The original program for the project assumed a construction period of four years (48 months). This was for the 960 MW scheme proposed in the 1992 Inception Report and possibly assumed that the project would be awarded as a variation to the 3rd Extension project that was ongoing at the time the report was written. The project has now been reviewed and the installed capacity will be larger with three instead of two turbines.
Furthermore, it will be necessary to enter into a new set of contracts and complete a new set of plant designs and this is likely to lead to some increase in construction time. The preliminary construction schedules indicate the first turbine generator could be completed within just over four years. The power station will now be completed between four and a half to five years after.
3.6.1 Construction Method of Tunnel 4 Raised Intake
After the final location of the intake has been determined, initial excavation for Option 2 would be carried out to establish a level platform around the proposed location at an elevation of 460 m on which the shaft construction can be carried out. It is envisaged that at this elevation the need for forming a cofferdam, that would extend the construction period for the shaft, would be minimized or avoided but grouting will be required.
Construction of the shaft would be by drill and blast methods. Cut slopes in the vicinity of Tunnel 4 will be need to be undertaken using controlled blasting..
The excavation volume of the intake area is estimated as approximately 900,000 m3 and it is expected that this excavation will be undertaken by dozing and ripping in the softer materials and by blasting in the stronger diabase. Work has been phased over a period of three years in periods of eight months when the water level is below 460 m, in order to minimize excavation and protect slopes.
The sequence for the Tunnel 4 raised intake excavations and associated construction program for intake option 2 have been provided in Figure 3.9 and Figure 3.10 respectively.
Tentative Construction Program for Mechanical and Electrical Plant
For the Project it is likely that the overall construction program (see Figure 3.11) will be controlled by the mechanical and electrical works. An outline for construction periods has been obtained from Alstom for a two unit station and they consider that the following periods would be appropriate:
• Supply Unit 1: 40 months;
• Supply Unit 2: 43 months;
• Install/Commission Unit 1: 13 months; and
• Install/Commission Unit 2: 13 months.
This gives total time for operation of the first machine of 53 months and the second machine of 56 months.
A third machine could be expected three months later i.e. 59 months following award of contract. This implies that first generation will be some five months later than had previously been envisaged with the last machine coming on line nearly five years following the notice to proceed.
Figure 3.9: Excavation Sequence for Tunnel 4 Raised Intake
Source: T4CJV
Figure 3.10: Construction Program of Intake Option 2
Source: T4CJV
Figure 3.11: Tentative Construction Program (Downstream Area)
Source: T4CJV
Powerhouse Civil Works
The civil work program has been developed independently from the above and it is estimated that the Power Station will be sufficiently advanced to start turbine erection some 26 months after the issuing of the Notice to Proceed. This will tie in well with the probable start of delivery of the first turbine components.
Once the first turbine draft tube is complete, it will be embedded and the concrete will form a base for spiral casing erection. Once the spiral casing has been completed and pressure tested the concrete works will continue upwards to form the generator foundations and the area will then be returned to the Mechanical and Electrical Contractors to allow them to erect the turbine internals and the generators. Each unit will proceed at roughly 3 month intervals.
The program envisaged is similar to that followed in many other large hydroelectric power stations and it is not expected to present any major difficulties. It might be possible to save some time but it is almost certain that it will be necessary to perform a full turbine model test and based upon previous experience it is considered that the indicative program put forward by Alstom is not unreasonable.
Switchyard and 500 kV Connections Civil Works
Assuming the studies confirm that the best solution is to simply extend the existing facilities it will be necessary to extend the 500 kV switchyard to accommodate the new switchgear bays and the associated incoming lines from the power station. It is proposed to use material from the powerhouse/intake excavation to extend the switchyard as this will reduce the quantity of material that has to be disposed of elsewhere.
It is expected that the connection between the new powerhouse and the extended switchyard will be by 500 kV transmission lines although alternative methods are being considered. It will be necessary to construct transmission towers within the tailrace but it is understood that the facilities exist in Pakistan for tower construction in rivers and similar areas. Therefore no problem is expected with this work. The time available to complete the switchyard civil works and the equipment installation is more than adequate and these works are not expected to be on the critical path for the project.
Penstock and Associated Civil Works
Before the erection of the penstock can begin it will be necessary to excavate a very significant trench to accommodate the pipeline. It will also be necessary to cut back the rock face behind the existing portal and also to the west of the existing outlet control structure. Significant slope support works are also expected to be required. It is proposed that the civil contractor will complete the majority of these works in advance of penstock construction. Construction of the penstock will proceed in sections leaving the connection to the exit from Tunnel 4 as late as possible to allow it to remain in service to release irrigation flows for the maximum period possible.