Posted by on 17 December 2009 | 0 Comments

As the debate regarding the use of concrete over plastic continues to remain high on the agenda across the water management industry, the very specific strengths, benefits and weaknesses of both materials are tested vigourously on a regular basis.

Dr Vasilios Samaras of Asset International Ltd, former graduate of the Polymer and Process Technology School, part of the Engineering department at Swansea University (one of only three Universities to have gained the 5* RAE rating), looks here at the specific benefits that can be offered through the use of plastic, in this case Weholite, in water storage systems and pipe infrastructure. Weholite is a large diameter, gravity or low pressure structured wall pipe made from high density polyethylene (HDPE) resin. The low-pressure, high-technology pipes are extremely 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 1.5 bar 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. A flexible pipe is by definition, a pipe which will deflect when subjected to external loads. It is a commonly held assumption that any deformation of a plastic pipe is inherently detrimental and indicative of failure of the pipe to perform properly. This is a fundamental misconception. On the contrary, it is actually an advantage, and demonstrates the strength behind the flexibility of Weholite and HDPE materials. Flexibility in buried pipes is a desired attribute. 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 which does not noticeably deflect, will be continuously subject to an increased load. The interaction of pipes with soil A buried pipe and its adjacent soil will attract earth loads and live loads in accordance with a fundamental principle of structural analysis: stiffer elements will attract greater proportions of shared load than those that are more flexible. In other words, the more flexible pipe will attract less crown load than the rigid pipe of the same outer geometry.

This is because the rigid pipe does not transmit the loads into the surrounding material but the loads are transferred through the pipe wall into the bedding and therefore are subjected to much greater loads than the flexible one. In general, flexible pipes rely upon their deformation from imposed loads 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 a case of a rigid pipe all the loads have to be resisted by the pipe. This simply means that in case of overloading, the flexible pipe simply deforms further whilst the rigid pipe fractures. Both flexible and rigid pipes require proper backfill, although the pipe/backfill interaction differs. In general terms, a flexible pipe offers significant structural benefits to the project designer. In many situations, a properly installed flexible pipe can be buried much deeper than a similarly installed rigid pipe because of the flexible pipe/backfill interaction. A rigid pipe is often stronger than the backfill material surrounding it, thus 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. The Weholite Pipe The Weholite pipe and the enveloping soil mass, working in tandem as a composite unit, is one of the most remarkably 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. This prism load 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.

Conclusion In summary, 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 simply 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 behavior between rigid and flexible pipes, demonstrates clearly that the flexibility can be characterised as a factor which 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. Box Out Section Asset International Ltd. has been instrumental in leading the debate on the use of HDPE over traditional materials and rather than just manufacturing and supplying the systems, takes responsibility for continually developing and testing the materials used. The process of assessing the long-term product benefits as well as improved performance and usability of the products, is a commitment the Company continues to make.

Asset is an active member of the British Plastics Federation’s Plastic Pipes Group. WEHOLITE is a WRc approved product and is manufactured in accordance with Pr EN 13 476 Standard Structured Wall Piping Systems of Polyethylene (PE) published by the European Committee for Standardisation. WEHOLITE pipe satisfies the requirements of a number of international standards and is also approved for the use in public water supplies across the UK.