How to Calculate the Bearing Capacity of Soil? Understanding Terzaghi's bearing capacity equations

Summary

This video explores the foundational principles of soil mechanics as established by Terzaghi's theories, including the penetration of hard bodies into soft materials and the concept of bearing capacity. It discusses key assumptions such as shallow foundations and the significance of shear and normal stresses in soil. The video further explains passive and active earth pressures, detailing how these forces influence structures like retaining walls. With a focus on practical applications, it promises to address the effects of groundwater in future discussions, encouraging viewers to engage and ask questions.

Takeaways

• 🔬 In 1921, scientist Branden published a study on the penetration of hard bodies into soft materials, forming the basis of Terzaghi's theory.
• 📏 Terzaghi's theory assumes that foundations are shallow, with the footing depth being less than or equal to its width.
• ⚖️ The theory also requires that the length-to-width ratio of the footing is greater than five, classifying it as a strip footing.
• 🔄 Terzaghi replaced the weight of soil above the footing base with a uniform circle based on the formula γ × df.
• 📉 The theory focuses on general shear failure, which involves three main stress zones caused by loads from superstructures.
• 💡 Shear stress is determined by soil cohesion and is calculated using friction angles and cohesion coefficients.
• 🧱 Active and passive earth pressures are defined by the movement of walls in relation to soil, as explained by Rankine's 1870 theory.
• 🔄 Horizontal stress in soil differs from vertical stress due to factors like the earth pressure coefficient and the soil's state.
• ⚖️ The passive earth pressure and cohesion resistance are essential in analyzing forces acting on structures such as retaining walls.
• 🔍 The video will continue to explore the effects of groundwater on bearing capacity in future discussions.

Q & A

What is the main theory discussed in the transcript?

The main theory discussed is Terzaghi's theory, which helps determine the ultimate bearing capacity of soil under shallow foundations.

What assumptions does Terzaghi make about foundations?

Terzaghi assumes that the foundation is shallow, meaning the depth of the footing is less than or equal to its width, and that the ratio of length to width is greater than five, which defines it as a strip footing.

What type of soil failure does the theory focus on?

The theory focuses on general shear failure, where the load from a superstructure causes uniform pressure leading to soil failure.

What are the two types of stress failures mentioned?

The two types of stress failures are shear stress, which depends on soil cohesion, and normal stress, which resists the downward movement.

How does Rankine's theory relate to earth pressure?

Rankine's theory states that lateral stress in soil is a function of vertical stress, helping estimate forces acting on structures like retaining walls.

What conditions define soil behavior against a retaining wall?

The three conditions are: soil at rest (minimal contact), active earth pressure (soil moves towards the wall), and passive earth pressure (wall moves towards the soil).

How is passive earth pressure estimated?

Passive earth pressure is estimated using the formula related to the shape area and specific weight, factoring in the earth pressure coefficient.

What components contribute to passive resistance in soil?

Passive resistance components include passive earth pressure, vertical component of cohesion, and surcharge contribution.

What are the 'triple zero' capacity factors mentioned?

The 'triple zero' capacity factors, named by Terzaghi, are n_q, n_c, and n_γ, which depend on pressure coefficients and the angle of friction failure.

What future topics are hinted at for exploration?

Future topics include the effects of groundwater on bearing capacity and differences between failure modes.