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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
Forensic EngineeringVolume 165 Issue FE1
Scour failure of bridgesMaddison
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
by a large number of other actions that affect river flows: these
include the placement or removal of artificial obstructions such
as weirs or training walls, exceptional rainfall and increased
runoff due to deforestation or urbanisation.
The river channel may change both in plan and longitudinal
section. Changes in section will occur as bed material is
transported along the river. Areas of deposition will change to
areas of erosion, either naturally or as a result of artificial
actions, and the river bed level will be lowered accordingly,
affecting the stability of any structure founded on that bed (see
Figures 1 to 3). The effect of channel instability on a bridge or
waterside structure may be to undermine foundations and
direct flows towards or behind the structure (Figure 4).
The same actions that cause changes in the longitudinal section
of a river will also cause changes in the plan. In flood plains,
erosion will cause a river to become sinuous. The banks of a
river on the outside of curves will erode and silt will be
deposited on the inside of curves where flows are slower. Thus,
the route of the main river will change over years or,
occasionally, dramatically in times of flood (Figure 5). Spans
of a bridge originally designed to be for flood-relief purposes
only may eventually become the main river spans and often the
piers involved will be founded at a shallower depth than those
of the original river spans (Figure 6).
2.2 Contraction scour
A local reduction in the width of the river can cause a general
lowering of the bed level by decreasing the cross-sectional area
of the river and increasing flow velocity, bed shear stress and
frequency of bed movement. A similar effect can also be caused
when river levels rise to above the soffit of a bridge or other
obstruction. Scour may occur in three ways.
(a) Channel contraction. The normal channel of the river is
reduced at a bridge by the presence of piers, abutments
extending into the channel and/or training walls, etc.
Local increase in sedimentsupply
Deposition of sediment
River bed
Figure 1. Natural raising of the river bed
Local reduction in sediment supply
Bed erosion (scour)
River bed
Figure 2. Natural lowering of the river bed
Distance downstream
Depositing phaseEroding phase
Stable
Cha
nnel
slo
pe
Figure 3. Longitudinal section of river bed
Bridge pier built in areaof deposition
Deposition
Erosion
Bridge pier becomesundermined when
river bed changes and deposition changes to
erosion
Figure 4. Bridge pier undermined by natural bed changes
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(Figure 7). The situation can be worsened by the presence
of heavy debris in the water.
(b) Flood plain and estuary contraction. Typically this will
occur when a railway embankment is built across a flood
plain. In times of flood, runoff from the flood plain is
channelled through a bridge opening with resultant
increase in flow velocity as the same quantity of flood
water is channelled through a relatively small cross-
sectional area. In this situation, flood water will seek any
opening in the flood bank and, as well as contributing to
scour in the main channel, scour may occur at flood-relief
arches, road bridges and similar structures. The same
effect happens when embankments are constructed across
tidal estuaries (Figure 7).
(c) Surcharging. Where the soffit of a bridge is lower than
high water level during a flood period, surcharging will
occur. Water will flow though the bridge opening, which
has a reduced cross-section not only due to the available
width of the river but also limited by the height of the
soffit. Using the equation Q 5 VA, the velocity will be
greater than if the height of the water was not restrained
by the soffit. As the level rises higher above the soffit, the
head of water increases the velocity.
When the increase in flow velocity through a bridge opening
because of channel contraction becomes sufficient to transport
bed material, the result will be an overall lowering of the bed
level at the bridge, probably across the full width of the river,
Original course
River meanders due to erosionMeander
StraightenedOriginal course
Bends are straightened due to deposition
Slow-moving river becomes braded
Figure 5. Changes to rivers in plan
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although this will depend on variations in the bed material.
This effect may be increased by the addition of flood plain
contraction.
Contraction scour may be observed after flooding has ceased
or silt may be deposited as the river slows, making the scour
difficult to detect.
2.3 Local scour
Local scour is a consequence of the presence of a structure in a
watercourse. The turbulence caused as water flows by a pier
will generally cause an uplifting effect at the nose, resulting in
erosion of the bed. The material removed from this area will be
deposited behind the pier as eddies form and the flow slows.
The mechanics of this action have been carefully studied by
hydraulics institutions in Britain, America and Europe and the
general result is as shown in Figure 8. Details of some of the
studies are shown in the literature (Hamill, 1999; Hoffmaans
and Verheij, 1997; May et al., 2002; Melville and Coleman,
2000). As the water strikes the nose of the pier, the direction of
flow is directed downwards causing material to be removed
from the bed at this point.
The shape of local scour shown in Figure 8 may be modified by
many factors. Most significant among these will be the shape
of the nose of the pier and the direction of flow relative to the
pier. If the pier is not aligned with the flow, local scour will
occur along the side of the pier that faces the flow. Hamill
(1999), Melville and Coleman (2000) and May et al. (2002) all
provide more detailed information.
In tidal waters, scour may occur on both ebb and flow of the
tide, resulting in scour at both ends of a pier. Scour may also
occur at ‘downstream’ ends of river piers when there is
significant backflow.
Local scour also occurs to weirs and protection inverts that
cross a watercourse. Typically, scour will start to develop at the
downstream end of an invert and, as the scour develops, the
weir effect increases until a deep hole develops. The scour then
gets under the invert and the trailing edge of the invert breaks
off. This process is progressive and can continue until the
invert breaks back to the bridge and the piers become
undermined (Figure 9).
The rapid movement of water caused by boat propellers and
water jet engines can also cause scour. A particular problem for
harbour walls can be the use of side-thrusters that are often
directed at the walls in close proximity.
Viaduct at time of construction
Viaduct after change of river position
River spans
Foundations exposed and undermined
River spans
Figure 6. Bridge piers at risk due to changes in the course of the
river
Flood plainFlood plain
Flood plain Flood plainEm
bank
men
tE
mba
nkm
ent
Con
tract
ed fl
ood
plai
n w
idth
Con
tract
ed ri
ver w
idth
Training wall
River flowArea of bed lowering
Figure 7. Contraction scour
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2.4 Scour sub-divisions
All three types of scour may be sub-divided into two types.
(a) Clear water scour. This often occurs at piers in relatively
low flows and is the simple removal of bed material by
flowing water. Upstream of a bridge, the water is not
transporting significant amounts of bed material (hence
the term clear water). At the bridge, bed material is
removed and transported away but no material from
upstream is deposited at the same time. Therefore, scour
holes formed remain present when flows subside and can
be seen during underwater inspection.
(b) Live bed scour is the continuous erosion and deposition
of bed material during periods of flooding. In its worst
case, the bed under a pier foundation will become fluid as
material is constantly removed and replaced. This type of
scour may be very difficult to detect. By the time a diver is
sent down to inspect the pier, the flow will have reduced
and the bed stabilised at a much higher level than the
maximum scour level during flood.
3. Case studies and investigations
3.1 Glanrhyd 1987: local scour, channel instability
and live bed scour
On 19 October 1987, the bridge carrying the Central Wales line
over the River Towy collapsed during a period of heavy rain and
flooding. At about 07:15, a passenger train ran on to the bridge
and fell into the swollen river. Four people died. The full report
of the accident was published by the railway inspectorate (DfT,
2009). The investigations undertaken included the following.
(a) A detailed underwater survey of the remaining parts of the
damaged piers undertaken by a diving team led by a
chartered civil engineer. The results of this survey and
other investigations concluded that the collapse was caused
by scour to the downstream end of pier 3 that undermined
the foundations, allowing the pier to settle and eventually
break its back (Figure 10). The survey showed that the pier
was originally constructed by driving timber piles to form a
cofferdam, making a base for the bridge foundations
within the cofferdam of ‘cemented river gravel’ and then
placing stone foundation slabs. Many of the timber piles
were missing and this had allowed the undermining to
progress below the foundation slabs.
(b) Evidence taken from Welsh Water Authority staff who
stated that the bed levels of the river in the vicinity of the
bridge changed from year to year and that a gravel bank
near the bridge was an intermittent and changing feature.
(c) Evidence taken from British Rail that indicated that the
depth and type of construction of the foundations of the
bridge were not known and had not been considered when
repairs were undertaken to the piers some 7 years previously.
(d) A study of the river flow was carried out by Hydraulics
Research Limited, some of the conclusions of which were
as follows.
(i) There was a ‘re-circulating zone’ or eddy at the
downstream end of pier 3.
(ii) Up to 17 000 t of sediment may have passed the
bridge during the 3 h of peak flows, indicating major
live bed scour with both erosion and deposition at
the bridge.
(iii) The depth of any anticipated scour at pier 3 could
have been between 0?75 and 2?2 m.
Overall, it seems that local scour at the downstream end of pier
3 was the main cause of the collapse, but elements of the report
suggest that channel instability may also have been a factor.
The results of the investigation into the Glanrhyd collapse was
the start of a complete review of scour risk to railway bridges in
the UK. The conclusions drawn included the following.
(a) The engineers responsible for the safety of the bridge
lacked thorough knowledge of the complex behaviours of
rivers such as the Towy.
(b) Foundation depths were not known.
(c) Remedial works previously carried out to defective bridge
piers increased the likelihood of scour damage because
the piers were widened and the shape of the cutwaters was
changed.
Elevation
Pier
Nominal bed levelScour hole
Scour hole
Flow
Pier Deposition of silt
Foundations
Plain
Figure 8. Local scour to a pier
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(d) The scour was probably live bed scour with the scour hole
refilling with sand/gravel as flows subsided. This made it
difficult for divers to find it during routine inspections.
Just 2 years later, there was another major railway bridge collapse
at Inverness, again due to scour. This did not involve loss of life
but severed the only railway that runs north from Inverness for
many months. A completely new bridge had to be built.
3.2 Beighton 2003: contraction scour
In 2003, a member of the public, walking along the bank of the
River Rother near Beighton (South Yorkshire) reported a
collapse of the railway bridge (Figure 11). It was found that the
pier and arch of one span on the bridge had partially collapsed.
Fortunately, arch bridges are inherently strong in compression
and even though the arch was severely damaged, trains had
been passing over the bridge without incident. However, all rail
traffic was diverted onto the track that was not affected by the
damage until temporary supports could be installed. Detailed
underwater inspections and investigations were undertaken
and the following information was ascertained.
(a) The incident was reported in the summer months but it is
likely that the scour occurred during flooding in the
Stage 1 _ as constructed
Foundations
Pier
Stage 2 _ scour development
Foundations
Pier
Invert
Invert
InvertFlow
Flow
Flow
FlowBed level
Minor scour toupstream face
Leading edge of invert undermined
Leading edge of invert undermined
Downstream edge of invert collapsesand scour undermines pier
Downstrean edge of invert collapses into
scour hole
Major scour hole develops at
downstreamend
Invert
Stage 3 _ damage to invert
Foundations
Pier
Stage 4 _ pier undermined
Foundations
Pier
Figure 9. Progressive collapse of invert
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previous winter. There is no public footpath under the
bridge so visits by the public are rare and the underside of
the bridge cannot be seen from the railway line.
(b) The bridge was constructed in three parts at different
times and with slightly different forms of construction
(Figure 12). At the point where the scour occurred, the
soffit level of the bridge dropped by about 300 mm.
(c) Local information provided by members of the public
and railway staff indicated that in times of flood, water
levels were seen to rise above the upstream soffit level.
There were also some indications on the bank to support
this.
(d) A detailed survey of the collapsed structure and the
surrounding river bed found that the bed of the river under
the bridge was composed of a thin layer of clay
(approximately 150 mm) overlaying sand and gravel.
However, the clay layer had been damaged. From the point
where the clay was found to be missing and the gravel was
exposed, an extensive scour hole had developed.
(e) The scour hole was deepest at the construction joint
between upstream and centre parts of the pier. The
foundations of the centre part of the pier were about
300 mm higher than the upstream and downstream parts
(Figure 13).
(f) As a consequence of the shallower foundations, the centre
part of the pier was undermined by the scour and had
collapsed into the scour hole. The arch above also
partially collapsed.
(g) Previous underwater reports showed that there was no
evidence of a distinct scour hole although the bed was
very slightly lower at this location.
(h) Side spans of the bridge had become silted up, reducing
their use as flood-relief spans and increasing the like-
lihood of contraction scour.
From the above information it was concluded that the most
likely cause of the undermining was due to contraction scour
during a period when the river was surcharging. At the bridge,
the width of the river is reduced by the piers of the bridge and
Downstream half of pierpart buried in sand deposition
Undermining beneathpier present
Remains of timber cofferdam
Upstream section pier 3
Water level
Random pier masonry
Downstream
Bed level 1 m from pier edge
Toe level of cofferdam
Bed level on pier
Figure 10. Glanrhyd, pier 3
Figure 11. Upstream face of River Rother bridge
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the silted-up side spans. During periods of flood when the
height of the river is above the level of the arch soffit, the
available cross-sectional area A is further constricted when
compared with the area of the open channel clear of the bridge.
Furthermore, at the point where the scour occurred, the soffit
of the bridge dropped, creating a funnelling effect and
increasing the head of water. As the same quantity Q of water
sought to pass through the restriction, the velocity V increased,
resulting in bed scour.
Whether or not the clay layer on the bed of the river was
natural or installed as an anti-scour measure is unclear, but it
had certainly been performing this function prior to the
incident. However, once it was breached, the granular material
below would quickly scour away and the clay layer would be
progressively removed by the flow.
The scour hole was sufficiently deep to undermine the centre part
of the pier but did not reach the level of the foundations of the
upstream or downstream parts of the pier, which remained intact.
The collapsed section of pier was eventually capped off and the
adjacent arches replaced with concrete beams (Figure 14).
3.3 Holyhead c. 1990: scour caused by bed lowering
and ship propellers
Holyhead harbour was developed by the London and North
Western railway over many years as their main port for
ferries to the Republic of Ireland. British Rail took over the
port upon nationalisation and further developments took
place. One part of the harbour was known as the cattle dock
and, as the name suggests, was used for the import of cattle.
By 1990, the trade in live cattle had ceased and the dock was
used for the temporary berthing of ferries that were awaiting
repair or their next duty. In January 1990 the dock wall
collapsed.
Following a detailed underwater examination, evidence indi-
cated that the wall, which was constructed of stone masonry,
had been built off the granite bed of the harbour. However, at
the time of the collapse it was founded on a narrow shelf of
rock above bed level (Figure 15). This shelf was showing signs
of erosion.
Investigations revealed that the bed level of the harbour had
been lowered many years prior to the incident to accommodate
larger vessels. Anecdotal evidence was provided by one of the
divers who had carried out this operation using explosives. The
operation had been successful but had left some of the harbour
walls founded on a shelf of rock about 500 mm above the bed
of the harbour. Furthermore, because explosives had been
used, the shelf was somewhat irregular in shape. It would
appear that as ships became larger and made more use of bow
thrusters, scour occurred at the base of the wall until a section
slid off the shelf and into the harbour.
Flow
Construction joint
Construction joint _ soffit lowers
Scour
River Rother
AbutmentAbutment
Pier
Pier
Pier
Pier
Figure 12. River Rother Bridge plan at river level
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3.4 Feltham 2009: scour caused by obstruction of the
river flow
On 14 November 2009, a small railway bridge carrying the
Windsor to London (Waterloo) line over the River Crane
collapsed. The incident was first reported at 22:03 when the
driver of a train crossing the bridge reported a dip in the
track. The incident was investigated and it was found that
the east abutment at the upstream end of the bridge had
collapsed into the river (Figure 16), taking with it a part of
the arch and leaving the railway track suspended above a
void (Figure 17).
The investigations revealed that the abutment collapsed due to
scour and a number of factors were involved (DfT, 2010).
(a) The abutment that collapsed was the original abutment
constructed in 1848. Foundations of this abutment were
shallower than the west abutment, which was constructed
later (in 1858). Core drilling carried out in 1991 showed the
east abutment to be founded at 0?65 m below river bed
level while the west abutment was 1?5 m below bed level.
(b) The shape of the river channel directed the flow towards
the east abutment, making it vulnerable to scour.
(c) In August, a member of the public took a photograph
showing the east span of the bridge to be obstructed by
floating debris.
On 28 October, an Environmental Agency inspector saw a
major blockage of the watercourse (Figure 18).
Approximate water level during flood periods
Soffit level of centre and downstream arches
Collapsing pier
Water level at time of inspection
Thin clay layer
Granular bed below clayScour hole
Foundations ofupstream arch
Foundation level of centre arch
Section through upstream arch
Centre arch
Change in soffit level
Upstream arch
Part plan of affected arch
Scour hole
Figure 13. River Rother Bridge
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At the time of the collapse, 5?5 m (65%) of the abutment’s
length was found to be unsupported where the material below
the foundations had been removed by scour.
The conclusions of the investigations were that the abutment
was already vulnerable to scour because it was on the outside
of a bend of the river and had substantially shallower
foundations that other parts of the structure. The debris in
Figure 14. River Rother bridge; collapsed centre pier removed
Rock bed
Bed level following blasting
Original harbour bed level
Masonry wall
Concrete slab
Rubble fill
Possible wash out of fill
Figure 15. Cross-section through Holyhead harbour wall prior to
collapse
Figure 16. River Crane bridge; collapsed abutment
Figure 17. River Crane bridge; track unsupported
Figure 18. River Crane bridge; debris blockage prior to collapse
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the river then caused river flow to be channelled directly
towards the abutments and the scour occurred during a period
of high river flows.
3.5 Cefyn viaduct: local scour
Situated on the Welsh borders near Wrexham, Cefyn viaduct
carries the railway across a wide valley and the River Dee. It is
a major multi-span masonry arch viaduct. During the course of
a three-yearly underwater inspection, it was found that the pier
located in the river was severely undermined by local scour.
There was no indication of contraction scour and the damage
had been caused by a typical form of local scour that had
removed bed material from below the upstream foundations
and caused the collapse of the cutwater though, fortunately, it
had not extended far below the main body of the pier and
repairs were able to be made. This viaduct is a major piece of
Victorian engineering and a collapse would have been a
catastrophe. The scour could have been prevented by the
provision of adequate bed protection (Figure 19).
3.6 Ruddington: scour caused by dredging
South of Ruddington in Nottinghamshire, the former Great
Central Main line to London crosses a small watercourse
known as Fairham brook. The Great Central Line was not
built until about 40 years after other main lines in the country;
the civil engineering is of the highest standard and the
foundations of their structures are generally large and deep.
However, when inspected, divers found that the pier of this
two-span bridge was in danger of collapse due to deepening of
the river bed (Figure 20). One span of the bridge crosses a farm
track and is over 1000 mm above the bed of the river in the
other span. When inspected, it was found that the foundations
of the south abutment and pier were exposed. The bottom of
the foundations could be seen but, fortunately, undermining
had not taken place.
Investigations showed that there was no local scour but that
the level of the river bed was consistently low for a significant
distance away from the bridge and that there were signs that
dredging had taken place. Contacts with the local drainage
authority were made and it transpired that a drainage and
flood-relief scheme had been undertaken. Although not carried
out at the bridge itself, dredging of the watercourse had caused
the bed levels at the bridge to be lowered by 1000 mm, putting
the bridge at risk. The risk was not only of undermining but of
the pier sliding sideways because of lack of horizontal
resistance to the forces transmitted from the farm track. Steel
sheet piling has now been driven around the pier and abutment
to stabilise the structure.
3.7 Staythorpe: changes to the course of the river
Staythorpe viaduct is a good example of how the exact amount
of bed scour can be detected by ‘forensic engineering’. This
railway viaduct, carrying the Nottingham to Lincoln line over
the River Trent near Staythorpe power station, had been
rebuilt in 1973 and new piers were constructed, each of which
consisted of concrete bored piles that were unlined below
ground but extended above bed level within pre-cast concrete
rings. The viaduct is located just downstream of a weir and is
subject to fast flows during flood periods.
Some 15 years later, during the course of routine underwater
inspections, it was discovered that the main flow of the river
was dividing and that a gravel bank had built up in the centre
of the river. The pattern of scour was complex and a full
contour plan of the bed was produced in order to fully
understand the situation. The worst area of scour was in fact
immediately downstream of the bridge on the outside of the
bend in the river. The bed at this point was over 4 m lower
than the average bed level. Up to 2 m of scour was present at
the bridge itself.
These results were confirmed by divers who took detailed
measurements of the piles. They were able to dive under a layer
of concrete that extended between the piles. At first, this was
found to be puzzling until it was realised that, below this layer,
the concrete of the piles was uneven and contained inclusions
of large gravel indicating that they had originally been
underground. Above the layer, the piles were cast within
concrete rings.
Pier 2
Bed level
Stones movedLoose stones lyingon bed
Scour hole
Flow
Void
Figure 19. Cefyn viaduct
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Fortunately, the original piling logs had been retained as part
of the as-built information for the viaduct and these showed
that the piles extended about 9 m below the original bed level
so the 2 m of scour could be reviewed in that context
(Figure 21). In this case, a thorough investigation proved that
there was no need to take emergency action. However, repairs
were subsequently carried out both to restrict further scour and
to provide protection to the exposed piles. The incident
demonstrated the benefit of deep piled foundations for river
bridges and the value of retaining as-built information.
3.8 Malahide: failure of causeway and protection
apron
On 21 August 2009, pier number 4 of Malahide viaduct
collapsed into the estuary (Figure 22). This viaduct carries the
main line between Dublin and Belfast. The collapse was
reported by the driver of a train that passed over the damaged
viaduct but fortunately crossed immediately before complete
collapse occurred.
Detailed investigations were able to prove that the masonry
piers of the viaduct were built on top of a stone causeway that
acted as a weir (RAIU, 2010). This causeway was maintained
in a fair condition for over 100 years by a regular regime of
replenishment of the stones, although during that period the
causeway elongated seaward due to migration of the stones. In
1967–1968, a major grouting scheme was undertaken to fill
voids in the weir. This scheme was reasonably successful but
more stones were discharged to fill scour holes on a number of
occasions up until 1996.
A hydraulic model of the bridge was built to investigate the
failure mechanism. The grouted layer, which was about
1500 mm thick, acted as an invert but, as scour occurred at
the seaward side of the causeway on the ebb tides, this became
undermined. The undermining continued in the manner shown
in Figure 9 until the invert between piers 4 and 5 collapsed, at
which time the scour began to undermine pier 4 until it failed.
Abutment Abutment
Farm track
Pier
Foundation
Water level
Original bed level
River bed
Timber piles
Figure 20. Ruddington Bridge: bed scour due to dredging
Steel trestle
Concrete pile cap
Pre-cast concrete rings
Bed level at time of construction
Exposed concrete piles
Bed level
Concrete piles below bed
Concrete spillage
2 m
Figure 21. Staythorpe Bridge pier
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4. ConclusionsWhat conclusions can be drawn from the investigations into
scour problems?
Firstly, it is noticeable that when a bridge collapses due to
scour, the causes of the collapse become clear quite quickly to
the team carrying out the forensic investigations. Although it is
often thought prudent to carry out detailed hydrological
investigations, in fact the causes are often almost self-evident.
If the same effort could be put into bridge management
regimes, collapses could be avoided.
Secondly, it is noticeable that on a number of occasions,
knowledge of the structures concerned was inadequate. In the
case of the Malahide viaduct, for example, the investigation
report (RAIU, 2010) states:
The dearth of documents available [to the responsible engineers]
meant they were not fully aware of the construction of the
Malahide Viaduct, and incorrectly assumed that the structure was
founded on bedrock… at the time of the accident, [they] were
unaware of the routine discharge of stones along the viaduct as this
process was not formally recorded, and there was an apparent loss
of corporate memory of knowledge.
Despite this statement, it did not take investigators long to
discover a wealth of information about the structure. Their
report includes original drawings of the viaduct and details of
recent remedial works. Had this information been made
available to the railway engineers, the collapse could have
been averted.
Thirdly, information made available to staff carrying out the
examination of underwater structures is often incomplete.
Similarly, when reviewing reports, it is essential that the
engineer has all known information about the bridge available.
When all such data was in the form of drawings and paper
records it is perhaps understandable that it was not reviewed
after every examination. However, computers now make it
easy to rectify this problem by taking the following steps.
(a) Establish, record and have available to the examining
team full details of foundation depths. If these are not
available from ‘as-built’ drawings, core drilling may be
required. Once this information is available, ensure that it
becomes routine to consider it when looking at under-
water reports and scour data.
(b) Develop a more sophisticated way of reviewing river bed
soundings. These are routinely taken at each underwater
inspection but for them to really predict the likelihood of
a bridge being affected by scour then the data should be
well handled. The results should show changes in time (by
comparisons with previous soundings) and geographical
changes. The latter will indicate if bed levels at the bridge
are lower than might be expected when compared with
upstream levels and will also show any migration of the
main channel. In the case of Staythorpe, for instance,
only by plotting bed contours over a significant length of
the river could the pattern of scour be established.
(c) Carry out a review of the inspection reports. This task
should be done by engineers with particular expertise in
scour.
Finally, it is important that repairs to scoured river beds and
scour protection works are not carried out without proper
engineering design. Whilst major anti-scour works such as steel
sheet piling will of course be correctly designed, simple tasks
such as filling scour holes are sometimes left to local teams to
carry out without any design input. This has been seen to result
in too small stones being placed and these usually end up some
distance downstream after the first flood.
REFERENCES
DfT (Department of Transport) (2009) Report on the Collapse of
Glanrhyd Bridge on 19th October 1987 in the Western
Region of British Railways. HMSO, London. See www.
railwaysarchive.co.uk for further details (accessed 02/09/
2011).
DfT (2010) Failure of Bridge RDG1 48 between Whitton and
Feltham. Railway Investigation Branch, DfT, Derby,
Report no. 17/2010. See www.raib.gov.uk for further
details (accessed 02/09/2011).
Hamill L (1999) Bridge Hydraulics. E & FN Spon, London.
Hoffmaans GJCM and Verheij HJ (1997) Scour Manual.
Balkema, Rotterdam.
May RWP, Ackers JC and Kirby AM (2002) Manual on Scour at
Bridges and Other Hydraulic Structures. CIRIA, London.
Figure 22. Malahide viaduct; two spans collapsed
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Melville BW and Coleman SE (2000) Bridge Scour. Water
Resources Publications, Highlands Ranch, CO.
RAIU (Railway Accident Investigation Unit) (2010) Malahide
Viaduct Collapse on the Dublin to Belfast Line on the 21st
August 2009. RAIU, Blackrock. Report no. R2010-004. See
www.raiu.ie for further details (accessed 02/09/2011).
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Scour failure of bridgesMaddison
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