X Международная студенческая научная конференция Студенческий научный форум - 2018

Introduction

The type of foundation is chosen based on a large number of factors. The problem of rational design of foundations is one of the most urgent in the field of construction. This problem is especially acute in the construction of complex engineering and geological conditions, in which the most expedient is the use of pile foundations. The share of costs for the erection of the underground part of buildings and structures in such ground conditions is up to 20%. The development of foundation engineering is directed towards the development of new, economical and reliable constructions of foundations and methods of their construction that ensure an increase in the load-bearing capacity of soils in the foundations, a more complete use of the bearing capacity of the foundation material.

The use of pile foundations has a long history, but the widespread use of driven piles in industrial and civil construction began only in the late XIX century. in connection with the appearance and development of reinforced concrete. For the immersion of reinforced concrete piles, powerful hammer rigs with steam hammers were built. In the course of the development of the technique of pile works, along with shock methods, vibration, indentation and their combinations began to be used.

Chapter 1. The theoretical part

1.1. Types of piles

Pile foundations are designed to transfer loads to the lower layers of the ground and there are two types:

1) Piles-racks cut through a thickness of weak or insufficiently strong grounds and rely on strong soils rocky, semi-local, coarse-grained, dense sandy soils, clay soils of solid consistency. The pile-rack transmits its entire load through the lower end, since with its small displacements-precipitations there is no mobilization of frictional forces along the lateral surface. The pile-rack works as a compressed rod in an elastic medium. Its load-carrying capacity is determined by the strength of the material itself for compression and the resistance of the ground below the lower end - the point.

2) Piles, supported by compressible soils, are pendent piles. They have displacements under the influence of loads much larger than piles, while friction forces appearing on the lateral surface are included in the work. In suspended piles, the load is transferred to the base not only through the lower end, but also by the lateral surface. The load on such a pile is determined by the sum of these two impacts. Thus, the pendant pile differs from the pile-rack in that it transfers the load from the weight of the structure not only with its lower end, but also with the lateral surface.

The pile-tower is similar to a column that rests on an incompressible ground and therefore its bearing capacity is determined only by the size of its cross-section. The suspended pile under the action of the load moves relative to the surrounding compressible ground, while on its lateral surface there is friction, which impedes the introduction of the pile into the ground. Therefore, the bearing capacity of the hanging pile depends both on the cross-sectional area and on the area of ​​the side surface of the pile.

Depending on the material, piles of wood, reinforced concrete, metal and combined are distinguished.

Wooden piles are currently rarely used. Such piles are usually made of logs. At one end, the logs are sharpened, the other is put on a metal yoke, which protects the wood from soaking during piling. The length of such piles, as a rule, does not exceed 12 m.

Reinforced concrete piles are more versatile, both in terms of assortment and shape, than wooden piles. Currently, most often used full-bodied square and hollow square or round. Square piles are made with cross-sectional dimensions from 0,2х0,2 to 0,4х0,4 m, length from 3 to 20 m.

Metal piles usually have a tubular cross-section, in rare cases - an I-beam or other complex cross-section, obtained during welding of rolling profiles.

Sometimes combined piles are used, the individual parts of which are made from different materials.

1.2. Methods of immersing piles in the ground

According to the method of immersion in the ground, pile driven piles are distinguished, immersed in vibration, pressed and screwed. The driven piles are dipped into the ground by pile hammers. Immersed by vibrating piles, it is advisable to use it in the presence of a series of sands saturated with water. Pressed piles are used where dynamic impacts can not be used to immerse them. Screw piles are advisable to use when the surface is covered with weak soils underlaid by low compressible ones, and for the construction of foundations working on pulling.

Piles made in the soil are often called stuffed. Printed piles are made of concrete and reinforced concrete. When manufacturing ramming piles in a stamped box, crushed stone is used, which is tamped into the soil foundation array at the bottom of the pile.

Stuffed piles are made with a cross-sectional diameter of up to 0.8 m and a length of up to 50 m as follows. The inventory pipe is lowered into the ground with the lower end closed by the lost shoe. After plugging the pipe into the ground, concrete is fed into it, and the hammer produces frequent impacts up and down; while from each blow the pipe rises by 3-4 cm and re-sinks by 1-2 cm. Thus, concrete is trampled, and the pipe is extracted from the ground.

Stamping piles are also vibrated piles, which are manufactured in a well formed by immersing the inventory tube vibrator, which is also closed with a lost reinforced concrete shoe. Concrete mixture is compacted by vibrating. The widened heel at the base of the vibrated pile is formed by extruding a shoe out of the casing and simultaneously a portion of concrete equal in height to 3-4 pile diameters and tamping them into the ground.

Vibro-stamped piles are made by filling the well with concrete and sealing it with a vibration shoe. Such a method of compacting concrete in the well provides for indenting the concrete into the ground and widening the heel of the pile stem with soil compaction around the pile.

If it is necessary to reinforce the pile in the casing, a reinforcing cage of four to six rods with a diameter of 14-20 mm and a spiral of wire with a diameter of 6 mm in steps of 150 mm is installed.

1.3. Interaction of piles with surrounding soil

When piling the pile into the soil, the soil particles are squeezed out from under its point to the sides and upwards. When the pile is submerged to a depth of less than 4d, the soil is raised against the surface of the base. The lifting of the surface of the base occurs at a distance (3-4) d around the pile. The amount of lifting the base depends on the moisture content of the soil.

With further immersion of the pile, only the internal pothor is observed, which leads to compacting of the soil within a cylindrical body with a diameter up to (3-5) d, depending on the type of soil. Under the lower end of the pile, a zone is formed, within which the soil density is maximum. The size of this zone depends on the type of soil and its strength properties. In sandy soils, after the pile is driven, the stress relaxation process begins in the over-stressed zone, the soil decomposition takes place and the size of the re-compacted soil zone decreases.

In water-saturated clay soils, the process of sinking the pile is accompanied by the destruction of structural bonds and the appearance of excessive pressure in the pore water, which also leads to the outburst of soil on the surface. This bulging is accompanied by a significant rise in the surface of the soil and lasts for several days after the pile driving process stops. A tense-deformed zone arises around the pile pile. Vertical compressive stresses have a maximum directly at the pile, decreasing in the radial direction. At a distance of about 3d from the axis of the pile, their value is insignificant and does not cause soil compaction. Therefore, in order not to apply stresses from neighboring piles, they are recommended to be located at a distance of at least 3d from each other.

For piles, which are supported by stronger soils, the distance between the pile axes in the level of their point is taken to be 1.5d.

The tangential stresses on the lateral surface of the pile increase to a certain depth, remaining then practically constant over the entire length of the pile stem. In a number of experiments, an increase in the frictional forces with depth was noted.

Chapter 2. Analytical part

2.1. Pile in the pipe

The problem of rational design of foundations is one of the most urgent in the field of foundation engineering. This problem is especially acute in the construction of complex engineering and geological conditions, in which the most expedient is the use of pile foundations. The share of costs for the erection of the underground part of buildings and structures in such ground conditions is up to 20%. The development of foundation engineering is directed towards the development of new, economical and reliable constructions of foundations and methods of their construction that ensure an increase in the load-bearing capacity of soils in the foundations, a more complete use of the bearing capacity of the foundation material.

In recent years, berthing facilities on metallic piles-shells have been widely erected, since they have many advantages. Steel piles better withstand dynamic loads and perceive large bending moments compared with reinforced concrete piles. The use of open steel tubular piles open from below helps reduce the amount and timing of construction work, labor and pile costs, due to a more rational operation of the cross section of the trunk under the design load. The main disadvantage of metal piles-shells is their corrosion. Reinforced concrete piles are economical, but their load-carrying capacity is low.

The technical task of the technology was the desire to combine the advantages of both types of piles. One of the options for such a merger is discussed below. In this technology, a steel tubular pile with an open bottom end is immersed in the ground and a grillage is built. After immersing the tubular pile, the longitudinal core is introduced into the soil core formed inside its cavity, and additional volumes of materials are introduced into the soil cells between the partitions and the walls of the pile, and additional energy is supplied, mainly to the lower part of the core. This strengthens the ground core, creates additional radial compressive stresses in the kernel core, provides additional friction and adhesion to the walls of the pile, and turns it into a quasi-monolithic deep foundation.

Depending on the area of ​​the section of the additional pile, we note the following advantages of the proposed technical solutions: - they combine the positive properties of two types of piles (steel tubular and reinforced concrete monolithic) and reduce their negative properties due to the fact that the tubular pile will work as a foundation for deep laying the area of ​​support on the ground equal to the cross-sectional area of ​​the "gross" tubular pile. The bearing capacity of such a foundation for the material will be composed of the bearing capacity of steel and reinforced concrete, they allow the creation of a foundation with a high bearing capacity with the help of conventional construction equipment. The immersion of internal additional piles allows to control the degree of hardening of the core during the erection of tubular piles by changing the number of additional piles, their diameter and the depth of immersion; - the method refers to environmentally friendly technologies, since it is only intended to immerse thin-walled (cutting) tubular piles. Pile elements are immersed in stages, and the influence of the dynamics of immersion of internal additional piles is localized by the ground core inside the tubular pile. In this case, internal piles have relatively smaller parameters, and, correspondingly, lower dynamics of their immersion.

Conclusion

The development of pile foundation engineering is extremely necessary in our country. Very often it is necessary to erect buildings in very difficult ground conditions, where the belt foundations can not give the necessary bearing capacity. Unfortunately, nowadays new developments in this field are rarely offered. I believe that the method "Pile in the pipe", described in my work is very interesting. It is interesting primarily because it combines only the positive aspects of materials that are used in this type of foundation. In practice, this method has not yet been applied, but I hope that in the near future this design development will find its application.

Bibliography:

SP 22.13330.2011 "Foundations of Buildings and Structures" dated 28 December 2010 No. 823 and enacted from 20 May 2011.

JV 24.13330.2011 "Pile foundations" dated December 27, 2010 N 786 and put into operation from May 20, 2011.

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