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The purpose of this research is to investigate the gain understanding of how embankments withstand large amount of load and factors that lead them to fail. With many readily available, my goal is to integrate and generalise findings , treatments and how systems are monitored in the United Kingdom.
Website bbmms.org for British history wrote about construction of locomotive in England in 1804 by Richard Trevithick and how railway was only seen as means of carrying goods. Steam engines with higher speeds were later discovered it was then realised that they could carry passengers quickly and cheaply. Investor saw railroads a profit making venture then laid many tracks all over the country and never really considered soil stability
20th century maintenance of embankment has become so apparent due to frequent failures occuring. Earthworks slopes in particular make up a large proportion of the UK transport network. Major (ultimate limit state) failure of such slopes causes disruption to network operation and frustration to the travelling public due to temporary route closure or delays to services while repairs are carried out Network rail a (government-created owner and operator of most of therailinfrastructure in Great Britain)identified 50% of the UK's 14,000km of railway cuttings and embankments to be in ‘poor' or ‘marginal' condition. The study found that 90% of failures occurred on assets in ‘poor' condition, but that half of the failures attributed to ‘marginal' or ‘serviceable' assets occurred as a direct result of wash out or drainage failures.
In an ICE article titled Harrison on earthwork slips, it talks about how early contractor built railway lines, there was not soil mechanics considered and used logical methods of building and maintaining tracks. Typical example was the cut and fill methods, thats the same methods used in the 20th century but disturbed soil was not full consolidated to ensure the soil bearing capacity was strong enough. When they ran out of filling material at one point they installed wood/ tranches at the bottom of the embankments then covered with fill this was a common practice. If they experienced wet soil and at times if they did not have enough men power they would used burnt ballast to light fire adjacent to the embankment to dry the material. This never strengthened the soil but increase the risk of embankment failure.
In the 21century ballast and its engineering behaviour have a key role in governing the stability and performance of railway. An article called Geotechnical properties of ballast and the role of geosynthetics in rail track stabilisation published in the ICE in 2006 discussed findings which were mainly about the deformation and degradation behaviour of ballast under static and dynamic loads. For them to understand the effects of the loads they focussed on the ballast physical and mechanical properties. Description of ballast function on the track
bed and qualities including size, angular particles, high specific gravity, high shear strength, high toughness and hardness ensured the embankment structure is maintained. It was also noted that the inconsistency between the actual particle sizes in the field and the greatly reduced particle sizes contributed to deformation behaviour on tracks. Since the main functions of ballast are to distribute the load from the sleepers, to damp dynamic loads, and to provide lateral resistance and rapid drainage. Permeability is regarded as an important factor because when it decreases, compaction and cementation increases with higher voids. Specific gravity and bulk density of the ballast are properties that play pivotal roles in rail track safety. In conclusion the article stated that ballast settlement was not regarded as a big problem as long as it is uniform along the length of the track and the main problem was the differential track settlement.
Causes of embankment failures
After finding relavant information regarding the embankment structure. Several literatures outlined causes as:
- Increase in rail traffic
- Animals burrowing (rabbits and badgers)
Proceedings of the Institution of Civil Engineers Engineering Sustainability 162 June 2000 a journal was published called Climate-change impacts on long-term performance of slopes
summarised some of the evidence that suggests climate change will have an effect on the behaviour of infrastructure slopes and their management, and provides an overview of future climate scenarios for the UK. It is suggested that increased rainfall has an influence on slope
stability. This was supported by the events involving intense rainfall in UK causing major floods in 2000/01 and over 100 slope failures occurred. 2007 was one of the wettest summers as well and there was similar effects. Drier summers were caused the same effect to high plasticity clay. Soft rock embankments are particularly vulnerable to climate change and changing rainfall patterns as they take up water, become heavy and fall, causing landslides. The study found that one of the most ‘at risk' areas was the South East due to its largely clay-based geology, which is prone to landslides.
A United Nationsclimate science panel called Intergovernmental Panel onClimate Change(IPCC) reported that the frequency of extreme weather events - wetter winters and drier summers - is set to increase with climate change over the next 50 years. This would be a global temperature increase of 1-2 degrees Celsius. This means that the storms of 2000-2001 could become normal in England and more embankment will be affected BBC reported
However, it is not just wet weather events that cause delays. The 2003 heat wave caused the shrinking and swelling of railway embankments, which caused many landslides and further disruption to travel.
Network rail noted in an article in the July 2007 article in the Proffessional Engineers journal that hot summers cause subsidence, which damages both travel networks and civilian properties. A separate study conducted by Network Rail found that around half of the UK's 20,000km railway embankments were in ‘poor' to ‘marginal' condition and at risk of landslides.
In another research in the ICE journal a lead author of the study, Fleur Loveridge, a PhD student, stated that climate change in the near future is ‘locked in'and is too late to change anything. The need to raise awareness and increase maintenance budgets has been raised, as well as supporting research to develop innovative engineering solutions to tackle the problems before they happen.
Referring back to the Climate change journal, describes how wetter winters are likely to increase or maintain soil wetness in the north and west of England and as for the summer soil moisture is reduced especially in the south of the country and therefore cause drier soils. The failure in high-plasticity clay embankments is a result of excessive shrinkage in hot weather in the presence of high water demand trees as well. The gradual degradation in clay strength and loss of suction caused by gradual ingress of water and successive shrink-swell cycles which cause surface cracking. Another scenario of hotter drier summers followed by periods of more intense rainfall increases the severity the problem.
The journal called Properties of Ballast published in the ICE 2006 talks about the bearing capacity, stiffness of subgrade and sub-ballast change seasonally and due to climate change aswell, effect can be seen especially in areas where intensive freezing periods and numerous freezing-thawing cycles occur. It was deduced that the seasonal loss in bearing capacity is about 30 to 60% related to the maximum value (in late autumn) even for ‘non-frost susceptible' unbound bases and sub-bases after experiments..
Another key influence on the water within is vegetation, it has beneficial effects including root reinforcement, prevention of positive pore water pressure build up and surcharging at the base of the slope. However, the Climate change journal stated that serious detrimental effects include loading the upper part of the slope, uprooting or overturning. The changing
seasonal demands for water by vegetation causes fluctuations in soil water content, increasing the problem related to shrinking and swelling. A temperature difference of 1-2°C, or
drought, will alter the composition of the plant community, its water use and rooting characteristics and therefore reasonable to assume that there will be some changes in the plant communities present on clay slopes as a result of climate change.
Using data set from the UK Meteorological Office (Bionics) programme derived climate scenarios on how UK climate impact the structure of soil. The report had four emission scenarios known as (SRES)A1, A2, B1 and B2 or low, medium-low, medium-high and high and three future time slices (30 years centred on the 2020s, 2050s and 2080s
Track drainage is considered as a major system placed on the trackbeds to essure the tracks are not affected by increase of pore water pressure. Network rail noted implications of poor in the drainage include:
- Deteriorating track quality - poor ride quality
- Discreet track geometry faults (e.g. twist & cyclic top) - risk of derailment
- Loss of lateral stability - increased risk of track buckling
- Ponding of water - damage to the formation
- Formation of wet beds - damage to the sleepers and bearers
Once the drainage has been compromised effects like cyclic Top can be visible due to the waves that start appearing on the track. The ‘wavy' top often associated with poor ballast and formation conditions
Reducing Railway maintainance journal published in the ICE talks about effects when water penetrate into the subbase the crushed ballast combined with water and material from concrete sleepers forms an effective grinding paste that will degrade the trackbed. Generally timber and steel sleeper are much more tolerant of poor ballast conditions than concrete. In the report it further talked about the process of cycle of deterioration which describe how soil deteriorate:
- Poor drainage cause fines to filter through ballast
- Ballast matrix becomes progressively ‘choked
- Drainage deteriorates as flow of water is impeded
- Presence of water reduces the bearing capacity of the formation
- Trapped water speeds sleeper and ballast degradation
- Track geometry deteriorates as ballast responds less well to maintenance
- Loading of ballast and formation from traffic increases as track geometry worsens ,
To understand the severity of embankment failures, case studies reviewed were as follows:
- Land slidesdes in Scotland (e.g. Stromeferry) and, in the winter of 2000/01, which was documented the wettest on record
- On the19th June 2007 in Worcestershire large amount of rainfall (166mm) triggered a sequence of events that led to nine major slope failures and 46 recorded developments along the embankments of the 18 mile route of Severn Valley Railway. One of the largest failures on the event the railway track bed was washed away and water proceeded to residential properties down the slope
- 15 January 2007 earth slope failures and resulted in passenger train derailment. The cutting failure was due to localised extreme weather, surface water and poor drainage systems on the site. (railwayarchive.co.uk)
- 100 slope failures in the southern region of Railtrack alone
- The summer of 2007 was one of the wettest summers on record
- and there were numerous slope failures A46 in Gloucestershire for several months.
- Drier summers effect on rail serviceability in the southern regions where embankments have been constructed of high-plasticity clays.6
- 2010 land slides that killed 8 people due to a derailment in scotland
With the increasing issues of embankment stability issues, there are a number of indicators to earth movements which warn track staff of the earth movement and possible problems before they develop. These include:
- Distorted fences
- Fence post tipping over when there is no sign of animals having to have pushed them
- Tree trunks tipping over
- Loss of cess
- Crack in ground surface
- Loss of top
The new review will involve designing modelling tools with the Met Office to predict what severe weather events might take place and which lines are most at risk. New flood maps will be produced using laser-based technology.
With railway tracks being affected with different factors, earthworks need to be investigated using different types of equipment thus:
- Auguer, percussion and rotary tools
- Penetration heads, loading plates and pumps
- Inclinometer tubes
- Resistivity meters
Remedial works help the stabilisation of embankments, as it is well known prevention is better than remedial work. Once an embankment is affected work need to be carried out first then monitored. Methods of treating failures can be categorised into different sections according to Railway Management:
- Reducing disturbing forces
- Increasing soil strength
- Controlling of water
- Mechanical support to resist deformation
Reducing disturbing forces:
- Retaining walls
- Soil pinning
- Track support
Increase soil strength:
- Stabilisation (binding)
- Geo Mats
Control of water
Mechanical Support :
- Geo grids
New Technology for Remediation and Monitoring embankments include:
Journal called Ground improvement techniques beneath existing rail tracks talks about different methods of monitoring tracks these include:
- For monitoring track displacements using sensors photosensitive array, mounted on the sleepers, and a laser, which was target outside areas that are influenced by train loading.
Ground-Penetrating Radar (GPR) was used to determine the variation of ballast depth:
- This is fixed on a locomotive then run over a length of the embankment. GPR is able to determine the depth of interfaces between dissimilar materials: this includes a strong reflection from the base of the ballast layer,
- A significant thickening of the ballast layer around the failure location was observed using GPR, which confirmed the existence of an ongoing stability problem in the are
- Waves travel easily through clean ballast and give a strong reflection from the underlying material or fouled ballast
- This consists of a sampling tube forced through the ballast with a hydraulic breaker. The sampling tube collects samples for testing and photography
The DP gives an empirical indication of the relative stiffness of layers in the embankment and underlying foundation. The DP has a drop weight that falls through a height driving a cone through the ground. The results are presented as the dynamic point resistance (qd) as the probe is driven into the ground (Scott Wilson Ltd, 2007).
(Track displacement and energy loss in a railway embankment journal Geotechnical Engineering 163 February 2010)
The principle of vibro compaction is based on the reduction of the pore content of granular soil in order to achieve a higher density combined with an increase of the stiffness of the soil. Moreover, settlements can be anticipated and longterm settlements are reduced. This method is economic and flexible solution, which easily adapts to varying ground conditions. The following geotechnical improvements are achievable:
- compaction and densification of the subsoil
- increase of the bearing capacity of the subsoil
- increase of overall soil stiffness and decrease of excessive settlements
- rapid consolidation of soft soil owing to the creation of vertical drainage lines
- improvement of the shear strength of the ground to decrease the risk of failure
- increase of the liquefaction stability and earthquake resistance of subsoil
- increase of the soil mass to mitigate ground vibrations and to reduce the basic resonance frequency of the soilstructure interaction system especially for high-speed railways.
Electrical resistivity tomography (ERT),
ERT profiles determined the presence and spatial extent of a significant layer of soft clay both beneath and to the east of the embankment, which could have a major impact on its long-term stability. ERT also detected steeply sloping bedrock close to the failure zone that is likely to have contributed to the long-term settlement of the embankment, which necessitated frequent re-ballasting.
Multichannel Analysis of Surface Waves (MASW
MASW confirms the presence of the steeply sloping bedrock in addition to determining the low stiffness (Gmax) values of the embankment fill.
Railway industry might consider new technology being introduced called Xi-Track
This is a product of two liquid component parts of a polyurethane system on site.
As it reacts, it is poured into ballast to form solid plastic. This helps reinforcing ordinary railway ballast whilest giving it superior strength and resilience, enhancing track support. It can be mixed at different proportions depending on the strength
Advantages include the ability of ballast to:
- Behave elastically (it helps control trasitional forces)
- Enables ballast to self drain with the help of a geo mat underneath that stops clay or soil infiltrating into the ballast
- Absorb noise
- Applied at any time of year
- Long service life
The mix can penetrate any given depth and sets in minutes,
After researching through a large number of books, articles, journals and case studies the embankment stability has been a real concern in England as soil structures are being affected by a large number of factors including weather, drainage and previous methods used to construct the embankments.
Railway is a multi-million industry and understanding how the embankments behave will help cost benefit analysis, end of life/ maintance cost and assest management of the infrastructure.
In the future proactive planning for climate change offers much better tax payer value for money than the inevitably large outlay when things go wrong.