Journal of Engineering Geology, Vol. 10, No. 4, Winter 2017

Effect of Pile Foundation on Natural Frequency of Soil Layer

Shirgir V., Ghanbari A.;

Department of Civil Engineering, Kharazmi University,

Tehran, Iran

Mohammad Amiri A., Derakhshandi A.;

Faculty of Engineering, Islamic Azad University, Science and

Research Branch, Tehran, Iran

Received: 26 Aug 2013 Revised: 30 Dec 2014

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Downloaded from jeg.khu.ac.ir at 11:32 IRST on Saturday October 28th 2017 [ DOI: 10.18869/acadpub.jeg.10.4.3839 ]

Downloaded from jeg.khu.ac.ir at 11:32 IRST on Saturday October 28th 2017 [ DOI: 10.18869/acadpub.jeg.10.4.3839 ]

Abstract

Free vibration of soil often occurs during earthquakes. Since the vibration caused by earthquake does not have (steady state harmonic vibration) continuity, the alluvium vibrates with its natural frequency between two natural seismic waves. This study evaluates the effect of piles on the period of free vibration of a soil layer using numerical method. In the first stage, using analytical equations for calculation of vibration period of a soil layer and a column with continuous mass, the results were analyzed by the software. In the second step, piles with the same dimensions and distance were added step by step, and the vibration period for the soil layer with piles was calculated. The friction or floating effects of the piles on alluvial soil vibration period was also examined. The results show that as the number of piles increases, the differences between the results of one dimensional analysis of alluvium soil and the results of the software become different, and this creates the need for specific arrangements for seismic analysis of this kind of alluvium (with inserted piles). The results also suggest that end-bearing piles have a greater effect on alluvial soil vibration period, and with increased amount of the floating of these piles, these effects decline.

Keywords: free vibration period, piles in alluvium, end-bearing and friction piles, numerical method

Introduction

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Downloaded from jeg.khu.ac.ir at 11:32 IRST on Saturday October 28th 2017 [ DOI: 10.18869/acadpub.jeg.10.4.3839 ]

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Studies show that soft sedimentary deposits reinforce geological movements resulting from the earthquake whose intensification happens in case of the synchronization of the earthquake period with the period of the structure, leading to damages [1]. So, the more accurate and realistic is the forecast of the period of soil alluvium, the safer the design of a structure against seismic loads. Piles are used as foundations of the structures built in a soft soil to improve soil properties and transfer structural loads to lower more stable layers. Increased strength of high structures sensitive to side loading in the foundations is another characteristic of such foundations. Interaction issues are generally considered in phenomena in which two objects with different hardness are next to one another. This issue is shown in pile foundations with regard to the difference of the material of the pile construction and the surrounding soil [1]. As a result of this phenomenon and different responses of objects, distribution of forces and anchors is different in soil and the construction. Displacement and vibrations resulting from seismic loads are not spared either. Studies show that the behavior of soil reinforced by group piles depends on the parameters such as the relative hardness of the soil to pile, the number of group piles, the ratio between the length of the pile and the earthquake wave length, and the distance between the piles. When reinforcing the weak soil (small relative hardness, few number of the piles in the group, and relatively small wavelength), the piles have no effect on the behavior of the soil, and in this mode, soil movement in the field of freedom can be used for the dynamic analysis of the pile [2]. This article set to examine the effect of piles on the period of free vibration of a soil layer using numerical method. This is done step by step by adding piles with the same distance and dimension and measuring the vibration period of the soil layer and piles. The endbearing or floating effects of piles on vibration period of alluvial soil are also discussed.

Literature review

Kamatchi et al. (2010) examined the effects of the depth of alluvium soil layers on the performance of buildings. The researchers studied some important parameters of earthquakes on constructions, suggesting that, considering the changes in the depth of the soil layer where the constructions is located, a major change happens in the base shear constructions and location change. Similarly, with changes in the depth of alluvium layer on which the construction is located, the construction response can vary in terms of construction performance (elasto plastic, etc.). The studies clearly show that due to the design spectrum proposed by seismic regulations that only observe the upper layer of soil (upper 30 meters), as being effected by soil reinforcement, the seismic performance of a building may not be predicted confidently. This reflects the fact that further investigations must consider the effects of the site type and the depth of the soil layer that have a significant effect on the building performance [3].

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Rovithis et al. (2011) performed a one-dimensional and analytical study on the response of a non-homogeneous soil layer. The researchers studied the shear wave propagation in heterogeneous environments and evaluated the results by changing the amount of heterogeneity. The studies show that for soils with low shear modulus on the ground surface, increased heterogeneity factor leads to the transfer of lower frequency mode vibrations and thus increases the seismic response. This may have significant practical implications for input movements of low frequency content [4]. Stamatopoulos (2014) studied the effects of non-permanent loads on seismic response of alluvial soil, indicating that soil preload reduces the period of the free vibration of the alluvial soil. These effects depend on the depth of the bedrock and alluvial soil type [5]. Sumio Sawada (2004) presented a semi-analytical equation to calculate the period of the free vibration of laminated sediments. Alluvial soil acts as a filter for seismic waves transmitted from the bedrock to the surface. Due to the cleaning effect, the frequency and amplitude of seismic wave and also the motor characteristics of the earth response are affected. Hence, investigating the effect of this on the seismic response of constructions and the alluvium is important [6].

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Chenari et al. (2012) studied the effect of shear modulus heterogeneity on the behavior of the response of natural alluvial deposits, suggesting that random heterogeneity has significant effects on the dynamic properties of natural alluvial material, and ignoring it would lead to overestimation of the fundamental frequency of alluvial deposits [7]. Ruiz and Saragoni (2011) investigated the free vibration of an alluvial soil during great earthquakes. They showed that during great earthquakes, the alluvial soil experiences its natural vibration many times through frequency [8]. Hadjian (2002) proposed an equation to calculate the free vibration frequency and vibration mode of multi-layer soil [9]. Christos Vrettos (2013) analytically examined the effects of hardness of different layers of alluvial soil on free vibration frequency, considering the nonlinear influences of alluvial materials. Some studies developed a transform matrix, and by comparing its results with finite element software reported satisfactory results [10]. By increasing the number of piles in a pile group, the period quotient of the system decreases. In other words, an increase in the number of the piles in a pile group increases the stiffness of the system whereas its period decreases [11]. Findings suggest that elements (like piles) that cause changes in the material and soil texture change the vibration characteristics of alluvial soil. The effect of piles on the vibration characteristics of an alluvial layer has received less attention, requiring more research to precisely examine its dimensions.

Verification of the results of finite element software

In the first step of validation, period of the free vibration of an alluvium with approximate dimensions of 120m30m was considered. The modeling of the alluvium in the software is twodimensional, and the results are compared with the values of analytic equation (1). The alluvial material properties are presented in Table 1. Figure 1 shows a schematic representation of the modeled alluvium.

Table 1 shows characteristics of the studied materials for this part.

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Equation 1 offers the value of the fundamental period of a homogeneous alluvium on a rigid bed rock using a one-dimensional analysis [1]:

4HS

T

(1) VS

Where: H S : depth of alluvium (m), VS : velocity of shear wave passing of alluvium material (m/sec).

B

s

H

s

S

S

V

Soil Stratum

B

ed Rock

B

s

قیمت: تومان