Foundation bearing capacity is the ability of the foundation to carry loads. The foundation is like a person carrying things. For example, a child can carry only a small weight—in engineering terms, low bearing capacity. The carrying pole is like the foundation, and the things being carried are like the superstructure loads on a building. An adult can carry a greater weight—in engineering terms, high bearing capacity. Foundation bearing capacity is equivalent to the weight a person can carry. The foundation also has requirements, equivalent to the carrying pole not being broken by the load.

Foundation settlement: This is equivalent to a person bending over under a heavy load. In engineering, this also needs to be minimized as much as possible—in engineering terms, controlling settlement and differential settlement.

In engineering, this is called uneven deformation, meaning the settlement of two foundations is different. This situation should be avoided as much as possible. The three failure modes of foundations and their corresponding curves are as follows:

This is an idealized model of general shear failure; actual conditions rarely match this.

In practice, engineering hazards are often caused by excessive settlement, especially uneven settlement, which is even more harmful. During engineering investigation and design, while ensuring bearing capacity, settlement must be a key consideration. Bearing capacity alone is insufficient and not very scientific.

For example, at collapsible loess sites, the natural soil often has high bearing capacity, but when the soil is soaked with water, significant collapsible settlement occurs. Therefore, preventing collapsible settlement is the main issue for loess foundations.

Another example is soil with a high void ratio or underconsolidated soft soil, where consolidation settlement is a key issue that must be considered and prevented. Considering only bearing capacity cannot prevent engineering accidents.

Typical diagram of sliding instability

Sometimes the stability of the foundation needs to be considered, such as at the edge of a slope.

Typical diagram of anti-flotation instability

For underground structures whose self-weight is less than the buoyancy force, anti-flotation stability must be considered.

Key issues that geotechnical investigation engineers need to consider:

Foundation stability, groundwater conditions, bearing capacity of foundation soil layers, treatment measures and recommendations, and the impact of special conditions.

Propose a foundation form that is safe and applicable, reliable in quality, economically reasonable, and environmentally friendly.

The "tofu-dreg building" incident was mainly caused by the construction unit not understanding foundation principles and relying on experience.

Key issues that structural design engineers need to consider:

Superstructure loads, foundation bearing capacity, foundation deformation, foundation type and punching shear resistance, shear resistance, etc., distribution of superstructure loads, base reaction conditions, uneven deformation conditions, etc.

And verification of bearing capacity under seismic conditions, etc.

Construction drawings must be produced.

The Leaning Tower of Pisa is a typical example of settlement.

No.1 Overview of Key Points of Foundation Bearing Capacity

No.2 Common Questions About Foundation Bearing Capacity

No.3 Basic Concepts and Terminology Development of Foundation Bearing Capacity

No.4 Application of Foundation Bearing Capacity

No.5 Explanation of the Principle of Depth Correction for Foundation Bearing Capacity

The leaning tower was caused by long-term uneven settlement of the foundation, while the "tofu-dreg building" was caused by piled soil and rainfall triggering shear failure of the foundation.

The correction of foundation bearing capacity is mainly considered from the perspective of shear failure, without considering settlement. That is, the settlement problem has little to do with foundation depth and width.

The foundation bearing capacity used in engineering, as well as the foundation bearing capacity tests, is actually a design value obtained by considering both deformation and strength. Since the test influence depth is only 1-2 times the diameter of the loading plate, the plate loading test cannot reflect the settlement of actual engineering, nor can it reflect the deformation parameters of deep soil layers. The settlement in the plate loading test is completely different from engineering settlement, especially for foundations with highly compressible soil layers beneath.

The "Technical Code for Ground Treatment of Buildings," the "Code for Design of Building Foundation," as well as foundation testing codes and loess codes; appendices of highway, railway, and other codes all contain test methods for various natural foundations and artificial foundations.

The principles are already in the mind map above and in soil mechanics. It is recommended that everyone master the core, and application will not be dogmatic or confusing. Soil mechanics is very important.