Understanding Pressure Distribution Under Rigid Foundations

When designing foundations, it's crucial to understand how the load from the structure is transferred to the soil beneath. This load transfer results in pressure distribution on the soil surface. The nature of this pressure distribution depends significantly on the flexibility of the foundation base and the type of soil.

Analyzing Rigid Foundation Behavior

An absolutely rigid foundation base is theoretical, meaning it does not bend or deform under the structural load or soil pressure. This rigidity forces the foundation to settle uniformly across its entire area. In contrast, soil is a deformable material.

Imagine pressing a stiff plate (the rigid foundation) onto a layer of soft material (the soil). Since the plate doesn't bend, every point on the bottom of the plate must settle by the same amount. However, the soil's resistance to deformation varies. This uniform settlement requirement, imposed by the rigid foundation, leads to a non-uniform distribution of pressure exerted by the foundation onto the soil.

Pressure Distribution on Soil Types

The exact pattern of non-uniform pressure depends on the type of soil:

• Granular Soil (like sand): For a rigid foundation on granular soil, the soil at the edges can move laterally more easily than the soil under the center. This reduces the support at the edges compared to the center. To maintain uniform settlement, the pressure tends to be higher at the center and lower at the edges.
• Cohesive Soil (like clay): For a rigid foundation on cohesive soil, the soil tends to shear more easily at the edges. The soil under the center is confined by the surrounding soil. To maintain uniform settlement, the pressure tends to be higher at the edges and lower at the center.

In both cases (granular and cohesive soil), the key takeaway is that the pressure distribution under an absolutely rigid foundation base is not uniform.

Evaluating the Options

Let's look at the given options based on our understanding:

1. On an absolutely rigid foundation base the pressure will be more at the edges of the foundation. This can be true for cohesive soil, but it's not a universally true statement for all soil types, and the question asks for a statement that is true in general for a rigid base.
2. On an absolutely rigid foundation base the pressure will be uniform. This is incorrect. As explained, the requirement of uniform settlement on a deformable soil leads to non-uniform pressure.
3. On an absolutely rigid foundation base the pressure will not be uniform. This statement is true. Regardless of whether the pressure is higher at the edges or the center (which depends on the soil type), the distribution is definitely not uniform when the foundation is rigid and the soil is deformable.
4. On an absolutely rigid foundation base the pressure will be zero at the center of the foundation. This is incorrect. Pressure is generally maximum or minimum at the center, not zero.

Therefore, the only statement that is consistently true regarding the pressure under an absolutely rigid foundation base resting on soil is that the pressure will not be uniform.

Conclusion on Foundation Pressure

Based on the analysis, the pressure distribution beneath an absolutely rigid foundation on soil is non-uniform because the rigid base forces uniform settlement, and soil deformation resistance is not uniform.

Revision Table: Foundation Mechanics Concepts

Additional Information: Factors Affecting Foundation Pressure

Several factors influence the pressure distribution under a foundation:

• Foundation Stiffness: Rigid foundations cause non-uniform pressure; flexible foundations tend towards uniform pressure on uniform soil.
• Soil Type: Granular soils and cohesive soils exhibit different pressure patterns under rigid foundations.
• Load Distribution: How the load is applied to the foundation affects overall pressure and settlement.
• Foundation Geometry: Size and shape of the foundation also play a role.
• Depth of Foundation: Influences the soil properties and confinement effects.

Understanding these interactions is vital for safe and economical foundation design, ensuring that soil bearing capacity is not exceeded and differential settlement is controlled.