For thousands of years, the North Sea has pounded the coastline of Scarborough, England to carve out some of the most dramatic seashore in the world. The constant battering requires these areas to have extensive sea walls in order to prevent major erosion and loss of existing infrastructure. Such is the case in Scarborough, where the sea walls protecting the Castle Headlands and Harbour were coming to the end of their useful life and were in need of repair to insure protection for years to come.
The Scarborough Borough Council approved the completion of a $40M dollar ($26M British Pounds) sea wall rebuilding campaign in order to protect their important coastline. The project safeguarded a 1-1/4 mile (2.1 km) stretch of coastline along the The Holms, Castle Headlands and East Pier region and was scheduled to take up to two years to complete. The contract for this task was awarded to Edmund Nuttall Limited and consultant High Point Rendell who assigned all drilling for the project to Ritchies Equipment Ltd.
The project called for installation of up to 308,560 tons (280,000 tonnes) of granite rocks from Larvic, Norway, weighing up to 22 tons (20 tonnes) each, to build rock armour protection around The Holms, Castle Headland and East Pier. Pre-cast interlocking concrete blocks called Acropodes were also used on Castle Headland and East Pier. These installations required the drilling of 605 x 34 inch (864 mm) piles into the seabed to a depth of 9 to 15 feet (3 – 5 m) in medium sandstone and mudstone. Of these 605 holes, 240 were drilled on the beach at low tide with the remaining 355 completed at sea.
Numa’s distributor in the region and Numa Regional Manager, Chris Beare, were instrumental with the support of this drilling project by providing great insight into the products and methods being used. Both have played a critical role in designing complete drill strings for the land and water-based drill operations. Similar in both set-ups was the use of Numa 240 hammers and 34 inch (864 mm) bits in order to provide the necessary combination of speed and performance to get the job done right.
The Numa 240 hammer was shrouded with a split sleeve to provide easier hammer disassembly with a large Scorpion unit available on site. The remaining drill string consisted of a 10-foot (3 m) joint of 12-inch (305 mm) pipe sleeved to achieve correct hole cleaning characteristics. Total weight on bit was approximately 8 tons (7.2 tonnes).
All beach drilling was carried out at low tide with a Casagrande B125 drilling rig, which tracked along a temporary beach road. Two 1500 cfm/150 psi (708 l/sec / 10.2 bar) compressors were used to generate up to 3000 cfm (1416 l/sec) of air with the hammer typically running at 125 psi (8.5 bar). All lubrication was provided by an ASL venturi type system supplying a marine and environmentally sensitive lubricant. To conduct drilling, the rig was accurately positioned over the pile location where care was taken to collar the hole and then drill to required depth. When the drilling was finished the rig is moved off the hole and a precast 30-1/2″ (775 mm) pile was located and grouted into position.
A Casagrande RM21 rig conducted all drilling out at sea and contained the same compressor, lubrication and the basic drill string set-up that was used on the beach, but with a longer upper string due platform height from sea level. The RM21 was mounted on a base which slided along the platform “Bull Bar” to each hole position.
Each hole required a 36-inch (914mm) temporary casing to be socketed into the seabed. Wen the casing was positioned, a crane dropped the hammer assembly and drill string into the hole. The rig was slid over the hole and connected to the drill string. Care was taken to collar the hole before drilling the socket 9 to 15 feet (3 – 5 m) into the sea floor. Upon completion, the drill string was removed for use on the next hole. A precast concrete and steel pile was then dropped into the drilled hole and grouted through built-in tremy lines. The temporary casing was removed and reused on the next hole.
The larvic granite was transported from Norway by large ocean going barges which anchored approximately 2,625 feet (800 m) offshore. The rock was moved to smaller barges for delivery to the project location. For the sea defenses around North Bay the rock was dropped on the sea bed/beach at high tide as close to the site as possible. At low tide heavy earth moving equipment adapted for handling large boulders maneuvered them into position against the existing sea wall. In the final stages the Acropodes are set in position.
Any project calling for the installation of 605 piles in two different environments requires proper planning and selection of the right drilling solutions to yield consistent, reliable results. The foresight of Ritchies to utilize both the experience and products offered by Numa paid off with an average penetration rate of 16 feet (5 m) per hour which enabled the group to meet their projected plan for pile installation. With these results, the Scarborough coastline along The Holms, Castle Headland and East Pier will be protected for generations to come.
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