(1) Watertightness of storage reservoirs. Geologists alone are competent to decide on the watertightness of storage reservoirs. Their decision is usually based on a detailed geological survey of large areas to make sure there are
no permeable rock strata through which water could seep out of the proposed reservoir.
In his classical treatise on dams and geology {Barrages et Geologie, Paris, 1933) M. Lugeon, one of the great pioneers of engineering geology, mentions two cases where geologists completely overlooked the danger of pervious rock. The two dams were built in Spain in the 1920s, but the reservoirs did not retain any water until expensive repairs were carried out.
The Camarasa Dam in Spain (1920) is founded almost entirely on fissured pervious dolomitic limestone (Lugeon, 1933) with impervious marl of the Liassic period several hundred metres below the dam crest. In 1924, Lugeon was called in as an expert geologist. He measured the loss at 11-26 m3/s and recommended that a grout curtain be driven under the dam itself and from lateral galleries at the level of dam crest. The depth of the curtain varied from 112 to 394 m and was 1029 m in length; 224 boreholes were drilled at a total length of 132 000 m (fig. 2.10).
1029 m
0 300 m
Fig. 2.10 Camarasa dam (Spain). Diagram of the grout curtain (Lugeon, 1933).
The grout curtain absorbed:
cement ashes sand and asphalt sawdust
gravel
40 734 tonnes 19 675 129 516 790 112
Conditions at the Monte Jaque dam in Spain on highly fissured limestone were no better (Lugeon, 1933). There the geologist recommended closing all the fissures from the upstream side with mortar. The work was done by hand. The same technique has been used more recently on the abutments of the Chaudanne and the Castillon dams in France.
A very similar problem occurred with the Dokan Dam in Iraq (Jones, Lancaster & Gillott, 1958). This high-arch dam (height 111m, crest length 350 m, radius of upstream face, 120 m) is founded on dolomite, approximately horizontally bedded shale, and higher up the valley sides, limestone. The left
Typical case histories 23 bank is a kind of peninsula against which the left abutment thrusts. The shaie is rather soft and the ground broken by faults, joints and large caverns.
Several methods for rock consolidation and sealing the dam were dis- cussed at different stages of the geological survey. One similar to that used at Monte Jaque was considered together with a clay blanket covering the reservoir bed upstream of the dam. Finally, the consultants decided on two large grout screens one on either side of the dam. On the left bank (peninsula) a 1348-m-long screen stopped water seeping through the narrow peninsula and the curtain on the right bank (1033 m) stopped leakage through the dolomite.
Some 375 000 m3 of rock were consolidated by injection with 1707 tonnes of cement and 471 tonnes of sand (Jones et ah, 1958).
For the curtains (4 528 000 ft2):
Total cement injected 45 000 tonnes Total sand injected 32 200 tonnes Length of boreholes 601 000 ft
which is slightly more than required for the Camarasa grout curtain. The cost was £2 400 000 in the 1950s.
In the three examples just given, the major responsibility lay with the geologists, because the final decisions depended on the location and per- meability of the rock stratas. More recently, responsibility for the design of grout curtains and rock consolidation has been given to the rock mechanicists. The design of the grout curtain has a direct bearing on the overall stability of the rock abutments; a problem of engineering rather than geology. Examples will be dealt with in detail in subsequent chapters on dam abutment design.
(2) Reservoir slope stability. It is well known that a rock slope, which is stable under natural conditions, may become unstable when submerged by an artificial storage reservoir with a varying water level.
As early as 1846, the French engineer Alexandre Collin developed a theory of the stability of clay slopes, which soil mechanics later extended by graphical and analytical methods. The stability of rock slopes is a far more complex problem, as rock masses may not have the relative homogeneity of slopes formed by loose material. The geologist will be called upon to determine the preferred lines of slip, possible faults, along which rock slides may occur.
An example is the tragic case of the Vajont rock slide (Jaeger, 1965a).
Several geologists, called in at an early stage, either completely ignored the danger of a rock slide or described it as a superficial movement of the rock masses of no danger to the reservoir. Several years later, a geologist (E.
Semenza) collaborating with a specialist in rock mechanics (L. Miiller) accurately predicted that the rock masses were in slow movement and they
correctly located the deep geological surface along which the slide would occur. Here too, a problem which in the past used to be considered only from the geological angle, is moving into the sphere of the rock mechanicists.
(3) Geology of dam sites. No major dam should be designed and built without a geologist first submitting a very detailed survey of the area, extending it well beyond the immediate vicinity of the dam foundations.
He should be given a detailed brief on which area to investigate and the type of dam which is to be erected.
The site of the La Jogne dam (1921) (fig. 2.11), the first high-arch dam to be built in the Swiss Alps, was carefully examined around the bottom and
pervious moraine sound rock
Fig. 2.11 The La Jogne dam (Switzerland). Leakage occurred through the pervious moraine; a trench filled with clay stopped the leak.
lower part of the rock walls of the La Jogne gorge. However, geologists did not discover a former valley, filled with moraine, located at a higher level but below dam crest. When the reservoir filled for the first time considerable losses occurred through this moraine. The reservoir was emptied hastily and a trench was cut through the moraine down to sound rock and filled with compacted clay.
The geologist in charge of investigating the site for the Malpasset dam carefully examined the bottom of the valley. He was probably not officially informed that the proposal for a gravity dam had been dropped in favour of a bold, thin-arch dam. Consequently he failed to comment on finding fissured gneiss masses at a higher level and the dam collapsed when they gave way under the thrust.
These are exceptional cases. In thousands of other dams collaboration between geologist and engineer has been excellent. The geologist has been able to produce the required information about the rock strata, their inclina- tion, thickness, contact zones, faults and the stability of the faults, and the mineralogy of the rocks and their chemical stability.
Information about the stability of faults is most important. Many dams have been successfully built across weak strata, so bridging faults (Bort dam in France, Bhakra dam in India), where the geologist has been certain that the area is stable. On the other hand, in an earthquake area to build across a contact zone or any fault likely to produce differential movements
Discussion 25 would be dangerous for dam foundations. (The Rapel dam in Chile does not cross any dangerous fault.)
The final estimate of the safety of the dam foundations is the responsibility of the designing engineer and the rock mechanicist. Their decision will depend on detailed research on the rock fissures, rock microfissures, shear strength of rock masses, faults, water passages and pressures; on rock strength and on strains and stress distribution in fissured rock masses, in addition to the findings of the geologists.
(4) Stable rock slopes for pressure pipes. Often the decision between an un- derground power-station with pressure shaft and galleries and conventional design with pipeline and power-house above ground depends on the cost, which is determined largely by the local geology. Sound rock inside the mountain and poor rock slopes for anchoring the pipeline would turn the scales in favour of the underground design. Stable rock slopes, providing a good anchorage for pipelines, would favour a conventional design. In such cases the advice of an expert geologist is invaluable to the designing engineer.