1122
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
were not designed by detailed analysis of discharge from
upstream catchments (although planning was thorough,
including years of field surveying and observations). However,
over long time periods and including possible climatic change
with increased rainfall, it is no surprise that the flow capacity of
the culverts may be exceeded from time to time. Human
activity, as urban development or construction of highways,
may also result in changed drainage patterns along the railway
line, increased discharge and reduced concentration time for the
catchment. ”Wear and tear” of the constructions and insufficient
maintenance may reduce flow capacity of culverts. Maintenance
may fail to detect damage or blockage of inlet or outlet, or
collapse of culverts inside embankments. One problem has been
that culverts are not extended when embankments are widened,
which may result in burying of inlet or outlet.
In 2011 damage was frequently caused by flash floods
following intense short-duraion rainfall. High discharge may
result in upstream damming of water due to the culvert being
unable to transport the discharge from the catchment, unless the
water finds alternative ways underneath or through the
embankment (which in turn destabilized several embankments
constructed of sand during the same period). Damming of water
will increase pore-water pressures in the embankment and lead
to collapse of a construction which needs drained conditions to
remain stable. Overtopping of embankments is particularly
disastrous, especially for embankments constructed of materials
that are easily eroded, as sand and silt. Embankments may
experience rapid and total destruction under such conditions. A
photo showing an example of complete destruction of the
embankment around a culvert is shown in Fig. 1. Note that the
embankment is mainly constructed of relatively fine, sandy soil.
Figure 1. Example of total destruction of embankment around culvert
near Ål, Bergensbanen railway line (Oslo to Bergen).
Settlements of the underground due to the weight of the
embankment may be considerable on soft marine clay, and also
results in deformation of the culvert. Settlements increase under
the centre, and decrease towards the foot of the embankment.
Horizontal sliding of culverts may also be caused by horizontal
soil pressures within the embankment. Displacements result in
the opening of gaps between stone blocks. Sprinkling of soil
from the embankment may result in cavities in the embankment.
Gaps may also result in water leaving the culvert, finding new
flow paths through the embankment, causing internal erosion.
Large deformations may lead to collapse and internal blocking
of the culvert. Vegetation transported during flooding may
block the inlet of culverts temporarily, which may not easily be
detected during an intense rainstorm. Landslides in the side
terrain can block the culvert and cause upstream damming.
For flash-flood events it is a problem that even regular and
frequent inspection of the railway during a critical situation may
be insufficient to detect incipient collapse of embankments.
Collapse of the embankment around a culvert may occur
between two consecutive inspections due to short-term intense
scours. For small catchments concentration times are short, and
the distance between existing meteorological stations does not
give sufficient information to forecast flash- flood events along
the railway lines (even when weather radar images are used as
supplement). In some cases, trains, unable to stop when a
collapsed section of the railway line was encountered without
any warning, have spectacularly passed the collapsed section of
the embankment on rails hanging in the air, as a suspended
bridge. In other cases, suspected embankment collapses have
been reported by the engine driver, who noticed unusual
behaviour of the train. The weight of the train then was the
remaining load necessary to initiate the collapse, which may
have occurred as rapid liquefaction of saturated soil volumes.
Improved maintenance, redesign and reconstruction of
culverts may reduce problems in the future. However, the high
number of culverts lines (tens of thousands) indicates that
similar collapses of embankments may still be a problem in the
future. Improved design of culverts with built-in safeguards
(increased cross section, double pipelines etc.) is possible, but
costly judged from normal maintenance budgets. Modernization
normally is restricted to already known problem areas. This will
keep existing culverts mainly unaltered and still contributing to
a high future risk.
2.4
Damage related to flooding
A second type of damages is caused by flooding in large rivers
and lakes along the railway. Settlements, local slope failures of
embankments, erosion along embankments and deposition of
fines are typical results of general water rise in rivers or lakes.
Slopes typically collapse when external water levels normalize.
During 2011 flooding was primarily a problem along the
Dovre line, the main railway line between Oslo and Trondheim,
and traffic was cut in periods. Due to the nature of flooding in
large rivers and lakes, these situations generally are less
dramatic than sudden destruction of embankments at culverts.
Water levels from regional flooding normally rise
comparatively slowly (when compared to flash floods in small
catchments), which allows evaluation of the situation as it
develops. Railway lines are normally resilient to such events,
and complete collapse will normally not occur. For regional
flooding the situation may be monitored as flooding develops,
and associated risk for train traffic be evaluated. The Norwegian
national system for flood warnings is well developed, and
flooding in large rivers and lakes should come as no surprise.
Based on regional warnings and weather forecasts mitigating
actions may ideally be well planned (e.g. reduced speed,
temporary closure of train traffic). The National Railroad
Administration also has introduced three alert levels for these
situations, based on weather forecasts.
2.5
Damages related to embankment slope failure
A third type of damage relates to slope failures in
embankments. Some embankments collapsed due to increased
supply of water in dikes upstream of the embankment.
However, several embankments collapsed where there were no
culverts, no dammed water upstream of the embankment, and
where no flooding occurred. Some collapses appeared rather
enigmatic at the first glance, and are interesting from a
geotechnical point of view. One slope failure occurred on an
embankment elevated about 5-6 m above a flat terrain, while
another occurred on an 8-10 m high fill across a ravine. For
these embankments water should ideally not be able to invade
the construction, however, this is exactly what happened. One
case is discussed in more detail in this paper.
