Balanced Pressure Drilling System deployed at Liang Tang Tunnel Project

Sigra’s Balanced Pressure Drilling System in use at the Liantang tunnel project

Background

The Liang Tang tunnel project comprises dual two-lane road tunnels, linking the Sha Tau Kok Road and Fanling Highway interchanges in Hong Kong, contracted primarily via a Dragages Hong Kong-Bouygues joint venture.

This tunnel project would require 4.8 kilometres of tunnelling through both soft soil and hard rock formations — the northern 500 metres consisting of soft soil tunnelling using an earth pressure balanced TBM, with cross tunnels also being excavated between the two main tunnels.

Enter Sigra: we provided specialist equipment to enable drilling and installation of the ground support and consolidation grouting, thus enabling the cross tunnel excavation through the soft soils to proceed. We installed the temporary ground support through pre-drilled holes.

Due to the nature of the formation, hydrostatic head acting on the tunnel, and the risk of liquefaction, all drilling and installation work had to be done under controlled conditions using borehole pressurisation equipment.

Sigra design brief

Our brief was to design well control equipment which would enable drilling and installation processes to be carried out safely, by:

  • Maintaining a pressure seal at the tunnel wall penetration
  • Providing a balanced pressure while drilling and grouting so as to

- Prevent inflow of water and soil into the tunnel

- Prevent excessive pressure being developed within the hole so as to avoid problems with ground liquefaction

We needed to design equipment for the restricted access within the tunnel for the drilling rig, as the main passageway could not be blocked from other construction traffic.

Three sealing modes were designed: a lip seal during drilling; an annular blow out preventer (BOP) to shut in on non rotating drill pipe; and a gate valve for extended shut in periods when the drill pipe was not in the hole.

Additional specifications include the ability to take tube-a-manchette cementing pipes, and a balanced pressure of between 50–250 kPa.

We based this equipment on Sigra’s well control system used in gas wells, with the key point of difference being the shortened length and precise choke used to control back pressure in the hole, while allowing passage of particles up to 40mm diameter.

The system we designed has been a key component in enabling the contractor to complete their cross tunnels.

For more information on Sigra’s tailored, custom solutions, please contact us here.