tunnels, power. stations and pumping plants ; and also main roads, large bridges and large buildings. On many of these kinds of projects as on many lesser ones, the systematic application of geology has become standard practice at all stages of development, from initial planning investigation, through detailed design to construction and maintenance. This has come about to a large extent through the increasing ability of geologists to apply their science in terms of the engineering requirements.

Large dams are among the most complex projects with which geologists have to deal, and they are particularly important since no risk of failure can be accepted. A sudden collapse releasing large volumes of water could cause catastrophe downstream.

To remove the risk, meticulous attention must be given to every possible source of weakness. The need to do this has been re-emphasized by several major dam failures overseas in the past 10 years due to geological defects which were not properly taken into account.

Malpasset Dam, a thin concrete arch of advanced design in France, collapsed due to sliding within the rock foundations supporting one abutment. Vajont Dam, a very high thin concrete dam, was over-topped by a flood wave 600 feet high thrown up by the sudden sliding of a huge block of rock from the valley wall into the reservoir;

the dam wall survived, but the sudden flood caused much damage and loss of life downstream. Baldwin Hills Dam in California, an earth wall, was breached by erosion initiated by differential movement along a fault plane allowing seepage to take place.

Therefore, in the siting, design and construction of a large dam many aspects of the geology of the site come up for consideration, including the strength of the soil or rock foundations and the effects of loading them by the combined action of the dam and the impounded water; assessment of the risk of movement along fault planes and of shaking of the structure by earthquakes; the quantity of stripping required to reach satisfactory foundations and the treatment necessary to make good local defects; the quantity and effects of leakage of water past the dam which might occur through the foundations and through the reservoir rim, and the grouting required to counteract such leakage; the means of passing floods safely through spillways; the stability of cut-slopes; the effect of changing reservoir levels on slope stability; the effects of changes in river regime on siltation of the reservoir, on erosion of the channel downstream and on raising the water table.

The task of the engineering geologist is to identify the geological factors which are significant to the engineering project and make sure that they are adequately investigated.