Structural Geology

The attitude of a rock (geological structure) is its three-dimensional orientation in space, that is, the direction in which a plane or line is inclined with respect to the horizontal and to north. The attitude of a bedding plane is expressed by its strike and dip.

Components of attitude

1. Strike
   • The direction of the horizontal line drawn on an inclined plane (the line of intersection of the plane with a horizontal plane).
   • It is measured as a bearing from north, for example N45°E.
2. Dip
   • The maximum inclination of the plane with the horizontal, measured perpendicular to the strike.
   • It has two components: dip direction (the direction in which the plane goes down, at 90° to the strike) and dip amount (angle of dip) (the angle between the plane and the horizontal).
3. For a line (such as fold axis or lineation) the attitude is given by trend (compass bearing of the line projected on the horizontal) and plunge (angle of the line below the horizontal).

             N
             |   strike
             |  /
        _____|_/______
       /    /|/      /|
      /    / /      / |
     /____/_/______/  |  dip angle (a)
     |   /-> dip direction (90 deg to strike)
     |__/___________|/

Expressing orientation

• Quadrant method: strike N30°E, dip 40° towards SE (written N30°E/40°SE).
• Azimuth (dip direction/dip) method: 120°/40°.
• Measured in the field with a Brunton or Clar geological compass, which gives strike with the compass and dip with a clinometer.
• Strike is always at right angles to the dip direction, so if the dip direction is known, the strike is dip direction plus or minus 90°.

A fault is a fracture in the rock mass along which the two blocks have moved relative to each other, parallel to the fracture surface. The displacement may be a few millimetres to hundreds of kilometres.

Evidences for recognition in the field

Direct evidences

• Displacement or offset of beds, dykes, veins, streams, roads or ridges.
• Repetition or omission (missing) of beds in a sequence.
• Slickensides (polished, grooved surfaces) and striations on the fault plane showing the direction of movement.
• Fault breccia, fault gouge or mylonite along the fault zone.
• Drag folds (bending of beds near the fault plane).

Physiographic (topographic) evidences

• Fault scarps (straight steep cliffs) and triangular facets.
• Linear valleys, straight stream courses and sudden changes in stream direction.
• Offset or beheaded streams, waterfalls and rapids along a straight line.
• Line of springs and seepages, and a sharp line of vegetation change.
• Alignment of sag ponds, lakes and hot springs.
• Abrupt change of rock type across a straight line, and earthquake epicentres in a line.#### Note No single evidence is final; the presence of several (such as slickensides, breccia and offset beds together) is used to confirm a fault.

Deformation is the change in shape, size or position of a rock body produced by the stress acting on it. Stress acts on the rocks inside the earth and when it exceeds their strength, the rocks bend, break or flow.

Causes

• Tectonic forces from movement of lithospheric plates (collision, subduction, spreading).
• Isostatic adjustment - uplift or sinking of crustal blocks.
• Gravity (sliding, slumping) and weight of overburden.
• Magmatic intrusion and volcanic activity.
• Cooling and contraction of the earth and thermal expansion.
• Compression, tension and shear are the three stress types caused by these forces.

Stages of deformation

1. Elastic - the rock returns to its shape when stress is removed.
2. Plastic (ductile) - permanent bending without breaking, producing folds.
3. Rupture (brittle) - the rock breaks when stress passes the strength, producing joints and faults.

Types of deformation and examples

Consequences

• Creation of mountains, valleys, basins, folds and faults.
• Earthquakes along faults, landslides and slope failure.
• Fracturing of rock reduces strength and increases permeability.
• Foundations, tunnels, dams and slopes are affected, so deformation structures must be mapped before construction.

A fault is a fracture in the rock mass along which the two blocks have moved relative to each other, parallel to the fracture surface. The displacement may be a few millimetres to hundreds of kilometres.

Engineering significance (effects and problems)

• Weak zone: crushed, sheared rock (breccia, gouge) has low strength, high compressibility and high permeability, so foundations may settle or fail.
• Leakage: a fault zone acts as a channel for water, causing seepage under dams and reservoir leakage.
• Tunnelling: sudden inflow of water, roof collapse, squeezing and heavy support requirement when a tunnel crosses a fault.
• Slope instability: a fault plane dipping out of the slope gives a ready sliding surface for landslides and rock slides.
• Differential settlement: a structure built across a fault with unequal rock properties on two sides.
• Seismic hazard: active faults can move again and cause earthquakes and surface rupture, so dams, bridges and buildings must keep away.
• Uncertainty in exploration: displaced or missing beds make the correlation of boreholes and the estimation of rock depth unreliable.#### Precautions
• Avoid building dams, big bridges and tunnels across active faults, or provide special design for expected displacement.
• Treat the fault zone with excavation, concrete plugs, grouting, and drainage.
• Map faults carefully by surface mapping, trenching, drilling and geophysical survey before design.

Method: the three-point problem. Three points at known altitudes define a plane. Find a point on the line AB with the same altitude as C; the line joining it to C is a strike line (horizontal line on the bed).

Given: B is the origin. A is 800 m north of B (600 m). C is 600 m east of B (500 m). B is at 200 m.

        A (600 m)
        |
        | 800 m
        |
        B (200 m) -------- C (500 m)   [600 m east]

Step 1: Strike line

Along AB the altitude changes uniformly from 200 m (B) to 600 m (A), a rise of 400 m in 800 m, i.e. 0.5000 m per m. The point D on AB with altitude 500 m (equal to C) is at a distance from B of

Line DC joins two points of altitude 500 m, so it is the strike line. DC runs from D (0, 600.0) to C (+600, 0).

Step 2: Strike direction

Taking east as +x and north as +y, C is at x = +600. Slope components: northward rise $b = (600-200)/800 = 0.5000$ and eastward rise $a = (500-200)/(+600) = 0.5000$.

Line DC runs 600 m east and 600.0 m south (from D to C). The acute angle it makes with the north-south line is

Because D to C goes toward the south-east, the strike is N45.0°W.

Cross-check from the slopes (altitude gain per metre): northward b = 0.5000, eastward a = 0.5000; the strike line is perpendicular to the gradient (a, b), giving azimuth 135.00°.

Step 3: Dip direction and dip amount

The bed is highest at A and lowest at B, so it dips away from the gradient direction (down the slope), at 90° to the strike. Dip direction azimuth = 225.00°, which is S45.0°W.

Answer: Strike N45.0°W; dip direction S45.0°W; dip amount 35.3°.

Method: the three-point problem. Three points at known altitudes define a plane. Find a point on the line AB with the same altitude as C; the line joining it to C is a strike line (horizontal line on the bed).

Given: B is the origin. A is 600 m north of B (400 m). C is 400 m west of B (300 m). B is at 100 m.

        A (400 m)
        |
        | 600 m
        |
  C (300 m) -------- B (100 m)   [C is 400 m west]

Step 1: Strike line

Along AB the altitude changes uniformly from 100 m (B) to 400 m (A), a rise of 300 m in 600 m, i.e. 0.5000 m per m. The point D on AB with altitude 300 m (equal to C) is at a distance from B of

Line DC joins two points of altitude 300 m, so it is the strike line. DC runs from D (0, 400.0) to C (-400, 0).

Step 2: Strike direction

Taking east as +x and north as +y, C is at x = -400. Slope components: northward rise $b = (400-100)/600 = 0.5000$ and eastward rise $a = (300-100)/(-400) = -0.5000$.

Line DC runs 400 m west and 400.0 m south (from D to C). The acute angle it makes with the north-south line is

Because D to C goes toward the south-west, the strike is N45.0°E.

Cross-check from the slopes (altitude gain per metre): northward b = 0.5000, eastward a = -0.5000; the strike line is perpendicular to the gradient (a, b), giving azimuth 45.00°.

Step 3: Dip direction and dip amount

The bed is highest at A and lowest at B, so it dips away from the gradient direction (down the slope), at 90° to the strike. Dip direction azimuth = 135.00°, which is S45.0°E.

Answer: Strike N45.0°E; dip direction S45.0°E; dip amount 35.3°.

Method: the three-point problem. Three points at known altitudes define a plane. Find a point on the line AB with the same altitude as C; the line joining it to C is a strike line (horizontal line on the bed).

Given: B is the origin. A is 800 m north of B (500 m). C is 400 m west of B (300 m). B is at 100 m.

        A (500 m)
        |
        | 800 m
        |
  C (300 m) -------- B (100 m)   [C is 400 m west]

Step 1: Strike line

Along AB the altitude changes uniformly from 100 m (B) to 500 m (A), a rise of 400 m in 800 m, i.e. 0.5000 m per m. The point D on AB with altitude 300 m (equal to C) is at a distance from B of

Line DC joins two points of altitude 300 m, so it is the strike line. DC runs from D (0, 400.0) to C (-400, 0).

Step 2: Strike direction

Taking east as +x and north as +y, C is at x = -400. Slope components: northward rise $b = (500-100)/800 = 0.5000$ and eastward rise $a = (300-100)/(-400) = -0.5000$.

Line DC runs 400 m west and 400.0 m south (from D to C). The acute angle it makes with the north-south line is

Because D to C goes toward the south-west, the strike is N45.0°E.

Cross-check from the slopes (altitude gain per metre): northward b = 0.5000, eastward a = -0.5000; the strike line is perpendicular to the gradient (a, b), giving azimuth 45.00°.

Step 3: Dip direction and dip amount

The bed is highest at A and lowest at B, so it dips away from the gradient direction (down the slope), at 90° to the strike. Dip direction azimuth = 135.00°, which is S45.0°E.

Answer: Strike N45.0°E; dip direction S45.0°E; dip amount 35.3°.

Method: the three-point problem. Three points at known altitudes define a plane. Find a point on the line AB with the same altitude as C; the line joining it to C is a strike line (horizontal line on the bed).

Given: B is the origin. A is 600 m north of B (500 m). C is 300 m east of B (400 m). B is at 100 m.

        A (500 m)
        |
        | 600 m
        |
        B (100 m) -------- C (400 m)   [300 m east]

Step 1: Strike line

Along AB the altitude changes uniformly from 100 m (B) to 500 m (A), a rise of 400 m in 600 m, i.e. 0.6667 m per m. The point D on AB with altitude 400 m (equal to C) is at a distance from B of

Line DC joins two points of altitude 400 m, so it is the strike line. DC runs from D (0, 450.0) to C (+300, 0).

Step 2: Strike direction

Taking east as +x and north as +y, C is at x = +300. Slope components: northward rise $b = (500-100)/600 = 0.6667$ and eastward rise $a = (400-100)/(+300) = 1.0000$.

Line DC runs 300 m east and 450.0 m south (from D to C). The acute angle it makes with the north-south line is

Because D to C goes toward the south-east, the strike is N33.7°W.

Cross-check from the slopes (altitude gain per metre): northward b = 0.6667, eastward a = 1.0000; the strike line is perpendicular to the gradient (a, b), giving azimuth 146.31°.

Step 3: Dip direction and dip amount

The bed is highest at A and lowest at B, so it dips away from the gradient direction (down the slope), at 90° to the strike. Dip direction azimuth = 236.31°, which is S56.3°W.

Answer: Strike N33.7°W; dip direction S56.3°W; dip amount 50.2°.

A joint is a fracture or crack in a rock along which there has been no (or negligible) displacement of the two sides.#### Geometric classification of joints Based on the relation of the joint with the attitude of the host rock (strike and dip of beds):

1. Strike joints - run parallel to the strike of the beds.
2. Dip joints - run parallel to the dip direction of the beds (at right angles to the strike).
3. Oblique (diagonal) joints - cross the bed at an angle to both strike and dip.
4. Bedding joints - lie along the bedding planes.

Based on the relation to folds, longitudinal, transverse and oblique joints are also distinguished. Joints that occur in sets are a joint set, and two or more sets form a joint system.

       strike joint   dip joint   oblique joint
     ----------------  |  |  |    /  /  /
     ----------------  |  |  |   /  /  /
     (parallel to      (along      (at an angle)
      strike)           dip)

Engineering importance

• Reduce rock strength and bearing capacity; blocks bounded by joints can slide or fall.
• Slope stability: joints dipping out of the slope or intersecting each other give wedge, plane and toppling failures.
• Seepage: joints are paths for ground water, causing dam leakage and tunnel inflow.
• Weathering starts along joints, so the rock deteriorates deeper.
• Quarrying: joints help in extracting blocks, and close jointing reduces block size.
• Tunnelling: joint orientation controls overbreak and roof falls; tunnel axis should be at a high angle to strike joints.
• Grouting and rock bolting are needed in highly jointed rock.

An unconformity is a surface of erosion or non-deposition that separates younger strata from older rocks and represents a gap (break) in the geological record.

Types

• Angular unconformity: younger beds lie over tilted or folded older beds at an angle.
• Disconformity: beds above and below are parallel, but a surface of erosion separates them.
• Nonconformity: sedimentary beds rest on eroded igneous or metamorphic rocks.
• Paraconformity: parallel beds with no obvious erosion, gap shown only by fossils.

 younger beds  ===========
 unconformity  ~~~~~~~~~~~  (erosion surface)
 older beds    \\\\\\\ (tilted)

Importance in civil engineering

• A basal conglomerate or weathered, soft layer at the unconformity is a plane of weakness and can cause sliding.
• The surface is generally uneven and weathered, so foundation depth and rock quality change suddenly.
• Seepage: water can move along the unconformity plane, so dams and tunnels may leak.
• Beds above and below have different strength, attitude and permeability, so differential settlement and stability problems arise.
• Wrong correlation of boreholes and rock depth, since beds are missing.
• Slope instability: sliding of younger beds over older tilted beds.Hence the position, depth and type of unconformity must be found by drilling before designing foundations, dams and tunnels.

Primary and secondary structures

Relationship of strike and dip

• Strike is the direction (bearing) of a horizontal line on an inclined plane.
• Dip is the maximum inclination of the plane, measured at right angles to the strike, in the vertical plane.
• Hence, strike and dip direction are always 90° apart, and the dip direction is perpendicular to the strike line.
• Strike is the contour line of the plane (all points on it are at the same level), and dip is the line of steepest descent.
• Any other direction on the plane has a smaller inclination, called the apparent dip.

        N
        |       strike line (horizontal)
        |     /
        |   /  
        | /  
   -----+-----------------> E
       /|\  
      / | \ dip direction (90 deg to strike)
         v
   Plan view; dip amount is angle in vertical section

The attitude is written as strike direction/dip amount and dip direction, for example N30°E/40°SE.

Strike from the measured dip direction

1. Measure the dip direction (bearing of the line of steepest descent) with a compass.
2. Strike = dip direction + 90° or dip direction - 90° (both give the same line, in opposite senses).
3. Convert to a quadrant bearing.

Example: dip direction N60°W (azimuth 300°). Strike = 300° - 90° = 210°, that is, 30° = N30°E (equivalent to S30°W). The strike is N30°E.

The strike is always at 90° to the dip direction.

Given: dip direction = S25°E, which is azimuth $180° - 25° = 155°$.

So the strike is N65°E (the same line is S65°W).

               N
               |        strike N65E
               |     /
               |   / 65
        W -----+-/---------- E
               | \
               |  \ 25
               |   v  dip direction S25E
               S

The strike line and dip direction are perpendicular (65° + 25° = 90°).

Answer: Strike = N65°E (S65°W). For the sandstone bed with the same dip direction in the net-diagram version, the same strike N65°E is plotted: draw the great circle (dip plane) with strike N65°E and a pole 90° from it, in the S25°E direction.

Fault

Direct evidences

• Displacement or offset of beds, dykes, veins, streams, roads or ridges.
• Repetition or omission (missing) of beds in a sequence.
• Slickensides (polished, grooved surfaces) and striations on the fault plane showing the direction of movement.
• Fault breccia, fault gouge or mylonite along the fault zone.
• Drag folds (bending of beds near the fault plane).

Physiographic (topographic) evidences

• Fault scarps (straight steep cliffs) and triangular facets.
• Linear valleys, straight stream courses and sudden changes in stream direction.
• Offset or beheaded streams, waterfalls and rapids along a straight line.
• Line of springs and seepages, and a sharp line of vegetation change.
• Alignment of sag ponds, lakes and hot springs.
• Abrupt change of rock type across a straight line, and earthquake epicentres in a line.

Fold

• Repetition of beds in reverse order on both sides of an axis (A B C D C B A).
• Opposite dips on two sides of a line (axis), either converging (syncline) or diverging (anticline).
• Outcrop pattern on a map: a series of curved or zig-zag outcrops (V-shaped patterns) with plunge.
• Age relation: in an anticline older beds are in the core; in a syncline younger beds are in the core.
• Drag folds and minor folds in the small scale, and variation of strike or dip along a traverse.
• Topography: ridges and valleys following the folded beds, and plunging noses on hill slopes.
• Cleavage or joints (fan or radiating pattern) related to the axial plane.

A fold is identified in the field by the following evidences.

• Repetition of beds in reverse order on both sides of an axis (A B C D C B A).
• Opposite dips on two sides of a line (axis), either converging (syncline) or diverging (anticline).
• Outcrop pattern on a map: a series of curved or zig-zag outcrops (V-shaped patterns) with plunge.
• Age relation: in an anticline older beds are in the core; in a syncline younger beds are in the core.
• Drag folds and minor folds in the small scale, and variation of strike or dip along a traverse.
• Topography: ridges and valleys following the folded beds, and plunging noses on hill slopes.
• Cleavage or joints (fan or radiating pattern) related to the axial plane.

A fold is a bend or wavy curvature of originally flat rock layers (beds), produced by compressive stress acting on plastic rocks over a long time.```text Axial plane | | (crest) ___ | ___ / \ | /
_/ |/ _ limb hinge limb

Parts: **limbs** (sides), **hinge** (line of maximum curvature), **axial plane** (plane dividing the fold symmetrically), **crest** (top) and **trough** (bottom). An upfold is an **anticline** and a downfold is a **syncline**.

Folds are classified on several bases. The two bases mentioned are explained here.

On the basis of convexity (upward or downward)

1. Anticline - an upward arch; limbs dip away from the axis and older beds are in the core.
2. Syncline - a downward trough; limbs dip towards the axis and younger beds are in the core.
3. Monocline - a local steepening in otherwise gently dipping beds, a single step.

On the basis of position of the axial plane

 Symmetrical   Asymmetrical   Overturned   Recumbent
    /\             /\           /\          ____
   /  \           /  \         /  \_       /____\_
  /    \         /    \       /  /  /      \_____/
 vertical axial  inclined    both limbs     horizontal
 plane           axial       dip same way   axial plane

Other bases: plunge (plunging, non-plunging), shape (chevron, box, conical).

Thrust

A thrust is a low-angle reverse fault (dip generally less than 45°) in which the hanging wall has moved up and over the foot wall under compression, bringing older rocks over younger rocks. Examples in Nepal are the Main Central Thrust (MCT), Main Boundary Thrust (MBT) and Main Frontal Thrust (MFT).

Importance of folds in civil engineering

• Foundation: fold limbs dip in different directions, so a dam or building on a fold must consider dip of beds. Dip upstream is favourable for dams; dip downstream with weak beds is dangerous.
• Tunnelling: in an anticline the roof is in tension (more joints, loose blocks, but water drains away); in a syncline the water collects and rock pressure is high. The crest and trough zone are more fractured.
• Reservoir: syncline can hold ground water, so leakage is low; anticline, if cut by a valley, may let water escape along dipping beds.
• Slope stability: beds dipping in the slope direction at an angle less than the slope can slide.
• Water supply: synclines form artesian basins; anticlines give petroleum and gas traps.
• Quarrying and exploration: thickness of a bed repeats on limbs, so outcrop pattern must be understood to estimate rock depth.

Engineering significance of joints

A joint is a fracture or crack in a rock along which there has been no (or negligible) displacement of the two sides.

• Joints reduce rock mass strength and bearing capacity, and the rock fails along them.
• They control slope stability: planar, wedge and toppling failures in cuttings, and dam abutments.
• Joints give paths for seepage, so water leaks from reservoirs and flows into tunnels and excavations.
• Weathering is faster along joints.
• In tunnelling they produce overbreak, roof falls and wedge failures; tunnel axis is best oriented across the joint strike.
• They make quarrying easier, but close jointing gives only small blocks.
• Grouting, rock bolts and shotcrete are needed in highly jointed rock.

Engineering significance of faults

• Weak zone: crushed, sheared rock (breccia, gouge) has low strength, high compressibility and high permeability, so foundations may settle or fail.
• Leakage: a fault zone acts as a channel for water, causing seepage under dams and reservoir leakage.
• Tunnelling: sudden inflow of water, roof collapse, squeezing and heavy support requirement when a tunnel crosses a fault.
• Slope instability: a fault plane dipping out of the slope gives a ready sliding surface for landslides and rock slides.
• Differential settlement: a structure built across a fault with unequal rock properties on two sides.
• Seismic hazard: active faults can move again and cause earthquakes and surface rupture, so dams, bridges and buildings must keep away.
• Uncertainty in exploration: displaced or missing beds make the correlation of boreholes and the estimation of rock depth unreliable.

An unconformity is a surface of erosion or non-deposition that separates younger strata from older rocks and represents a gap (break) in the geological record.

Types of unconformity

1. Angular unconformity - younger beds lie on tilted or folded older beds with an angular difference between them.
2. Disconformity - younger and older beds are parallel, but separated by an irregular erosion surface.
3. Nonconformity - sedimentary beds rest on eroded igneous or metamorphic (crystalline) rocks.
4. Paraconformity - parallel beds with no visible erosion; the gap is known from fossils only.
5. Local unconformity - small break in the same series due to local erosion.

 Angular            Disconformity     Nonconformity
 ___________        ___________       ___________
 young beds         young beds        sedimentary
 ~~~~~~~~~~~        ~~~~~~~~~~~       ~~~~~~~~~~~
 //tilted//         old beds          + + + + + +
 //older///         (parallel)        + igneous +
 ~~ = unconformity (erosion surface)
```The unconformity surface is often marked by a basal conglomerate and a weathered zone.

Definition

The Rock Mass Rating (RMR), or Geomechanics Classification, was proposed by Bieniawski (1973, revised 1989). It classifies a rock mass by giving a rating from 0 to 100 from six parameters, to estimate stand-up time, support requirement, cohesion and friction angle of the rock mass. It is widely used in tunnelling, slopes and foundations.

Parameters and ratings (Bieniawski 1989)

The first five are summed to give the basic RMR, and the sixth is the adjustment for orientation of joints relative to the engineering work (very favourable to very unfavourable).

Parameter notes

1. Intact strength - higher UCS gives a higher rating (>250 MPa gives 15).
2. RQD - percentage of core pieces longer than 10 cm in a run; higher RQD gives a higher rating.
3. Spacing - wider spacing of joints gives a higher rating (>2 m gives 20).
4. Condition - tight, rough, unweathered, unfilled joints give a high rating; soft gouge, open or continuous joints a low rating.
5. Ground water - dry rock gets the maximum rating; flowing water gets zero.
6. Orientation - favourable direction needs no correction; unfavourable joints reduce the rating.

Rock mass classes

Each class gives an average stand-up time, cohesion (kPa) and friction angle of the rock mass.

Apparent dip is the inclination of a bed measured in a vertical section that is not perpendicular to the strike. It is always smaller than the true dip.

Formula

Let $\delta$ be the true dip, and $\alpha$ be the horizontal angle between the strike line and the direction (section line) in which the apparent dip $\delta_a$ is wanted.

Derivation

        strike
   A --------------- B
    \      |       /
     \     | h(depth)
      \    |
       C ------- D
   true dip: CD is perpendicular to strike

Take a point on the strike line at height 0 and go a horizontal distance $x$ in the apparent-dip direction. The bed falls by $h$.

• Along the true-dip direction (perpendicular to strike): the horizontal distance is $d = x\sin\alpha$ for the same fall $h$, so $\tan\delta = h/d$.
• Along the apparent-dip direction: $\tan\delta_a = h/x$.
• Hence $\tan\delta_a = \tan\delta \cdot d/x = \tan\delta \sin\alpha$.

Procedure

1. Measure strike and true dip from the compass.
2. Measure the bearing of the section line and find $\alpha$ = difference between the two bearings (use the acute angle).
3. Apply the formula and take the inverse tangent.

Example

True dip = 30°, section line makes $\alpha = 45°$ with the strike.

Check: for $\alpha = 90°$, $\delta_a = \delta$ (true dip); for $\alpha = 0°$ (along the strike), $\delta_a = 0°$.

Relationship between strike and dip

• The strike is the bearing of the horizontal line on an inclined plane, and dip is the maximum inclination of the plane.
• The dip direction is always at right angles (90°) to the strike. So when the strike is known, the dip direction is strike ± 90° (the side must be seen from the field), and when the dip direction is measured, strike = dip direction ± 90°.
• Example: strike N30°E, dip 40° towards SE (azimuth 120°).

      N      strike
      |    /
      |  /
  ----+/------ E
     /|  \
        dip direction (90 deg to strike)

Apparent dip from the true dip

Apparent dip is the inclination of the bed measured along a vertical section that makes an angle $\alpha$ with the strike. It is calculated by:

where $\delta$ = true dip, $\delta_a$ = apparent dip, $\alpha$ = angle between the strike and the section line.

Example: $\delta = 30°$, $\alpha = 45°$ gives $\tan\delta_a = 0.5774 \times 0.7071 = 0.4082$, so $\delta_a = 22.2°$. The apparent dip is always less than the true dip, and is zero along the strike.

A joint is a fracture or crack in a rock along which there has been no (or negligible) displacement of the two sides.

Significance of joints

• Reduce strength and bearing capacity of the rock mass; blocks bounded by joints slide or fall.
• Control slope stability (plane, wedge and toppling failures) in cuts and dam abutments.
• Give paths for seepage, so reservoirs leak and tunnels get water inflow.
• Weathering is faster along joints.
• In tunnels they cause overbreak and roof falls; the tunnel axis should be at a high angle to the main joint set.
• Help quarrying of blocks, but close joints give small blocks.
• Grouting, rock bolts and shotcrete are used for treatment.

Significance of faults

• Weak zone: crushed, sheared rock (breccia, gouge) has low strength, high compressibility and high permeability, so foundations may settle or fail.
• Leakage: a fault zone acts as a channel for water, causing seepage under dams and reservoir leakage.
• Tunnelling: sudden inflow of water, roof collapse, squeezing and heavy support requirement when a tunnel crosses a fault.
• Slope instability: a fault plane dipping out of the slope gives a ready sliding surface for landslides and rock slides.
• Differential settlement: a structure built across a fault with unequal rock properties on two sides.
• Seismic hazard: active faults can move again and cause earthquakes and surface rupture, so dams, bridges and buildings must keep away.
• Uncertainty in exploration: displaced or missing beds make the correlation of boreholes and the estimation of rock depth unreliable.

Structural geology is the branch of geology that deals with the study of the geometry, form, origin and relationships of rock structures produced by deformation of the earth's crust, such as folds, faults, joints and unconformities.

A geological plane is a more or less flat surface in a rock mass that separates rocks of different properties or along which the rock can break or move. The common geological planes are:

1. Bedding plane - the surface that separates two successive beds of sedimentary rock.
2. Joint plane - the surface of a fracture along which there has been no movement.
3. Fault plane - the surface of a fracture along which the rock has moved.
4. Foliation / cleavage plane - planes of parallel alignment of minerals in metamorphic rocks.
5. Axial plane - the imaginary plane that divides a fold into two nearly equal halves.
6. Unconformity plane - an erosion surface separating younger and older rocks.

  ------------------  bedding plane
  ------------------
  ====/============   fault plane (with movement)
     /   |  |  |      joint plane (no movement)

The orientation of every plane is expressed by its attitude: strike (bearing of the horizontal line on the plane) and dip (maximum inclination, measured perpendicular to the strike).

        horizontal plane
        ______________
       /   strike    /|
      /____________ /_|  dip angle
     inclined plane

Dip amount

The dip amount (angle of dip) is the maximum angle that an inclined geological plane makes with the horizontal plane. It is measured in a vertical plane at right angles to the strike, with a clinometer. It ranges from 0° (horizontal) to 90° (vertical). The direction towards which the plane goes down is the dip direction.

Strike

The strike is the direction (compass bearing) of the horizontal line drawn on an inclined plane, i.e. the line of intersection of that plane with a horizontal plane. It is always perpendicular to the dip direction, for example N30°E.

Field identification of fold

• Repetition of beds in reverse order on both sides of an axis (A B C D C B A).
• Opposite dips on two sides of a line (axis), either converging (syncline) or diverging (anticline).
• Outcrop pattern on a map: a series of curved or zig-zag outcrops (V-shaped patterns) with plunge.
• Age relation: in an anticline older beds are in the core; in a syncline younger beds are in the core.
• Drag folds and minor folds in the small scale, and variation of strike or dip along a traverse.
• Topography: ridges and valleys following the folded beds, and plunging noses on hill slopes.
• Cleavage or joints (fan or radiating pattern) related to the axial plane.

True dip

The true dip is the maximum inclination of a bed to the horizontal, measured in a direction perpendicular to the strike.

Apparent dip

The apparent dip is the inclination of the bed measured along any direction other than the true dip direction. It is smaller than the true dip and is related by $\tan \delta_a = \tan \delta \sin\alpha$, where $\alpha$ is the angle between the strike and the section line.

Why apparent dip is needed in a cross-section

A geological cross-section is a vertical slice along a chosen line. The bed in this slice shows only the dip that exists in the direction of the section line.

• If the section line is perpendicular to the strike, the section shows the true dip.
• If the line is oblique to the strike, the bed in the section has the apparent dip. Drawing the true dip there would make the bed too steep, so the thickness, depth and position of the bed at depth would be wrong.

 Plan:        strike
        ----------------
          \ alpha |
           \      | true dip direction
 section    \     v
 line  -------> (apparent dip direction)

 Section along oblique line:
   true dip 40 deg (wrong)  /|
   apparent dip 22.8 (correct)/__|

Illustration by numbers

True dip = 40°, section line at $\alpha = 30°$ to the strike.

If the true dip of 40° were drawn, the bed would reach a depth much greater than the real depth, so a wrong depth of the rock bed is predicted for a tunnel or foundation. For $\alpha = 60°$ the apparent dip is 36.0°, and for $\alpha = 90°$ it equals the true dip (40°).

Hence, for oblique sections the apparent dip must be calculated (or taken from a chart or the stereographic net) before drawing the beds.

The strike is the direction of the horizontal line on the bed and it is always at 90° to the dip direction.

Given: Limestone bed dips towards S55°E. Dip direction = S55°E, azimuth = 125°.

These two azimuths (35° and 215°) are the two ends of one line, so the strike is N35°E (S35°W).

              N
              |
              |   strike line (N35E)
              |  /
     W -------+-/------ E
              |/
              |  dip direction (S55E)
              S

Answer: Strike = N35°E (S35°W).

Attitude of a rock

The attitude of a rock is its orientation in space, given by the strike (bearing of the horizontal line on the bed) and dip (maximum inclination, with dip direction and dip amount). For lines it is trend and plunge.

Strike calculation

Given: dip amount = 70°, dip direction = S20°E, azimuth = $180° - 20° = 160°$.

So the strike is N70°E (same line as S70°W).

              N
              |
              |   strike line (N70E)
              |  /
     W -------+-/------ E
              |/
              |  dip direction (S20E)
              S
```The dip amount (70°) does not change the strike; the strike depends only on the dip direction.

**Answer: Strike = N70°E; attitude = N70°E / 70° SE.**

The strike is the direction of the horizontal line on the bed and it is always at 90° to the dip direction.

Given: Quartzite bedrock dips 80° towards N10°E. Dip direction = N10°E, azimuth = 10°.

These two azimuths (280° and 100°) are the two ends of one line, so the strike is N80°W (S80°E).

              N
              |
              |   strike line (N80W)
              |  /
     W -------+-/------ E
              |/
              |  dip direction (N10E)
              S

Answer: Strike = N80°W (S80°E).

The strike is the direction of the horizontal line on the bed and it is always at 90° to the dip direction.

Given: Quartzite bed directed (dips) towards S20°W with a dip of 45°. Dip direction = S20°W, azimuth = 200°.

These two azimuths (110° and 290°) are the two ends of one line, so the strike is S70°E (N70°W).

              N
              |
              |   strike line (N70W)
              |  /
     W -------+-/------ E
              |/
              |  dip direction (S20W)
              S

Answer: Strike = S70°E (N70°W).

The bed is inclined towards the east, so the dip direction is due east (azimuth 90°). Strike is at 90° to the dip direction.

Hence the strike is north-south (N-S), written N00°E. The bed strikes N-S and dips 45° towards the east.

            N
            |  strike (N-S)
            |
     W -----+------> E  dip direction
            |
            S

Answer: Strike = N-S (0°/180°).

The strike is the direction of the horizontal line on the bed and it is always at 90° to the dip direction.

Given: Limestone bedding plane, dip direction N40°W, dip amount 64°. Dip direction = N40°W, azimuth = 320°.

These two azimuths (50° and 230°) are the two ends of one line, so the strike is N50°E (S50°W).

              N
              |
              |   strike line (N50E)
              |  /
     W -------+-/------ E
              |/
              |  dip direction (N40W)
              S

Answer: Strike = N50°E (S50°W).

The dip direction is at 90° to the strike, so it is found by adding or subtracting 90° from the strike.

Given: strike = N55°E (azimuth 55°), dip amount = 30°.

The strike alone does not say which of the two sides is lower; the dip direction is either S35°E or N35°W. Strike and dip direction are perpendicular (55° + 35° = 90°). In the field, the side is decided by looking at which way the bed goes down (or from the geological map). If the bed dips towards the south-east side, the dip direction is S35°E.

             N
         N35W |     strike N55E
              |   /
        \     |  /
         \    | /
    W ----\---+/------- E
           \  /\
            \/  \
            S35E  <- dip direction (alternative: N35W)

Answer: Dip direction = S35°E (if dipping to the south-east) or N35°W (if dipping to the north-west); dip amount 30°.

The strike is the direction of the horizontal line on the bed and it is always at 90° to the dip direction.

Given: Dip direction N40°W. Dip direction = N40°W, azimuth = 320°.

These two azimuths (50° and 230°) are the two ends of one line, so the strike is N50°E (S50°W).

              N
              |
              |   strike line (N50E)
              |  /
     W -------+-/------ E
              |/
              |  dip direction (N40W)
              S

Answer: Strike = N50°E (S50°W).

The strike is the direction of the horizontal line on the bed and it is always at 90° to the dip direction.

Given: Limestone bedrock dips towards N31°W. Dip direction = N31°W, azimuth = 329°.

These two azimuths (59° and 239°) are the two ends of one line, so the strike is N59°E (S59°W).

              N
              |
              |   strike line (N59E)
              |  /
     W -------+-/------ E
              |/
              |  dip direction (N31W)
              S

Answer: Strike = N59°E (S59°W).

Classification of folds

1. On the basis of convexity: anticline (upfold, older beds in core), syncline (downfold, younger beds in core), monocline.
2. On the basis of axial plane: symmetrical (upright, vertical axial plane), asymmetrical (inclined), overturned (both limbs dip the same way), recumbent (horizontal axial plane), isoclinal (parallel limbs).
3. On the basis of plunge: non-plunging (horizontal axis) and plunging (inclined axis) folds.
4. On the basis of shape: open, closed, tight, chevron, box, dome and basin.

Strike

The strike is the bearing of the horizontal line on an inclined plane (line of intersection of the plane with a horizontal plane). It is perpendicular to the dip direction.

Relation between apparent dip and true dip

The apparent dip is the dip measured in a direction oblique to the strike. It is always smaller than the true dip:

where $\alpha$ is the angle between the strike and the direction of apparent dip. When $\alpha = 90°$, $\delta_a = \delta$; when $\alpha = 0°$, $\delta_a = 0°$.

The strike is the direction of the horizontal line on the bed and it is always at 90° to the dip direction.

Given: Limestone bed directed (dipping) towards N20°E with a dip of 45°. Dip direction = N20°E, azimuth = 20°.

These two azimuths (290° and 110°) are the two ends of one line, so the strike is N70°W (S70°E).

              N
              |
              |   strike line (N70W)
              |  /
     W -------+-/------ E
              |/
              |  dip direction (N20E)
              S

Answer: Strike = N70°W (S70°E).

Method: the top (and bottom) of the fractured bed at three non-collinear points define a plane. The plane is solved by the three-point (strike-line) method, using the altitude at each borehole as the elevation of the bed.

Data (altitudes above M.S.L.): B at origin; A is 500 m due north of B; C is 400 m due east of B.

   A (300)          N
   |                |
   | 500 m
   B (280) ------ C (310)
        400 m

Step 1: Slope of the top surface

• A is higher than B by $300 - 280 = 20$ m over 500 m, so the northward rise is $20/500 = 0.04$.
• C is higher than B by $310 - 280 = 30$ m over 400 m, so the eastward rise is $30/400 = 0.075$.

Step 2: Dip amount

Step 3: Dip direction and strike

The bed rises to the north and to the east, so it goes down towards the south-west. Dip direction azimuth:

That is S61.9°W. The strike is at 90° to it: $241.93° - 90° = 151.93°$, i.e. N28.1°W.

(Strike-line check: the 300 m contour on the top surface passes through A. On line AB the 280 m level is at B, so the 310 m level lies beyond A; the 310 m contour passes C. Elevation 300 m on BC is at $400 \times 20/30 = 266.7$ m east of B. Joining A (0, 500) to (266.7, 0) gives a line with azimuth $\arctan(266.7/500)$ from north, i.e. $28.1°$ west of north, confirming N28.1°W.)

Step 4: Altitude of the fractured bed

Top of the fractured bed: 300 m at A, 280 m at B, 310 m at C. Bottom: 310 m, 290 m, 320 m. The bottom surface has the same attitude, and the bed is vertically 10 m thick, so its true thickness is $10\cos 4.86° = 9.96$ m (about 10 m).

The altitude of the fractured bedrock at any point $(x, y)$ east and north of B is:

Answer: The fractured, jointed bed lies between 280 - 290 m at B, 300 - 310 m at A and 310 - 320 m at C (above M.S.L.), with attitude strike N28.1°W, dip 4.9° towards S61.9°W (a gently dipping bed, thickness about 10 m).

Method: the three-point problem. Three points at known altitudes define a plane. Find a point on the line AB with the same altitude as C; the line joining it to C is a strike line (horizontal line on the bed).

Given: B is the origin. A is 600 m north of B (500 m). C is 300 m east of B (400 m). B is at 400 m.

        A (500 m)
        |
        | 600 m
        |
        B (400 m) -------- C (400 m)   [300 m east]

Step 1: Strike line

Along AB the altitude changes uniformly from 400 m (B) to 500 m (A), a rise of 100 m in 600 m, i.e. 0.1667 m per m. The point D on AB with altitude 400 m (equal to C) is at a distance from B of

Line DC joins two points of altitude 400 m, so it is the strike line. DC runs from D (0, 0.0) to C (+300, 0).

Step 2: Strike direction

Taking east as +x and north as +y, C is at x = +300. Slope components: northward rise $b = (500-400)/600 = 0.1667$ and eastward rise $a = (400-400)/(+300) = 0.0000$.

Since D coincides with B, the line BC itself is the strike line and runs due east-west, so the strike is E-W (N90°E).

Cross-check from the slopes (altitude gain per metre): northward b = 0.1667, eastward a = 0.0000; the strike line is perpendicular to the gradient (a, b), giving azimuth 90.00°.

Step 3: Dip direction and dip amount

The bed is highest at A and lowest at B, so it dips away from the gradient direction (down the slope), at 90° to the strike. Dip direction azimuth = 180.00°, which is due south.

Answer: Strike E-W (N90°E); dip direction due south; dip amount 9.5°.

Folds

A fold is a bend or wavy curvature of originally flat rock layers (beds), produced by compressive stress acting on plastic rocks over a long time.

        axial plane
            |
         ___|___
       /    |    \
 _____/     |     \_____  anticline (crest)
 limb       hinge   limb

Recognition of folds in the field

• Repetition of beds in reverse order on both sides of an axis (A B C D C B A).
• Opposite dips on two sides of a line (axis), either converging (syncline) or diverging (anticline).
• Outcrop pattern on a map: a series of curved or zig-zag outcrops (V-shaped patterns) with plunge.
• Age relation: in an anticline older beds are in the core; in a syncline younger beds are in the core.
• Drag folds and minor folds in the small scale, and variation of strike or dip along a traverse.
• Topography: ridges and valleys following the folded beds, and plunging noses on hill slopes.
• Cleavage or joints (fan or radiating pattern) related to the axial plane.

Influence of folds in civil engineering

• Foundation: fold limbs dip in different directions, so a dam or building on a fold must consider dip of beds. Dip upstream is favourable for dams; dip downstream with weak beds is dangerous.
• Tunnelling: in an anticline the roof is in tension (more joints, loose blocks, but water drains away); in a syncline the water collects and rock pressure is high. The crest and trough zone are more fractured.
• Reservoir: syncline can hold ground water, so leakage is low; anticline, if cut by a valley, may let water escape along dipping beds.
• Slope stability: beds dipping in the slope direction at an angle less than the slope can slide.
• Water supply: synclines form artesian basins; anticlines give petroleum and gas traps.
• Quarrying and exploration: thickness of a bed repeats on limbs, so outcrop pattern must be understood to estimate rock depth.

• Foundation and dams: dip direction of the limbs decides stability; beds dipping upstream are favourable for dams, while beds dipping downstream, with weak layers, may slide.
• Tunnels: crest of an anticline is fractured and loosened (tension), while a syncline trough collects water and has high rock pressure; the best tunnel lines cross the fold axis at a high angle.
• Reservoirs: a syncline holds water well, but an anticline can leak along the dipping limbs.
• Slopes: beds dipping out of the slope produce landslides.
• Water and oil: synclines form artesian basins; anticlines are traps for oil and gas.

   Anticline        Syncline
     ___              
   /     \       \         /
  /       \       \_____/
 limbs dip away    limbs dip towards

 Overturned         Recumbent
    /\                _______
   /  \_             /_______\_
  /   /               \_______/

A fault is a fracture in the rock mass along which the two blocks have moved relative to each other, parallel to the fracture surface. The displacement may be a few millimetres to hundreds of kilometres.

A fault is a fracture in the rock mass along which the two blocks have moved relative to each other, parallel to the fracture surface. The displacement may be a few millimetres to hundreds of kilometres.

               Fault plane
        FW (hanging wall)\
   ____________          \ ____________
  |            |          \|            |
  |            |           \   Foot wall |
  |____________|____________\____________|

Parts: fault plane, hanging wall (above), foot wall (below), dip, throw and heave.

Importance in civil engineering

• Weak zone: crushed, sheared rock (breccia, gouge) has low strength, high compressibility and high permeability, so foundations may settle or fail.
• Leakage: a fault zone acts as a channel for water, causing seepage under dams and reservoir leakage.
• Tunnelling: sudden inflow of water, roof collapse, squeezing and heavy support requirement when a tunnel crosses a fault.
• Slope instability: a fault plane dipping out of the slope gives a ready sliding surface for landslides and rock slides.
• Differential settlement: a structure built across a fault with unequal rock properties on two sides.
• Seismic hazard: active faults can move again and cause earthquakes and surface rupture, so dams, bridges and buildings must keep away.
• Uncertainty in exploration: displaced or missing beds make the correlation of boreholes and the estimation of rock depth unreliable.

A joint is a fracture or crack in a rock along which there has been no (or negligible) displacement of the two sides.

• Reduce strength and bearing capacity of the rock mass; blocks bounded by joints slide or fall.
• Control slope stability (plane, wedge and toppling failures) in cuts and dam abutments.
• Give paths for seepage, so reservoirs leak and tunnels get water inflow.
• Weathering is faster along joints.
• In tunnels they cause overbreak and roof falls; the tunnel axis should be at a high angle to the main joint set.
• Help quarrying of blocks, but close joints give small blocks.
• Grouting, rock bolts and shotcrete are used for treatment.

Genetic classification classifies a structure by the way it was formed (origin and nature of movement or stress).

Genetic classification of faults

By relative movement (genetic classification)

1. Normal fault - hanging wall moves down relative to the foot wall; caused by tension.
2. Reverse fault - hanging wall moves up relative to the foot wall; caused by compression. A reverse fault with a dip less than 45 degrees is a thrust fault.
3. Strike-slip (lateral) fault - blocks move horizontally along the strike of the fault; caused by shearing. May be right-lateral (dextral) or left-lateral (sinistral).
4. Oblique-slip fault - movement has both dip-slip and strike-slip components.
5. Rotational (hinge/pivotal) fault - one block rotates about a point.
6. Horst and graben - horst is a block raised between two normal faults, graben is a block dropped between them.#### Genetic classification of joints
7. Tension joints - formed by tensile stress (stretching), usually open and irregular.
8. Shear joints - formed by shear stress, in conjugate sets, smooth and closed.
9. Compression (release) joints - formed from compression and release of pressure after the removal of overburden; unloading or sheeting joints.
10. Cooling (contraction) joints - formed in igneous rocks during cooling, such as columnar joints in basalt.
11. Mural joints - cubic joints in granite from cooling and unloading.
12. Tectonic joints - related to folding and faulting, as longitudinal, transverse or oblique joints.

A fault is a fracture in the rock mass along which the two blocks have moved relative to each other, parallel to the fracture surface. The displacement may be a few millimetres to hundreds of kilometres.

Parts of a fault

• Fault plane - the fracture surface along which movement occurs.
• Hanging wall - block lying above the fault plane.
• Foot wall - block lying below the fault plane.
• Dip of fault - angle between the fault plane and the horizontal.
• Hade - angle between the fault plane and the vertical (90 degrees minus dip).
• Throw - vertical displacement; heave - horizontal displacement.
• Net slip - actual relative displacement along the fault plane.
• Fault line (trace) - intersection of the fault plane with the ground surface.

  hanging wall
   ________    fault plane
  |        |  /
  |        | /  <- dip
  |________|/_________
            /   foot wall
   throw (vertical), heave (horizontal)

Recognition in the field

Direct evidences

• Displacement or offset of beds, dykes, veins, streams, roads or ridges.
• Repetition or omission (missing) of beds in a sequence.
• Slickensides (polished, grooved surfaces) and striations on the fault plane showing the direction of movement.
• Fault breccia, fault gouge or mylonite along the fault zone.
• Drag folds (bending of beds near the fault plane).

Physiographic (topographic) evidences

• Fault scarps (straight steep cliffs) and triangular facets.
• Linear valleys, straight stream courses and sudden changes in stream direction.
• Offset or beheaded streams, waterfalls and rapids along a straight line.
• Line of springs and seepages, and a sharp line of vegetation change.
• Alignment of sag ponds, lakes and hot springs.
• Abrupt change of rock type across a straight line, and earthquake epicentres in a line.

Engineering problems

• Weak zone: crushed, sheared rock (breccia, gouge) has low strength, high compressibility and high permeability, so foundations may settle or fail.
• Leakage: a fault zone acts as a channel for water, causing seepage under dams and reservoir leakage.
• Tunnelling: sudden inflow of water, roof collapse, squeezing and heavy support requirement when a tunnel crosses a fault.
• Slope instability: a fault plane dipping out of the slope gives a ready sliding surface for landslides and rock slides.
• Differential settlement: a structure built across a fault with unequal rock properties on two sides.
• Seismic hazard: active faults can move again and cause earthquakes and surface rupture, so dams, bridges and buildings must keep away.
• Uncertainty in exploration: displaced or missing beds make the correlation of boreholes and the estimation of rock depth unreliable.

A fault is a fracture in the rock mass along which the two blocks have moved relative to each other, parallel to the fracture surface. The displacement may be a few millimetres to hundreds of kilometres.

Causes of faulting

• Tectonic stress (compression, tension, shear) exceeding the rock strength.
• Plate movements, uplift and subsidence of crustal blocks.
• Isostatic adjustment, volcanic activity and magma intrusion.
• Release of strain energy during earthquakes; also gravity sliding.

Classification

By relative movement (genetic classification)

1. Normal fault - hanging wall moves down relative to the foot wall; caused by tension.
2. Reverse fault - hanging wall moves up relative to the foot wall; caused by compression. A reverse fault with a dip less than 45 degrees is a thrust fault.
3. Strike-slip (lateral) fault - blocks move horizontally along the strike of the fault; caused by shearing. May be right-lateral (dextral) or left-lateral (sinistral).
4. Oblique-slip fault - movement has both dip-slip and strike-slip components.
5. Rotational (hinge/pivotal) fault - one block rotates about a point.
6. Horst and graben - horst is a block raised between two normal faults, graben is a block dropped between them.

 NORMAL                 REVERSE
  \ ____                    ____/
   \    |__          ___|    /
 ____\__    |      __|  /___/
      \ |___|        \/
 hanging wall down   hanging wall up
```#### Effects of fault on strength of slope
- The rock in the fault zone is crushed (gouge, breccia) and weak, so the slope strength is reduced.
- A fault plane dipping out of the slope is a ready-made sliding surface, leading to planar or wedge failure.
- Fault zones carry water, which increases pore pressure and reduces shear strength.
- Differential weathering along the fault gives deep weathered zones.
- The movement may be renewed by earthquakes, triggering landslides.

Genetic classification of faults

By relative movement (genetic classification)

1. Normal fault - hanging wall moves down relative to the foot wall; caused by tension.
2. Reverse fault - hanging wall moves up relative to the foot wall; caused by compression. A reverse fault with a dip less than 45 degrees is a thrust fault.
3. Strike-slip (lateral) fault - blocks move horizontally along the strike of the fault; caused by shearing. May be right-lateral (dextral) or left-lateral (sinistral).
4. Oblique-slip fault - movement has both dip-slip and strike-slip components.
5. Rotational (hinge/pivotal) fault - one block rotates about a point.
6. Horst and graben - horst is a block raised between two normal faults, graben is a block dropped between them.#### Physiographic evidences to recognise faults

• Fault scarps (straight cliffs) and triangular facets on a hillside.
• Linear valleys, straight stream courses and sudden bends in streams.
• Offset or beheaded streams, ridges and spurs; waterfalls and rapids in a line.
• A line of springs, seepages, sag ponds and hot springs.
• Abrupt change in vegetation or topography along a straight line.
• Sudden change in rock type, and alignment of earthquake epicentres.

Why documentation of faults is essential

• To locate weak, crushed zones and avoid them in selecting dam, tunnel and bridge sites, or to design the treatment.
• To find active faults that can cause earthquakes and ground rupture, for seismic hazard design.
• To correlate beds and rock depth correctly in borehole logs, since faults repeat or omit beds.
• To predict seepage and water inflow in reservoirs and tunnels.
• To assess slope stability and plan support, drainage and grouting.
• Documentation is also needed for the design cost estimate and for future maintenance.

Definitions

• Fold: a bend or wavy curvature of originally flat rock layers caused by compression, such as an anticline or syncline.
• Fault: a fracture in the rock along which the two blocks have displaced relative to each other.
• Joint: a fracture in the rock along which there has been no (or negligible) displacement.
• Thrust: a low-angle (<45°) reverse fault where the hanging wall moves up over the foot wall, bringing older rocks over younger.

Significance in tunnelling

Fold

A fold is a bend or wavy curvature of originally flat rock layers (beds), produced by compressive stress acting on plastic rocks over a long time.

   anticline        syncline
     ___          \       /
   /     \          \___/

Fault

A fault is a fracture in the rock mass along which the two blocks have moved relative to each other, parallel to the fracture surface. The displacement may be a few millimetres to hundreds of kilometres.It has a fault plane, hanging wall and foot wall; types are normal, reverse and strike-slip.

Joint

A joint is a fracture along which there is no appreciable displacement. Joints occur in sets and systems and may be tension, shear, or cooling joints.

Attitude

The attitude of a geological structure is its orientation in space, expressed by the strike (direction of the horizontal line on the plane) and dip (maximum inclination at 90° to strike, with dip direction and dip amount). For lines the attitude is the trend and plunge. Example: N30°E / 40° SE.

• Dams and reservoirs: an active fault can rupture and break the dam; fault zones give leakage paths and weak foundation, so sites on or near a fault are avoided or require major treatment.
• Tunnels and portals: crushed fault zones cause roof collapse, squeezing and inflow of water.
• Slopes and buildings: faults dipping out of a slope give sliding; buildings should not straddle a fault because of differential settlement and seismic risk.
• Bridges and roads: fault displacement can damage piers and alignments.
• Alternatives: site should be moved away from the fault trace (a setback zone of tens to hundreds of metres), or the foundation must be excavated and treated by concrete plugs and grouting.

An unconformity is a surface of erosion or non-deposition that separates younger strata from older rocks and represents a gap (break) in the geological record.

Stages in the formation of an unconformity

1. Deposition of older sedimentary beds in water.
2. Uplift / tilting or folding of the beds by earth movements, raising them above water level.
3. Erosion of the exposed surface so that an uneven surface is formed, representing the time gap.
4. Subsidence of the area below water again.
5. Deposition of new, younger beds over the eroded surface. The surface between old and young beds is the unconformity.

 1 Deposition   2 Uplift/tilt    3 Erosion      4-5 Subsidence +
 ========       ///////         _/\_/\_         new deposition
 ========       ///////        //////        ===============
 ========       ///////                      ~~unconformity~~
                                             //////// old

Engineering significance

• A basal conglomerate or weathered, soft layer at the unconformity is a plane of weakness and can cause sliding.
• The surface is generally uneven and weathered, so foundation depth and rock quality change suddenly.
• Seepage: water can move along the unconformity plane, so dams and tunnels may leak.
• Beds above and below have different strength, attitude and permeability, so differential settlement and stability problems arise.
• Wrong correlation of boreholes and rock depth, since beds are missing.
• Slope instability: sliding of younger beds over older tilted beds.

An unconformity is a surface of erosion or non-deposition that separates younger strata from older rocks and represents a gap (break) in the geological record.

Recognition in the field

• A break in the sequence: the rocks above and below have different attitude (angular unconformity) or different rock types and ages.
• An irregular, uneven, weathered or eroded surface with a basal conglomerate or a layer of fragments from the older rocks just above the surface.
• Missing beds: absence of some formations or fossil zones that should be present.
• Difference in the degree of folding, metamorphism or deformation of the beds above and below.
• Presence of soil (palaeosol), weathered zone, or iron staining at the contact.
• For nonconformity, sedimentary beds directly over igneous or metamorphic rocks; fossils of different ages above and below.

Problems created

• A basal conglomerate or weathered, soft layer at the unconformity is a plane of weakness and can cause sliding.
• The surface is generally uneven and weathered, so foundation depth and rock quality change suddenly.
• Seepage: water can move along the unconformity plane, so dams and tunnels may leak.
• Beds above and below have different strength, attitude and permeability, so differential settlement and stability problems arise.
• Wrong correlation of boreholes and rock depth, since beds are missing.
• Slope instability: sliding of younger beds over older tilted beds.

Planes of discontinuity

A discontinuity is any plane of weakness or break in the continuity of a rock mass that has little or no tensile strength. Main types:

• Bedding planes in sedimentary rocks.
• Joints - fractures without displacement.
• Faults and shear zones - fractures with displacement.
• Foliation and cleavage planes in metamorphic rocks.
• Unconformity surfaces and contacts between rock types.
• Dykes and sills boundaries, and sheeting joints in massive igneous rocks.

Their orientation, spacing, persistence, aperture, roughness, infilling and water condition control the strength and permeability of the rock mass.

RMR system (Bieniawski)

Rock Mass Rating gives a rating (0 - 100) as the sum of rating for five parameters, then an adjustment for orientation:

1. Uniaxial compressive strength of the intact rock (0 - 15)
2. RQD (3 - 20)
3. Spacing of discontinuities (5 - 20)
4. Condition of discontinuities (0 - 30)
5. Ground water (0 - 15)
6. Orientation adjustment (0 to -12)

Classes: I very good (81 - 100), II good (61 - 80), III fair (41 - 60), IV poor (21 - 40), V very poor (<21). It is used to find the support and stand-up time.

Q system (Barton et al.)

The tunnelling quality index:

where $J_n$ = joint set number, $J_r$ = joint roughness number, $J_a$ = joint alteration number, $J_w$ = joint water reduction factor, SRF = stress reduction factor. The three quotients represent block size, inter-block shear strength and active stress. Q ranges from 0.001 to 1000: below 0.1 is very poor, 1 - 4 poor, 4 - 10 fair, 10 - 40 good, above 100 extremely good. Q is used to select support type in tunnels and caverns.

Rock mass classification is the grouping of a rock mass into classes according to its quality and engineering behaviour, based on the properties of the intact rock and of the discontinuities (joints, bedding, faults). It is used to estimate rock strength, stand-up time and support requirements in tunnels, slopes and foundations.

Common systems:

• RQD (Deere) - percentage of core pieces longer than 10 cm; classes from very poor (<25 %) to excellent (90 - 100 %).
• RMR (Bieniawski) - sum of ratings for strength, RQD, joint spacing, joint condition, ground water and orientation; five classes, 100 to below 21.
• Q system (Barton) - $Q = \frac{RQD}{J_n}\frac{J_r}{J_a}\frac{J_w}{SRF}$, used in tunnel support design.
• Terzaghi's rock load classification and the GSI (Geological Strength Index).

Rock Quality Designation (RQD) was introduced by Deere (1964) as a simple index of rock quality from drill core. It is the percentage of the total core run that is made up of sound core pieces longer than 10 cm (4 inches).

• Only natural fractures are counted; breaks caused by drilling or handling are ignored, and the core must be at least NX size (54 mm).
• If cores are not available, RQD can be estimated from the number of joints per cubic metre ($J_v$): $RQD = 115 - 3.3 J_v$.

Classification

Example: in a 150 cm run, pieces longer than 10 cm total 105 cm, so RQD = 105/150 × 100 = 70 % (fair rock).

Use: RQD is a parameter in the RMR and Q systems, and it helps in judging foundation and tunnel support. Its limitation is that it ignores joint orientation, condition and the thickness of the pieces beyond 10 cm.

Discontinuities are planes of weakness or breaks in a rock mass across which the rock has little or no tensile strength. They separate the rock material into blocks and control the strength, deformability and permeability of the mass.

Types of discontinuities

• Bedding planes: surfaces between sedimentary layers.
• Joints: fractures with no visible displacement; they occur in sets (joint systems).
• Faults: fractures with displacement; they often have gouge, breccia or crushed rock.
• Foliation and cleavage planes: planar fabric in metamorphic rocks (slate, schist, gneiss).
• Shear zones and fracture zones, and unconformities or contacts between rock types.

Characteristics (ISRM)

1. Orientation: the attitude, given by dip and dip direction (or strike). Together with the slope it decides if a block can slide.
2. Spacing: the distance between adjacent discontinuities of one set. Close spacing gives small blocks and a weak mass.
3. Persistence (continuity): the length of the discontinuity trace; a persistent plane makes a more continuous failure surface.
4. Roughness: unevenness of the surface; rough surfaces give higher shear strength.
5. Aperture: the open distance between the walls. A wide aperture means low strength and high permeability.
6. Filling (infill): material between the walls (clay, sand, calcite); clay filling reduces shear strength sharply.
7. Wall strength (weathering): strength of the rock at the wall surface.
8. Seepage (water flow): water in the discontinuities.
9. Number of sets and block size.

These characteristics are logged in the field by scanline or window mapping and are the input for rock mass classification (RMR, Q) and slope and tunnel design.

Rock material

Rock material (intact rock) is the solid rock between the discontinuities, a block of rock without any fractures, as in a laboratory core specimen. It is described by its rock type, grain size, mineral composition and the strength found from laboratory tests (UCS, point load).

Rock mass

Rock mass is the rock in the field as a whole: the intact rock plus the system of discontinuities that cut it. Its behaviour is controlled mostly by the discontinuities, so its strength is much lower than that of the intact rock.

   rock mass = intact rock blocks + discontinuities
   ____ ____ ____
  |    |    |    |   each block = rock material
  |____|____|____|   gaps/lines   = discontinuities

Characteristics of discontinuities

1. Orientation: the attitude, given by dip and dip direction (or strike). Together with the slope it decides if a block can slide.
2. Spacing: the distance between adjacent discontinuities of one set. Close spacing gives small blocks and a weak mass.
3. Persistence (continuity): the length of the discontinuity trace; a persistent plane makes a more continuous failure surface.
4. Roughness: unevenness of the surface; rough surfaces give higher shear strength.
5. Aperture: the open distance between the walls. A wide aperture means low strength and high permeability.
6. Filling (infill): material between the walls (clay, sand, calcite); clay filling reduces shear strength sharply.
7. Wall strength (weathering): strength of the rock at the wall surface.
8. Seepage (water flow): water in the discontinuities.
9. Number of sets and block size.

These characteristics are logged in the field by scanline or window mapping and are the input for rock mass classification (RMR, Q) and slope and tunnel design.