One of the requirements for the CFM experiment is that the MI shear zone is sealed off at the tunnel surface. The seal needs to withstand elevated hydraulic heads for the prolonged period (i.e. > 4 years) of in situ experiments with radioactive tracers. The initial measure was to apply sealing resin directly onto the surface of the tunnel wall. The initial measure was to apply sealing resin directly onto the surface of the tunnel wall. As this proved insufficient, the megapacker was installed in October 2006 and modified in 2009.
The Figure shows the main design elements of the megapacker with a steel liner of 5 m length and large „O-ring type“ packers at each end. Each of the so-called Donut Packers (inflatable tunnel sealing packer - rubber type) has a sealing length of about 0.65 m. The two hydraulic packers had to be designed and manufactured specifically for the conditions at the CFM experiment site. The perfect fit and functionality required consideration of the tunnel itself and the facilities already in place. In particular, the steel liner has three ports closed with flanges for connections to surface packers at the tunnel wall.