Geotechnical Analysis: Pressure bulbs and Settlements

Geotechnical analysis is a crucial part of foundation design. Beyond verifying the stability of a foundation and the bearing capacity of the ground, it is vital to understand how stresses are distributed deep into the soil and what the resulting deformations will be.

Soil Pressures

Soil can be a highly complex medium: multiple irregular strata, heterogeneous particle sizes, water flow, varying permeabilities, and countless physical phenomena. This is why studying the stress state of the soil is essential to identify the primary behaviors that require attention and how to prepare for these challenges.

Natural Pressures

Natural pressures in the soil (geostatic stresses) correspond to the self-weight of the ground. In the simplest case of a single homogeneous soil stratum, the pressure varies only with depth according to:

A more complex case involves multiple regular soil strata with different densities, where the pressure still depends only on depth:

The most realistic case corresponds to terrain with multiple irregular soil strata, where pressures no longer depend solely on depth but also on the specific $(x, y)$ coordinates of a point:

In each case, the effective natural pressures correspond to the difference between total natural pressures and pore water pressure: $\sigma^\prime (z) = \sigma(z) - u(z)$. If a water table exists at a depth $z_w$, the pore pressure is calculated as:

Interesting observations can be drawn from effective natural pressure diagrams. For instance, the geometry of the isobars is related to the geometry of the soil strata with different properties. Furthermore, within the same stratum, the isobars will be closer together as the unit weight $\gamma$ of the stratum increases.

Induced Pressures

When a load is placed on the ground, it is distributed by dissipating into the depth of the soil. Points near the load experience a significant change in their stress state, while for a sufficiently distant point, the change is practically zero. The way pressures distribute at depth has been extensively studied for point loads or simple configurations, with the Boussinesq approach being the most widely used.

In practice, these problems are often studied graphically using various charts based on standard foundation geometries. However, studying pressure dissipation becomes complicated for complex foundation geometries or when considering the interaction between the pressure bulbs of different foundations.

An increase in induced pressures can be particularly delicate in certain cases, such as when there are nearby underground constructions, excavations, or slopes. It is important to study not only the individual pressure bulbs of each foundation but also their superposition, as in unusual cases, this could even lead to deep-seated soil failure. Consequently, analyzing pressure dissipation at depth becomes a high priority in most projects.

Soil Settlements

In structural design, there are many situations where strength and serviceability must be controlled independently. While these concepts are closely related, meeting the minimum requirements for one does not necessarily imply meeting them for the other. A typical example is the design of a long-span beam; it is not enough for it to simply not collapse; it must also avoid excessive deflections or vibrations. Soil behavior is similar: it is not enough for the soil not to "fail" in terms of shear strength; we must also verify how much the structure will "sink."

Immediate (or Elastic) Settlements

Immediate settlement occurs almost instantaneously upon the application of a load, before any change in the soil's water content takes place. In professional practice and in calculation software like Foundaxis, this phenomenon is usually modeled using linear elasticity theory (applying Young's Modulus $E_s$ and Poisson's Ratio $\nu_s$), assuming the soil behaves as a continuous and homogeneous medium.

However, from a detailed physical perspective, this "elastic" settlement is actually the sum of two distinct mechanisms:

• Particle Rearrangement (Inelastic Component): This is the predominant mechanism in granular soils. When a load is applied, soil grains slide and rotate to seek more stable and compact positions. This movement is permanent and irreversible.

• Particle Deformation (Elastic Component): This corresponds to the infinitesimal deformation of the soil particles at their contact points. This component is the only truly reversible part and represents a minor fraction (around 10%) of the total immediate settlement.

Note on Construction Execution:

Since design software assumes predictable elastic behavior, it is fundamental that the particle rearrangement component be "exhausted" before construction begins. This is achieved through a controlled compaction process. By compacting the ground, we mechanically force the grain rearrangement; thus, when the structure applies its real loads, the soil will already be in a state of density where deformations are minimal and remain mostly within the recoverable elastic range.

It is important to remember that the compaction strategy must match the soil's nature: for granular soils (sands and gravels), vibratory compaction is recommended to facilitate sliding between grains, whereas for fine-grained soils (clays), impact or pressure compaction is necessary to overcome the cohesion between particles.

Consolidation Settlements

This type of settlement is predominant in fine-grained soils with low water permeability. The physical mechanism behind this settlement is the progressive release of pore water pressure through the expulsion of water.

In low-permeability clay soils, it is difficult for water to move between soil particles. Upon the application of an external load, since water is incompressible and cannot escape quickly, the water initially takes the excess load through an increase in pore pressure equivalent to the applied load increase.

Over time, water slowly drains through the soil particles. Through this process, the load is slowly transferred to the solid skeleton, dissipating the pore pressure. Depending on the ground properties, this process can naturally take decades, meaning a building could continue to sink many years after its construction.

Absolute vs. Differential Settlements

While the comparison between elastic and consolidation settlements refers to the different soil mechanisms for deformation, the comparison between absolute and differential settlements refers to different ways of measuring that deformation. This is essential because different relative settlements can induce damage in different elements.

When we control absolute settlement, we are primarily concerned with the behavior of a structure within its external environment. This is relevant for evaluating utility connections (water, sewage, electricity, gas), as excessive settlement could damage or even disable a connection. It is also relevant for serviceability—ensuring the structure remains usable, for example, by maintaining a reasonable elevation difference with sidewalks and streets for pedestrian traffic.

When we control differential settlement, we are primarily concerned with the structure itself. When two points of a structure experience differential settlement, any structural element connecting those points undergoes a massive increase in internal stresses. The most tangible case is a simple frame where two columns are connected by a beam. The difference in settlement between the columns induces enormous shear forces and moments in the beam.

 

Discover how geotechnical analysis is integrated into the complete Foundaxis workflow:

Foundation Design Software: A Complete Guide for Structural Engineers 2026