The 5-Foot Sinkhole: Why Backfilling Your Sewer Trench Is Making It Worse

The Scenario

A homeowner recently asked:

My new construction home has a sewer trench that keeps collapsing. I’ve backfilled it three times, but now I’m staring at a 5-foot deep hole right next to my foundation—what is going on?

The GardenOwl Diagnosis

The Assessment

You bought a new build in 2022. Everything looked perfect until a small dip appeared in the lawn where the sewer line runs, drastically reducing its curb appeal and signaling the onset of The Subsurface Migration Syndrome. You did what most homeowners do: you tossed some dirt in it and re-seeded, unaware that you were simply feeding a subterranean "hourglass" where soil is physically vanishing rather than merely settling.

But the hunger wasn't satisfied. The ground kept eating the soil. Now, you are looking at a trench that is five feet deep, two feet wide, and threatening the stability of your foundation. Your builder says it’s “just settlement.” Your landscaper says “just keep filling it.”

They are both wrong, and following their advice is dangerous.

The Trap: Settlement vs. Migration

This is the classic confusion between Soil Settlement and Soil Migration.

Settlement happens when air pockets in disturbed soil slowly compress. It usually results in a shallow depression—maybe 2 to 6 inches deep—over the course of a year. It’s annoying, but it’s stable.

Migration is when soil physically moves from one place to another. If you have a hole that is five feet deep, that dirt didn't just compress; it vanished.

So, where did it go?

Think of an hourglass. The sand flows from the top bulb to the bottom bulb through a small opening. In your yard, the "top bulb" is your lawn, and the "bottom bulb" is likely the inside of your sewer pipe. You almost certainly have a cracked pipe, a separated joint, or a failed connection. Every time it rains or you run the water, that soil is washing into the pipe and being carried away to the city main.

If you keep dumping dirt in there, you are just feeding the hourglass. Eventually, this can cause a massive sewage backup or, worse, undermine the footing of your house foundation.

The Solution (Deep Dive)

Put down the shovel. Do not buy any more topsoil. Here is the exact protocol to fix this before it becomes a structural disaster.

1. The Camera Scope (Non-Negotiable)

Before you do any landscaping, you need a plumber who specializes in scoping. You need to verify exactly where the breach is. If the builder backfilled the trench with heavy rocks or construction debris (a common lazy practice), a heavy rock could have crushed the PVC line.

2. The Excavation and Repair

Once the break is located, the bad news is that the dirt has to come out again. The pipe must be repaired. If this is a 2022 build, this should absolutely be a warranty issue. Do not let the builder talk you out of this—a 5-foot void is not standard settling.

3. The "Lifts" Method for Backfilling

Once the pipe is fixed, how you put the dirt back matters. The reason many trenches settle (even without broken pipes) is that contractors dump 5 feet of dirt in at once and drive away.

To prevent this from sinking again, the soil must be compacted in lifts.

• Add 6–8 inches of soil.
• Compact it with a jumping jack or plate compactor.
• Add another 6–8 inches.
• Compact again.
• Repeat until you reach grade.

4. The Surface Fix: Grass vs. Rock

Now you are left with a scar on a slope. You have two choices:

Option A: Re-sodding. If you compact the soil properly, you can lay sod. However, since this is on a slope, you might face erosion issues before the grass roots take hold. Check out my guide on Why Topsoil Slides Off Your Steep Slope to understand how to stabilize this area.

Option B: A Dry Creek Bed. Since water already wants to travel down this slope, why fight it? You could turn this "scar" into a deliberate drainage feature. Line the trench (shallowly) with landscape fabric and fill it with 3-5 inch river rock. This allows water to move without washing away soil. It also solves the issue of trying to mow a steep, awkward strip next to the house.

This is similar to the logic we use when dealing with awkward dirt strips next to foundations—sometimes hardscape is smarter than grass.

Visualizing the Result

Fixing the pipe is the expensive, unsexy part. But once that is done, you have a blank canvas. Don't just throw grass seed down and hope for the best. You can use this opportunity to add a drainage swale that actually protects your foundation.

If you want to test this on your own yard, upload a photo to our Exterior Design App and see what this design would look like in your space. You can visualize a river rock swale versus a grass slope before you haul a single wheelbarrow of stone.