How to grout a geothermal borehole

Part Two: How to Mix and Pump Grout Effectively and Easily

By Brandon Wronski

In the last installment of the Geothermal Journal, we discussed the fundamentals of why a geothermal borehole is grouted and the various products that are used. The next question is how to mix and pump the grout effectively? Additives such as silica sand that increase thermal conductivity come with added cost in both material and labor as the difficulty to mix and pump the grout also generally increases. Many contractors charge more for grout mixes with higher thermal conductivity. Achieving the correct balance in the mix is also vital. Excessive water can negatively affect grout properties and thermal conductivity. It is also critical that the sand is evenly suspended and distributed in the mixture to ensure proper heat transfer.

Selecting the right grout mixing and pumping equipment can improve your efficiency and cut down on labor costs. Paddle mixers tumble materials from the outsides of the chamber to the middle where it is thoroughly mixed. They are easy to clean between batches, are simple to maintain, wear and tear is minimal and energy consumption is low making them relatively inexpensive to operate. Colloidal mixers, however, use a high-speed rotor which creates a vortex that circulates the heavy, unmixed material toward the outside walls of the mixing tank while the lighter material, such as water and partially mixed grout, is sucked in toward the throat of the mixing tank. The mixture becomes thicker and thicker until the entire mix is consistent.

Some contractors feel that colloidal mixers over-hydrate the grout, adversely affecting its properties and making it difficult to pump. However, the operator can control the amount of hydration by limiting the time the grout stays in the mixing tank — around 90 seconds per batch for most geothermal applications, depending on the mixer and products used. If you want the bentonite to hydrate in the bore, simply dump the product into the holding/pumping tank once it’s properly mixed. There it is gently agitated to prevent settling until you’re ready to pump. The operator just has to get a feel for how long it needs in the mixing tank to achieve the desired hydration. This is more of an art than a science as each batch is a little different, but timing is critical.

As discussed earlier, silica sand is often added to increase the heat exchanging properties of the grout. The high speed of the colloidal mixer prevents solids from settling in the bottom of the tanks — reducing the risk of clogging, as is sometimes the case with paddle mixers. Mixing silica sand into the bentonite is done rapidly and easily without having to use finely crushed silica which is more expensive. The time taken mixing the product is measured in seconds, not minutes; it’s that fast. Dry product can be poured quickly into the mixing tank with little risk of clogging the system.

A mix with 80 percent silica sand solids content is simple with a colloidal mixer, while with paddle mixers it is nearly impossible. A paddle mixer will require a higher percentage of bentonite, which affects the thermal conductivity of the mix compared with the higher silica content achievable with colloidal mixers. The speed of mixing and the low risk of clogging are the main advantages of a colloidal mixer. This reduces cost in labor, materials and, in some cases, even the length of the borehole, as the resulting grout mix is able to exchange heat more efficiently.

The next step is pumping the mix. Most paddle and colloidal mixers have, in addition to the mixing tank, a large capacity hopper and a pump. A two-tank system, mixing tank and pumping tank is ideal, as it allows for simultaneous mixing and continuous pumping. However, low-cost paddle mixers can get away with only a mixing tank. It should be noted though that while reducing equipment costs, this can also slow production. For pumping the mixed grout, piston pumps and progressive-cavity pumps are commonly used, as they are able to pump higher solids content and at higher pressures. Piston pumps are generally capable of higher pumping pressures.

Next, a tremie line is inserted to the bottom of the borehole through which the mixed grout is pumped. A helpful tip is to measure and mark the tremie line to correspond to the length of the borehole to ensure that it reaches the bottom of the hole and doesn’t just stop at a tight spot along the way. Generally, the tremie line is 1 1/4-in. diameter for most applications. A tremie reel, either manual or hydraulically powered, that will roll and unroll the coiled tremie line as it is inserted into and then removed from the bore can help to increase efficiency. As the grout is pumped down, it is important that the tremie line is submerged in the grout as it is being retracted to the surface to prevent any cavities in the grout column.

Although fairly simple in theory, in practice, grouting a geothermal borehole can have its fair share of challenges. If not done properly, the effort and money that went into drilling can be easily wasted. The good news is that using the right products and equipment and getting proper training can make all the difference.

Brandon Wronski is an equipment specialist with Hammer Drilling Rigs, based in Charlotte, N.C.

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