The Tower of Pisa in Italy is a widely known tourist spot that attracts visitors worldwide due to its distinct tilt. However, the millions of people who visit every year may not understand that the tower’s tilt results from soil subsidence – when the ground underneath the foundation sinks or settles, causing the structure above to tilt or lean.
Soil subsidence refers to the settling or sinking of the ground surface due to the compression or consolidation of the underlying soil, or the withdrawal of underground fluids (water, oil or gas). This phenomenon occurs due to natural effects that cause the soil layer to lose its shear strength, decrease density and displace or lose its permeability. Soil subsidence can have significant implications for the design and construction of deep foundations, such as piles and caissons, as subsidence can alter the soil-structure interaction and load transfer mechanisms, and increase the risk of failure or instability of the foundation system.
In the case of the Tower of Pisa, the soil subsidence was caused by the compression of the soft, clayey soil beneath the tower. The clay soil, which was initially wet and compressible, was compacted and consolidated over time, causing the ground to sink and the tower to lean. This process was exacerbated by the tower’s weight and the soil’s poor bearing capacity, which led to a significant tilt over time.
The tilt of the Tower of Pisa is a well-known example of soil subsidence and serves as a cautionary tale for engineers and architects. By understanding the soil subsidence potential of a site, and designing and building foundations that accommodate or mitigate its effects, engineers and architects can ensure the stability and safety of structures for generations to come.
Detailed analysis of a site’s subsidence potential are essential for deep foundation designers and builders.
Ground improvement should happen at the beginning of a project
Soil subsidence must be considered in every project phase – during the bid, design and construction. Detailed analysis of a site’s subsidence potential are essential for deep foundation designers and builders. It is essential to arrange ground improvement for the project site if the land doesn’t allow ground replacement. Moreover, ground improvement is required when the existing ground is reclaimed for different purposes. Alteration of site conditions achieves better soil performance and can reduce the project’s cost.
Ground improvement or modification should be made to increase shear strength and bearing resistance, increase the density of existing soil, decrease permeability, control deformations, increase drainage and consolidation, provide lateral stability, strengthen existing substructure and improve liquefaction resistance.
Construction companies dealing with soil subsidence of deep foundations should consider the following factors to fix the issue:
Identification of the cause: The first step is to determine the cause of the subsidence, whether it is due to soil compression, underground fluid withdrawal or other factors.
Assessment of the extent of subsidence: The magnitude and distribution of subsidence should be measured to assess its impact on the foundation system.
Foundation design modifications: If subsidence is anticipated, the foundation design may need to be modified to accommodate its effects and ensure the stability of the structure.
Ground improvement techniques: Ground improvement techniques, such as soil compaction, grouting or reinforcement, may be used to increase the strength and stiffness of the soil, and reduce the risk of subsidence.
Monitoring and maintenance: Regular monitoring of the subsidence and the foundation system is crucial to detect potential problems, and ensure corrective measures are taken.
It is crucial to work with experienced professionals, such as geotechnical engineers, and use state-of-the-art techniques and technologies to address soil subsidence of deep foundations effectively. Other considerations when selecting the correct ground improvement solution are environmental disturbances, application, ground conditions, access to the existing structure, function and costs.
Each project should include a soil assessment and a quality assurance plan. This will make all stakeholders aware of pre-planning, proper communication and available solutions.
Ground improvement technologies
Several technologies, which vary based on function, are available to address soil subsidence. Some of the most used are:
Ground improvement techniques: Techniques such as soil compaction, grouting and soil reinforcement increase the strength and stiffness of the soil, and reduce the risk of subsidence.
Vibro-compaction: Compaction of soil by applying controlled, high-frequency vibration. The technique increases the density and strength of soil, improving its bearing capacity.
Stone columns: This technique increases the soil’s bearing capacity, reducing the risk of settlement or subsidence. Stone columns provide vertical support for the structure above and help transfer the structure’s weight into the more competent soil layers below. They also help improve the soil’s permeability by reducing the consolidation rate and increasing its drainage capabilities.
Aggregate piers: These are installed by drilling 18- to 36-inch-diameter holes into the soil. Aggregate piers in seismic and sensitive areas give lateral stability and reduce dynamic settlement.
Deep soil mixing: This technique involves injecting a slurry of soil, cement and water into the ground to create columns of improved soil that can support the foundation system.
Pile foundations: Used to transfer the structure’s load to deeper, more stable soil layers, reducing the impact of subsidence on the foundation system.
Compaction grouting: This technique involves injecting a dense grout into the soil to compact, reinforce and reduce subsidence.
Slope stabilization: Techniques such as geosynthetics, retaining walls and soil nailing can prevent or reduce subsidence by improving the stability of slopes and retaining walls.
It is important to note that the most appropriate technology for fixing soil subsidence depends on the specific site conditions and the cause of subsidence. A professional geotechnical engineer can assess the site and recommend the most appropriate solution.
Canadian soil and soil subsidence
The most common type of soil in Canada is glacial till, which glaciers deposited as they advanced and retreated over the landscape.
Glacial till is a heterogeneous soil consisting of rock fragments, sand, silt and clay. It covers much of the Canadian landscape, particularly in the Prairie provinces and the Canadian Shield. This soil type is known for its high variability and low fertility, making it challenging for agriculture and urban development. However, glacial till also provides a solid base for construction and is an important resource for the mining, forestry and energy industries. Other common soil types in Canada include organic soils, peat and alluvial soils.
Soil subsidence could occur in glacial till soils. Still, the risk of subsidence depends on several factors, including the type of soil, the moisture content, the loading conditions and the presence of underground fluids. Therefore, it is difficult to say if there are more risks of soil subsidence in glacial till soils than in other soil types, as each has unique characteristics and subsidence potential.
For example, glacial till soils can be susceptible to subsidence due to soil compaction, especially if the soil is wet or has a high moisture content. However, some types of glacial till soils can also be relatively stable and resistant to subsidence.
On the other hand, organic soils, such as peat, can be highly susceptible to subsidence due to their low strength and compressibility. Alluvial soils, which are deposited by rivers or other water bodies, can also be prone to subsidence due to underground fluids, such as groundwater.
It is important to assess the subsidence potential of a specific soil type and site, and consider the particular site conditions, such as loading conditions and underground fluids, to determine the risk of soil subsidence. A professional geotechnical engineer can assess the soil subsidence potential and help design and build foundations that can accommodate or mitigate its effects. 
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