Predicting the residual strength available after liquefaction can be a major factor in the design of tailings dams and hydraulic-fill dams for stability. As strong-motion data accumulate in seismic areas, design acceleration levels tend to become progressively higher. Consequently, dams must be periodically reevaluated for safety, and remediation may become necessary. Analysis may show that liquefaction of at least part of the dam is probable, and remediation measures often hinge on the calculated stability of the structure assuming that there are zones within the dam that liquefy and the strength reduces to a residual value. In some cases, remedial measures may become so costly that the dam is instead taken out of service, disrupting water supply, power generation or mining operations. Thus, the prediction of residual strength can have an important effect on dam safety and operation decisions.
Residual strengths used in earthquake engineering practice are basically values back calculated from a limited number of case histories of liquefaction failures. There is considerable scatter in the data, probably due to the unique features of each failed embankment. Laboratory studies of residual strength after liquefaction are desirable to study this behavior under controlled conditions, and a large number of studies have been done. However, even for very similar materials, the values obtained vary widely, and tend to be much higher than those back calculated from field cases. It is very likely that the problem resides in the types of apparatus used, which generally do not have the capability of applying shear strains at a rate comparable to that of liquefaction slides, nor can they produce an amount of strain comparable to that observed in field cases.
This exploratory project will test the basic idea using a different approach. Earth scientists have for many years studied debris-flow slides in partly saturated natural materials, with a wide range of particle sizes. They have concluded that sliding behavior is a complex phenomenon controlled by particle/particle and fluid/particle interactions that are best interpreted as a form of viscous flow. However, a practical quantitative model of this behavior is still not available, due to the many variables involved. In the earth dams of concern, the range of particle sizes is generally smaller, and the material is saturated, which is a somewhat simpler case. This study measures the stresses around a small sphere pulled through liquefied soil, and analyzes the motion of the sphere in liquefied sand to see if it can be interpreted using a viscous flow (rheological) model. Both the rate of strain and the amount of relative strain are controlled to more closely resemble field values than in previous experiments.
If successful, this approach could lead to a better understanding of liquefied sand behavior during flow sliding, and provide a new experimental approach for studying the influence of different factors (e.g. fines content) on sliding behavior. Residual strength would thus not be modeled as a unique value, but would include a limiting threshold strength plus the strength gain due to the speed of viscous flow.
