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Fore-poling (ground improvement for tunnel excavation)


This technique used to improve the ground and limiting the decompression of the crown of the excavation for tunnels. Steel bars (dowels) or steel plates(forepoles) are installed in the upper third or quarter of the excavation. Dowels and forepoles installed ahead of the tunnel excavation from the face periphery. Figure no:1 showing the installation of dowels and forepoles before commencing with further excavation of tunnels to stabilize the soil ahead.
Steel bars or dowels are similar to the one used for rock dowels. Dowels are installed from within the tunnels. Dowels are placed above a lattice girder. Dowels will be positioned at an angle to the direction of excavation, as shown in figure no:2 if the ground is very dense and hard. The dowels will be installed in a predrilled hole. The predrilled hole will be filled with grout before installing the dowels. The dowels are installed to protect the crown against the afterfall (falling blocks of materials from the tunnel roof). The space between dowels depends on the size of the material block in the ground. As a general rule, it is higher than 20 cm. The length of the dowels is approximately 3 to 4 m. 

Figure 2


Forepoling plates or sheets mainly made from steel. The installation of forepoling plates is similar to dowels. Forepoling plates are pushed forward individually in the same manner for dowels. The forepoling plates are installed close to each other. These plates used for gravel, sand, and loose soil, where the soil expected to fall through the spacing between forepoling dowels.






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