Getting trench preparation right is one of the key elements of ensuring a successful clay pipe installation. However, a best practice approach to cutting, backfilling and pipe jointing must encompass critical Health & Safety considerations, writes Paul Wydell, Product Manager, Hepworth Clayware.

Contractors are faced with a range of challenges when it comes to installing below-ground drainage, balancing the need to minimise time spent on site to complete projects on schedule with site constraints and weather conditions. A best practice approach to trench preparation is essential however, to ensure that the dual requirement of ensuring that an installation meets regulations but also considers health and safety throughout to avoid risk to workers and buildings.

Excavation

Any buried service such as gas, electricity or water should be located prior to excavation and uncovered with extreme care. An often neglected task is where to place the excavated material to avoid boxing yourself in between the building and the excavation. Once excavation begins, all excavated material should be placed beyond the point of influence upon the trench wall. (usually greater than the depth of the excavation) Trenches should be kept free from water where possible. In addition, disturbance of the trench formation by foot traffic should be avoided. Top-soil should be put aside for later reinstatement.

Good safety practice is essential in trench preparation and excavation, as unsupported trenches can lead to fatalities. Excavations require adequate support to be installed throughout the process of excavation and during withdrawal when backfilling, plus barriers need to be strong enough to cope if someone falls against them. Continual checks and record keeping are the only way to ensure that staff and assets are protected. Good practice options for domestic trench excavation can include installing trench sheets and shoring supports or simple battering of trench sides to a 45° angle.

Bedding

Trench preparation is covered by Approved Document H of the Building Regulations, and BS EN 1610:2015 Construction and testing of drains and sewers. Both documents detail requirements for bedding and backfilling which vary according to the type of pipe used, as well as depth, trench width, subsoil type, and the loading expected. Certain installation approaches are available to rigid clay pipes which are not appropriate for other pipe materials and these must be established at the outset.

If the sub-soil can be accurately trimmed manually, such as sandy or friable sub-soils then a Class D Natural trimmed trench bottom can be created for clay pipes without using any granular fill. The trench bottom requires hand-trimming with a spade to support the pipe along its length, providing recesses to accommodate the jointing couplings, however this necessitates careful workmanship. This installation method takes the most time but additionally provides the greatest cost saving and least environmental impact.

A Class N bedding is used where the sub-soil cannot be accurately trimmed to support the pipe. The trench should be loosened 50mm below the pipe barrel and a bed formed of as-dug or granular material if suitable and well compacted across the full trench width. Shallow recesses to accommodate couplings should be made and the pipe barrel rested firmly on its bedding. Clay pipes can also be bedded in all soil conditions with a Class F granular flat bed or a Class B haunch granular bedding which rises up to half pipe level, unlike flexible pipes which require a full granular surround.

Pipe cutting

Time can be saved on pipe cutting by installing standard short lengths of 300mm, 600mm, and 1000mm primarily at manhole positions as rocker pipes or to adjust the pipeline length at manhole or junction positions. A lever action chain cutter is recommended to quickly and efficiently cut Hepworth 100mm and 150mm diameter SuperSleve clay pipes to non standard lengths as required on site, any edges removed with a trimming tool prior to jointing. The company’s technical department can advise on the appropriate chain cutter for each pipe size.

A powered masonry saw is recommended for the on-site cutting of non standard lengths of 225mm and 300mm diameter clay pipes. Before any pipe cutting operation is commenced, the operator must be trained in safely operating both the masonry saw and selecting and fitting the correct blade specification for the job such as a continuous rim diamond tipped blade suitable for cutting marble and hard ceramics, plus any specific manufacturer instructions should also be followed. Additionally ensure that additional and correct PPE is being worn for the task, protecting the operator against airborne dust by the wearing of goggles, gloves and a dust mast that prevents the respiration of crystalline silica (RCS) as an absolute minimum. Ideally use a saw with a water feed for dust suppression.

During the cutting operation the pipe should be positioned in a horizontal and stable position and supported to avoid damage to the blade or the pipe as the pipe separates.

Jointing

Effective pipe jointing requires a little experience and some basic best practice to be observed including checking for any damage to components to be used and ensuring that the seal inside the coupling and the outside of the pipe spigot is clean prior to jointing. Lubricate the pipe end chamfer almost to the required insertion depth, but not the coupling seal as this only increases the insertion force. Offer the lubricated spigot up to the coupling, and with the pipe slightly offset horizontally, push firmly whilst centralising the pipe in a single action until the joint is pushed home.

Testing

Hepworth Clay advise that a simple air test should be conducted as installation proceeds in accordance with Approved Document H and BS EN 1610:2015 and before any backfilling takes place. Our top tip is to conduct an air test every three pipes.

Backfilling

Best practice for backfilling entails compacting material by hand in layers not exceeding 100mmm, evenly on either side of the pipe until a minimum of 150mm coverage over the pipe is reached. Then further backfill should be placed in layers no greater than 300mm, with each well compacted. Mechanical compaction equipment should not be used until there is a minimum of 450mm of compacted material above the pipe crown.

Clay pipe has a life expectancy in excess of 100 years however following established best practice for trench preparation is a key part of ensuring that installations are fit for purpose, saving material, money and the environment.