Foundation Settlement: Causes, Soil Mechanics, and Rock Types

Understanding Foundation Settlement

The primary cause of foundation settlement is the compressibility of the underlying, load-bearing soil layers.

Uniform vs. Unequal Settlement

It is crucial to distinguish between two types of settlement. Unequal settlement, also known as differential settlement, can be dangerous to a structure's integrity, whereas uniform settlement is generally less critical.

Factors Leading to Uniform Settlement

• Foundations resting on frost-free soils.
• Thorough ground investigation confirming consistent soil properties.
• Pressures from neighboring loads do not overlap.
• Uniform loads and a consistent soil profile.
• Consistent soil class and uniform foundation types across the structure.

Causes of Unequal (Differential) Settlement

• Heterogeneous disposition of soil strata.
• Overlapping pressures from adjacent buildings.
• Fluctuations (rise or fall) in the groundwater table.
• Soil desiccation (drying out).
• Presence of natural or artificial eroded cavities.
• Subsidence or landslides in the area.
• Chemical modifications of the subsoil.
• Ground heave from freezing and subsequent settlement upon thawing.

Rock Characteristics and Formation

Microstructural Characteristics of Rocks

Rock structure is defined by the distribution and organization of its constituent elements (minerals), which can exist in an amorphous or crystalline state. This structure, along with its mineralogical composition, determines the rock's mechanical behavior and durability.

Examples of Rock Structures

• Holocrystalline: Composed entirely of crystals.
• Hypocrystalline: A mixture of crystalline and glassy material.
• Vitreous: Composed entirely of glass (amorphous).
• Clastic and Debris: Composed of fragments or grains of other rocks.

The Geological Cycle and Rock Formation

The material composing the Earth's crust undergoes continuous transformation processes. These processes cause significant changes in the composition, structure, and properties of rocks.

Igneous Rocks

Magma, a fluid mass of minerals deep within the Earth's crust, cools and consolidates to form igneous or "source rocks." Depending on the depth and speed at which the magma cools, they can be classified as plutonic (intrusive) or volcanic (extrusive).

Sedimentary Rocks

Existing rocks, such as igneous rocks, deteriorate through weathering. They are disintegrated, fragmented, and pulverized by the action of atmospheric agents. These fragments are then transported and deposited in a new location. Based on the deposition method, they can be classified as mechanical, chemical, organic, or volcaniclastic.

Metamorphic Rocks

Existing sedimentary or igneous rocks, when subjected to endogenous processes like intense heat and pressure, transform into metamorphic rocks.