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Instead of drilling boreholes, if conditions allow, we can simply “push” the collector piping into the ground. This technique, pioneered by Econic with our technology partner Lankelma, which has been mainly used in the past for site testing work can be much quicker and cheaper than standard drilling techniques. Typically, between 180 and 250m of vertical collector can be installed in a single working day. The Econic team have extensive experience in the use of this push technique and can advise on its suitability for your project.

How is it done?

A series of steel tubes (60mm diameter; 1-2m length) are pushed into the ground using hydraulic rams which are mounted within a large wheeled truck or crawler. The vehicles are normally 25-40 tonnes dead weight so site access is a consideration. The steel tube casing is capped with a disposable tip to prevent it becoming plugged with soil as it is pushed into the ground. As the casing ‘string’ is advanced, the soil is displaced around the tip and casing and no soil is produced at the surface for disposal, another advantage of this technique compared to rotary drilling where spoil disposal is sometimes an issue. As a technique it is normally also quieter and requires no backfill.

Lubrication

The casing string can be pushed ‘dry’ into the ground or lubrication can be used to reduce friction build up on the casing. Lubrication can be as simple as pouring water down the hole. Alternatively a lubricant (lean bentonite or biodegradable rapeseed oil) can be injected immediately behind the tip. The type and use of lubricants is dependent upon the ground conditions and overall GEC design.

Installation of the collector pipes

Once at depth (typically between 20 and 40m), a 40mm HDPE100 SDR 11 pipe with a fusion welded end cap is installed through the casing to its base. The pipe is then filled with water to provide additional dead weight and to increase rigidity which helps it get to the base of the hole. The tip is then detached and the casing pulled back very slowly over the first 2-3m, whilst maintaining downward pressure on the pipe to ensure tip depth is maintained. After this, the lateral stresses within the soil secure the installation tip and depth.

No backfill required - Thermal contact

As the casing is rapidly withdrawn, the soil squeezes back around the HDPE pipe and so leaves it in direct contact with the soil over the depth of the installation. This ensures that the installation has optimal thermal contact with the ground. Backfilling with high conductivity silica-bentonite grout is not required. This speeds up the process considerably compared to rotary drilling techniques.

Coaxial - Flow and Return system

Once the casing is withdrawn, the collector is pressure tested. A continuous length of 25mm HDPE pipe is installed through the 40mm pipe to provide the co-axial or ‘flow’ and ‘return’ arrangement. The heat transfer fluid runs down through the 25mm pipe and back up the 40mm pipe.

Header and Cross-Connections

A header is then attached to the top of the vertical collector, which is then in turn connected to any other individual vertical collectors within the array to form sub-arrays. The number of sub-arrays within the main GEC is dependent upon flow rates and determined as part of the detailed GEC design. All headers are cross-connected via a network of HDPEs laid within trenches excavated to depths between .08m and 1.2m below final ground level.

Once cross-connected, the entire GEC is purged and pressure tested, prior to backfilling and surface reinstatement. All buried pipework is then surrounded by compacted sand and a ‘warning - geothermal services’ marker with metal strip laid over the top prior to backfilling and reinstatement.

Single flow and return pipes are then run through the building wall and into the plant room. There are many possible configurations regarding cross-connection of borehole collector, field manifolds and multiple flow and return pipes into and out of the plant room. The final arrangement generally depends upon the number of boreholes, available space on site and available space within the plant room.

Once the collector has been installed and cross-connected, the complete GEC is again pressure tested prior to the backfilling and reinstatement of cross-connections trenches. Finally, the complete GEC is flushed with a biocide solution to disinfect it and then purged and filled with the heat transfer fluid.

Overall this is a very innovative technique that Econic has employed on some of the largest pushed ground collectors ever installed. If conditions allow it has several benefits that may be appropriate for your project including speed, no backfill requirement and lack of spoil and noise.