Chapter 12 — Geological Structures¶
Rock responds to stress in two ways: it folds if it is ductile, or it fractures if it is brittle. Both outcomes create the structural features that decide how a rock mass behaves.
12.1 Stress and strain¶
- Compression — squeezes rock; produces folds and reverse faults.
- Extension — stretches rock; produces normal faults and grabens.
- Shear — slides rock past rock; produces strike-slip faults.
12.2 Folding¶
Under compression, layered rock bends into anticlines (upward) and synclines (downward). Folding changes the dip of bedding — and dip is what the engineer measures to judge slope and foundation stability.
12.3 Fracturing and faulting¶
A joint is a fracture with no movement; a fault is a fracture with movement.
- Normal fault — extension; the hanging wall moves down.
- Reverse fault — compression; the hanging wall moves up.
- Thrust fault — a low-angle reverse fault; can carry strong rock over weak rock.
- Strike-slip fault — lateral movement along the fault's length.
Figure. A fault in intrusive rock on Quadra Island, B.C. A pink dyke is offset about 10 cm, right-laterally. © Steven Earle, CC BY 4.0 — Physical Geology, 2nd ed., Fig. 12.3.4.
Figure. The McConnell Thrust at Mt. Yamnuska, Alberta: Cambrian limestone thrust over Cretaceous mudstone. Where a thrust puts strong rock on weak rock, the weak unit controls the design. © Steven Earle, CC BY 4.0 — Physical Geology, 2nd ed., Fig. 12.3.9.
Figure. Graben and horst structures formed in extension. All the faults are normal faults. © Steven Earle, CC BY 4.0 — Physical Geology, 2nd ed., Fig. 12.3.6.
12.4 Measuring geological structures¶
The two measurements are strike (the compass direction of the intersection of a plane with the horizontal) and dip (its angle of inclination). Together they define the orientation of bedding, foliation or a joint set — the single most useful structural datum for a design.
Key points for the geotechnical engineer¶
- Structure, not rock type, often controls the design. A moderately strong rock with adversely dipping bedding is worse than a weaker rock without it.
- Faults are planes of weakness and possible groundwater paths — and they are discontinuities in the ground model.
- Thrusts can put strong rock over weak rock; the weak layer then governs settlement and stability.
- Strike and dip must be measured, not assumed, and must be recorded on every rock-mass description.
Terminology¶
| English | Vietnamese |
|---|---|
| stress / strain | ứng suất / biến dạng |
| fold / anticline / syncline | uốn nếp / nếp lồi / nếp lõm |
| joint | khe nứt |
| fault (normal / reverse / thrust / strike-slip) | đứt gãy (thuận / nghịch / chờm / trượt bằng) |
| graben / horst | địa hào / địa luỹ |
| strike / dip | phương / góc dốc |
Digest of Chapter 12, Physical Geology – 2nd Edition, by Steven Earle (CC BY 4.0).