To identify a coal outcrop, clear the weathered surface with a spade, measure the seam thickness at three points, and find its dip by measuring how far it drops over a known horizontal distance. A seam that drops 1 m over 5 m of ground lies under about 2 m of cover 10 m away from the outcrop, which sets how far you can follow it by open digging.
Part of Coal Surface Mining
Tracing a coal outcrop on the ground: cleaning it, measuring thickness and tilt, and converting those into the depth of cover along the line of the seam.
The Surface Of An Outcrop Lies About The Seam
Coal at the surface has been weathered by air and rain. It is dull, crumbly and often stained, and it burns worse than the same seam under cover. A true measurement of thickness and quality needs a clean face. Dig a trench into the bank until you reach firm, shiny, freshly exposed coal.
What You Need
- Spade and hand pick for cleaning the face
- A straight rod or tape marked in centimetres (Measuring Rods)
- A string line and a level or a plumb bob to read horizontal and vertical distances (Leveling)
- Stakes to mark where the seam crosses the ground
Method: Map The Seam In Three Measurements
Step 1: Clean a fresh face. Cut back into the bank until the coal is bright and unweathered. The weathered skin is thin; you should see the coal change from dull and powdery to firm.
Step 2: Measure thickness at three points. Measure the coal itself, from roof to floor, at three places along the outcrop. Note any bands of shale or clay inside the seam: these “partings” are not coal, and you subtract them. Thickness varies along a seam, so use the lowest of the three for planning.
Step 3: Find the dip. Stretch a level string horizontally from the seam’s top at the outcrop, and measure the vertical gap between the string and the seam where it meets the face a known distance away. Dip = vertical drop ÷ horizontal distance.
Step 4: Mark the strike. Strike is the line along which the seam stays at the same height, which runs along the contour of the hill. Stake it at intervals and you have drawn the line of outcrop.
Worked Example: How Far Can You Open-Cut Along The Dip?
Inputs: the seam drops 1 m over 5 m horizontally; the seam is 1 m thick; the parent article’s practical limit for hand work is rock overburden of about 2.6 m per metre of seam (derived in the open-cut article).
- Slope = 1 ÷ 5 = 0.2 (an angle of atan 0.2 = 11.3°).
- Cover at a horizontal distance x from the outcrop = 0.2 × x.
- Cover reaches the 2.6 m limit at x = 2.6 ÷ 0.2 = 13 m.
At 10 m from the outcrop the cover is 0.2 × 10 = 2.0 m. Beyond about 13 m down-dip, open-cut work stops paying and the choice is a drift or leaving the coal.
Common Mistakes
- Measuring thickness through the partings. Include a shale band and you overestimate coal tonnage.
- Using one measurement. Seams thicken and thin along their length.
- Forgetting that tilt is three-dimensional. Strike and dip are at right angles; dig along strike for even cover, down-dip for deepening cover.
Safety
A fresh trench in a bank is a trap
A cleaned face can slump onto the person at its foot. Cut it back to a slope or keep your trench shallower than 1.2 m, the support threshold used throughout this guide (see Excavation), and never stand in a trench with an overhanging lip.
What’s Next
- Learn next: Coal Types & Quality — judging the fresh coal you have exposed
- Combine with: Open-Cut Mining — digging it out from the surface