Flexibility equals strength

As the debate rages over the use of concrete or plastic, the benefits of both materials are vigorously tested. Dr Vasilios Samaras of Asset International looks at the benefits of plastic in water storage and pipe infrastructure.

A flexible pipe is, by definition, one that will deflect when subjected to external loads. It is commonly assumed that any deformation of a plastic pipe is inherently detrimental and indicative of failure to perform properly. But this is a misconception.

Deformation is an advantage. And it demonstrates the strength behind the flexibility of high-density polyethylene (HDPE).

Understanding how the flexible pipe relates to its neighbouring soil – thereby establishing a functional pipe/soil composite structure – is key to successful design.

Under load, even the most flexible pipes may only experience small amounts of deflection, provided they are installed correctly. In general, during the installation of a buried pipe, backfill is placed around the pipe in the trench. Irrespective of how well the backfill is compacted adjacent to the pipe (side-fill) during installation, further compaction develops with time.

Additional settlement then occurs in the side-fill resulting in an increase in deflection for a flexible pipe. A rigid pipe that does not noticeably deflect will be continuously subject to an increased load.

A buried pipe and its adjacent soil will attract earth loads and live loads in accordance with a fundamental principle of structural analysis. The more flexible pipe will attract less crown load than the rigid pipe. This is because the rigid pipe does not transmit the loads into the surrounding material. Instead, the loads are transferred through the pipe wall into the bedding.

Vertical loads

In general, flexible pipes rely on their deformation to mobilise the support of embedment material on both sides of the pipe. Their primary structural function is distributing the imposed vertical loads to the surrounding soil.

Only a small portion of the imposed loads are actually carried by the flexible pipe itself. Instead, load is transferred to the surrounding bedding material.

On the other hand, in the case of a rigid pipe, all the loads have to be resisted by the pipe. This means that, in the case of overloading, it fractures. Both flexible and rigid pipes require proper backfill, although the interaction between pipe and backfill differs.

In many situations, a properly installed flexible pipe can be buried much deeper than a similarly installed rigid pipe. A rigid pipe is often stronger than the backfill material surrounding it. So, it must support earth loads well in excess of the prism load above the pipe. Conversely, a flexible pipe is not as strong as the surrounding backfill. This mobilises the backfill envelope to carry the earth load.

The flexible pipe/backfill interaction is so effective at maximising the structural characteristics of the pipe that it allows the pipe to be installed in very deep installations, many times exceeding allowable cover for rigid pipes when identically installed.

Alternative solution

Weholite is a large-diameter, gravity or low-pressure structured wall pipe made from HDPE resin by Asset International. The low-pressure, high-technology pipes are tough, flexible, chemically resistant, and offer an alternative solution to other forms of traditional pipe systems, including concrete and GRP.

With Weholite, raw material properties have been combined with advanced product technology to create a lightweight engineered pipe with superior loading capacity. It is commonly used to convey liquids or air underground, above ground or under water in low-pressure applications up to 1.5bar internal pressure.

The material can be used across a range of applications, including surface drainage, foul sewers, inter-process pipework, culverts, attenuation tanks, ducting and outfalls.

The Weholite pipe and the enveloping soil mass, working in tandem as a composite unit, is one of the most synergistic systems in engineering. When the soil is properly compacted around the pipe, the load-carrying capacity of the pipe-soil system far exceeds the individual capacity of either component by itself.

In general, in the designing of polyethylene pipes, it is commonplace to assume that the overburden load applied to the pipe crown is equal to the weight of the soil column projecting above the pipe.

Often, this is referred to as prism load, which is a handy convection for calculating the earth pressure on the pipe when estimating vertical deflection, but the actual load transmitted to a pipe from the soil mass depends on the relative soil stiffness and pipe stiffness.

The dead load applied to a flexible pipe is considerably less than the prism load because soil-shear resistance transfers part of the soil loads that is directly above the pipe into trench sidewalls and embedment. This transfer is called arching, and this term is usually used to imply reduction in vertical loads.

The soil tries to follow the pipe downwards, but soil movement is restrained by shear resistance, along shear planes in the backfill soil, to be transferred in the adjacent soil. Therefore the amount of force exerted on the pipe by the backfill is less than the weight of the backfill soil mass, that is, less than the prism load.

Compared with rigid pipes, flexible pipes are versatile and have important structural performance advantages. Unlike rigid pipes, flexible pipes have excellent resistance to differential settlement.

Plastic pipes, when overloaded will deform further to generate greater passive earth pressures, until the system regains equilibrium. In contrast, overloaded rigid pipes are subject to fracture that can result in catastrophic failure.

The comparison of the behaviour between rigid and flexible pipes, demonstrates clearly that the flexibility can be characterised as a factor that increases the safety of a buried pipe and is not a disadvantage.

The strength of flexibility in combination with the unique properties of Polyethylene makes Weholite an ideal product for water solutions.

Dr Vasilios Samaras, Asset International. T: 01633 271906.

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