Jet grout has been, and continues to be, a successful means of ground improvement. It can serve many purposes – two of the most common are structural support and groundwater control.
Definition of Jet Grout
Jet grout is a method of ground improvement wherein a drilling machine injects cement, water and air into the existing underground. What results is a homogenous mass soil-cement mixture. It is effective in mixable soils – for example, excellent in gravels but ineffective in rock mass.
Where would I use is in my project?
It can serve a temporary need or a permanent need. Below is a test section (three overlapping jet grout columns)
- Bottom “plug” for a cofferdam to control groundwater from entering the excavation from below.
- Shaft construction – secant and/or tangent walls which alone can serve as a shaft wall and/or with reinforcement in it (such as casing pipe, rebar, or dywidag).
- Vertical wall as a closure between two sheetpiles that couldn’t be connected.
- Prevent soil and water from entering a shaft laterally where utilities penetrate the sides of your shored excavation.
- Installation of a thick wall and thick bottom plug to serve, cross-sectionally, as the area in which the pipe is installed. This bathtub effect allows the contractor to dig in a homogenous mass of cemented soil (the walls and the invert of the trench are all “dried in” and you’re digging in soil cement.
- Tunnel heading – serves as a “target” path for a tunneling machine to provide both stable and consistent soil to bore through.
- Foundation support to a structure. See the QR code below for a pictorial example.
- Load transfer under a pipeline – pipe laid in weak soils can be supported by vertical jet grout columns which prevent vertical displacement of the pipe.
- Cutoff wall to prevent migration of underground fluids.
How is it installed?
Keller does a great job here [QR] of showing, pictorially, how jet grout is installed. In simplest terms, here is how it is installed:
- Obtain a drilling machine with a hollow stem auger.
- Advance the drill to the desired elevation underground.
- Inject cement, water, and/or air through the rotating stem.
- Jet the grout (cement, water, and/or air) into the soil by starting at the deepest elevation and then pulling up the drill stem at discrete increments.
The result is a column of jet grouted soil.
I’ve been on projects for the past twenty (20) years which have used jet grout in the manners described above. To the left here is a trench with a jet grout bottom plug resisting about 20’ of head outside the sheetpile trench – look at the pipe subgrade, dry!
I’ve seen it be very effective, and I’ve seen it misapplied in a massive rock strata. It takes a plant to feed it (pumps, equipment, cement silos, et cetera), so it’s definitely a specialty operation, and one that takes a significant footprint near the installation location. It’s also messy. But overall, I’m a supporter!