Predicting the impact radius of dense sand piles in flowing soils

Master's thesis in the field of civil engineering (soil and foundation)

Abstract

Dense sand candles have been used as one of the most effective and economical methods in strengthening soil in order to prevent liquefaction, increase the bearing capacity and improve soils with various grain sizes, since 1950 in Japan and then in other countries of the world. The process of implementing dense sand piles is first by impact loads and then by vibration loads and finally according to the mechanism of their formation in the soil (shear deformation of the soil), nowadays it is done by static-rotational loads. The impact radius of dense sand piles is currently evaluated by experimental design methods. It is worth mentioning that in experimental methods, resistance characteristics at different distances from the center of dense sand piles are considered the same for the entire soil mass, contrary to reality. The hole expansion theory based on cylindrical hole expansion in an infinite soil mass with a very small initial radius, up to a specific final radius, is one of the most powerful and widely used methods in investigating related problems. In a way that can be used in the most complex design problems, including dense sand piles, taking into account the non-linear behavior of the soil. In this research, by modifying the hole expansion theory and considering the soil softening behavior in it, as well as providing simpler relationships and programming in the MATLAB7.1 software environment and numerical modeling in the PLAXIS2D.v8.2 software environment, the impact radius of dense sand piles has been evaluated and at different distances from their center and at all depths, we will be able to determine the volume strains, changes in the modulus of elasticity, values ??of relative density and porosity ratio due to deformation Let's examine the applied methods with proper accuracy in comparison with the experimental design methods and the possibilities of the previous theory of cavity expansion. At the end, the advantages and limitations of using the mentioned method are described in detail. Keywords: liquefaction, radius of influence, dense sand piles, hole expansion theory Chapter 1 1-1 General Due to the trend of population increase and the upward trend of construction of heavy residential and office structures in different regions, also due to the occurrence of damages Due to the high loss of life and money caused by earthquakes in areas with liquefied soils, strengthening of liquefied soils against this phenomenon is of particular importance.

The use of dense sand piles is one of the methods of strengthening soil in cases of liquefaction. In addition to increasing the resistance of loose sandy soils against the phenomenon of liquefaction, this method will be widely used in improving the resistance properties as well as stabilizing and increasing the bearing capacity of other types of clay, sandy and silty soils. The purpose of the above research is to use the theory of cavity expansion and consider the effect of soil softening in order to model the exact behavior of the soil during large deformations, as well as to simplify the existing relationships with the design objectives, to determine the impact radius of dense sand piles and finally It is common to present a comparison with the experimental design method. As will be mentioned in the following sections, in fact, in the above research, taking into account the behavioral characteristics of liquefied sandy soils and dense sand piles, more precisely, goals such as becoming more economical and increasing the efficiency of the above system compared to the current state are considered. We examine their strengths and weaknesses in detail and compare them.

(images are available in the main file)

1-2) liquefaction [1]

In the soil and foundation engineering dictionary, it means a state in which sandy soils lose their effective stresses and subsequently their shear strength due to the increase in pore water pressure[1]. This word was first coined by Mr. Kubo and Mogami (1953)[2].

The phenomenon of liquefaction caused by the process of reducing effective stresses due to the tendency to compaction in non-cohesive saturated soils in undrained conditions can be classified into two main groups: flow liquefaction [2] and cyclic mobility [3]. It has more destructive effects.. Instead, the phenomenon of liquefaction occurs as a cyclic movement in a wider range of construction and soil conditions, and its effects are classified from low importance to high damages.

(Images are available in the main file)

1-3) Stream liquefaction

This liquefaction state, which has the most important effects among all phenomena related to liquefaction, is due to the increase of static shear stress against the shear strength of the soil and only in loose soils, with Low residual resistance occurs [3]. Rupture caused by flow liquefaction is usually characterized by high propagation speed and long distances that the liquefied material often moves. For example, in figure (1-1), you can see the burial of a village after the occurrence of this type of liquefaction.

Cyclic movement

Cyclic movement, as opposed to flow liquefaction, occurs when the static shear stress is less than the resistance of the liquefied soil and under the effect of an earthquake, it exceeds the soil resistance in an instant due to the increase in pore water pressure. These deformations during an earthquake are caused both by cyclic stresses and by static stresses and include two distinct states, lateral diffusion[4] and surface liquefaction[5].

(Images are available in the main file)

1-4-1) Lateral diffusion

These deformations occur in gently sloping terrains or on surfaces without lateral support, such as the coastal walls of rivers or in coastal areas. Seas occur. In addition to loose clay soils, the above condition can also occur in dense clay soils [3]. The deformations that occur can cause damage to vital structures and important parts. In figures (1-2 and 1-3) you can see the damage caused to a bridge and a buried pipeline.

Surface liquefaction

Ruptures related to this liquefaction occur due to the upward flow of water that occurs during the depletion of excess pore pressure during an earthquake, and in order to achieve hydraulic balance, this rupture may occur after the end of ground tremors. One of the signs of this type of liquefaction is excessive deviation from the vertical state (Figure 4-1).

(Images are available in the main file)

Methods to deal with the liquefaction event

The methods to deal with the liquefaction phenomenon are divided into the following two general categories according to the way of operation:

1. Methods to prevent the liquefaction event (methods to prevent the increase of pore water pressure).

Considering that some soil modification methods achieve both of the above goals to some extent, therefore the mentioned classification is not correct in all cases and has been greatly simplified.

(Images are available in the main file)

1-5-1 General description of the methods of dealing with the liquefaction event. Prevention of the liquefaction event can be achieved by increasing the drained cyclic resistance, also by increasing the resistance to deformation or by depleting the excess pore water pressure. The resistance against liquefaction can be increased with the following factors, which we will describe below:

High density.

Appropriate granulation distribution to prevent liquefaction.

Stabilizing the skeleton of soil particles.

Decreasing the percentage of soil saturation. River:

Instantaneous depletion of excess pore water pressure.

Prevention of release of excess pore water pressure from the surrounding liquefaction layer.

Reduction of shear stress rate on the effective pressure of the load by increasing the effective pressure of the load.

Less shearing change of the ground during an earthquake.

Soil resistance methods against liquefaction and their principles can be seen in table (1-1)

Using the action of compaction, by increasing the density, the soil becomes resistant to liquefaction. The use of item number 7 also fulfills the above purpose. Research has also shown that by using compaction action, the lateral pressures in the soil increase.  

     In the method of soil placement, after the complete discharge of the liquefied soil mass, materials are used that do not have liquefaction potential in terms of granulation distribution (item no. 2). In this method, sand materials are usually used in terms of granulation.