EXTREMELY HARD rock hampered construction of bored pile walls for spillways on the new Merowe Dam in the Republic of Sudan.
The 65m high, 7.4km long dam, about 500km north of the capital Khartoum, will impound the River Nile over an area of more than 20km2 to provide power and irrigation. It is being built by the Merowe Dam Project Implementation Unit of Khartoum for the Sudanese Ministry of Irrigation and Water Resources.
The main contractor for the SDD164bn (£410M) project is the Chinese joint venture CCMD (the China International Water & Electric Corporation and Sinohydro Corporation).
German foundation contractor Bauer Spezialtiefbau won the contract to seal off the ground under the main earth core of the rockfill dam using a vertical diaphragm cut-off wall with a total area of 24,000m2. But first the firm had to install 5,700m2 of secant bored pile walls forming the foundations and scour protection for the wing walls of the spillway.
The extremely hard rock, which has a compressive strength of more than 200Nm, was not identified before piling began.
It caused high levels of wear and tear on the drilling tools so Bauer decided to bring in more drilling equipment capable of dealing with the difficult ground. And as well as rock-roller core barrels, it had to mobilise extra drilling rigs to site. To save time, one of the machines was flown in from Cairo.
In all, Bauer used eight rigs on site, including two Bauer BG30 rotary drilling rigs, a BGT12, BG14 and BG16 and three Hitachi cranes.
The extra kit meant piling work was finished on time at the end of December, allowing the river to be diverted for the next construction phase.
The diaphragm cut-off wall will be built in the second half of this year, using a Bauer BC30 diaphragm wall cutter and a BG60 grab unit. The Merowe Dam is due to be finished in 2009.