Numerical investigation and comparison of bearing capacity of groups of conical and cylindrical piles by 3D finite element method

Master thesis in the field of civil engineering, soil and foundation

In this research, the bearing capacity of the group of conical piles and the group of cylindrical piles with the same volume are calculated and compared. Due to the lack of numerical studies and modeling by software for conical piles compared to laboratory experiments in this field, in this research, the modeling of the piles group was done by PLAXIS 2012 3D software using the finite element method. Also, considering the importance of piles in a group compared to single piles, the main goal of this research is to investigate the behavior of a group of conical piles and compare it with a group of cylindrical piles of the same volume. For this purpose, the load-settlement diagrams of each pile group under an arbitrary load have been obtained, and the load capacity of the entire pile group has been calculated using this diagram and with existing techniques. Then, the pile group model is subjected to a load equal to its bearing capacity for the second time so that the characteristics of the pile group, including the state of tension around the block of the pile group, the settlement of the pile group, the efficiency of the pile group and so on, can be determined. and finally made a comparison for both groups of conical and cylindrical piles of the same volume. To calculate the efficiency of each pile group, a single pile from that group is modeled separately and loaded as before to obtain the load capacity of the entire single pile from each group. Modeling has been done in sandy soil and with concrete piles in real scale. The parameters that have been varied in these modelings to make the effect of their changes clearer are: the angle of taper, the angle of internal friction of sandy soil, the angle of expansion and the coefficient of lateral pressure of the soil.

Key words: conical pile, finite elements, bearing capacity, three-dimensional modeling, numerical integration

Chapter One

1-1- Introduction

Piles are structural members of wood, concrete, steel or other materials that are used to transfer surface loads to lower levels in the soil. This transfer is done by distributing the load along the body of the pile or directly transferring the load to the lower layer through the tip of the pile. Usually, all piles transfer the load as a combination of body friction and tip resistance, except for cases where, for example, the pile penetrates a very soft soil to reach a hard bed. Piles are usually used for the following purposes:

1- Transferring the load of massive structures into the underlying layers. (Vertical and lateral loads)

2- Resistance against upward tensile force or overturning, for example, for uniform foundations under the water surface or to keep bridge foundations against overturning caused by lateral loads such as wind.

3- Compaction of soft and non-adhesive layers by a combination of displacements by pile volume and pile movement in a vibrational manner. These types of piles may be pulled out later.

4- Settlement control, when the strip or wide foundation is on the soil under which there is a layer with high compressibility.

5- Hardening the soil under the foundation to control the vibration amplitude and natural frequency of the system.

6- As a higher reliability factor for bridge foundations or supports.

7- In offshore structures. To transfer loads above the water surface to the subsoil. Sometimes to control ground movements (e.g. landslides). Piles may break under vertical and lateral loads. Also, buckling failure may occur in long members.

A pile foundation is much more expensive than a strip foundation and almost more expensive than a wide foundation. In every case of foundation design, studies on local soil characteristics are very important. Because based on these studies, it is necessary to carefully decide whether piles are needed and, if so, whether more piles or longer piles are needed.

Pile bearing capacity means the maximum load placed on the pile, so that the pile can settle to an acceptable level under its effect. Considering the carrying capacity to a greater extent than the actual value, may have caused heavy damage to the structures or their general deterioration. On the other hand, considering it to be less than the actual value, causes the dimensions of the pile to become too large and makes it uneconomical. The bearing capacity not only depends on the mechanical characteristics of the soil and environmental conditions, but it is also a function of its shape, dimensions, material and implementation method.. Obtaining a general relationship for calculating the bearing capacity of piles, even considering a limited number of effective factors, leads to very complex differential equations that cannot be solved in general and only in some limited cases. Although, nowadays, useful analytical methods such as the finite element method have increased the trend towards numerical analysis methods. Of course, it should be noted that the results of finite element modeling or similar methods must be verified with laboratory results to be usable.

1-2- Conventional geometry of some in-situ piles

An in-situ pile is formed by digging a hole in the ground and filling it with concrete. This hole may be dug in the form of a box or created by driving a shell or wall tube in the ground. The wall pipe can be driven in the ground by a steel shaft; so that the steel shaft releases the wall pipe when it rises.

Figure (1-1) shows some commonly used in-situ piles. It should be noted that these candles are generally of three types. (1) shell or wall (2) without shell and (3) pillar base.

(Images are available in the main file)

In Figure (1), items (a) and (b) are piles without shell. Items (b) and (c) are pillar piles up to 35 m long. Candles (c) to (h) are of the shell type, among which (f) and (g) are conical candles and (h) is a stepped conical candle.

Variable cross-section candles are candles that have larger cross-sections in the head part than the tip. One of the types of piles with variable section is the pile with variable circular section, which is cone-shaped in appearance and is known as conical pile (Figure 1 - items f and g). Cone-shaped piles have a fundamental advantage over cylindrical piles, and that is that in the case of downward friction, the axial force of the pile decreases from the tip of the pile to the tip of the pile, so in terms of cross-section, smaller sections are needed to transfer the structural load to the soil as we go down the pile, which we see in conical piles. This saves the volume of materials and the correct distribution of materials in the pile. Also, for piles under lateral load, when the lateral load is applied to the pile head, from the pile head to the tip of the pile due to the transfer of load to the soil, the amount of horizontal force in the structural body of the pile is reduced. For a foundation to be defined as a pile, it is necessary that the ratio of depth to average width or radius is at least equal to 6. According to this definition, the maximum angle that can be considered for a conical pile is 46.9 degrees. Most of the relationships that have been published for piles are related to cylindrical or prismatic piles, while for conical piles no specific theoretical relationship has been presented that can determine their behavior in soil. However, in the last three decades, there has been an increase in interest in studying the behavior of conical piles.

(Images are available in the main file)

1-3- How piles are placed in the soil

Piles are placed in the soil through several methods:

1- Driving the pile into the soil by successive fixed blows on the top of the pile. Using a pile hammer. This method creates significant local noise and vibrations that may not be allowed by local regulations or environmental organizations and may also cause damage to surrounding structures. This method is usually silent. This method is more useful for driving in soils with low adhesion.

3- Jacking the pile. This technique is more useful for short and hard piles.

4- Digging a hole and placing the pile in it or more commonly, filling the hole with concrete.