Structural Geology

Record the orientation convention with every measurement. The right-hand rule is one common convention; dip direction and dip-and-direction notation avoid ambiguity when discontinuities are used in engineering analysis. Strike and the horizontal dip direction are perpendicular, while dip angle is measured in a vertical section normal to strike.

Here, azimuth increases clockwise from north: north is $000^\circ$, east $090^\circ$, south $180^\circ$, and west $270^\circ$. Under the right-hand rule (RHR), the dipping side is $90^\circ$ clockwise from the recorded strike. Thus RHR strike $045^\circ$ and dip $30^\circ$ describe dip direction $135^\circ$; strike $225^\circ$ would select the opposite dipping side. An undirected strike line alone does not distinguish these planes.

Record whether north is true, grid or magnetic, together with the measurement convention and any correction. Bearings referenced to different north directions must be reconciled before comparison.

• Anticline: a fold with older strata in its core.
• Syncline: a fold with younger strata in its core.
• Monocline: a step-like bend between more gently dipping strata.
• Dome and basin: approximately circular or elliptical upwarps and downwarps, respectively.

An upright anticline commonly has an antiform shape and an upright syncline a synform shape. Inverted or refolded successions can break that shape/age association. Establish stratigraphic younging from suitable field evidence, such as graded bedding or other reliable facing indicators.

Folded bedding may influence tunnel alignment, slope blocks, groundwater pathways or foundation continuity. A fold label does not establish joint persistence, water pressure, support demand or an inherently favorable alignment.

• Normal fault: the hanging wall moves down relative to the footwall in the dip direction of the fault.
• Reverse fault: the hanging wall moves up relative to the footwall. A thrust fault is a low-angle reverse fault; no single angular cutoff applies to every classification scheme.
• Strike-slip fault: the principal relative motion is horizontal and parallel to the fault strike; state the viewing convention when naming lateral sense.
• Joint sets: groups of similarly oriented discontinuities. Their spacing, persistence, infill, roughness, aperture, and water conditions help define block size and rock-mass behavior.

Fault cores and surrounding damage zones can affect strength, deformation and groundwater differently. A clay-rich core may impede flow while connected surrounding fractures conduct it. Investigate the actual materials and connectivity; a thrust-fault label does not prove expansive clay, bearing failure or a required treatment.

Joint aperture is opening normal to the surface; observable shear displacement along it supports a fault interpretation. Joint sets group similar orientations, while a joint system can include several sets. A mapped fault's geometry alone does not determine its activity or the timing of its last movement.

The hanging wall lies above an inclined fault surface; the footwall lies below it. For pure dip slip, normal motion lowers the hanging wall and reverse motion raises it. The relative displacement vector lies along the fault, so vertical throw is generally smaller than slip magnitude.

For strike slip, look across the fault: motion of the far block to your right is right-lateral; motion to your left is left-lateral. State the view and correlate displaced features before interpreting sense. Oblique slip combines components and needs a fuller displacement description.

Use a stated projection and hemisphere consistently. The great circle represents the plane; the pole represents its normal. Equal-angle stereographic and equal-area nets use different radial scales. The figure here is lower-hemisphere, equal-angle stereographic.

For a nonhorizontal plane dipping toward azimuth $D$, the lower-hemisphere pole trends opposite to $D$ and plunges $90^\circ-\delta$. Thus a plane dipping $45^\circ$ toward $135^\circ$ has pole trend $315^\circ$ and plunge $45^\circ$. A horizontal plane's pole is vertical, with no unique trend. A vertical plane's poles lie on opposite rim points; state the selected convention.

Plotting discontinuity sets with the slope-face orientation supports geometric comparison. It does not supply missing persistence, release surfaces, shear strength or groundwater data.

• Planar sliding: a persistent discontinuity dips toward and daylights in the slope face; its orientation must be compatible with the direction of possible sliding.
• Wedge sliding: two discontinuities intersect, and their line of intersection may daylight through the face.
• Toppling: steep discontinuities dip into the slope and may form columns or blocks that rotate outward.

A simple kinematic screen compares orientation and daylighting geometry. Friction, cohesion, persistence, water pressure, loads, block release, and the selected analysis method govern whether a geometrically possible mechanism is stable.

For an oblique intersection line, compare its plunge with the apparent slope-face dip in that line's trend, not automatically with the face's larger true dip. The line must point out of the face and satisfy the actual spatial release conditions. A friction-cone shortcut is not a complete two-plane wedge force analysis.

Any lateral direction window used in a simplified planar screen must be stated as an assumption or method criterion. Do not invent a universal angular tolerance or factor-of-safety requirement. A negative screen for one mechanism also leaves other mechanisms and uncertainty to be assessed.

Let $\beta$ be the acute horizontal angle from the strike line to the section direction. The section perpendicular to strike shows true dip; an oblique section shows apparent dip. Use the measured section azimuth and the same strike convention when reconstructing a geologic section.

The equation below gives a magnitude for a nonvertical plane. For a directed section, a trace descending toward its chosen azimuth has positive signed apparent dip; reversing the section direction reverses that sign. Treat strike as a line when finding acute $\beta$, including bearings that wrap through north.

The direction-limit formula assumes a straight alignment actually follows the plane, so its grade is the section trace's grade. It is not a general tunnel-design rule. If the permitted inclination equals or exceeds true dip, every direction meets that inclination constraint. A horizontal plane also meets any nonnegative inclination limit. Excavation, drainage, access, support and operating requirements still need independent assessment.

Two nonparallel section observations can supply information that a single exposure cannot. For the special case of sections directed east and north, define positive angles as descending east and north, respectively. The plane's downward gradients in those directions are the tangents of the measured angles. Their vector sum gives the steepest descent.

This construction assumes both measurements belong to the same planar feature and use a common horizontal datum, north reference and sign convention. It is not valid for combining unrelated joints or curved bedding. A horizontal plane has zero gradient in both sections and no unique dip direction.

• FHWA, Context Sensitive Rock Slope Design Solutions, Chapter 2 — Federal Highway Administration; structural mapping and rock-slope kinematic analysis.
• USGS, FGDC Digital Cartographic Standard for Geologic Map Symbolization — FGDC-STD-013-2006; planar, linear, fold and fault mapping conventions.
• USGS Bulletin 1206, Geology of the Bridgewater Quadrangle — page 49 distinguishes antiform/synform shape from inverted stratigraphic folds.
• USGS, General Information Product 16, Introduction — fault movement and the observer convention for lateral slip.
• FHWA, Technical Manual for Design and Construction of Road Tunnels — Civil Elements — FHWA-NHI-09-010; investigation and rock-tunnel ground characterization.