Alternative installation method for drilling vertical boreholes to install collectors for a ground source heat pump

Drilling vertical boreholes into the ground to install collectors for a ground source heat pump (GSHP) usually involves a rotary drilling rig utilising a variety of drilling bits, and water based drilling fluids. Typically a 100mm to 180mm diameter hole is drilled to accommodate U-shaped collectors and both the spoil and drilling fluids emerge from the ground as the drill is advanced to depth. It can be a messy, noisy and time-consuming operation.

However, there is another way to install ground collectors that is far cleaner, quieter and faster. This method – which Econic have pioneered with their partner Lankelma Green Energy – involves a 33-tonne truck containing 36 tonne capacity hydraulic rams that pushes a tubular steel casing with a sacrificial leading tip into the ground to produce a hole about 60mm in diameter. Because the casing is advanced using a static hydraulic push technique rather than drilled ‘dynamically’ into the ground the soil is displaced around the casing as it is advanced. As a consequence, there is no spoil and little noise.

Once the casing has been inserted into the ground, they take a 40mm pipe, fusion welded at the end, and slide it down the middle of the casing. The casing is then pulled out and, within a few days, the ground’s elastic properties ensure that it contracts back around the pipe.

A 25mm pipe is inserted down the inside of the 40mm pipe to form a concentric collector, which looks similar to a coaxial cable, rather than the u-pipe typically found with conventional installations. The heat transfer fluid (monopropylene glycol) is pumped down the centre pipe and it comes out of the bottom and up the annulus between the 25mm and 40mm pipes. As the transfer fluid rises through the annulus, heat is transferred from the surrounding soil and through the pipe.

The casing can be pushed ‘dry’ into the ground or lubrication can be used to reduce friction build up on the casing and therefore enable us to achieve greater depths. Lubrication can be as simple as pouring water down the hole. Alternatively, a lubricant such as bentonite (a plastic clay material), polymer based fluids or biodegradable rapeseed oil can be used.

There is mess – the process is equivalent to ‘pushing’ a nail into, rather than hammering or drilling a piece of wood. And it is considerably quieter, with no vibration or potential ground loss through excessive ‘overbreak’ or ‘flighting’ or borehole collapse, all commonly associated with drilling. This makes GSHP installation using this technique a practical proposition next door to housing estates, for example, or on other sites particularly sensitive to noise.

It is also a far quicker operation – typically achieving about six to eight pushes to about 30m depth – amounting to around 180-240m a day compared with 50-100m for a conventional rotary drill.

Finally, grouting with high conductivity silica-bentonite grout is not required which speeds up and simplifies the process, reduces costs and provides direct thermal contact between the collector pipes and the surrounding soil in comparison to rotary drilling.

Being faster and involving less disruption, it is usually a cheaper solution than a traditional drilling method.

However, pushed collectors are not the right solution in every location due to underlying geology and the available space. Econic help advise the best form of ground collector on a project by project basis.

The “push” process step-by-step:

• 63mm diameter, 1m to 2m long steel tubes are pushed into the ground using powerful hydraulic rams mounted on a specialised wheeled truck or crawler. 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.

• Once at a depth, typically, of between 20m and 40m, a 40mm pipe with a fusion welded end cap is installed through the casing to the bottom. The pipe is filled with water for additional dead weight and increased rigidity. The tip is then detached and the casing pulled back slowly over the first 2-3m, while maintaining downward pressure on the pipe to ensure tip depth is maintained. After this, the lateral stresses within the soil secure the installation tip at depth.

• As the casing is withdrawn, the soil squeezes back around the pipe and leaves it in direct contact with the soil over the entire depth of the installation. This ensures that the installation has optimal thermal contact with the ground.

• After the casing has been withdrawn, the collector is pressure tested. A continuous length of 25mm pipe is installed through the 40mm pipe to provide the co-axial or ‘flow’ and ‘return’ arrangement. The heat transfer fluid for the ground source heat pump runs down through the 25mm pipe and back up the 40mm pipe.