Flow channeling in soft clays and its influence on consolidation

The flow channeling phenomenon in soft clays and its influence on consolidation process is studied. Flow channeling refers to the fabric change caused by the imposed flow across a layer of soft soil. The primary objectives and scope of the research are to study whether the formation of the concentrated flow channels is beneficial in field applications for densification of soft soils and how the channeling phenomenon affects the consolidation characteristics of soft soils.; The problem was addressed by a comprehensive experimental program and by numerical analysis based on the finite strain consolidation theory. Experiments conducted on Speswhite china clay included seepage-induced consolidation tests, tests with imposed flow against gravity, long column tests, and centrifuge tests. A hypothetical but realistic field case was also analyzed to evaluate the relative effectiveness of some of the existing methods for densification of soft soils and the effect that the channeling phenomenon could have on the field performance.; The major findings and implications are: (1) After channel formation, no further consolidation under the seepage force is observed; (2) The channels are not stable, they will close when the imposed flow is removed and reopen when the flow is again imposed; (3) For a given soil at a given initial void ratio, the flow velocity is the controlling factor for channel formation. The lower the initial void ratio, the lower the velocity causing channeling; (4) The finite strain consolidation theory is not applicable to the conditions in which the flow channels are formed; (5) A nominal surcharge at the top of the sample or soil layer prevents the formation of the flow channels; (6) Practical limitations in a pumping scheme in the field prevent the creation of the high enough flow rates to cause channeling unless the soil layer is less than a meter thick; (7) The centrifugal testing of soft soil is affected by the gravity level dependency of compressibility characteristics in the low effective stress range potentially leading to the invalidation of the scaling relations.; In summary, flow channelling is not a phenomenon of much practical significance since the field conditions are seldom, if ever, conducive to the channel creation. However, the channels can be formed in the laboratory experiments unless measures are taken to prevent their creation. If they remain undetected in the laboratory testing the collected data will lead to the erroneous conclusions. The research identified conditions under which the channels will form in the laboratory and provides the methodology for their prevention.