Sheet 04 · new runs

New runs: levels, bedding and testing

A fall budget built up from the one invert that cannot move, then the trench, the bedding, the backfill and the five tests that close the job.

Sheet 04 · back to the defect schedule

Setting out from the connection upwardsLevels

Section through a pipe trench showing, from the bottom up, the granular bed, the pipe with haunching to the springings, the granular surround over the crown, selected fill, and the general backfill to ground level ground level general backfill, compacted in layers selected fill granular surround pipe haunch granular bed cover trench width
The layers are specified separately because they do separate jobs: the bed takes the pipe off the trench bottom, the haunching stops it rolling and carries load into the sides, the surround protects it during backfill, and the selected fill keeps stone off the crown.

A new run is set out from its connection and worked upwards, never the other way about. The invert of the connection is a fixed figure: it is already in the ground and it cannot be moved. Everything above it — every chamber invert, every gully, every stack base — is a sum built on that one level, and the whole design either fits in the fall available or it does not.

So the first sheet on a new run is a fall budget. Length by length, the gradient wanted is multiplied out to a fall in millimetres, those falls are added up from the connection, and the result is compared with the invert the highest point actually needs. Where the budget does not close, something gives: a slacker gradient on part of the run, a deeper connection, a pumped section, or a different route.

The trench, and holding its sides upExcavation

Trench width is a working figure, not a minimum: it has to let someone bed and haunch the pipe properly, which is a wider hole than the pipe needs to pass through. Beyond a shallow depth the sides are either battered back or supported, and a trench nobody can safely enter is one nobody enters — the pipe is laid and bedded from the surface or the hole is made safe first.

Water in the trench is dealt with before the pipe goes in. A sump at the low end and a pump running while the bed is placed is normal on the lower ground; placing granular bedding into standing water is not, because the fines wash out of it and the bed stops being a bed.

Bedding the pipeClasses

Bedding is chosen for the pipe material, the depth, the load above and the ground the trench is cut in. The classes below are the ones that come up on this kind of work; which applies is a design decision recorded on the drawing, not a choice made in the trench.

  • Rigid pipe Granular bed only

    Single-size material under the barrel, nothing round the sides. Firm even ground, shallow cover, a rigid pipe.

  • Line held Bed and haunch

    Granular to the springings. Holds the pipe in line while the surround goes in and spreads load into the trench sides.

  • Flexible pipe Full surround

    Granular from below the barrel to over the crown. Flexible pipe, soft or variable ground, or deeper cover.

  • Shallow cover Concrete surround

    Cover too shallow for anything else, a trafficked slab above, or a structure crossed. Movement joints line up with the pipe joints.

  • Dry even ground As-dug bed

    Trimmed trench bottom, nothing imported. Only where the ground is even, free of stone and stays dry.

Backfill in layersBackfill

Backfill goes in layers with each layer compacted before the next, and the first layers over the pipe are selected fill — graded material with the stone taken out, placed by hand and tamped at the sides rather than driven over the crown. The compaction plant stays off the pipe until there is enough cover over it to take the plant.

The tests that close the jobTesting

The tests that close a drainage job are short, boring and conclusive, and every one of them is recorded on a sheet with the date, the run, the node references and the result. The sheet is what an adoption submission, a building control inspection or a buyer’s surveyor actually reads.

Tests carried out on a new run, and what each proves
Air testPressure held in a plugged length and watched for a drop over a set period. Quick, and it finds an unmade joint. A fail is confirmed by water test before anything is dug up, because air tests are sensitive to temperature.
Water testThe length filled to a head above the pipe, left to absorb, then topped up and measured. Slower, but it gives a figure for loss rather than a yes or no.
Deformation testA mandrel pulled through a flexible pipe. Proves the pipe has not been squeezed out of round by the backfill.
Infiltration checkThe run watched with the ground water up. Proves nothing is coming in, which an air test cannot show.
CCTVThe finished run surveyed and coded. Proves line, level, joints, debris left in the pipe, and that the laterals are connected where the drawing says.