The understanding of foundation conditions of major dams in a realistic manner before embarking on actual construction is a must to avoid risks and failures. The statistics have shown that most of the failures of structures are caused by the failure, of foundations. This necessitates studies of geological discontinuity of foundation rocks. In the Sixth five Year Plan (1980–85) a major irrigation development programme has been launched in Madhya Pradesh, the heart of India. A few examples of projects where difficult foundation conditions were encountered are described below.
Il est absolument necessaire de saisir d'une manière realiste les conditions defondations des barrages importants avant de se lancer à la construction proprement dite si l'on veut eviter les risques et les ruptures. Les statistiques montrent que la plupart des ruptures des ouvrages sont dues aux ruptures des fondations. Cela exige des etudes sur la discontinuite geologique des roches des fondations. Dans le cadre du sixième plan quinquennal (1980–85), un vaste programme de developpement pour l'irrigation a ete lance au Madhya Pradesh, au coeur de l'Inde. Quelques exemp1es d'amenagements où l'on a rencontre des conditions difficiles de fondations sont decrits ci-apres.
Das Verstandnis der Fundamentbedingungen größerer Staudaamme in einer realistischen Weise vor Beginn der aktuellen Bautatigkeit ist unbedingt erforderlich, um Risiken und ein eventuelles Scheitern zu vermeiden. Statistiken haben gezeigt, daß die meisten Staudammbrueche auf Fundamentschwachen zurueckzufùhren sind. Deshalb ist ein Studium der geologischen Trennflachen der Fundamentfelsen erforderlich. Der 6 Fuenfjahresplan (1980–85) enthalt ein größeres Bewasserungsentwicklungsprogramm in Madhya Pradesh in Mittel-Indien. Einige Projektbeispiele, bei denen schwierige Fundamentbedingungen angetroffen wurden, werden beschrieben.
Forty metre high Tawa dam with drainage area of 5789 Sq.km. is located on the tributary of the Narmada river. The foundation rock of the dam consisted of fine to coarse grained gritty sand stone intercolated with shale and carbonaceous shale. The foundations dipped at 15° to 25° in the upstream direction. The sand stones were thick bedded and the bedding joints were most prominent. Percolation tests conducted in the right transition indicated that these joints were tight. The shale layers were one cm. to 1.5 m thick and extended from 3 to 60 metres. The results of unconfined compression tests were generally low and 50 percent of the tested samples showed crushing strength values less than 90 kg/cm2. A few samples crumbled after soaking in water. The weak spots were distributed in the whole foundation area and they were not concentrated in any specific zone. The crushing strength values varied erratically and there was no improvement with depth. The results of in situ shear tests on shales indicated that though the predominant value of ø was about 30°, there was possibility of occurrence of ø of about 15° and cohesion of 0.54 kg/cm2. Thus the stability of Tawa dam was likely to be governed by sliding along shale bands. Triaxial shear tests with Mohr's envelope gave minimum value p=45° and c=6 kg/cm2 for sand stones. The values of modulus elasticity of masonry was 0.135 to 0.225 6 kg/cm2. This necessitated non-linear analysis of foundation stress. Because of the low crushing strength of the foundation sand stones the conventional factors of safety of 4 to 5 against crushing were not obtained. Corresponding values with 75 and 90 percent reliability were 37 and 11 kg/cm2 respectively. These crushing strength values compared to the applied stress of about 18 kg/cm2 produced quite low factors of safety. The condition would be still more adverse with non-linear stress analysis which would give stresses about 2 to 3 times the linear stresses. Due to the low shear strength of the shales factor of safety against sliding along the assumed shale bands were also rather low. With the solitary minimum value of 15°, the shear friction factors were just higher than unity. The usual recommended values of 4 with average values of 'c' and ø were not obtained.
In 48 metre high Barna dam with drainage area of 1176 Sq.km. located on the tributary of the Narmada river, the presence of clay seams in the foundation necessitated a review of design against sliding along the weak bedding planes below the foundation. The preliminary geological investigations did not reveal the presence of clay seams which were observed during subsequent geological investigations and drilling. The majority of the clay seams occurred at 1 to 3 metre vertical intervals in the foundation with their thickness varying from about 2 to 4 cm. and dipping about 15° towards the downstream at the bedding planes with sand stone. The aerial extent of the clay seams was varying from block to block and estimated as 40 percent in the blocks 8 to 11, 50 percent in block 12 and 70 percent in the blocks 13 to 16 of the foundation area of each block.
