Deformation

Deformation

1. Orientation of tectonic forces

   • normal faulting - caused by tensional forces that stretch rock and pull it apart
   • reverse faulting - caused by compressive forces that squeeze and shorten a rock
   • thrust fault - reverse fault with a shallow-dipping fault plane
   • strike-slip faulting - shearing forces
   • oblique-slip faulting - caused by a combination of shearing & compressive/tensional forces

2. Engineering significance of faults

   • large earthquakes are caused by the rupture of one or more faults
   • general construction impact
     • fault zones provide sources for large seepage flows, concrete deterioration can be caused by the attack from sulfuric acid that results from the reaction of oxidizing surface waters with sulfides

3. folding of rock layers - direction of folding and by the orientation of the fold axis (called a plunging fold if not horizontal) & axial place

   • Effects of folds on rocks
     • stress inducement - causes tension cracks, shear cracks and drag folds
   • types
     • anticlines - layered rocks bent upward into arches
     • synclines - bent downward into trough
     • recumbant/overturned fold - the fold is so distorted that it is overturned

4. Joints

   • Effects
     • cause problems on slopes - when slopes are cut at a steeper angle than the joint planes, the joint emerges/ daylight in the slope face and a slip can occur
     • allows water and air to penetrate rock, permitting deep weathering
     • Springs may occur on hillsides where joints reach the surface
   • Sheeting - roughly parallel to the land surface & may cause landslides
     • 1. Removal of overburden and rebound
       • Earth surfaces are eroded & relieve vertical stress but lateral stress is reduced proportionally. Horizontal stresses become aligh=ned with a current ground surface as the vertical stress drops to zero at this boundary. Thus, large surface-parallel compressive stresses can be generated through exhumation that may lead to tensile rock fracture. The frequency of sheeting joints is related to the depth of overburden. The thinner the rock cover, the more pronounced the sheeting. This suggests a connection between the removal of overburden by denudation and the development of sheeting joints
     • 2. Thermoelastic strain
       • Shrinking of cooling large homogenous igneous masses. For large daily rock surface, temp variations stresses created during heating cause the near-surface zone of rock to expand and detach in thin slabs.
   • Hexagonal
     • Shrinkage joints produced by drying out of sediments or by cooling of molten lava
     • Columnar jointing common in the lava where shrinkage cracks develop around regularly spaced centres towards which tensional stresses coverage. Once the crack develops it continues to grow, which is perpendicular to the surface of the flow. Interferences between neighbouring centres leads to a honeycomb pattern of hexagonal joints
   • Basics
     • joint set - joints that run parallel, joint system - 2 or more sets that intersect at a constant angle, dominant joints are primary joints & the rest are secondary, systematic - planar & parallel, non-systematic - orientation is irregular
     • joints are formed through failure of rock masses in tension or in shear or both, rupture surfaces formed by extension thend to be clean and rough with little detritus, they tend to follow minor lithological variations