The Daniel-Johnson dam, formerly named Manicouagan-5 (Manic-5), is located 800 km northeast of Montreal (Québec) in Canada. This is the largest multiple-arch concrete dam in the world consisting in 13 arches and 14 buttresses. Since its construction in 1969, various type of cracks have appeared on both downstream and upstream faces of almost all the arches of this concrete dam. As a part of the Daniel- Johnson's dam safety assessment program, Hydro-Québec is currently performing structural analysis of the dam with advanced numerical models to better understand and predict the dam behaviour. Because the buttresses transmit all the arch loads to the rock mass, determination of the properties of the dam foundation should not be neglected in dam global safety analysis and also to insure that reliable input parameters are used in the dam numerical simulations. This paper summarizes the past and the more recent investigations of this foundation used to assess its safety and stability. Moreover, the use of rock mass classification to estimate the deformation modulus, based on empirical equations, gives values in the range of 30 to 40 GPa that are higher than expected values. Using those values in the numerical simulations for dam structural analysis gives an excellent correlation with displacements monitoring of the buttresses.
Daniel-Johnson dam is the largest multiple arch dam in the world consisting in 13 cylindrical arches and 14 buttresses (Figure 1). Except the central main arch, and those immediately adjacent to it, all the arches are similar in shape. The concrete thickness at the base of the main arch reaches 25 m and the other arches have 8.5 m in thickness. The construction took place between 1962 and 1968 and filling of the reservoir, which covers an area of 2000 km2, was not completed until 1977. Before the end of the dam construction, various types of cracks started to appear. A series of cracks are located at the base of almost all the arches. Those cracks, which daylight at the upstream face, plunges in the downstream direction but do not reach the downstream face. Those are called plunging cracks. Another series, called oblique cracks, are very fine cracks that are visible at the downstream face (Bulota et al., 1991; Saleh et al., 2002). At the time, different actions were taken to insure safety of the dam: in-depth analysis, monitoring, drainage and grouting. As a part of the safety assessment plan, inspections on a regular basis are made during dam operation. The dam performs very well and is structurally sound. In 2014, Hydro-Québec had to submit a detailed safety assessment study for this dam, including the rock foundation, as required by the Quebec Government.