Gas-Permeable Seal Test (GAST)

Saturation with high pressure water injection has been temporarily stopped due to an unexpected outflow of water through the main duct on January 2014. A series of activities took place to identify the point of leakage at the main duct, including a gradual increase of the water pressure in the system to 6 bars with continuous visual inspection inside the main duct. No water inflow was observed at this pressure and it was decided not to continue with the pressure increase before countermeasures against such an event in the future were taken.

The remediation concept selected consists of:

1. closing of the main duct with the installation of a tight well head at the front end with water and gas-tight throughputs for the instrumentation cables
2. backfilling the main duct with a low-permeability material to prevent any potential loss of bentonite or sand/bentonite mixture in the future or a connection through the risers

Main duct inspection with video camera

 
Preparation for well head emplacement

Well head is installed at main duct mouth

Sealing of cable ducts in upper well head

Gas-Permeable Seal Test (GAST)

The sand/bentonite seal has been saturated through the sand filter on the side of the concrete plug:

• Phase I - Saturation of the injection filter (11th  – 17th July 2012) 
• Phase II - Pressurization Phase: The pressure was increased in steps until a manometric height of around 3.5 m was reached (17th July 2012 – 29th April 2013)
  The pressure was increased in steps until a manometric height of around 3.5 m was reached
• Phase III - Saturation Phase (29th April 2013 – 29th January 2014). A rate injection is performed
• Leak event (28th January 2014)
• Depressurisation and remediation (29th January 2014 – June 2015)
• Phase VI – Remediation and Resaturation (July 2015)

Longitudinal cross section of the GAST experiment with instrumentation sections Sxx and levels Lx (not all sections and levels shown). Superposed is a vertical section of interpolated pore pressures as observed on 31.12.2013.

a) Injection phases and water flow, b) head space relative humidity, c) S/B pore pressure, d) total pressure at tunnel surface. Sensor code in legends: [Sensor type]_[Section]_[Level] (see figure above for positions).

Gas-Permeable Seal Test (GAST)

The pre-mixed sand/bentonite material for the GAST (Gas-Permeable Seal Test) experiment at the Grimsel Test Site is emplaced and compacted with different methods to ensure correct emplacement around the fragile instrument assemblies. (photos Comet)

Emplacement of pre-mixed S/B material

Distribution of S/B material

Compaction with Wacker hammer

Compaction with vibration plate

Gas-Permeable Seal Test (GAST)

The experiment design was finalized after numerous material evaluations had been done in the first phase of the project.

The gas-permeable seal itself consists of 23 horizontal layers (10 cm in height) of compacted sand/bentonite (S/B ratio 80/20) with a length of 8.0 m and a diameter of 2.3 m. Vertical sand filters on either end serve for initial water and later gas injections. The S/B is surrounded by granular bentonite material (GBM) to avoid water or gas bypass. On the back end the setup is closed off against the tunnel face by a concrete element. Towards the open tunnel there is a pressure containing concrete plug.

Bulk material parameters

The setup facilitates 17 instrumentation sections with sensors to monitor the sealing behaviour during saturation and the gas transport capacity in the later gas invasion phase as well as the effects on the host rock (hydraulic) and the curing and movements of the concrete plug.

List of installed sensors.

a) Instrumentation tree and compaction of a lower layer of sand/bentonite. b) Cross-section through 3D CAD model showing surfaces of selected S/B layers and sensor positions c) Experiment layout with instrumentation sections indicated in red.

3D cutaway model of the GAST experiment