French Drain Repair

French drain repairs are often required when a french drain becomes clogged. French drain repairs can be costly and are not always 100% effective, so it is generally better to follow a french drain maintenance schedule which will increase the french drains effective life and reduce the likelihood of repairs being required.

French drain repairs are most commonly required when the french drain has become clogged. This can occur for a number of reasons as detailed below.

French Drain Clogged with Debris

The inlet and the top of the filter material may become blocked with debris, leaf litter or mud. This can usually be rectified with a quick french drain cleanout whereby the debris, mud and leaf litter is removed along with the top 100mm of filter material. The filter material is either replaced or better can sometimes be washed and relaid.

French Drain Clogged with Sediments from Inlet

The filter material can become clogged with mud and sediments entering the french drain through the inlet with the runoff water. In this case the full depth of filter material must be removed and replaced or washed and relaid. This can be prevented in the future with some kind of sediment interception method such as a filter strip. Where this is not possible a geotextile filter fabric could be installed at a depth of 200mm-300mm into the filter material. This will prevent the full depth of filter material becoming clogged in the future, though the filter material topping above the geotextile will require much more frequent inspection and maintenance as this will continue to clog over time.

French Drain clogged with Sediments from Surrounding Soils

The filter material can become clogged with fine particles and sediments which have entered the french drain from the surrounding soils. In some cases groundwater flows can cause fine particles to migrate from the surrounding soils into the filter material. Over time this will cause the filter material to become clogged. Again in this cases the french drain repair must consist of the removal and washing or replacement of the full depth of the filter material. Before the filter material is relaid a geotextile filter fabric should be installed around the french drain in order to prevent fine sediments from migrating into the french drain. Note that depending on the design of the filter fabric and the size of the sediments this may reduce or even prevent the runoff water from infiltrating into the surrounding soils. In cases where this infiltration was included in the design as a mechanism for disposing of runoff water, the design of the french drain may need to be reviewed. Further information on the design of geotextile filter fabrics can be found in our French Drain Filter Fabric post.

French Drain Perforated Pipe Clogged

As well as contamination of the filter material, fine sediments entering the french drain from either the surrounding soils or through the inlet can also cause clogging of the perforated pipe. The perforated pipe can become clogged in two ways. The perforations in the pipe can become blocked preventing the water from entering the pipe quickly enough, causing the french drain to flood during rainfall. In this case it is unlikely that the pipe can be cleared from the inside so it will need to be removed and replaced. Alternatively, the sediments may be small enough to enter the pipe through the perforations. In this case the sediments may slowly build up inside the pipe causing it to lose hydraulic capacity and eventually clog up entirely. In this case it may be possible to clear the pipe using jetting or rodding methods similar to those used for traditional drain pipes. This is one reason why it is important that the french drain details include for access points to inspect and maintain the perforated pipes. Further details are included in our French Drain Details post.

French Drain Maintenance

French drain repairs are always relatively costly, disruptive and are seldom 100% effective. It is always much easier and cheaper to ensure that the french drain is designed and detailed correctly, installed correctly and maintained correctly. This will lead to a much more durable french drain. Minor oversights regarding the detailing and installation of the french drain can cause the drain to perform poorly and require frequent repairs. Further information on the correct construction of french drains is included in our French Drain Installation post.

Get your copy of the CivilWeb French Drain Design spreadsheet now for only £20.