Jet Grout and Soil Mixing – The 101

Jan 27, 2025 | Construction

Executive summary. Soil improvement is a broad all-encompassing term which describes both an engineering solution and a construction method. It’s a solution for both permanent and temporary structures. Read on to become instantly educated.

Now, the “what”. What soil improvement is, is a mechanical means of changing the engineering properties of soil to carry some sort of load or to prevent unwelcomed water. The load could be the vertical load from a permanent structure or the lateral load induced on an excavation sidewall of a hole 40’ deep. The groundwater that could be anywhere from a nuisance to a catastrophic event may need to be controlled.

Let’s start with jet grouting. For this article you’re reading now, we are concentrating on two specific types of soil improvement: jet grout and soil mixing. These two soil improvements are very similar in that they both transform soil into a grouted mass. The jet grout process puts a small diameter auger into the ground (less than 6” in diameter), and then shoots a high pressure stream of cement and water into the adjacent earth. This high pressure cement and water combination (as it is squirting into the sidewall of the shaft just drilled) then develops a column of grout soil – the diameter of the grouted column varies from a foot to, say, five feet. When you put enough of these overlapping vertical columns together, you have a homogenous mass of grouted earth. This mass may take the form of a giant mat at the bottom of your building wherein this mat prevents water from coming up from below into an excavation (this installation would be called a bottom plug), or perhaps it will serve as a sidewall to prevent soil from caving into an excavation. Below is a jet grout installation used to support an 84” pipe (from below) as well as to prevent water intrusion (from below and the sides). Jet grout is so versatile. It can be used to build sidewalls and it can be used in bottom plug applications. It can be vertical or it can be installed on a batter (or a slant). Jet grouting is very useful in tight locations where you have underground obstacles, say in urban environments where there are many underground utilities.

Now on to soil mixing. Soil mixing also transforms soil into a grouted mass. The main difference is that soil mixing does not develop a grout shaft, it creates one with certainty. It drills a completed hole; it doesn’t develop one by force of grout spray.

Take a look at the photo on the immediate left. Look at how wide the auger is. This auger, with certainty, disturbs a column of soil, say, 8’ in diameter. After this shaft is drilled, then cement is mixed into the drilled soil (notice I said mixed in with the soil – this instllation does not displace the soil in the shaft and replace with cement, rather it disrupts all the soil with a premix and them adds cement). So, as just described, you are left with an 8’ diameter drilled shaft comprised of soil and cement (and water to set off the cement). And again, with overlapping shafts you have created a homogeneous mass of grouted soil. This transformed soil prevents groundwater intrusion and serves as a bearing surface. Below is a soil mix bottom slab design.

The actual final installation, of the drawing shown immeidately above, is photographed below.

My story. I’ve been involved in the installation of tens of thousands of cubic yards of jet grout and soil mixing on numerous projects. These installations have served me well, and poorly. The key for me has been knowing when to use which technology. It may not be the cheapest alternative, but when it works, it’s a lifesaver. The engineering concepts are simple and some of it is art over science.

Work safe!

 

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