In the densely populated Thonburi district of Bangkok, a municipal utility upgrade required installing a 300mm high-density polyethylene (HDPE) water main beneath a major six-lane arterial road. Open-cut excavation was not a viable option: the road carries over 45,000 vehicles per day, and closing it for the estimated 12-day open-cut schedule would have caused intolerable traffic disruption and business impact. Horizontal directional drilling (HDD) was selected as the trenchless alternative.
The bore path was designed to run 4.5 meters below the road surface, with entry and exit pits located in vacant lots on either side of the arterial road. A utility survey using ground-penetrating radar (GPR) confirmed the absence of existing fiber optic, power, or gas lines along the proposed alignment — a critical step, as Bangkok's underground utility network is notoriously dense and poorly documented. The design included a 1.5-degree entry angle and a gentle arc peaking at the midpoint of the crossing.
The pilot hole was drilled using a 110mm drill head equipped with a walkover tracking system. The drill operator monitored depth and azimuth continuously, maintaining the designed path within a tolerance of 0.3 meters vertical and 0.5 meters horizontal. The soft clay formation drilled easily, at rates of 40-50 meters per shift, but the team encountered a 15-meter sand lens at the 200-meter mark, where circulation loss required increasing bentonite concentration to maintain borehole stability.
After pilot hole completion, the bore was reamed in three passes: first to 200mm, then to 350mm, and finally to 450mm — a pre-ream that provided ample clearance for the 300mm HDPE string. Reaming fluid was a bentonite-polymer slurry at 45 seconds Marsh funnel viscosity, optimized to carry cuttings while minimizing hydrofracture risk in the soft overburden.
The 350-meter HDPE pipe string was assembled on a leased vacant lot beside the exit pit and pressure-tested to 1.5 times operating pressure before pullback began. The pullback operation commenced on the morning of day five and completed by mid-afternoon, with the drill rig registering a steady pullback force of 12 tons, peaking briefly at 18 tons during the sand lens section. The pipe was pulled through without incident, and final pressure testing confirmed no leaks or deformation.
Urban HDD projects succeed or fail in the planning stage. The GPR utility survey and careful bore path design — avoiding the known sand lens by adjusting the arc depth — prevented what could have been a costly borehole collapse. Contractors who skip the utility survey or drill the pilot hole without continuous tracking routinely hit existing underground lines, turning a 6-day project into a 6-week dispute.
We supply HDD drilling rigs from 20kN to 600kN thrust/pull-back, suitable for urban utility crossings up to 600mm pipe diameter, with full operator training available.
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