OHN10103300491
Numerical Analysis of Inclined Micropiles Behavior Under Seismic Load
Seyyed Mehdi Radeghi Mehrjou
M.Sc of Civil Engineering, Geotechnical Engineering,Azad University of Central Tehran Branch, Tehran, Iran
Email: mehdi_ [email protected]
ABSTRACT
Performing human made that lead to structures damages have occurred due to earthquakes and seismic load.
Studying the existing structures and the methods of retrofitting has an important role in earthquake-prone countries. Nowadays, micropiles are commonly used in structures as a practical element to retrofit the seismic behaviors. In this paper, inclined micropiles under the earthquake load were analyzed by finite difference method using FLAC 3D. The approach considers an idealization for micro pile-soil system in three dimension model in order to evaluate logically their behaviors when the earthquake occurs. The micropile is assumed to be linear elastic and the soil is elastoplastic material. The micropile's failure is controlled by the Mohr- Coulomb criterion and all the micropiles were analyzed on the load of earthquake recorded in Tabas, Iran, 1978. The results obtained from the numerical analyses were compared with available other data, indicating a satisfactory agreement. The results show that the inclined micropiles, due to being in the soil, give more hardness to it. So these cause less displacement in comparison with vertical ones. Also the micropile-soil interactions in performance of inclined one were salient.
Keywords: micropile, inclined micropile, soil improvement, seismic behavior, micropile-soil interaction, finite difference methods.
1.INTRODUCTION
For using the micropiles in seismic retrofitting or seismic zones we need to know about analysis of the seismic- induced response for the micropiles with inclined elements. In fact , because the stiffness and resistance of vertical micropiles to lateral loading is small, using inclined micropiles is the potential alternative for inertial forces and for making sure about the stability the foundations under seismic loading. But using the micropiles in seismic area has limitation which is for designing the piles , because due to several researches, the function of inclined is not suitable. The inclined piles may be have a big energy on piles or if the inclined of caudles is not symmetric , permauent rotation may develop due to varying stiffness of the pile group in each direction.
According to the French recommendation (AFPS) [1] using the inclined piles in seismic areas is forbbidened, but reinforcement of soil can be include inclined elements. The seismic reccomodation (Eurocode EC8) [2].
Indicates that inclined piles shoulden't be use for transmitting lateral loads to the soil, but if these piles are used , they should design bending loading. On the other hand , according to Gazetas and Mylonakis's report [3]
curreutly the different observation is recorded which is shown that inclined piles, in certain cases , has a good function for the structure they support and the piles. One of the observation which support it happens in Kobe earthquake. Which one of the few quay-walls that survived the disaster in Kobe harbor was a composite wall which is relying on inclined piles and the near wall supported on vertical piles was totaly devastated. Moreover, centrifuge tests and pseudo- static analysis which is doue by Juran et al [4] showed that pile inclination cause first the decrease on both the pile cap displacement and bending moment at the pile cap connections and second the increase in axial force on piles.
2.Analysis Method
FLAC [5] is a 3-dimensional explicit finite – difference plan in mechanics compalation engineering for simulating the behavior of 3-dmensional soil structures based on rock and the other undergoing plastic flow
explicit , Lagrangian which is used in FLAC ensure that plastic collapse and flow are modeled properly. Because no matrixes are shaped, big 3-dimensional calculations can be made with no excessinve memory requirements. It suggests the perfect analysis tool for solution of 3-dimentsional matters in geotechnical engineering. The aim of grid generator is to make cays the creation of all required physical shapes in the model. First, grid generator is made with radial cylinder shape to model radially graded mesh through the cyliandrical shaped tunnel and cylindrical mesh to model the pile. One of the noticeable issues in grid generation is that an physical boundaries in model simulation should be difined before that the solution begins.
Ofter the model generation, the boundary situations are assigned. The boundry situations in numerical modeling includes the worth of field variables like displacements at the boundry of the numerial grid. The initial situation reproduce in-situ state of stress in ground before the start of excavation or construction. The information about the intial state comes from field measurement but when they are not available , the model can be run for possible situations.
Although, there are number of constraining factors when the range is potentially infinite. This analysis is supposed to be linear and elastic. The moher-coulomb plasticity model is used for materials when conquered to shear loading but it depends on the major and minor principal stresses and there is no effect on intermediate principal stress, and , mohr –coluomb elements for cohesion and friction angle are available more than geo- engineering materials.
Quiet boundaries are used in the model to got energy at the boundaries. The Lysmer and Kuhlemeyer is boundaries scheme consist of dashpots attached independenty to the normal and shear direction's boundary – viscous normal and shear traction that is directly in to equation of motion of the grid points which lies on the global coordinate direction or inclined bounders in the normal and shear directions.
The resistance of the cohensive soil in pile driving and end bearing is related to the soil undrained shear strength. The skin friction which is distributed the pile shaft is recognized using: x .cu( Where cu is undrained shear strength and is adhesion factor).
3. Numerical Analysis
Model geometry and soil properties and micropile:
3.1. Soils
All models are assumed elastic properties of the soil. Soil parameters used in the modeling are summarized in the table below as well as the dimensions of the model are shown in the following figure.
1700 kg/m3 Density
2.7 mpa Bulk modulus
66.6 mpa Shear modulus
20m 40m
20m
3.2. Micropiles
All micropiles used in the model are 10 cm in length, 10 meters in diameter so that the configuration of the Modeling micropile over 20 meters and 10 meters are the coordinates micropile. Micropile behavioral characteristics and physical properties are summarized in the table below.
21 Gpa Modulus of elasticity
0.3 Poisson's ratio
78.5 cm2 Area
490 cm4 Moment of inertia
130 Gpa Shear stiffness of the contact
130 Gpa Normal stiffness of the
contact
10 Interface friction angle
This paper examines the make up of the two groups will be micropile:
A) Group micropile 2 ×1 B) Group micropile 2 ×2
Also, in each of these two effects being willing micropile groups were studied. Micropile tilt in three states and 70 ˚ 80 ˚ 90˚ and were considered.
Group micropile 2 ×1 with angle 70° [6] Group micropile 2 ×2 with angle 70° [6]
Group micropile 2 ×1 with angle 90° [6] Group micropile 2 ×2 with angle 90° [6]
Also micropile distance in five diameters micropile models is considered. Micropile In addition, as Head of micropile block on micropile group to be able to effect over time on group micropiles considered. Head as broad micropile and soil has been placed on the long term effects on the soil, regardless the freedom of micropile. There are also effects on the structure of matter that is micropile Group is solely confined structures. So a mass of 10 tons on any of micropile is placed on the head.
3.3. Loading
In this paper, the time harmonics of Tabas earthquake is used by Mavroeidis and Papageorgiou[7] . Harmonic Installation model is mentioned in only one direction. Equations and parameters used for the earthquake and the resulting pulse shape is as follows:
Tabas earthquake simulations by the equation for the unknown values are substituted like below:
Earthquake harmonic load
3.4. Output Analysis
In this article we will examine the effect of tilt micropile groups. Because every moment of this micropile various forces occur, therefore, to compare the forces micropile different groups to compare these forces will be discussed might react. In the following chart, covering shear forces, bending moments and axial length micropile 1 × 2 groups are shown. It should be noted that the figures presented below are all the internal forces due to earthquake loading harmonics.
As can be seen in the figure, increasing the amount of tilt in micropile cover increases in shear. It covers the increased positive shear occurs. Push the negative shear micropile with increasing tilt in less than micropile to increase with less intonation. This type is caused because the negative shear load model is created first and eventually moved on to the final and lasting micropile cut is positive.
Time(sec) Acceleration (m/s2)
Comparison of axial force created by the group's desire micropile different angles (figure above), we know that the tilt increases, the axial force in micropile increases. Since the earthquake, there ciprocating motion is upward from the depths of the earth, it would be expected that micropile are placed inclined, the more thrust to receive.
Figure illustrates this idea is well. As can be seen, in piles that is more inclined, axial forces are more in both positive and negative spectrum. These micropile show more spinality against the earthquake forces, so that it controls load generated of quake in the direction of its length and this causes that the relocation of the head of micropile and also the area of the soil in their sides would be less than another cases. The following table below shows the maximum displacement to the maximum displacement at the top of the base model and the model in different scenarios tilt micropile is presented.
The ratio of maximum displacement to the maximum displacement at the top models in the
floor model 1
× 2 Micropile
angle to the horizon
0.44 90°
0.42 80°
0.41 70°
The ratio of maximum displacement to the maximum displacement at the top models in the floor model 2
× 2 Micropile
angle to the horizon
0.43 90°
0.42 80°
0.41 70°
Chart review compared with the bending moment caused by the earthquake in micropile in it, we should remember that the maximum positive bending moment in micropile with greater tilt caused this to happen while the negative bending moment with fewer rates increase the positive moments.Comparison with the internal forces caused by the angle of inclination in micropile different arrangement 2×2 we know that the same results were obtained in micropile the 2 × 1, the new arrangement is achieved. Following the results of the seismic analysis micropile 2 × 2 group shows. As can be seen, the results are quite similar to results of previous groups.
It represents the make up of this group micropile difference in the angle of inclination of the micropile does not, and can be followed by examining the nature of desire in a particular micropile and generalize it.
4. Conclusions
Micropile wanting due to their placement in the soil, make soil more spinality and this causes a shift in the upper levels of the soil (the ratio of vertical micropile) is lower. Also, with increasing tilt, push the shear force and the axial force increases. It should be considered that the maximum positive bending moment generating in micropile with more inclination angle and this occurs in case that the negative bending moment increases with less rate in comparison with the positive one.
5. Refrences
[1] AFPS. Association franc¸aise de Ge´nie Parasismique. Recommandations AFPS 90. (1990), Presses des Ponts et Chausse´es.
[2] Eurocode EC8. Structures in seismic regions. (1994), Part 5. Foundations, retaining structures, and geotechnical aspects.
[3] Gazetas G. Mylonakis George. (1998), “seismic soil–structure interaction: new evidence and emerging issues”. Geotechnical Earthquake Engineering and Soil Dynamics, Geo-Institute ASCE Conference, Seattle; 3–6 August.
[4] Juran, I., Benslimane, A., and Hanna, S. (2001). "Engineering Analysis of The Dynamic Behavior of Micropile Systems." Transportation Research Record, 1772, Paper No. 01-2936; p. 91–106.
[5] ITASCA “FLAC - Fast langrangian analysis of continua, (1996) Version 3.4.” ITASCA Consulting Group Inc., Minneapolis, Minnesota.
[6] Radeghi Mehrjou, S.M., (2011),”Evaluation of seismic behavior of inclined micropiles with FLAC3D” , M.Sc Thesis, Azad University of Central Tehran Branch, Tehran, Iran.
[7] Bulletin of the Seismological Society of America, (2003) Vol. 93, No. 3, pp. 1099-1131.
