Strength and deformation characteristics of cement-treated Bangkok clay | |
| Author | Uddin, Md. Kamal |
| Call Number | AIT Diss. no. GT-94-01 |
| Subject(s) | Strains and stresses Clay--Thailand--Bangkok |
| Note | A dissertation submitted in partial fulfilment of the requirement for the degree of Doctor of Engineering, School of Engineering and Technology |
| Publisher | Asian Institute of Technology |
| Series Statement | Dissertation ; no. GT-94-01 |
| Abstract | Stress-stra.in behaviour and strength characteristics of soft clay treated chemically with a hardening agent, namely cement, were investigated through a comprehensive series of laboratory tests. The program addresses the effect of ahe variability of the treated clay in tterms of quantity of the hardening agent, the pre-shear consolidation pressure, the stress conditions imposed during testing, the drainage condition and the time dimension. The base clay used were samples of soft Bangkok clay with an organic content of 5.6°/o. Due to high organic content, a considerable amount of cement was required to neutralize the large reserve of potential acidity or buffering capacity of the base clay. The tests for soil properties, UC Tests and Oedometer Tests were conducted and monitored up to 6 months; while the Triaxial Tests and Constant Stress Ratio Tests were performed at 1 and 2 months' curing time. The pre-shear consolidation pressure was varied from SO kPa to 2000 kPa in the triaxial tests •. The UC Tests involved samples having Oo/o to 40% cement content with curing period ranging from 1 week . to 6 months; zonal demarcations established to delineafe the degree of effectiv.eness r~ndered by the parameters of cement content & curing time indicate that 10°/o to 15°/o can be regarded as the most effective quantity of hardening agent with curing time of 1 to 2 months. The Oedometer Tests (performed with cement content= 5°/o to 25% & curing time=l to 6 months) reveal that the cement treatment caused substantial improvement of consolidation properties, apparent enhancement in preconsolidation pressure, reduction of compression index (Cc) and concurrently raised the value of the coefficient of consolidation (cv)· Normalized intrinsic compression curves off erred confirmation of the hardening effect in the void index plane. The prevalent role of pre-shear consolidation pressure was manifested to annihilate the cementation effects attributing ductility to the treated matrix. On the basis of triaxial stress-strain characteristics, stress path, pore pressure generation and volume change behaviour, it was discerned that the main effect of cement treatment is to modify the behaviour of the soft clay from normally consolidated to overconsolidated state. The degree of overconsolidation is influenced by the cement content, the curing time, the pre-shear consolidation pressure and. the applied stress path. The initiai, peak and residual stress - - states have curved projections on the (q, p) and (e, In p) planes, which refer to the characteristics generally associated with overconsolidated behaviour. The existence of a small strain domain denoted as 'Initial Small Strain Phase' was ascertained based on the characteristics of undrained stress paths and from the (q, Ev) relationships of drained tests, establishing a family of loci in the (q, p) stress space which governs the onset phase transformation of the treated clays. The convex nature of these loci indicates that the degree of initial small strain behaviour is abated by the increase in pre-shear consolidation pressure. Beyond the small strain domain and up to the curved failure envelope, a work hardening & elasto-plastic type of behaviour possessing large strain, is observed. Based on the phase transformation behaviour, the stress paths were generalized into 5 categories.·.The regions of elastic & plastic strains and the boundary of commencement of plastic strains were identified. The deployment of plastic volumetric strains evinces the existence of an 'Elastic Nucleus' where plastic volumetric strain is negative. (ii) The normalized pore pressure exhibited a bi-linear trend with the stress ratio, Tl propounding the possibility of predicting undrained stress path & pore pressure development. Sets of 'Overall Failure Envelopes' were established covering both drained and undrained conditions. The stress-dilatancy relations on the Spatial Mobilized Plane (SMP) exhibit a remarkable phenomenon such that the stress ratio, 't/aN, and the normal & shear strain increment vector, -dEN/dy, constitute a specific correlation between treated and untreated clays. In Constant Stress Ratio Tests, strain paths produced sets of stress-dilatancy relationships. The loci of the transition points of bi-linear strain paths obtained from Constant Stress Ratio tests are similar to the limit state curves for natural clays. These loci - - are generally curved and convex to the p -axis in the ( q, p) plane. Heavily overconsolidated (rigid) behaviour was observed inside these loci. The characteristic's configurations of these loci were found to expand with parametric enhancement of cement content and curing time. Probing the characteristics of pore pressure generation, strain variation and volume change at each stress ratio, it was found that the behaviour of cement treated clay can be explained by sub-dividing the ( q, p) stress space into 3 zones corresponding to the behaviour of untreated clay, the behaviour of treated clay and the softening part. A composite conceptual model to describe the mechanical behaviour of cement treated clay has been proposed in which three zones [(A) Stiff Zone (B) Zone of Small Strain & Large Strain and (C) N.C. Zone] and four sub-zones [(i) Elastic Nucleus (ii) Small Strain Phase (iii) Large Strain Phase & (iv) S.train Softening Phase] have been envisaged • The treated clays have been found to be strain softened after failure in an 'Unstable Phase' with the residual stress state lying on a failure envelope nomenclatured as 'Destructured Envelope', which is close to the CSL of the untreated clay. At this envelope, the cohesion is destroyed and the treated clay then behaves as purely frictional material. Prediction of strains of cement treated clays was attempted based on the Incremental Stress-Strain Theory of ROSCOE and POOROOSHASB (1963). The strains predicted were found to be in general agreement with the experimentally observed strains. |
| Year | 1994 |
| Corresponding Series Added Entry | Asian Institute of Technology. Dissertation ; no. GT-94-01 |
| Type | Dissertation |
| School | School of Engineering and Technology (SET) |
| Department | Other Field of Studies (No Department) |
| Academic Program/FoS | Geotechnical and Transportation Engineering (GT) |
| Chairperson(s) | Balasubramaniam, A.S.; |
| Examination Committee(s) | Bergado, Dennes T. ;Noppadol Phien-wej ;Pichai Nimityongskul ;Kamon, M.; |
| Scholarship Donor(s) | Asian Institute of Technology Partial; |
| Degree | Thesis (Ph.D.) - Asian Institute of Technology, 1994 |