What is the Bearing Capacity of Soil? I Geotechnical Engineering I TGC Ask Andrew EP 4

Summary

In this episode of Tensar Ground Coffee, Andrew explains the concept of bearing capacity in soils, using a practical demonstration with sand. He details how soil can fail under vertical loads, distinguishing between granular materials like sand and cohesive materials like clay. The video covers how bearing capacity is influenced by soil permeability, settlement behavior, and how water content can affect the strength of both sand and clay. Viewers are given an accessible breakdown of the mechanisms behind soil failure and how these concepts are applied in ground engineering.

Takeaways

• 😀 Bearing capacity is the soil's ability to support loads without failing, with failure occurring when the load exceeds the soil's strength.
• 😀 The failure mechanism in soil is caused by shear forces, where the soil moves around the foundation, leading to displacement and heaving.
• 😀 Sand compresses quickly under load due to its high permeability, leading to immediate settlement. This is different from clay, which settles more slowly.
• 😀 Water saturation in sand reduces its bearing capacity, as it weakens the soil's strength. In clay, saturation delays settlement but increases long-term bearing capacity.
• 😀 When a vertical load is applied to soil, the load initially transfers to pore water in clay, slowing down settlement but increasing bearing capacity over time.
• 😀 In sand, bearing capacity is typically high due to the immediate increase in effective stress when load is applied, resulting in higher strength.
• 😀 Concentrated loads, such as those from cranes or narrow footings, can cause bearing capacity issues, especially in loose or saturated sand.
• 😀 Clay has a more gradual bearing capacity response due to its slow water dissipation, with strength developing over time as pore pressure decreases.
• 😀 The shear failure mechanism occurs when the foundation sinks while surrounding soil pushes upward, creating a shear plane in the soil.
• 😀 Engineers use specific equations to calculate bearing capacity in both sand and clay, accounting for the soil’s effective stress and load conditions.

Q & A

What is the definition of bearing capacity in soil engineering?

Bearing capacity is the maximum vertical load a soil can support without undergoing failure. It is a failure mode where soil under a vertical or near-vertical load eventually shears or deforms, rather than just compressing.

How does sand behave under an applied vertical load compared to clay?

Sand, being granular with high permeability, settles almost immediately under load, whether dry or saturated, and can fail if loose or heavily loaded. Clay, on the other hand, is cohesive and compresses slowly; initial load is carried by incompressible water in its pores, leading to delayed settlement and a gradual increase in bearing capacity over time.

What visual demonstration was used to explain bearing capacity in the video?

The presenter used a bowl of loose sand and a bottle as a mock foundation. Applying the load to the sand showed initial settlement, followed by heave around the edges as the soil failed, illustrating the shear plane mechanism.

What causes the heave around a foundation during soil failure?

Heave occurs because, under excessive vertical load, the soil beneath the foundation moves downward, displacing surrounding soil upwards. This movement forms a shear plane and demonstrates the failure mechanism associated with bearing capacity.

Why do sand and clay have different bearing capacities?

Sand's high permeability allows immediate dissipation of water in voids, so effective stress increases quickly and provides strength. Clay's low permeability delays effective stress development because water cannot escape quickly, resulting in lower short-term bearing capacity but potential long-term improvement as pore water dissipates.

What is the main failure mechanism of soil under vertical load?

The main failure mechanism is the formation of shear planes. The soil beneath the foundation moves downward, while surrounding soil is displaced upward or sideways, allowing the particles to follow the most efficient path and relieving stress through shearing rather than pure compression.

Under what conditions is sand more likely to fail in terms of bearing capacity?

Sand may fail if it is particularly loose, subjected to highly concentrated or narrow loading (such as crane outriggers), or if it is saturated, which reduces its effective strength.

How does the effective stress in soil relate to bearing capacity?

Effective stress is the stress carried by the soil skeleton, excluding pore water pressure. Bearing capacity depends on effective stress because soil strength, particularly in granular materials, comes from friction between particles. Higher effective stress generally increases bearing capacity.

Why does clay's bearing capacity increase over time?

Clay's bearing capacity increases over time because the initial load generates excess pore water pressure that gradually dissipates as water slowly flows through the clay's small pores. This allows the soil skeleton to compress and gain effective stress, increasing its strength.

What practical considerations should engineers keep in mind regarding bearing capacity?

Engineers should consider soil type, density, moisture content, and load concentration. For loose or saturated sand, concentrated loads may risk failure, while in clay, short-term failure is critical but long-term strength may improve. Proper foundation design must account for these factors to avoid settlement or shear failure.

How does particle movement in granular soils affect bearing capacity?

In granular soils like sand, particles move around under load rather than compressing, allowing the soil to form shear planes and redistribute stress. This movement provides resistance and contributes to bearing capacity, making sand relatively strong unless it is loose or saturated.