Folds, faults and joints

Rocks rarely lie flat and unbroken. Over geological time, tectonic forces fold them, fracture them and move them along faults. These structures decide where water flows, where slopes slide and where tunnels and dams meet weak ground, so mapping them is a core task of engineering geology.

The orientation of a plane is recorded by its strike, the direction of a horizontal line on it, and its dip, the angle and direction of steepest slope. Folds bend layers into anticlines, arched up, and synclines, troughs. Rocks at fold hinges are often fractured, and the steep limbs can present bedding planes dipping out of a cut slope. A cross-section that is not perpendicular to strike shows an apparent dip smaller than the true dip.

Faults are fractures along which the two sides have moved. Normal faults form in extension, reverse and thrust faults in compression, and strike-slip faults by sideways movement. Fault zones often contain crushed rock and clay gouge, which is weak, compressible and can carry or block groundwater; tunnel boring machines and drill-and-blast headings alike slow down there. Active faults, those that have moved recently, are a hazard in themselves for dams and bridges.

Joints are fractures with no significant displacement. They occur in sets, often in three roughly perpendicular directions, and are the main control on rock mass quality. Stereographic projection of their orientations against a slope or tunnel shows which blocks and wedges can slide. Spacing, persistence, roughness and infilling of each set are logged as well, because they control strength and permeability.