When digging a trench, attention to the following points will make subsequent mapping and sampling much safer and more convenient.
- Cut back both sides of the top of the trench for one bucket width and to a depth of 50–100 cm. This prevents loose unconsolidated sur face material from falling into the trench.
- Stack all topsoil and any loose surface material from the trench on one side of the opening; stack all bedrock material to the other side. This facilitates making a quick assessment of the trench material from the bedrock spoil heaps and will permit a bulk sample to be taken if required. When re-filling the trench (a normal environmental requirement) the spoil should be replaced in reverse order so that the topsoil is preserved on top.
- If the trench is deep (i.e. cannot be easily climbed into or out of) and more than 50 m long, provide an access ramp at its midpoint.
- Most trench wall collapses take place in the first few hours after digging or else after heavy rain. With deep trenches, it is therefore advisable to leave them for at least 24 h before entering and not to enter them immediately after rain.
- In any case, never enter a deep trench unless accompanied by another person who should remain outside the trench and be ready to provide assistance if necessary.
- Before entering any trench, but particularly an old one, walk it out along the surface to check for incipient wall cave-ins. If in any doubt, do not enter! There is generally plenty of information to be obtained from the spoil heaps along the trench edge; the walls of old trenches are often covered in grunge anyway and certainly not worth risking one’s life for.
Author: Roger Marjoribanks
