Hydraulic thrust ?
Hydraulic thrust forces occur at changes in direction, reductions in diameter (bends, tees, tapers) and at the ends of pipelines carrying water under pressure. They can be high and must be counterbalanced by appropriate anchored joint systems, or by anchor blocks.
Thrust forces occur in a pressurized main:
– at any change in direction (bends, tees),
– at any change in diameter (tapers),
– at each end (blank flanges).
These localized thrusts must be counteracted to prevent joint separation:
– either by using anchored joints,
– or construction of concrete anchor blocks.
The table below gives the thrust forces for 1 bar pressure. (For other pressures, multiply the pressure in bar by the site test pressure).
|DN||Thrust F in daN for 1 bar|
|90° Bends||45° Bends||22 1/2° Bends||11 1/4° Bends|
Anchoring water pipes – Why?
Hydraulic thrust forces occur at the location of changes in direction, reductions in diameter (bends, tees, tapered sections) and at the end of pipelines carrying pressurized fluid. These forces may lead to joint separation on the pipeline unless they are counteracted by using concrete anchor blocks or anchoring devices.
Hydraulic thrust forces can be very severe and must be counteracted by suitable anchoring devices or concrete anchor blocks.
|Value of coefficient K depending on the type of fitting|
(S’ smaller section)
Greater freedom in designing networks
Phasing out concrete anchor blocks
Anchoring technologies are increasingly taking the place of concrete anchor blocks, which have many drawbacks including their weight and size.
- Space on work sites
The larger the diameter of the pipeline, the bigger the anchor blocks required. This can lead to real problems as the limited space available under ground has to be shared by a great many networks (such as gas, sewage, telecommunications and cable networks).
- Trench opening time
Good concreting practices require a maturing time of 28 days before applying a load. Even if this time can be shortened, it constitutes a major constraint that is no longer acceptable in urban areas.
- Long-term risks of destabilization
These risks may be due to natural causes, such as non-homogeneous soils or irregular ground, nearby digging for work on other networks, especially in urban areas. These factors affect the stability and, therefore, the durability of concrete structures and raise the fear of possible junction separations.
- A heritage that is hard to manage
Major dismantling works have to be carried out when modifications or servicing are required on a pipeline or, later on, when a pipe has to be removed at the end of its life.
Anchoring: A modern approach to water supply networks
The utilization of anchoring solutions is growing fast in most countries all over the world. These solutions offer significant advantages:
- Limiting space requirement underground
Pipelines fitted with anchoring systems take up no more space than pipelines without anchoring. This leaves space for other networks and, what is more, helps reduce the volumes of material to be excavated.
- Reducing logistic constraints
For reasons such as accessibility and cost, it is not always easy to bring in several cubic metres of concrete to make anchor blocks. Pipeline laying rates are often limited by the rotation of trucks delivering concrete. Anchoring devices are light and easy to transport to the installation site, whether it be in the city, in the country, or in remote mountainous or desert regions.
- Quick installation and setting into service
Anchoring systems are extremely quick to install, especially the Rapid Vi system. What is more, they can be submitted to hydraulic tests immediately after being installed.
- Proven stability and durability
The operation of anchoring systems relies on a combination of their intrinsic resistance to joint separation and friction with the soil. PAM's recommendations on anchoring lengths take into account the type of soil and the risks of works conducted in the vicinity of the pipes. The anchoring systems receive the same level of corrosion protection as the pipes and fittings.
- Possibility of dismantling
Pipelines can always be dismantled with tools supplied by PAM, without entailing long and extensive civil engineering work.
Greater flexibility for network acceptance procedures
Pipe laying and site acceptance procedures have been speeded up and have reached an unprecedented level of reliability thanks to anchoring devices.
- No longer any need to wait for concrete to set
Pipes are ready for pressure testing as soon as the anchoring devices have been fitted.
- Doing away with test anchor blocks
There is no longer any need to make test anchor blocks for the individual testing of pipeline segments.
- Shorter segments can be tested
It is now possible to test shorter lengths of pipeline so it is easier to locate and solve any problems that may arise and trenches can be refilled more quickly.
PAM anchoring devices can be tested up to their allowable test pressure (PEA) during acceptance tests.
Anchoring solutions to meet increasingly strict pipe-laying requirements
The various anchoring solutions can be adapted to respond to even the most difficult pipe-laying situations:
- Casing pipe-laying, road crossing, tunnels, bridges,
- Directional drilling or pipe bursting replacement (UNIVERSAL Ve),
- Pipe-laying in poor soil or submerged ground, etc.
Anchoring and sustainable development
- Save on materials: joints weighing just a few kilos can replace several tons of concrete.
- Save space: thanks to the compactness of these systems.
- Save on transport (for earth fills and concrete).
- Save time.
- Save wood, as formwork for concrete anchor blocks is no longer needed.
What length of pipeline should be anchored ?
The technique is based on the principle of anchoring joints over a sufficient length on both sides of a region of hydraulic thrust, such as a bend, in order to harness soil/pipe friction forces to counteract the thrust force.
The calculation of the length to be anchored does not depend on the anchoring system used. It depends on the test pressure, the pipe diameter and the parameters shown in the two figures, C & D.
Anchoring lengths (in m) for average soil and 10 bar test pressure.
- the laying conditions,
- the quality and compaction of the earth fill,
- uncertainty regarding the physical characteristics of the earth fill.
In case of ground with an average mechanical resistance, consisting of gravel or silty sand, with an internal friction angle of 30°, for a zinc or zinc-aluminium coated pipe with pore-sealer and a safety coefficient of 1.2 and a test pressure of 10 bar, the lengths to be anchored are indicated in the table above.
Where applicable, allowance should be made for any partial presence of groundwater by correcting the weight of the full pipe by applying the corresponding Archimedes' value.
Use of concrete anchor blocks is the most commonly applied technique for containing the hydraulic thrust of socket and spigot mains under pressure. Its use is now in sharp decline.
Various types of concrete anchor blocks can be designed, depending on the configuration of the main, the strength and type of soil, the presence, or absence, of significant amounts of ground water. The block contains the hydraulic thrust forces:
- either by friction on the soil,
- or by bearing against the ground.
In practice, anchor blocks are designed by taking into account both the friction forces and the soil reaction against their bearing surfaces.
If the construction of concrete anchor blocks is prevented either by congestion problems or by low strength ground, the technique of joint anchoring or joint restraint can be used.
Dimensioning (usual cases)
The volumes of concrete suggested in the following tables are calculated with both the soil friction and ground bearing support in mind, for the most common types of soil encountered. If trenches subsequently need to be excavated in the vicinity of the anchor blocks it is advisable to reduce the water pressure during the work. The design assumptions are given below.