Detailed geological investigation of the chalk cliffs above Brighton Marina allowed engineers to design stabilisation measures to prevent further failures. Max Soudain reports from the Sussex coast.
In early April 2001 the Asda supermarket in Brighton Marina received an unexpected delivery - 2000m 3of chalk, clay and raised beach deposits came tumbling down from the cliff behind it, knocking over a water tower and ending up in the store's offices.
This was just one (albeit the largest) of 15 'significant' failures in the 300m section of cliff behind the marina in 2001, following the extreme wet weather of the previous winter.
Although the cliff had no history of failure, Brighton and Hove City Council had already brought in consultant High Point Rendel in 1998 to develop a management strategy after concerns were raised over the risk to the public and marina residents.
Jon Palmer, an independent consultant working for High Point Rendel, explains: 'The risk changed dramatically over the winter of 2000 and 2001. But the cliffs were already in a high-risk category because of the narrow undercliff walk and the coast road that runs just behind the clifftop. And while there was no recorded instability, there had been some isolated rock falls and some minor rock removal.' The cliffs have been protected from their chief enemy, the sea, since the 1930s, when a seawall and groynes were built along the 5km stretch between Brighton city centre and Saltdean to the east, creating the undercliff walk which is used by up to 2700 visitors a day. The cliffs were also reprofiled at the same time to give them a 70° face.
Engineers had little to go on because of the lack of detailed information on historic failures.
Any evidence of rockfalls before the sea defences were built had been removed by reprofiling. This led to a qualitative assessment based on observations and expert judgement.
A detailed visual survey of the lithology, structural geology and weathering of the cliffs was carried out as well as an assessment of the human risks, taking into account the proximity of property and roads and the condition of the sea wall. This allowed stabilisation work to be prioritised.
Standing between 25m and 40m high, the near-vertical cliffs mainly consist of Cretaceous Upper Chalk, with residual soil and dry valley Head and Coombe deposits occurring intermittently along the clifftop. The Head deposits comprise clay with chalk and flint gravel and the Coombe deposits range from a chalky loam to a chalky rubble with flint gravel in a sandy clay matrix.
The chalk is subhorizontally thickly bedded (2° to the west) with four main joint sets and some weathering at the clifftop, especially around the dry valley deposits. Palmer explains that for stability, the two most important joint sets are the vertical orthogonal joints striking the cliff face at 70°, creating lateral release surfaces; and joints dipping out of the face at 50° and striking it at 90°, creating plane failure surfaces.
'This combination of weathering, joint sets and vertical cliffs is the reason for past rockfalls and the retreat of the cliff line, ' he says. 'While there are no recorded failures from these two joint sets, apart from after some excavations, the risk assessment still identified this type of failure as a possibility.'
Historic instability would have occurred 'bottom-up' because of the sea undercutting the cliff. Because the sea defences prevent this, instability tends to start at the cliff top and work its way down. So, while the defences almost halted cliff regression, they have not completely stopped failures.
The west end of the cliff line comprises mixed Coombe and chalk debris deposits infilling an abandoned Pleistocene cliff which ran at about 90° to the present cliff line. This is the section that dramatically failed in April 2001.
'This 100m section cannot be stabilised because of its geological significance, ' says Palmer. It is designated as a site of special scientific interest and cannot be altered, ruling out remedial work.
A fence has been installed behind Asda to prevent debris hitting the store and this part of the undercliff walk has been closed until further notice. A new access to the promenade is being built east of the protected section.
The 16-week, £350,000 stabilisation contract for the rest of the cliff finished in August. 'The cliff was divided into two sections, ' Palmer explains. 'The top 7m weathered zone and the structured chalk below.' The cliff here is 21m high above the sea wall.
'The original plan was to reprofile the upper section to 45°, then place geogrid protection, ' he says. 'But we weren't allowed to do that on environmental grounds, so it was decided to reprofile to 60° and install soil nails.' The weathered section is supported by 570, 6m long, Ischebeck Titan 30/11 hollow grouted soil nails installed on a 2m diamond grid.
To stop failure along the main joints in the cliff face, any loose or unstable areas were removed and rockbolts installed to secure large chalk blocks. Steel rock mesh from Maccaferri was then placed over the face to restrict rock falls and to stop the major flint falls that are common after heavy rain.
Reprofiling and rock face trimming was done by Uckfield-based main contractor CJ Thorne, working from the western end.
Temporary fencing was erected below the work area to protect the marina residents. This was moved along the sea wall as work progressed.
'A couple of thousand metres of material was removed, ' says CJ Thorne contracts manager Chris Webb. 'Trimming was very difficult, with anything below the top 7m done by hand.' Up to 14m 3chalk blocks were prised off the face using bars and sent tumbling to the foot of the cliff, where material was taken off site.
'This was one of the biggest concerns on the project, ' Palmer confirms. 'It was very difficult to form the slope as the chalk wants to break along the joints.' In the cliff face, 16mm and 20mm diameter galvanised rebar rockbolts supplied by Ischebeck were installed and grouted in to support chalk blocks. Fifty-seven are 7m long and 10 are 4m long.
'In one or two areas patterned bolting has been carried out, ' adds Palmer.
Soil nailing and rock anchoring was carried out by a crew of abseilers from WT Specialist Contracts, working from the top of the cliff.
'The 30/11 soil nails are selfdrilling with a sacrificial bit, ' explains WT Specialist Contracts director Andreas Ramsauer.
These were installed by a rig suspended on the arm of a long reach excavator working over the cliff edge.
Lower down, the rock anchors were installed by abseilers using hand drilling or from a cherrypicker working from the undercliff walk. Up to 45 anchors were installed in one day.
The poor start to the summer affected work. 'In the first four weeks of anchoring work we had pretty diabolical weather, ' says Palmer. But even high winds and driving rain did not prevent the abseiling team installing between 20 and 25 anchors in a 12-hour shift.
'If the wind is gusting the team can work in up to 35 knots, if the wind is steady the limit is about 45 knots, because they can brace themselves against the cliff, ' says Ramsauer.
The final part of stabilisation was to install the steel mesh. This is secured to the face using 614, 1m long steel 'staples' on a 2m grid. The top and bottom of the mesh is secured by a 16mm diameter steel cable pinned into the cliff, with the mesh folded over and stitched together.
Palmer says monitoring of the cliff will continue. Chalk experts from Brighton University will do more work, including a geophysical investigation, to try to explain the sudden rush of failures last year. Further surveys of the next stretch of cliff to the east have already begun.
'Instability is certainly to do with the heavy rainfall, ' he says.
'It may be due to general weathering since the slope was cut back 70 years ago which means the cliff is now reaching instability or it may be the extreme rainfall - there are no clear hydrogeological reasons.'