GPS & structural monitoring: deformation studies of Koyna Dam

GPS World, Nov, 2004 by Madhav N. Kulkarni, Nisha Radhakrishnan, Deepa Rai

The safety of large engineering structures demands monitoring of the deformation patterns of these structures and their surroundings. Over the past four years, a team of researchers from the Indian Institute of Technology Bombay has used GPS to monitor the Koyna dam in Western India for structural and crustal deformation. This article describes the GPS network, the methodology followed, and some significant results from this research project.

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Construction of large engineering structures such as dams, bridges, and high-rise buildings, is essential for the development of a nation. However, under excessive loading, such structures are subjected to deformation, potentially causing loss of lives and property. Hence, the safety of these works, especially dams, demands periodic monitoring and in-depth analysis of their structural behavior, based on a large set of variables that contribute to the deformation. In fact, the deformation itself forms the most relevant parameter to be monitored.

Several factors contribute to the deformation of these structures. The most important of all is the result of the varying stresses and strains developed in the structure due to the effects of local crustal movements.

Crustal movements cause shifts in the relative location of points situated on the dam body and in the vicinity of the dam. Other contributing factors include type of construction materials, wind forces, temperature variations, settlement of soil, and fluctuations in the load due to vehicular traffic.

In measuring the effect of these forces, modern researchers are increasingly turning to high-precision GPS positioning as a critical tool in their efforts. In India, the GPS team of the Indian Institute of Technology Bombay (IITB) has conducted dam and crustal deformation studies in the seismically active region near Koyna dam, Western Maharashtra, with the first author serving as the principal investigator under a research project funded by the Department of Science and Technology, Government of India (DST). The team has established a GPS network for this purpose and observed over ten epochs, from December 2000 to October 2004. The observations taken on the dam structure and the surrounding area have been processed, evaluated, and analyzed at IITB for the purpose of determining the rate, magnitude, and nature of the deformation.

Koyna Dam, Western India

Geologists traditionally characterized the Indian peninsular plate, one of the oldest continental blocks on the earth's surface, as a stable rock that remained immune from any major seismic disturbance. But the December 11, 1967, earthquake at Koyna, with a magnitude of 6.9 on the Richter scale, contradicted this belief and evoked interest on the part of geologists, geodesists, dam experts, and engineers. Fortunately, the dam in the region withstood this significant seismic activity without any significant damage. This seismic occurrence in the central Indian shield caused serious introspection among geo-scientists and led to various studies dealing with the stability of the dam structure and the surrounding regions.

The Koyna Dam, a rubble concrete structure, is an 85-meter high, 800-meter long, rock-filled structure with 872 square kilometers of catchment area. The dam has been well-instrumented for observations of structural behavior, right from the design stage, and this fact has proved to be very useful because of the continuing tremors in the zone after the 1967 event. The instrumentation includes various gauges embedded in the dam structure or installed around the dam body, such as uplift pressure cells, thermometers, piezometers, stress and strain meters, and coordimeters.

Apart from this, Survey of India established a traditional geodetic network in the region in 1976 and monitored it over a few years. More recently, the use of GPS for carrying out dam and crustal deformation studies has been introduced. The main objective of the project described here is to study the seismologically disturbed region of Koyna area and the behavior of the dam body, using geodetic GPS technique. This has included establishing a dense GPS network in the identified seismically active area, repeat observations, detailed GPS data processing using scientific software, and estimation of parameters responsible for deformation.

Koyna GPS Network

In order to carry out structural and crustal deformation studies, an extensive GPS network comprising 34 stations has been established in the vicinity of Koyna Dam. The GPS network set up to monitor the deformations in the region is detailed in Figure 1.

The distribution of the GPS stations is as follows: one base station on the dam axis, established approximately at the center and the topmost point of the dam, six stations on the dam axis (three on either side of the base station), three stations on the dam pier (two on the upstream (U/S) side and one on the downstream (D/S) side), three stations on the dam base (all on D/S side), one old geodetic triangulation (GT) station connected as reference at the Jangli Jaigad Hill Station, situated approximately eight kilometers north of the dam, two U/S stations near the reservoir, eight stations (fault points) along the fault line D/S of dam, three back site stations (all on D/S side), two safety stations (all on D/S side), and five deformation stations (all on D/S side).