CAUSES OF FOUNDATION SETTLEMENT

The causes of foundation settlement are rarely due to the design (or under design) of the structure itself. More commonly, damages is caused as charge occurs within the foundation soils that surround and support the structure.

The following briefly describes are common causes of foundation settlement:

1. Weak bearing soils

2. Poor compaction

3. Change in moisture content

4. Maturing trees and vegetation

5. Soil consolidation

6. Wetting and softening of soil

1. WEAK BEARING SOILS: some soils are simply not capable of supporting the weight or bearings pressure exerted by building foundation. As a result, the footings press or sink into the soft soils, similar in theory to how a person standiting in mud sinks into soft wet clay.

In such cases, footings may be designed to spread the load over the weak soils, there by reducing potential foundation settlement.

However, the majority of settlement problems caused by weak bearing soil occur in resigned based upon general guidelines and not site specification soil information.

2.  POOR COMPACTION: placement of a fill soil is common practice in the development of both commercial and residential subdivisions. In general, before a foundation can be constructed on a plot, hilltops are cut down and valleys are filled in order to create buildable lots. Properly and compacted fill soil can provide adequate support for foundations, and are sometimes brought in from off-site locations. When fill soils are not adequately compress under a foundation load resulting in settlement of the structure.

3. CHANGES IN MOISTURE CONTENT: extreme changes in moisture content within foundation soils can saturated foundation soils which often loads to softening or weakening of clays sits. The reduce ability of the soils to support the load results in foundation settlement. Increased moisture within foundation soils is often a consequence of poor surface drainage around the structure, leaks in water lines or plumbing, or a raised ground water table. Soil with high clay content also have a tendency of shrink with loss of moisture. As clay soil dry out, they shrink or contract, resulting in a general decrease in soil volume. Therefore, settlement damage is often observed in a structure supported on dried out soils is commonly caused by extensive drought-like conditions, maturing trees and vegetation (see next section), and leaking subfloor heating, ventilation, and air conditioning (HVAC) systems.

4. MATURING TREE AND VEGETATION: maturing trees, bushes and other vegetation in close proximity to a home or building are common causes of settlement. As tree and other vegetation mature, their demand for water also grows. The root systems continually expand and can draw moisture from the soil beneath the foundation. Again, clay-rich soils shrink as they loss moisture, resulting in settlement of overlying structures. Many home and building owners often state that they did not have a settlement problem until decades after the structure was built. This time frame coincides with the maturation and growth of the trees and vegetation. Foundation closer the surfaces are more often affected by soil dehydration due to tree roots than are deep, basement level foundations. As a general rule, the diameter of a trees root system is at least as large as the trees canopy.

5. SOIL CONSOLIDATION: consolidation occurs when the weight of structure or newly-placed fill soils compresses lower, weak clayey soils. The applied load forces water out of the clay soils, allowing the individual soil particles to become more densely spaced. Consolidation result in downward movement or settlement of overlying structures. Settlement caused by consolidation of foundation soils may take weeks, months, or years to be considered “complete.” As this occurs, the foundation will experience downward movement-sometimes at an uneven rate. This leads to cracks and structural damage.

6. WETTING AND SOFTENING OF SOIL: Foundation soils usually wet and soften in three scenarios:

i. Heavy rain and flooding: when clay soil contacts water, they hold on to it and become very soft. This soft soil is not good load-bearing soil, and heavy objects will sink down into it.

ii. Poor drainage: if water is allowed to “pond” next to be home due to poor soil grading, clogged gutters, or some other factor, the soil will absorb the water. If the soils around the home are clay, then they will often, and the home may sink.

iii. Plumbing leaks and broken water lines: plumbing leaks under or around a home can also saturate the soils around a home, and potentially weaken their load-bearing capacity.

1. FOUNDATION UNDERPINNING: sometimes referred to as “peering,” this technique involves driving steel piers (either helical or standard “push” piers) into the earth beneath a settled foundation until each pier reaches solid, stable soil or bedrock. A bracket at the top of each pier is connected to the foundation, enabling the installer to raise and stabilize the settled masonry. Underpinning is effective for raising settled foundation walls and slabs.

2. POLYMER INJECTION: As expending “geotechnical” foam can be injected into looser or weak soil to consolidated and stabilize the soil while also improving its load-bearing characteristics. Polymer injection is sometimes used in combination with underpinning. By itself, it’s an effective technique for raising settled sidewalks, walkways and concrete slab.

3. GROUT PUMPING: sometimes referred to as mud jacking, this technique involves pumping a liquid concrete “slurry” into void beneath settled concrete. Grout pumping can be used to raise sunken sidewalks and slabs, or to fill voids left after raising s settled foundation with piers.