Deep Foundations Institute of India (DFI of India) held a webinar under its Groundwork program on “Foundation Treatment for Concrete Gravity Dam: A Case Study of Sardar Sarovar Dam” on 18 January 2022. It covered topics like understanding and analyzing geotechnical and geological features of dam sites, project challenges presented by the strata, solutions and treatments.
Importance of geotechnical and geological features of dam sites
Moderator Dr. R. Chitra, Director, Central Soil and Materials Research Station, Government of India, began the session by quoting some historic developments of dams in India, dating back to the 12th century, and which are still standing the test of time. She informed that Maharashtra has the largest number of dams, despite the region having problematic expansive soil, and the challenges faced during the construction of dams in other regions as well.
“1971-2000 saw many dams being built and serving the country well. We also witnessed some dams failing because of the presence of shear zones, poisonous gases, massive rock falls, tectonic stresses, variation in lithology, etc,” She emphasized the fact that when we are in a hurry, we fail to give due attention to the geotechnical and geological features of the dam site, which leads to its failure. “However, we are now adopting the required quality control measures, not ignoring warning signs, framing proper contracts, and undertaking thorough analysis and documentation.”
Foundation Treatment for Concrete Gravity Dam
Vivek Kapadia, Director, Sardar Sarovar Narmada Nigam Limited, Gujarat, gave a presentation on the construction challenges of the dam. Highlighting the dam’s importance, he informed that it is providing irrigation to the largest Culturable Command Area (CCA) in India - a whopping 1.8 million hectares. In fact, the entire state of Gujarat benefits from this dam, which serves a triple purpose of providing irrigation, domestic water supply, and hydroelectric power generation. It is built on the topography of Deccan Trap, which is one of the oldest basalt rock formations on earth. The main challenge while designing the dam were the dykes and faults in the strata.
The Sardar Sarovar Dam has a mild curvature of 7600m radius, a length of 1210m, and a height of 128m. The service spillway has 23 radial crest gates, and the auxiliary spillway has 7 gates on the left flank.
The special feature of this dam is that the canal from the dam cannot be provided directly from the reservoir because of topographical challenges. So, four small rockfill dykes were needed, which formed four small reservoirs or ponds; these were connected through links and then the canal was laid out towards the north within the state of Gujarat. Two hydropower plants were built: one at the canal head hydropower plant (CHPH) through a saddle dam on the right bank, and the other is a riverbed powerhouse (RBPH) near the dam with 6 turbines which are underground. (Fig. 1).
Figure 1: Project planning
Bore logs out of 30 exploratory drill holes showed presence of lava lows and river channel fault. The strata consisted of dolerite dykes and crushed calcined and weathered trap. The fault in the river channel was exposed by drilling a 0.91m diameter calyx hole up to a depth of 36m in the year 1966-67. The fault was found to be 5 to 7m wide and of a reverse-type footwall block forming the left bank. Material obtained in the fault zone was consolidated gritty material and resembled weathered or soft rock. The detailed cross-section is as shown in Fig. 2. The important point to be noted is that this topography had constant movements causing 3 to 4 seismic events every year.
Figure 2: Geological details
Treatment of Reverse Fault
Treatment included removing fault zone material and filling it with high strength mass concrete. Photo-elastic studies were carried out for determining the depth of concrete plugs. These concrete plugs were provided below 5 monoliths as the fault traversed in a skew direction from upstream to downstream. An inspection cum drainage gallery was provided in concrete plug for fault with its alignment parallel to strike direction of the fault zone. Stress meters, strain meters extensometers and other monitoring equipment are placed in the gallery for continuous evaluation. Since the area has high seismic activity, hammock reinforcements were provided consisting of two layers of 36mm diameter high yield strength deformed steel bars @ 250mm c/c parallel to dam axis in one direction and parallel to strike of fault in the other direction.
Treatment of Sedimentation
A patch of sedimentary consisting of 7 layers of sandstone was found which had much less strength as compared to basalt. Concrete shear keys were provided in two layers: each in transverse and longitudinal directions for integrating the basalt and sandstone thus avoiding sliding of the layers (as shown in Fig. 3). This treatment using shear keys was inspired from the famous Itaipu Project in Brazil, which had encountered the same problem for its foundation. 2/3rd height of the keys was made up of concrete and 1/3rd height was made up of colcrete.
Figure 3: Shear keys to prevent sliding
The dam’s foundation planning and the tests conducted involved a 30 year long exploratory period - from the British era up to post-Independence. Today, a wide range of instruments including inclinometers, pressure meters, strain gauges, etc. are present within the structure of a dam for round-the-clock monitoring, and more such modern instruments are being developed.
The basalt rock is a very slippery area for a dam’s construction, hence, the Factor of Safety against sliding is necessary to ensure a life of more than 250 years for any well-constructed dam. A remarkable fact of the Sardar Sarovar Dam was that such tedious exploration and calculation was done at a time when no software was available.
The session ended with a Q&A on the technicalities of the dam by engineers from many parts of the world and a quote of Swami Vivekananda by Vivek Kapadia: “Take care of the means and the end will take care of itself.”
About Deep Foundations Institute (India)
DFI of India (DFII) implements various programs for advancement of Indian foundation industry. Groundwork is DFII’s student outreach program, which provides students pursuing geotechnical courses various educational programs delivered by international experts, internship programs, project site visits, networking opportunities with domain experts, etc. Its goal is “Laying the Foundation for the Next Generation Foundation Practitioners”.