Geotechnical Engineering has advanced to the present stage that various types of earth structures can be designed and constructed safety and economically in most instances. However, in some cases, difficulty arises either in the form of failure during construction or after many years in existence. The soils in which these problems occur include but are not limited to highly sensitive clays and stiff fissured clays of various geological origins. These clays possess pronounced macroscopic and microscopic structures that control the strength and deformation properties. Macroscopic structures are visible features that include fissures, joints, stratifications and other discontinuities in an otherwise intact soil mass. Microscopic structures would include soil fabric and cementation bonds. A typical soft clay deposit usually is composed of a weathered crust at the top that is fissured and thus macroscopic structures are dominant and soft clay at depth in which microscopic structures are significant. The properties of these clays are complex, having a stress-strain relationship that exhibits a peak strength and a post peak decrease in strength, a non-linear failure envelope, strength anisotropy and a significant decrease in strength with a slower rate of testing or longer time to failure. This paper explores the implications of microscopic and macroscopic structure on stability problems and the conditions under which difficulties arise. Results of laboratory and field tests together with case histories show that the dominant effect of a macroscopic structure is exhibited in the reduction of undrained and drained strength with the sample size. The mass strength, whether in the undrained or drained condition, is only a fraction of the intact strength. Design analysis for stability conditions should therefore start with the mass strength at initial time followed by a reduction in strength as time progresses. A case history of an embankment founded on stiff fissured clay on which it failed after 32 years is analyzed in detail to illustrate progressive development of plastic zones with construction details and time. The effect of cementation bonds in influencing the strength properties of soft clays is studied by artificially deposited bonds using the electro-kinetic process and examination with the electronic microscope. It is shown that in addition to the classical increase in strength with decrease in water content, a strength increase occurred with time due to the deposition of cementation bonds by diffusion. An important bonding agent is identified and its effect on bond strength is compared with bonding in natural clays. As the height of an embankment founded on a sensitive clay deposit is increased, a plastic zone will develop and increase in size. The pore pressures at a point will increase at a greater rate when the point is engulfed by the plastic zone as a result of bond breakage. Concurrently, the strength will drop to the post-peak state. Case histories of embankments on these clays are analyzed to illustrate the propagation of the plastic zone in controlling the foundation behaviour at imminent instability. The difference in performance of embankments with different geometries in the same clay deposit is investigated. It is shown that the stability and subsequent strength changes are controlled by the loading geometry and extent of the plastic zone. Finally, design considerations are suggested to accommodate the effects of the macroscopic and microscopic structures in these clays.
