The Teton Dam: rhyolite foundation + loess core = disaster IAN SMALLEY

The Teton D a m was a large embankment dam built in Idaho, USA, in 1975. I t failed catastrophically in 1976. T w o major causes of failure were the fissured rhyolite terrain on which the dam was built and the brittle pemzeablle loess used for the core. Thus two geological factors combined to produce an engineering disaster. The complex of factors involved in the failure is still being discussed by engineers and geologisis.

Construction of the Teton Dam in Idaho was completed in 1975 and the dam failed in June 1976 while the reservoir was being filled for the first time. The failure is still being discussed. James Sherard, a dam engineer who participated in the post-failure discussions, has said th,at ‘the Teton Dam failure is one of the most important single events in the history of dam engineering’. Two geological factors appear to have played key roles in the failure; the rhyolite rock on which the dam was constructed was deeply fissured and the dam core was made from loess that covered the local landscape. The dam was built by the US Bureau of Reclamation, an organi- zation with an excellent reputation as a builder of dams, but in the complex system of site selection, design and construction fatal flaws occurred. In retrospect, it appears that the site was wrong, the design was unsuitable and the construction techniques were inadequate, but underpinning all this is the amazing fissured rhyolite tuff of the foundation and the uiicom- pactable and definitely permeable loess used for the core.

Choice of site The Teton Dam was an embankment dam, an earth dam with an impermeable core. It was designed with a very large core, and because loess was locally available it was used as the core material. The embankment dam is currently popular because it is relatively cheaip to construct and it suits foundation conditions which would be difficult for other dam types. In the twentieth century, concrete dams in their various forms gained in popularity until the 1930s (the great Hoover Dam on the Colorado River was the star of the concrete dams:), but since then they have become less popular. This is possibly due to a better understanding (of the behaviour of embankment dams with the developments in soil mechanics and engineering geology, combined with improvements in earth- moving machinery and the decrease in quality of

the remaining sites. Good sites for dams are proving increasingly hard to find.

The farmers of eastern Idaho had enjoyed an abundant supply of water from wells in the very permeable mass of volcanic rocks under their land, which is conveniently covered by a thick layer of loess, rich in minerals that will grow a good crop, given the water for irrigation. The loess is possibly the best agricultural soil avail- able in North America; its youth means it contains fresh minerals to provide nutrients and its silt-size means a good soil structure. Add water and the three constituents of agricultural success are achieved. One well in the area was reported as producing 140 litres a second, but the water was deep and a lot of energy was needed to pump it up. The regional water table is 60-150 m below ground level, and down- stream of the dam site the Teton river loses water into the volcanic rocks. Under the reservoir area, sediments have created a perched water table; and one of the factors that led to the area being considered suitable was the fact that the river was not losing water to the rocks over this stretch.

The dam was located in a steep-walled canyon incised by the Teton River into the Rexburg Bench, a plateau of volcanic origin draining into the Snake River Plain, Idaho (Fig. 1). The rocks

~ i ~ . 1. The T~~~~ D~~ before failure, looking upstream. (Photo: US Bureau of Reclamation.)

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exposed in the canyon walls are almost entirely of volcanic origin. At the dam site they consist primarily of intensely layered and jointed rhyo- lite with minor inclusions of basalt, breccia and welded ash-flow tuff. At the right abutment (that's right looking downstream - Fig. 2), in the failure area, the prominent bedrock joint systems are generally flat-lying upstream and approximately vertical downstream. The joints are closely spaced, conspicuously open and

0 50 lOOm Centre section of embankment \ /

Grout holes 1 I Zone 1 : loess core zone Zone 2: selected sand, gravel

and cobbles Zone 3: mixelloneous fill Zone 4: selected sib, sand,

gravel and cobbles

Fig. 3. Schematic cross-sections of the Teton Dam. Top: the centre portion, founded on alluvium. Bottom: the abutment section founded on jointed

. It was Zone 1, right (west) abutment, that failed.

Fig. 2. General plan of the Teton Dam, based on the original design drawings of the US Bureau of Reclamation, with dimensions in feet (1 ft = 0.305 m). The failure initially occurred in the right abutment, where the dam meets the canyon wall. On 5 June 1976 the water level reached 5309 ft, more or less at spillway level - then the failure happened.

unfilled, the partings commonly being 6- 50 mm. The vertical joints downstream from the dam axis strike across the canyon at an angle of 45" with the canyon wall, bearing roughly N20"W. Hence they provided multiple planes of freely discharging, gross-leakage capacity under the right abutment but practically no such conveyance capacity around the left abutment.

During construction, some very wide fissures and caverns were found in the right abutment. A haul road on the right side of the canyon revealed a group of 16 near-vertical fissures with widths of 25-915 mm. During excavation of the 21-m-deep cut-off trench to the right of the spillway, two extremely large fissures were exposed, about 25 m apart. They crossed the cut-off trench nearly 200 m from the spillway; and one, about 1.2 m wide, was explored by a geologist, who was able to go a distance of about 30 m upstream, downstream and below the trench floor. In the river bottom, accumulated alluvium of sand and gravel was up to 30 m deep. A cut-off trench, backfilled with the local loess, sealed off the full depth of the alluvium.

Design and construction The dam was designed by the US Bureau of Reclamation. It was a compacted, central-core, zoned, earth-and-gravel-fdl embankment (Fig. 3). Its gross height above bedrock in its cutoff trench was 126 m; it was 950 m long at the crest, and it had a total fill volume of 7650000 cubic metres.

Foundation seepage control in the intensely jointed abutment rock was intended to be effected by 21-m-deepY narrow, excavated trenches in the rock, backfilled with compacted loess, and by a deep grout curtain beneath a grout cap in the centre of the Zone 1 (central- core) contact, flanked closely on each side by a row of shallow, consolidation grout holes. The grouting principle is very straightforward; deep holes drilled in the rock are injected with a cement-slurry sealant which disperses to seal local cracks. Grouting is more of an art than a science and good grouting design is difficult. The Hoover Dam itself needed a vast amount of additional grouting to prevent seepage. The cut- off trenches at the abutment sides of the Teton Dam served the same purpose as the grout curtain, to prevent seepage under the dam; but to work efficiently they needed to be filled with the right sort of impervious material.

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The construction of the core was carried out very carefully. The loess was placed in layers 15 cm thick, and each layer was compacted by 12 passes with a heavy sheepsfoot roller. H[ad the core material had a higher clay-mineral content, all would probably have been well; but the silt- rich loess could not really be adequately compacted, and once in place it tended to be very easily eroded by leaks.

Loess The dam core (Zone 1) was composed of the local loess. The volume of the core fill was 3 965 466 cubic metres, which was morle than half the volume of the whole dam. The mistake was to go for the economy of using the local material, which was not at all suitable for a dam core. The Idaho loess is some distance from the main US loess deposits in Iowa, Illlinois, Nebraska and adjacent Great Plains states, but it is still more or less typical loess. It contains a mixture of volcanic ash from the western volcanoes, but this does not materially affect the properties.

Loess is an airfall deposit with a narrow size range concentrated in the coarse-silt region (say 20-60 vm). In North America it largely consists of primary mineral particles (quartz, feldspar, etc.) which have been reduced to silt size by glacial grinding. Additional material has been supplied from the Rocky Mountains by cold weathering; the youngest loess tends to be of this mountain variety (Fig. 4). The mountain loess occurs close to the Rockies - for example, in Nebraska to the east and in Idaho to the west. In Nebraska this recent mountain loess overlies the Brady soil, which has been dated at about 8000 BP; so it is a very young soil material, ideal for agriculture.

Loess is dominated by the silt-sized primary mineral particles, which determine many of its properties. For example, the interparticle contacts are of a hard, short-range variety, which means that loess has a low plasticity (and high plasticity is a very desirable property for dam cores). Loess makes excellent bricks, the reason being that the rigid mineral skeleton, the silt particles in contact, does not allow shrink- age. The loess usually contains enough clay- mineral material for the brick to be satisfactorily fired but not enough to cause shrinkcage or warping. This low clay-mineral content is fine for brickmaking (most British loess is called 'brickearth'), but it is the very opposite of what is required for a dam core. The core of an embankment dam needs to be flexible and waterproof (impermeable), and the clay minerals confer these properties. A compacted loess core may be initially impermeable, but it is brittle and dilatant. A dilatant system (first described by Osborne Reynolds in the 1880s) is composed

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of rigid particles in contact and it must expand when deformed. A dilatant core stressed by settlement would tend to lose its hydraulic integrity and allow passage of water. Because the inter-particle contacts are of the hard, short- range type, the system erodes easily (the loess lands in north China erode for the same reason). Flowing water detaches particles, and tunnels develop.

One of the post-failure investigations measured the plasticity index (which in effect indicates the clayeyness of a material) of the Zone 1 core material as 3, an incredibly low value. A normal clay soil might have a plasticity index of 30-50, while very clayey materials (containing smectite clays) might have values of 300-500. This very low plasticity value meant that a tiny change in the water content had a huge effect on the mechanical properties.

Failure Storage commenced in early October 1975. The auxiliary low-level outlet (a tunnel under the

Fig. 4. Sketch map of Idaho, USA. The younger loess in Idaho is largely mountain loess (M) derived from the Rocky Mountains and carried into the state by the Snake River. The Palouse area in Washington State next door is a well-known loess region, and the famous Spokane floods moved vast amounts of loess material.

Fig. 5. Looking downstream through the break in the Teton Dam after the crest of the water had passed.

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surface just upstream from the enlarging hole in the downstream face. By 11.20 a.m. the eroded hole in the dam was so large that bulldozers sent to fill it sank into the flow, and at about 11.55 a.m. the dam crest was breached as a complete failure occurred. By 6.00 p.m. the reservoir was virtually empty, having released about 308 million cubic metres of water with an estimated peak outflow in excess of 28 300 cubic metres a second. The resulting flooding down- stream caused the loss of 14 lives and about 400 million dollars’ worth of damage (Figs 5-7).

Fig. 6. An aerial view of Rexburg, Idaho, looking south, showing wide- spread flooding after the Teton Dam failure.

right abutment) was in operation during the entire filling period, releasing flows that were fairly constant at about 8.5 cubic metres a second until 4 May 1976. On that date, in response to the occurrence of unusually heavy runoff, which promised to fill the reservoir more rapidly than had been planned, the discharge was increased. It eventually reached 27 cubic metres a second on 28 May, rather in excess of the rated capacity. This capacity was small in comparison with the combined design capacity of the main river-outlet works, located under the left abutment, which, with the auxiliary outlet, totalled 120 cubic metres a second. The river-outlet, however, was incomplete up to the time of dam failure and hence was unavailable for participation in the planned control of the reservoir filling schedule. The actual filling, which was intended to be no faster than 0.3 m a day had reached over 1 m a day by 5 June.

On 4 June there was no evidence of appreci- able leakage as late as 9.00 p.m. On 5 June, shortly after 7.00 a.m., small seepages were seen emerging at the downstream toe and about one- third of the way up the right groin of the dam. By 7.30 a.m. the outflows were muddy, and by 8.30 a.m. the muddy flows had increased to around 0.8 cubic metres a second, coming from the right abutment dam contact. By 10.30 a.m. the point of emergence of muddy leakages had progressed up the right groin to a level about two-thirds of the height of the dam. Soon after- wards, a whirlpool formed on the reservoir

Fig. 7. The view through the gap left in the Teton Dam, from a spot on the island formed by rock and gravel piled over the powerhouse by the flood waters.

Enquiries and investigations Two principal engineering investigations of the failure were undertaken. The first was that of the Independent Panel to Review Cause of Teton Dam Failure, appointed by the US Secretary of the Interior and the Governor of Idaho. It reported on 31 December 1976. The second was the US Department Teton Dam Failure Review Group. This panel submitted two reports, in April 1977 and January 1980. Both investigations pointed to failure in the right cut-off trench, where water coming through the rhyolite fissures eroded the compacted loess. The major design deficiency was the failure to provide some seepage-control filter zone between the compacted loess core and its contact with open joins and fissures in the excavated rhyolite face downstream of the grout curtain.

To quote the words of the Independent Panel: ‘The fundamental cause of failure may be regarded as a combination of geological factors and design decisions that, taken together, permitted the failure to develop. The principal geologic factors were (1) the numerous open joints in the abutment rocks, and (2) the scarcity of more suitable materials for the impervious zone of the dam than the highly erodible and brittle windblown soils.’

A geologist looking back over the whole affair might say that geological problems were glaringly obvious, but the engineers did not seem to appreciate them. I think that this is an oversimplification, but there was certainly a failure to appreciate the hydraulic consequences of the complex of cracks and joints and fissures in the rhyolite and, perhaps more culpably, a lack of appreciation of the special nature of the Idaho loess. One or two post-failure discussions have listed lessons learned, but a slightly worrying aspect is that they all tend to be lists of engineering shortcomings, suggesting that the geological problems may be still unappreciated.

Ian Smalley is Professor oj-Applied Geomorphology at Leicester University and a member of the Geo- technical Engineering Research Group at Lough- borough University of Technology.

22lGEOLOGY TODAY January-February 1992