Grouting the borehole for many first-time installers is a steep learning curve. At best, it is time consuming; at worst, the borehole can be ruined! Drilling contractors typically spend the majority of their research time on the drilling and assume the grouting will be simple compared to drilling the borehole. While this premise is generally correct, grouting still requires the correct setup with the right equipment, mix and training.
Why do we need to grout the borehole after installing the loop? It is tempting to look at the drill cuttings or drilling mud and just use that in the bore around the loop. Can that be done? In short, NO! Here are some of the reasons. The ground we stand on feels solid and stable, but think of it as a stiff fluid; it moves with time and seasons, and so does the loop pipe due to thermal expansion and contraction. Over time, a pipe can wear against the sides of the bore or sharp drill cuttings and start leaking. So a smooth cover of grout gives the pipe protection. The grout also protects the aquifer from contamination from the surface.
|RigKits crews work a geothermal jobsite. Proper grouting can protect loop installations and groundwater resources. Source: RigKits LLC|
We tend to think of this from the standpoint of the coolant in the loop leaking out, but that is not the major risk here. The ground strata have stabilized over thousands of years. The ground filters contaminants from the surface like lawn chemicals and dog waste, so that no contamination reaches the aquifer. Our new borehole, especially one that has been air drilled and back filled with cuttings, can act as a superfast highway for the undesirable materials on the surface to get down into the drinking water. Imagine we just put weed killer on the lawn, when it rains the surface water with freshly added poison drains down our borehole into the aquifer, straight to our water well which we pump back up and give to the kids to drink, saying it’s better for them than all those sugar-filled soft drinks! And so, in short, ALWAYS grout the borehole.
So, now we understand why we need to grout. But how do we grout? Where do we start? What type of grout is needed and how much? The engineer will have given the drilling contractor the number of boreholes with diameter and depth. Also, the thermal conductivity of the grout will be specified and maybe the materials to be used. The number of loops and their diameter is also required. With this information, the volume of the bore less the volume of the loop to be inserted can be calculated, and thus we know the total volume of grout required. However, this does not allow for any cavities in the bore, so it is safer to order 10 to 30 percent more grouting material than required, depending on ground conditions. What is not used can be saved for the next contract.
What type of grout should we use? The thermal conductivity of the grout is a major consideration in designing the whole system: The better the conductivity, the more efficient the system. But this also depends on the energy that the ground can release and/or absorb (ground load). If the loop field is too small, it cannot absorb enough solar energy to keep up with the heating requirements of the system or dump enough energy back into the ground on the cooling cycle. So, a high thermal conductivity of say 1.2 Btu/hour x feet x degrees Fahrenheit (2.08 Watts/minute x degrees Kelvin) may not be needed if the ground is not a good conductor of heat. The good news is that the engineer will (and should!) have worked this out for you when designing the system.
Now, we get to the tricky bit. Not all mixes are created equal! Cost, thermal conductivity and ease of mixing are factors to consider. The easiest to mix and lowest cost mix may not have the thermal conductivity you need. Everything comes at a price! The bentonite grout that is commonly used is a good insulator and easy to pump, but not a good conductor of heat at 0.37 to 0.43 Btu/hr x ft x deg F (0.64 to 0.74 W/m x deg K). It needs an additive, typically silica sand, to increase its energy transfer ability from the ground to the loop and vice versa. Not just any sand will do: course or fine, sharp or smooth all effect mixing,
pumping and costs. The target particle size should be 50-70 mesh (0.3-0.2 milimeter). A larger size particle will be harder to pump and more difficult to mix. If there is too much water in the mix, the sand settles out. If there is too little it is hard to mix and pump, and is more likely to cause clogging. A smaller particle size is not so prone to settling, but does require more mixing time and is more expensive and can lower thermal conductivity. So the “Goldilocks” size of 50 to 70 mesh, is “just right.” Also, the sand needs to have 99 percent or higher silica content. The calculations from the grout provider will most likely be on this quality of sand and anything less will reduce the effectiveness of the grout.
There is another additive we have not mentioned: carbon or graphite. It is easy to mix with the bentonite and has amazing thermal conductivity of over 1.5 Btu/hr x ft x deg F. (2.6 W/m x deg K). It is often supplied pre-mixed in the bag with the bentonite. If it seems to be the solution we have been looking for, why are we not all using this stuff? The reason is price: It’s expensive. It is used in specialist applications and, if the ground is capable of
supporting the higher heat load, then the length of the loop can be reduced and this offsets the cost of the graphite.
You may also have heard about cement grouts. Why are these not used? They are used, but I do not think they offer the same protection of the loop pipes that bentonite does. Cement, once cured, is much harder and resists bending. The ground has no problem flexing the slender cement,around the loop pipes resulting in a fractured and/or leaking loop. Why does cement not resist the fracturing if it is harder than the bentonite? The cement is able to grip the loop pipe and so, when the cement does crack, the expansion is concentrated into a very small area and this can rip apart the loop pipe. The bentonite-based grouts are more flexible and gently move with the ground thus avoiding a rupture. The cement-based grout, however, is ideal to protect the top of the bore near the surface. Over a short distance of 10 feet (3 meters) or so, the harder grout will protect the loop from tree roots and accidental damage from future ground works without potentially causing a rupture and damage.
In our next article, we will look at the types of pumps available to mix and pump the grout.