Stability of embankments subjected to large number of cyclic loads


Soils under road or rail embankment are subjected to millions of low amplitude cyclic loads. Similar large number of load cycles are seen in wind and wave loaded storage structures and below machine foundations. These load levels are not high enough to cause soil liquefaction or particle breakage, but the significantly large number of cycles can alter the soil fabric and cause significant volumetric strain accumulation over time. Under the best case scenario, this can cause the embankment’s strength to increase, thus permitting even greater axle load over them. However, some soils and in situ stress states will not permit improvement and otherwise strain – soften if the threshold cycle is crossed. It is therefore desirable to understand how the cyclic stresses and the number of cycles affect the post cyclic strength and stiffness of different soils. Also of significance is to be able to quantitatively estimate the improvement in shear strength of soils that have been subjected to such large number of cyclic load.To study the soil behaviour under millions of load controlled cycles, triaxial tests were performed in the Advance and Dynamic soil testing (ADsoil) Laboratory at IIT Bombay. Traditionally, strain and pore pressure accumulations are modelled as empirical functions of the cycle number. They have limitation as they do not fully model the soil behaviour. A rather correct approach (used in this study) is to develop model which has a theoretical material behaviour background and stronger parameter definitions to capture the entire soil response. Further investigation into the post recompression soil behaviour is related to the excess pore pressure developed during the cyclic loading. The restructured strength and modulus improvement can be computed from established material behaviour and verified using the experimental data.