The State government has achieved a breakthrough by completing excavation works of twin tunnels of Poola Subbaiah Veligonda Project on Tuesday (January 23). The excavation works of the first tunnel were completed in 2020, while those of the second tunnel were completed on January 23, 2024. The government is confident of releasing water through the project in the ensuing monsoon season.
The tunnel boring machine at tunnel two of Poola Subbaiah Veligonda Project. | Photo Credit: ARRANGEMENT
The Veligonda project is envisaged to provide irrigation water to 4.47 lakh acres and drinking water to 15.25 lakh population in drought-prone fluoride affected upland areas in 30 mandals of Prakasam (23), Nellore (5) and Kadapa (2) districts.
It comprises Nallamalasagar Reservoir drawing the Krishna floodwaters of 43.50 tmc ft above +840.00 feet (+256.032 M) from the foreshore of Srisailam Reservoir through the twin tunnels and feeder canal.
Water by gravity
Of all the new projects contemplated on floodwaters of the Krishna since 2004 in the combined State of Andhra Pradesh, the Veligonda is the only project which draws water by way of gravity using twin tunnels, Shashibhushan Kumar, Principal Secretary, Water Resources, told The Hindu on Wednesday.
Veligonda twin tunnels originate on the left flank of Kollam Vagu in the foreshore of Srisailam Reservoir. They run beneath Nagarjuna Sagar Tiger Reserve in Nallamala Hills with an overburden ranging from 50 m to 500 m and exit near Kottur Village, near Dornala in the Prakasam district.
The excavation of twin tunnels is proposed with two custom-made Tunnel Boring Machines (TBM) in order to minimise the disturbance that may occur to the wildlife during the excavation of tunnels.
The twin tunnels run almost parallel to each other. The distance between them at their inlet is 45 m and at their exit is 168 m. The Head Regulators of the tunnels are located at their inlets.
All the clearances of the Union Ministery of Environment, Forest and Climate Change, including Wildlife Clearance, were obtained prior to the commencement of tunnel boring operations.
To enhance the rate of progress of tunneling work, tunneling by Drill Blast Method was also adopted along with TBM tunneling.
The cross-section of tunnels is circular when excavated with TBM and it is inverted ‘D’ when Drill Blast Method is adopted.
Tunnel – 1
Tunnel - 1 including its Head Regulator is designed with an internal diameter of seven metres for a length of 18.8 km to carry 85 cumecs/3,000 Cusecs of water from the foreshore of Srisailam Reservoir to feeder canal.
The tunnel excavation commenced on September 2008 and was completed son November 2020. Out of the total completed length of 18.8 Km, excavation using TBM is 17.747 Km and Drill Blast Method (DBM) is 1.053 Km.
Tunnel – 2
Tunnel - 2 including its Head Regulator is designed with an internal diameter of 9.2 m for a length of 18.787 km to carry 243 Cumecs/8580 cusecs of water from the foreshore of Srisailam Reservoir to Feeder Canal. This tunnel excavation commenced in 2009 and was completed on January 23, 2024.
Out of the total completed length of 18.787 Km, excavation using TBM is 11.847 Km and Drill Blast Method is 6.940 Km.
In the Drill Blast Method of tunneling for 6.940 km, benching portion to a depth of 2 m at the bottom of tunnel is in progress for a length of 4.900 Km.
Mr. Shashibhushan Kumar said that the benching portion to a depth of 2 m at the bottom of Tunnel - 2 is in progress and will be completed before the ensuing monsoon season. The balance lining work in Tunnel – 1 & 2 may take about one year time and is not at all a hurdle for release of water in the ensuing monsoon season. He said that the compensation to the Project Displaced Families (PDF) of submersible villages in Nallamalasagar Reservoir will be settled before impounding the reservoir.
The project was commenced in the State during the regime of the then Chief Minister Y.S. Rajasekhara Reddy and continued till date.
