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Foundations – Bridging a Drain Run

There may be occasions where existing services clash with the position of a proposed foundation. An example might be a drain run which runs from the back of a house and crosses the intended line of the concrete foundations for a new extension. However, this may not be an issue if it is dealt with correctly.

You will need to have the alterations approved and inspected to ensure that the new detail is viable and acceptable. The following guide details how a drain run might be accommodated in this situation.

Building Load

The foundations take the load of a building or structure and transfer and distribute this to suitable load bearing ground beneath it. So, where a drain run or service pipe crosses the path of the foundations, a suitable means of bridging will be required to protect the pipe from any load.

Foundation Depth

It should be noted that the base of the foundation on either side will need to be deeper than the base of the service pipe to ensure that no lateral pressure acts on it. If the drain run (for example) is found to be lower than the bottom of the proposed foundations, then the trench on either side will need to be excavated deeper so that the bottom of the foundation is deeper than the drain run. The trench for the foundation will be stopped sufficiently short of the drain run on either side. The reason for this is to ensure that the drain itself remains undisturbed and that the existing support for the pipe work isn’t compromised.

Shuttering Around Drain

Undisturbed Drain Run Shuttering ply used to retain concrete

Shuttering can be used to hold back the concrete on either side when it is poured so that a void is left around the drain run. Once the concrete has cured, the area over the drain can be backfilled. If, as in our example, it’s an old drain run and the surrounding area has been left undisturbed, most of the depth can simply be back filled with previously excavated spoil. A 150mm layer of consolidated hardcore and sand blinding is then laid as a base for the concrete forming the bridge.

Steel Work

Steel beams spanning the drain run Close up showing steel and mesh

Steel work can then be positioned to bridge between the two sections of the foundations. In the accompanying images you can see how two steel beams were positioned over the gap. Commonly, a couple of ‘I’ beams can be used for this. The length of the beams will be such that their ends have sufficient bearing on the concrete work either side of the gap. You can also see in the images how the steel beams have been wrapped in steel reinforcement mesh. This ensures an adequate bond is achieved between the steel work and the concrete. A minimum of 50mm of concrete all round the steel is needed to protect it.

Concreting Around Steel Work

Shuttering ply to retain concrete Once the work has been inspected and passed by the District Surveyor, the steel work can be encased in concrete. Simple timber shuttering is placed around the area to create a form for the concrete. This needn’t be anything elaborate – shuttering ply of a suitable height along the sides and ends, and suitable bracing will suffice. The formwork only needs to retain the concrete while it goes off. Using the same proportions of cement, sand, and aggreagate as for the foundations, the concrete is mixed. This is then poured into the formwork and worked to ensure there are no voids and air pockets are released. The upper surface is tamped down and left flat and level ready to receive the brick work.

Once the concrete has cured, the drain run is completely protected from any pressure. Loads acting down at this point are transferred by the steel work to the adjacent foundation concrete and the ground beneath.
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