CASE STUDIES OF DAM FAILURES: A GEOLOGICAL INSIGHT
ABSTRACT: Case studies of past dam failures reveal the lack of dam-foundation-rock interaction study. Role played by the connection between the contact surfaces of rocks during loading history is still questionable. Parameters like deformation modulus, normal and shear stiffness, cohesion, friction at joints between rocks play a very important role, for which geological features like faults, folds, etc must be considered in the study for the static and dynamic analysis of dams. Special techniques which incorporates discontinuum should be used for analysis, as complex behavior of rock mass is predominantly affected by the presence of structural discontinuities.
1. INTRODUCTION
Today the face of the earth is dotted with small and large dams and reservoirs contributing in a variety of ways to complex requirements of an expanding technologically advancing civilization. In the total history of dam construction certainly many hundreds, and, if small embankment dams are included in the count several thousand dams have failed. There are no accurate records of most of the failures and extent of damage that resulted from the torrential inundation of lower areas as a consequence of their failure. Dams and reservoirs and the foundations on which they rest inevitably undergo changes with time. Some of these changes are slow and subtle and don’t reveal their existence unless precisely and constantly monitored. Others, such as those caused by earthquakes, landslides and unexpected floods are of short duration and usually cannot be predicted, in spite of the fact that the quantity of knowledge conciliating their prediction is constantly undergoing improvement. The responsibility for the construction of a dam with maximum provisions for the safety and constant critical surveillance of the dam, reservoir and foundation during its lifetime no longer is confined to the engineers who built the dams, but is shared by those who have special knowledge of hydrology, geophysics, geology, and rock and soil mechanics although every precaution may be taken to design and construct a dam with provision for generous margins of safety during excavation and treatment of the foundation. Within the boundaries boundaries of the reservoir, it must be recognized that the dam and reservoir behind it create dead load and water pressures that did not exist previously. Accordingly, the material in the foundation, abutment and the reservoir site requires constant monitoring after construction and reservoir filling. 2. DESIGN AND ANALYSIS
Two distinct stages in the design of dam are well recognized. The first stage is that of preliminary design when the overall hydrologic, topographic, economic and even political factors involved are examined in relation to costs and benefits. The geologic, topographic and hydrologic factors as well as the availability of materials of construction and labor cost decide the location and type of the dam. Though, the overall economy of the project, to a large extent, depends on the decisions taken at this stage, the analysis is essentially based on the judgment of experienced engineers based on the available data. In the second stage more thorough investigations of the selected site are carried out and detailed analysis of the structural stability of the dam is conducted. It is at this stage and at later stages, when special problems connected with foundation and super structure crop up, where numerical methods of analysis play a very important role in assisting design decisions. Dams are structures of great importance. It is therefore, necessary to analyze them as accurately as possible, for the failures of a dam may prove to be catastrophic. Also, considering the large amount
involved in the construction of dams it is worthwhile investing a small fraction of it, analyzing them with the best available techniques, even if it means more effort and higher cost of analysis. An accurate analysis and design helps to obtain an optimum section of the dam that may result in net saving in cost. An accurate method would would be one, which involves all those those parameters that have significant effect on the results of the analysis. To evaluate the safety of a structure subjected to seismic excitations, the method of analysis and permissible stresses under earthquake conditions are important. Based on the past research, it has become clear that dams have their own characteristic behavior at times of earthquakes and are not solely dependent on earthquake acceleration. Structures surrounded by water exhibit different dynamic characteristics than those vibrating in air. This is because of the interaction of water with the structure which results in hydrodynamic pressures and makes the determination of dynamic forces complicated. When a disturbance occurs in the reservoir dam system, the water behind the dam is set into oscillations. These oscillations result in impulsive and convective type of pressures of which convective pressures are usually insignificant in magnitude and are neglected. The impulsive pressures which are experienced by a dam in the event of an earthquake are termed as hydrodynamic pressures. So it is obvious that the reservoir of the dam also participates and interacts with the dam when it is subjected to dynamic loading such as earthquake. This interaction is known as fluid structure interaction. It is therefore apparent that the structural response due to loading of any kind will depend both on the properties of the structure and foundation. Studies conducted for the gravity dam design at several sites have indicated that foundation structure interaction effects play an important part in the behavior of dams, which is quite often ignored in the design. Foundation interaction effects not only alter the dynamic properties of the system due to elongation of time periods but also alter the static stresses due to self-weight and water pressure. Thus the inclusion of its effect in the study of seismic response of any structure, particularly for dams, gains due importance in the present days and requires a thorough understanding. The interaction effect of the foundation rock and the dam and of the reservoir has a significant effect in the response calculation of the dam. The response of any system comprising of more then one component is always interdependent. 3. FORCES
Many of the forces, according to IS: 6512-1984 8 which must be considered in the design of the dam structure fall into two categories, first, forces such as deadweight and water pressure which are calculated directly from the unit weight of the materials and the properties of the fluid pressure, secondly, forces such as uplift, earthquake, silt pressures and ice pressures which can only be assumed based on the assumed degree of reliability. The intensity, direction and location of these forces must be estimated after consideration of all available facts and, to a certain extent, must be based on judgment and experience. Analysis has been carried out in the past considering foundation as a rigid base and water in the reservoir as incompressible. Since this did not reflect the actual behavior of the dam-foundationreservoir system, analysis procedures considering the flexibility of the base and compressibility of the base and compressibility of the reservoir have been devised. The consideration of the fluid structure interaction between the dam and the reservoir in the subsequent analysis procedure alters the response of the system. The dynamic characteristic of the system and characteristics of the ground motion affects the fluid –structure interaction effect on the seismic response of the dam. Available results show that response of the structure is further reduced due to the damping effect of the soil. To represent the effect of fluid-structure and soil-structure interaction, the structure, the reservoir and the foundation is idealized and modeled in such a way that the results obtained are reliable and at the practical applications.
But the history of dam failure highlights some major dam failures that failed in spite of suitable measure take in design and construction of dam. Dam failures are of particular concern because the failure of large dams has the potential to cause more deaths and destruction than the failure of any other man-made structure. In our literature we have many but some which are more significant are presented below: among them major failure are named as St.Francis dam failure, Malpasset Dam failure, Teton dam Failure and many more. 4. DAM FAILURES 4.1 St. Francis Dam
St. Francis Dam was a 200-ft high concrete gravity arch dam. In 1920’s world-leading geologist of those times found no fault with the san Fransquito rock as shown if Figure 1. The dam was built squarely over the San Fransquito fault, although the fault had since been inactive. Two divergent rock types were separated along the distinct plane, making a sharp contact between the Pelona Schist and Vasquez formation. Several cracks appeared in the dam throughout 1926-1927. Some began to leak. But investigators felt that they were typical of gravity dam and hence declared it safe. The failure of the dam took place on March 12-13, 1928 killing around 420 people after this breaching, leading to feeling of a strange shaking of the ground, the sound of crashing, falling blocks as recalled by the only survivor about half a mile after the dam failure. Remains of the dam were as shown in Figure 2.
Figure 1: St Francis Dam before Failure
Figure 2: Dam after Failure (Roger, 2006)
Failure mechanism, which has been predicted by many researchers through back analysis, reveals the following mechanism shown in Figure 3. Some noticeable leakage started from the transverse cracks developed some years ago and also from the base of the left abutment. Followed by a high orifice flow of water from the base of the right abutment, which led to the loss of entirety of the left abutment, which was carried across the downstream face of the dam. This led to shearing off the large concrete blocks. As a result, the dam tilted and rotated to that side, allowing water to enter the transverse crack triggering a chain-reaction. The geologists were impressed by a sharp contact created by the fault with sedimentary rock on one side and metamorphic on other. The main cause suspected is the hydraulic piping that may have occurred on the bed along the fault as the Sespe beds were subjected to disintegration when submerged in water. The red conglomerate was underlying the right abutment; the failure is thought to begin in the area along the old Fransquito fault. Another major cause of failure was political promise, as the dam height was raised by 11% without increasing the base width.
4.2 Teton Dam
Teton Dam was a 305-foot high earth fill dam across the Teton River in Madison country, southeast Idaho. It was located in the eastern Snake River plain, a broad tectonic depression, underlain by rhyolitic, basaltic, late-cenozoic volcanic rocks. The dam was in the steep walled canyon cut by the Teton River into a relatively smooth to gently undulating silt covered with volcanic upland. The river flows southwestward of the dam site. On the left side of the canyon bottom, an erosional
Stage I
Stage II
Sage III
Stage IV
Stage V
Stage VI
Figure 3: Failure Mechanism of S t.Francis Dam (Rogers, ppt on Reassessment Reassessment of St.Francis Dam Failure)
remanant of an intra-canyon basalt flow which overlies the tuff. On the right side of the canyon, the basalt and tuff are overlain by thick accumulation of young alluvium. al luvium. The tuff in the right r ight abutment was foliated and strongly jointed. The joints consisted of both high and low angle joints. It was designed as a multipurpose facility that would provide irrigation water, flood protection, electrical
power and water based recreation. The level of seismicity se ismicity was very low in the t he eastern ea stern Snake River 11 and active faults were not known at or near the dam site. The failure of the dam took place in Idaho in June 1976, with a sudden release of water of its reservoir during the first filling of the reservoir. It was a significant event for geotechnical engineer because dam of such a height had never failed before. Failure of this dam started by the development of two small springs downstream on the right abutment, followed by another small spring at the toe of the dam on the right abutment. After which a major leak was discovered and in a short time there was a gush of water release and the crest of the embankment fell into the water and dam was breached. Figure 4 shows the section of the dam before and after failure of the dam. There are many phenomenons, which can be considered as being the triggering mechanism by which the flood path was created through the impervious core and the key trench fill- the basic cause of failure. Playing a key role in this aspect of failure were undoubtedly the specific nature of the joint system in the rock on the right abutment and the highly errodable nature of the zone-I fill material. Thus, of paramount importance was the possibility of leakage flow occurring immediately above the core-to-rock interface to loosen and erode the compacted silt from zone 1 as depicted by the Figure 5.
Figure 4: Cross section of dam before and after failure (Seed & Duncan, 1981)
Figure 5: Possible Failure Mechanism of the Teton Dam (Seed & Duncan, 1981) 4.3 Malpasset dam
Malpasset dam was a 60m high double arch concrete dam in southern France near Franjus. It was located in the crystalline tannerson massif, 80 Km from Nice. The dam was located in a gorge w here the Reyran river flows from north to south as shown if Figure 6. There was calcic garnet present which indicat dicated ed the the sed sedim imen enta tary ry orig origin in of the the roc rock. k. The The roc rock k at at the the dam dam site site cons consist isted ed of schist schistos osee foliation and gneissic foliation. Schistose foliation at the macroscopic level consisted almost exclusively of platy or needle like crystals, which had a parallel or sub parallel orientation. Gneissic foliation consisted of alternating bands of schistose and granular layers. Rock at this site was, in fact, a remarkable remarkable discontinu discontinuous ous with the the general dip dip of foliation foliation between between 300 to 500 towards towards nd downstream side and the right bank. The failure of dam took place on 2 December 1959. Investigators visited the dam on 2 nd December afternoon to avoid detrimental effects of spilling; as a result they opened the bottom outlets gates to control the rising of the reservoir. No vibrations were
observed. In the evening of the same day the dam failed with a sudden single movement 1. Figure 7 shows the possible failure mechanism that led to the failure of the dam. As can be seen from the figure, it is clear that the presence of the parallel discontinuities with variable zones of permeability were the major reason for the failure 8. As a result large amount of high uplift pressure developed which led to the wedge type failure. Figure 8 shows the more detailed three-dimensional failure mode.
Figure 6: Geology of the Site (Londe, 1987)
Figure 7: Possible Failure Mechanism
Figure 8: Three Dimensional Failure Mechanism (Londe, 1987) 4.4 Vajont Slide Catastrophe Catastrophe
The valley of the Vajont river, a tributary of the Piave river located in southern dolomites in northern Italy, which was dammed by an arch dam constructed from 1957 through 1960. The double arched 276 m high dam was supported on the steep flanks of a deep canyon, which cut into dolom ite limestone of the Malu and Dogger i.e. Jurassic information. Failure 4 took place in October 1963 when a rock mass having a volume of approximately 300 million m 3 slided into the reservoir causing one of the greatest catastrophes in history of dam construction. Wa ter swept across the dam crest reaching a height of 100m above the crest level-popularly known as SEICHE EFFECT. On one hand, the slide developed unexpectedly as far as mode was concerned. On the other h and it was said that the slide was expected based on three years of monitoring as from first partial impounding a phase of accelerated movements were observed. It can be seen that the lower part of the western portion of the slide was displaced along an existing discontinuity seen more in terms of kinematics than of geological structure and not so much along the actual surface. The sliding process happened due to the joint water thrust provoking strong horizontal thrust towards the lake and the pore pressure in the intermediate filling i.e. softening of clayey material in the base of the moved mass. It has been rightly said, “Man learns little from success but a lot from failures”. The failure of dam is the result of complex concourse of causes and mechanisms. Above study of failures of dam highlights the cause of failures of dam, reservoirs not due to design and analysis but due to inadequate knowledge of geology and importance of geology. It should be understood the role of of geological engineering which becomes more important with the increasing trend of dimensions of such a structure in height, depth, volume of reservoir. Thought detailed study was carried out in and around the area of dam, reservoir and foundation but analysis and design of dams were not done
Figure 9: Vajont Dam Before and After Failure (Tzamtzis& Asteris, 2004)
taking into account the discontinuous nature of discontinuities underneath the dam. Strong emphasi s should be placed on site investigation acquiring thorough knowledge of rock structure, type of rock, type of weakness planes, their orientation and so on. Nevertheless, that every dam site has a unique feature which are difficult to account for purely analogy. The functi on of good analysis is to extend the designers experience, judgment and intuitions in a rational manner. Physical models still are desirable for special studies but they are limited in their ability to simulate the initial state of stress and ground water regime, the seepage forces due to fi lling of reservoir and most importantly in their ability to consider the interaction. The problem of foundation and abutment treatment for high embankment dams on rock foundations remain one of the most critical aspects of dam design. Techniques which take into account these interaction affects and foundation discontinuities include the much deve develo lope ped d Fin Finit itee Ele Eleme ment nt Prog Progra rams ms (FEM (FEM), ), Dist Distin inct ct Elem Elemen entt Pro Progr gram amss whi whicch include Universal Distinct Element Code (UDEC), 3 Dimensional Distinct Element Code (3DEC), FLAC, Discontinuous deformation analysis (DDA), Modal Methods (CICE), Momentum Exchange methods etc. should be used to analyze such problems in details. 5. CONCLUSIONS
Rock discontinuities play a very important role in the study of dams or other important structures such as tunnels, slope stability problem and other man-made structures, which play a very vital role in our lives. In such structures the interface discontinuities, where the assumption of rigid connection between the contact surfaces is questionable. If, throughout the loading history, perfect bonding is maintained, the presence of an interface does not offer any difficulty in the analysis of the system. However, if, at some point in the loading, the bond breaks down and there is a relative movement of the two mating surfaces, where special solution techniques must be employed. The analysis of such discontinuous system is compounded by the sliding and separation that may occur along the interfaces between adjacent blocks. In general, this occurs at shear levels that are significantly lower than the limiting shear of the block materials. Consequently, an analysis that assumes perfect bonding at interface, would predict an over- or –under estimation of the response of the structure. Therefore, special linear or non-linear analysis technique like FEM, DEM etc, that take into accounts for the effect of these discontinuities on the response of the system must be employed to study the behavior of the problem. 6. REFERENCES
1. Bellier J, Londe P & Langbein J (1970) The Malpasset Dam Proc., Evaluation of Dam Safety, ASCE Safety, ASCE , Asilomer. 2. Chadwick W.L (1977) Case study of Teton Dam and its Failure, Proc 9th Int. Conf. Soil Mech. Found. Engg Engg , Case history Volume, Tokyo. 3. Chandrashekhran A.R and Samant B.B.S (1974) Dynamic behavior of concrete gravity dams including foundation interaction, VII Symposium of EQ Engg.U.O.R. 4. Chowdhury R (1978) Analysis of the Vajont slide – new approach, Rock Mech, J. Int. I nt. Soc. Rock Mech., Mech., Vol 11. 5. Chopra A.K and co-workers (1982) Hydrodynamic and foundation interaction effects in frequency response functions for concrete gravity dam s, EESD, s, EESD, 82, 82, Vol.10, No.1. 6. Lotfi et al (1987)- A technique for analysis of dams due to earthquakes, EESD, 87 , Vol.15, No.4. 7. Londe P (1987) - The Malpasset Dam Failures, Proc. of Int. Conf. on Dam Failures, Engineering Geology, Geology, 24,295-329.
8. IS: 6512-1984 Criterions for Design of Solid Gravity Dams, Bureau of Indian Standards, New Delhi. 9. Milanovi P (2003) Geological Engineering and Large Structures, ICGGE Bled 2003. 2003. 10. Seed H.B. & Duncan J.M. (1987) Proceeding of the 10 th Int. Conf. on Soil Mechanics and Foundation Engineering, Volume 3, Stockholm. 11. Seed H.B. & Duncan J.M. (1987)- The Failure of Teton Dam, Proc. of Int. Con f. on Dam Failures, Engineering Geology, Geology, 24,173-205. 12. 12. Tzamtzis A.D. A.D. & Ast Aster eris is P.G. P.G. (200 (2004) 4) FE Anal Analys ysis is of Comp Comple lex x Dis Disco cont ntin inuo uous us and and Joi Joint nted ed Structural Systems Part 1, Electronic 1, Electronic Journal of Structural Engineering , Vol. 1.
