slurry in under-slab tunneling

What is Slurry and How is It Used in Under-Slab Tunneling?

Don’t you just love industry language that means nothing to you? Like what in the heck is slurry? Sounds like a cross between a snow flurry and a . . . er . . . somethin’ else cold

If you’re wondering about the role of slurry in under-slab tunneling, then this article is for you, and P.S., slurry is not cold and has nothing to do with snowy weather.

Anchor Foundation Repair

At Anchor Foundation Repair, we use slurry all the time for a number of our primary services, including under-slab tunneling. So we can talk about slurry all day long until we’re all sleepy and *slurring* our words. 

We promise to keep this one short and sweet though. This article will quickly define slurry. We will talk about its use and benefits as part of the under-slab tunneling process used to access and repair plumbing leaks under your slab foundation.

What Is Slurry? What is Slurry Used For?

slurry mixture used for under-slab tunneling

Slurry is a mixture of topsoil, Portland cement, and water. Specific to our area, we use a Brazos River topsoil that is sifted and fine-filtered before mixing with the cement and water. The topsoil choice should be tailored to your area if you’re not here in the Brazos Valley.

If slurry sounds a little bit like mud, that’s because it IS a little bit like mud. The consistency of the slurry is different depending on its purpose in a repair project. The slurry is made thinner and more watery for filling purposes and thicker and with less water for lifting purposes.

Slurry is used in foundation repair, under-slab tunneling, and concrete repair. So in foundation repair and under-slab tunneling, slurry is used as a filling material and is of a thinner consistency like pancake batter. In concrete repair, it is used as a lifting agent and is more of a cookie dough consistency. Anyone else feeling hungry now?

For this article, we are focussing in on the use of slurry in under-slab tunneling only. So a more watery slurry mixture is used for part of the “filling in” process after the tunnel is no longer needed.

How Slurry is Used in Under-Slab Tunneling

spacious under-slab tunnel
Ya gotta fill all of this in . . .

Under-slab tunneling is a way to get to plumbing problems under your slab foundation. Tunneling is a less messy and less disruptive solution to jack-hammering through your foundation from above to fix an under-slab plumbing issue. Sometimes under-slab tunneling is used along with foundation repairs but it can also be done on its own just to repair plumbing lines under a home.

A tunnel is a 3-foot by 3-foot square tunnel dug in the ground under your home to reach whatever plumbing location needs repairs. It’s made big enough for a plumber to crawl in there and fix your plumbing from below. 

Afterward, the tunnel needs to be filled back in with fresh soil. Then, mud-pumping in the slurry helps to completely fill in all the air gaps and extra spaces that were created by the tunnel. 

The slurry is mud-pumped under your home in the filled-in tunnel the same way as with foundation repair. Temporary pipes are set up, and the slurry is pressurized and pumped under the home with the pumping machine. Then the surface pipes are removed and the slurry hardens to a firm solid material similar to sandstone.

The sandstone-like dried slurry material is very strong and can support a lot of weight on the horizontal, you know . . . like a house resting on top of it.

Benefits of Slurry Used in Under-Slab Tunneling

filling tunnels with slurry

Mud-pumping slurry with under-slab tunneling has similar benefits to its use with foundation repairs. The slurry fills up all the space and minimizes opportunities for future foundation settlement because of the remnants of a tunnel being dug under the home.

Another similar benefit to foundation repair is to help prevent erosion and washout in the former tunnel area. An extra benefit for tunneling and plumbing repairs is that the slurry also helps to secure and lock in the position of the plumbing drain lines at the proper pitch when secured with proper plumbing fasteners. 

Plumbing drain lines are carefully sloped to ensure that they drain the *bad water and stuff* away from your home. Using slurry makes sure that the pipes don’t shift or come out of alignment from how they were installed after the repair.

To put this in a nice little list, here are the main benefits of using slurry in under-slab tunneling:

  1. Fills up all air gaps and voids under home for strength
  2. Minimizes opportunities for future settlement by filling the space
  3. Prevents erosion and washout in the former tunnel
  4. Secures and locks in the proper pitch of drain lines

I don’t know about you, but all this seems like it makes great sense to use slurry after tunneling under your home. You make a big hole under your home, why not fill it up and secure it the best you can? This slurry stuff is starting to sound sensical, ain’t it?

Are there Alternatives to Slurry in Under-Slab Tunneling?

Great question. Well, the main alternative to using slurry after tunneling is to NOT use slurry at all and just backfill with dirt. Someone who digs a tunnel for you could fill it back up with the old dirt that got taken out and probably add in some new dirt as well.

The dirt in our area is made from a lot of expansive clay soil. It’s gloppy and sticky when wet and rock hard and crumbly when dry. In other words, clay is difficult to deal with and doesn’t just pack itself back in very easily when wet or dry. Voids are inevitable if all someone is using is the old dirt and a bit of new to fill in a tunnel.

clay soil is not great for reuse in tunnels
Clay soil is . . . unpleasant . . .

There would be air pockets and areas could collect both water and pests. Nature doesn’t like spaces and always tries to fill them with something. So if the spaces don’t get filled with dirt or slurry right away, they will get filled with something else eventually. It just might not be something to your liking . . . or in the best interest of the stability of your foundation . . .

More About Under-Slab Tunneling and Its Cost

digging under-slab tunnels in BCS

Let’s see . . . maybe now you’re really interested in under-slab tunneling and want to know more about it. You’ve probably already got some basic information about the tunneling process. So maybe now you want to know how much tunneling costs.

At Anchor Foundation Repair, we have been repairing foundations and tunneling under them too for 35+ years in Bryan, College Station, and other Brazos Valley communities like Brenham and Navasota. We like to give homeowners as much info as possible so that they can make the best possible decisions for their homes.

For more information about the costs of under-slab tunneling, check this out: “What Is the Average Cost for an Under-Slab Tunneling Job?