The dynamic nature of earthquakes introduces a unique set of stresses on structures. Imagine the ground beneath a building not just shaking but moving laterally, creating a scenario where the foundation’s ability to support the structure above is critically challenged. This lateral movement is especially perilous for buildings on unstable soil or those constructed without adequate reinforcement. As the ground moves, it prompts a series of structural responses – from shear walls bracing against the motion to the entire building swaying in an attempt to dissipate the energy.
The Most Common Earthquake Effects on Foundation
The following are the most frequent effects of earthquakes on foundations:
Surface Faulting and Ground Rupture
Surface faulting and ground rupture are common during earthquakes. Fault displacement, which refers to the relative movement of the two sides of a fault surface, is measured in a specific direction. This movement can result in both vertical and horizontal displacements.
The primary manifestation of fault displacement is fault rupture, where the ground surface breaks along the trace of a fault. This occurs when accumulated stress along the fault exceeds the strength of the rocks, causing them to fracture and move. These fractures can extend for several kilometers and can cause significant damage to buildings and structures built on or near the fault.
Liquefaction
Liquefaction is a phenomenon that occurs during earthquakes and can lead to significant damage and hazards. It refers to the process where loose, water-saturated sandy soil transforms from a solid state to a liquefied condition due to increased pore water pressure caused by ground shaking.
The typical subsurface condition susceptible to liquefaction is loose sand recently deposited or placed, often with a high groundwater table near the ground surface. During an earthquake, the ground shaking causes the loose sand to contract, increasing pore water pressure. This increase in pressure causes an upward flow of water to the ground surface, emerging as mud spouts or sand boils.
Effects of Liquefaction:
Liquefaction can have several effects on the surrounding area. One of the most significant is the sinking or falling over of structures built on top of liquefied soil. The loss of strength and stiffness in the soil can cause buildings and infrastructure to collapse or tilt, leading to severe damage and potential loss of life.
Liquefaction can also result in lateral movement of slopes, creating flow slides. This occurs when the liquefied soil flows downslope due to the increased pore water pressure. Flow slides can be highly destructive, as they can carry away buildings, roads, and other structures in their path. They can also cause landslides and further damage to the surrounding environment.
The severity of liquefaction and its effects depend on several factors, including the magnitude of the earthquake, the depth of the liquefied soil layer, and the proximity of structures to the affected area. Areas with a high potential for liquefaction are often identified through geological studies and assessments, allowing for better preparation and mitigation measures to be put in place.
Slope Movement and Ground Settlement
Lateral movements caused by earthquakes can have various effects on the Earth’s surface, including slope movement and settlement. In particular, loose soils such as cohesionless sand and gravel are highly susceptible to settlement due to the ground vibrations generated during seismic events.
Slope Movement
Slope movement refers to the sliding or collapsing of inclined surfaces, such as hillsides or slopes, triggered by seismic activity. When an earthquake occurs, the ground shaking can cause the loose soil particles to lose their frictional resistance and slide down the slope. This can result in landslides or slope failures, which can be highly destructive and pose risks to human lives and infrastructure.
Settlement
On the other hand, settlement refers to the downward movement of the ground surface due to compression or consolidation of underlying soils. During an earthquake, the ground shaking can cause the loose soil particles to rearrange and settle, resulting in a decrease in the elevation of the ground surface. This settlement can lead to uneven or sunken areas, affecting the stability of structures and infrastructure built on top.
The severity of slope movement and settlement caused by earthquakes depends on various factors, including the magnitude and duration of the seismic event, the soil characteristics, and the slope’s steepness. In general, larger and longer earthquakes can cause more significant slope movement and settlement. Additionally, the soil type plays a crucial role in determining the severity of these lateral movements. Loose soils with low cohesion, such as sandy soil and gravel, are more prone to sliding and settling than cohesive soils like clay.
The proximity of structures to the affected area also influences the impact of slope movement and settlement. If structures are built on or near slopes that are prone to movement, they are at a higher risk of damage during an earthquake. Therefore, it is important for engineers and urban planners to consider the potential for lateral movements when designing and constructing buildings in seismically active areas. Measures such as reinforcing slopes, using retaining walls, and implementing proper drainage systems can help mitigate the effects of slope movement and settlement caused by earthquakes.
Translation and Rotation
An unusual feature of earthquakes is the translation and rotation of objects. Translation and rotation are two unusual features that can occur during earthquakes. While most people are familiar with the shaking and ground displacement caused by seismic events, they may not be aware of the additional movement that can take place.
Translation
Refers to the linear displacement of objects during an earthquake. This means that not only does the ground move horizontally or vertically, but buildings, structures, and other objects can also undergo a change in position along a straight line. This can result in buildings shifting or moving laterally, potentially causing damage or collapse.
Rotation
On the other hand, refers to the twisting or turning of objects during an earthquake. This means that structures can experience a change in orientation or alignment, leading to structural instability. Rotation can be particularly dangerous for tall buildings or structures with irregular shapes, as it can affect their overall stability and structural integrity.
The occurrence of translation and rotation during an earthquake depends on various factors, including the magnitude and duration of the seismic event, as well as the characteristics of the structures and objects involved. The intensity of these movements can vary greatly, ranging from slight shifts in position to significant displacements and rotations.
To mitigate the effects of translation and rotation, engineers and architects employ various design strategies. These may include incorporating flexible materials and joints into buildings to allow for movement, using strong and durable construction methods, and implementing specialised seismic isolation systems.
In addition to designing structures that are resilient to translation and rotation, it is also essential to consider the surrounding environment. For example, if a building is located near a fault line or on unstable ground, measures such as reinforcing the soil or using deep foundation techniques can help minimise the risk of translation and rotation.
The Behaviour of Different Footing Types During Earthquakes
The lateral movements imposed by seismic activity have different effects on different types of footings.
Strip and Stump Footing Systems
In areas prone to seismic activity, houses with strip and stump footing systems are particularly susceptible to damage during earthquakes. These raised floor systems, characterised by distinctive crawl spaces, lack the necessary resistance to lateral movements caused by seismic events. As a result, the damages incurred by these structures tend to be more severe compared to other types of foundation systems.
One key reason for the vulnerability of strip and stump footing systems is the lack of shear resistance in the concrete stumps. During an earthquake, these isolated posts can collapse or tilt, leading to structural instability. Additionally, inadequate bolted connections between the house and the footings can cause the structure to slide or even fall off the foundation.
The age of the residence also plays a role in its vulnerability to lateral movements during earthquakes. Older houses may have weakened timber stumps due to rot or termite damage. Sometimes, the concrete strip footings may be unreinforced or weakened due to prior soil movements, making them more susceptible to cracking during an earthquake.
Slab-on-Ground Footing System
On the other hand, structures built on slabs tend to hold up a bit better compared to other types of construction when it comes to lateral movements caused by earthquakes. This is primarily because the slab on ground is typically stronger due to steel reinforcement and monolithic construction.
Steel reinforcement within the concrete slab provides additional strength and stability during seismic events. These reinforcements help distribute the forces exerted by the earthquake, reducing the risk of cracking or failure. Additionally, the monolithic construction of the slab creates a more unified and cohesive structure, enhancing its resistance to lateral movements.
Furthermore, houses with slab-on-ground foundations often incorporate other design features that contribute to their resilience during earthquakes. For example, including shear walls in the construction helps absorb and dissipate seismic energy, reducing the impact on the structure. Shear walls are typically made of reinforced concrete or steel and are strategically placed within the building to provide additional strength and stability.
Another critical factor in the resistance of slab-on-ground foundations to lateral movements is the continuous contact between the timber bottom plate and the concrete foundation. This connection helps transfer loads and forces more effectively, minimising the risk of sliding or tipping during an earthquake.
Conclusion
Earthquakes pose unique challenges to building foundations, with lateral movements being a critical concern. The impact varies significantly depending on the footing type, with strip and stump systems showing greater vulnerability compared to slab-on-ground foundations. Understanding these differences is crucial for engineers, builders, and homeowners, especially in seismically active areas. By recognizing how various footing types respond to lateral forces, we can make informed decisions about foundation design and retrofitting, ultimately enhancing the resilience of structures against earthquake damage. As our knowledge of seismic effects on foundations continues to evolve, so too must our approaches to creating safer, more stable buildings in earthquake-prone regions.
