Liquefaction can have various effects on pile foundations, which are prone to experience this phenomenon. Loose saturated soils like loose sandy silts and sandy soils can undergo significant loss of strength due to liquefaction. Tests such as the standard penetration test and cone penetration test can determine whether a particular soil is susceptible to liquefaction.
Liquefaction is a phenomenon that occurs when loose saturated soil loses its strength. Pile foundations are particularly vulnerable to this phenomenon. Loose sandy silts and sandy soils are the most susceptible types of soil. Tests like the standard penetration test and cone penetration test can help identify whether a soil is at risk of liquefaction.
The effects of liquefaction on pile foundations can be significant. As loose saturated soil loses strength, the stability of the foundation can be compromised. Therefore, it is crucial to determine whether the soil is susceptible to liquefaction before building pile foundations. The standard penetration test and cone penetration test are useful in predicting whether a particular soil will experience liquefaction or not.
Fig.1: Effect of Soil Liquefaction on Pile Foundation
Fig.2: Liquefied Soil Around Bridge Piers
Effect of Liquefaction on Pile Foundation
Liquefaction can have significant impacts on pile foundations, resulting in two major effects: buckling of piles and lateral spreading of sloping ground.
Buckling of piles is a common phenomenon in liquefiable soils where piles can deform and buckle due to lateral spreading or ground settlement. This can be particularly problematic for long and slender piles, as they are more susceptible to buckling under the load of the structure. Buckling of piles can lead to serious damage to the foundation, compromising the overall stability of the structure.
Lateral spreading of sloping ground is another effect of liquefaction on pile foundations. This occurs when the soil loses its strength and stiffness, causing the ground to move laterally. This can be especially dangerous for structures built on sloping ground or in areas with a high water table. Lateral spreading can result in the displacement of the entire foundation system, leading to foundation failure or collapse of the structure.
In summary, liquefaction can have significant effects on pile foundations, including buckling of piles and lateral spreading of sloping ground. It is important to consider these factors when designing and constructing foundations in areas susceptible to liquefaction. Adequate measures such as ground improvement, soil reinforcement, and proper foundation design can be taken to mitigate the risks associated with liquefaction.
Buckling of Piles in Liquefiable Soil
Pile foundations are often used to support large structures because they provide excellent support when the soil is firm. However, if the soil is soft and the load is heavy, the pile can buckle under the weight. Buckling is not a common problem for pile foundations, but it can happen under certain conditions.
Pile foundations carry loads through skin friction and base capacity. Skin friction is generated by horizontal stress around the surface of the pile, which acts like a strut and provides lateral support. However, when the soil liquefies due to an earthquake, the lateral support is lost, and skin friction can no longer occur. According to Eurocode-8 recommendations, the strength of liquefied soil should be ignored. Therefore, if a pile foundation is supporting a large axial load and loses its lateral support due to soil liquefaction, buckling is highly probable.
Another scenario in which pile foundations can buckle is when the pile toe is fixed in bedrock, and the axial load it supports is much larger than the Euler buckling load of an equivalent column. The Euler buckling load (PE) is calculated using a specific formula.
The pile’s ability to resist buckling is dependent on its flexural rigidity and equivalent length, which is based on the pile’s end conditions. If the design load exceeds five times the Euler load, the pile is considered vulnerable to buckling. The Euler equation can be applied to the pile if it is free of imperfections and the load acts solely on the center of the pile. If the pile does not meet these conditions, the computed buckling load will be significantly reduced, necessitating a larger factor between the design load and the Euler buckling load.
The slenderness ratio, which is determined by dividing the equivalent length by the radius of gyration, is another indicator of the pile’s potential for buckling. If the soil surrounding the pile is expected to liquefy, the slenderness ratio can be used to determine the possibility of buckling. If the slenderness ratio exceeds 50, the pile is highly likely to buckle, whereas if it is less than 50, the pile is assumed to be safe.
Fig.3: Buckling of Pile Foundation due to Soil Liquefaction because of Earthquake Shaking
Lateral Spreading of Sloping Ground
Lateral spreading refers to the downward movement of soil after it has liquefied due to earthquake shaking. This movement generates considerable lateral force due to the passive pressure created by the soil wedge at the upper side of the slope. Piles are commonly used as a means of stabilization at the downstream and upstream of earth dams. However, these piles need to withstand significant lateral forces generated by soil passive pressure, particularly when the entire slope experiences lateral spreading. Slopes with as little as 3o are likely to experience lateral spreading after soil liquefaction, as liquefied soil lacks sufficient shear resistance. The lateral spreading can be several meters wide, generating significant passive earth pressure. The lateral spreading situation is exacerbated when a non-liquefied soil layer is present above the liquefied layer, causing it to retain excess pore water pressure for a longer period. The movement of the non-liquefied soil layer laterally across the liquefied layer increases the lateral load exerted on the pile. The non-liquefied soil layer may control the total imposed loads, with the lateral loads due to liquefied soil being comparatively small.