Drains can be placed as a trench or fin at the toe of a cutting, between the cutting and the pavement construction (Fig. 13.23). These act to lower the water in the cutting both increasing the stability of the slope and also reducing the water arriving at, and the pore water pressure in, the foundation of the pavement. Typically these cut-off drains also perform the function of lateral pavement drains (see Section 13.4.1). However, they differ from lateral pavement drains because they are designed to collect water from both sides of the trench or fin.
In porous ground where water tables are high, or high rainfall is anticipated, the drain must be designed to carry relatively large, continuous flows unlike normal lateral pavement sub-surface drains. This may have implications for the design dimensions of the carrier pipe at the bottom of the trench, for the regularity of points at which the trench is emptied to a surface water body or other outlet, and for the accessibility for maintenance. The last point deserves emphasis — if a normally operative cut-off drain ceases to function, water pressures will quickly rise in the toe of the cutting slope and in the pavement foundation, leading to rapid pavement deterioration and reduced cutting slope stability. In general, fin drains are less desirable when large flows have to be carried away as their capacity is normally less than that of a comparable trench drain.
Longitudinal cut-off drains may also be installed in sidelong earthworks so as to prevent water from ever arriving at the road’s construction. Figures 13.24 and 13.25 give examples.
