Essay About Geol411 Joints And Free Surface

Geol411 Joints and Veins
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GEOL411: Joints and VeinsI) Introduction: * Joint = planar fracture or crack in a rock, without shear displacement(Shear fractures do not produce the features found in joints) * Why Study Joints:- Affect strength of rock bodies on escarpments.- Can lead to rock falls.- Affect regional permeability of rock bodies; direct groundwater or oil flow.- Affects permeability of reservoirs.- Controls natural erosion.  – Controls caves.- Paleostress indicators: form normal to σ3; the joint plane will contain both σ1 and σ2. II) Joint Sets and Systems:* Systematic vs. Non-Systematic Joints:- systematic joints:  there are several with a similar orientation and approximately equal spacing; tend to be planar        – non-systematic:  random-like arrangement and spacing; may be curviplanar. *Joint set = a group of systematic joints   * Joint system = two or more joint sets (may have formed at very different times!)         – Orthogonal joint systems: dihedral angle ~ 90˚        – Conjugate joint systems: dihedral angle < 90˚ (often 30 to 60˚)                 [Do not confuse with conjugate fractures!] [pic 1]  * Timing relations – the first-formed set of joints will truncate later sets, because joints         cannot cross a free surface (free surfaces cannot transmit shear stresses, and must         be principle planes of stress!). [pic 2]         - e.g. for orthogonal joint sets, the “rungs” of the ladder-like pattern formed later        - e.g. grid pattern suggests alternating stress field and concurrent cracking (σ2 and                         σ3 may have been similar in magnitude and flipped back and forth) * Joint spacing:         - Experiments suggest that joints form sequentially.        - The often regular spacing of joints in sed rocks is explained by the formation of                         one joint relieving tensile stresses for a critical distance on either side of the                 joint, causing a “stress shadow”.         - Width of stress shadow depends on bed thickness.        - This explains why joints are usually closer spaced in thinly bedded sediments. [pic 3]         - Spacing depends on:                 [1] bed thickness                [2] strain (more strain requires more joints)                [3] stiffness (Young’s modulus)                [4] tensile strength.  III) Surface morphology of joints * Plumose structure:  Joint surfaces show a feather-like pattern.         (This is best seen in fine-grained rocks.) [pic 4]         - Origin = small dimple where joint initiates (commonly an inclusion)        - Mirror zone = very smooth zone around origin        - Mist zone = area where surface roughens, around the mirror zone        - Hackle zone = contains “barbs” which curve away from plume axis. Note barbs                         point towards the origin.        - Plume axis is often wavy and diffuse.         -Arrest lines may be seen – concentric rings around the origin where a joint                         formed in stages. * Plumose structure forms due to:        (i) rock inhomogeneity        (ii) changing stress field at crack tip as joint propagates.  * Twist Hackle:  If a joint is not // principal plane of stress (inclusions, bedding planes,         etc.) then crack tip pivots into a new orientation. Joint may split into an en echelon         array (called “twist hackle”).