Understanding your land drainage requirements
Before starting with the design and installation of land drainage, it’s important to understand the conditions leading to the requirement for a drainage system. This can usually be determined through the excavation of trial pits in the affected area, with the findings determining the type of drainage required and driving the specific design.
Types of land drainage
Where soils are concerned, there are two types of drainage system:
- Groundwater drainage
- Shallow drainage
Which is applicable to specific sites is entirely dependent on the drainage characteristics of the soil and whether a permeable soil layer is present.
Where no permeable layer is present then shallow drainage is required. The aim of shallow drainage is to improve the drainage characteristics of the mainly impermeable soils.
Where a permeable capable of allowing free water flow is present then a groundwater drainage system will be required to assist in the movement of water.
Pipes such as WavinCoil provide a low resistance path for effective water flow. They are vital in long runs (>30m) where the free flowing properties allow easy movement of the water collected. Especially where water volumes are high, using pipe offers a much more effective means of drainage than a simple granular filled trench.
Which size pipe is required depends on a number of factors including:
- Expected rainfall
- Soil properties
- The area of land to be drained
In groundwater drainage systems, the use of a piped system (utilising WavinCoil) is recommended. This should be installed at the depth of the permeable layer to facilitate the effective removal of groundwater.
Once the trench is excavated, clean aggregate should be used to surround the WavinCoil pipe. This aggregate should be to a minimum depth of 300mm from the bottom of the drain and should cover the pipe. The purpose of this stone is to provide a clean connection to the permeable soil layer in which it sits. With groundwater drainage, no water is collected from the surface, so it is not necessary to extend the granular layer to the surface. The remaining depth of trench can be back filled with as dug soils.
In shallow drainage, piped systems are used as the collector system, collecting the water drained through disrupted soil drainage. Mole/gravel mole drainage has a limited life span so provision of a robust collector system allows successful reinstatement of the drainage as and when required. They should be installed in trenches of 0.75-1m deep with the pipe installed within a clean aggregate layer. Unlike groundwater drainage, in this application the aggregate is brought up to within 10cm of the surface to ensure good connection with the mole/gravel mole drainage channels.
Maintenance of drainage systems
As with all drainage systems, maintenance is vital to ensure long and successful operation. WavinCoil is easily cleaned using rodding or, more preferably, jetting techniques. Jetting, performed from the outlet, quickly and effectively removes debris and deposits from the inside of the pipe and the clears the perforations allowing effective inflow of water into the pipe.