Properly drained foundation or retaining walls don't have to be built as strong or as expensively as those without drainage. Providing drainage of soil behind these walls serves two purposes: It minimizes leakage by carrying water away from the wall, and more importantly it reduces the lateral pressure on the wall. Lower lateral pressure permits the use of thinner or less heavily reinforced walls. While drainage behind underground walls has traditionally been provided by backfilling with specially graded mixtures of sand and gravel, prefabricated drainage matting or drainage board attached to underground walls is now also being used. These materials are called in-plane drains because they form a drainage plane between the soil and the wall. Prefabricated in-plane drains usually consist of two elements: a filter fabric that lets water in but keeps soil out, and a drainage core with open channels that allow the groundwater to quickly flow to the bottom of the drain. Some means of carrying away water at the bottom of the in-plane drain is also required, just as it is for aggregate drains. A perforated drain pipe is the most common method, but other methods are also available.


The same filter fabric will not work equally well with all soils. Particle sizes of soils vary, and the rate at which water seeps through soils varies with different types of soil. The appropriate filter fabric to use then depends on the type of soil that's encountered. Filter fabric must do three things: allow water to flow into the in-plane drain with little head loss, keep soil particles from entering the drain, and resist clogging.


The core of the in-plane drain must be able to carry groundwater from the filter fabric to the drain outlet faster than the water can leave the soil. The amount of water that the core material can drain must be equal to either the maximum seepage rate of the soil or the maximum rainfall, whichever is smaller. The core must also have a crush strength high enough to resist lateral earth pressures.