Civil Engineering Reference
In-Depth Information
(1976)
A 90 m (300 ft) high zone-filled earth dam was constructed in a steep walled canyon
eroded by the Teton River in Idaho (Figure 2-3). The typical cross-section (Figure 2-
4), shows a wide silt core, with upstream and downstream shells consisting mainly of
sand, gravel and cobbles. In the main section of the dam, the impervious core was
keyed into the foundation alluvium 30 m (100 ft) deep to serve as a cut-off trench.
Lesser cut-off trenches were excavated at both abutments through the permeable rock.
Reservoir filling commenced in November 1975 at an intended rate of about 0.3 m (1
ft) per day. Delays in completing the construction of outlet works combined with
heavier than expected spring melt run-off resulted in a filling rate up to 1.2 m (4 ft)
per day in May 1976.
The dam failed on June 5, 1976, when the water level in the reservoir was at
an elevation 9 m (30 ft) below the embankment crest and 1 m (3 ft) below the
spillway crest. Breaching of the dam crest and complete failure was preceded over a
period of two days by increasing quantities of seepage. This seepage was observed
initially 460 m (1500 ft) downstream and later on the downstream face of the dam
(Figure 2-5). Noticeable increase in seepage rate from the face of the dam adjacent to
the abutment about 40 m (130 ft) below the crest occurred during the morning of June
5. By approximately 10:30 am, the flow rate of seepage increased to about 0.4 cu m
per sec (15 cu ft per sec). This quantity continued to increase as a 1.8 m (6 ft) diam-
eter “tunnel” formed perpendicular to the longitudinal axis of the dam. By 11:00 am a
vortex was observed in the reservoir. The seepage flow rate increased rapidly from
this time onwards, accompanied by progressive upward erosion of the “tunnel” crown
(Figures 2-6 through 2-9). The dam crest was breached at about 11:55 am, with
complete failure of the dam ensuing. The flooding downstream after the failure of the
dam resulted in the loss of 14 lives and caused an estimated $400 million in damage.
Lessons Learned
The failure of Teton Dam was important in that it was the tallest dam to have failed.
It provided important lessons relating to the need for instrumentation; the need for
protective filters to prevent uncontrolled seepage erosion; the design of cut-off
trenches; consideration of the impact of frost action; and the importance of adequate
compaction control criteria and methods.
References
Chadwick, W.L. (1977). “Case Study of Teton Dam and its Failure,”
Proceedings of
9th International Conference on Soil Mechanics and Foundation Engineering
,
Case History Volume.
Delatte, Norbert J. (2009),
Beyond Failure: Forensic Case Studies for Civil
Engineers
, ASCE Press, Reston, VA, 223-234.
