In areas where soil cover is thin, the location and testing of bedrock mineralisation is made relatively straightforward by the examination and sampling of outcrops. However in locations of thick soil cover such testing may involve a deep sampling program by pitting, trenching, or drilling. Pits and trenches can be a quick and cheap way of obtaining lithological, structural and assay information in areas of shallow cover. Pitting to depths of up to 30 m is feasible and, with trenching, forms the simplest and least expensive method of deep sampling but is much more costly below the water table. Drilling penetrates to greater depth but is more expensive and requires specialized equipment and expertise that may be supplied by a contractor. Despite their relatively shallow depth, pits and trenches have some distinct advantages over drilling in that detailed geological logging can be carried out and, if necessary, undisturbed samples collected.
1) Pitting: Pitting is usually employed to test shallow, extensive, flat-lying bodies of mineralization. An ideal example of this would be a buried heavy mineral placer. The main advantage of pitting over a pattern-drill programme on the same deposit is that pits are capable of providing a very large volume sample. Large sample sizes are necessary to overcome problems of variable grade distribution, which are a characteristic feature of such deposits. In areas where the ground is wet, or labour is expensive, pits are best dug with a mechanical excavator. Pits dug to depths of 3–4 m are common and with large equipment excavation to 6 m can be achieved. In wet, soft ground any pit deeper than 1 m is dangerous and boarding must be used. Diggers excavate rapidly and pits 3–4 m deep can be dug, logged, sampled, and re-filled within an hour. In tropical regions, thick lateritic soil forms ideal conditions for pitting and, provided the soil is dry, vertical pits to 30 m depth can be safely excavated.
2) Trenching: Trenches are usually employed to expose steep dipping bedrock buried below shallow overburden, and are normally dug across the strike of the rocks or mineral zone being tested. Excavation can be either by hand, mechanical digger, or by bulldozer on sloping ground. Excavated depths of up to 4 m are common.
SAFETY AND LOGISTICS IN TRENCHING
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 as shown in Fig.1. This prevents loose unconsolidated surface material from falling into the trench (and on to the head of any geologist below!).
- 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.