The device of a bored foundation with a grillage

Recommendation points

• Classic pile technology
• Common misconceptions or how to do it right
• Drilling and clean-up process
• Pile cutting
• Concrete injection
• Immersion of the frame
• Strapping and filling grillage

Under unfavorable geological conditions, the pile-grillage foundation is almost the only way to provide reliable and durable support for buildings. Today we will talk about the technology that is used where shallow piles are simply not able to solve the problem.

Classic pile technology

The foundation on bored piles can be called one of the oldest and, at the same time, the most technologically advanced types of foundations for capital buildings. In essence, the process of building a foundation is reduced to drilling a well, installing a reinforcing cage and pumping concrete with its compaction..

Despite their obvious simplicity, bored piles require close attention at every stage of the installation process. Since the filling of cavities with concrete occurs without visual control, the likelihood of error is quite high, and a defect in the production of even one in ten piles can make the entire foundation unusable..

Nevertheless, subject to all the rules of the technological cycle, the bored foundation is a truly universal solution. Such a foundation helps to level the flaws in the geomorphology of the site, allows you to build buildings even on muddy, heavily heaving and unstable soils, or on areas with a significant slope. The rationale for the construction of a bored foundation may be the proximity of other buildings or residential buildings, which is why the vibration immersion method is not allowed.

Common misconceptions or how to do it right

Basically, errors in the construction of a bored foundation are the result of an insufficiently high-quality technical base for performing work. The technology has a number of problem areas:

1. The well must have a rigid casing, which excludes soil shedding from the walls. Casing pipes (retrievable, not retrievable, with a lost tip) can be used for this, as well as special bonding solutions, for example, based on refractory clay.
2. The drill auger must be equipped with a “finishing” head that does not leave any loose soil residues at the bottom of the well. In some cases, they resort to packing the bottom of the well. It is also possible to use screws, which, due to the design features, themselves seal the walls of the well..
3. Concrete injection must be carried out under excess pressure due to the impossibility of vibration shrinkage. At the same time, air cavities and remaining fragments of soil float to the surface.
4. The processes of drilling, casing, filling and submersion of the frame occur inseparably from each other, the well cannot be in intermediate states for a long time.

Bored piles allow you to effectively and quickly equip a reliable foundation, but only in the case of full compliance with the installation of technology and full mechanization of the process.

These and other reasons have spurred the development of several separate drilling and filling technologies. When performing work by a specialized contractor, the degree of customer participation is reduced to a minimum, which cannot be said about the independent construction of such a foundation. Therefore, you should take note of the range of technical solutions and try to reproduce the basic principles to the best of your strength and technical capabilities..

When independently arranging a foundation with bored piles, using simple means of mechanization, it is impossible to completely control the quality of each pile and guarantee the reliability of the foundation as a whole..

Drilling and clean-up process

The two basic technologies for the installation of bored piles are called CFA and DDS. The first involves drilling with a continuous full auger, which partially seals the borehole walls in the downhole area. The second technology uses the principle of drilling a well and does not imply excavation. It is evenly crushed along the walls, giving them additional strength.

Installation of piles using CFA technology. 1 – drilling a well (on soft or crumbling soils, the casing pipe is simultaneously fed after the drill); 2 – lifting the auger with soil and filling the well with concrete under pressure; 3 – laying the reinforcing cage

The main disadvantages of these technologies are the need to attract special drilling rigs, as a rule, very technological. There are no analogues for independent work at such a high speed. A replacement for CFA technology can be called conventional crane-boring machines used when drilling holes in limestone or when installing power transmission towers.

To make a well using the DDS technology, the same technical means can be used, but with the replacement of the drill head with a cone. In this case, the base of the drill rig must have a significant mass, because the auger sinks with significant excess force. Naturally, you cannot do without the use of hydraulic jacks.

Installation of bored piles using DDS technology. 1 – drilling without excavation and compaction along the borehole walls; 2 – lifting the drill with gradual filling of the well with concrete under pressure; 3 – installation of the reinforcing cage

In both cases, the final well quality remains rather low. The main problem here is crumbling soil, which can only be completely removed from the bottom by special sweeping augers. It is possible to suggest compaction of the bottom of the well using a massive concrete beam, which is thrown from the surface, ramming a loose embankment.

Without fail, after 10-15 compaction cycles, the bottom should be covered with incompressible material (crushed stone, ASG) and compacted again. In reality, it is possible to arrange wells up to 6–8 m deep, in deeper wells the walls crumble during ramming, and all work turns into Sisyphean labor. Otherwise, high cleanliness of the piles can be achieved only with the use of bucket drills or cleaned by hand..

Pile cutting

Casing helps protect the well even from partial collapse. Casing can be dispensed with only when two factors are combined:

1. At the time of installation, the water table is below the heel of the pile.
2. The soil is composed of dense sedimentary rocks without interlayers of sandy loam, quicksands and aquifers.

On soils with a fluid consistency or with abundant water flow into the well, casing is strictly recommended. For it, vertically movable pipes (VTP) are used. These are reusable inverter products with which each drilling crew is fully equipped. However, in contrast to the means of mechanization, HVHTs are not leased, they will have to be purchased separately. It is cost-effective to do this when arranging a field of 30-50 piles or more.

Casing pipes are collected in sections and plunged into the well as drilling progresses, usually with a slight lag behind the hole. Under special geological conditions, such as the presence of quicksands and mudflow pockets, the pipes can run ahead of the bottom. The pipes remain in the well until the concrete injection is complete.

Concrete injection

CFA and DDS technologies use a hollow drill. Through the axial hole, the concrete is pumped into the borehole by means of a pump, while the filling and extraction rates of the drill are carefully controlled. So in the array of piles, it is guaranteed that no soil impurities are formed, and due to excess pressure, all cavities and irregularities in the walls are filled.

The need for gradual pumping of concrete from the bottom is associated not only with the requirement of complete filling without the formation of cavities. When the concrete mixture is dumped from a height of more than 5–6 m, its stratification is guaranteed, and it is even more difficult to inject the wells partially filled with groundwater in this way. A way to solve this problem with an independent device of a bored foundation can be called fall rate absorbers built into the reinforcement cage. However, they require careful calculation, mainly due to the difficulty of filling the areas under the blades..

It is possible to simulate the operation of a hollow drill using concrete pipes. These are also inventory items that cannot be rented separately from the drilling complex. A set of a channel for filling consists of several pipes with a length of 1 to 4 m and a receiving funnel, the volume of which is calculated according to the size of the well.

Installation of pipes for pumping concrete is carried out immediately after the end of drilling. They are removed either as the well is filled with concrete mixture, or at the very end. In the process of pumping concrete, in any case, it is necessary to organize control of the mixture consumption. After the concrete level rises to the height of one VPT, the casing is lifted with jacks and the upper vacant section is removed. Some boring machines have devices that simultaneously lift casing and concrete pipes. It should be remembered that as a result, the concrete level will gradually decrease and it must be replenished.

Immersion of the frame

The reinforcing frame can be immersed either before filling the well with concrete, or after final filling. The first option is the simplest, you just need to securely fix the reinforcement, protect it from shifts and withstand protective layers. The method of immersing the frame in a flooded well is considered to be more optimal, because this way it is guaranteed that no unfilled areas are formed. However, this is fraught with difficulties..

The main one is the requirement for high structural strength of the frame and its ability to maintain shape and spatial position. Typically, factory-made reinforcement systems are used for bored piles. Another difficulty is that the submersion of the frame occurs before the casing is raised. In this case, much more force has to be applied to extract the VAC, which often cannot be done without vibration and half-rotary action. It is almost impossible to do this without special equipment..

Strapping and filling grillage

Bored piles are poured to form a pit, the bottom of which is located no higher than the upper dense layer of sedimentary rocks. The tails of the reinforcing frame are left free for bandaging with the grillage reinforcement. After pumping the concrete, they are washed with water. The connection with the reinforcement of the grillage tape is carried out by means of L-shaped and T-shaped bent anchors with an overlap of at least 50 nominal diameters of the working reinforcement.

There are two options for the location of the grillage. It can be buried and rest on a dense layer of clay, or it can be completely above the ground without support. In the first variant, the influence of installation errors is very large. For example, if the well has not been properly cleaned up, after the soil is compacted under the heel of the pile, it ceases to perform its supporting function and, conversely, loads the foundation even more..

When the grillage is located above ground, it performs the function of distributing the load over the pile field. This is considered the best solution for civil engineering: the piles easily tolerate settlement and over time the entire structure becomes stable. But such a foundation requires careful design in terms of load capacity. The second disadvantage can be called the difficulties in the construction of the formwork, because there is no natural fencing with soil. Usually, it is practiced to fill a dump of soil, on which the lower plane of the formwork rests. The side surfaces are shielded with ordinary board shields and anchored in the upper soil layer.