The general intent of peripheral freezing is to minimize the amount of frozen ground to be excavated. The frozen wall, of appropriate thickness and strength, is constructed for the most part outside of the excavation but extends some distance inside the future excavation surface so that the freshly exposed face is stable. Peripheral freezes must be formed with sufficiently watertight bottoms to ensure that excessive groundwater leakage will not develop as an upward flow into the unfrozen ground inside the frozen barrier.
Typical candidates for peripheral freezes are shafts, large circular, open excavations, horizontal tunnels and small connections between structures.
Vertical shafts are the most common application for peripheral freezes. For deep mines, no better method of sinking production shafts through deep, water-bearing ground has yet been established. In mine shafts on the order of 10 to 20 feet in diameter, excavations have been carried out to depths of over 2,700 feet within the protection of un-braced frozen walls.
There are several advantages of ground freezing unique to the construction of shafts:
- The freeze can be implemented through the soil/rock interface, which is often the most difficult geology in which to create a groundwater cut-off by other methods.
- A frozen wall, by design, is continuous into the underlying cut-off and resists the loads imposed by full groundwater and soil pressures.
- Proper instrumentation can provide assurance of the integrity of the freeze to full depth prior to excavation.
Large Circular Open Excavations
Conceptually, this application is very similar to shaft freezing, but shallower and wider. Pump stations and other structures up to 200 feet in diameter have been constructed within frozen walls of this type. For larger excavations, multiple rows of freeze pipes may be necessary to develop the required wall thickness. For rectangular structures, an elliptical shape is employed to mobilize the compressive strength of frozen soil.
Since frozen ground can be created with any freeze pipe orientation, ground freezing is a very effective stabilization tool for tunneling operations. Vertical freeze pipes can be installed to create a frozen arch through which tunneling can proceed. The sides of the arch extend to an underlying cut-off below the tunnel invert, while short pipes are used to freeze above the tunnel crown. Vertical pipe installation can also be used to create a full face tunnel freeze.
Horizontal pipes installed from jacking pits can be used to create a frozen cylinder that is parallel to the axis of the tunnel. In this instance, a mass freeze approach may be preferable to a peripheral freeze, except for relatively large diameter tunnels, to avoid the mix of frozen and unfrozen ground encountered during excavation.
Ground freezing can be utilized to facilitate connections between non-interlocking or disjointed structures such as a cross passage between two 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. Freezing works well in these situations because small, irregularly-shaped, hand-mined excavations can be performed under the cover of the frozen ground without internal lining or support and without the need to handle seepage water from within the restricted or confined excavation.