Ground freezing is recognised by mining and civil engineers as undoubtedly the most effective and consistently reliable method of providing temporary support and of preventing groundwater from flowing into deep excavations. Its use has numerous benefits including the fact that it does not deplete aquifer reserves by continuous pumping nor does it pollute or alter the groundwater regime in any way thus eliminating any adverse affect on adjacent structures, installations and populous.
the basic principle of ground freezing, which was first used in south wales circa 1862, is to circulate a cold medium through a suitable pipe system, which cools the strata and thus converts the in-situ pore water to ice. The ice is so formed bonds adjacent soil particles together and forms an impermeable structure with increased strength characteristics.
freezing is adaptable to any shape, size or depth required. The largest excavation in the uk, supported totally by frozen ground, being 40 metres in diameter and 40 metres deep through unconsolidated alluvial deposits adjacent to a tidal estuary. Another novel use for ground freezing was to support and protect the foundations of a 14th century bridge during pipe laying, as required by contract there was no detectable movement of the bridge.
the Freezing expedient
Many civil engineering projects have to control unstable and/or water bearing ground for the construction of sub-surface structures.
The choice of the groundwater control process is particularly important in urban areas where adjacent structures, water abstraction and/or habitation may be affected particularly as the depth of construction increases.
Freezing has been used as an expedient to facilitate the construction of all types of structures to depths in excess of 600 metres.
The plot of process versus particle size indicates the versatility of freezing as it covers the complete range of soil types so long as they contain sufficient moisture to freeze.
The decision to freeze
In assessing the relevance of the Freezing process as a constructional aid for particular circumstances account must be taken of its advantages and limitations comparative to alternative processes which may be considered.
Advantages
- 1.Strength
Soil strength is increased many times and frequently excavations are rendered stable without any further temporary support. Frozen soil can be likened to a weak concrete or rock therefore complete stability can be assured.
2.Impermeability
Impermeability is common to all frozen soil types thus the requirement for dewatering or pumping is eliminated.
3.Stability
Settlement associated with vibration during piling and removal of fines due to pumping are eliminated.
4.Non-polluting
Freezing does not pollute the ground water or affect its level in any way.
5.Versatility
Freezing satisfactorily stabilises and excludes ground water in all types of ground from fine micro-grain to fissured rock structures.
6.Safety
A safe working environment is provided eliminating hazards associated with toxic chemicals compressed air working excessive noise and the possibility of pumping contaminated groundwater.
Cost effectiveness
Ground Freezing is a very cost effective and time saving process. In all too many cases the client has only realised this when he has committed to another stabilisation technique that has failed to give the performance expected.
The incidence of such decisions can be substantially reduced if the client carries out an informed selection process to determine his choice on the basis of -
- The ability of the chosen process to control the ground conditions expected and those which could possibly occur.
- The reliability of the chosen process to perform to programme and budget
- The degree of safety provided by the chosen process for personnel, construction and the environment.
- The conditions, which the chosen process will provide within which to construct a quality, assured structure.
- Value analysis after weighting the above criteria depending on the degree of comfort provided.
Refrigeration systems
There are two widely used methods of providing refrigeration energy to freeze the ground. 1.Conventional system
The conventional system consists of a primary refrigerate gas, usually ammonia, being compressed and conducting its cold energy on evaporation to a secondary coolant, usually calcium chloride brine which is circulated through the freeze tube system to cool the strata. The refrigeration plants used for this operation are self contained and mounted on skid units for easy transportation. In operation the ammonia does not leave the confines of the skid unit and is protected by a series of safety systems to protect against failure. This type of system can operate at temperatures as low as -40 degrees celsius and are suitable for the larger projects where freezing will be carried on for several weeks. Bdf operates a large fleet of these units that vary in capacity from 80000kcal h to 455000kcal h at the operating temperature of -26 degrees c.
2.Cryogenic systems
As a rescue operation and where the construction period is of short duration the cryogenic system is generally used. This system uses an expendable refrigerant usually liquid nitrogen that is applied directly to the freeze tubes where it evaporates at -196 degrees c and the resultant gas is exhausted to atmosphere. Liquid nitrogen is used in cases where remedial action is urgent and for small compact projects where time is of paramount importance. It may also be appropriate to use liquid nitrogen where water movement or saline condition preclude normal methods within a reasonable period of time, the liquid nitrogen is delivered to site by road tankers and stored in vacuum insulated vessels of up to 100 tonnes capacity. Distribution of the liquid on the site is then effected through vacuum insulated liquid lines to the closely spaced freeze tubes so that the freeze wall formation can be completed in typically 3-7 days. A comprehensive monitoring system is used to ensure a safe operation. The system has been successfully used for example at a tunnel face beneath a river and over 1000 meters from an access shaft.
If you require more information on ground freezing techniques or you wish to discuss the possibility on using ground freezing on one of your projects then please do not hesitate to contact John Belton by phone fax or e mail.
