Hanging driven piles. Features of the use of hanging piles

29.01.201827.05.2017

Unstable soil is a common problem encountered by many builders when building foundations. To obtain solid foundation, piles are used on soft ground. Their length compensates for the lack of support, and the frictional forces of the side surfaces against the ground help to hold them in the soil. Depending on the nature of the layer and the method of interaction with soil bedding, there are hanging piles and piles of posts.

Difference Between Hanging and Rack Piles

The first category includes supports that rest on compressible soils. Their structure allows, with the help of the lower end and the lateral surface, to redirect the load to the soil base.

The second category includes supports based on poorly compressible soils, which include clay and coarse rocks with dense sandy filler.
Hanging piles and rack piles have a similar structure and appearance, but they radically differ in the principle of operation and basic technologies for ensuring the safety of the future structure.

Hanging structures are used in work with strong layers of soil lying at great depths. The task of such supports is to pass through the thickness of the weak soil and to anchor with the lower end in a strong soil layer. Achieving this goal is possible only with a competent calculation of the length of the hanging pile, which takes into account the following indicators:

The density of the material it consists of.
Working length.
Characteristics of soil features: mobility, looseness, moisture saturation.
Estimated load of the future building: the greater its value, the deeper the hanging pile should be installed.

The principle of the pile rack

A pile rack is a structure up to 20 m long, which passes through unstable soil layers and is fixed in a hard surface at a depth of 0.5 meters or more. Unlike a hanging one, this support prevents a gradual settlement of the building, exerting a load on the solid ground not with its whole body, but at the expense of its lower part. Rack piles are widely used for laying pipelines and in large-scale industrial construction. Such supports can also be used in areas with unpredictable seismic activity.
To calculate the optimal pile length, the posts are guided by the following indicators:

soil strength;
the ability of a solid foundation to carry the mass of a future building;
material for making piles of the rack.

Advantages and disadvantages of supporting structures

Depending on the material of manufacture, wood, concrete, reinforced concrete, metal, solid, and hollow products are distinguished. An important selection criterion is the installation method, which subdivides the supports into driven, boring, pressed, screw and rammed.
Foundation piles are considered to be more reliable than hanging piles, which are not supported by a supporting layer of soil.
There are several ways to eliminate this significant drawback:

Increasing the cross-sectional area.
Increase in length.
Expanding the diameter.
Adding more supports per unit area.

However, such methods do not justify themselves in terms of economy and practicality.
So, an increase in the size of a product due to a change in the cross-sectional area, length and diameter, leads to additional costs of money and effort: the purchase of additional large-sized lifting equipment, the cost of related building materials, the hiring of qualified workers, a general rise in the cost of the foundation, the price of which is the cost of each element.
The thoughtless use of additional supports significantly makes the structure heavier and leads to settlement of the structure. In turn, the uneven subsidence of the foundation entails unpleasant consequences, which begin with the appearance of cracks on the walls and end with the destruction of the structure.

Thus, strengthening the foundation is a laborious process that requires certain professional skills and the availability of special equipment. The selection of support products is based on initial requirements and a feasibility study.

Weak soils are a very big problem in construction. In order to secure and secure the entire structure, two basic technologies were developed using piles:

First technology. Hanging piles are supports that rest on directly compressible soil.
Second technology. The pile post is a rather long structure that penetrates the weak layers and enters the solid layers of the soil for at least half a meter.

Each of these technologies has its pros and cons, therefore, considering the topic, we propose, without delving into the intricacies, to go through the basic points.

Two technologies - two entities

Let's try to understand the installation problem on soft soils. Rack piles and hanging bases, with all the external similarity, will differ in their principle of operation.

So:

The pile-pile resembles a compressed rod, which transfers all the load from the carried mass to the low-compressive layers of the soil. In this case, in fact, it is the lower part of the structure, and not its entire body, that exerts the redistribution of loads on the ground. When calculating, it is necessary to pay attention to the strength of the soil, and precisely its ability to bear the mass of the structure, as well as the strength of the material from which the pile is made.
Hanging piles act in a completely different way. In this case, the pile is supported on directly compressible soils, and not on strong deep layers. The piles carry the load due to the friction of the entire body of the product, not just the tip. And this is a significant disadvantage of this design.

Ways to solve the problem

To solve the problem of increasing the reliability of hanging piles, several technological developments have been developed and implemented, here are some of them:

Increasing the diameter of the piles, and in some cases the length;
Increase in "bushiness" of supports per unit area of \u200b\u200bthe building foundation;
Expansion of the heel of the product. In this case, the friction area at the end of the product increases;
Applications modern technologies by the type of discharge-pulse.

However, practical application has shown a not very bright prospect of such techniques:

First, any increase in the size of the product leads to an increase in the cost of the entire construction, since the price of the entire foundation is made up of the prices for each element. In addition, it is worth remembering that the thicker the pile, the more difficult it is to drive it into the ground;
Secondly, the number of "driven" supports for such reinforcement can make the structure heavier both literally and financially, but not bring the desired benefit:
The bushy structure is subject to more settlement than one pile;
The length of a standard support is 7 meters, while the draft of a single pile and a bush is approximately the same, but there is a paradox.

Interesting to know! There is an interesting practical observation. If the distance between the piles is more than three diameters, then the settlement of a single support and a bush is approximately the same; with a decrease in this distance, the shrinkage of the foundation increases sharply.

New technologies

At first glance, new technologies are faced with an insoluble task, namely, increasing the bearing capacity of existing pins. And this is on labile, weak soils. The purpose of this technique is to increase the frictional forces of the surface of the piles and the soil located around them.

The instruction for applying the technology assumes the following procedure:

In the existing foundation, with a step of 1.5 to 2 meters, the soil is drilled and compacted under high pressure to form hydraulic fracturing cavities. Drilling is carried out in the interpile space and at the base of the installed piles;

Note! Under the influence of high pressure, the soil is compacted near the existing foundations; to achieve the maximum result, drilling is carried out to a depth of one meter to two and a half deeper than the level of occurrence of the main piles. In addition to the usual direction of filling with a fast-hardening solution, multidirectional injectors are used, which allow you to scoop up the soil just under the already installed structures.

In existing buildings, if available slab foundation injectors are inserted across the entire tiled field with the capture of squares from one and a half to one and a half meters, to two to three meters. All operations take place in the basement;
The injection of the solution occurs in stages, the ideal is the filling of the mines going and adjacent to each other. Thus, there is a purposeful displacement of the soils to a specific support and support of this newly formed cushion with a kind of reinforcing belts made of cement mix.

Positive characteristics of this method:

Compaction of the soil with reinforcement elements occurs without complicated earthwork, the use of heavy equipment and additional construction crews;
Blowing the cement mixture under pressure allows to increase the friction force between compacted soils and the surface of native piles, which in turn will increase the effectiveness of the bearing capacity of the structure by 2 times;
The proposed technology will make it possible to repair the foundation of an existing building and enhance the capabilities of the hanging supports. Since the classical technology presupposes the construction of new piles and redistribution of the load on them, which is practically unrealistic, in the new method this inconvenience is leveled.

When constructing foundations on soft soils, piles of various types are used. They differ in material of manufacture, immersion method, dimensions and cross-sectional shape. By the way of interaction with soil layers, such structures belong to one of two groups - hanging piles or racks.

In the first case, the pillars do not rest on a solid foundation, but on compressible soils, transferring loads by their lateral surfaces and a tip. The second group of piles is held by low or incompressible soils, which take on forces from the tip or extended heel of the foundation shafts. With this interaction, the soil resistance forces on the side walls of the foundation supports are not taken into account in the calculation of their bearing capacity.

Types and features of piles

By installation method in design position both hanging and rack piles are subdivided into:

hammered;
pressed in;
stuffed;
drilling;
screw.

They are concrete and reinforced concrete, metal and wood, hollow and solid. The hanging pile does not rest on the bearing soil layer, like the foundation pillars, and this is its main disadvantage. In order to increase the reliability of the "unsupported" foundation, several methods are used:

an increase in the cross-sectional area or length of the pile column;
adding the number of piles per unit area;
camouflage heel widening device.

But these techniques do not always justify themselves. For example, an increase in size always leads to additional material investments, an increase in the labor intensity of work, and also to the need to order more powerful and expensive equipment. The use of additional piles in the bushes can lead to the opposite effect, since the weight of the structure will pull with it a greater settlement of the structure. By the way, decreasing the pitch between point supports leads to the same result. But in some situations, the option of increasing bushiness saves the situation and turns out to be more suitable than setting pile rack.

The widened heel is advantageous in that it becomes possible to additionally support the hanging trunk on the ground. However, for driven piles, in this case, there are some inconveniences. The technology does not allow her to be immersed.

Uneven settlement of the foundation entails the appearance of cracks on the walls of the building and its possible destruction.

The length of the hanging piles is determined depending on the design load of the structure and the "weakness" of the soils lying on the site. The larger the first component and the smaller the second, the deeper the pile supports are installed. In some cases, they are made composite.

Stand piles rest on a solid layer of soil, transferring loads from the above-ground part of the structure to it. Often they are recessed some distance into the depth, which increases the reliability of the foundation and prevents any settlements. The length of the barrels can be 20m or more.

Calculation of a suspended driven pile

The bearing capacity (F d, kN) of hanging, as well as piles of shells, is determined by the formula specified in the corresponding section of SNiP 2.02.03-85. The calculation is based on several real indicators. These include:

u -the perimeter of the cross-section of the driven support in meters;
A -the area of \u200b\u200bthe pile post, shell or the largest section of the camouflage broadening in square meters;
H i -the thickness of each of the soil layers located in the immediate vicinity of the lateral surfaces of the driven pile support, in meters.

The calculation also includes tabular values:

R- the calculated value of the resistance of the soil layer located under the lower part of the pile, in kPa (Table 1);
Fi - the calculated values \u200b\u200bof the resistance of each of the soil layers in contact with the lateral surface of the foundation support, in kPa (Table 2);
Y c \u003d 1 - coefficient that determines the operating conditions of the structure;
Y cr - the same, under the pile tip (Table 3);
Y cf - the same, on the lateral surface, taking into account the method of deepening the support (Table 3).

The calculation itself for the compressive load is carried out according to the formula:

F d \u003d Yc * (Ycr * R * A + u * Σ Ycf * Fi * Hi).

that have inclined side faces are calculated using a slightly different formula, which takes into account the value of the slope and different values \u200b\u200bof the cross-sectional areas of the foundation column.

Another calculation of the bearing capacity of the driven pile and the shell, which does not provide for excavation, is performed for the pull-out load. The designations here are the same as in the previous formula, but the coefficient of working conditions ( Yc) has other indicators. When the support is deepened to 4 meters, it is 0.6, and more than 4 meters - 0.8. It should be noted that these values \u200b\u200bdo not apply to the foundation for the power transmission line supports. The formula for the calculation is as follows:

F du \u003d Yс * u * Σ Ycf * Fi * Hi

Driving and drilling types of hanging piles are calculated using the same formulas, but taking into account the conditions of the well construction and concreting of the wellbore, which affect the coefficient γ cf... It is determined from Table 5 of the same SNiP. The norm also describes some other features of the calculation.

Pile racks

Them load bearing capacity determined by a simplified formula:

F d \u003d Yс * R * A.

The letter designations here correspond to those previously described. The only difference is that the value of R, which determines the resistance of the soil under the lower part of the pile of the rack, is not taken from tables, but is calculated using formulas.