ABSTRACT
Larger savings are possible in the absence of uplift if the upstream face is also inclined, thus mobilizing the vertical water load on it for sliding stability. If the latter, in terms of the inclination of the resultant of the forces, is maintained the same as with the vertical upstream face, a buttress dam with 50 per cent (horizontal to vertical) slopes on both faces permits a theoretical reduction of the dam's mass of 64 per cent. At 100 per cent upstream and 33 per cent downstream slopes the reduction is as much as 89 per cent. Also the eccentricity of the resultant is greatly improved which moves from the downstream edge of the "middle third"
in case of the vertical upstream face almost to the centre of the section with the 100 per cent upstream slope. Correspondingly, the vertical stresses are almost uniform over this section, although fives times larger. Except for high dams, this is welcome because it permits a better use of the strength of the construction materials. Forerunners These basic considerations were, of course, unknown to the Roman engineers who introduced for the first time not only the arch1 but also the buttress dam. They may have developed its concept from buttressed earth retaining walls. In fact, some of their buttress dams, like the ones in the Wadi Megenin (Sidi Gelani)*, 40 km south of Tripoli, Libya2, and at Olisipo near Lisbon, Portugal, are only partially and irregularly propped up rectangular walls which are rather slim. Closer to true buttress dams are those of Iturranduz, 30 km southwest of Pamplona in northern Spain3 and of Esparragalejo, 10 km northwest of Merida in southwestern Spain4. While the water retaining element in the former consists of vertical walls placed between the buttresses at about their upstream thirds, these are interconnected by upstream arches at Esparragalejo, which is thus the oldest known multiplearch dam. The flat vertical arches show only on the downstream side, whereas the steeply stepped upstream face is straight. The structure was built of rubble concrete-faced with masonry and was carefully reconstructedin!959.
