Characterization of a reconstituted low plasticity silt

Abstract

During a major upgrade of the wastewater infrastructure in Skibbereen, Co. Cork, Ireland, there were many challenges during construction. Many of these difficulties were associated with the soft silt ground conditions. Consequently, soil samples were sourced from the location and transported to MIT for the purposes of this research. This thesis investigates the characteristics of the low plasticity silt encountered at Skibbereen to allow an understanding of the soil behavior and help explain some of the difficulties encountered during construction. The research program is divided up into two major components: 1) Index property testing; and 2) Engineering property testing. The index property testing defines the material as a well graded low plasticity silt with very low salt and organic contents. An extensive testing program is undertaken using Constant Rate of Strain (CRS), Hydraulic Conductivity, Ko-Consolidated Direct Simple Shear (CKoDSS), and Isotropic and Ko-consolidated undrained triaxial compression (CIUC and CKoUC) tests to define the engineering properties of the material. The experimental investigation finds that undercompaction is the best reconstitution technique for this material and it is almost impossible to test intact samples. The CRS tests demonstrate that the compression ratio increases with an increase in stress level but the recompression and swelling ratio are not significantly affected. The hydraulic conductivity results fit into the expected range for silt material and they are dependent on stress level. The DSS tests demonstrate that the undrained strength ratio ... increases with an increase in stress level. A similar trend is found for the peak arctan ... values. Overconsolidated tests prove that the SHANSEP framework can be successfully used for this material and the cyclic tests demonstrate that an increase in density through shearing will provide a dramatically higher su/a'v and greater ductility. The triaxial compression tests indicate that ... ill increase with an increase in stress level. In the CIUC tests, ductility and the degree of dilation increase with an increase in stress level. However, the CKOUC tests provide more consistent data at all stress levels and the su/a', friction angle, and modulus is lower than the CIUC counterpart.