What is the primary difference between a fault and a joint in terms of rock mass mechanics?

A fault involves significant relative displacement of rock masses along a fracture, often creating a zone of crushed rock (gouge). A joint is a fracture where no measurable movement has occurred, primarily affecting the rock's fragmentation and permeability.

How does the stability of a tunnel differ when placed in an anticline versus a syncline?

Tunnels in anticlines benefit from compressive stresses that close joints and natural drainage away from the center. Synclines are riskier because they trap groundwater and the strata dip toward the tunnel, increasing the risk of roof collapse and water inflow.

Why is a strike-parallel slope excavation more dangerous than a strike-perpendicular one?

When an excavation is parallel to the strike, the entire dip of the bedding plane is directed toward the open face, facilitating plane failure. In perpendicular excavations, the rock layers are 'edge-on,' providing much higher structural stability.

What is the 'Daylighting' error in slope stability analysis?

Daylighting occurs when a structural plane (like a bedding plane) dips at an angle shallower than the slope face, causing the plane to intersect the air. This removes the physical restraint at the toe of the slope, making sliding highly probable.

Why is it a mistake to rely solely on the compressive strength of intact rock samples for dam design?

Intact rock samples do not account for discontinuities like joints and faults. The actual rock mass strength is significantly lower because failure will occur along these weak structural planes long before the solid rock material crushes.

What error occurs if pore water pressure in joints is ignored during stability calculations?

Ignoring water pressure leads to an overestimation of the factor of safety. Water pressure reduces the effective normal stress on the joint plane, which directly decreases the frictional resistance available to prevent sliding.

Define 'Dip' and 'Strike' in the context of geological mapping.

Dip is the steepest angle of descent of a tilted bed relative to the horizontal. Strike is the horizontal line representing the intersection of the tilted bed with a horizontal plane, always perpendicular to the dip direction.

What is an 'Unconformity' and why is it significant for engineering?

An unconformity is a buried erosional or non-depositional surface separating two rock masses of different ages. It represents a major structural discontinuity where physical properties like density and strength can change abruptly.

What does RQD (Rock Quality Designation) measure?

RQD is a measure of the degree of jointing or fracture in a rock mass, calculated as the percentage of drill core pieces longer than 10 cm. A low RQD indicates a highly fractured, poor-quality rock mass.

What is 'Fault Gouge' and how does it affect dam foundations?

Fault gouge is the finely ground, clay-like material produced by the grinding of rock during fault movement. It is highly erodible and compressible, making it a major risk for seepage and settlement in dam foundations.

Influence of Geological Structure | Pearson Edexcel A-Level Geography

Coastal Landscapes & Change

Influence of Geological Structure

Summary

Geological structures, including folds, faults, joints, and unconformities, define the mechanical behavior and hydraulic properties of rock masses. Understanding these structures is critical for engineering stability, as they represent planes of weakness where failure is most likely to occur under induced stress.

1. Definition & Core Concepts

Geological Structure refers to the physical arrangement and disposition of rock units, resulting from tectonic forces, cooling, or sedimentary processes.
Dip is the angle of maximum inclination of a rock layer measured from the horizontal plane, while Strike is the compass direction of a horizontal line on the surface of that layer.
Discontinuities are any interruptions in the physical continuity of a rock mass, such as joints, faults, or bedding planes, which significantly reduce the overall rock mass strength compared to intact rock samples.
Folds are bends in rock strata caused by compressive stresses, categorized primarily into Anticlines (arch-like shapes) and Synclines (trough-like shapes).

Diagram showing the relationship between strike, dip direction, and the inclined plane of a rock stratum.

2. Underlying Principles of Rock Mass Behavior

The Scale Effect dictates that the strength of a large-scale rock mass is governed by its structures (joints/faults) rather than the strength of the rock material itself.
Anisotropy is a fundamental principle where the physical properties of the rock (like strength and permeability) vary depending on the direction of measurement relative to the geological structures.
Effective Stress in jointed rock is heavily influenced by pore water pressure within the discontinuities, which acts to reduce the normal stress holding the blocks together, thereby increasing the risk of sliding.

3. Methods & Techniques for Structural Analysis

Stereographic Projection is used to plot the orientation of planes and lines in 3D space onto a 2D circular grid (Schmidt or Wulff nets) to identify dominant joint sets and potential failure modes like daylighting.
Kinematic Analysis involves checking if the geometry of the geological structures allows for movement (sliding or toppling) under gravity, independent of the actual forces involved.
Core Logging and RQD (Rock Quality Designation) provide quantitative measures of structural frequency by calculating the percentage of intact core pieces longer than 10 cm relative to the total drill run.

4. Key Distinctions in Structural Features

5. Exam Strategy & Tips