Lateral Drain Design

The most common type of large scale field drainage scheme involves the installation of a number of lateral perforated pipes installed in a permeable backfill. The permeability of the backfill and the perforations in the pipe allow rainwater to quickly percolate into the trench, through the backfill and into the lateral drain pipe. The water is then conveyed down the pipe into a main drain which in turn conveys the water to an outfall.

The CivilWeb Field Drain Design Spreadsheet can be used to accurately and quickly design all kinds of field drains.

Depth

The lateral drains need to be installed at a depth which will prevent them from being damaged by loading from plant and equipment on the surface and deep enough to avoid damage from soil implements. In general a depth of around 750mm below the soil surface is sufficient. Unless there is a specific need, for example due to a deeply rooted crop, the lateral drains should not be installed significantly deeper.

Where lateral drains are used to lower the groundwater table, deeper lateral drains can lead to increased spacings between drains. In these cases lateral drain depths of around 1.2m to 1.5m are common. The optimum depth and spacing of these drains should be acquired through calculation.

Drain Spacing

Drain spacing can be calculated using the CivilWeb Field Drain Design Spreadsheet. The spreadsheet will calculate if a proposed spacing is adequate by comparing the capacities of the lateral drains and the total capacity required. This is done in accordance with ADAS 345 as explained in the following section.

Typical spacings of around 20m to 25m are common for field drains installed in permeable soils. This is close enough to allow the rainwater to percolate through the soils to a lateral drain without waterlogging, even during prolonged wet weather.

Where heavy clay or impermeable soils are present field drains would need to be installed very closely together in order for the rainwater to percolate to a drain quickly enough to prevent waterlogging in wet weather. In these cases a system of mole drains is generally more appropriate, as explained below. These can be installed in conjunction with lateral drains installed at between 40m and 80m if required.

In some cases lateral drains are installed without imported permeable backfill. The trench is simply backfilled with the arisings after the perforated pipe has been installed. This is cheaper for a single trench but would require the spacings between lateral drains to drop to around 10m in order to prevent the risk of the area becoming waterlogged during prolonged wet weather. Generally it is more efficient to install lateral drains with permeable backfill at greater spacings.

Permeable Backfill Material

Generally the lateral drains should be installed with a permeable backfill material placed above and around the perforated pipe. This backfill material allows the rainwater to quickly percolate through to the perforated pipe. This increases the speed at which the rainwater can be removed from the soil and allows any runoff water to be transmitted from the surface vertically into the pipe.

One exception is where lateral drains are installed to lower the groundwater table in permeable soils. In this case the permeability of the soil below the pipe is more important than the permeability above, so the trench can simply be backfilled with the arisings.

The backfill also helps to keep the pathway to the pipe open. As the rainwater percolates towards the pipe it will deposit small sediments, particularly in the area around the pipe. If there is no permeable backfill installed this area can become blocked with sediments restricting the amount of water which can enter the pipe. This can lead to flooding during heavy rain.

Permeable backfill does add a significant cost to field drainage schemes, as shown in our costings section below. Even though the spacings for pipes with permeable backfill can be larger, it still tends to increase the overall costs of the scheme.

The permeable backfill should ideally extend up to the cultivated layer. This will allow rainwater to enter the pipe as easily as possible. Where mole drains or subsoiling are included, there must be a connection between the permeable backfill layer and the mole drains or the fissures caused by subsoiling.

The permeable backfill material needs to be structurally sound and should be primarily large coarse gravel, typically between 5mm and 50mm size. The percentage of fines within the material should be as low as possible and must not exceed 10%. This grading of material and lack of fines will ensure that the backfill is sufficiently permeable.

Installation

Lateral field drains can be installed using special plant designed for this purpose. These machines will excavate the trench, install the pipe and backfill automatically. This is the quickest, safest and usually cheapest method of installation. They can also be set to lay the drainage pipes at a consistent gradient which improves the hydraulic performance of the pipes.

Lateral drains can be installed by hand or by general construction plant but there are several disadvantages to this. The trench will need to be wider in order for the pipes to be installed which leads to more permeable backfill being used. It also takes longer and usually costs more.

The CivilWeb Field Drain Design Spreadsheet can be used to accurately and quickly design all kinds of field drains.