Buildings, parking lots, roadways, sidewalks, and other structures all depend on the soil beneath them for support. When that soil moves or compresses after construction, the result can be settlement that affects the performance and condition of the structure above.
Some settlement is expected and can be accounted for during design. Excessive or uneven settlement, however, can contribute to cracked concrete, uneven pavement, foundation problems, drainage issues, and other costly repairs.
Understanding why soil settlement occurs is an important part of evaluating a site before and during construction.
What Is Soil Settlement?
Soil settlement occurs when the ground beneath a structure decreases in volume or shifts downward under a load. This movement can happen gradually over months or years, or it can become noticeable relatively soon after construction.
One of the biggest concerns is differential settlement, which occurs when one portion of a structure settles more than another. Uneven movement can place additional stress on foundations, slabs, pavement, utilities, and other components.
Several site and construction conditions can contribute to settlement.
Poorly Compacted Fill
Improperly compacted fill is a common cause of settlement on construction sites.
When soil is placed to raise a site or replace unsuitable material, it generally needs to be installed in controlled layers and compacted to meet project requirements. If fill is placed too thickly, contains excessive moisture, or does not receive adequate compaction, voids can remain within the soil.
Over time, the weight of a building, pavement, traffic, or even the soil itself can compress these loose areas.
Field density and compaction testing can help verify whether the placed material meets the requirements established for the project.
Changes in Soil Moisture
Water can significantly affect the behavior of soil.
Some soils expand when they absorb moisture and shrink as they dry. Repeated wetting and drying cycles can cause movement beneath foundations and pavement. Other soils may lose strength when they become saturated, increasing the potential for settlement under a load.
Moisture changes can result from several sources, including:
- Poor site drainage
- Plumbing or utility leaks
- Changes in groundwater conditions
- Heavy rainfall
- Improper grading
- Downspouts discharging near foundations
Proper drainage and an understanding of the site’s soil conditions can help reduce these risks.
Weak or Compressible Native Soils
Not every site naturally contains soil capable of supporting the planned development.
Loose soils, soft clays, organic material, undocumented fill, and other unsuitable materials may compress significantly when subjected to the weight of a building or pavement system.
A geotechnical exploration performed before construction can help identify these conditions. Depending on the findings, recommendations may include removing unsuitable material, improving the soil, using engineered fill, modifying foundation design, or considering other site-specific solutions.
Consolidation of Clay Soils
Certain fine-grained soils can experience consolidation settlement.
When a new load is placed on saturated clay, water within the soil pores may gradually be squeezed out. As this happens, the soil particles move closer together and the soil layer compresses.
Unlike settlement caused by immediately compressing loose fill, consolidation can take place over an extended period. This makes understanding subsurface conditions particularly important for projects involving significant structural loads or thick layers of compressible soil.
Erosion and Loss of Supporting Soil
Settlement can also occur when soil is physically removed from beneath a structure.
Water moving through or across a site can erode supporting material, creating voids beneath pavement, sidewalks, slabs, or foundations. Poor drainage systems, leaking pipes, failed stormwater infrastructure, or concentrated runoff can all contribute to erosion.
Signs may include sinking pavement, voids beneath concrete, cracking, or localized depressions.
Improper Subgrade Preparation
Even suitable soil can create problems when the subgrade is not properly prepared before construction.
Before pavement, slabs, or foundations are installed, the underlying soil may need to be graded, moisture-conditioned, compacted, or otherwise prepared according to the project specifications.
Soft or unstable areas identified during construction may need additional evaluation before work continues. Addressing these conditions before concrete or pavement is placed can help prevent more difficult repairs later.
Increased Loads on Existing Soil
Soil conditions that adequately supported an undeveloped site may behave differently once substantial loads are introduced.
Buildings, retaining walls, heavy equipment, pavement, storage areas, and new fill all place additional stress on the underlying soil. If those loads exceed what the soil can adequately support, settlement may occur.
This is one reason geotechnical engineers evaluate both the subsurface conditions and the anticipated loads associated with a proposed development.
How Can Soil Settlement Be Reduced?
Reducing settlement risk begins with understanding the site.
Geotechnical exploration can provide information about soil types, groundwater conditions, strength, density, and other subsurface characteristics. Engineers can use this information to develop recommendations appropriate for the proposed construction.
During construction, materials testing and observation can help verify that earthwork is being completed according to project requirements. This may include monitoring fill placement, testing soil moisture and density, and evaluating questionable subgrade conditions.
Testing does not eliminate every potential construction problem, but it provides valuable information that allows project teams to identify issues and make informed decisions before they become larger problems.
Watch for the Warning Signs of Settlement
Settlement may become visible through cracking or movement in the completed structure. Potential warning signs include uneven floors, foundation cracks, doors or windows that begin sticking, depressed pavement, cracking around utility structures, or areas where water begins collecting unexpectedly.
Not every crack or uneven surface indicates a serious settlement problem. However, significant or progressive movement should be evaluated to determine the underlying cause.
Start With a Better Understanding of the Ground
The performance of a building or pavement system begins below the surface. Understanding the site’s soil conditions before construction—and verifying earthwork as the project progresses—can help reduce uncertainty and identify potential problems early.
Quality Testing & Engineering (QTE) provides geotechnical exploration, construction materials testing, environmental testing, and related engineering services for projects throughout the St. Louis Metro and surrounding areas.
If you’re planning a commercial, industrial, municipal, or residential development, contact QTE to learn how geotechnical and construction testing services can help you better understand the conditions beneath your project.