When excavation is planned that extends below the foundation level of an adjacent structure and soils are not conducive to conventional concrete pit underpinning, jet grouting offer a viable alternative, with the dual advantages of providing both underpinning and excavation support in a single operation.
Jet Grouting uses high-pressure, high-velocity jets to hydraulically erode, mix and partially replace the in situ soil or weak rock with cementitious grout slurry to create an engineered soil-cement product of high strength and low permeability. This method can be performed above or below the water table and in soil conditions ranging from coarse sands and gravels to highly plastic clays. Various geometries of soil-cement product can be created, though for underpinning/excavation support purposes overlapping columns are typically used.
The three basic jet grouting systems are single-fluid (grout only), double-fluid (grout and air), and triple-fluid (grout, air and water). Selection of the most appropriate system is dependent on the in situ soil characteristics and the application.
Jet grouting is accomplished using a purpose-built system consisting of a drill rig, drill steel incorporating a jet grout monitor, high-volume mixing and batching equipment, high-performance pumps, large air compressors, and an automated data acquisition system.
A jet grout column is constructed by advancing a borehole to the bottom of the planned treatment zone using rotary or rotary percussive drilling methods. Grout injection then commences as the tooling is extracted with controlled rotation and withdrawal speeds. The process effectively erodes the soil and mixes it with the injected grout slurry. Installation of individual columns is sequenced to allow sufficient curing time prior to installation of intermediate overlapping elements.
- Underpinning/earth support
- Groundwater cut-off
- Closure of conventional excavation support systems
Advantages & Limitations
- Can be performed above or below the water table
- Can be performed in most soils
- Low permeability
- Angled jet grouting can facilitate construction in areas where access to the target soils is limited by overhead obstructions
- Can be performed around sensitive structures
- Local obstructions such as cobbles or boulders can result in “shadowing” leading to incomplete soil-cement geometries.
- Considerable volumes of spoil are generated that must be channeled to a holding pit and allowed to set up before disposal in accordance with the project specifications.