Jet grouting is a trenchless technique that is enabling construction companies to shore up excavations more effectively and with less noise and vibration than conventional shoring methods. This article describes what jet grouting is and how it is being used in shoring applications.
What Is Shoring?
Shoring is a technique that has long been associated with open trench construction to support the side walls of an excavation. Regulations stipulate that any excavation deeper than five feet should be re-enforced in order to prevent collapse and danger to workers. This re-enforcement is defined as shoring.
Open Trench Shoring Techniques
Soldier pile and lagging is a technique where piles are driven into the ground around the perimeter of the excavation. As the excavation progresses, lagging planks are laid against the pile and backfilled so that the pressure of the retained soil is transferred to the piles.
Soil nails are steel rods, which can be driven into the side wall of an excavation at regular intervals with temporary fencing placed over the surface of the ground during excavation. Upon completion of the excavation, nails are grouted into position if necessary and the grouting coat applied on top of the temporary fence.
Hydraulic shoring uses hydraulic jacks to apply pressure to trench shields on opposite walls of the excavation, thus providing an active force against the retained soil. Pneumatic jacks can also be used for this purpose.
What is Complex Shoring?
Excavations that are planned close to existing buildings have the potential to destabilise their foundations. This creates an engineering concern to ensure that the shoring put in place for the excavation is sufficient to secure the foundations of the buildings.
In some cases, the proximity of existing buildings or infrastructure requires minimum vibration and noise during construction. This is due to the presence of office workers and the need to avoid disruption of business activities. Conventional shoring techniques do not meet these criteria, which has led to the development of trenchless techniques to install shoring before the excavation begins.
Trenchless techniques like jet grouting are ideally suited to these projects. Soil conditioning or stabilization may be needed during these stages. (Learn more in "When Ground Improvement is Needed During Trenchless Rehabilitation.")
What is Jet Grouting?
Jet grouting is a trenchless construction technique that is used to mix a cement grout with loose soil in order to create a stable column in the ground. This stable column provides support to buildings or forms part of a complex shoring solution for an excavation.
The simplest form of jet grouting consists of two nozzles diametrically opposite from each other just above the drilling tool. High pressure grout is pumped through these nozzles while the drill string rotates at speeds of 15 to 25 RPM. At the same time, the drill is lifted upwards at a rate of 100 to 500 mm/min. The principle is that the grout mixes with the surrounding soil, forming a hardened cylindrical shape around the drill string. Excess soil is lifted to the surface by the upward motion of the drill.
Two further technological developments from this simple form of jet grouting involve the use of air, or water and air. Compressed air surrounding the grout jet into the soil, minimises the reflection of excess soil back into the core and makes it easier to move that soil to the surface by the presence of air in the mixture. Water can be used to greatly increase the diameter of the grout inclusion so that less passes are needed to create the same structural effect in the ground. (Read more in "A Look at Hydro Vacuum Excavation.")
It is also possible to increase the strength of the jet grouted inclusion by adding a re-enforcing element into the inclusion after grouting. Steel cylinders or H-beams can be inserted into freshly grouted inclusions or drilled into place once the grout has set.
Examples of Jet Grouting in Complex Shoring?
Jetted columns can be used as shoring piles in the same way as the soldier pile and lagging is described above. Columns can be re-enforced and they can be drilled at an angle to create the support required to retain soil as an excavation is dug.
Jet Grouting in Vevey, Switzerland
This technique was used in the construction of a new chamber to house heat exchange units and pumps for the Nestle Inc. headquarters in Vevey, Switzerland. An excavation of a 20m x 20m x 6m chamber was required right next to the headquarters building. Due to the requirement to minimize noise and vibration during construction, jet grouting was chosen as the shoring technique.
Jet Grouting in Surry, BC
In the construction of a residential tower block in Surrey, British Columbia, engineers encountered a soil mixture of weak fills, peat and soft silty clay that is difficult to manage using conventional shoring techniques. Jet grouting was chosen for these technical reasons as well as economic advantages. A total of 345 jet grouted columns were installed for the project. The column depth was set at 15 meters and every alternate column was re-enforced with a steel H beam.
Conventional shoring would have been executed in 2m deep stages and problems would have been encountered in the migration from one soil type to another. Jet grouting saved considerable time on the project, solved the technical problems and was cheaper to execute.