1.2. Shear Strength

Soil failure usually occurs in the form of “shearing” along internal surfaces within the soil.

Soil is a material that can fail when it is subjected to certain stresses. The ugliest form of this failure is when the soil breaks apart along internal surfaces, which is referred to as 'shearing.' Understanding how and why this happens is crucial because it helps engineers to predict when soil might fail and how they can design structures to prevent this from occurring.

Imagine building a sandcastle at the beach. If the sand is too dry and the tide comes in, the castle might collapse. This is somewhat similar to how soil can fail under weight or stress. If we understand how the sand (soil) behaves in these conditions, we can build better sandcastles (structures).

Thus shear strength is "The capacity of a material to resist the internal and external forces which slide past each other."

Shear strength is defined as the ability of a material — in this case, soil — to withstand forces that are trying to make its particles slide against one another. This definition helps us understand why some soils can support heavy structures better than others; the stronger the soil’s shear strength, the more load it can carry without failing.

Think of how you can push two books stacked on top of each other. If they are sitting on a smooth table, they might slide easily. But if they are on a rough surface, it takes more force to slide them. The rough surface gives the books a higher shear strength against sliding.

• Engineers must understand the nature of shearing resistance in order to analyze soil stability problems such as:
  • Bearing capacity
  • Slope stability
  • Lateral earth pressure on earth-retaining structure.

Understanding shear strength is vital for engineers as it impacts several aspects of civil engineering — particularly when it comes to analyzing how well soil can hold up structures or withstand various environmental conditions. For example, determining 'bearing capacity' helps engineers know how much weight a foundation can support, while analyzing 'slope stability' helps prevent landslides.

Imagine a book on a shelf: if the shelf is weak and the book is heavy, it could collapse. Engineers need to figure out how strong the shelf (the soil) is so they can safely support the book (the structure) without fear of it falling.

Soil derives its shear strength from two sources: – Cohesion between particles (stress independent component) – Frictional resistance and interlocking between particles (stress dependent component).

Shear strength of soil comes from two primary components: cohesion and friction. Cohesion refers to the forces that hold particles together, such as chemical bonds. Friction depends on how those particles interact and can change with stress — more pressure can increase friction. Together, these two components dictate how strong or weak a soil is under shear stress.

Imagine two friends holding hands (cohesion), preventing one from sliding away from the other. If more friends (pressure) join in a circle and push (friction), everyone holds tighter. The more cohesive forces and friction there are, the stronger the group stays together against sliding apart.

Internal Friction angle (φ) is the measure of the shear strength of soils due to friction.

The internal friction angle (φ) is an important concept that describes how much friction helps a soil resist shearing. This angle helps engineers quantify how different soil types will behave under load. The larger the angle, the greater the resistance to sliding, indicating that the soil has better shear strength.

Think of a pile of sand. If you try to push a handful of sand down a steep slope, the angle at which you can push it before it slides down relates to the internal friction angle. A higher angle means more resistance; in other words, the sand holds together better.

Gravity generates stresses (force per unit area) in the ground at different points. Stress on a plane at a given point is viewed in terms of two components:

• Normal stress (σ) : acts normal to the plane and tends to compress soil grains towards each other (volume change)
• Shear stress (τ): acts tangential to the plane and tends to slide grains relative to each other (distortion and ultimately sliding failure).

Stresses within the soil are caused by the force of gravity acting on it, leading to two types of stress: normal stress and shear stress. Normal stress pushes soil grains together, potentially causing them to compact, while shear stress works to make them slide past one another. This understanding helps engineers predict how soil will behave under various loads.

Imagine pressing down on a soft sponge (normal stress) and then trying to push it sideways (shear stress). The way that sponge reacts gives insight into how soils behave under construction loads, where they may compact in one way while also potentially sliding in another.

--