How to install a clay drainage system

Planning

Depending upon whether the new pipeline to be installed is to be a drain (serving a single property) or a sewer (serving two or more properties) will dictate which authority, regulation and guidance that should be followed with respect to its design and installation. In general terms, private drainage should follow the Building Regulations Approved Document H. Where England, Wales, Scotland and Northern Ireland each have a local version of their regulations required for building. Conveniently, covering similar geographical areas sewer installation guidance should follow that of Sewers for Adoption in England, Welsh Ministers Standards in Wales, Sewers for Scotland in Scotland and Northern Ireland Water for Northern Ireland.

Design

System design utilises the documents mentioned above plus additional guidance in the form of BS EN 752, Drains and sewer systems outside buildings to create a topographical design layout and hydraulic design. Where structural design is led by BS EN 1295 Structural design of buried pipelines under various conditions of loading and BS 9295 Guide to the structural design of buried pipelines these documents will help form the basis of a workable and self-maintaining drainage system for many years to come.

One of the many benefits of clay drainage is its long asset life derived in the main from its durability originating in product strength. Vitrified clay pipes are load-bearing structural components, whose intrinsically high strength is determined by material properties and unique manufacturing process. This strength is verified in the factory by continual testing and is delivered to the site for incorporation into the permanent works.

Excavation

Before beginning excavation, all underground services should be located and uncovered with extreme care, not simply limiting the search to the usual suspects of gas, electricity and water.

The trench should be adequately supported during excavation, according to the depth and width of trench being dug and diameter of pipeline being installed. This can take the form of simple trench sheets with manually adjustable trench props all the way up to trench boxes with hydraulic rams to hold the trench walls upright and stable. Providing substantial protection for the ground workers.

The excavated material should be placed an appropriate distance away from the trench wall. The point of influence of excavated material loading on the trench wall usually starts at a horizontal distance approximately equal to the depth of the trench. In other words, within the 45 – 90 degree angle from the base of the trench.

Where possible, trenches should be prevented from filling with water and protected from disturbance caused by footfall which will reduce the amount of support provided by the trench base. The only solution is to dig deeper and reinstate with hardcore and pipe bedding material. Adding time and cost to the project.

Bedding

For a subsoil that can be trimmed manually, then a class D natural trimmed trench bottom can be created without any granular fill. The trench should be trimmed along its length with recesses to accommodate the jointing couplings and to ensure that the pipe is fully supported. Whilst this method of installation takes the most time, it is also the most efficient both financially and environmentally. Where the excavated material can be adequately compacted it can be returned as backfill to the trench for maximum environmental benefit and cost-saving.

Alternatively, the pipe can be surrounded by recycled aggregate to BS EN 13242 creating a class S bedding. The structural suitability of the aggregate can be determined by a compaction fraction test to determine the ability of the aggregate to provide additional structural support to the pipeline. The lower the fraction the better the aggregate. Compaction fractions lower than 0.15 (even though it’s a decimal!) are best.

Primary aggregate from quarries and dredging should only be used with clay drainage where recycled aggregate is not available.

The strength of clay pipes means that pipe bedding surround is not normally there to augment the strength of the pipes but to protect it from the surrounding ground from subsequent movement and improve the speed of installation.

Jointing

Starting at the downstream end, lay the first pipe in the trench providing temporary bracing to resist the jointing action of subsequent pipes.

In order to create an effective pipe joint, you should first check for any damage on the components to be used and ensure that the seal inside the coupling and the outside of the pipe spigot is clean. The pipe end chamfer (but not the coupling seal) should be lubricated to the required insertion depth. The lubricated pipe spigot should be offered up to the coupling with the pipe offset slightly horizontally. The pipe should then be pushed firmly whilst centralising in one action until the joint is pushed home, this should be heard and felt with a satisfying clunk.

Pipe cutting

When installing pipe at manhole positions as rocker pipes or to adjust the pipeline length at a manhole or junction positions, installation time can be minimised by using standard short lengths of 300mm, 600mm and 1000mm. When non-standard pipe lengths are required, Hepworth SuperSleve pipes of 100mm and 150mm can be cut on site using a lever action chain cutter for 225mm and 300mm pipes a powered masonry saw is required. Operators should always be trained to safely operate the saw and select the correct blade. PPE including goggles, gloves and an appropriate dust mask should be worn. Ideally the saw should also have a water feed to help supress the dust.

Testing

An air test should be conducted on Hepworth clay installations in accordance with Approved Document H and BS EN 1610:2015. This should be conducted on every three pipes before any backfilling takes place. This will highlight any incorrectly made joints before backfilling takes place making any remediation quick and easy.

Backfilling

Backfilling should include compacting material by hand in layers not exceeding 100mm, evenly on either side of the pipe until a minimum  coverage of 150mm over the pipe is reached. Additional backfill should then be added in layers of less than 300mm, compacting each layer in turn. The first layers will need to be compacted manually. Once there is at least 450mm of compacted material above the pipe crown, mechanical compaction equipment can be used.

Following best practices when laying clay pipe and taking time to ensure that the trench is fully prepared will ensure that the pipe is fully functional with a life expectancy of beyond 100 years. In the long run this will help to save material, money and the environment. Delivering a clay drainage system with a low whole life cost and a high whole life value.