Influence of a Thin Horizontal Weak Layer on the Mechanical Behaviour of Shallow Foundations Resting on Sand

Maurizio Ziccarelli,Marco Rosone

doi:10.3390/geosciences11090392

Abstract

The presence of minor details of the ground, including soil or rock masses, occurs more frequently than what is normally believed. Thin weak layers, shear bands, and slickensided surfaces can substantially affect the behaviour of foundations, as well as that of other geostructures. In fact, they can affect the failure mechanisms, the ultimate bearing capacity of footings, and the safety factor of the geotechnical system. In this research, numerically conducted through Finite Element Code Plaxis 2D, the influence of a horizontal thin weak layer on the mechanical behaviour of shallow footings was evaluated. The obtained results prove that the weak layer strongly influences both the failure mechanism and the ultimate bearing capacity if its depth is lower than two to four times the footing width. In fact, under these circumstances, the failure mechanisms are always mixtilinear in shape because the shear strains largely develop on the weak layer. However, the reduction in the ultimate bearing capacity is a function of the difference between the shear strength of the foundation soil and the layer. The presence of a thin weak layer decreases the ultimate bearing capacity up to 90%. In conclusion, this research suggests that particular attention must be paid during detailed ground investigations to find thin weak layers. Based on the obtained results, it is convenient to increase the soil volume investigation to a depth equal to four times the width of the foundation.

Highlights

Foundation soils are frequently characterized by secondary or “minor” constitution details, such as interfaces, shear bands, and thin lenses of materials
The paper presented the results of a numerical study aimed to assess the influence of the presence of a thin horizontal weak layer on the mechanical behaviour of shallow foundations resting on sands
The weak layer strongly affects both the failure mechanism and the ultimate bearing capacity qlim if its depth from the ground does not exceed a critical value of about 2 ÷ 4 times the footing width B

Summary

Introduction

Foundation soils are frequently characterized by secondary or “minor” constitution details, such as interfaces, shear bands, and thin lenses of materials. They are considered to be induced weak layers if they have geochemical (produced by leaching in the flow of water rich in mineral salts) or geotechnical origins (e.g., in strain-softening sediments where progressive failure can be achieved or in thin strata with increased pore water pressure and hydraulic conductivity) Due to their limited thickness, these minor constitution details are frequently undetected from direct (i.e., observation trenches and boreholes) and indirect (e.g., seismic and electrical surveys) geotechnical investigations. They can have a strong impact on the mechanical response of both shallow and deep foundations [17,18,19,20,21,22,23] In this regard, it is worth mentioning that tests performed, both under single gravity (1 g) [24] and in geotechnical centrifuges up to 40 g [25], on small-scale physical models of a strip footing resting on a dense sand bed including a thin weak layer showed that the latter can strongly influence both the failure mechanisms and the ultimate bearing capacity. The study of the problem by means of a numerical perspective based on Finite Element (FE) modelling will make it possible to approach the problem from a more general point of view and to analyse many cases by varying the soil parameters in a wide range of realistic values

The Problem Considered

Results and Discussion

Effect of the Weak Layer on the Failure Mechanism

Effect of the Weak Layer on the Ultimate Bearing Capacity

Conclusions