Your browser is no longer supported

For the best possible experience using our website we recommend you upgrade to a newer version or another browser.

Your browser appears to have cookies disabled. For the best experience of this website, please enable cookies in your browser

We'll assume we have your consent to use cookies, for example so you won't need to log in each time you visit our site.
Learn more

WELL SORTED

SLOPE ENGINEERING

New drainage techniques are saving coastal homes at Castlehaven on the Isle of Wight, where landslips have damaged property and local services.

Damon Schünmann reports.

Nature is exacting a punishing toll on the picturesque rolling countryside and the inhabitants of the south coast of the Isle of Wight.

The locals, or caulkheads as those born there are known (although no one seems to know why), have endured a long history of coastal instability. But now landslips are damaging infrastructure such as roads, power lines and drainage, as well as buildings on the slopes at Undercliff near Castlehaven.

The slips are periodically triggered by a combination of susceptible geology, high groundwater levels and coastal erosion. Each high tide has contributed to a recent cliff recession rate of 0.6m a year: historical maps and data show that 40m has disappeared since 1862.

David Hattersley, resident engineer for consultant High-Point Rendel, explains: 'The problems are the massive slides. They're not only a threat to property but also the main tourism route linking Ventnor to Freshwater via Niton.

One of them sliced right through the Undercliff coastal A road one mile east of Niton.'

Many houses were in immediate danger.

'About 120 properties would have been affected if the scheme hadn't gone ahead, and there are also insurance problems, ' Hattersley says. It has been estimated that if left unchecked, the damage would have led to losses of £18M at 2001 prices.

The slips have had a striking effect on the contours of the slopes, creating a stepped hillside as they fall away to the cliffs below. The cliffs themselves clearly show ongoing erosion, with piles of glauconitic greensand collecting at the base of the exposed face.

Main contractor Team Van Oord is tackling the landslip problem with a two-pronged approach. Site agent Stephen Ashman says: 'We are reducing the lubrication of the slip plane and dropping the water table to reduce the weight of the ground.'

Analysis indicated that groundwater levels needed to be lowered throughout the winter to equal summer depths of 5-12m below ground.

The first issue is being dealt with by the installation of 1.7km of near-surface slotted drains arranged in two main defensive lines running along the slope known as the Undercliff.

These remove surface run-off and near-surface groundwater and are laid in gravel-filled trenches 3-6m deep.The trenches are lined with geotextile that allow through water but not fines. An additional polyethylene membrane on the downslope side of each trench prevents leakage once water has entered from upslope.

Hattersley claims the method for lowering the water table is one that has not been seen in the UK before.

Siphon wells and electro-pneumatic wells are installed at 6m intervals along the trenches to depths of either 20m or 25m.

Both types of well are produced by French groundwater lowering specialists TP.GEO, whose work foreman Sylvain Lefaucheux describes the benefits of the siphon wells: 'They work in complete silence, require no energy, create no pollution and they only require cleaning two or three times a year to prevent fines from building up in the system.'

The siphon wells are arranged in seven runs along the trenches and the electro-pneumatic wells in an eighth. The wells are formed by rotary drilling 200mm diameter boreholes. The well screen is inserted and the annulus is filled with 2mm to 4mm gravel.

Each line of wells feeds into an outlet manhole and the pipe from these also sits in the geotextile-lined trenches. The outlet manhole for each of the seven lines of siphon wells also contains an accumulator for each well.

These unpowered accumulators include a flushing process that allows water to flow at high rates and stops it at low rates. Once water flow begins to increase, any bubbles that have collected in the system are flushed out. The siphon stays permanently primed because the inlet and outlet pipes remain submerged in reservoirs of water at all times.

In some cases electro-pneumatic wells were needed because the contours of the land and the level of the water table would not allow a siphon system to function. To power these, a compressor station has been installed in an underground chamber as the maximum noise permitted by the compressor under the contract is 23dBA.

This has required very high levels of dampening. Ashman says: 'The energy in the air is lost as it travels through the baffles in the air vents. Duty air [the air powering the pneumatic wells] is drawn into the compressors but it is a small quantity compared to the cooling air for ensuring the compressors don't overheat.'

It is not a straightforward project.'I've done a lot of drainage jobs but this one is very technical. In some of these [drainage trenches] there are four different pipes, ' Ashman adds.

Although the geology is Gault Clay overlaying Lower Greensand, the ground varies.

Hattersley explains: 'Because of the slips over hundreds of years, we've hit all sorts of material such as boulders. You're never sure when you're going to come across landslide debris.'

A range of monitoring instruments has been installed to keep a close eye on what is happening in the ground. Vibrating wire piezometers are being used to measure the build-up of water pressure in the ground at depths of 15m, 25m and 35m. Five observation wells with data loggers have also been installed to measure the standing water levels together with two inclinometers to measure any ground movement.

Flow loggers are being used to measure the amount of water being discharged through the outfall and a weather station has also been built. These will help provide a more accurate picture of long-term slope movement.

At the end of the installation project there will be an 18-month maintenance period, after which the client, Isle of Wight Council, will take responsibility for the entire monitoring system.

The scheme is subject to a number of environmental, technical and financial constraints.

English Nature and the Environment Agency set construction limits to avoid environmental damage to both the coastal slopes and the foreshore reef.

Van Oord began work on the £4.5M contract for the Department for the Environment Food and Rural Affairs in November 2003 and it is expected to be completed next month.

However, the job is the first of what may turn out to be a series of stabilisation schemes needed along the south coast of the island.

Work to protect the Lower Greensand cliffs being cut away by wave action has now been completed. This included construction of a rock armour revetment that stretches 550m across Reeth Bay and 140m along the Castlehaven headland.

Rather than the exposed cliff face, waves now hit the 3-6t carboniferous limestone rock armour which sits in two layers on a 5-500kg rock core, completing a coastal defence designed to last 50 years. Wave energy is further dissipated as it compresses the air in the pockets between the rocks.

'One of the things we had to satisfy English Nature on was the protection of the reef while we were bringing in the material for the revetment, ' says David Hattersley, resident engineer for consultant High-Point Rendel.

A heavy duty geotextile placed against the cliff face extends below the rock of the revetment and toe, which is excavated into the underlying sandrock. The front face of this protective line has a 1 in 1.5 slope and reaches a crest elevation of 5m above sea level.

Hydraulic Research Wallingford tested the design using a model to withstand the worst waves that would occur on average over a 100-year period.

The drainage system in the slopes above feeds down to the Castlehaven headland, and the outfall pipe feeds through the revetment as it empties into the bay.

Have your say

You must sign in to make a comment

Please remember that the submission of any material is governed by our Terms and Conditions and by submitting material you confirm your agreement to these Terms and Conditions. Please note comments made online may also be published in the print edition of New Civil Engineer. Links may be included in your comments but HTML is not permitted.