Where trench formation has little bearing strength and won’t support pipe bedding material effectively, it’s necessary to provide a stable formation before pipe laying. Such conditions usually occur in peat, silty ground, soft clays, running sand, or in artificially-filled ground. Although trench formations are sometimes stabilised with concrete, it’s unlikely to guarantee long-term stability, so a flexible bedding construction is the preferred method.

The trench and manhole base should be over-excavated up to 600-800mm, depending on the bearing strength of the ground. Small particle sized hardcore of less than 75mm is then compacted in layers to form a firm trench bottom. A 50mm layer of lean-mix concrete is then placed as a blinding layer and the pipe is then laid on 150mm bed of granular material above the blinding layer.

The extra depth of granular bedding material of 150mm for sleeve-jointed pipes (rather than the usual 50mm in good ground) is required because of the hard and inflexible nature of the constructed trench bottom using lean mix concrete.

The pipe bedding construction requirements are calculated in the normal way by using the Clay Pipe Development Association (CPDA’s) Bedding Tables or Simplified tables of external loads on buried pipelines. It’s important that ‘wide trench’ design criteria are used, because ‘narrow trench’ conditions can’t be guaranteed in this situation, because of the practicalities that trench walls of soft ground may continue to fall into the trench making it wider or irregular in profile than originally designed thereby increasing the loads locally on the pipeline.

Dependent upon the final traffic and fill loads various bedding configurations providing increasing levels of support can be used to optimise the cost of the installation, a class ‘F’ bedding, has granular bedding material placed under the pipe and selected excavated backfill material placed around and up to 150mm above the pipe and compacted before the main trench backfill is placed. A class ‘B’ granular bedding extends half way up the pipe to the springing points, then excavated material placed above, or a class ‘S’ full granular surround extending up to 150mm above the pipe could be utilised, the additional imported bedding material will either partially or wholly replace the selected excavated backfill material.

Where groundwater exists at a level above the interface, between the hardcore and the new stabilised trench bottom, a procedure detailed in Waterlogged ground (below) should also be applied. The geotextile should surround both the material in the base of the trench as well as the pipe bedding material.

Alternatively, a stable pipeline base can be constructed via a geogrid reinforced granular mattress below the bedding. The unique interlock mechanism between grid and aggregate creates a supporting mattress which provides an efficient load spread into the weaker ground, and also helps to control any longitudinal differential settlement which may take place.