New jumbos key to Vietnam hydro schedule
New jumbos key to Vietnam hydro schedule Dec 2009
Sandvik news release
Drilling contractor Song Da on the Dakmi 4 hydropower plant in Vietnam has taken delivery of two purpose-ordered Sandvik DT820 jumbo drill rigs to recover the tunnelling schedule on the contract.

Facing tough conditions in Vietnam

The 172 MW hydro scheme is located about 2½ hours drive west of Vietnam's third city Da Nang, close to the Cambodian border, and is centered on a 1,450 hectare reservoir on the Dakmi River created by a new concrete dam of 87m high x 420m long at the crest. But it is rock conditions in the 3.37km long tailrace tunnel that are creating major problems for main contractor Ho Chi Minh City-based IDICO and tunnelling partner Song Da. Poor rock comprising sediment and badly fragmented and decomposed granite, complying with Vietnamese standards of Class II, 1B, 1A2, 1A1 and edQ + eQ, are being encountered.
At the power house portal conditions were particularly poor. With drilling starting at the beginning of May, advance rates were very slow and arduous. Up to 40 x 4.5m long rock bolts in two rows at an angle of 15° were installed together with wire mesh and shotcrete plus erecting steel girders around the portal entrance. Rock conditions continued to cause problems 10m into the tunnel. As a result the contractors elected to halt proceedings and open a new 195.7m long temporary adit to allow access into the proposed main tunnel.
By the end of May the adit had been completed allowing Song Da to open two new faces - one towards the inlet portal and the second back to the powerhouse and outlet portal. Achieving advance rates of 4.5m/day in the adit, Song Da is hoping to meet a similar schedule in the main tunnel.
While rock conditions have improved in the new workings, some rock bolting is still required and the contractor is confident lost time can be made up with the new Sandvik rigs. Both units were ordered by the local Sandvik dealer in Vietnam Open Asia Heavy Equipment.
The two Sandvik D820 jumbos feature two booms and a basket and are fitted with Sandvik 4.9m shank adapters and couplings. At the 43.2m2 tunnel faces at the outlet, Song Da is drilling 115 holes including the three 102mm diameter burn holes. For these, Sandvik R32 reaming bits are being used. According to Song Da Plant Manager Le Van Hung, "these are far superior to the American bits being used for the 44mm diameter charge holes. We are in the process of changing over to Sandvik drill bits for all the drilling operations," he said, adding that the Sandvik cemented bits have "a longer life, drill 'truer' straight holes, and provide increased productivity."
Inlet tunnel
Progress by the second Sandvik D820 jumbo in the larger 49.34m2 inlet tunnel heading has been halted by a tunnel collapse. Following the clean up of an earlier collapse, a second cave-in at about 145m into the drive resulted in a 40m high by about 5m diameter hole to the surface. Concrete segments of 35cm and 20cm thick and meeting 250 MAC standards are being installed around the perimeter of the tunnel wall to recover the situation. The decision to infill the cavity with either rock or concrete was still under discussion at time of writing.
As a tunnelling specialist, Song Da is beginning to standardise its large fleet of equipment with Sandvik rigs and rock tools. In addition to the two DT820's delivered earlier this year for Dakmi 4, two DX700 rigs were ordered also for the Sekaman 3 hydro project in Laos. The company also has a Sandvik DP1100, a Ranger 700, and CHA550 drill rigs.

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