........ published in NEWSLETTER # 57
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........ published in NEWSLETTER # 57
GENESIS AND PROPERTIES OF COLLAPSIBLE SOILS
by Professor E. Derbyshire, Royal Holloway University, London (U.K.) Dr. T.A. Dijkstra, University of Sussex and Professor I.J. Smalley, University of Technology, Loughborough (U.K.)
Soil collapse forms a major hazard in large parts of eastern Canada, the United States, eastern Europe, China and southern Africa. The sediments involved include young marine clays and lake sediments, wind-deposited materials (loess and some sands), volcanic ash, and some residual soils. Human activities continue to increase in regions underlain by collapsible soils, so that the hazards posed are increasing in both relative and absolute terms. Collapsible soils are metastable. A granular material with angular particles compacted on the dry side of optimum can form a structure which is capable of further densification. This is a phenomenon known from the full particle size range - from clays to coarse rock aggregates. However, the classic collapsible soils are natural materials in which particle type and sedimentation mechanism combine to produce collapsibility. The collapsing soils problem is exacerbated by a paucity of descriptive terms.
This book (NATO ASI SERIES C468) brings together the work of international researchers from several scientific fields, including civil engineering, engineering geology, stratigraphy, geomorphology and soil science. A number of authors show that the consequences of collapse are usually more serious in materials with high sensitivity. Development of high sensitivity is usually a post-depositional process arising from (a) decrease of the remoulded strength, and (b) increase of the undisturbed strength. Sensitivity may be decreased by weathering or diagenetic processes which produce swelling clay minerals and/or oxide minerals and/or agents which increase particle interaction in disturbed material. The fabric of collapsible soils can be characterised using micromorphological techniques, these being a useful aid in explaining and predicting collapse. In the case of subaqueous flowslides, comparisons between quantitative simulations and field observations highlight the influence of soil density, slope angle and slope height. Causes and mechanisms of collapse are specified for different soil types including soft sensitive clays, loose unsaturated or saturated sands or silty sands, and loess. Although different in detail, it is shown that the behaviour of these different soils has many common features and requires quite similar approaches to design. For stability analysis, the shear strength parameters must be defined for the post-collapse condition once an equilibrium has developed between effective stress, voids ratio, and shear stress. The advantages and limitations of a number of remedial strategies to minimise collapse in loess are discussed. A new working definition of soil collapse emerges which emphasises the structural strength of the grain skeleton as a unifying factor. Given that collapse in the transition from an open metastable packing to a closer, more stable packing, then the open structure of quickclays, loess and sands is a unifying entity. Regional views of the collapsibility of loess also emerge clearly: eastern European, Chinese and North American, in particular.
The timeliness of this volume is most evident in that it throws into relief several important communication gaps, the greatest being between the micromorophologists (soil scientists) and the remainder. This volume signposts strategies for future research into the collapse phenomenon, including some longer-term research objectives requiring interdisciplinary collaboration. The work summarised in this volume suggests that, in future, special attention should be given to (1) determining and describing soil and sediment structures; (2) finding ways to determine changes in structure; (3) improving mineralogical description; and (4) mapping collapsible soils. A future ARW might well address these questions.
Reference books: C115, C149, C468, E70, E147, E172, G32, I23, I38