«Last Modified 4/2/2015 IV-6. Seismic Risks for Embankments Key Concepts There have been very few instances where an earthquake has damaged an ...»
Last Modified 4/2/2015
IV-6. Seismic Risks for Embankments
There have been very few instances where an earthquake has damaged an embankment
dam enough to result in the uncontrolled release of reservoir water. Many embankment
dams are exposed to earthquake shaking each year, but either the damage caused by the
earthquake has not been extensive enough, or in the rare cases where damage was
extensive, many of the reservoirs were, by chance, low at the time of the earthquake, so uncontrolled releases did not happen. The failure probability estimation procedures described below are built upon standard analysis techniques used to predict responses of soil to dynamic loading and upon observations from case histories of embankments that have been exposed to earthquakes.
Dynamic loading can cause permanent deformation if the stress changes cause shear or tensile strength to be exceeded. Loose, saturated, cohesionless soils, when subject to earthquake shaking and initial shearing, can contract as the soil particles are rearranged.
Since the water within the pore spaces is virtually incompressible, this results in an increase in pore water pressure and decrease in shearing resistance. If the pore pressure increase is enough to reduce the effective stress to nearly zero, the soil is said to have liquefied, and the soil experiences a significant reduction in shear strength. Extensive shear strength reduction beneath an embankment slope can trigger a flow slide which, in turn, can result in a very rapid dam failure. In dense, saturated cohesionless soils, large shear displacements may not occur. Instead, the temporary occurrence of excess pore water ratios of 100 percent (or initial liquefaction) is accompanied by the development of limited strains, resulting in progressive and incremental lateral spreading of slopes.
Whether or not the soil of an embankment or its foundation liquefies completely, pore pressure can increase, resulting in a decrease in shearing resistance. If enough reduction occurs, over a sufficient extent, large deformations can result. Translational failure can occur if the entire foundation beneath an embankment liquefies and the reservoir pushes the embankment downstream far enough to create a gap it can flow through. Overtopping erosion failure can occur if crest deformations exceed the freeboard at the time of the deformations.
If the deformations do not result in an immediate release of the reservoir, the embankment can be cracked or disrupted to the point where internal erosion can occur through the damaged remnant. This failure mechanism can occur with or without liquefaction. There are many ways in which cracking can occur due to seismic shaking, such as differential settlement upon shaking, general disruption of the embankment crest, offset of a foundation fault, or separation at spillway walls. See Chapter IV-4 Internal Erosion Risks for Embankments and Foundations for other conditions that may make a particular dam more susceptible to transverse cracking and subsequent internal erosion.
Compacted embankments are typically not considered susceptible to liquefaction upon shaking and initial shearing. Dense, cohesionless soils tend to dilate upon shearing, which increases the pore space between soil particles and reduces the pore pressures. Most IV-6-1 Reclamation and USACE embankment dams are compacted, so the focus of liquefaction studies tends to be related to loose foundation soils.
However, hydraulic fill embankments may be susceptible to liquefaction or pore pressure increases. Fine-grained soils, while not strictly “liquefiable,” may be susceptible to strength loss during an earthquake. Two aspects of a fine-grained soil's shear strength behavior can require investigation: 1) the anticipated peak magnitude of earthquakeinduced shear loading when compared to a soil's undrained shear strength determined from monotonic loading; and 2) sensitivity, which is the potential for a reduction in the undrained shear strength due to the effects of many shearing cycles or very large monotonic strain.
If active faults or faults capable of co-seismic displacement cross an embankment dam foundation, the potential exists for foundation displacement that cracks or disrupts the dam core or water retaining element as well as transition zones or filters. The cracking can initiate concentrated seepage, and the translational movement can create locations where there would be unfiltered exit points for the seepage. Both conditions would increase the likelihood for failure from internal erosion. Shearing of a conduit passing through an embankment dam as a result of fault displacement can result in transmission of high pressure water into the dam, leading to increased gradients and potential for internal erosion. At the time of the 1906 San Francisco Earthquake, Upper and Lower Howell Creek Dams were located on the San Andreas Fault and holding water. Lower Howell Creek Dam which had a conduit through it failed, but Upper Howell Creek with no conduit did not. The presence of the conduit might have made the difference.
Seiche waves can be generated by large fault offsets beneath the reservoir, by regional ground tilting that encompasses the entire reservoir, or by mass instability or slope failure along the reservoir rim. “Sloshing” can lead to multiple overtopping waves from these phenomena.
Important Case Histories Relatively few dams have actually failed as a result of liquefaction, internal erosion through seismically induced cracks, or other seismic failure modes. However, a few case histories provide relevant insights.
Lower San Fernando Dam (1971) The upstream slope of Lower San Fernando Dam failed during the 1971 San Fernando Earthquake (Seed et al. 1975). Intact blocks of embankment material moved tens of feet on liquefied hydraulic fill shell material (Figures IV-6-1 and IV-6-2). There was evidence to suggest the slope failure took place after the shaking had stopped. Fortunately, a remnant of the dam remained above the reservoir water level at the time, and the dam did not breach.
Sheffield Dam: 1925 Sheffield Dam failed during the Santa Barbara earthquake of 1925 (Figure IV-6-3).
Although there were no witnesses to the breach, it was believed that the sandy foundation soils which extended under the entire dam liquefied and that a 300-foot long section of IV-6-3 the dam slid downstream, perhaps as much as 100 feet (Seed et al. 1969). The dam was located quite close to the city of Santa Barbara, and a wall of water rushed through town, carrying trees, automobiles, and houses with it. A muddy, debris-strewn aftermath was left behind. Flood waters up to 2 feet deep were experienced in the lower part of town before they gradually drained away into the sea. No fatalities were reported.
Figure IV-6-3. Sheffield Dam after 1925 Earthquake (Courtesy of National Information Service for Earthquake Engineering, University of California, Berkeley) Cracking of Dams Exposed to Loma Prieta Earthquake (1989) Harder (1991) describes the damage that occurred to 35 dams exposed to the Loma Prieta Earthquake. The Loma Prieta Earthquake was a magnitude 7.0 earthquake with approximately 7 to 10 seconds of strong shaking. Dams exposed to less than 0.2g did not experience damage. Dams exposed to peak ground accelerations between 0.2g and 0.35g either experienced no damage or developed longitudinal cracks. Transverse cracking was only noted in dams exposed to greater than 0.35g, although 7 of 19 dams exposed to this level of shaking experienced no damage, 7 of 19 dams experienced either minor or longitudinal cracking, and only 5 of 19 dams experienced transverse cracking. Only Austrian Dam suffered severe damage.
Austrian Dam (1989) Austrian Dam was severely cracked and damaged by the 1989 Loma Prieta Earthquake (Forster and MacDonald 1998), with peak ground accelerations estimated at 0.5g to 0.6g from the nearby magnitude 7 event (Figures IV-6-4 and IV-6-5). Longitudinal cracks that were 14 feet deep (based on trenching) formed just below the dam crest on the upstream and downstream slopes. Transverse cracks formed at both abutments, 1 to 9 inches wide, and the embankment separated from the concrete spillway wall, opening a gap of about 10 inches. Fortunately, the reservoir was low at the time of the earthquake, and no subsequent internal erosion ensued.
Figure IV-6-4. Settlement and Cracking at Austrian Dam in Area of Spillway Wing Wall (Courtesy of Sal Todaro) Figure IV-6-5. Plan of Transverse and Longitudinal Cracking at Austrian Dam (Forster and MacDonald 1998) IV-6-5 San Fernando Power Plant Tailrace Dam (1994) A small embankment dam forming the tailrace for the San Fernando power plant was shaken by large ground motions during the 1994 Northridge earthquake. The earthquake occurred early in the day, and the tailrace dam was intact when power plant personnel left for the day. The next morning, the dam had failed (Davis 1997). The tailrace concrete lining had buckled in several locations. It was suspected that a layer of loose sand beneath the dam, identified by CPT data, liquefied, and piped through the gaps in the concrete lining undetected, slowly throughout the day.
Steps for Risk Evaluation
The general steps for evaluation of seismic risks for embankments are as follows:
Develop detailed site-specific potential failure modes Develop event trees to assess the potential failure modes Establish loading conditions for earthquake ground motions and associated magnitudes, as well the coincident reservoir level Evaluate site conditions and develop representative characterization of the embankment and foundation materials Perform a screening by evaluating the load combinations and site characteristics to determine if seismic potential failure modes will be significant risk contributors If the potential failure mode can’t be screened out, then perform the following for each selected earthquake and reservoir load combination.
Estimate the likelihood of liquefaction of any foundation or embankment materials Calculate the likelihood of no liquefaction (one minus the probability of liquefaction) Estimate the residual strength of the materials that may liquefy or may experience strength loss Estimate the deformation of the embankment given liquefaction Estimate the deformation of the embankment given no liquefaction occurs For overtopping, assess the estimated deformation, and estimate a probability of overtopping. Different estimates are made for the various reservoir (freeboard) and earthquake combinations represented in the event tree. Complete the event tree nodes following procedures similar to flood overtopping failure modes. See Chapter IV-2 Flood Overtopping Failure.
For cracking, assess the estimated deformation, and determine the likelihood of developing transverse cracks. Estimate the depth and width of the cracks, and complete the event tree similar to the failure mode of internal erosion through cracks. See Chapter IV-4 Internal Erosion Risks for Embankments and Foundations.
The probability for each node in the event will be determined by expert elicitation considering all of the more likely and less likely factors associated with that node. See Chapter I-6 Subjective Probability and Expert Elicitation.
IV-6-6 Seismic Potential Failure Modes The following are generic descriptions of how a dam might fail due to these potential failure modes. For a specific dam, additional details would be needed in the descriptions, as described in the Chapter I-3 Potential Failure Mode Analysis.
Deformation and Overtopping Severe earthquake shaking causes loose embankment or foundation materials to contract under cyclic loading, generating excess pore water pressures (i.e., liquefaction occurs).
The increase in pore water pressure reduces the soil’s shear strength. (This could also occur as a result of loss of strength in a sensitive clay.) Loss of shear strength over an extensive area leads to slope instability and crest settlement. Crest deformation exceeds the freeboard existing at the time of the earthquake. The depth and velocity of water flowing over the crest are sufficient to erode materials covering the downstream slope.
Headcutting action carves channels across the crest. The channels widen and deepen.
Subsequent human activities are not sufficient to stop the erosion process. The embankment breaches and releases the reservoir.. If the seismic deformation is great enough for the crest to settle below the reservoir level, overtopping can be initiated. This mostly pertains only to dams that have a small amount of freeboard at the time of the earthquake. If the freeboard at the time of the earthquake is small enough, this failure mode could also occur without liquefaction, particularly if there is soft or sensitive clay in the foundation.
Deformation and Transverse Cracking at the Crest Severe earthquake shaking causes loose embankment or foundation materials to contract under cyclic loading, generating excess pore water pressures (i.e., liquefaction occurs).
The increase in pore water pressure reduces the soil’s shear strength. Loss of shear strength over an extensive area leads to slope instability, deformations, and crest settlement. However, crest deformation does not exceed the freeboard existing at the time of the earthquake. Open and continuous transverse cracks form across the crest and through all zones of the dam deep enough to intersect the reservoir. The depth and velocity of water flowing through the open cracks are sufficient to erode the materials along the sides and across the bottom of the cracks. Material from upstream zones is not effective in sealing the cracks (by being transported to a downstream zone or constriction point where a filter would begin to form). Headcutting action carves channels across the crest. The channels widen and deepen. Subsequent human activities are not sufficient to stop the erosion process. The embankment breaches and releases the reservoir. This failure mode can also be initiated without the requirement for liquefaction. If the seismic deformation is great enough for cracking to extend to the below the reservoir level, internal erosion can be initiated. Again, this mostly pertains only to dams that have a small amount of normal freeboard, such as a water supply dam that is kept full most of the time.