What is Tunnel Forepoling, Purpose and Methods

Tunnel Forepoling defines the placement of near-horizontal steel bars/ pipes/ section around the periphery of the heading to form a rock support cage. in areas of the weak zone. In other words, To support the excavation roof or Tunnel crown.

Forepoling shall be applied to rock and soil conditions which tend to produce overbreak, collapses or material inflow immediately after excavation. It shall be applied locally or systematically, as the circumstances require for the safety of the works and for preventing overbreak.

Forepoling

Fig.1 – Forepoling

The installation of Forepoling always requires the erection of steel ribs. They shall be driven from the supporting frame in a slightly upwardly inclined direction at the crown of the heading and should penetrate at least half a set beyond the next excavation cycle.

It shall be properly supported by the steel rib and the shotcrete above the steel rib. Therefore, the shotcreting of the gap between steel rib and the shotcrete sealed rock surface along the area of Forepoling shall be completed after the installation of fore poling.

Forepoling

Fir.2 – Fore Poling

Spacing between consecutive fore poling pipes or bars around the crown of the excavation profile shall bot exceed the maximum distance specified on the approved design drawings, and shall be reduced if the actually prevailing geological conditions at the tunnel face required to do so.

Great care shall be taken to prevent the disturbances of face boards and supports in general during the fore poling cycle.

Tunnel Forepoling method is generally adopted for tunnelling in Soft rock under the following conditions

  • The existence of Overburden
  • Poor Ground Conditions

Three different methods of fore poling  are

  1. Sub-horizontal jet grouting method
  2. Spilling Method and
  3. Pipe Roofing Method

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Also Read: Construction Methodology of Box Culvert

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