Structural Geology | Fault

Faults in Geology for Civil Engineers
Introduction
In this session, we discuss faults, a critical geological structure that civil engineers must understand. A fault occurs when forces acting on rocks cause them to break and move along a fracture. These movements can impact construction projects, so it is essential to recognize and classify faults.

Deformation and Faulting
Deformation occurs when rocks are subjected to forces. There are two types of deformation:

Ductile deformation: The rock bends or folds without breaking.
Brittle deformation: The rock breaks, leading to the formation of faults.
A fault is a fracture along which the rocks on either side have moved. If the rock moves along a fault, the process is called faulting. The energy released during faulting often results in earthquakes, as the rock breaks suddenly, releasing stored energy.

Structure of a Fault
When rocks break, the break occurs along a plane, called the fault plane, which is a three-dimensional surface. The fault plane separates the two rock blocks, which can move vertically, horizontally, or diagonally.

Fault Terminology
Fault Plane: The plane along which the rock breaks.
Fault Zone: A zone where the rocks around the fault are crushed due to movement.
Shear Zone: A zone where ductile deformation occurs, causing a smooth movement along the fault.
Slickenlines: Striations or scratches on the fault plane caused by the movement of rocks along it. These marks help geologists determine the direction of movement.
Dip and Slip
Dip: The angle between the fault plane and the horizontal surface.
Heave: The horizontal displacement of the rock.
Throw: The vertical displacement of the rock.
In some cases, the movement can be diagonal, in which case we refer to the displacement as slip, which includes both horizontal and vertical movements.

Importance of Faults in Engineering
Faults can cause significant disruptions in the geology of a region, affecting construction projects. For example, when tunneling or laying foundations, encountering a fault means that the geological conditions change abruptly from one side of the fault to the other. This variation in rock type can impact the stability of structures like tunnels, bridges, or buildings.

Example
Consider constructing a tunnel across a fault. As you dig through the fault plane, you will likely encounter a different rock type than expected, which can complicate the engineering process.

Types of Fault Blocks
When a fault occurs, it separates the rocks into two blocks:

Hanging Wall: The block above the fault plane.
Footwall: The block below the fault plane.
To visualize this, imagine standing near the fault plane. The block where your head would touch is the hanging wall, and the block beneath your feet is the footwall.

Classification of Faults
Faults can be classified based on several factors:

Apparent movement: The direction in which the rock blocks appear to move.
Relation to fault attitude: The angle of the fault plane relative to the surrounding rocks.
Direction of slip: Whether the movement is horizontal, vertical, or diagonal.
Mode of occurrence: Whether the fault is high-angle, low-angle, or medium-angle.
Common Fault Types
Normal Fault: The hanging wall moves down relative to the footwall.
Reverse Fault: The hanging wall moves up relative to the footwall.
Strike-Slip Fault: The blocks move horizontally past each other.
Conclusion
Understanding faults and their characteristics is essential for civil engineers, as faults affect the stability of construction projects. Being able to identify fault planes, fault zones, and different types of movements is crucial for ensuring safe and sound engineering solutions.