The Drainage Trenches application allows sizing a drainage system based on drainage trenches: it is possible to size the structure, calculate the spacing, the time factor, and the drain flow rate.
Drainage trenches are used to stabilize shallow translational landslides, generally frequent on gentle slopes in cohesive soils, and for which other interventions are often impossible or not economically viable.
Figure 1. Drainage trench section
The draining function of the trenches is carried out by the filling material, generally consisting of coarse-grained soil, in order to have a high difference in permeability between the surrounding soil and the filling material.
Trench execution
The width at the base of the trenches is generally between (0.5 ÷ 1) m, and depends mainly on the equipment used for the excavation, as well as the maximum reachable depth, which must be at least equal to that of the slip surface.
Trenches can be shallow, with depths up to (4 ÷ 5) m, or deep, and in this case can even reach 25 m in depth.
Operationally, if work inside the trenches is expected, it is necessary to evaluate the stability of the excavation walls for depths greater than 2 m, and it is good practice to limit the excavated section as much as possible and fill it rapidly.
Figure 2. Typical installation of a trench on an infinite slope
Drainage trenches must be built longitudinally to the slope, in the upstream-downstream direction, and it is important to remember that these must be built starting from downstream, in order to remove water without carrying out further interventions.
A proper trench construction requires the presence of a geotextile filter separating the coarse-grained filling material from the surrounding soil, thus preventing the transport of fine material from clogging the trench.
Construction phases
The installation phases of a drainage trench can differ depending on the technology used by the company performing the work and the geometry chosen during sizing, but they are generally divided into:
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Excavation with mechanical means starting from downstream
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Laying the geotextile
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Spreading the gravel bed and laying the slotted pipe
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Filling with draining material
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Covering with impermeable material