Wells and trenches are dug for laying cables and/or pipelines, making foundations, and underground maintenance work on cables and/or pipelines.
Risks
The collapse of banks and soil-retaining structures are among the greatest risks. Counter measures depend heavily on the space used, the composition of the soil, and groundwater levels. Leaks in chemical industry sites may result in gases and vapours that are heavier than air collecting in wells and trenches. The soil may also be contaminated.
Standards and guidelines
If a well or trench is deeper than 1 m, shoring, jacks or sheet pile walls must be used, or a safe and stable bank must be dug; see table.
The well or trench must have sufficient safe accesses and exits and/or escape routes, using ladders for example. Trenches wider than 80 cm must be equipped with proper bridges.
Smoking and other naked flames (e.g. welding burners) are prohibited close to wells and trenches with pipelines containing gas or other flammable substances, unless measures have been taken to ensure the exposed working area is and remains gas free.
Handrails or fencing at least 1 m high must be placed alongside work floors, platforms and other places where people work or walk, and in such a way that there is no risk of objects falling into the well or trench.
It is important to prevent those not involved or vehicles from ending up in the well or trench during the work. Proper fencing, and lighting if necessary, are very important.
Fencing in accordance with road sign legislation must be placed on and alongside public roads.
If there is contaminated soil or groundwater present, the necessary protective measures will have to be determined before the start of work.
|
Soil type |
Depth in m below ground level |
Bank not steeper than |
|
Sand or loam |
||
|
solid, untouched |
1.00 to 1.50 |
3.00:1.00 |
|
solid, untouched |
1.50 to 2.50 |
1.50:1.00 |
|
solid, untouched |
2.50 to 4.00 |
1.25:1.00 |
|
loose or agitated |
1.00 to 4.00 |
1.00:1.00 |
|
very solid, untouched |
1.00 to 1.50 |
vertical |
|
very solid, untouched |
1.50 to 2.50 |
2.00:1.00 |
|
very solid, untouched |
2.50 to 4.00 |
1.25:1.00 |
|
Clay |
||
|
solid, untouched |
1.00 to 1.50 |
vertical |
|
solid, untouched |
1.50 to 2.50 |
1.50:1.00 |
|
solid, untouched |
2.50 to 4.00 |
1.00:1.00 |
|
loose or agitated |
1.00 to 1.50 |
1.50:1.00 |
|
loose or agitated |
1.50 to 4.00 |
1.00:1.00 |
Table 1. The relationship between the type of soil in the work location, the depth of the well or trench, and the slope of the bank
Risk-limitation measures
Before digging starts, it must be checked if there are any gas pipelines, water pipelines or electric cables in or near the excavation location (via the Cable and Wiring Information Centre ‘Kabels en Leidingen Informatie Centrum – KLIC’). Industrial transport pipelines must also be taken into account. It is recommended that a test trench is dug by hand first.
On a case-by-case basis, an expert must determine and if necessary calculate in advance which measures against collapse are necessary, taking into account possible adverse effects, such as:
- water problems caused by the height of the groundwater or rain, frost, thaw or leaks;
- heavy loading in or alongside the trench or well due to storage of soil, material, equipment, adjacent buildings or a road;
- soil which is not homogeneous or has a layered structure;
- vibration, caused for example by a digger, pile driver, or heavy road traffic.
Banks and soil-retaining structures must be checked daily and/or after any suspension of work due to bad weather, and repaired if necessary.
Soil-retaining structures may only be removed when there is no more risk of soil collapsing.
Excavators and the like must be kept a distance from the trench or well that is sufficient to avoid any risk of collapse of the banks. Hydraulic excavators and draglines must therefore be set up with the chassis or caterpillars perpendicular to the longitudinal direction of the trench.
If the edge of a slope is a surfaced road, the hardened surface must be removed to a distance of at least 50 cm from the edge (unless closed shoring has been inserted which protrudes at least 5 cm above the road surface).
Strips at least 50 cm wide and free of soil and materials must be kept next to a slope that is deeper than 1 m (unless closed shoring has been inserted which protrudes at least 15 cm above the road surface).
Depending on the work, personal protective equipment must be worn (including helmet and safety shoes).
When laying or installing sewer pipelines, wells or formwork, the worker must not be under or in range of the pipe, well or formwork. Laying or installing these must be done in such a way that workers can stay at a safe distance. This can be done by using guide ropes or special hoisting tools (such as a pipe lifting hook).