Tarbela Dam

The struggle for Tarbela Dam was one of the great engineering epics of the twentieth century. The ten year saga eventually ending in an uneasy ¿success?, but not without the whole edifice teetering on the brink of complete collapse half a dozen times.



Tarbela blocks the Indus River as it carries the snow-melt from the Karakoram range through Pakistan's North West Frontier Province, just east of Peshawar. In its time Tarbela Dam was the largest manmade structure in the world. And, with 11,000,000 acre-feet of water held behind the 450 foot high earthdam, poised upstream of 40 million people, the consequences of failure were simply unimaginable. The team of specialist consultants included: Arthur Casagrande, Barry Cook, Wendell Johnson, Stanley Wilson, and Skip Hendron.

The main dam was built on top of the river gravels which were ~ 700 feet thick and therefore any type of cutoff trench, or even grout curtain, was out of the question. So to preserve downstream stability and keep seepage losses low, the bottom of the reservoir was covered by a blanket of fine material. This "impervious" upstream blanket as an extension of the sloping core is shown in red on the construction drawing. The blanket was designed/meant to absorb 90% of water energy acting on the base of the dam; consequently, at the upstream toe of the embankment the design hydraulic gradient through the blanket was i = 9.

During the first attempt to fill the reservoir this blanket was punctured by several hundred sinkholes; in the next few years 385 more appeared.

Hodge was resident there for two years as the senior technical engineer for TAMS, the New York design firm. As geotechnical Divisional Head he was responsible for directing the operations of the following supervisory Sections: Design and Construction Review; Instrumentation; Soils Testing; Geology; and Micro-seismicity. In this capacity as the engineer responsible for the dam, his daily interpretation of monitoring devices and embankment observations were the basis for deciding the settings for the gates which controlled the reservoir storage.


Hydraulic Control Structures

Water is gentle and compliant when we approach it calmly: it can break our bones if we hit it too hard. It's all a matter of speed. The immense power of water can be best appreciated by seeing the damage wrought by a discharge such as the 300,000 cusecs from the main spillway shown on the right.

Outlet water, flowing past a derailed discharge gate, cavitated and tore out the concrete lining of one of the four 40 foot diameter power intake tunnels. Its collapse, resulted in 12 million cubic yards of the upstream face of the earthdam being lost through the breach.

To stave off earthdam failure the two available tunnels were opened wide. As a consequence, their stilling basins, designed to absorbed two million horse power from the 115 mph water jets, were destroyed.

Nature was forgiving that year and sent down the lowest flood on record, allowing the basins to be repaired, but only in time to be destroyed a second time. It took the indomitable will of John Lowe III and the superb construction resourcefulness of Impregilo of Milan to force these structures to their feet again in time for the next flood.

Hodge was assigned the additional responsibility of supervising hydraulic model testing carried out in Pakistan during rebuilding the damaged stilling basins, and then the monitoring of their subsequent behaviour.

With such vengeance on both sides, the stilling basins were reinforced to a level which all but defied physics to wreck them again, at least as far as we think we understand what nature allows. Anyway, blocking a river in its natural course can only be a temporary affair at best.


Sinkhole through Main Embankment

The sinkhole concern climaxed when, with the pond just 5 feet from filling for the first time, when routine side-scan-sonar imaging of the blanket and upstream face of the dam recognized a sinkhole-like depression in the earthdam itself, 150 feet below water level. The reservoir had to be dumped, with fatal results for the senior politician who had promised irrigation water to downstream farmers - his main supporters.

In the three week window made available by taxing the hydraulic control structures to their limit against the oncoming annual flood from the spring runoff, the depression was seen to be indeed a sinkhole. It was investigated and an attempt made to seal it by grouting. With flood waters rising faster than the low-level hydraulic control structures could dealt with them, the hole was backfilled with crushed rock. Hodge decided on this material so as to facilitate re-access after the flood passed.

It is likely it was the prayers of the Pakistani staff that saved the dam this time.


More Lessons from Tarbela

In the years that followed Hodge came to have a better appreciation of some of these experiences. For instance, now with a better understanding of the hydrodynamics of 2-phase physics, it becomes clear what was the basic flaw/oversight that triggered the initial Stilling Basin failures that set the scene for much of what was to happen in the following years. Also, the question as to whether the recent disastrous inundation of the riverbanks along the Indus River downstream of a dam – a dam which was designed to prevent such flooding – was due to a further undisclosed mishap at Tarbel. Consequently, as time permits, an article to be entitled "More Lessons from Tarbela" will appear on this website. Among other things, this will provide an interesting comparison with the more recent events at B.C. Hydro's Bennett Dam, a interesting topic which will also be given its own room here on another page.