Why a sidewalk panel sinks: soil, water and voids under the slab

The short answer

A sidewalk panel sinks when the ground under it stops holding it up. The American Concrete Institute (ACI) defines settling as the lowering in elevation of sections of pavement due to their mass, the loads imposed on them, or shrinkage or displacement of the support. Under a sidewalk, that support is the subgrade, which ACI defines as the soil prepared and compacted to support a structure or a pavement.

The sources point to three ways it fails: soil that was not compacted evenly, water that collects under the slab and carries the base away, and clay that shrinks as it dries. Each leaves different clues, which the table on this page sorts. The cause matters more than the step itself, because lifting a panel back up fills the space under it without changing what made it. The Federal Highway Administration (FHWA) warns that if a sagging sidewalk is mudjacked, "the original unstable base may cause continued sagging."

What holds a panel up

FHWA's 2013 guide to maintaining pedestrian facilities says the type of soil under a sidewalk may be the greatest determinant of whether it fails before the end of its projected lifespan. It cites a Cincinnati study that found a greater correlation between sidewalk failure and soil type than with nearby trees. An adequate subgrade, it says, may deter many failures by providing stability and good drainage, and should be designed for local soil conditions and policies.

As general best practice, FHWA cites Canadian national guidance: a subgrade of uniform material compacted to at least 98% standard Proctor density, with 4 to 6 inches of free-draining granular base under the sidewalk. Those are general figures, not a Texas or city rule. They show what the support is supposed to be: even, firm and able to drain.

Cause 1: soil that was not compacted evenly

FHWA lists "poor base soils and sub-base preparation causing differential settlement" among the reasons sidewalks fail, alongside tree roots, heavy vehicles on sidewalks not designed for them, and concrete that is too thin. It says a slab can rise, subside or tilt because of expansive native soil, frost or thermal expansion, and that this could also be due to non-uniform compaction of the subgrade.

FHWA points to driveways. It says transverse cracks, the ones across the walk, occur due to non-uniform subgrade compaction, especially where vehicles cross the sidewalk. The National Concrete Pavement Technology Center's preservation guide, written for highway pavement, says settlements are most usually associated with fill areas and over culverts, and lists poor consolidation among their causes. The sidewalk cracks guide covers what each crack direction points to.

Cause 2: water under the slab

Guidance written for highway pavement describes water carrying material out from under a slab and leaving it unsupported. The pavement guide says pumping and loss of support occur beneath concrete pavements when three factors are present: an erodible base or subbase, excessive moisture, and significant slab deflections. It says loss of support can develop beneath slab corners and edges for the same reasons.

ACI defines pumping as the ejection of water, or water and solids such as clay or silt, along joints, cracks and pavement edges, when a slab moves down under loads after free water has collected in the base or subgrade. The pavement guide says accumulated fines or staining next to joints and working cracks are evidence of pumping and possible voids. Both describe pavement under traffic loads, which a sidewalk does not carry in the same way, so treat mud or silt at a joint as a clue to raise with a contractor, not a diagnosis.

FHWA says proper drainage matters for sidewalks. It recommends a slight cross slope to prevent pooling, and says a sub-drain behind the curb may be considered to carry water away from the base.

Voids can also connect to pipes and structures. FHWA says it is important to identify the reason for a void before mudjacking, "so that mud is not inadvertently pushed into storm sewers or other utilities." One Midwestern city's inspection criteria, reproduced in FHWA's guide, give damage deemed caused by public infrastructure, such as sewer and water mains, manholes and catch basins, a category of its own. If a low panel sits beside a manhole, storm inlet or other utility structure, tell the city or the utility what you see.

Cause 3: clay that shrinks as it dries

FHWA describes tensile shrinkage as deformation while the soil underneath shrinks as its moisture content decreases. It says the term "sagging" is commonly used for sidewalks where an entire slab or set of slabs drops below its initial grade, and that the forces behind it are more accurately explained as that shrinkage.

One widespread soil in the Dallas area and south to San Antonio does this. A USDA sheet on the Houston Black soil says it occurs on about 1.5 million acres in the Blackland Prairie, which extends from north of Dallas south to San Antonio. It says the soil's highly expansive clays shrink and swell markedly with changes in moisture content, and that in metropolitan areas its very high shrink-swell potential commonly limits building site development. The Natural Resources Conservation Service (NRCS) description of the series says that when dry it has cracks 0.5 to 4 inches wide reaching 12 inches or more below the surface, open for 90 to 150 cumulative days in most years. Water enters it rapidly when it is dry and cracked, and very slowly when it is moist.

The same clay also swells. FHWA describes slabs heaved by swelling clay, so clay can raise one panel and drop another. Those descriptions are of a soil series, not of any address. Whether a given walkway sits on shrink-swell clay is a question for the site visit. The expansive clay guide covers where that clay occurs in North Texas and how its seasonal movement lifts, drops and cracks a walk.

Roots, and panels that only look sunken

A step next to a tree may be a raised panel, not a sunken one. FHWA says raised pavements are commonly caused by tree roots. Before treating a step as a sunken panel, check whether the panel beside it has risen instead: one that stands above both of its neighbors near a tree is a root question, covered in the tree roots guide. FHWA says mudjacking is only effective on sunken sidewalks.

What to ask a contractor

FHWA says care should be taken to identify the cause of settlement, and to make sure it has been addressed, before mudjacking. The pavement guide says that if the underlying causes are not addressed, the same distress "will once again resurface." So ask:

• What caused it, and what did you see that says so? Compaction, water and clay call for different follow-up.
• Has the cause been dealt with? FHWA rates mudjacking most suitable where confidence is high the slabs will not sink again, and least suitable where the underlying problem cannot be counteracted.
• Where does the void lead? Ask how they will make sure material is not pushed into a storm sewer or utility line.
• If the panel is replaced, how will the base be prepared? FHWA says many sidewalk failures come from poor subgrade or tree roots, and that a replacement that ignores them has a shortened life. In its description of replacement, the grade is adjusted, fill is sometimes added, and the base is mechanically tamped where fill is added.

Concrete leveling explains how foam and mudjacking lift a settled panel and where lifting stops working. The mudjacking vs. foam lifting guide compares the two materials, and sidewalk replacement covers panels too broken to lift.

What to ask the city

Who must repair a sidewalk along a street depends on the city, and this page does not say. The Dallas guide quotes the City's own code, and public vs. private sidewalk repairs covers the general question. Check with the city that the address is in before you hire anyone or wait for a city crew.

Next steps

Record each low panel, the height of the step, what is beside it (a driveway, a utility cover, a tree) and where water runs with the walkway inspection checklist. The method comparison sorts grinding, lifting and replacement, the inquiry preparation guide gathers what a contractor needs, and you can ask about your project when ready.