In modern megacities, there is an increasing trend towards a more rational use of space and the compaction of buildings. These circumstances dictate certain conditions to construction companies. On the surface, less and less free sites remain, which forces developers to resort to the construction of underground structures. Among other things, there are some objects that are more rational to build underground. This includes large warehouses, shopping and entertainment complexes, as well as garages. But underground construction is a rather time-consuming process, which provides for the presence of certain experience and appropriate equipment from construction companies.
The solution of the problem described above can also be complicated by the fact that the soil is very heterogeneous, it can contain voids of different sizes, underground water currents. Sometimes, when examining the territory for development, it turns out that the rocks are quite weak. It happens that underground are all kinds of tunnels of engineering systems that are not mapped. At the same time, it is often necessary to work in cramped conditions, since the foundations of neighboring buildings are located quite close to the construction site, and the walls of high-rise buildings do not allow the cranes to fully deploy.
Solving the issue of the construction of underground structures
Depending on what the hydrogeological characteristics of the area are and how deep the premises will be, underground construction can be done in one of several ways. The most common are the "wall in the ground", the method of the lowering well, as well as the open method. The first technology in modern realities is quite common and still continues to rapidly gain popularity, because it can be used to solve the problem in cramped conditions without bothering the foundations of buildings located nearby.
Technology principle
The wall in the ground is built according to a rather simple principle, which provides for the preparation of the trench and excavation. Further, enclosing structures are constructed in the formed voids , for this, as a rule, reinforced concrete is used. Under the protection of the obtained systems, internal structures, for example the floor and other elements, are equipped.
Varieties of method
The technology "wall in the ground" can be divided into several subspecies, such as: trench and pile. The first is the use of monolithic concrete and reinforced concrete sections, with the help of which a single wall is formed. The pile method involves the installation of bored supports, which are located in a continuous row. They allow you to form a strong enclosing structure. Whatever technology is used, it is more promising in comparison with alternative methods of erecting underground structures. It is advisable to use it when reconstructing existing buildings for any purpose.
Application area
A wall in the ground can be used when there is a need to erect anti-filter curtains, subway tunnels, garages, warehouses, underpasses, tanks, all kinds of sedimentation tanks, road interchanges, as well as the foundations of buildings for various purposes.
Wet and dry methods
Given the strength of the soil and its moisture level, builders can choose the wet or dry construction method. The latter is not so expensive, because for him there is no need to prepare a clay solution. However, it can be resorted to only if there is confidence in the strength of the soil and the absence of underground currents. Wet technology is an ideal solution for the construction of large facilities in water-saturated unstable soils. If the construction is accompanied by the described conditions, then sometimes there is a need for additional strengthening of the walls of the trench. Ultimately, durable and reliable rooms are obtained.
Thixotropy
When a wall is installed in the ground, the technology may include the use of a wet method, in which such a thing as thixotropy is important. This property is inherent in a clay solution, which has the ability to restore its original form without mechanical stress. Due to this, a correctly selected suspension will gain strength at the construction stage and liquefy from vibrational influences. This allows you to insure the walls of the trench from deformation. The highest thixotropic qualities are characteristic of bentonite clays.
If we consider the additional characteristics of such solutions, then it is worth paying attention to their water-repellent quality. After the suspension has hardened, hydrostatic pressure will act on the surface of the walls , which contributes to the formation of a waterproof film. Its thickness can vary from 1.5 to 5 millimeters, this is enough to protect the structure from water. Claying the walls allows you to save on the reduction of the tongue of the tongue. This is one of the many advantages of the described technology.
Applied Equipment
When creating a wall in the ground, the technology involves the use of appropriate equipment. It allows you to dig a trench. For this, a continuous device is most often used. A similar result can demonstrate a cyclic approach. For the formation of trenches, earthmoving machines are usually used, namely: bucket, plow, milling rigs, draglines, rotary and impact drilling rigs, grabs, and also backhoe shovels. The listed equipment will be quite enough to get a wall in the ground, which can be deepened by 100 meters. The conditions may be completely different. The “wall in the ground” method most often assumes that the width of the trench will be equal to the limit of 1 to 1.5 meters. In some cases, projects are drawn up in which the width reaches 2 meters.
Cases of inappropriate methods
Undoubtedly, the described technology has many advantages, however, we can distinguish situations where the use of the method is impractical. The construction of the “wall in the ground” is not carried out if there are strong underground currents in the soil, with loose soil, and also when the dilapidated masonry is located on the site. The technology should not be used when there are metal islands, as well as large fragments of concrete. When there are voids and cavities in the soil, work on the described technology should not be started either.
Impervious curtains
Manipulations to create impervious curtains can be considered as simple as possible. They are performed using heavy and hard clays, as well as monolithic concrete. The purpose of the curtains is to protect the object from water. Most often, such elements are used in the equipment of dams and digging pits. In the latter case, curtains are necessary to prevent water from entering the cavity. Workers will not be faced with the task of lowering the groundwater level, which is a rather time-consuming procedure. If we compare the curtain with step-down installations, the latter act temporarily while work is ongoing. Structures with curtains will not be afraid of the most powerful groundwater flows.
Capture Options
Before the foundation "wall in the ground" will be built, you need to calculate the length of the grab. Some factors will influence this parameter, among them:
- trench stability;
- design features and functionality of the structure;
- the type of equipment that is used to develop the trench;
- calculated concreting intensity.
Technology of work
The construction of a wall in the ground begins with the drilling of a well, after which trenches are prepared, which are simultaneously filled with a solution. The next step will be the installation of reinforcing cages, as well as concrete pipes. The final manipulations include the displacement of the clay mortar by supplying concrete mixture by means of a vertically moving pipe. Trenches can be developed full-length or in separate sections. Reinforcing frames are based on steel rods with corrugation. The resulting system should be 12 centimeters smaller than the width of the trench. Elements are wetted in water before installation, as this reduces the amount of adhering clay and increases adhesion to concrete.
Concreting
The construction of the wall in the ground involves concreting, which is carried out by the method of a movable pipe. The latter has a diameter ranging from 270 to 300 millimeters, while the wall thickness is 10 millimeters. Given the volume of the pipe, the neck is selected, and wads can be made of burlap.
Gripper limiters
The construction of the wall in the ground may involve trench deepening of 15 meters or less. In this case, pipes with a diameter of 50 millimeters less than the width of the trench should be used. 5 hours after concreting, the elements must be removed, and the resulting cavities are poured with a mixture. If the depth of the trench is greater than the mentioned parameter, then there will be a need to install a limiter. Its task is performed by a metal sheet, which is fixed to the reinforcing cage. The canvas can be strengthened by welding beams to it.
Increase in productivity
When the “wall in the ground” method is used in the construction process of a rather large object, and the grab length is more than 3 meters, it may be necessary to supply concrete mix of huge volumes. In this case, it enters through pipes, and for faster and easier laying, the plasticity of the solution is increased by plasticizers. The composition is poured in such a way that its surface overlaps the entire structure by 10 centimeters. This is required in order to be able to subsequently remove the contaminated concrete layer, because it will have a large amount of clay. Sealing will need to be done using special equipment, which is mounted on a concrete pipe. If its length is more than 20 meters, then it is recommended to use two vibrators.
Those pipes that will be located on the border of the grips are always removed. It is important to determine the extraction time correctly. If this is done too early, the edges of the shell may be damaged. If the extraction is too late, the pipe may get stuck between the concrete and the ground. In order to exclude such processes, sheet metal is often used instead of a pipe, with which you can create permanent, non-removable lintels. They must be welded to the reinforcing cages. To protect the mouth of the trench from deformation and shedding, it is necessary to equip the foresail, which is the head of the trench.
About soil pressure
If you need to find out what soil pressure is on the wall at a depth of z, then you can use the following formula: PR = PS + PQ, where PS is the lateral pressure intensity at a specified depth from your soil weight, taking into account the stratification of the layers, the effect of water, and also effective clutch; PQ is the lateral pressure intensity at said depth from surface loads. If, according to the design, the foresail is located on a specially formed dump above the surface of the earth, then the value is taken with a minus sign.