Piling, South Coast, Devon

Jurassic Coast, Contiguous Piled Wall, Branscombe, Devon

Project Brief:

The site is situated on the side of Branscombe cliff, in Lyme bay. The cliff is made up of three strata; Chalk, Pre-Cretaceous Upper Greensand and Mercia Mudstone with gypsum from the Triassic era.

Originally Branscombe grew from mining gypsum and the property we are working on was terraced workers cottages, with the look out on the end being a coastguard station. The Jurassic coast is a world heritage site, and the rock and cliff formations have been changing for millions of years, with land slip a very common problem. Only 1km down from the site is Hooken cliffs, where in the 1800’s a landslip took place containing a few million tons of chalk slipping off the Triassic Mercia Mudstone. This continual changing coastline means we need to find modern answers to solve a very old problem.

This project comprised of installing a Contiguous Piled Wall in front of an old coastguards lookout. The lookout, which is no longer used by H M Coast guard and is now part of private home, has been converted to suit the clients needs with a large decking, cantilevered over the cliff edge. In front of the building was mass poured concrete to retain the cliff edge and support the decking, unfortunately due to weather erosion the concrete has cracked up and is slowly slipping down the cliff, leaving the footings of the old lookout exposed. In an attempt to prevent any further movement, the whole cliff has been rock anchored and netted down while works continue. With the original decking removed, scaffolding had to be erected to allow for the piling rig to work in front of the building.

The scaffolding had to support a 4t point load and every other support was placed on a rock anchor. The machine we used for this job is a PPS Unwin Kitten Rig which is capable of working in restricted access conditions, and very powerful and able to bore through the tough layers of limestone. The Kitten is also able to install Temporary Cased Piles, which we are using for the bearing piles 1.5m from the centre of the Contiguous Piled Wall. This places them in the middle of the scaffolding, and any spoil or concrete would fall down the cliff causing a safety hazard. 

Site access and weather conditions also made this a challenging job for our team. All our equipment and reinforcement had to be delivered by 7.5t lorry as roads to site were very narrow. as the winter storms of 2014, with high tides and storm force 10 and 11, for some days the wind speeds reached 90mph with driving rain on the unprotected cliff face work had to stop as just standing up became a problem. Although we were unable to work on site, we helped clear debris from the road, restaurant and secured loose boats and peoples belongings. The high tides and high winds had washed most of the service road away. This road leads to several beach chalets which were also damaged, and only 25m below our site. Storm force 10 and abnormal high tides caused major problems on the beach front. The service road and some of the chalets have been washed away.  

Scope of works:

* Site canteen and welfare facilities provided by main contractor.
* Protect all walls and drive where we are working.
* All services located and isolated.
* As we are working on a cliff face all safety handrails must be installed and checked.
* Mobilise PPS Unwin Kitten Rig, by 7.5t lorry.
* PPS Unwin Kitten Rig chained with tether while on scaffolding.
* Prefabricated 6H16 – H8 helicals @ 200mm pitch, delivered to site by us.
* Pile positions set out by main contractor.
* Safe route for concrete deliveries planned, and lorry walked to site by banksman.

Site setup, with lifting frame to lower heavy reinforcement
Site setup, with lifting frame to lower heavy reinforcement.

The Contiguous Piled Wall continues along the front of the garden, although here we are working in between four cantilever beams which hold the decking over the cliff. We are installing three Piles in each section. The spacing between the Cantilever beams was too small, so the third pile was installed behind the face of the wall giving it the same lateral restraint.

Unwin PPS KItten Rig on 4t point load scaffold
Unwin PPS Kitten Rig on 4t point load scaffold

Reinforcement, consists of two 6H16 – H8 @ 200mm pitch prefabricated cages, which are spliced together and then lowered in safely, with our specially designed cage lowering frame and winch.

Kitten Rig tethered to rock anchor, at all times.
Kitten Rig tethered to rock anchor, at all times.

The Contiguous Piled Wall is installed at 600mm from the centre of the piles, to the face of the Look Out, top of reinforcement is finished at 1200mm from piling platform to allow for protrusion into 600mm x 600mm R/C capping beam.

Piling on the cliff face.
Piling on the cliff face.
Temporary Cased Piles.
Temporary Cased Piles.

Installing 300mm dia Temporary Cased Piles here with the Kitten Rig Prevents the spoil and concrete from falling down the cliff face, and means no operatives need to come off the scaffold.

Temporary Cased Pile installed between scaffold and 1500 from center of Contig wall.
Temporary Cased Pile installed between scaffold and 1500 from center of Contig wall.

Casing is socketed into the clay to form a seal and the pile is then formed with the casings extracted straight after concreting. These piles will have pile caps formed on top of them, which will support the new decking. Site is left clean and tidy after each day, with the access to the main house kept open and the drive power washed.

For more information on our Piling Systems and Services or helpful advice please Contact us. 

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