Jet grouting (ground improvement)

A ground-improvement technique: a slim rod is drilled down and, on the way up, a cement grout is jetted at very high pressure that erodes the soil and mixes it in place, forming a column of soil-cement. Side by side, the columns strengthen, seal or underpin the ground — even beneath existing buildings, with little room and low vibration. The risk to control is claquage, fracturing and lifting the ground instead of mixing it.

Jet grouting is a ground-improvement technique: a slim rod is drilled down and, on the way up, a cement grout is jetted at very high pressure that erodes the soil and mixes it in place, creating a column of soil-cement. By placing columns side by side the ground is strengthened, sealed or underpinned, even beneath existing buildings.

The heart of the technique is the very-high-pressure jet (hundreds of bar) leaving nozzles on the rod: its energy breaks up the soil around the hole and, as the rod rotates and rises, remixes it with cement. A cylindrical column is formed, its diameter set by pressure, speed and soil type. The soil is not replaced: it is transformed.

To make bigger columns the jet is assisted: single-fluid uses grout alone; double-fluid wraps the jet in a ring of air that increases its reach; triple-fluid erodes first with a water-and-air jet, then fills with grout. More fluids, bigger columns — up to a couple of metres — and more control.

It is used to consolidate loose soils, form cut-off walls, base plugs of excavations and delicate underpinning. The risk is claquage: too high a pressure can fracture the ground and lift what is above, instead of mixing. Pressures, flows, the rising spoil and the movements of nearby buildings must be controlled, with trials and monitoring.