Drainage in reclaimed lands

Drainage in reclaimed lands

Drainage means natural or artificial extraction of water from the ground surface or of underground water. Ground often needs to be ameliorated through diversion of underground water or storm waters for improving agricultural practices, constructing buildings and structures.

Drainage on irrigated areas implies a complex of hydraulic facilities (pipes, canals, wells, pumping stations, etc.) usage of which ensures lowering of the level and flowage of groundwater and its diversion outside the irrigated area.

Drainage is also used for protection from water penetration into structures, preservation and reinforcement of building footings, reduction of seepage pressure upon the framework. Drainage is also indispensable for maintaining sites and roads of a respective site in dry condition, prevention of rotting of cultivated plants’ roots, protection of foundations and basements from excessive moisture. Local drainage systems are used when groundwater lowering in the construction area cannot produce a required effect or does not make economic sense.

To organize drainage, a design of drainage system is to be drawn up. Based on that they determine the location of drains, their burial depth, slopes, layout of canals, select necessary components and materials. Depending on the depth of groundwater that moistens the area, drainage of the site surface (surface drainage) or underdrainage can be carried out.

Dewatering drainage is used to drain overmoistened lands; desalting drainage, to control soil salinization in arid zones; aeratic drainage (rarely used by itself) to intensify gas exchange in heavy clay soils.

Dewatering drainage is needed for lands where groundwater level is higher than the drainage rate. Mainly, they use horizontal systematic (uniformly distributed over the irrigated area) tubular drainage system, a system of pipes in the form of solid drainage lines. Groundwater flows into the joints between the pipes or into pipe holes under pressure difference; therefore, the drain laying depth should be deeper than the drainage rate. Drainage system is used more intensively during “critical periods” of intensified feeding of groundwater (generally, in spring and autumn).

Drains with the interval between them B (in meter), depth of laying b (in meter) and diameter d (in meter) are supposed to divert the melt water came to the soil for t days after snow melting and lower the groundwater level down to drainage rate а (in meter); Н stands for water reserves in snow cover (in m³/ha); Т stands for the occurrence depth of water confining stratum surface (in meter); Д stands for drains; Тр stands for trenches; Р stands for pressure conventionally taken as zero.

Generally, for mineral soils b is accepted as 1.1 m; for peat swamps, as 1.3-1.5 m.

The spacing between drains (within a range of 15-50 m) is determined by the following formula:

where

Б₁ is a drain imperfection value in terms of completion/formation (“hanging”) degree,

Г₁ is the same for (tile) drains’ receiving surface, i.e.

s is the length of drainage pipe (in meter);

с is the width of butt slots or protection of joints (in meter);

H₀ is the mean productive head (in meter);

q is the period-average drainage modulus (in m/day) estimated through balance calculation in terms of the water quantity needed to be diverted and time of its diversion (usually, 0.5 < q < 1,5 l/s per 1 ha, at that 1 l/s per 1 ha = 1/116 m/day);

к is coefficient of permeability (in m/day).

The water that came to drains flows from its source to its outfall, to which end drainage lines (not longer than 150-200 m) are weathered at a grade of at least 0.002. The water flows from the drains to open or closed (made from pipes) collectors (at most 1000 m long), and from there it is discharged to the main canal and diverted outside of the drained area.

To drain mineral soils they use mainly tail drainage pipes 4-10 cm in diameter, 33 cm long. They are laid on the bottom of trenches (or on boarded racks in unstable soils) end to end (width of the joints is at most 1-2 mm), covered by filter medium (moss, glass fiber, glass fiber cloth) and then the trench is covered up with earth. The life duration of tile drainage is 50-80 years. On peat swamps, it can be used only after preliminarily drainage of the sites by means of open canals; here wooden-box drainage is also used: it is made from rectangular pipes hammer together from planks providing for a longitudinal slit trench.

Plastic drainage has prospects: plastic pipes with longitudinal slit trenches or rounded holes are laid in open trenches or formed after cartridge (mole press), i.e. trenchless drainage that allows fully mechanizing its laying process. To speed up diversion of surface water or perched water on soils of poor permeability, a system of closed collectors is used: tile pipes are put in the trench 0.8-1 m deep laid across the slope and covered up with earth taken from the topsoil or with other water-permeable material. The collectors’ length is 150-200 m; the spacing between them is 30-60 m.

Desalting drainage is used for lowering of groundwater level, diversion of flushing water and salt control on the irrigated areas in arid zones. Its operation is divided into two periods: land reclamation period (1-3 years) during which a layer of 1-2 meters gets desalted and saline groundwater is diverted; and operational period, when achieved results are maintained. On irrigated lands, horizontal and vertical desalting drainage is used. Horizontal and relatively deep (about 3 m) continuous regular tubular drainage, which is complemented by open canals about 1 m deep at a distance of about 20-50 m during the land reclamation period is the most practical. Drains made from tile, asbestos-cement or plastic pipes (the two last types are perforated) are laid into a trench, bestrewed with sorted-out gravel, and covered up with excavated earth. Their length is advisable to be up to 1 km, bed slope is at least 0.001, spacing between the drains is 150-400 m. The critical condition for successful operation of horizontal desalting drainage is gravity manner of drainage network and normal fault of drainage water into an outfall drain. If gravity fault is not provided, pumping stations need to be installed.

Drainage system

Drainage system is an engineering facility for collection and removal of infiltrated and ground waters.

Drainage system is a branched structure of pipes (drains) laid around the periphery of a land plot and connected to each another and drainage pits; it is intended for protection of a particular area from excess moisture.

By using drainage system, soil water balance regulation problem can be resolved.

Components of drainage system

Drainage pipes (drains) are the main and key component of the drainage system; they perform the function of catchwater and water discharge needed for drainage of an area.

In this case, drainage represents a branched system of interconnected pipes (drains) that are laid with a gradient in the direction of catchwater (canal, ditch, pond, drainage pit) around or along a particular facility (or land plot). Each drain has a special network of holes situated at certain spacing between one another on its walls. Such a pipe system absorbs water from soil and diverts it outward the land plot.

Use of drainage pipes allows solving the problem of protection of an area and facilities on it from damages caused by excessive moisture (frost, formation of mould, puddles and ice mound), preventing submergence of basement and underground accommodations, and rotting of plant rootage.

Drainage pipes can be divided into the following main categories:

Asbestos-cement pipes. To use these pipes as drainage ones they make sawcuts 3-7 mm wide arranged staggered-order at 100-200 mm interval (depending on soil characteristics) in their upper side.

Ceramic pipes are made from clay (with the application of additives, if necessary). There are pipes with perforation, slit trenches and seamy outer surface (their grooves conduce to increase of absorbing capacity).

Polymeric pipes are pipes made from high-pressure or low-pressure polyethylene.

Pipes made from porous materials (clayite-glass, polymer concrete) are characterized by that water percolates into them through the pores in the walls, but not through special holes.

Drainage pipes are laid according to prefabricated plan of drainage system. Minimal gradient of the drainage pipes according to constructional norms is 2 mm per running meter in clay soils and 3 mm per running meter in sandy soils per running meter. In practice, to ensure good water flow the pipe gradient is made 5-10 mm per running meter. In compliance with these requirements, trenches are cut the bottom of which is leveled and rammed with pebbles (filter bed is made).

Installation of pipe network is carried out by means of reducing fittings, i.e. couplings, adapters, T-joints, bent pipes. Drains are covered up with several layers of water permeable materials (e.g., textile fabrics): initially washed crushed stone or gravel is placed, next sand, at last dug earth is laid on top. The covering’s thickness is within the rage from 100 to 300 mm (the less permeable the surrounding soil, the thicker the covering). Filters are used to avoid siltation of the drains and clogging up of the perforation holes; filters are made from geotextiles (when reclaiming sandy or sandy-loam soil) or coir fiber (when draining clay, loam, and peat).

If the system’s carrying capacity needs to be increased, a combination of several drainage pipes of different diameters can be laid into the trench.

Drainage pits are placed in the points of pipe bents that giving direction to flowing water or functioning as water drainage sumps. Drainage pits serve also as maintenance centers. Particularly, they are used for flushing drainage pipes.

Drainage pumps intended for pumping out water are used when it becomes impossible to remove accumulated waste waters from the drainage system in natural way. Such pumps are installed in drainage pits and designed for operation in submerged condition: their electric part is waterproof and securely isolated. Pump models differ in terms of capacity (pumping speed) measured in cubic meters per hour. Another important point is the diameter of the pump outlet pipe. The more is this diameter, the more is maximal size of solid particles let trough the pump.

Main types of drainage

Bed drainage system is laid in the foundation of a protected building structure immediately on water-bearing soil. At that, it is hydraulically connected with a tubular drain (subsoil artificial channel for collection and diversion of groundwater) located outside the foundation at a distance of at least 0.7 meters from the surface of a structure wall. Bed drainage system protects the structure both from waterlogging by groundwater and damping with capillary moisture. Bed drainage is widely applied at construction of underground structures built on low-permeable soils as well as at drainage of “hot” shopfloors of the right-of-ways of heat supply networks and chimney (which are to be securely isolated from ingress of moisture even in capillary form).

Wall drainage system consists of drainage pipes (with filter package) laid in waterproof ground outside the structure. As a rule, wall drainage is applied when the structure foundation is set on waterproof soil.

Annular (perimeter) drainage system is installed along the contour of a protected building or plot. Operation of annular drainage system is based on lowering of groundwater level within the protected contour; this ensures protection from impoundment of underground structures and parts of the building. Depth of such lowering depends on penetration of pipes, galleries or the filter parts of wells relative to groundwater table as well as on the protected contour’s size. Annular drains are placed at a some distance from the building, thanks to which they can be installed after the building has been constructed. For that matter, annular drainage compares favorably with bed drainage that can be installed only simultaneously with the construction of the building.

Drainage types according to design features

Horizontal drainage is a system of tubular and gallery drains, canals, and flumes. Tubular drain is a combination of drainage pipes with one or several layers of filter package. These layers are put in order to avoid jamming of the pipes with drainage ground particles. Surface inlets are arranged to control the tubular drains. Gallery drain is a pipe of large cross section with water intake holes and with package. Ditches are generally used in small villages, where groundwater level can be maintained at a depth of 1.5 meters. In stable soils, ditches are made as a rule in the form of sloping trenches; and in unstable ones, in the form of precast concrete constructions.

Vertical drainage is a system of wells connected by a collector through which water is pumped out by a pump unit or an individual pump on each well.

Combined drainage is a combination of a drain and a number of blow-out wells.

Drainage of hydraulic structures (reservoirs, sluice gates, etc.)

Drainage systems are usually installed at downstream of hydraulic structures. In earth dams, drainage is completed in the form of various structures, for example as follows:

drystone drain at the downstream toe;
blanket drainage (downstream drains) inside the dam;
strip and pipe drainage in its base.

In concrete dams built on bedrock foundation, drainage represents a system of vertical drains falling into longitudinal galleries; from there water is diverted into the tailrace canal. In concrete dams built on earth foundation, drainage with backflow filters is used.