Briefly explain using bullet points four factors that can lead to volume changes

Question

Briefly explain using bullet points four factors that can lead to volume changes in soils and result in foundation settlements. Explain and compare potential Ultimate Limit State failure mechanisms of gravity and cantilever retaining structures. Use sketches to illustrate your answers. Outline the process for obtaining design values as part of ground characterisation, using shear strengths measured in triaxial tests as an example. Describe the steps employed, types of factors applied and a strategy for dealing with parameter variability.

Explanation / Answer

1.Weak Bearing Soils

Often times, soils simply can not support the weight or bearing pressure of a building's foundation. As a result, the footings of the foundation will sink into the soft soils, similar to a person standing in mud. In such cases, special footings are designed to spread the load over the weak soils, in attempts to reduce potential foundation settlement. Unfortunately, the majority of settlement problems caused by weak bearing soils occur in residential construction, where the footings are designed based upon general guidelines rather than site-specific soil information.

2. Poor Compaction

When developing commercial or residential property, usually hilltops are cut down and valleys are filled with soils from off-site locations, to create flat land for build-able lots. With that said, the placement of properly compacted fill soils can provide adequate support for foundations. When fill soils are not adequately compacted, they can compress (settle) under the load of a foundation resulting in damage to the structure.

3. Changes in Moisture Content

The lack of moisture, or an excess of moisture within soils, can result in foundation settlement. When there is a buildup of moisture, the foundation soils will saturate leading to soft, weak clays and silts. The soil will then lose it’s load bearing capabilities, the foundation will sink and settlement will occur. Increased moisture within foundation soils is usually due to poor surface drainage around the structure, leaks in water lines or plumbing, or a raised groundwater table.

Soils with high clay content are more likely to shrink with the loss of moisture, resulting in a general decrease in soil volume. Which is why settlement damage is often found in structures supported on dried out soil. Usually, the cause of dry soil is due to extensive drought conditions, or maturing trees and vegetation. It can also be attributed to leaking subfloor heating, ventilation, and air conditioning.

4. Maturing Trees & Vegetation

When in close proximity, maturing trees, bushes and other vegetation can be a cause of foundation settlement. As trees and other vegetation mature, their root system along with a demand for water grows. The roots pull water from the soil, eventually causing it to dry out and shrink over time. Those foundations closer to the earth's surface are more often affected by soil dehydration than basement level foundations. The same goes for clay-rich soils over more solid sediment. When settlement problems occur decades after a house was built, a lack of moisture caused by roots is the likely culprit as the time-frame coincides with the maturation of trees and vegetation. As a general rule of thumb, the diameter of a tree's root system is at least the size of it’s canopy.

5. Soil Consolidation

When the weight of a structure or newly placed fill soils compress, the lower, weaker clay soil is consolidated. The individual clay soil particles become very dense as the load from above forces the moisture out. When soil consolidation occurs, your foundation experiences a downward movement at an uneven rate which results in cracks and structural damage. It could take weeks to years to see the results of settlement caused by consolidation of foundation soils.

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