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 infrastruture. 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 needed repair to ensure protection for years to come.
The Scarborough Borough Council approved the completion of a
$40 million sea wall rebuilding campaign in order to protect its important
coastline. The project safeguards a 1 1/4-mile stretch of coastline, and took
two years to complete. The contract for this task was awarded to Edmund Nuttall
Ltd. and consultant High Point Rendell, which assigned all drilling for the
project to Ritchies Equipment Ltd.
The project called for the installation of 308,560 tons of
granite rocks from Larvic, Norway, weighing up to 22 tons
each, to build rock protection around The Holms, Castle Headland and East Pier.
Pre-cast interlocking concrete blocks called Acropodes were used on Castle
Headland and East Pier. These installations required the drilling of
605-inch-by-34-inch piles into the seabed to a depth of nine feet to 15 feet 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, H&F Drilling, 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 had
played critical roles in designing complete drill strings for the land- and
water-based drilling operations. Similar in both set-ups was the use of Numa
Champion 240 hammers and 34-inch bits in order to provide the necessary
combination of speed and performance to get the job done right.
The Champion 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 joint of 12-inch pipe sleeved
to achieve correct hole-cleaning characteristics. Total weight-on-bit was approximately
All beach drilling was carried out at low tide with a
Casagrande B125 drilling rig, which tracked along a temporary beach road. Two 1,500
cfm/150 psi compressors were used to generate up to 3,000 cfm of air, with the
hammer typically running at 125 psi. All lubrication was provided by an ASL venture-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 was moved off the hole, and a precast 30 ½-inch
pile was located and grouted into position.
A Casagrande RM21 rig conducted all drilling out at sea, and
contains the same compressor, lubrication and the basic drill string set-up
that was used on the beach, but with a longer upper string due to the platform
height from sea level. The RM21 was mounted on a base that slid along the
platform to each hole position. Each hole required a 6-inch temporary casing to
be socketed into the seabed. Care was taken to collar the hole before drilling
the socket 9 feet to 15 feet into the sea floor. Upon completion, the drill
string was removed for use on the next hole. A precast concrete and steel pile
then was dropped into the drilled hole, and grouted through built in tremie
lines. The temporary casing was removed and reused on the next hole.
The larvic granite was transported from Norway by large barges that anchored
approximately 2,500 feet offshore. The rock was transferred 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 were 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. In the early stages
of this project, it was clear that the foresight of Ritchies to utilize both
the experience and products offered by H&F Drilling and Numa had paid off.
Ritchies realized an average penetration rate of 16 feet per hour, and was able
to meet its 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.