The stabilisation of soft clay soils is intended to increase their shear strength and to reduce their compressibility. The possibility exists of using geophysical methods to monitor changes in these properties. Laboratory experiments were carried out on stabilised clays to study the relationships between shear wave velocity, and hence small strain shear stiffness, and shear strength or one-dimensional compressibility. One artificial clay, Speswhite kaolin, and two natural clays, from Malaysia and Sweden, were used as the base clays. Either ordinary Portland cement or a 1: 1 mix of the cement with unslaked lime was added to the base clays in order to stabilize them. In the first part of the investigation, samples of stabilised clay were initially subjected to a non-destructive bender element test to obtain the shear wave velocity and then to an unconfined compressive strength test or vane shear strength test. It was evident that small stabiliser amounts (less than 10 % of the dry weight of the base clay) could significantly improve both the strength and stiffness of the originally soft material. In addition, good correlations between the shear strength and the shear wave velocity (or small strain shear stiffness) of the stabilised clays were established. In the second part of the investigation, an instrumented oedometer was used to simultaneously monitor shear wave velocity and one-dimensional compression during tests on samples cured for a set period. Lateral stresses were also measured. Complementary tests were conducted in standard oedometers, to study the effect of the curing period. In these tests yield stresses were identified and corresponded to the onset of changes in shear wave velocity. After yield, the constrained moduli could be correlated with shear wave velocity. Tests were also carried out on samples of clay in which a central stabilised column had been created. Equal strain predictions of the compression of these samples, based on the results of separate tests on the two components, were relatively successful. The results of the research suggest that shear wave velocity measurements could be useful in practice to enable the shear strength and post-yield compressibility of stabilised clay soil to be estimated.

The empirical concepts for the liquid limit and plastic limit of fine-grained soils have been in use since they were originally defined by Atterberg in 1911, introduced into use by Terzaghi and modified by Casagrande. The liquid limit by fall-cone has been the preferred method since 1975 and is currently described in BS1377: Part 2: (1990). It seems a logical step that the plasticity index should also be determined using the fall-cone. It is at present defined as the mathematical difference between two separate tests:- the current thread rolling plastic limit test and the cone penetrometer liquid limit test, both as described in BS 1377: Part 2: (1990). This research, whilst taking into account the findings of others is empirical in nature. Laboratory testing is carried out by the author to produce data for analysis from samples of natural remoulded UK soils. The initial testing programme involves the determination of cone penetration and undrained shear strength by laboratory shear vane. Determination of plastic limit, (thread-roll method), and liquid limit, (fall-cone method), is also carried out for each soil sample. Other tests include clay content, particle density and dry density. Samples are also prepared and tested for clay mineralogy by x-ray diffraction. A correlation is discovered linking density at the cone penetromer liquid limit with plasticity index. Consequently, additional samples are obtained and tested for index limits and density only. A new method of determining plasticity index from fall cone and density determination is proposed. A survey is conducted whereby volunteers are asked to carry out liquid limit and plastic limit tests on standard soil samples. Additionally, extra weighings are required. Analysis of these data enable a direct comparison to be made between plasticity index determiations produced by the colunteers using both the current method and the proposed new method. It is concluded that the new method, in addition to being faster, gives plasticity test results comparable wioth and more reproducible than results obtained using the current method.

Acoustic emission (AE) is a natural phenomenon that occurs when a solid is subjected to stress. These emissions are produced by all materials during pre failure. In soil, AE results from the release of energy as particles undergo small strains. If these emissions can be detected, then it becomes possible to develop an early warning system to predict slope failure. International research has shown that AE can be used to detect ground deformations earlier than traditional techniques, and thus it has a role to play in reducing risk to humans, property and in mitigating such risks. This thesis researches the design of a system to quantify the AE and calculate the distance to the deformation zone, and hence information on the mechanism of movement. The quantification of AE is derived from measuring the AE event rate, the output of which takes the form of a displacement rate. This is accurate to an order of magnitude, in line with current standards for classifying slope movements The system also demonstrates great sensitivity to changes within the displacement rate by an order of magnitude, making the technique suitable to remediation monitoring. Knowledge of the position of the shear surface is critical to the planning of cost effective stabllisation measures. This thesis details the development of a single sensor source location technique used to obtain the depth of a developing or existing shear surface within a slope. The active waveguide is used to reduce attenuation by taking advantage of the relatively low attenuation of metals such as steel. A method of source location based on the analysis of Lamb wave mode arrival times at a smgle sensor is summansed. An automatic approach to source location is demonstrated to locate a regular AE source to within one metre. Overall consideration is also given to field trials and towards the production of monitoring protocols for data analysis, and the implementation of necessary emergency/remediation plans.