Evaluation of the Effects of Liquefaction on Concrete Pile

ANSYS is finite element software. In the present work finite element model is employed herein to determine the displacements in a concrete pile under liquefiable and nonliquefiable soils. In the present study parameters used are the variation in stiffness, different soil layers and residual strength of the liquefied soil. The different soil layers used are dense sand and loose sand. The study shows that the total displacement increasing with respect to depth of the concrete pile. The displacement of concrete pile is more for which is having liquefiable soil compared to nonliquefiable soil because when liquefaction takes place soil loses its strength and stiffness therefore such piles will have more displacement. KeywordsPile, Liquefaction, Stiffness, Displacements,


INTRODUCTION
Pile foundations are the most popular form of deep foundations and pile foundations are the part of the structure used for both onshore and offshore structures. They are often used to transfer large loads from the superstructure into deeper, competent soil layers particularly when the structure is to be located on shallow, weak soil layers. Piles are long and slender members which transfer the load to deeper soil or rock of high bearing capacity avoiding shallow soil of low bearing capacity the main types of materials used for piles are wood, steel, concrete. Piles made from these materials are driven, drilled or jacked into the ground and connected to pile caps. Under liquefiable and nonliquefiable soils using the advanced finite element software package ANSYS.

Pile Behavior under Earthquake Loading:
The loading requirements imposed by seismic events on piles require different geotechnical and structural design of these elements compared with the static design of pile under static loading. Let us consider the case of pile foundations in level ground comprising a soft, horizontal soil layer in which no soil liquefaction occurs, overlying a stiff, horizontal soil layer. The pile foundation passes through the soft layer and rests on the stiff soil layer. The earthquake motion can be transmitted from the stiff soil stratum into the softer layer and this motion can be amplified as it propagates through the softer layer, is transferred to the pile and onto the superstructure. This sets up structural vibration in the superstructure. As these vibrations are being set up in the superstructure, it will impose inertial loading on the pile cap. This inertial load has to be carried by the piles. If the inertial load is large, then piles can suffer significant lateral displacements. Further, depending on the stiffness of the superstructure and the pile cap's bearing capacity, the pile cap can either be prevented from suffering any rotation or some rotation can occur. If the pile cap rotations are prevented then the piles will undergo elastic bending and in extreme cases plastic hinges can form at the pile/pile cap. On the other hand, if the pile cap can undergo some rotation, then the piles will suffer elastic bending but the pattern of bending is opposition to the previous case. These types of behavior will be dominated by the stiffness of the shallow soil layer as the lateral inertial load on the piles will only displace the soil in this region. If the shallow layers in this region are soft, this can be accomplished easily. On the other hand, if the shallow layer below the pile cap comprises stiffer/competent soil then the lateral displacement of piles and the rotation of the pile cap will be smaller, but the piles may attract large lateral loads that oppose the inertial loads due to mobilization of the passive soil pressures in this region.
Pile foundations are primarily designed to transfer vertical loads from the superstructure to the bearing stratum. For this reason, piles are relatively vulnerable to lateral loads such as those imposed by ground shaking during strong earthquakes. In the case of soil liquefaction, this vulnerability is particularly pronounced since the loss of strength and stiffness in the liquefied soil results in a near complete loss of lateral support for the embedded piles. It is known from previous earthquakes that liquefaction can cause very large loads on pile foundations, both from inertial loads from the superstructure and from lateral displacements of liquefied soil. The extensive damage and failure of piles have affected numerous bridges, buildings and storage tanks in the past.

NUMERICAL STUDY
The finite element analysis software ANSYS, is used in this study. The pile is modeled using a 20-node brick element which is solid 186 elements. A fixed support condition is provided at the bottom of pile. For the concrete pile, the young's modulus is 27386 MPa and Poisson's ratio is 0.3.

Verification and Validation
Consider pile foundations are to be designed for a bridge pier. The axial load on the pile cap from the pier is expected to be 7580 kN. Considering 1m diameter of pile and 16 m length, 8m thick loose sand layer and 8m thick dense sand layer. The water table is expected to be at the ground level. The saturated unit weight of the loose sand layer is 17 kN/m 3 and that of the dense sand is 19 kN/m 3 . The critical state friction angle for both these soils is initially taken to be 32°. The design earthquake is an L1 event, having a magnitude M = 6 generating a peak ground acceleration of 0.2g.
Step Where, Ab = Pile base area σb = Effective overburden pressure Nqbearing capacity factor that can be estimated using Therefore the base resistance at the pile tip is Qb = 0.7854 x 128 x (40-1) Qb =3920.72 KN Shaft friction of the pile The shaft capacity may be obtained by estimating the shear stress generated along the shaft which can be calculated as τ =K σ'tanδcv where Ksis a earth pressure coefficient σ'is effective vertical stress at a given elevation δcvfriction angle between the pile material and the soil. Ks depend on type of pile and installation method (driven or cast in situ piles). Broms (1966) related the values of Ks and δcv to the angle of shearing resistance of the soil φ as shown in Table 5.4. In order to obtain the shaft capacity due to skin friction, the shear stress must be integrated over the surface area of the pile using the following equation, Where r is the pile radius L is the length of the pile. Table 2. Values of earth pressure coefficient Ks and pile-soil friction angle δ [6] . The stiffness of each pile in the pile group is determined by considering eurocode 8 provisions [7].   Step 3: Reviewing the Results (post processing): The results of the static analysis can be seen in post processing phase of ANSYS.

Step 4: Verification and validation
It is imperative to do some checks to make sure that the numerical results are accurate. Numerical results can either be compared directly to empirical data or they may be compared to theory.
Verification of Maximum Total Displacement The ANSYS results will be verified by comparing them to the results obtained using Winkler spring approach in the preanalysis. The ANSYS simulation outputted 0.035817m for the total displacement of the beam at x =16m for nonliquefiable soil while the calculation from the pre-analysis yielded 0.03528m. The ANSYS simulation outputted 0.13802m for the total displacement of the beam at x =16m for 4m dense sand as liquefiable soil while the calculation from the pre analysis yielded 0.12319m. The ANSYS results very closely match the calculations from the pre-analysis, thus the simulation has been verified. and studied the effect of liquefaction on piles. In the present study, the analysis is done for M30 grade of concrete pile having 1m diameter and 16m length using ANSYS software. The parameters considered are different soil layers such as dense sand, loose sand and effect of liquefaction.

Effects of non liquefiable soil layers
In the analysis, finite element 3D models are created for single layer and two layers of soil. Single layer consist of 16m length of dense sand and two layers consist of 8m dense sand at the bottom and 8m loose sand at the top as shown in fig 3. 3D finite element analysis is performed, after applying the loading conditions to the final model and stiffness at each 2m depth around the pile. The result shows that the total displacement increases with respect to depth of pile from bottom to top of the concrete pile. The maximum displacement is at the ground level and it is zero at the 16m length because it is assumed as fixed at the bottom. The curves shows in fig 4 analytical and ANSYS results, the ANSYS results are near to the analytical results and zero at the bottom. The result of these analyses shows a significant difference in the displacements due to different soil layers and stiffness. Fig 5 shows displacements for two layers nonliqefiable soil, the analytical results are compared with ANSYS results, the results are almost same but the maximum displacement of analytical result is 21% less than ANSYS results.

Effects of liquefiable soil layers
In the analysis, finite element 3D models are created considering different soil layers as liquefiable soil. 3D finite element analysis is performed, after applying the loading conditions to the final model and stiffness at each 2m depth around the pile. Figure 6. Shows the displacement of the pile having single layer of soil that is dense sand 8m length from the bottom of pile to ground is considered as liquefiable soil.

Discussion
To see the effect of liquefiable soil layers, the total displacement of the pile versus depth of the pile is presented in figures 7. The curves indicate that the total displacement increases with respect to depth of pile from bottom towards top of the concrete pile. The analytical results are compared with ANSYS results, the results are almost same but the maximum displacement of analytical result is 30% less than ANSYS results. The analytical curve is linearly decreasing up to 8m depth after that increasing slightly and zero at the bottom because of fixity. The displacement of concrete pile is more for which is having liquefiable soil compared to nonliquefiable soil because when liquefaction takes place soil loses its strength and stiffness therefore such piles will have more displacement.

Comparisons of model results for liquefiable and nonliquefiable soil layers
The Type of concrete piles and its descriptions are given in table 5.  1. CONCLUSIONS After compiling and analyzing the results from each analysis, the following conclusions can be made. • The total displacements are increasing with respect to the depth of pile from bottom towards top of concrete pile in both liquefiable and nonliquefiable soil layers. • The concrete piles in nonliquefiable soil layers, the analytical results are less than model results and as the depth increases the values are almost same. • The concrete piles in liquefiable soil layers, as the depth increases from top to bottom of pile the displacement of pile decreases.
• By comparing the results of concrete pile under liquefiable & nonliquefiable soil layers it can be seen that liquefaction of the upper soil layer, reduces the lateral stiffness of the foundation substantially, lengthens the natural period of the foundation, reduces acceleration at the top of the foundation, and increases the lateral response of the foundation substantially.
• Comparing the liquefiable soil models A, B, C, D and E with Control model H1, the displacement in other models are having 75% to 90% more than the control model displacement.
• The structural load and the surrounding soil stiffness does have a significant impact on the overall behavior of the pile under the liquefiable & nonliquefiable soil layers and must be considered when designing a pile under similar conditions.