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Seepage-control measures

Water losses in canals replenishing groundwater contribute to waterlogging and salinization of valuable irrigated areas, reduce system performance, and consequently, lead to increase in water withdrawal, in the sizes of head works, canals and structures on them (at pumping irrigation, water losses cause as well rise in energy consumption), rise in construction and operational costs. Therefore, canal water loss control is carried out on a regular basis.

Seepage-control measures are undertaken to reduce water losses to seepage from irrigation canals when irrigation system performance is lower than the desired or canals run through soils with high permeability coefficient (more than 0.1 m/day).

The following means are used for controlling canal water seepage: seepage-control lining of canals made from cast-in-situ concrete, reinforced-concrete slabs, asphalt, rock, clay; watertight barriers beneath canals made from polymer films (film-ground, concrete-film, etc.), clay and loam, bentonitic clays, etc.; decrease of permeability coefficient in the result of natural and artificial mud fill of canal, deep and shallow soil compaction, petrolization, alkalinization, gleization, mechanical dispersion of soil, etc.

Seepage-control measure type is selected depending on the combination of hydrogeological conditions, canal extension, soil filtration characteristics, size of required enhancement of losses and presence of locally available material. Accepted seepage-control measures are to be substantiated by feasibility study.

The efficiency of seepage-control lining is determined by: the quantity of water saved; service life; possibility of construction work mechanization; inputs of financial, material and manpower resources; complexity and cost of operation, repair and restoration; improvement of the meliorative condition of lands; increase of agricultural harvest due to expansion of irrigated areas or augmentation of water supply to and improvement of meliorative condition of lands; improvement of system operating conditions; decrease of the scope of canal cleaning and drainage excavation works; possible decrease of the scope of earth works; reduction of the number of connecting structures in the irrigation network.

Facing from bituminous materials is made in the form of canal surface lining and concealed screen. Surface lining is made in the form of uniform coating of the canal bottom and sides by a layer of asphalt concrete (38 cm thick) heated up to 140C, with its subsequent compaction reaching a volume weight of 2.2 t/m3.

Bitumens of petroleum grade are applied for lining.

Screens made from polymer films 0.20.6 mm thick are used in soils where seepage-control measures are required to be carried out. Film screens are installed only closed according to trench, perimetric, and combined schemes.

The trench scheme is applied on cohesive and sandy soils; the perimetric scheme is used for screen installation on slopes on any type of soil, at that on crushed stone soils film is laid on an undercoat made from fine-textured soil; the combined scheme is used for large canals where their water waves may destroy their sides.

Ground lining made from clay and loam is used when the canal bed is formed of sandy-loam, sandy, gravelly, pebbly and other soils the permeability of which is higher than that of lining.

Ground lining is made in the form of open canal lining and in the form of concealed screens coated by protective layer of loose ground.

Canal permeability coefficient can also be lowered by decreasing the active porosity of soil through natural and artificial mud fill, deep and shallow compaction, alkalinization, gleization, and other measures.

Source: mse-online.ru

Canal lining works, types of lining (concrete. reinforced concrete, synthetic films and materials, etc.)

Concrete and reinforced-concrete lining is widely applied for controlling water losses to seepage from canals of any size, protection of bed against erosion and overgrowing. Hydraulic concrete prepared of plasticized, hydrophobic, durable Portland cement of grade no less than 400 is used for the installation of lining. The thickness of cast-in-situ concrete lining depends upon the conditions of its application and comes to 1025 cm.

In canals with discharge of 1-10 m3/s, the ratio of slopes for cast-in-situ concrete lining is adopted as 1.5, and for precast lining as 1-1.5.

Foundation of lining should be stable and reliable enough. Depending on hydrogeological conditions, it is prepared by steeping of filling and loosely packed grounds, compaction of filled and natural ground, levelling of slopes and bottom, installation of special bedding and drainage, treatment of soils by herbicides.

Draining bedding for concrete lining is installed at the most dangerous places: on hillsides located near human settlements, large enterprises and important facilities as well as on landslide sites. The thickness of the drainage layer is made no less than 10 cm.

Precast reinforce-concrete slabs are used: for single-layer and multilayer seepage-control linings; if the usage of precast reinforced concrete is advisable according to feasibility study; if lining of improved durability is required; at the construction in arid hot areas where it would be difficult to install precast concrete linings of proper quality; in the case of available construction industry facilities and to remove the seasonality of construction works; when carrying out construction works in hard-to-reach and underdeveloped areas.

When installing screens made from polymer films, it is advisable to use ground or reinforce-concrete slabs, which are to be laid on waterproof film, as protective layer.

Multilayer linings with water-proofing course are used at critical sections of the canal, where soil wetting is not allowed and, at the same time, higher margins of stability and safety are required.

Flat thin-walled slabs made mainly from stressed reinforced concrete, 49 m long and 0.63 m wide, are used for precast canal lining.

The joints of single-layer precast canal linings are to be made rigid and flexible. Their quality determines the size of seepage losses.

To reduce the number of joints, it is advisable to use as big as possible slabs so that one slab should cover the entire side slope from its top to bottom. The gap between the slabs is to be left 10 cm for rigid joints and 4 cm for expansion joints.

The following jointing materials are to be used for sealing the joints: asphalt joint filler (bituminous mastic); liquid-thiokol-based sealant; epoxy-thiokol-based sealant which ensures impermeability and flexibility of joints required for concrete and reinforce-concrete slabs.

Source: mse-online.ru

The most durable and long-lived lining is that made from cast-in-situ and precast concrete, reinforced concrete. Longitudinal and transverse construction, shrinkage and expansion joints are made in such canal linings. Construction joints are arranged every 3-4 m, and expansion joints every 10-12 m. Joints are filled by mastic compound (thiokol, self-adhesive sealant, etc.) for sealing.

The concrete layer in linings is to be 7-15 cm on average soils; up to 18-20 cm on soft soils at canal slope ratios of 1-1.5. If a vertical slope (T=0) needs to be installed, revetment walls are to be built.

Seepage-control lining efficiency is estimated by the quality of the lining itself and the quality of the works for installation of foundation under the lining (special bedding made from soil or lean mix concrete, leveling of canal bed, etc.)

Lining made from cast-in-situ concrete or reinforce-concrete is efficient on canals with steep slopes, because it has not only water-proofing characteristics, but also protects the canal against erosion. However, this lining has some drawbacks: concrete cracking at considerable temperature fluctuations; concrete destruction as affected by salts. Installation of such linings on collapsible soils is not advisable.

In recent years, different polyethylene films have been applied as watertight material. Ground-film coating has become widespread. When being installed in the canal bed, film is laid and then covered with protective soil layer up to 1 m thick.

To strengthen the waterproofing effect at concrete lining of irrigation canals, various films are effective, which are underlaid the concrete lining. Reinforced concrete slabs of standard sizes are also used for seepage control. The facing slabs are to be 6 cm thick, 6 m long, and 1, 1.5 or 2 m wide. When installing the protective revetment from slabs, special attention should be paid to their interlocking joints. Applied design of joints and technology of their implementation does not always provide their full water permeability. On large irrigation canals (water depth is more than 5 m), large-size prestressed slabs of 600074080 mm size have been used recently as seepage-control lining.

Asphaltic lining is used as seepage-control lining by far more rarely than concrete one, in spite of its higher permeability and freeze- and salt resistance properties. Its major drawback preventing from its wide use is its remarkable plasticity at high temperatures, which requires increasing canal slope ratios up to 2.5-3; this results in decline in land use ratio and requires increasing the sizes of canal structures. The asphalt layer thickness recommended when installing such lining is 2-5 cm. But even with such a thickness, weed vegetation often breaks through the lining, which lowers the quality of water-proofing protection and destroys the coating.

Impermeable screen (membrane) represents a low-permeable layer of soil or artificial material protected by a rather thick layer of canal bed ground from exposion to sunlight and from temperature fluctuations. Generally, they are made from clays, particularly from bentonitic or artificial materials, for example from various types of films. Screens are also made from polymer materials. To protect them from damage by plants and rodents, these screens are strengthened by capron threads. This also increases the roughness of the screen material and allows slightly decreasing the angle of canal slope ratio.

Seepage-control measures are to be chosen so that to provide: required permeability of canal ground; sufficient durability and reliability of structure, its resistance to temperature exposure and damaging effect of weed vegetation. In addition, the possibility of mechanization of operations should be provided for construction works and cost-effectiveness of seepage-control measures.

The funds spent for seepage-control measures are repaid quickly, because their implementation ensures great water saving; this rises the irrigation capacity of the water source, prevents degradation of meliorative condition of a given irrigated area, reduces the scope of the works related to the construction of irrigation network and structures on it due to reduction of gross expenditure, and rises the ratio of the use of a given irrigated area.

Final choice of water-proofing protection type is to be made according to engineering and economic comparison of available options. At that, the network (canal) performance should be estimated before and after the implementation of seepage-control measures; achieved water saving and its effect upon the cost of pumping station, water-intake structure, reduction of canal sections; improvement of meliorative condition of a given irrigated area.

Source: Agricultural hydrotechnical reclamation. / Bogushevskiy, A.A., Golovanov, A.I., Kutergin, V.A., et al. Edited by Markov, E.S. Moscow, Kolos Publishing House, 1981 (in Russian)

Compaction and stabilization of canal bottom and sides

One of the ways to control water seepage for irrigation canals is to reduce soil permeability; this can be made by compacting the soil. Soil compaction is carried out by means of special compactors, rammers and vibration machines. Compaction allows reducing seepage by 70-75%, but it can be implemented only on cohesive soils.

Seepage losses are also reduced considerably with changing seepage characteristics of canal bed soil by soil compaction and mud fill. Canal bed soil can be compacted by different methods: rolling; impact, and shooting.

Rolling is carried out by special compactors: tamping/sheepsfoot; sectional/harrow; and smooth-wheeled rollers.

Use of tamping rollers gives the highest efficiency; these machines allow compacting soil down to a layer of 60-70 cm.

Impact compaction is carried out by impact compactors (rammers) or excavator plates; in this case, the maximal layer of compacted soil comes to 1 m.

In recent years, canal bottom and slope soils have been got compacted by pinpoint blastings.

Effect of the compaction lasts for several years, after which the compaction needs to be renovated. When compacting the soil, its optimum water content should be provided.

Mud filling implies washing out of soil silt particles that come along with irrigation water to the upper layer of the canal bed soil.

Installation of concrete and reinforced concrete lining reduces seepage losses by 85-95%; installation of asphalt lining, by 80-90%; screens (made from clay), by 60-80%. With canal bed soil compaction and mud filling these losses reduce by 50-70%.

Source: Agricultural hydrotechnical reclamation. / Bogushevskiy, A.A., Golovanov, A.I., Kutergin, V.A., et al. Edited by Markov, E.S. Moscow, Kolos Publishing House, 1981 (in Russian)