The VelociFoam method creates a barrier layer of rigid closed-cell Polyurethane Foam between a pipe surface and the surrounding soil or backfill systems.
This layer can have several functions, grouped into two connected areas: mechanical protection and geotechnical performance.
The single most important technical improvement over classic buried pipe systems is increased protection for the pipe itself.
Research in Norwegian hydropower shows that the overwhelming majority of buried pressure pipe failures occur due to incorrect installation, rather than faulty pipe. There are two main failure modes: direct impact damage on composite pipe surfaces and corrosion on metal pipes, caused by destruction of coatings, which both lead to an inability to withstand water pressures and leakage or hydraulic explosion.
A 100mm / 4-inch layer of PU foam creates a very resilient barrier that negates almost all mechanical effects of backfill, whether native or imported.
Even very sharp objects, with a great weight of overhead cover, will find it very hard to penetrate the foam layer to impact the pipe surface. As an example, the diagram below shows that a very sharp, and quite unlikely piece of granite, would hardly penetrate the surface of the PU foam, even under quite disadvantageous conditions.
In a normal trench, with 1 meter of high density backfill, a sharp stone will not penetrate more than 4mm below the surface of the foam. For the same stone to penetrate all the way through the foam would require nearly 10 cubic meters of solid granite as backfill.
During laboratory tests at Normet in 2016, the actual resistance to penetration was higher than calculated. This is probably caused by the penetrating object being subjected to torque force due to the anisotropic foam cell structure, which causes the forces to deviate and encounter greater resistance, due to compression from the entire foam mass.
The consequence of these mechanical properties mean that VelociFoam allows the use of native backfill, or excavated masses, because the foam isolates the pipe from possible impact-related damage.
Buried pipelines, particularly those under pressure, must be stabilised and supported by a combination of engineered backfill and stable surrounding soils. Failure to do so leads to differing degrees of system failure, such as rupture and ovalization.
Aside from the effects of liquid pressure in the pipe, other factors affecting stability and performance are dynamic loading and buoyancy. Less stiff pipe types, such as plastic HDPE (High Density Polyethylene) which is otherwise an ideal candidate for low pressure water systems, suffer from poor resistance to ovalisation when buried under roads. This leads to a loss of hydraulic performance, cracking and leaking, and ultimately complete failure of the system.
Unrestrained joint systems are cheaper, but require very careful backfilling, with laborious and time-consuming compaction, which often loses effectiveness over time, even when installed correctly, which is often not the case. When the backfill systems fail, pipe sections move apart from each other in the coupling, leading to leakage and then dangerous hydraulic explosions, that are powerful enough to destroy major roads.
Thrust Anchor Blocks
Pipe systems often require bends to negotiate terrain challenges, both vertical and horizontal. These bends experience thrust forces that try to force the pipe to move in the direction of the apex of the bend. In large, pressurised pipe systems, these forces must be counteracted, or the pipe may move; this is a problem even with restrained or welded joint systems, but with unrestrained systems, the problem leads to failure of the couplers / joints. Historically, where thrust forces exceed the ability of the surrounding soil to counteract them, engineers use a concrete block to take up the thrust forces through ‘anchoring’. VelociFoam eliminates the need for thrust blocks in all but the most extreme circumstances.
The technique was first tested in small scale thrust simulation tests, and then implemented by the Norwegian state-owned hydropower giant Statkraft in 2016, with complete success. The system has been running for 2 seasons, with performance that exceeds prediction according to the current models by 400%.
The VelociFoam method transforms unrestrained pipeline into a fully-restrained pipeline, using the bearing strength and frictional resistance of the soil-foam interface, to turn the entire pipeline into a single thrust block.