conducted by the Railway of Australia (ROA) were 2200 timber sleepers in Australia were studied, the following were found to be the major causes of failure to Timber sleepers; fungal decay, end splitting, termites, still sound, sapwood, shelling, rail cut, weathering, spike kill and knots (Ferdous & Manalo, 2014).
The report goes further to elaborate that, 53% of failures were caused by fungal decay, 10% were caused by end splitting and 7% was caused by termite attacks, whilst the other factors were only responsible for the balance of 30% (Ferdous & Manalo, 2014).
The most dominant mode of timber sleeper failure is the Fungal decay.
This is due timber’s susceptibility to bio-deterioration which may result from many micro-organisms as timber is an organic material. Timber sleepers are mostly under Fungus attack during rainy seasons or in environments characterised by moisture (Ferdous & Manalo, 2014).
Figure 3.1: Wooden sleeper failure as a result of fungal decay
MPHIL: Engineering Management Page 45 According to Konstantinos Tzanakakis, wooden Sleeper failure due to end splitting arises when the sleeper is subjected to large transverse shear loading (Tzanakakis, 2013). This failure may also be experienced during the insertion of a screw-spike, used for the fastening apparatus which fastens the rail to the wooden sleeper.
Figure 3.2: End splitting of wooden sleeper
MPHIL: Engineering Management Page 46 According to “a strong hold for insect research”, Termites are eusocial insects mostly feed on dead plant material and cellulose, generally in the form of wood, leaf litter, soil, or animal dung. in an event where a termite attacks timber, it normally consumes all the materials which contain cellulose and the resultant damage is permanent (Tyagi & Veer, 2016).
Figure 3.3: Wooden sleeper under Termite attack
3.3.2 Failure mechanism of steel sleepers
There is very limited research on steel sleeper’s failure mechanism;
however, it is well recorded in the engineering body of knowledge that;
corrosion and fatigue cracking are the main reasons why steel sleepers are inferior.
Because sleepers are in direct contact with the substructure, steel sleepers are thus exposed to the risk of corrosion when installed in environments where the supporting soil or ballast is characterised by high concentration of salty elements (ARTC, 2009).
Fatigue failure may result from repeated stress caused by cyclic loading as well as the location of the rail-seat which is subjected to heavy shear that is vulnerable to fatigue cracking (Ferdous & Manalo, 2014).
MPHIL: Engineering Management Page 47 Figure 3.4: Corrosion in a steel sleeper
Whilst corrosion and fatigue cracks are the main steel sleepers’ failure mechanism, steel sleepers are also undesirable due to the following reasons:
high electrical conductivity
difficulty of packing it with ballast
can only be used for specific rails which the sleeper would have been manufactured for
3.3.3 Failure mechanism of concrete sleepers
Concrete sleepers are the mostly used type of sleepers in the world. In South Africa, mono-block pre-stressed concrete sleepers are the most used type of sleepers. These sleepers are not without any risk of failure, various studies across the world confirmed how concrete sleepers are so susceptible to failure. In their International concrete sleepers and Fastening System Survey, Dyk et al. ranked the most critical failure concerns internationally as follows:
MPHIL: Engineering Management Page 48 Table 3.1: Ranking of concrete sleeper failure modes
According to the results of the survey, Tamping damage, shoulder wear, cracking from centre binding and cracking from dynamic loads are the major failure modes, internationally.
According to Article published in the Journal of Materials in Civil Engineering, rail-seat deterioration is the most common failure mode for modern pre-stressed concrete sleepers world-wide. The article went further to report that, this type of failure is primarily caused either of the following; hydraulic pressure cracking, rail-seat abrasion, hydro abrasive erosion, freeze thaw cracking and chemical deterioration ( Bakharev &
Struble, August 1997).
According to a Failure mode and effect analysis of concrete ties study that was presented in the 9th International heavy haul Conference in China, a Rail-seat abrasion is caused by the relative movements between the concrete rail seat and the rail pad during train operations. This situation is perpetuated by the gradual wearing away of the cement paste from the concrete due to the frictional forces as the abrasive fine particles and the water penetrates the rail-seat pad interface (Zeman , et al., 2009).
Another major failure mode of concrete ties is that of High-impact loading.
According to a study conducted in Australia, a high-magnitude wheel load can cause bending cracks in concrete sleepers if applied infrequently and
MPHIL: Engineering Management Page 49 for a short duration. The cracks of this nature are mostly detected in the sleeper’s mid-span and they eventually reduce the sleeper’s flexural stiffness.
The engineering body of knowledge contains a lot of literature on concrete sleepers’ failure modes and effects analysis, published from various parts of the world.