During a stability testing campaign on a gravity dam built between 1909 and 1911, core sampling was carried out in order to characterise the physical parameters of the structure’s masonry and foundation. These boreholes were supposed to be used as drains to lower the water pressure. When boring over a 15-m length from the dam’s downstream platform (penetrating the body of the dam along 5 metres then dipping 10 metres into the foundation), an active fault was crossed. The flow rate tapped rose dramatically from 6 to 20 m³/h in less than an hour.
The project’s prime contractor asked the construction company to install a plug at the bottom of the drain so as to prevent damage to the masonry by circulating water. The equipment available on-site initially forced the contractor to place this device at the drain outlet (where both flow rate and pressure were high). Flowing water appeared in the vicinity of a second drain. The contractor installed another plug and channelled this flow downstream. The dam operator then decided to drop the retention height. Despite drain valves being open, this lowering step could only get underway on 15 December due to heavy rains during the two days prior. One plug was positioned at the bottom of the borehole on 14 Dec using outside equipment. Close monitoring was introduced, with twice-a-day site visits including weekends, plus enhanced analysis. The leak stopped after injecting 180 litres of a cement-bentonite grout into the borehole (cored volume: 60 litres). Following this incident, the works specifications were adapted: a special pressure monitoring protocol underneath the structure and piezometric readings on the downstream platform were implemented after each drainage step and in the event of rainfall.
The joint water mission, combining the services of various State agencies was notified. A crisis cell, featuring the dam operator, the prime contractor, its engineering consultant and the construction company, was convened for this occasion.
New disorders related to surveying work appeared on 9 March 2013 after the dam had been filled with water (ARIA 43577).
