title: Undrained shear strength of ultra-soft soils admixed with lime
Creators: name: Al-Alwan, Asad A. Khedheyer
Publisher: name: University of Glasgow
Year: 2019

description: This thesis describes the results of a study on the undrained shear strength (Cu) of ultra-soft clay soils in admixtures of calcium hydroxide (slaked-lime). The pozzolanic gains in strength over time, over periods as long as one year were recorded. The undrained shear strengths were measured primarily using penetration tests: a Tinius Olsen desk-top compression machine was modified to conduct these constant-rate of strain tests, using circular disc penetrometers. Measured bearing resistances were interpreted in terms of undrained shear strengths: data from the literature, as well as some finite element analyses, were employed to establish the necessary depth-dependent correlations. The strength testing programme was supplemented by triaxial compression and vane shear tests. The parametric study of the factors affecting the strength of lime-admixed clay slurries included soil type, water content, lime content, curing time, and curing temperature. The results show how the rate of strength gain is affected by soil mineralogy. The greatest strength gains can only occur if sufficient clay fractions are present to utilize any unbound additive and conversely sufficient additive is present. For clays, samples prepared at the same water content/ liquid limit ratio (W=w /wLL) produced approximately the same undrained shear strength after one year of curing. Tests were also conducted on remoulded samples: as expected, these admixed soils have high sensitivity. However, remoulding is not achieved without the expenditure of considerable work. Moreover, the remoulded strengths remain some orders of magnitude higher than their untreated counterparts. Diffusion of additive from the admixture into surrounding water was observed; this was manifest in softening of the near-surface material and over a period of one year extended to depths of the order of 10 cm depending on lime content. Curing temperature has a significant effect on the rate of strength development. Lower curing temperatures retard strength development while higher temperatures have the opposite effect. The Arrhenius model for the rates of chemical reactions describes this temperature dependent phenomenon very satisfactorily. Finite element studies, including small-strain Lagrangian and coupled Eulerian-Lagrangian large-displacement formulations (incorporated within ABAQUS) were conducted to investigate whether penetrometer data interpretation required consideration of the finite size of the test chamber. These numerical results tended to confirm the experimental finding that penetrometer disk diameters up to 30 mm were sufficiently small to be unaffected by constraints imposed by the test chambers. In addition, oedometer testing was carried out on both intact and remoulded samples. The former revealed the existence of reasonably well-defined “yield stresses”, which were found to correlate well with the corresponding undrained shear strengths. The compression and swell indices were found to be largely dependent on soil type and correspondingly unaffected by lime content.