Drainage Information

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Drainage Systems

The sole function of any drainage system is to convey foul/waste effluent or surface/storm water to a suitable outfall. Typically this outfall is the public sewerage system.

Historically the surface water and foul sewers were often one in the same, being known as a 'combined system'. In recent years authorities have, wherever possible, encouraged the use separate storm and foul drainage systems. This has the benefit of both reducing the amount of waste needing heavy duty sewage treatment, but also stops a huge surge in volumes at treatment plants during heavy storms.

Where possible surface water drainage systems are no longer terminated into highway drains but emit to a purpose built soakaway in the ground. Soakaways cannot be used in clay ground.

In many areas of the UK, house builders and contractors generally find it confusing that Drainage Authorities may require separate storm and foul drainage systems. Particularly to new housing and office developments that are subsequently joined together at the curtilage of the site only to then join a 'combined drainage system' in the highway. The thinking behind such practice is that if the main drainage infrastructure in a town is an antiquated combined system and needs upgrading, then it will be a simple matter to split the foul effluent and storm water at the point where it exits the site.

Foul drainage systems.

The fundamental difference between a foul and storm drainage system is that a foul drainage system is sealed at the point of connection to appliances by a water filled 'trap' created by a 'u' bend. It is note worthy that the humble W.C. (water closet) is generally accepted as the worlds most important invention. Until the invention of the water closet, the smells and vermin emanating from drains throughout a town or city were unrestricted in any way. The simple water filled 'u bend' created a way that solids could freely enter a drainage system whilst keeping the end of the pipe fully sealed at all times...eureka! In Victorian times, the once prevalent cholera and typhoid epidemics were eliminated at a stoke once this simple principle had been adopted.

To avoid traps being broken by the siphonage that is created within a moving sealed system it is essential to allow air to enter the system at some point. A tall vent pipe at the head of a drain run is typically the solution to this, whereby any smells emanating are at roof level away from windows and doors. In recent years anti siphon traps with air admittance valves have reduced the need for vent pipes in some situations. Indeed 100mm soil pipes can at times terminate within houses, e.g bathroom cupboards, if fitted with air admittance valves.

Items normally requiring trapped outlets are WC's, baths, basins, kitchen sinks, shower trays, and washing machines.

Waste pipes are terminated via 38mm and 50mm plastic waste pipes outside a building and it is normal for these to be collected by a trapped gully with a grating.

Storm Water systems

Storm water drainage systems are typically used for collecting the water from a rainwater system of gutters and down pipes used around the roof of any building. In rainwater systems the standard size for plastic guttering on housing in the UK is 115mm to 125mm with downpipes being 65mm to 75mm. On industrial buildings 150mm to 200mm diameter guttering is not uncommon, with 100mm and 115mmm heavy duty plastic downpipes.

Where rainwater pipes join drain pipes at ground level it is normal to connect via a simple slow bend with the joint being made with a rubber drain adaptor or cement flaunching.

Drain pipes.

Drain pipes are either made from clay or plastic. Standard clay pipes are usually in 1.6m lengths joined by a plastic collar with sealing rings. The largest UK producer of clay pipes is Hepworth and the pipe brand is Hepsleeve. Historically all pipes were made of clay, whereby each pipe has a spigot on one end and a join was made with either tar or cement. Clay pipes are extremely resilient to chemical attack and do not suffer from distortion if they inadvertently come under load. Being relatively brittle however they can suffer from cracking if impacted and can then subsequently leak.

Plastic drain pipes are supplied in 3m and 6 m lengths and are either joined by a plastic pipe collar or the pipes can have sealed spigots. The longer length of plastic pipes create less joints and thus it is faster to install. Plastic has excellent impact resistance. Plastic pipes are not adviseable in contaminated ground whereby they could suffer chemical attack.

Most private housing drainage is nominal 100mm pipes laid in an even bed of 10mm pea gravel. Pipes should always be laid with an even fall. The standard gradient for 100mm pipes is a fall of 1:80. A gradient of 1:100 can be acceptable on long straight runs. If flows are likely not to exceed 1l/sec then the gradient of a 100mm pipe should increase to 1:40. Some may ask why a pipe with less flow needs a steeper gradient; this is because fluid runs down wet pipes easier than dry pipes and in low flow situations much fluid is used in creating the wetted area on the pipe and hence there can be a tendency for effluent to sit in the pipe. The self-cleansing velocity of a pipe is regarded as 0.7 l/sec.

150mm pipes are used where larger effluent volumes are expected and these are appropriate to a minimum of 5WC's. (note; larger pipes require a larger wetted area.)

150mm plastic drain pipes or 150mm clay pipes would commonly be used on the surface water drainage to industrial units where heavy storm flows are expected.

Good drainage design will always allow roddable access to pipe runs. Commonly a rodding eye is located at the top end of a run, or a roddable drain gully.

Where the ground is likely to suffer from settlement, steeper gradients should be adopted.

Backfill to pipes should be granular material, usually pea gravel, and any compaction adjacent to the pipe should be by hand and not mechanical compaction.

Sewer Pipes.

In major drainage infrastructure spun concrete pipes are used and these can be of diameters from 300mm pipes to 2400mm pipes. A full range of concrete pipe fittings and concrete pipe bends are available. Pipes are typically joined by a captive rubber pipe gasket or a sliding gasket. Pipe lengths are typically 2.5m long.

Elliptical drain pipes are also manufactured which are used in storm and foul application since they have better hydraulic performance thereby allowing shallower gradients. The elliptical concrete pipes are joined with an elastomeric captive gasket joint. The types of spun pipes are often associated with stream and river diversion works and culvert works.

Manholes, Access Chambers and Inspection Chambers.

For foul drainage, good practice will make provision for access points at all major changes of direction. Access chambers tend to be of plastic construction and about 300mm diameter and shallow depth (450mm).

Inspection chambers tend to be 600mm diameter and normally occur where branches join the main run. Most companies now make standard plastic inspection chambers with additional inspection chamber rings that can be added to allow for increasing depth requirements. Inspection chambers at 600mm diameter however are not designed for a person to stand in them and thus are not suitable over depths of about 1 metre. The terminology between inspection chambers and manholes is often interchanged. Steel or cast iron inspection chamber covers fit directly onto the inspection chamber rings.

For drain intersections at deeper depth, a manhole is constructed. Typically these are built from Class B semi-engineering bricks laid in a concrete base. Clay channel pipe, and clay channel junctions are used to pass the effluent and storm water through the manhole chamber and these are then 'benched' with a strong sand and cement mortar. Step irons are built into the manhole wall to facilitate access. Historically all manholes were made of brickwork.

Current practice is now to construct manholes from precast concrete rings, sometimes known as concrete section rings, chamber rings or manhole rings. In smaller manholes, the rings are rectangular 1000 x 600 or 100 x 750. Circular rings are available as standard from 900 diameter to 3000 diameter.

Larger section rings are typically lifted into position with slings by crane or rough terrain forklift. When in position the lifting holes are filled in with manhole plugs. The manhole rings have tongue and grooved joints to allow jointing with mortar or flexible jointing compound. A manhole cover slab or manhole reducing slab sits on the top manhole ring which the supports the chosen manhole cover.

Within very deep manholes there can sometimes be a landing slab, or manhole landing slab to create a safer access inside.

Manhole Covers

Standard domestic manhole covers are usually pressed steel manhole covers or cast iron manhole covers. Most covers will withstand light duty vehicular traffic. Chequer plate manhole covers are of medium duty with heavier steel and ductile iron manhole covers are available up to a load capacity of 60 tonnes.

External recessed manhole covers are available to enable the cover to be disguised with paving or block paviours. Internal double seal recessed manhole covers are used in internal applications where drain access is necessary within a building. The cover is then screed and overlaid with the floor covering of the room. Lifting pins are required if the cover has to be removed.

Disguising manhole covers with flower pots, gravel etc is regarded as very poor practice since owners always think the will remember the location of the manhole but rarely do so. Manholes are rarely visited but their use is absolutely vital as some drains are literally impossible to rod or carry out drain jetting without them.

Storm Attenuation Tanks

In order to avoid surcharge of public sewers, it is common for the Local Planning Authority to insist upon a storm attenuation tank to be installed in the storm drain system just prior to the point of egress of storm water from the site. A drainage engineer or drainage consultant will be required to carry out a storm water calculation to predict the likely highest possible flow of storm water to be produced by the site. Such calculations are based upon data of rainfall occurring in 10, 20, and even 30 years worst-case storms. A measurement of all hard surface areas such as roofs and drained paving is then made in order to arrive at the peak flow likely to be encountered.

The attenuation tank typically will have a 100mm diameter and inlet both located at the bottom of the tank. In a storm, as the outlet pipe flow becomes restricted due to general system overload, the storm water from the site backs up and gradually fills the tank. Such freak storms tend to retain a peak flow for little more than 10 or 15 minutes and the tank is designed to fill during this period and then discharge after the storm.

Septic Tanks

Where main drainage access is not available it is common to install a septic tank. The tank basically allows solids to be separated from fluids and the fluids then discharged into the nearby ground. Bacterial action reduces the volume of solids but the tank will require pumping out by storage tanker every 6-12 months. Septic tanks do not work in clay ground since only soils such as sand, gravel and chalk allow adequate percolation to the surrounding ground.

Prior to building a septic tank it is normal to carry out percolation tests whereby a 300mm diameter hole is dug 1m deep and then filled 300mmm deep with water. The porosity of the soil is then calculated from percolation tables. From these tables, and depending on the usage of the septic tank the required outfall percolation pipes or soakaway size can be calculated. Percolation pipework is commonly laid in a herringbone fashion.

The capacity of a septic tank is calculated as:

Capacity in litres = (180 x person in dwelling) + 2000

The water emitting from a septic tank is not in any way regarded as clean water. This water nearly always enters the ground water table and often local streams or rivers. The Environment Agency and Local Authorities are increasingly discouraging of Septic Tanks and any outfall may need their consent.

Sewage Treatment Plants

Packaged waste water treatment plants are a progression from septic tanks and accelerate the treatment process whereby pure water is discharged which is acceptable for steams and into the water table. These systems are often called biodisc plants, biodisc systems or biodisc treatment plants. Small treatment plants can cost as little as £3000 to purchase. They still require percolation drainage or a means of water discharge. Sewage treatment plants are likely to phase out the use of septic tanks in future years.