Drainage filters

Drainage protection against silting

Main types of drain deformation are silting, ochration (ochre sedimentation) and overgrowing by plant roots. Protective filter materials are used to prevent inflow of the soil particles washed out by groundwater into the drainage pipes.

Protective filter material (PFM) has two functions: it protects drain from silting with small soil particles and enhances its water intake capacity. It sort of enlarges the outer diameter of drains, promotes water motion along pipes to their holes, and reduces pressure losses for water entry to drains. The second function of PFM is particularly important for drainage of heavy poorly-permeable soils. With the application of filters in heavy soils, the water intake capacity of drains becomes 2…3 times as high as drains without filters.

Protective filter material is to trap soil particles that can silt drainage pipes (carry-over of small particles should not exceed 5…7%); not be clogged with minor soil particles; not create additional resistance to water inflow into pipes; have sufficient durability for manual and mechanized laying of drainage; meet sanitary condition requirements for safe working; be chemically and biologically resistant; not pollute drainage water; be reliable for estimated drainage service period; and its cost should be economically sound.

PFM filtration coefficient during drain operation is to be higher than the filtration coefficient of drained ground: at least 5 times as for sand; 10 times for peat; and 20 times for clay.

Depth (thick) filters are more preferable for increasing the water intake capacity of drainage. In clays, the filter thickness is to be at least 2 mm; thin PFMs are laid in 2…3 layers.

Protective filters are especially needed during the first 1…2 years of drainage operation, because with the lapse of time small filtering domes naturally form next to drainage joints and pipe perforation holes due to carry-over of small soil particles; they function as filters.

The following materials meet the requirements applicable to filters to the fullest extent possible: mineral ones are represented by gravel, sand, crushed stone, clinker, etc.; organic ones are represented by moss, scobs, heather, straw mats, poorly decomposed peat, divot, milled peat (in the form of crumbs with particles of 1…5 cm in size) and mats made from it based on polyethylene screen; artificial mineral are represented by glass cloth, polyethylene cloth, nitrone and chlorine cloth, etc.

The most widespread material in our country is 0.8…1.5-mm-thick glass cloth from fibers of 14…18 micron in diameter. Enforced armor-clad glass cloth is allows laying it mechanically. Its thickness is 0.7 mm, and weight of 1 m2 is at most 100 g. Cloth is reinforced by glass filaments.

Polyethylene cloth made from low-density polyethylene is very durable and permeable. Its thickness is 1…1.2 mm, and weight of 1 m2 is 0.22…0.24 kg.

Glass cloth, nitrone, chlorine and other PFMs are supplied in rolls. On the site, rolls are cut with saw or knife into stripes of desired width depending on the pipe diameter and protection method.

According to occupational health and safety requirements it is obligatory to work with glass cloth by using work gloves, safety spectacles and gauze bandage in order to protect skin and respiratory apparatus from glass particles.

Risk of silting of closed (subsurface) drainage is highest in pulverescent soils and clay sands and lowest in peats. In the latter, drains will not be silted even with the interface gap sizes coming up to 8 mm and more, if the pipes are laid in a dry trench.

The following methods are applied to protect ceramic pipes 50 mm in diameter from silting by synthetic materials: wrapping of pipe joints on their top and sides with bands 7…10 cm wide; all-over wrapping with a single strip 30 cm wide; laying of pipes on a dunnage strip 15 cm wide with a cover with strips 7…10 cm wide on top; all-over protection of pipes with two strips (with dunnage width of 15 cm and cover width of 25 cm). When applying such methods, need for PFM comes to 0.09; 0.30; 0.40 and 0.21 m2 respectively per meter of drain length.

It is recommended to wrap plastic pipes beforehand (prior to laying) with a strip 20…25 cm wide with at least 3…5 cm overlap. Consumption of PFM made from rolled materials is at average 400 m2 per kilometer of drains.

In pulverescent loams, sandy loams and sands, the joints of ceramic pipes and plastic pipes are coated with a layer of protective material along the full length. In clay and loamy soils and well-decomposed peat, it is recommended to apply depth filter; roll materials are necessarily to be covered with a layer of vegetable soil (topsoil) at least 20 cm thick.

In floating sand, pipes are to be shelved, protected with 2-3 layers of PFM and covered with at least 30 cm thick topsoil. In lowland peats, pipes are protected with filter strips on their tops and sides and covered with dry peat.

Special enveloping devices are equipped on placing machines (rackers) to protect plastic drains against silting.

In recent years, filtering couplings made of plastic, e.g. low-density polyethylene, are used instead of protective filtering materials for connecting pipes. Coupling is a hollow cylinder in the middle part of which there are two beads-locking pins; between them there are water intake holes protected by adhered glass cloth strip. The locking pins serve as back stops for pipes and make their couplings flexible. The tapered ends of the coupling are corrugated diametrically for tight joint with ceramic pipes. The coupling length for pipes 50 mm in diameter is 62 cm, wall thickness is 1.2 mm with 12 holes (slots), and the coupling weight is 12 g. There are other designs of filtering couplings.

Though couplings facilitate the process of mechanical construction of drainage system, allow increasing labour productivity and reducing manual labour at drainage laying, it is not recommended to apply them too often, because couplings reduce the water-transmissive capacity of drains because the cavity gets narrow creating additional resistance to water flow as well as reduce the water-capturing capacity of drainage. If they are used, the space between drains is to be lessened by 10…15%. Outer sleeves are more preferable.

Silting of drains can occur also because of deposition of ferrous compounds in pipes and filtering materials. This type of silting is called drain ochration. It occurs as a result of sedimentation of ferrous compounds from groundwater under the influence of chemical reactions and iron bacterium activity.

Iron comes into drains along with groundwater in the form of nitrous oxide and complex organic compounds. Ferrous iron oxidizes due to contact of water with the atmospheric oxygen present in drain. Generated Fe(OH)2 turns into ferrous hydroxide Fε(ΞΝ)3 that deposits in the form of brown friable gelatinous mass. Then it gradually constringes and sits tight on the pipe walls and filter. Ochration appears when the content of ferrous iron exceeds 3 mg/l; it makes itself felt especially strong (the mouth reaches of drains and collector can become entirely silted within 1…3 years) when the content of iron is more than 8…14 mg/l as well as in acid soil with pH lower than 5.

To fight against drainage ochration, the following preventive measures are carried out: capture of groundwater flows and particular springs with high content of iron by means of an open canal; intensive preliminary land drainage (3…5 years prior to laying of drainage) with high content of iron by means of open canals; arrangement of concealed wells; increase of the diameter and slope gradient of drains up to 0.006 and collectors to 0.002 and over, which provides carry-over of formed ferriferous compounds; installation of submerged collectors and waterlogged drains to reduce air exchange in drains; application of inhibitors (lime, gypsum) in drainage trenches; deep lime treatment and loosening of soil for iron oxidation before its inflow into drains. With high content of iron in soil, the space between drains is changed by 10…15%.

During the operation of drainage, drains and collectors are rinsed by drain flushing machine.