Royal Oak Landslip Repairs

Royal Oak Landslip Repairs - Official Report

History

Just south of Greet Tunnel on the Cotswold side of the track, the line is in a cutting and the side has been subject to slipping for many years. There is evidence of remedial works being done in BR days but with limited effect. There was a manhole at the top of the cutting to collect the water from the fields but by 2011 it looked like this:

By 2015, it had deteriorated to this:

By September 2017, sufficient funds were available to make a partial repair to the damage. This involved constructing a new manhole at the top of the slope connected to the drains from the fields together with a new outlet pipe taking the water down to the track drains. In this picture, the new manhole can be seen above the excavator arm with the new outlet pipe being laid under the arm:

This shows how close the slip was to the railway boundary together with the new manhole and drains indicated by the stone: 

Because of the limited funds, a proper reconstruction of the slope was not possible so the existing soil was reformed to provide a smooth slope:

Unfortunately the winter of 2017/2018 provided unusually wet especially over the Christmas period and the reformed slope was undermined by water flow causing a new slip. As the ground slipped down, it took the outlet pipe with it so that the water from the manhole was now flowing on to the slope making things worse. The ground was so soft and precarious, it would not have been safe to attempt an emergency fix at this time:

Conditions worsened until the new manhole was completely exposed:

It took the months of dry weather to dry out the ground and by this time, further funds were available. A consultant, David Symonds Associates, was engaged to prepare a design and drawings and then three firms were invited to tender to complete the repair works. After evaluating the tenders, Walsh Construction was awarded the contract. As a bonus for the railway, they had just had a project delayed so were able to start on site just a week later on 17^th September 2018 to take advantage of the continuing dry weather. The railway had already negotiated access across the fields with the neighbouring land owners and Walsh starting constructing an access road immediately:

In order to protect the site in case of further rain, the first task was to install new drainage in one of the fields so that any water was collected and sent to the manhole before it could soak the slipped area. A series of pipes surrounded in stone and membrane were laid in a herring bone pattern and then the topsoil was replaced:

Whilst the small excavator carried out this work, its larger companion finished off the access road and then started excavating the soil from the slipping area and putting it into a stockpile. After discussions with our consultant, this material was declared unsuitable for reuse and was removed from site:

The soil at the toe of the embankment was excavated down until hard clay was encountered below the slipped material. During this work, a clear slip zone could be seen in the face of the excavation between the lower hard clay and the softer upper clay sloping back up the embankment.  Water was percolating along this zone lubricating the surfaces and therefore aiding gravity to move the upper material down. This zone was in a bowl or valley shape and the excavation was taken beyond this on each side to ensure that it was removed. The excavation was brought up in a series of benches with the excavated material being passed from one excavator to the other and thence to the stockpile:

The shape of the slip zone determined the size of each bench and once most of the failed material was removed, work started on rebuilding the slope. A geotextile membrane was spread over the hard clay to keep the new fill separate and the stone was placed on top in layers and compacted with either a roller or a large ‘Wacker’ plate:

Initially only a thin layer of stone was placed to hold the membrane in place and protect the exposed good clay in case of rain whilst excavation continued at higher levels:

Then more stone was placed and graded to the required slope:

As the level of the stone fill rose, the new pipe to connect the top manhole down to the track drains was installed:

As the benches rose, the working area got smaller and a Wacker plate was used instead of a roller:

At this stage, the topsoil that had been saved was spread over the surface of the stone:

In order to hold the topsoil in place, a membrane was laid on top and pegged down:

As the stone got close to the final level, the drain along the crest of the embankment was reinstated surrounded in free draining stone and a membrane:

Here the stone fill is being placed before excavating for the crest drain visible in the background:

After the drainage was completed, the slope was topsoiled and the protective membrane laid before the entire area was hydro seeded:

New fencing was then erected, the temporary access road removed and the field reinstated:

A view of the completed works:

As a bonus, the material used in the temporary access road was taken around to Working Lane to improve our access up towards the tunnel:

Report and photographs by Alastair Watson.