Drainage of Structures

a system of drains (pipes, boreholes, and subterranean tunnels) intended to collect and draw off groundwater from structures. As distinguished from drainage of farmland, drainage of structures is used to prevent the seepage of water, to reinforce foundations, to reduce filtration pressure on the structure, and to protect foundations from erosion by seepage. These tasks can frequently be dealt with through general drainage of urban areas (industrial sites, airports, and so on), for which the same systems are used as when draining farmland. When general reduction of the level of ground water at construction sites cannot provide the necessary effect or is not economically justified (as a rule, the water level should be 3 to 3.5 m below the earth’s surface), local systems of drainage of structures (bed, layer, and circular) are used.

Bed drainage of structures is achieved through the use of a so-called filtration bed, which is laid in the foundation of the structure, directly on the water-bearing soil. The filtration bed is hydraulically linked (by pipes or filtering material) to a tubular drain located outside the foundation not less than 0.7 m from the plane of the structure’s wall (Figure l,a). Bed drainage of structures completely protects the building not only from seepage of groundwater but also from capillary moisture. It is widely used in protecting underground structures built on loose soil of low permeability, as well as in draining high-temperature shops, heating systems, and flues into which no moisture should penetrate.

Layer drainage consists of drainpipes with filtration cushioning layers. The pipes are laid on water-resistant soil outside the structures to be protected. This type of drainage is used only when the foundation of the structure being protected is located on water-resistant soil.

Circular drainage of structures is constructed along the outlines of a building or area in which a number of structures are located. The operation of circular drainage of structures is based on lowering the level of groundwater within the protected contours, ensuring protection against seepage into underground installations and building units. The extent to which the groundwater level is lowered depends on the depth at which the drainpipes, the tunnels, or the filtering units of the wells lie relative to the subsoil water table, as well as on the size of the outline to be protected. Circular drains are placed at a certain distance from the building; as a result, the drains may be laid after the structure has been erected. In this sense, circular drainage of structures is advantageous in comparison with bed drainage, which is laid as the structure is being erected.

A distinction is made among horizontal, vertical, and combined types of drainage of structures, depending on the design features. Horizontal drainage uses pipe or tunnel drains, ditches, and gutters. Pipe drains are a combination of drainpipes with one or more sheets of filtration cushioning layers of sand and gravel (Figure l,b), arranged to avoid silting of the pipes by particles of soil. Manholes, usually made of sectional reinforced-concrete rings, are installed for inspection

Figure 1. Horizontal drainage of structures: (a) bed drainage, (b) pipe drains for circular drainage; (1) original level of groundwater, (2) lowered level of groundwater, (3) filtration bed for bed drainage, (4) drainpipe, (5) filtration cushioning layer, (6) level of basement floor

of pipe drainage systems. Tunnel drains are pipes of large cross section, usually made of sectional reinforced concrete, with water intake openings and a cushioning layer. Ditches are mainly used in areas of small settlements, where the maintenance of groundwater at depths of up to 1.5 m is permissible. Under conditions of firm soil, ditches are made in the form of sloping trenches; in soft soil, gutters of sectional reinforced concrete are used.

Vertical drainage of structures is a system of wells joined by a sewage conduit, through which water is pumped out by a single unit or by a separate pump in each well. Sometimes the sewer is equipped with a trap, by means of which water sucked in from the wells enters the sewer (Figure 2). Combined drainage of structures is a combination of a horizontal drain—usually a tunnel drain—and a number of self-draining wells.

Figure 2. Vertical drainage with trap water discharge: (1) trap piping, (2) absorption tube of trap, (3) removal of air to vacuum pump, (4) discharge of water to pumping unit, (5) air collector, (6) tubular wells, (7) intake chamber, (8) tunnel

Drainage systems for hydraulic-engineering installations (dams, locks, and so on) are usually constructed in their lower units. The drainage system for earthen dams uses various devices (a drainage prism in the lower bank, a drainage mattress in the body of the dam, or banded and pipe drainage systems in its foundation). In concrete dams with rock bases, the drainage system for the dam body uses a system of vertical drains emptying into longitudinal tunnels, from which the water passes into the tail water. In concrete dams without a rock foundation, drainage of structures with reverse filters is used.