RSSB
INS & RST Delivery Unit
TITLE: BALLAST PROJECTION
1.
Reason for this note
This topic concerns a relatively recently encountered phenomenon, in which
track ballast becomes airborne underneath a high speed train running in normal
weather conditions, strikes the under surface of the train leading to damage,
sometimes of a catastrophic nature.
It is different in nature from well-documented incidents in winter, when large
masses of snow or ice, accumulated around bogies, in under-train cavities and
couplings etc, fall onto the track ejecting ballast stones and which then hit the
train.
It is also distinct from British Rail’s problems with HSTs 25 years ago, which
were investigated in great detail at BR Research. Amongst the issues causing
the problem was ballast being catapulted up from the sleeper ends and
ricocheting off the sides of two passing trains. Pieces of broken windows then
caused a cascading effect down the length of the trains. Airflow effects were
however felt to be a secondary contribution to this.
2.
Background
The phenomenon of ballast projection had always been linked to railway
operation in winter conditions. The mechanism can be simply stated as:

Snow and ice is accumulated on the train;

Eventually, due to vibration or to melting, an ice accumulation falls down;

It hits the ballast;

Ballast stones are projected outwards and also towards the train;

Stones hitting the train might: (i) cause damage to the train underbody and (ii)
gather increased momentum from impact with the train;

These stones may then hit the ballast again setting up a chain reaction.
Railway operators have encountered this phenomenon for decades. Due to this,
SNCF impose speed reductions on high speed trains if there is snowfall or the
track is covered by snow. In Germany, DB lowered the ballast level in between
the rails “some centimetres” below the sleepers. This measure ensures that
falling ice will generally hit the sleepers instead of ballast, and the sleepers will
contain any displaced ballast better.
In general, these measures work satisfactorily and ballast projection due to winter
conditions is no longer considered a significant problem for high speed railway
operation.
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In contrast to this, it was totally unanticipated when ballast projection events
occurred in 2003, leading to train damage under non-winter conditions without ice
and snow.
During ICE 3 homologation tests in Belgium and France, a total of three ballast
projection incidents were recorded at train speeds of up to 300 km/h. There had
been no similar incidents before with ICE 3 in Germany, or with TGV in Belgium
and France, even at much higher train speeds.
It was surmised that the phenomenon was a system interaction problem. The
mechanism was thought to consist of the following sequence of events:

Ballast stones start to move due to a combination of aerodynamic and track
dynamic effects;

The stones are accelerated by the induced airflow under the train;

Rolling stones hit obstacles and are projected upwards;

Stones hit the train and gain a large increase of momentum from impact with
the train;

Stones hit the ballast and project more stones upwards and, thus, instigate a
chain reaction.
Shortly after the ICE 3 problems, another incident occurred during 300 km/h test
runs of the ETR 500 on a new section of the new high speed line between Rome
and Naples. Since there had been no problems with ETR 500 reported up to that
point on other Italian high speed lines, there was a suggestion that the design of
the new line contributed to the phenomenon.
The reported incidents caused damage to the trains involved, costing several
hundreds of thousands of Euros each. The ICE 3 train underbody had to be
modified to enable operations in Belgium at significant cost.
3.
Research into ballast projection
There has been a significant amount of research into ballast projection since the
reported incidents, firstly in the DeuFraKo Aerodynamics in Open Air Project
and more recently in the EU funded AeroTRAIN Project, in both of which RSSB
has been involved, (although not directly in the studies of ballast projection).
This research has concentrated on trying to understand the aerodynamic and
mechanical forces acting on ballast stones during the passage of a high speed
train; the way in which moving ballast stones interact and become airborne and
the probability of ballast impacting on trains. Methods applied included:

1/7th model scale test measurements of the airflow in the region between a
generic bogie-intercar gap and the track;

wind tunnel tests of full scale sleeper-ballast configurations;

tests involving catapulting ballast stones into a box of ballast with the
resulting motion of the stones being captured by high speed camera;
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
complementary computational fluid dynamic simulations;

full scale measurements of the airflows and pressures generated on the
ballast by passing high speed trains;

development of theoretical models of ballast movements;

development of a ballast projection risk model.
The research has confirmed that the problem is a complex, stochastic
phenomenon.
Within the AeroTRAIN Project, draft proposals have been made, based on the
research undertaken, for a measurement technique for the assessment of
vehicles in relation to the risk of ballast projection. This methodology is likely to
be incorporated in CEN railway aerodynamics standard EN14067.
4.
Summary of research findings
The research has determined that the basic sequence of events leading to
ballast projection is as was originally envisaged. The critical influence of the
design of the train underbody, as it affects the pressure field generated by the
train on the ballast and the airflow induced by the train on the ballast surface,
has been determined. In addition, the influence of ballast height relative to
sleeper tops is also better understood.
Despite the ballast projection phenomenon being a complex combination of
high speed train design, track design and probability, there are some rather
simple outcomes of the research that can be stated.
1. Ballast projection is associated with train speeds over 250 km/h.
2. The underbody design of the train is a significant parameter. Two
aerodynamic effects that are important are the pressures generated by the
train on the ballast and the induced airspeed under the train. The existence
of cavities, such as occur near the bogies (particularly for trains with
distributed traction1), generalised underbody roughness with low clearances
between the train and the track ballast all exacerbate the problem.
Smoothing the train underbody profile permits lower clearances to the
ballast level and reduces the likelihood of ballast projection, as well as
beneficially reducing aerodynamic drag
3. High ballast levels, with ballast above sleeper levels, also contribute to the
problem as these, too, reduce clearances between the ballast and the
moving train, increasing air speeds under trains and make ballast stones
more vulnerable to aerodynamic effects.
NB Le Pen and Powrie [1] have undertaken research into the lateral sliding
resistance of railway track and have investigated the relative contributions of
base, crib and shoulder ballast. Their work suggests that friction between the
sleeper/crib ballast interface is the mechanism that contributes between 37%
1
It is not known why this is so or what is the mechanism involved, only that such trains have
featured in the reported incidents.
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and 50% of the total lateral sliding resistance, in combination with contributions
from the crib and shoulder sleeper ballast interfaces. The inference is that
increasing the ballast in the crib, above the sleeper tops, would not increase the
lateral sliding resistance to any great extent. In addition, increasing the ballast
shoulder height is much less effective at increasing lateral sliding resistance
than increasing the shoulder width. This suggests that there are no good
reasons for increasing the ballast height either alongside the track or between
the rails.
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5.
1.
References
Le Pen, L.M. & Powrie, W. Contribution of base, crib and shoulder ballast to
the lateral sliding resistance of railway track: a geotechnical perspective. Proc.
IMechE,, pp 113-128, Part F Journal of Rail and Rapid Transit (2011)
Terry Johnson
Aerodynamics Engineer
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