Abstract
The presence of weak layers in geotechnical systems, including soil or rock masses, both natural and man-made, is more frequent than is normally believed. Weak layers can affect both failure mechanisms, in drained and in undrained conditions, as well as in static and seismic conditions, and the safety factor. In the present study, conducted numerically using the finite-element method (FEM) Plaxis 2D code, the influence of a horizontal thin weak layer on stress and strain distribution, on failure mechanisms and on the overall stability of an embankment was evaluated. The results obtained prove that when the weak layer is located at a significant depth from the foundation plane, the failure mechanisms are normally mixtilinear in shape because the shear strains largely develop on the weak layer. As a result, the safety factor highly decreases compared to the same case without a weak layer. Then, in the presence of weak layers, even embankments that, if founded on homogeneous soils, would have very high global safety factors (higher than 2) can become unstable, i.e., the safety factor can become unitary. So particular attention must be paid during detail ground investigations to finding thin weak layers.
Highlights
Natural soils and rock masses, as well as earthworks such as backfills, can include minor structural features
The phi-c reduction procedure effect of the weak layer on mechanical behaviour, and in particular on the deformationwas applied to the safety forthe thesafety considered mechanisms atcalculate failure
The influence of the weak layer increases with the difference between the shear the failure mechanism can decrease the safety factor up to 60% of the corresponding safety strength of the weak layer and the foundation soil
Summary
Natural soils and rock masses, as well as earthworks such as backfills, can include minor structural features. Minor structural features can have a very small thickness and, for this reason, may pass undetected when usual ground investigations are carried out. With reference to their difficult identification, Terzaghi (1929) [1] termed these features as “minor geologic details”. In his studies, Terzaghi highlighted the great influence of weak strata on the mechanical behaviour of geotechnical systems and their potentially detrimental effects on the safety of earth dams. They include thin shear bands and sliding surfaces brought about by past instability processes, pre-softened zones within homogeneous soil masses, thin syngenetic seams and laminae such as those occurring in varved clays, structural discontinuities, stratified soils, contact surfaces—sometimes slickensided—among successive strata in earth embankments
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