The ground freezing process converts in situ pore water to ice through the circulation of a chilled liquid via a system of small-diameter pipes placed in drilled holes. The ice acts to fuse the soil or rock particles together, creating a frozen mass of improved compressive strength and impermeability. Brine is the typical cooling agent, although fast acting liquid nitrogen can be used for projects where the freeze only needs to be quickly established and maintained for a short period of time or in emergency situations.
OPEN FACE & TBM TUNNELING APPLICATIONS
Over the past 20 years, ground freezing has become a versatile tool for both soft ground and hard rock excavation and tunneling.
Vertical Shafts: Ground freezing offers several advantages unique to the construction of vertical shafts. The freeze can be implemented perfectly through the soil/rock interface, often the most difficult geology in which to create a groundwater cut-off by other methods. Proper instrumentation can provide assurance of the integrity of the full depth freeze prior to excavation.
Vertical and Horizontal Arches: Ground freezing can be utilized to ensure stability of sequentially excavated (SEM) or hand-mined tunnels. The freeze pipes may be installed from the surface, either vertically or battered, or they may be installed horizontally from access shafts to create a frozen envelope parallel to the tunnel alignment.
Cross Passages and Connector Tunnels: Ground freezing can be utilized to facilitate connections between structures such as a cross passage between twin tunnels or a mined connection between deep structures. The frozen ground will conform to adjoining subsurface installations or obstructions, if necessary, to provide a composite cut-off structure.
Mixed Face Conditions: Mixed face conditions for a hard rock tunnel boring machine (TBM) can be alleviated by creating a frozen mass that encompasses the tunnel face, presenting a homogenous, strengthened zone to allow tunneling to proceed safely.
Safe Havens: Mass ground freezing of a designated area along the tunnel alignment allows for a ‘safe haven’ for the tunnel boring machine to undergo inspection and cutter head changes.
Launch and Retrieval Blocks: When ground freezing is the method used for groundwater control and support of excavation for deep shafts, the freeze can be extended via additional freeze pipes to create a targeted stabilized mass for cutting of the eye for TBM launch or retrieval.
Additional candidates for ground freezing include circular or elliptical, open excavations, which might be required for pump stations and other large structures. Ground freezing can also be used for environmental purposes to control/minimize groundwater extraction and contaminant plume movement.
Projects of particular note include the East Side Access Northern Boulevard Crossing, Port of Miami connector tunnels and Access Shaft # 3 in Buenos Aires.
Contact us today to see if ground freezing is the right option for your next tunneling project.