The Austin Dam, also known as Bayless dam, was designed as a gravity dam but failed catastrophically on 30th September 1911, within two years of its construction. Later on, it was recognized by the engineers that it failed due to the sliding of the foundation.
The foundation of the dam was constructed on sedimentary rocks and the depth of the foundation was a mere 1.2 m, which was significantly less. The orientation of the failed blocks of the dam and other site evidence pointed towards a sliding failure. The critical interface for sliding was a shale contact located at a shallow depth below the base of the dam.
Stability analyses based on shear-strength tests on the foundation showed that the dam was safe against a bearing capacity failure but unsafe with respect to sliding and overturning. The lowest factor of safety of 0.6 was observed against sliding. The major factors contributing to the failure include low shear strength of the foundation materials, inadequate provisions for reducing the uplift pressure, and weak concrete.
The failure of the Austin Dam is recognized as the sixth-worst dam failure in the history of the US in terms of loss of life. In this article, we have discussed the geology of the dam site, its construction details, the main reasons for the dam failure, and the lessons learned about dam safety.
1. Geology of the Austin Dam Site
Austin Dam is located in the Deep Valleys section of the Appalachian Plateau’s physiographic province of Pennsylvania. The following points describe the geology of the Austin Dam site:
- The region is a deeply dissected plateau, having extensive areas of steep slopes separated by narrow ridges.
- The deeply incised valleys in the section with hilltops as much as 152 m above the valleys originate from fluvial erosion.
- The site of the Austin Dam is about 16 km south of the glaciated high plateau's section, the nearest approach of Pleistocene continental glaciers.
- In the area of Austin Dam, the geologic structure consists of gently plunging anticlines.
- Jointing is common, with valley stress-relief joints striking parallel to the valley wall and constituting a prominent set.
- Fractures and rock deformation were prevalent in valley floors and valley walls.
- Valley stress relief is a result of the downcutting and formation of the valley. Often, flat-lying sedimentary rocks near the surface have horizontal stresses equal to or greater than the vertical stresses due to overburden.
- River downcutting removes horizontal support from valley walls and reduces the vertical pressure on valley floors.
- The erosion process results in tensional features in the valley walls, most often taking the form of open sheeting-type joints oriented nominally parallel to the strike of the valley.
- In the valley floor, the removal of overburden results in a zone of compression with deformational features that include arching and buckling of beds, open bedding planes, bedding plane faults, and low angle thrust faults.
2. Construction Details of the Austin Dam
Before the construction of the Austin Dam, a smaller dam had existed at the same site. The owner of the dam (Paper Mill and the Bayless Pulp) decided to construct a bigger dam to increase their production. The proposed dam was a concrete gravity dam. It was designed to resist gravity and hydrostatic forces through friction against the foundation. The owners wanted to finish the construction of the dam before winter.
The Austin dam was 161 m and 166 m long at the base and the crest, respectively. The height of the dam was 15 m at the crest and 13 m at the spillway location. The thickness of the dam was 10 m at the base and 0.76 m at the crest. The dam was designed at a capacity of 750,000 m3 of water by blocking the Freeman Run Valley.
Cyclopean concrete was used in the construction of the dam. The concrete consisted of boulder-sized rock pieces of sandstone with other content of standard concrete. A total 12,000 m3 of Cyclopean concrete was used in the construction of the dam. Twisted steel reinforcements of 32 mm diameter spaced at 1 m apart were used to reinforce the dam.
The foundation of the dam consisted of thin layers of shale rock sandwiched between sandstone. A total of 6500 m3 of soil was excavated for foundation construction, and compaction grouting was used to cement together the loose material. A 1.2 x 1.2 square meter size footing was provided for the base of the dam, and abatements were provided up to 6 m into the bedrock.Â
As expected earlier, the construction of the dam could not be completed by winter. However, the owners insisted on carrying on the construction during winter, and thus the concrete construction was undertaken below freezing temperatures. After the completion of all the construction work, on 1st December 1909, a large crack occurred through the dam, and the boulders of cyclopean concrete got fractures. Since the dam was empty and the foundation of the dam did not have a visible settlement, the contraction in the concrete was considered to be the cause of cracking.
3. Condition of the Austin Dam Before Failure
Six weeks after the completion of the dam, the dam reservoir was filled with water. On 17th January 1910, heavy rainfall occurred and the snow melted as warm temperatures after the rainfall caused the water over the spillway of the dam to flow. As a result, heavy seepage was observed at the toe of the dam, which resulted in the cracking of the downstream face of the dam.
To reduce the water level in the dam reservoir, the owners decided to use dynamite to blast the upper portion of the dam crest. However, as soon as the water level receded, the crest of the dam was repaired. After that, again, the water was filled to the normal pool level.Â
During the blasting event, the dam had suffered a considerable amount of damage. A total of six prominent vertical cracks had developed. As the dam was constructed without construction joints, the resulting cracks divided the entire portion of the dam into seven separate segments. However, to release the water, the blasting was necessary because the dam had no gates to release the water.
4. Details of the Austin Dam Failure
The Austin Dam failed on 30th September 1911. A hole was formed at the west abutment of the dam and due to massive force of water, a concrete block of the dam got pushed and slid downstream. A large wave rose between the reservoir and the downstream of the dam, and flooded the Austin town. The remains of the failed dam are shown in Figure-5 and Figure-7.
As the water reached the Austin town, it destroyed a lumber mill and picked up the logs as debris. The impact of the wave was so huge that most of the town’s buildings got destroyed. Figure-8 shows the extent of the damage in Austin.
The fast-moving water ruptured a large gas main and set it on fire, burning much of the wreckage left by the flood. The city’s fire protection system had been destroyed in the flood. Furthermore, 5 km downstream to the dam, the wall of water completely destroyed the town of Costello. The town of Wharton, 13 km past Costello, was also heavily damaged, but didn’t have any casualties.
5. Main Reasons for Failure of Austin Dam
Two separate inquiries were launched, one by the Pennsylvania State Water Commission and another by the District Attorney of Potter County where the town of Austin was located. The dam engineer, Chalkley Hatton, asserted that his design would have been sound if the foundation had not been undermined by seepage. He claimed to have told the owners to build a concrete cutoff trench upstream but the recommendation was not followed.
Hatton also blamed poor construction practices of placing concrete at below freezing temperatures and not allowing enough time for the concrete to cure before the dam was put into service. Even by the standards of 1909, the investigation for the dam foundation was inadequate, and the construction methods were insufficient. The cyclopean sandstone boulders were weak themselves, as shown by the fractures through them when the dam first cracked.
Investigations also showed that the cracks corresponded to cold joints between the concrete cast on different days. The faces of all the cracks but two were discolored, indicating that they had occurred sometime before the dam failure. Foundation excavations showed that considerable water seepage had undermined the foundation. A block of underlying rock had slid downstream along with the dam itself. Because of the seepage, the hydrostatic force lifting up the gravity dam (reducing friction) turned out to be much greater than assumed in the design.
An independent engineering news organization’s editor, Schmitt, also investigated the disaster. He surveyed the wreckage in the field, mapping the final location of the blocks. Schmitt identified two prominent breaches in the dam, one 30 m from the left abutment and another at the same distance from the right abutment. At each breach in left and right abutment, two blocks had been pushed about 9 and 30 m downstream, respectively.
5.1 Role of Geology
The site geology and the in-situ materials explain the sliding failure. Remarkably upfolded and crumpled masses of laminated shale strata suggest a tremendous downstream pressure between two failed concrete sections.
The site geology consisted of a thin layer of sandstone between two layers of shale. The dam foundation rested on top of the sandstone. The sliding plane developed in the lower shale layer, and the upper sandstone layer slid along with the concrete blocks. The sandstone layer, which the designer had thought made an appropriate foundation, was only 0.6 m thick.
The geology played a significant role in the disaster at Austin. Had the dam’s foundation been taken deeper into bedrock, it may have prevented the under seepage that ultimately brought about the high uplift condition that caused the instability in the structure.
Uplift pressure reduces the effective weight, and therefore the stability of a concrete dam. When the effective weight of a dam is reduced, the horizontal force of the water load can move the structure. In addition, had the foundation been taken below the shale layer, the dam’s stability would have been significantly improved.
5.2 Role of the Foundation
The failure of this dam was not the result of poor workmanship, but due to the poor judgment in determining its foundation requirements. The bigger mistake made while building this dam was expecting the rock foundation to be impervious.
The Bayless Mill imposed severe constraints on the dam design. If the dam had a foundation cutoff wall that would extend far enough into bedrock, it would have prevented seepage, but the idea was overruled because of the additional expense.
5.3 Role of the Dam Authorities
The operation of the dam was also clearly at fault. Once cracks appeared in the dam, more robust measures should have been taken to ensure the safety of the downstream community. The first crack was more than an adequate warning, but the construction continued without any changes. The implications of the second crack were also ignored.
Blasting holes in the dam, which they did to bring down the water level, was a great, absurd blunder that probably weakened whatever remained of the structure’s integrity. When the authorities disregarded the engineer’s recommended repairs, the dam’s fate was sealed. Though the reservoir should have never been filled after cracks were observed in December 1909, it definitely should have been entirely abandoned after the near disaster of January 1910.
6. Impact of the Failure of the Austin Dam
Pennsylvania had suffered two dam failures with substantial loss of life, South Fork Dam in Johnstown and Austin Dam, just over two decades apart. Not surprisingly, soon after the failure of Austin Dam, legislation was enacted in Pennsylvania calling for state supervision of the design, construction, and operation of dams and reservoirs.
Thus, a comprehensive dam safety program was initiated in Pennsylvania to prevent the recurrences of similar disasters. The failure of the dam at Austin raised awareness regarding dam safety and taught valuable lessons regarding the prevention of such tragedies in the future.
7. Lessons Learned
There were clearly two critical problems with the Austin Dam. The first one was the inadequate foundation design, and the second one was the shoddy concrete construction. The mode of failure clearly shows that the dam slid into separate blocks. This evidence suggests that the most crucial factor was the low friction against the dam base.
It is clear that a gravity dam resting on shale would have resisted water pressure indefinitely, if the dam had not cracked. However, the poor quality of the concrete construction also played some part. The cracks and weakness of the concrete divided the dam into blocks, and once the weakest block began to slide, the breaches widened. Cracks within a dam allow hydrostatic pressures to work to break the dam apart.
Time and economic pressures compromised the safety of the dam. The decision to omit the foundation cutoff wall from the original design, combined with the refusal to pay for the concrete cutoff trench upstream, doomed the dam to a sliding failure. The rush to complete the dam before the winter led to the freezing of the concrete and early freezing considerably reduces the strength of concrete.
Concrete shrinks and if the shrinkage is restrained, as with a long gravity dam, it will crack. As a result, control joints or designed cracks are generally put in concrete structures. In concrete dams, control joints are provided with water stops to keep water from penetrating into the dam and weakening it. The cracks that divided the Austin Dam occurred pretty much where control joints should have been provided, dividing the failed dam into blocks of roughly equal sizes.
FAQs
The reservoir of the Austin Dam was designed to hold up to 750,000 cubic meter of water.
The Austin Dam was designed as concrete gravity dam with a height of 15 m high.
The Austin Dam failed due to sliding failure. The critical interface for sliding was a shale contact located at a shallow depth below the base of the dam.
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