Poorly prepared and compacted subgrade causes cracks and settlement in landscape slabs.

Why slabs crack and settle—and how to stop it before it starts

A concrete slab in a landscape seems straightforward: pour, cure, and use. But there’s a quiet, stubborn chain of events that can turn a smooth patio, driveway, or walkway into a trouble spot. If you’ve ever watched a driveway crack not in a neat line, or seen a slab that looks a little uneven after a heavy rain, you’re not alone. The root cause often hides under the surface, in the ground that bears the load day in and day out.

Here’s the thing most landscapers in Nevada keep in mind: the condition of the subgrade—the soil right beneath the slab—is the real boss of the project. It’s not glamorous, but it’s foundational. When the subgrade is properly prepared and compacted, the slab has a fighting chance to stay straight and strong for decades. When it isn’t, you get cracks, settlement, and, occasionally, a whole pile of headaches.

What causes cracks, settlement, and structural trouble?

Let’s break it down with a simple but very honest diagnosis: the primary culprit is a poorly prepared and compacted subgrade. Think of the subgrade as the foundation of a house, only on a smaller, outdoor scale. If you pour on top of loose, void-filled soil, what happens? The concrete bears the weight, but the ground beneath it shifts, compresses, or sinks in spots. The result is uneven support. The slab can crack as stress concentrates where the ground is weakest, and over time you may notice settlement—areas that sit lower than the rest, like a living room floor with a dip.

To make this click, imagine you’re laying a garden path on a bed of soft soil. If you walk on that path for a while, you’ll feel some spots compress first, others stay firm. The path isn’t truly flat anymore. A concrete slab behaves the same way. If the base isn’t uniform and solid, the forces from the load are distributed unevenly, and stress lines form. Over months or years, those stresses translate into cracks, popping joints, and, in the worst case, visible settlement that makes the surface look like it’s winking at you.

It’s worth noting that other issues can sneak in and worsen the picture, but they’re usually secondary. Poor drainage can push water under a slab and weaken the subgrade over time. Overwatering during curing can alter moisture in the nearby soil, which can affect how the ground compacts. An improper concrete mix can affect surface durability and the way joints crack. Still, the core problem tends to be that weak, uneven substrate beneath the concrete.

A quick tour of the “why” behind the symptoms

• Voids and loose pockets under the slab: If the subgrade isn’t compacted evenly, there are holes, gaps, and soft spots. When the slab receives weight, it bears down on those voids, and the surface above shows it as cracking or a hollow-feeling edge.

• Differential settlement: Not all parts of the subgrade settle at the same rate. One corner of a patio might drop a little, pulling against the rest of the slab. That mismatch creates stress cracks as the concrete tries to stay intact.

• Poor load distribution: A slab isn’t just a flat sheet. It’s a heavy structure that wants a steady, uniform foundation. Uneven base materials don’t distribute load evenly, so the concrete ends up “working” in ways it wasn’t designed to.

• Ground moisture shifts: In arid climates, moisture in the soil can swing a lot. Clay soils, for example, expand when wet and shrink when dry. If the subgrade swings, the concrete slab feels that push and pull, showing up as cracks or settlement.

Not the only culprits, but the ones you’ll see first

• Drainage: Water is a stealthy enemy. If the slab doesn’t have proper slope and drainage away from the surface, water can pool near edges, soften the subgrade, and invite settling. It’s not the primary offender, but it often makes a bad situation worse.

• Overwatering and curing practices: If you’re curing the concrete with water on a hot Nevada day, you’re helping the surface set nicely, but you might also affect the surrounding soil moisture. The goal is a balanced moisture environment that doesn’t destabilize the soil right under the slab.

• Concrete mix and workmanship: A mix that doesn’t meet the right specifications or joints that aren’t properly spaced can lead to surface cracking. It’s a piece of the puzzle, but it’s not usually the root cause of structural failure—that honor goes to subgrade prep.

How to protect slabs from this fate: practical steps you can take

Before you pour, here’s a mental checklist that blends Nevada conditions with solid engineering ideas. Think of it as a compact guide you’ll keep in your tool belt.

1. Inspect and select the right subgrade

• Remove vegetation, roots, and organics that could decay and shift.

• Check the soil type. In many Nevada landscapes you’ll encounter clay, silt, and caliche-like layers. These behave differently than clean fill.

• Look for existing voids or soft spots. If you feel “bounce” under your foot when you push on the soil, that’s a red flag.

2. Define an appropriate depth and subbase

• Excavate to a depth that accommodates the slab plus the base, with room for a stable subbase.

• Use a well-graded granular subbase (crushed rock, gravel) to create a uniform, stable layer. The goal is a consistent bearing surface.

• Consider a geotextile fabric under the subbase where soils are suspect. It helps separate soils and reduces mixing.

3. Achieve proper compaction

• Compact in layers, not in one thick pass. Each layer should reach the target compaction before the next goes on.

• Use a plate compactor or roller appropriate for the base material and the slab size.

• Check compaction with a proctor-like test or in-field verification as your crew would do in Nevada’s warm season. Consistency is the name of the game.

4. Grade for drainage and slope

• Place slope away from structures and toward proper drainage spots. A slight setback at the edges helps keep water from pooling at joints.

• Plan around the landscape’s drainage patterns—gently redirect water where you want it to go, not where it could undermine the slab.

5. Control joints and reinforcement

• Use control joints to plan where cracks will occur. Proper spacing helps cracks follow a predictable line and remain less disruptive.

• In larger slabs, consider reinforcement options and consult structural guidelines to see if fiber reinforcement or dowels are appropriate for your design and soil conditions.

6. Mix, cure, and protect

• Ensure the concrete mix is suitable for the exposure conditions and load. High-strength or specially formulated mixes can improve durability, especially in Nevada’s varying climates.

• Protect the curing environment. Temporary shading, misting, or moisture retention measures can help the concrete cure evenly while the surrounding soil stabilizes.

7. Monitor and maintain

• After installation, monitor for signs of movement or water issues. Address drainage problems or soil shifts early.

• Seal joints as needed and inspect edges that receive winter or monsoon-season stress.

Nevada-specific realities to keep in mind

Desert soils aren’t always what they appear. Some areas hide clay, salt, or caliche layers that resist compaction and shift under load. The climate adds another layer of complexity: temperature swings, occasional freeze-thaw cycles in higher elevations, and irrigation practices that can alter soil moisture in the near-term. A solid subgrade plan isn’t one-size-fits-all. It’s a thoughtful recipe that accounts for soil tests, site conditions, and the expected load from the slab and the landscape around it.

A practical way to frame the risk

• If you can imagine a slab as a sleeping cat, a solid subgrade is the sturdy bed that makes the cat happy and still. A soft bed or an uneven surface will cause the cat to squirm and wake easily. The same idea applies to concrete: a flat, stable base keeps the slab calm and reduces the chance of cracks and settlement.

• If you see signs of movement after rain, don’t assume it’s just the surface. Look beneath. Wet conditions can reveal a weak subgrade or poor drainage plan. Address the root cause, not only the symptom.

Real-world signs that you might be dealing with a compromised subgrade

• Cracks that radiate from a corner or along a single axis, as opposed to a neat grid.

• Spalling at the edge where the slab meets the base, suggesting differential movement.

• Uneven or sunken areas that feel slightly hollow when you tap them.

• Water pooling near joints or along the slab’s edge after a rain, indicating drainage issues that compound soil instability.

A closing thought

You don’t have to be a philosopher to see this clearly: a slab’s fate isn’t sealed by the concrete itself. It’s shaped by the ground beneath it. In landscape projects, especially in Nevada’s varied soils and climate, the subgrade’s strength and uniformity are the quiet heroes. When you treat the base with respect—thorough soil assessment, proper subbase, careful compaction, and deliberate drainage—you’re investing in long-term performance.

If you’re exploring this line of work in Nevada, you’ll hear the same message again and again from seasoned crews: the root of successful slab construction is meticulous site preparation. It’s not glamorous, but it’s essential. And the payoff? A smooth, durable surface that stays true—crack-free and steadier as time goes by.

So next time you’re planning a patio, sidewalk, or driveway, pause at the edge of the trench and ask yourself: is the ground beneath this slab ready to bear the load? If the answer isn’t a confident yes, it’s worth sharpening the prep plan. A little extra effort up front saves a world of trouble later—and that’s the kind of wisdom that makes a landscape project shine.