How to Create Mine Cross Sections
A plan view shows where things are. A section shows what shape they are. In an open pit the questions that actually need a section are geometric — how steep is this wall from toe to crest, how deep is the cut at this station, how has the face moved since last month — and each of them depends on choosing where the section is cut. This guide covers that choice, not the volume arithmetic that follows it.
What a section is cut from
A section is the intersection of a vertical plane with a surface. Draw a line across the terrain, sample the elevation along it, and the result is a profile of the ground under that line.
The useful sections carry more than one surface on the same frame: this month's survey against last month's, the survey against the design, or the survey against the original ground. A single profile describes a shape; two profiles on the same axes describe a change.
Where to cut them
Perpendicular to the wall. A section taken at an angle to the face reads a longer, shallower slope than the wall actually has — the geometry is projected. For any angle measurement the line has to be perpendicular to the strike of the face.
Along the haul road, not across it. A road is described by its longitudinal profile: the grade the trucks climb. A section across it only shows the cross-fall.
Through the deepest point, and through the shallowest. The extremes bound the answer. A section through the middle of a cut is representative of nothing in particular.
At fixed stations for anything that will be compared. If the section is going to be repeated next month, it has to be at the same chainage and the same orientation, otherwise the difference between the two includes the difference in where they were cut.
Spacing, when a run of sections is needed
Sections at a regular interval along an alignment are how earthwork quantities are computed in road and trench work, and the interval controls the accuracy. Wider spacing misses the variation between sections; the error is systematic, not random, and it grows where the ground changes fastest.
The practical rule is that the interval should be short enough that the ground between two sections is close to linear. On uniform ground that can be tens of metres; through a break in slope it needs to be much tighter, and a section should be placed at the break itself.
The volume methods that consume these sections — average end area, prismoidal — and the differences between them are covered separately in the guide on cross-section cubature.
What the section has to state to be usable
A section without its metadata cannot be checked by anyone else. The station or chainage, the orientation of the line, the vertical exaggeration if any, the date of each surface shown, and which surface is which.
Vertical exaggeration deserves particular care. Stretching the vertical scale makes a shallow feature readable, and it also makes every slope on the drawing steeper than it is. If the exaggeration is not written on the drawing, the next person will read angles straight off it and be wrong.
Frequently asked questions
Where should a cross section be cut in an open pit?
Perpendicular to the wall being measured, through the deepest and the shallowest points of the area of interest, and at fixed stations for anything that will be repeated. A section cut at an angle to the face reads a shallower slope than the wall actually has.
How far apart should sections be spaced?
Close enough that the ground between two of them is approximately linear. Uniform ground tolerates a wide interval; where the slope breaks, the interval has to tighten and a section should be placed at the break itself. Spacing that is too wide produces a systematic error, not a random one.
What is vertical exaggeration and why does it matter?
It is stretching the vertical scale relative to the horizontal so shallow features become readable. It also makes every slope on the drawing look steeper than it is, so the exaggeration factor has to be written on the section — otherwise angles read straight off the drawing will be wrong.
Can a section show how much a face has advanced?
Yes, if both surveys are drawn on the same section at the same station and orientation. The horizontal distance between the two profiles at a given elevation is the advance at that elevation. If the section line moved between the two dates, the difference includes that movement.
Sources
See how cross sections work in STREAM
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