EARTH RETENTION AND ANCHORS
Ground Modification Methods
The most versatile of all ground improvement methods, grouting has long played a significant remedial role in ground improvement. Over the years, the introduction of new grouting technologies, refinement of established techniques, and the development of more sophisticated equipment have expanded that role to include site improvement for new construction.
The two grouting methods typically used for earth retention and excavation support are permeation grouting and jet grouting.
Permeation Grouting is defined as the flow of a low-viscosity grout (sodium silicate, acrylate or polyurethane) into the pores of the soil without displacing or changing the soil structure. The characteristics of the ground are modified with the hardening or gelling of the grout. Depending on the requirements of the project, grout materials can be temporary or permanent.
Structural permeation grouting increases the strength and cohesion of granular soils, and is used for a range of construction-related purposes. These include increasing stand-up time to allow tunneling to take place without significant ground loss and soil stabilization for excavation, utility and footing support.
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. Grouting can be performed above or below the water table and in most subsurface stratigraphies from cohesionless soils to highly plastic clays. This technique is most cost-effective when both excavation support and underpinning of structures abutting the excavation is required.
The primary use of ground freezing for excavation support is in the sinking of deep shafts. The peripheral frozen cylinder performs the dual functions of groundwater control and earth support, allowing shaft excavation without the need for internal bracing and sheeting.
Peripheral Ground Freezing with liquid nitrogen has also been used where excavation support is required for just a short time to facilitate removal of abandoned underground tanks and other structures. This technique is particularly advantageous where the excavation is to take place close to sensitive structures that cannot tolerate construction-generated vibration.
Mass Ground Freezing to facilitate excavation, while not common, has been successfully utilized where ground control in particularly challenging subsurface conditions was crucial. Mass freezing can also be an attractive option where safety is an overriding project consideration