Pile foundations are likely to experience liquefaction and its effects on such foundation are in several forms or ways. Effects of liquefaction on pile foundation is discussed.
Liquefaction is a phenomenon in which loose saturated soil such as loose sandy silts and loose sandy soils will strength substantially. There are tests such as standard penetration test and cone penetration test to specify whether a certain soil is expected to suffer from 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
Following are the effects of liquefaction on pile foundations which are discussed in details:
- Buckling of piles in liquefiable soil
- Lateral spreading of sloping ground
Buckling of Piles in Liquefiable Soil
Piles are considerably long element and the soil around it offers an excellent support. However, if the soil is soft and the supported load is large, then it is likely that the pile would buckle.
It should be known that buckling of pile foundation is not common and the aforementioned conditions should be meet in order for the pile to suffer buckling.
Pile foundation usually carries loads through skin friction and base capacity. The former is created by horizontal stress around the pile surface area. The horizontal stress would act similar to strut with smaller spacing and hence a lateral support is generated.
When the soil around the pile foundation liquefied due to earthquake affects, then the loss of lateral support is highly probable and consequently skin friction would no longer occur. Based on the recommendation of Eurocode-8, the strength of liquefied soil should be ignored.
So, buckling of pile foundation is very likely to occur when it supports large axial loads and loss its lateral support due to soil liquefaction.
Pile foundation is anticipated to buckle if pile toe is fixed in a bedrock and the axial load supported by the pile is substantially large in comparison with Euler buckling load of an equivalent column.
The Euler buckling load (PE) is computed according to the following expression:
EI: pile flexural rigidity
Le: pile equivalent length based on pile end conditions
The pile is assumed to be vulnerable to buckling unless the computed design load is smaller than the Euler load by a factor of five.
The Euler equation provided above may be utilized on the condition that the pile does not have imperfections and the load acts completely on the pile center.
If such condition does not meet, the computed bucking load will considerably decrease. That is why large factor is provided between design load of the pile and Euler buckling load.
Slenderness ratio, which is equal to equivalent length divided by radius of gyration, is another indicator by which potential buckling of the pile can be determined.
If the soil around the pile is expected to liquefy, then slenderness ratio can be used to check the possibility of bucking. If slenderness ratio is greater than 50, then pile bucking is highly possible whereas the pile is assumed to be safe in the case where slenderness ratio is less than 50.
Fig.3: Buckling of Pile Foundation due to Soil Liquefaction because of Earthquake Shaking
Lateral Spreading of Sloping Ground
The movement of soil in the downward direction after the soil has been liquefied due to earthquake shaking is termed as lateral spreading. This lateral spreading creates considerable lateral force because of passive pressure created by the soil wedge at the upper side of the slope.
Piles are used as a stabilization mean at the downstream and upstream of earth dams. These piles commonly need to withstand substantial lateral forces generated by soil passive pressure while the entire slope suffer lateral spreading.
It is reported that, slopes with as minimum as 3o are highly possible to experience lateral spreading after soil liquefaction. This is because liquefied soil does not have adequate shear resistance.
It should be known that lateral spreading is usually in the order of several meters which is adequate to generate great passive earth pressure. The lateral spreading situation would be worse when non-liquefied soil layer is available above the liquefied layer, especially when such layer causes the liquefied layer to retain excess pore water pressure for longer time.
When the non-liquefied soil layer on the liquefied layer move laterally, the lateral load exerted on the pile would be increased. The non-liquefied soil layer may control the total imposed loads and the lateral loads due to liquefied soil is small.