INVESTIGATION OF LONG-TERM STRENGTH PROPERTIES OF PARIS AND BEAUMONT CLAYS IN EARTH EMBANKMENTS. FINAL REPORT

Shallow slope failures in compacted highly plastic clay embankments have been a common problem along Texas highways. Recent studies had shown that shallow slides might be the result of cyclic wetting and drying that takes place in the field. The purpose of this study was to examine the effect of wetting and drying on the long-term strength properties of compacted highly plastic clays. The impact of pore water pressures on the long-term stability of earth embankments was also investigated. Consolidated-undrained triaxial compression tests with pore pressure measurements were performed on laboratory prepared soil specimens of clay from two embankments that experienced shallow slope failures. Triaxial test results indicated that a reduction in the effective-stress shear strength parameters occurred after compacted specimens were subjected to cyclic wetting and drying in the laboratory. Triaxial tests also showed that the long-term strength properties of compacted highly plastic clay embankments may be measured in the laboratory by conducting strength tests on laboratory prepared, normally consolidated specimens. Results obtained from X-ray diffraction analyses confirmed that normally consolidated specimens and specimens subjected to wetting and drying had a similar clay structure. Slope stability computations, performed for the two embankments under consideration, revealed that the reduction in strength due to wetting and drying partially explained the observed failures. Stability analyses also confirmed that significant positive pore water pressures may have existed at failure. Pore water pressure conditions at failure were back-calculated for an additional 34 shallow slope failures in Texas. The results of the stability analyses were used to establish recommendations for predicting pore water pressure conditions at failure for design of future embankments.