Drainage and Dispersion

Polieco Products

Pipes with non-woven fabricdrainage and dispersion of rainwater into the ground. This material, also called geofabric or geotextile, is used to prevent soil infiltration.

It is generally used to:

  • cover slotted pipes. Many companies already supply the drainage pipe with non-woven fabric sleeve in order to facilitate pipe laying and reduce possible installation errors;
  • cover the section of excavations where the dispersion cells are placed. In this case, the geotextile must be laid crosswise to the length of the excavation, and at the joints of the fabric there must be an overlap of about 50 cm, and then the cells are completely wrapped.

Geotextile can be made in a variety of weights depending on its application; the non-woven fabric for drainage must have an air mass ≥ 150 g/m2 so that it promotes the release of water but hinders the ingress of solid materials.

Slotted pipes 
Dispersion cells
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Pipes with non-woven fabric
Absorbing Manholesslotted pipes, an important ally for the drainage and dispersion of rainwater into the ground is the absorbing manhole.

The absorbing manhole is nothing more than a vertically buried, slotted pipe; installation should not be too deep, so a pipe with a stiffness class of SN 4 kN/m2 will suffice.

The absorbing manhole, in order to perform its task, must have slots along its entire circumference.

The manhole can have a bottom or not, this depends on the expected amount of rainwater and the desired rate of dispersion. Rainwater storage manholes generally have a bottom that allows fluids to slowly flow into the ground through slots on the circumference. In bottomless absorbing manholes, the water tends to pour into the ground immediately, much of it will disperse vertically but much will seep through the side slots.

Rainwater manholes are therefore composed of:

  • Conical reducer to fit on the riser. This tapers the riser to the size of a crawl space;
  • Fitting inserted by a gasket on the outer wall of the riser to allow PVC or polyethylene pipes, which are also slotted, to be inserted;
  • 360-degree slotted riser forming the body of the absorbing manhole;
  • Closure plate that is welded to the inner wall of the riser.

Since it is a pipe, the final height of the manhole can be reduced on site with the appropriate tools to change the length of the riser as desired.

Drainage manholes combined with drainage trenches form a complete system for draining and dispersing rainwater into the ground.

Absorbing Manholes
Our Dispersing cells
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Absorbing Manholes
Drainage Pipesmicro-slotted drainage pipe are that it collects excess water and evacuates it. For the first function, it is necessary that they have slots such that ground water can enter the pipe along the entire drainage line; while for evacuation, it is necessary that the pipe has a sufficient cross-section that it is free of occlusions and that it is laid with a slope such that the water can transit. In rainwater drainage pipes, collection is ensured by the slots in the pipe itself. The slots in a micro-slotted pipe can be arranged around the entire circumference (at 360°), or only on the top (at 220°), allowing the bottom surface to collect water as it is free of slots. The width of the slots is crucial to the purpose of a drainage pipe: it must be narrow enough to prevent large particles from entering the pipe, which could result in clogging, quickly rendering the drainage function ineffective. While the slots must also need to be wide enough to allow water to flow out fairly quickly. It has been found that the ideal slot width of a drainage pipe is 2 mm. The slots can be perpendicular to the pipe or longitudinal or diagonal to the axis of the pipe. With slotted pipes, the slotting must be in the corrugation groove so that the two peaks of the corrugation prevent the surrounding material from obstructing the slot. The combination of the thickness together with the length of the perforation allows the slotted drainage pipe to act as a filter, thereby separating the water from the suspended silt-soil. Polieco produces different types of corrugated drainage pipe:
  • POLIDREN: piping in coils up to an outside diameter of 200 mm
  • DRENOSEWER: polyethylene pipe in straight lengths up to an outside diameter of 200 mm
  • DRENOPAL: tubing in straight lengths up to an outside diameter of 1200 mm
 

Drainage and Dispersion Pipes                            Designing a Drainage System

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Drainage Pipes
Building a drainage trenchNowadays, we are witnessing a push for urbanisation due to strong industrial and civil expansion, which on the one hand increases the amount of water that needs to be disposed of and on the other hand reduces the amount of land that can be used for water absorption.

For this reason, drainage systems are increasingly being developed underground with trenches or tunnels that aim to remove excess water by dispersing it in the soil.

There are various methods to disperse water in a surface, the simplest being to build a drainage trench. This technique creates a permeable “path” that the water will follow until it slowly dissipates.

In order to construct an effective trench, an excavation must be created in the ground between 500 centimetres and a maximum of 1 metre wide; the depth depends on the running surface. Drainage trenches are in fact divided into:

  • surface: maximum depth of 5 metres;
  • deep: the trench excavation can be as deep as 25 metres.

In order for a trench to drain soil, non-woven fabric (or geotextile) must be placed around the trench to prevent soil from clogging the trench. It must then be filled with coarse-grained material (sand or gravel) so that water can pass through it. Finally, the excavation must be covered with waterproof material.

The choice of backfill material is crucial to the success of the drainage system as it is the contrast in permeability between the trench soil and the surrounding soil that determines the direction of the water and thus its dispersion.

To facilitate the inflow of water into the trench and its subsequent dispersion, cracked pipes and/or dispersing cells can be inserted into the excavation.

 

Discover the drainage systems                            Why you should drain land

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Building a drainage trench
Artificial drainsArtificial drainage of poorly draining soil is achieved by means of a network of small underground conduits known as drainage pipes, which are introduced into the permeable or poorly permeable soil, and collect and promote the evacuation of excess water without the need for special shaping of the overlying soil surface.

In order to properly investigate the design of a drainage system, it is necessary to identify groundwater infiltration. An in-depth study of the surface and underground hydrology of the territory must frequently be carried out, also through the statistical processing of rainfall, hydrometric and phreatometric data, together with a geopaedological investigation aimed at ascertaining the physical-chemical characteristics of the soils, mainly the permeability coefficient and the depth of the first impermeable layer.

There are no rules to follow for draining the soil, but it must be analysed on a case-by-case basis. For example, the body of a motorway (or a high-traffic road) generally consists of an impermeable carriageway, side verges and a central dividing wall, which is almost always permeable. Rainwater, filtered through the draining soil, will eventually cause damage to the stability of the road if it is absorbed by the road's support structure. In these cases, drainage systems should be positioned longitudinally, along the edges of carriageways and in the centre of the traffic island.

In roads built halfway up the hillside, i.e. part earthwork and part relief, the technique of intercepting drainage is used by placing the drains upstream of the road in order to avoid infiltration between the impermeable zone and the fill.

Polieco's drainage solutions                            Designing a drainage system

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Artificial drains
Designing a Drainage SystemWhether it is agricultural or civil drainage, in order to correctly approach the design of a drainage system, it is necessary to have a set of basic data and knowledge, which can be obtained through case-by-case studies and surveys. To create a draining soil, it is necessary to identify and establish the source of water infiltration, ascertain the percolating characteristics of the soil, determine the permeability coefficient K and the depth of the first impermeable layer.

For the drainage of agricultural or civil land, it is always necessary to have a complete plano-altimetric survey of the area, on which the heights of the land, existing buildings and infrastructures (road network, irrigation network, other watercourses, inhabited areas, etc.) are highlighted. In the case of home perimeter drainage or foundation drainage, for example, information on the extent of the green area around the house or the depth of the foundation can be obtained from the survey.

The first step in designing a drainage system is to determine the specific flow rate to be drained.

Excess water in the soil may originate from:

  • rainfall;
  • groundwater or deep recharge waters;
  • irrigation.

Here we focus only on water generated by meteoric events.

Water generated by rainfall is classified on the basis of its different final destination into:

  • surface run-off water: that which flows and streams to the surface;
  • groundwater: that which penetrates the ground and flows into it;
  • interflow: water that is intercepted by ditches and underground channels and is forced to take an underground route in order to reappear on the surface after a short journey through the ground.

The calculation of the amount of rainwater that must be evacuated by the drainage pipes is performed on the interflow: in fact, both surface runoff and deep recharge water, not being stagnant, generally do not decrease the mechanical stability of the soil. The amount of water for each destination varies depending on the type of soil structure and texture; as a guide, it can be stated that 5% of rainfall constitutes surface runoff and 95% filters into the ground, of which 25% is deep recharge and 70% from interflow that is captured by drainage systems. In this case, therefore, the pipes or drainage cells should be sized for a specific flow rate of 70% of the rainwater.

Once the flow rate to be drained has been established, the diameter of the drainage pipes can be defined using Visser's empirical formula:

d = 0.0209q0.375 A0.375 J-0.375

where:

d: internal diameter of the drainage pipe (cm)

q: specific flow rate of rain to be drained (mm/day)

A: the area of land affected by drainage (m2)

J: slope of the drainage pipe (%)

In addition to the flow rates, the degree of filling (maximum height of the water with which the drainage pipe will operate) must also be established, and the slopes to be adopted, in relation to the altimetric course and the level of the collectors where the drainage pipes enter; all this is necessary to ensure a regular flow of water.

The speed of the water must be such that any deposits inside the drainage pipe are removed (speeds above 0.5 m/sec are generally recommended). This can be achieved by arranging the drainage pipes with slopes that are also dissimilar to the slope of the ground surface but in any case greater than 1%.

The following guidelines must be followed for the correct and effective installation of a cracked pipe:

  • spread a sheet of non-woven fabric around the walls of the excavation;
  • create a bed of selected drainage material (gravel with a grain size of 3-5 mm) with a thickness of 10 cm, thus preventing the top of the rib from resting on the excavated soil;
  • use small grain size drainage gravel as backfill material around the pipe;
  • backfill with it up to 40 cm above the extrados of the pipe;
  • cover the section with non-woven fabric;
  • complete backfilling with topsoil.

Design your Drainage Plant                            Discover our Solutions

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Designing a Drainage System
The importance of draining soilsIn construction, it is important to drain soils properly to prevent them from causing damage to the infrastructure.

Groundwater infiltration is responsible for 80 per cent of cracks in buildings, works of art and artefacts.

When it comes to stormwater, it is the roads that suffer the most deterioration. In fact, if the water is not properly collected and drained into the ground below, it will cause damage to the stability of the road as it is absorbed by the road's supporting structure. Any work on the surface will therefore be useless because stormwater will continue to cause cracking until the subsoil is dry.

Every soil has its own natural drainage system; however, the ratio between rainwater inflow and its dispersion depends on its characteristics. Those soils where there is no underground water table or there is an excess of water require an artificial drainage system.

What is an artificial drainage system and how does it work?

According to the dictionary, it’s the “process by which water from a layer of soil or incoherent matter that is soaked in it are drained by means of special items (channels with permeable or perforated walls, trenches, tunnels)”.

Draining a soil therefore means creating a network of small underground conduits (drain pipes) that collect and promote the removal of stormwater without having to shape the above surface.

On the other hand, the function of stormwater dispersion in the soil is different. In this case, the purpose is not to collect and remove groundwater, but rather to disperse stormwater that is generated by the presence of impermeable surfaces.

 

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The importance of draining soils