A study of volumetric behaviour of compacted clayey soils in the void ratio, moisture ratio and net stress space

2017-03-01T00:58:36Z (GMT) by Islam, Md. Tanvirul
The volumetric behaviour of unsaturated soils is complicated than saturated soils. Depending on the state paths involving loading/wetting/unloading, compacted unsaturated soils can exhibit swelling, collapse, collapse followed by swelling, swelling followed by collapse and swelling pressure development. While significant advances have been made in modelling of the hydromechanical behaviour of unsaturated soils, it is still difficult to predict these behaviours using most methods that use suction as a constitutive variable, since the required testing effort is overwhelming on one hand and it is difficult to take into account field soil variability on the other. In contrast, the Monash – Peradeniya – Kodikara (MPK) framework proposed by Kodikara (2012) uses void ratio – moisture ratio – net stress space accompanied with the Loading Wetting State Boundary Surface (LWSBS) to explain/predict these paths and is relatively simple and has the potential to be applied to practical problems with relative ease. The current research project mainly focuses on a comprehensive validation and extension of the MPK framework and demonstration of its application to practical problems. A comprehensive series of tests are performed on statically compacted soils for the validation of the MPK framework. Two soil types, namely lightly reactive kaolin and more reactive clay referred to as Merri Creek soil, are used in the testing. The soils were prepared with different moisture contents from dry state and were statically compacted at constant water contents to obtain the void ratio-moisture ratio-net stress constitutive surfaces as well as soil specimens for the state path tests. The state path test results of yielding under loading, collapse under wetting, swelling pressure development and change in yield pressure due to wetting are explained with the MPK framework. Despite the difference in the degree of reactivity, both soils followed the concepts of the MPK framework reasonably closely. In addition, some published data from the literature were also analysed within the framework, highlighting that the framework is valid, regardless of the degree of reactivity of the soil. No suction was measured in these experiments, as it is not essential to explain most volumetric behaviour as per the MPK framework. Dynamic compaction is commonly used to construct structural fills for various geo-infrastructures. The current practice is to specify a minimum dry density and moisture content criterion to be used in the field on the basis of Proctor compaction carried out in the laboratory. Nonetheless, we still do not have practical methods for predicting the behaviour of compacted clay under the expected mechanical and environmental loadings. Current theories are difficult to apply in practice due to difficulty in determining the necessary parameters. In this thesis, the MPK framework is extended to analyse the dynamically compacted soils. Similar to statically compacted soils, a significant number of experiments were performed on the dynamically compacted lightly reactive kaolin and reactive Merri Creek soils at constant moisture contents. Since the compaction stresses were unknown for the dynamic compaction, recompression of the soil specimens from compacted soil was used to establish the relevant LWSBSs. Subsequently, independent tests were undertaken highlighting that the MPK framework could predict well the behaviour of dynamically compacted soils under loading/unloading and yielding, collapse during wetting, change of loading yield stress after wetting, and swelling pressure development during constrained wetting. The value of the approach is that the testing methods are straight-forward, do not require specialised equipment and the testing times are much shorter. In addition, the uncertainty that laboratory dynamic compaction may not relate directly to the field roller compaction can be addressed with the developed framework. Soil specimens obtained from field soil pads compacted by the actual rollers can be used to establish the corresponding LWSBS. This information will allow the direct prediction of the likely behaviour of field compacted fills under the expected environmental and mechanical loadings. Another addition to the MPK framework is achieved by incorporating loading/wetting suction within the void ratio-moisture ratio-net stress space. Initially, two hypotheses are proposed to present the suction contours on and inside the LWSBS by analysing several datasets from the literature. Subsequently, a mathematical representation is provided to establish full suction profile within the void ratio-moisture ratio-net stress space for both kaolin and Merri Creek soils. Although suction is not essential for the application of the MPK framework in many practical problems as demonstrated, yet knowing the suction profile within the void ratio-moisture ratio-net stress space is essential to complete the hydro-mechanical picture in the volumetric space. This extension will allow the development of constitutive models as well as soil water characteristic curves more rationally in future. Finally, the MPK framework is applied to analyse the performance of a conceptual compacted clayey fill. The heave/settlement results from the MPK framework and the results available from literature are compared qualitatively for both laboratory behaviour and field scale behaviour of compacted soils. It is found that the complex volumetric behaviour due to major wetting events are easily explainable using the extended MPK framework. It is observed that the initial operational void ratio (or the initial operational density) and the operational stress are the two most important parameters that govern the volumetric behaviour of compacted unsaturated soils.