Scour failure of bridges Brian Maddison CEng, MICE Independent Civil Engineering Consultant, Former Engineering Director of Bridgeway Consulting Limited, Nottingham, UK In recent years there have been several bridge collapses in the United Kingdom and Republic of Ireland that have been caused by scour. Towns in the north west of England have been cut off and loss of life occurred. Major railway lines have been closed for extended periods. Although scour is basically the removal of bed material due to flowing water, it has a number of different causes and takes different forms. The paper describes the different forms of scour and looks at a number of case studies to illustrate the different ways in which scour has caused structures to collapse or require protection. The case studies are of railway bridges and are drawn from official investigation reports and underwater examinations carried out by the author. The paper concludes by illustrating ways in which failures due to scour could be avoided by the employment of good bridge management systems. 1. Introduction Bridge failures are fortunately rare, but every year the pages of New Civil Engineer contain details of a collapse that has occurred somewhere in the world. In many cases these collapses could have been avoided by an adequate bridge management regime that included good inspection, assessment and maintenance procedures. One specific type of failure that from time to time causes sudden catastrophic collapse of bridges is the undermining of foundations due to bed scour. Recent examples of collapse due to bed scour are included in this paper and others can be found in the reference documents listed. Bed scour is the transport of bed material by the flow of water and is present to some degree where the river bed or seabed is formed of granular material. Scour increases as flow rates increase and therefore the actual collapse of structures due to scour often occurs during periods of extreme flow, either due to flooding or exceptional tides. Of course, this is exactly the time that direct observation of the foundations of a structure is not possible and therefore a collapse may be put down to an ‘act of God’. A good inspection regime that includes bed measurement and engineering analysis can find indications of developing scour before the situation becomes critical. If this is followed up with well-designed remedial works, undermining of the structure, even in extreme conditions, may be prevented. The basic relationship between flow quantity Q, velocity V and cross-sectional area A is expressed by the equation Q 5 VA. Thus, velocity increases as the flow quantity increases or the available cross-sectional area of the watercourse reduces. The velocity will determine whether bed material of a particular particle size will be transported. This paper essentially deals with practical aspects of scour that can be observed during examination. For a full understanding of scour, it is recommended that the work of May et al. (2002) is referred to. Further insight can also be gained from the works of Hoffmaans and Verheij (1997), Hamill (1999) and Melville and Coleman (2000). Different types of scour are dealt with and illustrated with actual examples, including recent bridge collapses as well as cases where developing scour problems have been found during inspection. The paper draws on the author’s own experience as an engineer/diver in carrying out underwater inspections and developing an underwater inspection regime for British Rail. The cost of one single bridge failure can be immense in terms of disruption to road or rail traffic and even loss of life as well as in purely monetary terms. A small enhancement to the inspection and maintenance regimes for vulnerable bridges can prevent many costly failures. 2. What is scour? Bed scour is a very simple concept but takes a number of different forms. It may be a natural occurrence or due to man- made changes to a river. 2.1 Channel instability (also referred to as natural scour) All scour is the result of the transportation of bed material by the watercourse. Channel instability is a natural phenomenon and is the result of the erosion and deposition of bed material, which occurs gradually under normal conditions or very quickly during floods. Rivers that transport considerable amounts of bed material are most prone to scour and channel instability. These include sand-bed rivers and upland gravel bed rivers. The natural changes that occur will also be affected Forensic Engineering Volume 165 Issue FE1 Scour failure of bridges Maddison Proceedings of the Institution Civil Engineers Forensic Engineering 165 February 2012 Issue FE1 Pages 39–52 http://dx.doi.org/10.1680/feng.2012.165.1.39 Paper 1000016 Received 19/12/2010 Accepted 22/07/2011 Keywords: bridges/failures/maintenance & inspection ice | proceedings ICE Publishing: All rights reserved 39
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Scour failure of bridgesBrian Maddison CEng, MICEIndependent Civil Engineering Consultant, Former Engineering Director ofBridgeway Consulting Limited, Nottingham, UK
In recent years there have been several bridge collapses in the United Kingdom and Republic of Ireland that have been
caused by scour. Towns in the north west of England have been cut off and loss of life occurred. Major railway lines
have been closed for extended periods. Although scour is basically the removal of bed material due to flowing water,
it has a number of different causes and takes different forms. The paper describes the different forms of scour and
looks at a number of case studies to illustrate the different ways in which scour has caused structures to collapse or
require protection. The case studies are of railway bridges and are drawn from official investigation reports and
underwater examinations carried out by the author. The paper concludes by illustrating ways in which failures due to
scour could be avoided by the employment of good bridge management systems.
1. Introduction
Bridge failures are fortunately rare, but every year the pages of
New Civil Engineer contain details of a collapse that has
occurred somewhere in the world. In many cases these
collapses could have been avoided by an adequate bridge
management regime that included good inspection, assessment
and maintenance procedures. One specific type of failure that
from time to time causes sudden catastrophic collapse of
bridges is the undermining of foundations due to bed scour.
Recent examples of collapse due to bed scour are included in
this paper and others can be found in the reference documents
listed.
Bed scour is the transport of bed material by the flow of water
and is present to some degree where the river bed or seabed is
formed of granular material. Scour increases as flow rates
increase and therefore the actual collapse of structures due to
scour often occurs during periods of extreme flow, either due to
flooding or exceptional tides. Of course, this is exactly the time
that direct observation of the foundations of a structure is not
possible and therefore a collapse may be put down to an ‘act of
God’.
A good inspection regime that includes bed measurement and
engineering analysis can find indications of developing scour
before the situation becomes critical. If this is followed up with
well-designed remedial works, undermining of the structure,
even in extreme conditions, may be prevented.
The basic relationship between flow quantity Q, velocity V and
cross-sectional area A is expressed by the equation Q 5 VA.
Thus, velocity increases as the flow quantity increases or the
available cross-sectional area of the watercourse reduces. The
velocity will determine whether bed material of a particular
particle size will be transported. This paper essentially deals
with practical aspects of scour that can be observed during
examination. For a full understanding of scour, it is
recommended that the work of May et al. (2002) is referred
to. Further insight can also be gained from the works of
Hoffmaans and Verheij (1997), Hamill (1999) and Melville and
Coleman (2000).
Different types of scour are dealt with and illustrated with
actual examples, including recent bridge collapses as well as
cases where developing scour problems have been found during
inspection. The paper draws on the author’s own experience as
an engineer/diver in carrying out underwater inspections and
developing an underwater inspection regime for British Rail.
The cost of one single bridge failure can be immense in terms of
disruption to road or rail traffic and even loss of life as well as
in purely monetary terms. A small enhancement to the
inspection and maintenance regimes for vulnerable bridges
can prevent many costly failures.
2. What is scour?
Bed scour is a very simple concept but takes a number of
different forms. It may be a natural occurrence or due to man-
made changes to a river.
2.1 Channel instability (also referred to as natural
scour)
All scour is the result of the transportation of bed material by
the watercourse. Channel instability is a natural phenomenon
and is the result of the erosion and deposition of bed material,
which occurs gradually under normal conditions or very
quickly during floods. Rivers that transport considerable
amounts of bed material are most prone to scour and channel
instability. These include sand-bed rivers and upland gravel
bed rivers. The natural changes that occur will also be affected