Trench excavation and backfilling depend on the following parameters:
– characteristics of the main (type of joint and diameter),
– nature of the soil (presence or absence of water),
– laying depth.
The laying recommendations given below are those usually prescribed for ductile iron pipes.
After making a thorough study of the type of environment, and obtaining agreement from the various Utilities (Telecom, Gas, Water ...), the contractor marks out on site the route and profile of the main to be laid, complying with the project as prescribed, and ensures that the proposals and the conditions under which they will be implemented are in accord.
Prepare for the breaking of road surfaces by pre-cutting the edges of the trench to avoid damaging the neighbouring areas. The width is slightly greater than the trench width. Excavation is usually carried out with a mechanical digger, suited to the pipe diameter, the type of ground and depth of laying.
Depends on the DN, the type of soil, depth of laying and the methods of shoring and compaction. Care is taken during the work:
– to stabilize the walls, either by battering or shoring,
– to clear the edges of the excavation of lumps of rock or clods of earth, to prevent them from falling,
– to deposit the excavated material so as to leave a 0.4 metre space between the pipe and the trench.
Fascicule 71 prescribes in article 47 that: “Trenches are prepared at every point to the depth indicated by the longitudinal profile. Unless otherwise specified, the normal trench depth is such that the depth of backfill above the crown of the pipe is not less than 1 meter” This depth is justified by the need to protect against frost damage.
Types of soils
Soils can be divided into three main classes, based on their cohesion:
Extremely cohesive, making excavation difficult but not precluding all possibility of collapse. Cracks are sometimes present, which can result in complete chunks falling.
- Friable soils
By far the most common. These exhibit a certain amount of cohesion, which allows them to hold together for a while during excavation. This cohesion can change very rapidly under the influence of factors already mentioned (water ingress, nearby equipment movement, etc.): collapse is possible.
- Non-cohesive ground
This is ground lacking any cohesion, such as dry sand, muds or freshly restored backfill. These collapse almost instantaneously. Special procedures are needed for working in this type of ground. Protection against the danger of collapse is therefore essential: either by sloping the trench sides backwards or by shoring the trench sides.
The precautions to be taken also depend on the situation (urban or rural), and the depth of laying.
Rarely used in urban situations because of the space needed, it consists of giving the walls an outward slope known as “the angle of slope”, which must be close to the internal friction angle of the soil. This angle varies with the type of soil.
There are numerous shoring techniques: it is important to study and adapt them before starting the work. Shoring must be used in cases prescribed in existing regulations, or, in general, when demanded by the nature of the ground.
The most common shoring techniques:
– prefabricated wooden panels (joined or single),
– wooden or metal sheeting,
– pile driven sheets.
Whichever technique is used, the earth pressure has to be taken into consideration. Panels must be capable of resisting a thrust over their whole height.
The trench bottom must be levelled to comply with the longitudinal profile of the main, and all stony protrusions or rubble must be eliminated. Ensure that the pipe rests on uniformly distributed soil. Joint holes need to be excavated for EXPRESS and anchored joints Ve, to facilitate assembly.
Presence of water: excavation must be from downstream to upstream, to allow the water to drain by itself from the trench bottom.
If the trench passes through ground flooded with water (water table) it may be necessary to remove the water from the trench by:
– pumping it out (directly from the trench or a sump at the side),
– dewatering with probes of filter wells.
Pipe bed, pipe surround and backfill
The trench bottom provides the pipe foundation. In cases where the native soil is well broken up and relatively homogeneous, the pipes can be laid on the trench bottom, as previously described. It is essential to ensure that the pipes are properly bedded on the soil, particularly in the case of large diameters. If the trench bottom does not lend itself to direct laying, a bed of pea gravel or sand must be laid, over an approximate thickness of 10 cm.
Refer to the section on soil depth of cover for details of the different types of surrounds and backfills, in terms of:
– environment (earth loading, wheel loading, backfill quality),
– pipe diameter,
– types of soils encountered.
Two types can be distinguished:
– a pipe supporting surround (to resist any ovality in the case of large diameter pipes), consisting of soil free from stones, etc., or brought-in material, compacted on the sides;
– a protective surround (in the case of very heterogeneous soils) consisting of stone-free soil or sand; this surround can act as both protection and support.
This is usually uncompacted infill with the original trench soil (away from roads), or by brought-in compacted material (beneath roads) when required.