Six Definable Particle Types in Engineering Soils and Their Participation in Collapse Events: Proposals and Discussions

Abstract

Six definable particle types need to be recognised in engineering soils. Failure to appreciate the true nature of soil and to treat soils crudely as a ‘black box’ has led to many different types of failures. The basic five types that need to be recognised are (related to the R-size diagram, see Figure 1) AA’ active clay minerals, BB’ inactive clay minerals, C fine cohesive primary minerals, D silt (at R approximately equals 1), E sand. We estimate that at about 200 microns and above the cohesive forces can be neglected, so that truly granular soils start at 200 microns and extend to 60 mm. We propose 60 mm as the upper limit of truly granular behaviour, which has been arbitrary chosen for the discussion purposes. From 50 to 200 microns we propose that a sixth definable sub-region type occurs, where R<l but cohesive forces are still significant.

Many misconceptions have occured because of over simplification in soil type recognition for example, a soil from region D was used as the core material of the Teton Dam and such as made a significant contribution to its collapse and failure Quickclays have only recently been recognized as region C materials, having previously been considered as type A or B materials. This significantly delayed the understanding of their formation and failure modes. Collapsing soils occur in regions C, D and E; the basic requirements are inactive particles (usually primary mineral particles), an open structure ( often by slow-fall sedimentation) and short range bonds. We have tentatively defined three ideal collapsing soils: Soar Sand (at point E), Loughborough Loess (at D) and Quebec Quickclay (at C). These are defined in structural and mineralogical terms only, for the purposes of duscussion.