Tunneling (2)

II] TUNNELLING TROUGH IN DECCAN TRAP BASALT (OR IN COMPACT BASALT OR IN AMYGDALOIDAL BASALT OR IN A JOINTED ROCK)

                                                                                                                                      1) Joints are also natural divisional planes in the rocks. It is main source of trouble during tunneling.

                                                                                                                                      2) Because of fragmentation it leads to roof falls, also passage way for ground water.

                                                                                                                                      3) More massive rock better will be for tunneling.

                                                                                                                                       4) Hard rocks are suitable for tunneling point of view but it is necessary to determine nature and spacing of natural divisional planes like joints in them.  

                                                                                                                                     5) Rock with closely spaced, open, regular and consistent. Joints or thin bedded rocks will be more troublesome than those with broadly spaced, irregular and inconsistent jointing.

                                                                                                                                      6) In case of horizontal bed or lava flows the condition are ideal if there is a soft bed underlain and hard bed overlaid. Tunnel should be cut in the soft rock formation so that hard bed is act as floor and ceiling of floor.

                                                                                                                                    7) In Deccan trap area two main types of basalt occure a) compact basalt b) Amygdaloidal basalt. These two basalt are different in jointing. The compact basalt are well jointed where as Amygdaloidal basalt are unjointed.

                                                                                                                                       8) Tunnel in compact basalt often very difficult. While the Amygdaloidal basalt very high rank suitable for tunneling, because of it is unjointed. They allow excavation to design section without overbreak, are stable, also completely impervious.

                                                                                                                                     9) Examples :- i) A large part of Koyna 3^rd stage tail race tunnel has been excavated through a closely jointed . Compact basalt, constant roof fall, heavy overbreakes and leakage of water through joints tunneling very difficult and expensive.

                                                                                                                                   ii) Bassein creck tunnel carry the water from vaiterna river to Bombey city. In this tunnel there is also tremendous inflow of water through jointed compact basalt.

           Tunnel excavated through amygdaloidal basalt all without any trouble, all of them are totally dry inspite of being in heavy rain fall areas.

                                                                                                                                      i) Head race tunnel of Koyna Hydro electric project 3^rd stage.

    ii) 3^rd line in Thal Ghat on Bombey –Delhi line.                  

    iii) Bor Ghat on the Bombey – Puna line of central railway line about 20 tunnels are through amygdaloidal basalt.

            Totally different behaviour of two varieties of same rock type basalt in tunneling because of the difference in jointing.