Monitoring signs of potential movement rather than installing extra remedial solutions saved Network Rail considerable bother on a tricky slope in Essex. Gemma Goldfingle reports.
Unstable embankments have long been a concern along the rail lines of the busy London to Norwich commuter route in Manningtree, Essex.
One 580m stretch, 1km west of Manningtree station, has a steep slope ranging from 1 in 1.10 to 1 in 1.67, and runs on the valley side of the River Stour.
Despite the location being only 90m away from a coastal flood plain, there is no drainage at the toe of the downline embankment.
The section was identified as being at risk of failure, and part of the embankment had been given a “poor condition” rating in previous earthworks inspections.
With signalling and overhead line electrification equipment also suffering a loss of support, Network Rail decided to act.
“London to Norwich is one of our highest priority, prestige lines. It is popular commuter route so we invest a lot of money to keep it running,” says Network Rail acting route civils asset manager for the south east territory Derek Butcher.
Consultant Gifford was called in to determine the cause of the failure. “Analysis showed a minor variance in groundwater levels has a big impact on slope stability,” says Gifford associate Louise Hellawell.
“The section changes from a cutting to an embankment as the land falls towards the river Stour. As well as changes in water levels there are also variances in natural ground conditions which move from cohesive to granular. This combination is extremely risky.”
The embankment itself consists of ash/ballast up to 1.8m thick overlying up to 6.5m of cohesive made ground.
“There are changes in water levels and natural ground conditions move from cohesive to granular. This combination is extremely risky.”
The first 80m at the western end of the embankment lies on sandy alluvial clays directly overlying the London clay.
These deposits then change to granular deposits of sands and gravels for about 140m, and the remainder of the embankment being remediated lies on the cohesive alluvium overlying the granular river terrace deposits, which in turn overlie the London clay.
Ground investigation had taken place prior to Gifford’s involvement on the project.
A gabion toe wall, allowing regrade of the existing steep embankment to a shallower profile, was deemed the most appropriate decision based on safety, environmental impact, access and cost.
But it was then discovered that the embankment toe area was 1.5m narrower than originally thought.
Building a gabion wall and regrading at this steeper angle would have reduced the factor of safety to below 1.3, the level agreed with Network Rail.
The gabion wall design had already been developed, however, and moving to an alternative design would delay the works on this important line.
Buying extra land would have been costly and time-consuming, so the team devised a more inventive solution - using the observational method approach.
“We were confident it would be sufficient to stabilise the slope.
As the factor of safety was over 1.15 the client agreed to go ahead with the wall, using an observational method to improve safety,” says Hellawell.
The observational approach, developed from the Observational Method, is a straightforward approach focusing on prediction, monitoring and feedback allowing the use of factors of safety less than 1.3.
Possible modes of failure are assessed and controlled, which allows solutions with a lower factor of safety, which may be less expensive and time consuming, to be used.
Along with a monitoring and review strategy, Gifford also has a contingency soil nail design, should monitoring reveal that the slope is failing.
“The aim of the monitoring is to predict movement. We are not looking for major movement but signs that movement over the serviceable limits may be likely,” says Hellawell.
The slope stability analysis indicated that groundwater levels have a clear impact on the slope stability.
As the groundwater level in the embankment increases, the embankment slope becomes more unstable with lower factors of safety.
A combination of piezometers and standpipes were installed to monitor groundwater level change and consequently give an indication of the overall slope stability.
Four piezometers with data loggers and four standpipes are installed across the site.
Two piezometers and two standpipes are within the embankment at made ground level and the others extend further down the superficial, river terrace deposit.
The instruments are manually monitored at predefined frequency.
These installations, located in exploratory holes, are at approximately 30m intervals.
Additional inclinometers through the embankment and survey points on the gabion wall are used to assess shear movement.
Track geometry and quality monitoring data (in a form such as colour coded quality data) is used to assess settlement under the track.
Soil moisture deficit (SMD), defined as the amount of water per unit surface area that soil will absorb before further precipitation cannot be stored, is also used to predict failure.
Historically, periods of low SMD correlate well with those major slips that have occurred in the south-east of England since 1988.
“It is a really useful indicator of potential failure in clays. Network Rail has been using Met Office data of high pore pressure locations to identify at risk areas since 2000. It is an effective early warning system,” says Network Rail’s Butcher.
Using the slope stability analysis, trigger levels are determined for each instrument.
Green, amber and red zones have been designated where groundwater levels were seen to rise and factors of safety fall. Gifford proposed a series of interventions to be taken in each zone.
In the event of monitoring revealing a severe drop in the factor of safety, Network Rail can decide on whether the contingency plan of installing 10m soil nails along the embankment should be carried out.
All gabions along the lower rows of the 2-3m-high wall contain a plastic sleeve, installed at a pre-defined location and angle during filling of the gabion basket.
This allows soil nails to be installed quickly and efficiently without pre-drilling. “Despite diameters and angles
of the nails being predetermined there is some flexibility in this solution,” says Hellawell. “Positions can be missed, numbers or lengths of nails can be varied. It allows us to tailor the solution to the specifics of the slope.”
Monitoring has been carried out since the gabion wall was completed in August last year.
So far, despite the heavy snow and rain experienced over the winter months, the factor of safety has remained steady, staying in the “green zone”, meaning no remedial action has been necessary.
“We were always confident that the gabions were a practical first step. This activity was always one of managing the risk effectively,” says Hellawell. “Hopefully the soil nail solution will not be used and we will save the client the cost of carrying out unnecessary work.”
Network Rail will continue to monitor the site each month and the need to continue monitoring will be reviewed on an annual basis.
“We are always looking at clever ideas to be more efficient,” says Butcher.
“The observational approach will not be appropriate on every site as monitoring can be time-consuming, but on this occasion it has proved an efficient way of working and has saved over £1M.”