Behavior of a Compacted Residual Soil at a Quasi-Saturated State

Compaction control in the field is strict regarding the degree of compaction and compaction water content. The control of the placement water content presents some difficulties in regions where the meteorological conditions are such that the soil is frequently above the specified water content. This can be at the compaction site or even at the borrow pit area. Traditionally, a positive pore water pressure is expected for compacted sedimentary soils. The case of compacted residual soil requires some further investigation. In some cases, the construction of wet embankments may be the only alternative. Theoretically, when the soil is compacted in the wet branch, although it is still unsaturated, the value of suction is below the air entry value and/or its degree of saturation is above 80%. In this case, its mechanical behavior is controlled by the effective stress principle and the soil is said to be at a quasi-saturated state. An experimental study in a compacted residual soil of the city of São Paulo was performed with water content above the optimum. All the specimens are at a quasi-saturated state. The analysis was performed by interpreting the soil water retention curve together with triaxial tests. CW and CIU tests were performed in order to obtain the shear envelope. The use of a high capacity tensiometer allowed measuring the suction during the tests. The results show that the residual soil compacted at optimum water content and above presents a suction that is below the suction at the air entry value. The results of shear strength tests indicate that the value of φbis similar to φ', which supports the hypothesis of treating the soil according to the effective stress principle. The analysis of the data allowed concluding that the soil tested could be compacted above the optimum water content provided specific care is taken.