Combined drainage system implies the system of horizontal drains (open or closed) with vertical wells. The purpose of vertical wells is to capture (drawn out) water-bearing horizon and relieve the pressure head in the lower aquiferous stratum, which allows considerably increasing the space between the drains. At that, the wells at the outlet section are connected with the drains at the water elevation level in them.

Combined drainage system is used in the cases when vertical or horizontal drainage cannot provide required capture of groundwater. This type of drainage is used both in homogeneous and stratified soils of different permeability and large thickness of filtering stratum. Structurally, the horizontal drain of combined drainage system is similar to the drain of horizontal drainage system, and drain holes are similar to drainage wells of vertical drainage.

Structure of combined drainage::
1 – ground surface; 2 – low-permeable stratum; 3 – water-bearing stratum; 4 – screen of well-amplifier;
5 – well-amplifier; 6 – open and closed horizontal drains

Combined drainage system is generally used when horizontal drain is located in low-permeable strata, and this is why it is not efficient enough.

At operation of combined drainage system, a combination of horizontal drain with vertical pressure-relief wells, a complex three-dimensional flow forms in a drain-adjoining zone and its deformations occur both in vertical and horizontal planes. However, locality of the sharp flow deformation zone close to combined drainage makes it possible to apply of the flow coefficient (filtration resistance) method to estimate its hydrodynamic imperfection.

In general, to design horizontal and vertical drainage under unstable filtration conditions it is necessary to take into account irrigation periods and inter-irrigation periods, because upon the completion of irrigation stabilization of infiltration hump does not yet come in inter-drain line. At that, calculated relationships are rather complicated and inconvenient for practical application.