Ground freezing is a soil stabilization technique carried out by continuously refrigerating the soil. Methods, applications and advantages of ground freezing is discussed.
There are numerous ways and techniques of soil stabilization for deep excavation or tunneling. Some of them are grouting, heat treatment method and many more. One of the common and popular method used in the recent times is ground freezing technique.
History of Ground Freezing TechniqueThe method of artificial ground freezing was found out by German scientist F. Hermann Poetsch, in 1883. It was first used in America in Chapin Mine Company in Iron Mountain, where freezing was performed to a depth of 100 feet. However, providing groundwater control and excavation support for shaft sinking remains the primary application. In fact, for deep shafts, no better method has yet been established.
Conditions Where Ground Freezing is Most Effective
- Ground where penetrability by drilling, jet grouting, clamshell excavation, or other vertical cut-off tools is limited.
- Filled ground and ground containing man-made obstructions.
- Virgin ground containing cobbles, boulders, or an irregular soil/rock interface.
- Ground that has been disturbed due to unstable conditions or water inflow.
Principles of Ground FreezingThe major principle of this method is to convert the water into ice by external freezing methods to create a water seal and strengthen the soil. The effectiveness of freezing depends on the presence of water to create ice, cementing the particles and increasing the strength of the ground to the equivalent of soft or medium rock. If the soil has doesn’t enough amount of water to fill all to pore when they freeze, then it may be necessary to provide extra water so that the pores are complete sealed. This method is very effective in the places where the ground is made up of silts. Other methods of grouting cannot be undertaken due to very fine pores. The strength achieved by the ground after installing this method depends on freeze temperature, moisture content and the nature of the soil. After the initial freezing has been completed and the frozen barrier is in place, the required refrigeration capacity is significantly reduced to maintain the frozen barrier. When the ground water is transformed into ice, the expansion takes place which is negligible, around 9% expansion is observed which doesn’t impose any serious stress or strain on the soil. As this is an artificial method of refrigeration, a uniform freezing can be imposed on any type of soil which offers great security when compared to various grouting methods. As in all ground treatment techniques, adequate site investigation is necessary to allow the best system to be chosen and to design the appropriate array of freezing tubes and select plant of adequate power. Once the freezing process has begun, monitoring is required to ensure formation of the barrier wall and to verify when freezing is complete. During the drilling process, temperature-monitoring pipes are installed to measure the ground temperature.
Types of Ground Freezing MethodsThe types of ground freezing methods used for temporary support of a tunnel heading are discussed below:
- Indirect Method
- Direct Method
Indirect Method of Ground FreezingThis type of freezing method is commonly used in every place for the stabilization of tunnel headings. In this method, a secondary coolant is circulated through tubes which are driven into the ground.
Direct Method of Ground FreezingThis method is further divided into two types:
1. Direct, by circulation of the primary refrigerant fluid through the ground tubesIn this type of direct ground freezing method, only Ammonia is used in the freezing process. The process is same as that of indirect method, but here only one coolant is used. The ammonia is compressed and passed into the tubes driven into the ground. This ammonia freezes the tubes which in turn freezes the surrounding ground. Time required for this process is same as that of indirect process, but the efficiency is higher when compared to indirect method. The choice will depend on plant availability, estimates of cost and perhaps personal preference.
2. Direct, by injection of a coolant into the ground, such as liquid nitrogen
Advantages of Ground Freezing Technique
- Temporary underpinning of adjacent structure and support during permanent underpinning.
- Shaft sinking through water-bearing ground.
- Shaft construction totally within non-cohesive saturated ground.
- Tunneling through a full face of granular soil.
- Tunneling through mixed ground.
- Soil stabilization.
Disadvantages of Ground Freezing Technique:
- Very expensive.
- Needs continuously monitoring.
- Volume expansion of water during freezing, leading to soil heave and thaw settlement.