The high strength of the strut is designed to both resist the load and act as a support - essentially preventing collapse of the excavation walls and keeping everything in equilibrium

An earlier article published in the Engineer’s Journal considered the ‘basic principles’ behind Groundforce’s hydraulic struts. This piece continues the theme but takes a more in-depth approach to the subject by discussing fluid compression, installation pressure and the modulus of elasticity.

The first article briefly introduced the concept of ‘installation pressure’ and how pressure in the struts increases between two opposing walls. Once a strut has been installed correctly and the excavation commences, the retained ground applies a load onto the strut. The high strength of the strut is designed to both resist the load and act as a support; essentially preventing collapse of the excavation walls and keeping everything in equilibrium.

## Hydraulic fluid in compression

Looking at the hydraulic ram in isolation, the diagram below shows the direction of load as it travels along the piston rod, into the piston and finally through the column of hydraulic fluid.

A common misconception is that hydraulic fluid and other liquids are incompressible. They are, however, subject to a small amount of compression when under load. This type of compression is common for all types of material (including metals) but the extent to which this happens varies depending on what is known as the ‘modulus of elasticity’ , ‘Young’s Modulus’ or ‘E value’ of the material. See the two values below for comparison:

Esteel = 210 000 N/mm²

Ehyd = 160 000 N/mm²

## Establishing the modulus of elasticity

To define the above, the lower the E value the more compressible the material. The Ehyd value given above has been derived from data that Groundforce has gathered through a series of rigorous tests which involved setting a strut in a test rig, applying a load to it and measuring the subsequent compression. The test result values were then set into the formula below to determine the ‘modulus of elasticity’ of hydraulic struts:

E = stress = σ = F / A

strain     ε     δL /L0

A stiffer support with a higher E value tends to attract more load than a less stiff one. Given two identical excavations; one with a structural steel frame and one with a hydraulic system, the steel frame would be more heavily loaded. This means that in some scenarios, it may be possible to use a smaller hydraulic frame than a steel one as the load it needs to resist is not as high; this could prove advantageous in sites where space is a limiting factor.

To reduce the amount of compression in a strut, a process described as ‘pre-stressing’ is introduced. This is when the anticipated load (calculated by design) is put into the system at the point of installation before the ground has been excavated. This reduces any additional load that you would expect on the frame as the excavation proceeds and it is this additional load that will cause the compression.

It is possible to induce a relatively accurate amount of pre-stress into the struts by simply increasing the installation pressure accordingly. If we consider that pressure is equal to force divided by area, the linear relationship can be used to determine the actual installation pressure required for a specific load in any strut type.

 Groundforce Struts Strut Type Strut Capacity* Load/1000 psi (69bar) (kN) (Tonnes) (kN) MP50 530 8.6 86 HSK80 800 13.8 138 MP125 1250 21.5 215 HSK150 1500 34 340 MP250 2500 44.2 442

*Using non-standard pumps it is possible to achieve a pre-stress in a single prop of approximately 80 per cent of its axial capacity

Groundforce offer specialist solutions to the construction industry, dealing in shoring equipment, piling equipment, confined space entry, vacuum excavation, pumping, trenchless technology, pipe testing and rehabilitation.