Curtain grouting stops water infiltration into buried structures by injecting chemical grouts around the structure to encapsulate it. This is typically done with an expansive grout, particularly if there are voids outside of the structure, or a non-expansive, permeation grout, if there are not voids outside of the structure.
How Curtain Grouting Works
The goal of curtain grouting is for the selected grout to create a waterproof barrier around a structure. The most appropriate grout is determined by the soil type and presence (or lack of) and uniformity of voids. Typical grouts include acrylate, colloidal silica, single component polyurethane resins, and plural component polyurethanes.
This chart shows the general framework for identifying the most appropriate curtain grouting material. The two primary criteria are soil permeability and whether or not there are bulk voids adjacent to the structure.
| Acrylate | Colloidal Silica | Prepolymers | CJGrout Polyurethane | |
| Soil Permeability | low (silty sands) | low (silty sands) | moderate (open graded sand) | high (clean gravel) |
| Bulk Void Presence | low (no bulk voids) | low (no bulk voids) | limited | limited to massive |
Acrylate Curtain Grouting
Acrylate is exceptionally low viscosity–it can be difficult to distinguish from water. Because of this, acrylic grouts are know for their ability to very uniformly permeate fine grained soils, such as silty sands and silty clays. Similar to resins, acrylic grout set times can be easily adjusted by varying the catalyst dosage.
In place, acrylic grouts range from the consistency of a cooked egg white to a hard silicone rubber. Cured acrylic grout is highly elastic. Acrylate doesn’t expand, so it’s not a great choice for grouting where there are large voids. If there are large voids, acrylate grouting is sometimes performed as secondary grouting after a primary grouting program using CJGrout plural component polyurethanes, which can expand up to 30 times their liquid volume.
Colloidal Silica
Colloidal silica is a mineral grout. Therefore, when it gels, there is no heat generated. The viscosity of colloidal silica grout during placement is around 5 centipoise. Without using a viscometer, it’s not possible to distinguish the grout from water. This facilitates uniform permeation at low pressure into dense and/or fine grained soils without fracturing.
Once cured, colloidal silica is a somewhat friable gel. Due to its very low viscosity, colloidal silica will pass through non-woven geotextile, so is not a great fit for installation adjacent to drainage board that needs to stay functional. Usually curtain grouting is required due to the lack or failure of drainage systems, so this is usually not a concern
Prepolymer Grouting
Prepolymer chemical grouts are particularly advantageous for grouting gravels and course sands. Sand treated with prepolymer chemical grouts can achieve unconfined compressive strengths greater than 1000psi. Prepolymers don’t permeate find sands, silts or clays very well.
Catalyst dosing controls set times. Prepolymer permeation grouts react with existing moisture in the ground. They expand a few times their liquid volume, which allows for reducing the amount of material needed to treat a given volume of soil.
Prepolymer grouts are also excellent for addressing high velocity water flows through soils without washing out. The reaction time can be set to be nearly instantaneous from contact with groundwater.
Prepolymers can be either hydrophobic or hydrophilic. Both types require water to react and cure. Hydrophilics effectively absorb water during reaction, so remain elastic after expansion. This gives them higher elongation than hydrophobics, but lower compressive strength, and can lead to dimensional instability if the environment dries significantly. Hydrophilics tend to be lower viscosity, and can ‘chase’ water through soils during placement. When permanently below groundwater, hydrophilics are an excellent choice.
To ensure dimensional stability, use hydrophobics above grade or groundwater. But, know that elasticity is lower, so dynamic environments may require subsequent retreatment.
CJGrout Polyurethanes
Plural component polyurethanes aren’t generally designed to permeate soils. So, they depend on the presence of voids to perform well during curtain grouting. Most plural component polyurethane curtain grouting is done in the stormwater market; failed culverts typically have large voids outside of them.
Because plural component polyurethanes, such as CJGrout-35NHV61, are relatively high viscosity, and expand significantly during installation, are great for getting massive leaks down to weepers or drippers. Touch up grouting with acrylate or prepolymer can then stop any residual leaks.
Advantages of Curtain Grouting
Curtain grouting is an excellent alternative to excavating to waterproof a structure. Compared to excavating for waterproofing, curtain grouting is:
Cleaner
Curtain grouting is generally done through small diameter (less than 1″) holes. Therefore, drilling is clean and fast. All of the various types of grouts cure very quickly, so don’t make a big mess, and can easily be cleaned up if they leak back into the structure during the grouting process.
Less Disruptive
Compared to excavation, curtain grouting is very non-disruptive. Some installations are done from the surface using driven tubing, others by drilling through the wall(s) being treated from inside the structure. Regardless, avoiding excavation and all of its disruption, is the primary goal of curtain grouting.
Faster
Curtain grouting is pretty much guaranteed to be faster than excavating a structure to install a surface-applied waterproofing system.
Other Curtain Grouts
Alternative curtain grout materials include sodium silicate and microfine cement. Microfine cement is very economical at scale, but not as clean as resin or acrylic grout. Sodium silicate is exceptionally effective in sands, and similar in price to acrylic grouting. Neither is dependent on moisture exposure for long term dimensional stability.
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