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The Uma Oya Project (UOMDP) passed a milestone event with the breakthrough of the Link Tunnel on January 13, 2017.
The UOMDP includes two reservoirs, namely the Dyraaba and Puhulpola reservoirs. These two reservoirs are connected by a 3.871 km long Link Tunnel, essentially diverting water from the Puhulpola reservoir to the Dyraaba Reservoir. Water from the Dyraaba reservoir is then diverted through the Headrace Tunnel of 15.6 km length and finally through a vertical shaft, of 600 m height, to the underground Powerhouse at Karandagolla near Wellawaya, adding 120 MW of electricity to the national grid. The water then discharges from the tailrace tunnel of the Powerhouse directly to the Alikota Ara for irrigated agriculture in the lower Uva area.
The construction works in the UOMDP is carried out by the Contractor, Farab Co., of Teheran, Iran under an EPC type Contract Agreement. The Employer, the Ministry of Mahaweli Development and Environment has engaged the Geological Survey and Mines Bureau (GSMB) and the Central Engineering Consultancy Bureau (CECB) to monitor the Contractor’s performance and compliance with requirements in the Scope of Works and adherence to established standards and specifications in this type of construction. The Contractor has employed Mahab Ghodds of Iran in joint venture with Poyry of Switzerland for the overall design and construction supervision of the project. Amberg Engineering of Switzerland has been appointed as specialist consultant responsible for the tunneling work. There are many subcontractors responsible for the specific areas of construction of the project creating a complex and integrated team approach. The Link Tunnel is being excavated by traditional drill and blast by the Excavation Team.
The Link Tunnel of 3.781 km length is of horse shoe shape with excavated dimensions of 4.7 m width, 4.95 m height and having a face of 20.15 m2 in cross section. The invert level of the intake at the Puhulpola reservoir portal is at 980.5 MSL with the invert level at the outlet at the Dyraaba portal of 976.9 MSL with a slope of 0.013%. The tunnel is designed for free flow.
The Link Tunnel excavation activities were planned to be completed within a period of 13 months. Excavation activities started in September 2014 unfortunately work was suspended in February 2015 for political, social and financial reasons. With the recommencement in November 2015 the Link Tunnel excavation was completed by Jan 2017 taking an effective construction period of 14 months.
Excavation of the Link Tunnel progressed from both portals and the excavated rock volume is 78,000 m3. In general a substantial volume of the excavated rock was used in concrete production for the roller compacted concrete and conventional concrete for other parts of the project in particular roller compacted concrete (RCC) for the main dams in Dyraaba and Puhulpola.
There are a number of young local graduates (Mining Engineers and Geologists) working for the Contractor and Consultants. These young men and women work under the direct supervision of experienced local and foreign expatriate professionals. The experience from the Link Tunnel construction surely has contributed much to the development of these young local graduates in tunneling and geotechnical engineering. The construction team also included surveyors who executed the important aspect of setting out and monitoring the tunnel trace resulting in precision of the holing through of both ends of the tunnel. The success of the Link Tunnel construction is undoubtedly due to the combined efforts of both local and expatriate Mining Engineers, Geologists and Surveyors.
In tunneling works, a significant feature that negatively impacts the progress of the work is water ingress in the tunnel excavation. In the case of the Link Tunnel excavation, only minor water ingresses were encountered. However, with timely action these ingresses and related issues were quickly resolved. Water ingress was limited to manageable quantities and this did not have a major impact on construction progress or the local environment.
The excavation of the tunnel was by the conventional drilling and blasting method with a maximum of 3.0 m as the allowable drilling length per cycle. Typically the ground conditions were excellent requiring only temporary and primary supports such as rock bolts, shotcrete with wire mesh, but some lattice girders were also required in one small fault zone.
With the rock excavation complete, the remaining activities for the Link Tunnel are completion of the final shotcrete line and invert concrete. And construction of the portal structures at both ends, control gates and trash track at the Puhulpola Portal and a cascade system at the Dyraaba portal.
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