What is the shear strength of soil?

by Andrew Lees, on 04-Mar-2021 03:44:45

What is the shear strength of soil?

This is a key question for ground engineers and is vital to any design project. The reason it’s so important is that at some point, everything that is built makes contact with the ground – and transfers load into it. Understanding what influences the strength of a soil and quantifying it, are always the first steps to be taken.

Ground Coffee 'Ask Andrew' Episode 5: Andrew Lees explains what the shear strength of a soil is. 

What exactly is meant by 'strength of a soil'?

Soil strength is used in soil mechanics to describe the force to be overcome in order to fail a soil by shear. Soil strength depends on many elements, one fundamental consideration being the type of soil. The particles of most soils are essentially incompressible, and soil masses have no tensile strength. Soils fail when one block of soil moves relative to another block and the soil particles at the failure plane move over each other. This is what is known as shear. When particles move across each other, the resisting (or shear) force is friction. The shear resistance, or shear strength is related to the physical characteristics of the soil, including particle size, shape, distribution and orientation, but also the stresses acting on the soil at that location.

So a key element of shear strength is friction. For objects in contact, friction force along a plane varies with the pressure acting perpendicular to the plane (known as normal stress). As normal stress increases, so too does frictional resistance, or shear stress. This underlines that soil strength is not a single figure but is dependent upon the stresses that are acting upon a soil. For granular soils the relationship between shear stress and normal stress is a straight line, defined by an angle (ø) known as the friction angle.

When considering soil strength, it is important to know the friction angle, as well as the stresses which will be acting on the soil. When comparing different types of granular soil, the friction angle is the single property that defines strength. The friction angle is critical when sourcing materials for structural fill in reinforced soil structures and important for applications such as working platforms and road foundations. It must also be remembered that any change in friction angle may necessitate a re-design.

Clay soils are also composed of particles, although the particles are extremely small. There are electrostatic charges (attractive forces) acting between these fine particles, and surface tension from pore water holding particles together even without the application of external confining forces, hence clay soils have some shear strength even when normal stress is zero. This additional strength is known as apparent cohesion. It is not however a fundamental soil property. When considering the strength of granular soils, even those with some clay content, cohesion can mostly be ignored as it is the friction angle which is key.

The influence of ground water on the shear strength of soil

The presence of ground water also has a marked influence on soil strength. Soils can be saturated, where all void spaces between particles are filled with water, or partially saturated when a percentage of air bubbles are present within the void spaces. The pore pressures generated affect particle to particle stress and hence friction between soil particles. When a load is applied to saturated soil, the pressure of the pore water (water in the spaces) immediately increases because water is incompressible. A granular soil has relatively large interconnecting voids between particles. When – as is usually the case in construction – loads are applied slowly onto a granular soil, water can be assumed to drain away freely, dissipating any increased pore pressure and allowing the applied load to be transferred to the soil skeleton. Consequently pore water effects can be ignored.

Clay reacts differently as the voids are microscopically small and poorly interconnected. Water can therefore only move at much lower rates and drainage is very slow, so when a load is applied to a clay soil, the pore water pressure is unable to dissipate. As water is incompressible, the pore water pressure carries the load and the particle to particle stresses in the clay do not increase. The short-term saturated shear stress of a clay is therefore a constant value referred to as the undrained stress, denoted by cu or su When considering the bearing capacity of clay soils, the undrained shear strength is critical. This is extremely common in the UK where soils with high clay content are found on many construction sites. Over time, water will drain from clay and the pore pressure will slowly reduce and hence the strength of the clay soils will increase – however this is a very long-term effect.

Soil state's impact on sheer strength of soil

A further factor affecting granular soil strength is the degree of particle compaction or soil state. When a load is applied to loose, uncompacted soil, the particles will move closer together as the soil contracts. After contraction of the soil, shearing takes place as particles begin to move over each other. The soil shear strength increases as the particles compact, ultimately continuing at a constant level and constant density or volume.

Where granular soils are already densely compacted, little or no contraction takes place and particles are interlocked. As the load increases, before particles can shear over one another, they have to move apart along the shear plane, unlocking the interlock. This is known as dilation. The shear force required to overcome dilation is the ‘peak strength’ (øpeak). After dilation, particles can move over each other more easily, requiring a lower shear force than at peak, which is referred to as strength at constant volume øcv. In a situation where the soil is not expected to shear – for example the structural fill in a reinforced soil wall – the peak strength should be used in design. If addressing a high deformation condition, the preferred option would more likely be the strength at constant volume. The important consideration is to ensure that the materials report provides the appropriate strength for the design purpose.

To find out more about our TensarTech Systems for earth retaining structures, embankment foundations and working platforms, you can visit our website here

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