Bangkok:

Din Daeng - Don Muang Tollway - Independent Engineer/Site Supervisory Services

Chronology:

• 12/87: Call for tender
• 08/89: BOT contract signed (25-year concession)
• 1991: Commencement of work
• 08/94: Completion of work
• 10/94: Open to traffic
• 04/96: End of maintenance period

Construction:

Traffic on the existing expressway at ground level (volume approx. 180 000 vehicles per day) was not to be interrupted; it was therefore necessary to effect special traffic management measures, adopt special construction methods &, above all, carry out construction work at night. 

Standard viaduct cross-section: a concrete deck slab on top of 10 T-girders placed on cross-beams which are supported by 2 arms of Y-shaped piers - both an aesthetic & an economical solution. 

Pre-boring with steel casings to a depth of 5 m was required prior to the driving of any spun piles, so as to avoid upheavals in the existing pavement below. Static dynamic loading tests were performed on floating piles & on piles embedded in dense sand layers; in both cases the service loads are largely carried by skin friction. 

Pre-cast concrete spun piles (800 mm external dia., 250 tons capacity) were driven into the dense sandbearing layer beneath existing median strip (depth of 30 to 40 m) using 12.5 ton hammers (drop height 1.20 m). The standard pile caps (covering 11 piles) are 5 m wide, 8.5 m long & 1.5 m thick. 

Temporary sheet piling was inserted to facilitate excavation in the restricted working area; also, 6 special steel shutters were inserted to aid casting (permitting a rate of 5 Y-shaped piers per week).

A prefabrication yard was set up near the construction site; because of the restricted working area there, offsite prefabrication was essential:

• surface area: 6 500 m2
• gantries: 1 x 140 t/2 x 70 t
• workshops
• site laboratory
• main offices
• 2 concrete batching plants (each with capacity of 60 m3/hour)

Rate of production of yard: 78 T-girders cast per week, 2 000 t of reinforcement per month.