You’ve already made a big call. Tearing up the floors inside your home to reach a broken drain line isn’t happening, not with your new flooring and your family living there, so you’ve chosen to tunnel under the slab and reach it from outside instead.
Tunneling is a practical and homeowner-friendly choice. But there’s still that little voice in the back of your head . . . what if something goes wrong down there?
At Anchor Foundation Repair, we dig hundreds of feet of tunnel a year here in the BCS area, and several thousand feet over our 40+ years in business. We’ve learned to sidestep most of the problems that come up under a home. Most, not all. A few can’t be fully avoided no matter how careful the crew is, and you deserve to know about them before you hire anyone.
In this article, we’ll cover what those risks are, how likely each one is to happen, and how the right contractor keeps them from landing on you. First, though, a quick word on why tunneling is worth choosing in the first place.
Why Homeowners Choose Under-Slab Tunneling
Since you’re reading an article about what can go wrong, there’s a good chance you’ve already chosen tunneling. So we’ll keep this part quick. The reason so many homeowners land here is simple: tunneling gets the repair done while keeping the mess, and your daily life, mostly out of the picture. Here’s what makes tunneling worth it for homeowners:
- No jackhammers ripping through your floors and no dirt piling up inside
- Your slab stays whole and uncut
- You keep living in your home while the work happens
- No flooring to tear out and replace afterward – can be a significant extra cost
- In some cases, your homeowner’s insurance may help cover the cost of the access
Those are real upsides. But even the best option on the table isn’t a risk-free one. No matter how carefully a crew works, digging under a home comes with a handful of things that can go sideways. So to be fair, let’s discuss those downsides.
Here are the seven we’ll walk through, ordered from the most serious down to the merely annoying:
- A tunnel cave-in
- A long rainy stretch that floods the tunnel
- More tunneling than expected
- The tunnel heading the wrong direction
- Erosion and settlement from a bad tunnel backfill
- Chipping into the concrete slab
- A nicked water line under the slab
1. A Tunnel Cave-In
Let’s start with the most serious one and not sugarcoat it. When you dig in the dirt the wrong way, the ground can collapse. A tunnel cave-in can trap, crush, and bury a person underneath your home. It can be fatal.
This is exactly why tunneling is skilled, careful work and not a job for anyone with a shovel and a free afternoon.
The soil has to be read correctly, the opening has to be the right size and shape, and the crew has to understand how the ground behaves as they move through it. The federal safety rules for this kind of excavation come from OSHA, and they exist because the danger is real.
A trained, experienced crew that respects the soil is your single best protection here. When you hire a tunneling contractor, you aren’t paying for a hole in the dirt. You’re paying for people who know how to dig it without anyone getting hurt.
How Often Does a Cave-In Happen?
Extremely rarely. Across our decades of work, proper technique keeps this from ever happening. But rare is not the same as never. A trench collapse near Milford, Ohio in late 2025 took the life of a 28-year-old worker, a sobering reminder that this is serious business. It stays rare only because good crews treat it seriously every single time. Ok . . . enough with the scary stuff and on to the more everyday . . .
2. A Long Rainy Stretch Floods the Tunnel
Texas weather has a bipolar disorder: really soggy or really droughty. Plus, your need for tunneling won’t time itself perfectly. When a long, rainy stretch settles in during your project, water can seep or pour into an open tunnel and turn solid dirt into muck. A flooded tunnel is slow going, and a job that should have wrapped up in a few days can stretch out while everything gets pumped out and dried back into working shape.
That extra water can prolong your timeline and your patience right along with it. The good news is that a prepared crew watches the forecast and can set up temporary drainage to keep the worst of the water out of the work zone.
Rain is the one variable nobody controls, but a contractor who plans with the season instead of ignoring it keeps your tunneling job timeline under control.
How Likely Is a Rain Delay?
It depends on the time of year and even on your specific yard. Some seasons and some low-sitting lots are far more prone to it than others. During rainier stretches, temporary drainage measures can cut the chances way down, so this one is very manageable when someone’s actually paying attention to the sky.
3. You Need More Tunneling Than Expected
Here’s a frustrating one. The crew tunnels to the plumbing line break that was diagnosed, the plumber makes the repair, and everyone figures it’s handled.
Then another leak turns up further “upstream” that was not apparent the first time around – leaks tend to hide other leaks. Now you need more tunneling to reach it, and more tunneling means more cost.
This comes up a lot with older homes still running their original cast iron drain lines, which start to fail in more than one place once they begin to deteriorate. It’s usually the plumber, not the tunneling crew, who calls for the extra digging once they can finally see the condition of the line up close or perform a hydrostatic pressure test to check the line for other leaks.
It’s rarely anyone cutting corners. It’s the nature of old pipe . . . when one section has failed, its neighbors are often not far behind. Also, you want to make sure that no other under-slab leaks are present. No use covering it all back up only to find another one a bit later because it’s causing foundation settlement and other issues.
How Often Does This One Come Up?
Another leak and more tunneling comes up fairly often. It simply can’t be avoided, because there’s no way to know about a second break until you’re already under there fixing the first one. Pressure testing the line after each repair is what confirms whether everything is truly sealed or whether there’s more leak to chase.
4. The Tunnel Heads the Wrong Direction
The drain lines under a slab can be a maze. On paper it looks straightforward. In the ground, pipes turn, split, and run in ways that don’t always match what’s generally expected.
Every so often, the available information points the crew toward one spot . . . and the actual problem turns out to be somewhere else entirely.
Honesty Alert: It happens, and it’s no fun for anyone involved. The important thing to understand is that this usually traces back to how the leak was located and diagnosed in the first place, not to the digging itself.
That distinction matters for your wallet. Choosing a plumber with real under-slab experience to pinpoint the failure before anyone digs is your best defense. And if a misdiagnosis does send a crew the wrong way, make sure the terms are clear up front so you’re not the one stuck paying for someone else’s mistake.
How Often Does the Tunnel Go the Wrong Way?
It’s uncommon, but it does happen. The fix is accuracy at the very start: the better the leak is located before a shovel touches the ground, the smaller the chance the tunnel ends up chasing the wrong direction.
5. Erosion and Settlement From a Bad Tunnel Backfill
Putting the dirt back the right way matters almost as much as fixing the leak. When a tunnel is done, that empty space under your home has to be filled so the ground stays solid. Skip it, or just shovel loose dirt back in, and you’re looking at years of erosion: sunken spots, drainage headaches, and a landscaping problem that won’t quit.
Worse, if that erosion gets bad and widespread enough, the lost support can lead to foundation settlement with your home.
That’s where mud pumping comes in if your contractor uses this method. Mud-pumping fills the tunnel with a pressurized mixture of Portland cement, topsoil, and water that hardens to a sandstone-like consistency, packing every void so nothing washes out later. It takes more effort and material than loose dirt, which is exactly why some contractors skip it.
Our ground makes it worse. The Brazos Valley sits on expansive clay that swells and shrinks with the seasons (you can look up your own lot’s shrink-swell rating on the USDA’s soil survey). Give that restless soil a void to move into, and it will.
If you remember one thing from this whole list, make it this: ask whether mud pumping is part of the backfill. It’s the difference between a tunnel that disappears cleanly and one that can haunt your slab for years.
How Common Is Erosion After Tunneling?
This is the most frequent problem on the list, but almost only with crews that don’t mud pump the voids. The settlement it can cause is impossible to pin to a number, since so many factors feed in. So why gamble? Making sure mud pumping is in the plan takes the worry off the table.
P.S. Have we mentioned that we always mud-pump our tunnels??
6. Chipping Into the Concrete Slab
Sometimes the broken drain line is partially wrapped in concrete where it meets the slab or a beam. To reach it, the crew has to chip that concrete away. This adds a little time, a little cost, and a bit of noise to the day.
Honesty Alert: This one isn’t really something “going wrong.” It’s just a part of the job that shows up when the pipe happens to sit right up against the foundation. No one can know ahead of time exactly where a failure will land relative to your slab’s design and beam layout, so the smart move is to expect the possibility and roll with it.
Because it can add to the bill, it’s worth understanding the kinds of extra costs that can pop up on a tunneling job before you begin, so nothing catches you off guard.
Think of this one less as a mishap and more as a “just in case.” If it happens, it’s completely normal. If it doesn’t, even better.
How Often Does Chipping Come Into Play?
Often enough that you should plan for it, though it’s entirely case by case. It comes down to where your particular pipe failed and how your foundation was built. There’s no way around it when the concrete is in the way . . . if it has to be done to reach and repair the leak, it has to be done.
7. A Nicked Water Line Under the Slab
Underneath a slab, drain lines and copper water lines often run right alongside each other. Even with a careful crew, a water line can sometimes get kinked or nicked while the work goes on around it. When that happens, it needs to be repaired right away so you’re not adding a fresh leak to the list.
There’s an important distinction worth knowing here. There’s a real difference between incidental contact, which can happen to anyone working in tight quarters underground, and actual negligent damage.
It’s a fair thing to talk through with your contractor ahead of time so you both understand how that situation would be handled if it ever came up.
A good contractor won’t pretend this never happens. They’ll be straight with you about the possibility and clear about who handles what if a line does get nicked.
How Often Does a Water Line Get Hit?
It’s not an everyday event, but across all the jobs in a given year, it comes up a handful of times. Given how closely those two lines share space down there, it’s less a matter of carelessness and more a matter of plain geography.
How Do You Avoid All This When Tunneling at Your Home?
Here’s the reassuring part. Almost everything on this list comes down to a single thing: who you hire.
You can’t control the weather or the maze of pipes under your slab, but you can absolutely control the crew you trust to work down there. Do a little homework up front and most of these worries shrink to almost nothing.
Three things in particular are worth digging into before you sign anything.
Vet the Company Before You Commit
Look into their experience, their reviews, and how long they’ve been tunneling specifically. Ask how many feet of tunnel they dig in a typical year. This is not the kind of work you want a company learning on your home.
Understand Their Tunnel and Backfill Process
Ask exactly how they fill the tunnel back in once the plumbing is done. If the answer is anything other than mud pumping, keep asking questions. This one part of the process is what separates a clean finish from years of erosion, so it’s worth confirming in plain terms.
Ask How They Set Your Plumber Up for Success
Tunneling and plumbing are two different jobs, and they have to work together well. A good tunneling crew makes the plumber’s work easier and safer and communicates clearly so the right spot gets reached the first time. How a contractor handles that partnership tells you a whole lot about them.
Do those three things and you’ve handled the bulk of the risk before a single shovel touches the dirt. The best crews welcome these questions . . . they’d honestly rather you ask now than wonder later.
Ready to See How Tunneling Gets Done Right?
So there you have it: seven things that could go wrong under your slab, from a serious cave-in to a simple nicked water line, and how likely each one really is. Most range from rare to manageable, and nearly all come down to hiring a careful, experienced crew that does the job right.
We’ve spent more than four decades digging tunnels under Brazos Valley homes, refining the process so these problems stay the exception, not the rule. We mud pump our backfill, plan around the weather, and treat the safety of our crew and the care of your home as non-negotiable. Our goal is for you to feel confident, not nervous, about what’s happening beneath your feet.
Now that you know what could go wrong, the natural next step is seeing what going right looks like. Walk through our tunneling process, step by step to see how a proper job unfolds from first shovel to final fill. And when you’re ready for a real answer about your own home, reach out anytime.