Jet grouting is a construction process using a high kinetic energy jet of fluid to break up and loosen the ground, and mix it with a thin slurry.
It is not truly grouting but rather a hydrodynamic mix-in-place technique producing a soil-cement material.
The concept first appeared in the late fifties in the United Kingdom. Single, double and triple jets carry the necessary hydrodynamic energy to break up and mix the soil with the slurry. It is suitable for mass treatment, linear treatment and inclusions, in soils of good to poor groutability.
Jet grouting makes use of three physical processes, singly or in combination:
- the very high speed jet loosens the soil
- the jetting fluid washes some of the soil to the surface
- the slurry adds a binder to the soil mix
The sequence of work is usually as follows:
- A small (100-200mm dia.) hole is drilled to the required depth,
- A high pressure (several dozen MPa) fluid is pumped through one or more small (1-10mm) nozzles on a monitor at the foot of a 70-100mm dia. drill string,
- The drill string is slowly raised and rotated to form a column of soil cement.
During jetting, material in excess of the soil cement mix must rise freely to the hole collar (to prevent the excess material fracturing and disturbing the surrounding ground). It is removed from site as it emerges.
The result (diameter, composition and strength of the columns) is dependent on drill string rotation and raising speeds, jet pressure and flow, grout mix; soil type, grain size distribution, composition and compactness; and jet configuration (single, double or triple jet set-up).
In the single jet configuration, the jetting fluid is grout which performs the three functions of loosening the soil, removing the excess, and providing the binder.
A significant proportion of the kinetic energy of the jet is lost through friction in the soil suspension and the excess material may become too viscous to rise freely to the hole collar.
If the very high velocity grout jet is sheathed in a cone of air, the radius of action of the jet in the same soil is substantially increased.
In the double jet configuration, the air improves performance in removing in situ soil by acting as an air lift.
In the triple jet set-up, soil loosening and removal is effected by water and air jets together, independently of the incorporation of the binder, which is delivered by a simultaneous jet of low pressure grout (at a few MPa) emerging from a low nozzle.
The high pressure pump and circuit in the triple jet configuration carries only water. There must also be a second, low pressure grout pump, and a three-line drill string.