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Swabi Scarp Tunnel Construction Phase, 1995-1997 (on-going)
The Swabi Scarp Irrigation Scheme has its origins back in the days of Empire. Coming from the north, part of the Swat River was diverted to create an irrigation scheme over The Swabi Scarp. This scheme comprised a main supply tunnel (The Benton Tunnel), several minor tunnels and many miles of canals and feeder channels. The scheme was finally completed around 1911.
To improve the capacity of this scheme, to improve saline areas and to provide additional water for power generation, it was agreed that an additional supply tunnel was to be built and the Benton Tunnel improve and repaired.
On behalf of the Swabi Scarp Consultants in Pakistan rock mechanics recommendations were made on support assessment of all tunnels, rock slopes and excavation prodedures. In addition, a Construction Manual was also developed for use by both the contractors and the Supervisory staff.
The geology of the area is mainly granite with some schist at the oulet end of the tunnels.
Building the second tunnel is, ordinarily, littl problem. However, the old Benton tunnel was only 30m away running parallel at a progressively lower elevation as the tunnel goes downstream. Little was known of its condition and it was known that failures had occurred in the Benton tunnel as recently as 1994. It was essential then that due care was taken during excavation.
Above, is one of the two Tamrock drilling jumbos used during excavation. The Outlet portal shown on lower down the page has been formed in shist. Softer than granite the schists generated few problems, although several water inflow areas were encountered, some causing delay.
Adjacent to the outlet of the Benton tunnel the new outlet structure is being excavated for the construction of a connecting gate. Although the rockk is relatively good, the excavation techniques actually used have created a number of excavation problems.
Tunnelling in the granite caused few problems. Although the massive rock structure did give rise to long persistant joints in the crown which required some support measures. In general rock mechanics problems were few, although had the worst fears of a link between the tunnels been correct, then the scale of problems may have been different. The main tunnel is now excavated but several areas still require treatment. Work on the repairs to Benton Tunnel is about to start.
The contractor has generally taken a fairly pragmatic view on rock support measures. Turning bays were used to enable trucks to muck the tunnel. This method added to support considerations producing larger openings at several points along the tunnel. These were only allowed at places opposite the good quality granite reported in the Benton tunnel running alongside. Vibration monitoring was also completed. The tunnel is now to be partly lined with shotcrete when the granite is poor or when drill and blast has caused excessive damage to the walls or roof.
Two photographs here show the inlet and outlet of one of the several minor tunnels almost completely full during irrigation periods. |