Engineering behaviour of artificially cemented sand : cement treatment versus biocement treatment

Abstract

The engineering properties of soil can be enhanced through artificial cementation which is realized by premixing or injecting chemical agents such as cement, lime and gypsum in soil. Biocementation of soil using biogenic cements has also been developed in recent years.In this thesis, the engineering behaviour of sand cemented using either cement or biocement was investigated. Cement-treated sand was prepared by mixing uncemented sand with Portland cement, while biocement-treated sand was made using the low-pH-one-phase-injection method via the microbially induced calcite precipitation (MICP) process. The differences and similarities in the cementation effects of the two different cementation materials and cementation processes were analysed.Drained and undrained shear behaviour of cemented sand were studied under both axisymmetric and plane-strain conditions using either triaxial or plane-strain apparatus. Under a drained axisymmetric condition, the stress-strain behaviour of cemented sand becomes brittle, and its volumetric deformation becomes dilative as compared with the ductile and compressive behaviour of the loose parent sand under the same conditions. The cementation effect was also affected by the effective confining stress. The lower the stress level, the more significant the cementation effect. Compared with cement-treated sand, biocement-treated sand with similar biocement content is more brittle with higher tangent modulus, but less dilation in general. The cementation effect and its mobilization could be analysed using an energy equation. The effect of cementation due to cement or biocement was dominated only at the early phase of shearing. During a drained triaxial test, the cementation effect was mobilized first while the dilation was restricted by bonding.With further shearing, bonding due to cementation began to break and the contribution of dilation increased. When dilation ceased to develop further, the shear Summary viii strength would be controlled mainly by friction. For both cement or biocement, cementation contributed mainly to the increase in the effective cohesion, but not much to the effective friction angle. The bonding or yielding stress increases with the increase in cement/biocement content. The bonding provided by biocement is stronger than that by cement.Under drained plane strain conditions, the bonding provided by cementation could be degraded during K 0 consolidation when the consolidation stress was higher than the yielding stress of the cemented soil. As a result, the stress-strain behaviour of cemented soil was affected by the consolidation state. When the mean effective consolidation stress was smaller than the mean yield stress, the shear stress versus shear strain curve of cemented sand could still be affected significantly by cementation as indicated by a steep peak in the stress-strain curve and the contractive volumetric strain. On the other hand, when the mean effective consolidation stress was greater than the mean yield stress, the stress-strain behaviour of cemente...