**1. Introduction (Characterisation of liquefaction behaviour)**

The liquefaction of loose, saturated sands, particularly cohesionless soils is caused by earthquake shaking or cyclic (monotonically increasing) undrained loading. The early work in liquefaction soil in the laboratory apparently emerged from the experience of the Fukui earthquake in 1948 in Japan [1]. It was regarded as a milestone from researchers since its devastating failures were prevalent following the major earthquakes in Niigata, Japan and Alaska, USA, in 1964 [2–4].

Soil liquefaction has been responsible for extremely damaged structures and foundation piles of bridges and buildings and has resulted in severe loss of strength and stiffness of saturated cohesionless soil. Liquefaction was first introduced by Hazen [5] that he used to describe the 1918 collapse of Calaveras Dam in California [6]. Typically, liquefaction occurs when a deposit of loose saturated sand layers are subject to shaking during a seismic event, the progressive build-up of excess pore water pressure and the stiffness of the liquefied soil drops to a value of near-zero. The reduction in strength and stiffness of liquefied soil often leads to permanent deformation in sloping grounds, commonly termed as lateral spreading (a few centimetres or metres) or flow failure (hundreds of meters). Flow failures have been observed in a number of hydraulically filled earth dams, constructed tailings dams and in coastal and/or offshore areas [6]. This hydraulic problem was observed as secondary and progressive liquefaction surrounding the majority of slides as a result of the generation of excess pore pressure and the upward flow of water almost immediately prior to the initiation of the slide flow [6]. This also may lead to formation of sand boils, which have been illustrated by Ishihara [7] in terms of the relative thicknesses of liquefiable and overlying non-liquefiable layers in case history data from the 1976 Tangshan and 1983 Nihonkai earthquakes. In this respect, Huang and Yu [8] also classified the liquefaction related damage to soils and foundations during earthquakes in the first part of the twenty-first century in: ground subsidence, lateral spread, and damage induced by buoyancy (uplift).

Laboratory studies carried out to investigate the liquefaction susceptibility and conventionally evaluate the undrained behaviour of sandy soils under monotonic shearing. This tendency is generally expressed in terms of void ratio (e) and relative density (Dr). In this state, sand is flowing under constant shear stress at constant effective minor principal stress and at constant volume [6, 9–11]. Poulos [11] included the requirement of constant velocity, the "steady-state of deformation," and the relationship between the steady-state effective stress and the void ratio i.e. the "steady-state" line. The response for very loose sand shows fully contractive behaviour is reached at large strains, as delineated in **Figure 1**. This behaviour reported as "spontaneous liquefaction" and also known as flow. In in the case of sand with slightly higher density, the strain softening is followed by the strain hardening and the sand recovers its strength and restores stability. This type of behaviour was first called "limited liquefaction" by Castro [9] and known as limited flow.

In medium-dense and dense sands exhibiting dilative behaviour, ever-increasing shear stress is needed to induce shear strain and eventually obtain the steady state

**Figure 1.** *Monotonic behaviour of different sands: (a) effective stress path; (b) shear stress–shear strain relation [12].*

**183**

τ σ

**Figure 2.**

*0*

*´ /* <sup>d</sup> *: Shear stress ratio).*

*The Dynamic Behaviour of Pile Foundations in Seismically Liquefiable Soils: Failure…*

of deformation (no flow) [12]. Kramer et al. [6] indicated that at relative densities greater than those corresponding to the steady state line (slightly greater than 44%), the soil exhibit dilative behaviour, with no potential for liquefaction.

**Figure 2a** and **b** illustrate a typical cyclic loading test on loose sand (2a) and dense sand (2b) using a torsional shear apparatus. The upper graphs describe the time history of cyclic shear stress ratio applied to constant-amplitude cyclic loading, while the lower graphs show the development of shear strain and the generation of excess pore pressure with time. In the early stages of loading the effective stress path moves to the left and the shear strain is negligible. However, as the loading progresses, the pore pressure builds up until the stress path begins to cross the phase transformation line identified by Ishihara [7] and eventually reaches a value equal to the initial confining pressure, which is called cyclic mobility. It can be seen that in both sand the effective confining stress decreases, but the shear strain increases in a

The liquefaction potential of the soil is generally estimated by comparing the anticipated earthquake loading and its inherent liquefaction resistance. This comparison is most commonly base on cyclic shear stress amplitude usually normalised by initial vertical effective stress and known as a cyclic stress ratio (CSR) for loading and a cyclic resistance ratio (CRR) for resistance. The potential for liquefaction is expressed in terms of a factor of safety against liquefaction, FL = CRR/CSR. If the FL > 1.0 soil profile can be safe against liquefaction. Standard penetration test (SPT) and cone penetration test (CPT) are the two empirical methods that use to obtain the cyclic resistance to liquefaction. The susceptibility of soil deposits to liquefaction is determined by a combination of various factors such as soil properties, geological conditions and ground motion characteristics. The soil's CRR is also affected by the duration of shaking (which is correlated to the earthquake magnitude scaling factor, MSF) and effective overburden stress (which is expressed through a Kσ factor). The evaluating of the soil liquefaction potential based on the

The seismic response of a soil profile is strongly influenced by the effective stress of an earthquake ground motions. The nature of ground motions at sites containing potentially liquefiable soils can affect the potential damage to pile foundations. The

*Stress path behaviour and stress–strain curve of (2a) loose sand and (2b) dense sand from the cyclic torsion* 

γ

*: Shear strain,* 

*shear test (Ishihara, [7]) (Dr: Relative density of the soil, K0: Coefficient of earth pressure,*

*DOI: http://dx.doi.org/10.5772/intechopen.94936*

slower manner for dense sand.

SPT and CPT values are well explained in [12].

### *The Dynamic Behaviour of Pile Foundations in Seismically Liquefiable Soils: Failure… DOI: http://dx.doi.org/10.5772/intechopen.94936*

of deformation (no flow) [12]. Kramer et al. [6] indicated that at relative densities greater than those corresponding to the steady state line (slightly greater than 44%), the soil exhibit dilative behaviour, with no potential for liquefaction.

**Figure 2a** and **b** illustrate a typical cyclic loading test on loose sand (2a) and dense sand (2b) using a torsional shear apparatus. The upper graphs describe the time history of cyclic shear stress ratio applied to constant-amplitude cyclic loading, while the lower graphs show the development of shear strain and the generation of excess pore pressure with time. In the early stages of loading the effective stress path moves to the left and the shear strain is negligible. However, as the loading progresses, the pore pressure builds up until the stress path begins to cross the phase transformation line identified by Ishihara [7] and eventually reaches a value equal to the initial confining pressure, which is called cyclic mobility. It can be seen that in both sand the effective confining stress decreases, but the shear strain increases in a slower manner for dense sand.

The liquefaction potential of the soil is generally estimated by comparing the anticipated earthquake loading and its inherent liquefaction resistance. This comparison is most commonly base on cyclic shear stress amplitude usually normalised by initial vertical effective stress and known as a cyclic stress ratio (CSR) for loading and a cyclic resistance ratio (CRR) for resistance. The potential for liquefaction is expressed in terms of a factor of safety against liquefaction, FL = CRR/CSR. If the FL > 1.0 soil profile can be safe against liquefaction. Standard penetration test (SPT) and cone penetration test (CPT) are the two empirical methods that use to obtain the cyclic resistance to liquefaction. The susceptibility of soil deposits to liquefaction is determined by a combination of various factors such as soil properties, geological conditions and ground motion characteristics. The soil's CRR is also affected by the duration of shaking (which is correlated to the earthquake magnitude scaling factor, MSF) and effective overburden stress (which is expressed through a Kσ factor). The evaluating of the soil liquefaction potential based on the SPT and CPT values are well explained in [12].

The seismic response of a soil profile is strongly influenced by the effective stress of an earthquake ground motions. The nature of ground motions at sites containing potentially liquefiable soils can affect the potential damage to pile foundations. The

#### **Figure 2.**

*Stress path behaviour and stress–strain curve of (2a) loose sand and (2b) dense sand from the cyclic torsion shear test (Ishihara, [7]) (Dr: Relative density of the soil, K0: Coefficient of earth pressure,* γ *: Shear strain, 0* τ σ*´ /* <sup>d</sup> *: Shear stress ratio).*

relevant characteristics of ground motions are the frequency content, amplitude, and duration, and they can provide insight into the effects of ground motion duration on liquefaction hazards. The initial characteristic site period for a simple layer (ground of thickness H) with constant initial shear wave velocity, Vso, is given by Tso = 4 H/ Vso. The velocity is related to frequency f and wavelength λ by *v= f*λ . The relation between vs and the SPT N value indicates the soil type [12]. The effects of liquefaction and generation positive of pore pressure leads to decrease in the effective stresses and the shear modulus of the soil. As a result, there is a reduction in soil stiffness which, in turn, increases prevalence of low frequency motions. The accelerations recorded of the Wildlife liquefaction array in the 1987 Superstition Hills earthquake (NS component) at the ground surface and at 7.5 m depth and the associated excess pore water pressure measured at 2.9 m depth are shown in **Figure 3**.

In each case, a clear and gradual stiffness degradation associated with an increase in the pore water pressure can be observed. Acceleration time history of the surface record showing clear visual evidence of the high-frequency portion of the motion from the beginning of the record (about 18 s); which is consistent with a series of isolated high-frequency pulses of acceleration (see numbers) [12]. Kramer et al. [14] reported that these pulses have amplitudes smaller than those of the pulses that occur prior to liquefaction, but in some cases, the peak ground acceleration of the entire motion is produced by a strong dilation pulse occurring near, or even after, the initiation of liquefaction. Hall et al. [15] examined the transient vibration characteristics of two 2 × 2 pile-group models based on the wavelet. They found that liquefaction causes a decrease in structural frequency, whose reduction depends on the rate of excess pore pressure build-up, whereby high rates ("fast liquefaction") lead to greater reduction, i.e. up to 51%.

#### **Figure 3.**

*Acceleration time histories time histories and associated pore water pressure of north–south components at wildlife site during the 1987 Superstition Hills earthquake (Zeghal and Elgamal, [13]) (numbers show high-frequency pulses of acceleration).*

**185**

**Figure 4.**

*The Dynamic Behaviour of Pile Foundations in Seismically Liquefiable Soils: Failure…*

Ozener et al. [16] used of Stockwell spectrograms and indicated that as a result of the changes in stiffness; the response of a liquefied soil is often markedly differ-

( ) <sup>20</sup>

Where z is the depth (0–20 m) and w(z) = weighting function (10–0.5z) and F is a function of the liquefaction resistance. Sites with LPI > 15 have increasing susceptibility to liquefaction and potential for severe damage. The risk of liquefaction tends to decrease with depth while if LPI < 5, the effects are minor due to increasing

Piles are a particular type of deep foundation generally constructed to support heavily loaded structures to transfer the loads from superstructures to the deeper layers of soil, relying on both skin friction and tip resistance [19]. Piles are also used in seismic-prone zones comprising loose to medium-dense sandy soil or soft clay. However, during earthquake shaking when the soil around the pile loses much of its stiffness and strength due to liquefaction, the pile will act like a long laterally unsupported column and could buckle under the high axial load from the superstructure, affecting the foundations. Collapse and damage of pile-supported structures due to liquefaction have been observed after many major earthquakes [2–4]. The observations from many historic cases indicated that the failure of foundations

During earthquakes, the response of pile-supported structures to liquefiable soils would depend of the stiffness of the pile foundation type, the response of the soil surrounding the pile, and the soil-pile interaction effects. The analysis of this response requires accurate characterisation of the interaction effects include the inertial loading exerted by the superstructure and the kinematic loading induced by the soil surrounding the pile. **Figure 5** illustrates four critical stages of loading on

*LPI F w z dz* <sup>=</sup> . ∫ (1)

The liquefaction potential index (LPI) of Iwasaki et al. [17] is an integral number of factors of safety (FS) values weighted by depths of soil layers, which has increasingly been used for assessing the severity of liquefaction hazards [17, 18].

0

*DOI: http://dx.doi.org/10.5772/intechopen.94936*

This is expressed by Eq. (1):

effective stress.

**2. Pile failure**

ent before and after triggering of liquefaction.

occurred at unexpected locations (see **Figure 4**).

the piles during a seismic liquefaction-induced event.

*(a), (b) Buildings in Niigata city and (c) Building in Kobe city [12].*

*The Dynamic Behaviour of Pile Foundations in Seismically Liquefiable Soils: Failure… DOI: http://dx.doi.org/10.5772/intechopen.94936*

Ozener et al. [16] used of Stockwell spectrograms and indicated that as a result of the changes in stiffness; the response of a liquefied soil is often markedly different before and after triggering of liquefaction.

The liquefaction potential index (LPI) of Iwasaki et al. [17] is an integral number of factors of safety (FS) values weighted by depths of soil layers, which has increasingly been used for assessing the severity of liquefaction hazards [17, 18]. This is expressed by Eq. (1):

$$LPI = \bigcap\_{0}^{20} F.w\left(z\right)dz\tag{1}$$

Where z is the depth (0–20 m) and w(z) = weighting function (10–0.5z) and F is a function of the liquefaction resistance. Sites with LPI > 15 have increasing susceptibility to liquefaction and potential for severe damage. The risk of liquefaction tends to decrease with depth while if LPI < 5, the effects are minor due to increasing effective stress.
