Civil Engineering Reference
In-Depth Information
Freezing lances
Frozen ground
Direction of tunnel
construction
Potential
flow path
for water
Tunnel face
Figure 4.1
Potential flow paths when in-tunnel ground freezing is used at the
tunnel crown
•
It is totally removable (unlike grouting) - although there can be an
adverse reaction in some soils.
Limitations:
•
The time required to achieve ground freezing can be many weeks
depending on the ground and groundwater conditions.
•
Flowing water causes heat drain and can prevent the ground freezing.
The limiting flow rate depends on the type of freezing being used (see
below). For example, if a two phase brine freezing process is used, a
maximum flow rate of 2 m/day can be tolerated, whereas for a direct
process using liquid nitrogen, the maximum flow rate is 20 m/day.
•
The boreholes must be accurately positioned to create a continuous
frozen zone.
Care must be taken as there is the potential for the ground to heave
during the freezing process and subsequent settlement at the end of the
freezing process (ITA/AITES 2007). Ground heave is related to the frost
susceptibility of the ground. In coarse grained soils, the frost susceptibility
is low as the permeability is high. This means there is less heave because
the water can drain as the freezing progresses. Conversely, in fine grained
soils the frost susceptibility is high as these materials have a low permeability
and therefore there is more heave as the water does not drain during the
freezing process. Ground heave can be limited by controlling the speed of
the freezing process and the sequence of freezing.
There are two methods of ground freezing:
•
Two phase method (closed)
- Figure 4.2a.
In this method, a primary
refrigerant (ammonia or freon) is used to cool a secondary fluid (usually
brine).
•
Direct process (open)
- Figure 4.2b.
In the direct process liquid nitrogen
is used to freeze the ground. The nitrogen is passed down the freeze
pipes and then allowed to evaporate into the atmosphere. This direct
process is good for short-term or emergency projects. Liquid nitrogen
is likely to be the only effective method for freezing pore water in fine
grained soils.
