Soil water characteristic curve and shear strength of soil with bimodal characteristic

Many residual and colluvial soils show bimodal grain-size distribution (GSD). Some soils with bimodal characteristic of GSD result in bimodal characteristic of soil-water cahracteristic curve (SWCC), whereas others result in unimodal characteristic of SWCC. Modelling GSD and SWCC with a continuous function is advantageous. The GSD model can be used to classify the soil and predict the SWCC. On the other hand, the SWCC model can be used to obtain soil properties, such as: the air-entry value, residual water content and water-entry value. Many research works have been carried out on development of GSD and SWCC model. However, most of them were limited to soils with unimodal characteristics. The objective of this research is to study the mechanical behaviour of unsaturated soil, particularly the SWCC and shear strength of the unsaturated soil with bimodal characteristics. Several mathematical equations were proposed in this study, such as: the equations for best fitting the GSD, SWCC and shear strength of the unsaturated soil with bimodal characteristics, the equations for estimating the SWCC of soil with bimodal GSD and the equation for estimating the bimodal shear strength of the unsaturated soil. SWCC tests and unsaturated consolidated drained triaxial tests were carried out on nine different sand-kaolin mixtures which were compacted on dry density associated with wet of optimum (95% maximum dry density), optimum water content and dry of optimum (95% maximum dry density). The SWCC test results showed that the specimen with bimodal GSD does not always produce bimodal SWCC, but they can also produce unimodal SWCC, depending on soil density, void ratio, water content and ratio of coarse to fine particles within soil specimen. The specimen with bimodal GSD and consists of high percentage of fine particles is likely to have unimodal SWCC. This happens since the percentages of small pores are much higher that those of big pores within this soil specimen. In addition, the saturated water content of this soil specimen will be higher than the saturated water content of the soil specimen with bimodal GSD, but consists of low percentage of fine particles. The specimen with bimodal GSD and high dry density will contribute in the low air-entry value (AEV) and the steep slope of SWCC. The CD triaxial test results indicated that specimen with bimodal SWCC had a higher shear strength, a lower axial strain at failure and a higher stiffness than that with unimodal SWCC. The results from CD tests also showed that b angle of specimen with bimodal SWCC at suction between AEV1 and AEV2 was less than b angle at suction less than AEV1. However, the relationships between suction and shear strength at suction between AEV1 and AEV2 were still linear. This might be attributed to the reduction of area of water within macropores of specimen with bimodal characteristic at suction between AEV1 and AEV2. However, micropores within soil specimen with bimodal characteristic were still in saturated condition. At suction beyond AEV2, b angle of specimen with bimodal SWCC started to decrease non-linearly. This happened since the area of water menisci within micropores started to decrease at suction beyond AEV2 due to the desaturation of micropores of soil specimen with bimodal SWCC. The proposed GSD, SWCC and shear strength equations for soil with bimodal characteristics were evaluated with data from laboratory tests carried out in this study and published data from literatures. The proposed best fitting equations were shown to best fit GSD, SWCC and unsaturated shear strength of soil with bimodal characteristics successfully. In addition, the parameters in the proposed equations represent the variables of GSD, SWCC and unsaturated shear strength of soil. The proposed estimating equations were shown to estimate the SWCC of soil with bimodal GSD and to estimate the bimodal shear strength of unsaturated soil successfully.Doctor of Philosophy (CEE