1. Introduction
1.1 Main challenge and purpose of the abstract

After finalizing the excavation and support installation in the main caverns of the Uma Oya MPDP (Multi-Purpose Development Project), cracks in the shotcrete lining of the caverns and in the connecting galleries occurred. This happened during the ongoing work on the inner concrete structure of the caverns. The designers had to give proof to the client that these events will not endanger the integrity of the structures during construction and service life.

As one of the main hazard scenarios potential buckling failure along pre-defined rock slabs in the cavern's side walls was recognized. The potential slabs were formed by the specific discontinuity pattern with sub-vertical joints almost in parallel to the caverns. In case of buckling failure, additional load would have been transferred into the rock pillar between the caverns.

The decisions taken on site during construction were based on long-term monitoring data of which actual examples can be presented. The focus is on readings from load cells on anchors, extensometers and crack meters.

1.2. General outline of the project

The Uma Oya MPDP in Uva province of Sri Lanka is a classical hydro power plant with the additional benefit conveying water from the central mountain region to the dry south part of the island. The scheme consists of two catchment areas confined by each a dam. These areas are linked via a low pressure conveyance tunnel of 3.9 km length. A 15.3 km long headrace tunnel starting from the Dyraaba dam guides the design discharge of 19.5 m3/s past a surge shaft, a valve chamber, a vertical pressure shaft with a length of 630 m, an underground powerhouse with an adjacent transformer cavern both bearing an overburden of almost 700 m and a tailrace tunnel of 3.7 km length.

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