Understanding why soils fail

Summary

This video introduces the fundamentals of soil mechanics, explaining the importance of understanding soil strength for geotechnical applications such as foundations and retaining structures. It covers key concepts like shear stress, particle interlocking, and cohesion in soils. The Mohr-Coulomb failure criterion is discussed to show how soil strength is represented, and the friction angle is emphasized as a critical parameter in determining a soil’s ability to resist shear forces. The video concludes by highlighting the ideal characteristics of soils used in construction, setting the stage for future discussions on practical geotechnical applications.

Takeaways

• 😀 Soil mechanics is crucial for geotechnical applications like designing foundations and earth-retaining structures.
• 😀 The capacity of soil is determined by its shear strength, which is the primary factor in soil failure.
• 😀 Unlike other materials, soils fail due to excessive shear stresses, not compression.
• 😀 Shear stresses cause soil failure when the interlocking particles slide past one another.
• 😀 Soils can exhibit frictional shear strength and cohesive shear strength, with cohesion being mostly relevant for clay.
• 😀 The frictional strength of soil depends on pressure: compression increases interlocking, while water reduces friction.
• 😀 Soil strength can be represented by the Mohr-Coulomb failure criterion, showing how normal stress and shear stress relate.
• 😀 The Mohr Circle represents the strength of soil under loading, with each point in the soil mass drawing a separate circle.
• 😀 A soil’s failure occurs when the Mohr circle touches the failure envelope, initiating sliding failure at that point.
• 😀 The friction angle determines the strength of coarse-grained soils, and higher friction angles resist higher shear stresses.
• 😀 Angular, well-graded, compacted soils with high friction angles are ideal for foundation and backfill applications.

Q & A

Why is understanding soil mechanics important for geotechnical applications?

Understanding soil mechanics is crucial because it helps determine the soil's capacity and limits, which are key to designing stable and reliable foundations and earth-retaining structures.

What is the main cause of soil failure in geotechnical engineering?

Soil failure in geotechnical engineering is primarily caused by excessive shear stresses, rather than compression. Shear stresses lead to the sliding or rearranging of soil particles, which results in failure.

How do shear stresses affect soil compared to other materials like steel or concrete?

Unlike materials like steel or concrete, which fail due to the breaking of molecular bonds under shear stress, soils are particulate materials. Their shear strength comes from the interlocking of soil particles, and failure occurs when these particles slide past each other.

What is the role of particle interlocking in soil’s shear strength?

The interlocking of soil particles provides resistance to shearing forces. When the shear stresses are too large, the particles begin to slide, leading to failure. Without interlocking, the soil would behave more like a fluid than a solid.

What is liquefaction, and when does it occur?

Liquefaction is a phenomenon where soil behaves like a fluid due to a loss of interlocking between particles. It typically occurs when soil is saturated with water and subjected to sudden stresses, such as during an earthquake.

What is cohesive strength, and how does it differ from frictional strength in soil?

Cohesive strength refers to the 'stickiness' or internal bonding of soil particles, such as in clay. In contrast, frictional strength arises from the interlocking of particles and is more significant in coarse-grained soils like sand and gravel.

How does the pressure on soil affect its shear strength?

The frictional strength of soil increases with pressure because the particles become more interlocked under compression, making it harder for them to slide past one another. Conversely, when soil is submerged underwater, the buoyant force can reduce the friction between particles.

What is the Mohr-Coulomb failure criterion, and how is it used in soil mechanics?

The Mohr-Coulomb failure criterion is used to represent the strength of soil graphically. It plots normal stress on the x-axis and shear stress on the y-axis, showing how the soil's shear strength varies with different stress conditions. The failure line indicates when the soil reaches its limit.

What is the Mohr Circle, and how does it relate to soil failure?

The Mohr Circle represents the stress conditions within the soil. As the soil is loaded, the Mohr Circle grows until it touches the failure envelope, which indicates that failure has occurred at that point in the soil mass.

What is the significance of the friction angle in soil strength?

The friction angle is a critical parameter in soil strength, representing the resistance to shear stresses. A higher friction angle indicates that the soil can resist greater shear stresses, which is particularly important when designing retaining walls and foundations.