The geological and geotechnical characteristics of the proposed site for General Mugica multipurpose dam (Mexico) strongly conditioned its basic and executive design. The presence of a paleo depressed relief wrought in volcanic rocks (andesites and volcanic breccias) covered by deposits of fluviovolcanic origin (lahars), from the quaternary age, on the right margin of the project, imposed restrictions in terms of the load capacity of the foundation, deformability and permeability. This condition on the right margin forced to modify the initial concept of a comprehensive RCC dam about 90 m height, for an RCC mixed dam on the left margin and river bed, and clay core rockfill on the right margin. The permeability problems of the lahar and the implications of groundwater flow (hydraulic gradients and uplift pressure) in downstream slope stability were resolved by a drainage gallery excavated in the lahars deposit downstream of the dam axis.
The geological model of the project was defined by running 26 exploration wells, geophysical and geoelectrical prospecting and detailed geological mapping. Samples were collected through core drills and cubic ones for the implementation of rock mechanical tests (simple compression, diametric compression, triaxial compression and characterization) .
The geological model in Figure 1 and 2 shows that the foundation of the dam on the left margin and riverbed is developed on volcanic rocks of high strength, low deformability and very low permeability, while on the right margin there are lahars sediments, of medium strength, low deformability and medium to high permeability.
The unlike geomechanical characteristics of the dam foundation deducted in the model, defined that the best solution for the closure would be a mixed dam, RCC type on the left margin and riverbed and a clay core rockfill dam on the right margin. A 3D Percolation model performed on the right margin showed high hydraulic gradients and uplift pressures downstream slope of the hillside that put at risk the stability. Several alternative solutions were assessed but the problem was solved by building a drainage tunnel excavated in the lahar downstream of the dam axis and to a unique geometry of the dam core.