I can honestly say "I knew that", because that's what one of our long-time readers, netudiant, suggested some time ago.
From Yomiuri Shinbun (5/30/2013):
Control contaminated water by impervious wall of frozen soil, the government committee is to request
As a drastic countermeasure against the contaminated water that continues to accumulate at Fukushima I Nuclear Power Plant, the government committee on contaminated water treatment countermeasures (chairman Yuzo Onishi, professor emeritus at Kyoto University) has decided to propose the installation of the "shield wall of frozen soil", whereby the soil around the reactor buildings will be frozen in order to stop the groundwater from leaking into the buildings.
The final decision will be made in the meeting on May 30, and the government will tell TEPCO to implement the plan.
400 tonnes of groundwater per day are leaking into the reactor buildings, and it is the biggest contributor to increasing contaminated water.
The impervious wall of frozen soil was proposed by a large general contractor. Rows of pipes will be driven into the ground, and coolant at dozens of degrees below zero Celsius will be circulated in the pipes to freeze the soil around the pipes to a certain depth. That will become the shield that will block the movement of groundwater. Even the underground structures won't be much of an impediment, and the wall could be set up in 1 to 2 years.
Unlike the impervious wall made of concrete, the wall of frozen soil will be built in a short period of time by burying all the pipes and freeze the soil all at once. Therefore, there will be little danger that the contaminated water leaks out from the reactor building, as could be the case if the level of groundwater inside the wall gets lower during the construction of the concrete wall.
That large general contractor is Kajima, who has also created the unmanned debris removal system for the Reactor 3 operating floor. Kajima is famous for huge civil engineering projects like tunnels, and this method of freezing the soil is very well known to them. The other major general contractors, Taisei and Shimizu also submitted their proposals, which involved concrete walls like TEPCO has been contemplating and included a study of overseas examples of such impervious walls.
Let's see what TEPCO says, because, as "Happy", unnamed former worker and journalist Ryuichi Kino said in the AP interview, cost-saving comes first for TEPCO. This frozen soil method looks faster and cheaper than the concrete wall or underground bypass.
From TEPCO's 4/26/2013 document on "How to prevent groundwater from coming into the reactor buildings" pages 36-47 , Kajima's excellent presentation (compared to the other two):
Plane view. Surround the reactor buildings and turbine buildings of Reactors 1-4 with frozen soil wall of about 1,400 meters in circumference.
How to create frozen soil wall.
How continuous wall is possible with frozen soil wall.
According to Kajima (from the presentation),
As long as the frozen soil remain frozen, the coefficient of permeability is zero (100% block of groundwater from flowing in).
Even if power is lost, the wall remain frozen for several months up to a year.
The wall self-repairs.
Small footprint for installation equipment (2x2 meters, as opposed to 15x15 for normal wall construction), easy to remove.
There is very little contaminated soil resulting from the construction of the wall.
Construction of frozen soil wall won't interrupt decommission work
By regularly replacing the coolant and pipes, the wall can be used for a very long time.
Remote monitoring of temperature is possible.
Continuous wall is possible.
Almost too good to be true!