In colder climates, deep drains (“cut-off’ drains) are used to reduce local frost damage by intercepting the flow of groundwater and seepage water under the road structure, usually where there is a crossfall (see Section 13.4.4). The depth is usually at least the design frost depth (e. g. in Finland, this is between 1.5 and 2.2 m). Lesser depths may be used if there is very low permeability soil below. Deep drains can be installed beneath open ditches but then some gravel cuts may be needed to connect the drain and the structure. However, to ensure the fastest drainage of the road structure during the thawing period, the best location is connected to the structure under the inner slope, see Fig. 13.27. A particular benefit over an open trench is that such a drain doesn’t get clogged with ice. In most cases the need for a deep drainage or cut-off drain is due to longitudinal variation in the permeability of the subsoil. For example, a rock or belt of clay may change the flow direction to flow under the road bed (see also Section 13.3.7).
In road rehabilitation, the trench for a deep drain is made with a narrow bucket at the toe of the road embankment slope, a drain pipe is laid near the bottom (filter fabric first if necessary), initially filled with drain gravel or crushed stone #5-10 mm and followed by a top filling with coarse gravel or similar material, or crushed rock. The sides should have a low permeability lining. Narrow channels should be blasted through rock thresholds. Inspection wells can be made, for example, from plastic culvert material at 400 mm diameter with a cover on the top and some silt storage at the bottom, at about 50 m intervals. A cover flap should be used at the drain outlet opening. The water from this outlet is led to a lateral ditch, to a diversion ditch or into a rainwater sewer. A wide gravel outlet can ensure a safe discharge in the case of a local blockage of the outlet especially if the drain opening is located in a low gradient area. A sunny position for the outlet also decreases the risk of freezing. If the collected water has to cross the road, a separate pipe is usually constructed, instead of a culvert, to avoid freezing problems.
Among the possible benefits of deep drainage are:
• decreased growth of vegetation and thus some lower maintenance costs;
• better support of the slopes and pavement edges;
• improved traffic safety since open drains can be shallow and the slope gradients low;
• reduced probability of cracking at pavement joints, especially for narrow roads; and
• sheet ice should be eliminated.
Because deep drainage may also lower the groundwater table in the long term, the risks and disadvantages for the environment have to be evaluated separately.
