Test piles: in which cases is necessary, the methods used

In drafting any house, among other things, it is necessary to determine and what will be its foundation. There are several types of grounds for buildings. The most reliable type of foundation is the pile. These structures are usually built on soils with a weak bearing capacity. In this case, the piles are buried in the ground until they reach dense layers. Such a solution allows to give the building maximum stability and completely excludes the possibility of movement, and consequently, destruction of load-bearing structures. Before erecting this type of foundation, a procedure such as a pile test is often performed. Such an exercise is being carried out in compliance with the standards set by SNiP and GOST.

What is a pile foundation

For the first time the base of this type was built under the house in 1838. This foundation was initially used only when building buildings on water and marshy areas. Later, the piles began to be placed under houses in the permafrost zone and simply on not too strong soils. In our time, foundations of this type can be built, including on the slopes.

The basis of the construction of such a foundation are long supports, actually called piles, connected by a horizontal grillage. The latter can be both a concrete solid plate and a tape. In the ground support, depending on their design, either clogged or screwed.

Pros and cons of pile foundations

Piles, the methods of field testing which are different, are made only of very strong materials. Usually it is steel or reinforced concrete. The main advantages of pile foundations are:

• Possibility of use on unstable soils;

• Possibility of carrying out work under any weather conditions;

• Relatively low cost;

• High degree of reliability.

Among other things, the pluses of such foundations are attributed to the fact that the buildings erected on them practically do not shrink. Disadvantages of pile foundations are very few. But they are, of course, available. The disadvantages of such foundations are first of all:

• Need for the use of special equipment during erection;

• Impossibility of arrangement under the building of the basement floor.

Basic technologies for determining bearing capacity

Before the construction of the foundation of the project organization, among other things, you should determine and with such parameters as the diameter and length of the piles. Supports in the first place should, of course, reliably hold the structure. But at the same time during the erection of the foundation one should also try and avoid unnecessary expenses.

In order to find out exactly what kind of safety factor a particular support has, different methods can be used:

• Mathematical calculation, taking into account the requirements of SNiP;

• Static testing of bored piles and steel;

• Dynamic test.

The latter two methods are called the pile test by piles. It is possible to carry out such procedures at any stage of construction - during exploration, during design, when the supports themselves are immersed, etc.

In most cases, when determining the bearing capacity of supports, the calculation of their resistance by the formula, and the test are carried out simultaneously. Further, the lowest of the values obtained is taken as the basis. This allows you to eventually build the most reliable foundation. Specific methods for testing piles in the field are usually selected depending on the composition of the soil.

Formula by SNIP

This technique for determining the degree of pile resistance to loads is the simplest and at the same time the least accurate. The formula for calculating the strength of the support looks like this: Fdu = R * A + u * Σ γcf ∙ fi ∙ h, where:

• R - resistance of soil of a composition under the sole (determined by the table);

• A is the area of the base of the pile;

• Fi - average value of the resistance of the lateral surface of the support (depends on the material of manufacture);

• U is the perimeter of the pile section;

• Hi is the thickness of the soil layer.

For bored piles, whose diameter can reach up to 1.5 m, and the length - up to 40 m when building a house on a clayey soil, these parameters will have the following values:

• R for clay soil - 794 kPa.

• A = π ∙ d2 / 4 = 3,14 * 0,8 / 4 = 0,5 m2.

• U = π ∙ d = 3.14 * 0.8 = 2.5 m.

• Σ γcf ∙ fi ∙ hi = 222 (determined using table values of fi and hi).

Substituting these data in the formula, we can obtain the following result: Fdu = 794 * 0.5 + 2.5 * 222 = 952 kN = 95.2 tons.

When static testing of piles is required

This technique makes it possible to determine the bearing capacity of supports as accurately as possible. However, static tests are usually quite expensive. It is also a drawback of this technology that it takes a very long time. From the beginning of the work to obtaining a specific result using this technique usually takes at least a week. But more often this time interval is even longer.

Static tests are usually carried out only in case of special need. For example, when reinforcing foundations in the event of a threat of collapse of buildings of old construction. Clayey soils are also often tested for this technology. Methods of field testing of piles in such areas can use different, including dynamic ones. But static gives still the most reliable result. The same applies to loose soils (from construction debris) or woodlands.

How are static tests conducted: the general scheme

Determine the load-bearing capacity of piles using this technology by modeling the mechanism of their operation in a specially designated area - with the worst ground conditions for a particular site. This allows you to get the most accurate result. Field tests of soil with piles on static technology are carried out in several stages. First, the supports are jammed into the ground to a conditional level. Then they are allowed to stand for 3-20 days, depending on the composition of the earth. This is necessary in order to restore structural bonds in the ground.

Further, the test pile is step-by-step (steps), also with the mounting, loaded, each time measuring the draft using special tools. In melted soils, the total duration of this procedure is usually about 2 days, in permafrost - about 10. The required load during testing of piles can be created using the following settings:

• With a hydraulic jack, anchor piles and a beam system;

• With the cargo platform used in the quality of the jack stop;

• With a calibrated load;

• Combined.

In addition, static pile tests can be performed using pulling loads. In this case, the jacks are used as the main equipment.

Preparatory activities

Begin work to determine the load capacity in accordance with the static method, according to the standards, only after the strength of concrete piles reaches 75%. Supports are preliminarily inspected for chips and other defects. Only qualitatively manufactured piles are allowed to be tested. If necessary, their heads are strengthened using an outer cage. In certain cases, it is possible to seal the chips in a cement-sand mixture.

One of the piles is prepared as a reference before testing. Before blocking it, among other things, check for straightness. Also, carefully examine the place of its docking for wear. Deviations in a straight line at such supports, according to specifications, should not be more than 10 mm along the entire length in any of the planes.

In addition to inspection, the testing of piles involves carrying out this kind of preparatory work, such as saturation of the soil with moisture. After all, the land under the house in the future will not always be dry. Consequently, the load on the pile may be higher than under normal conditions. To moisten the soil during testing around the perimeter of the site, trenches with a width of 0.5 m and a depth of 1-1.5 m are excavated. Sometimes wells are drilled (usually in the amount of 3 pieces). Their diameter is usually 20 cm. The wells are also located at the edge of the site.

During the tests, ensure that the water level in the mines and trenches remains constant. The minimum soak time of SNiP soil is determined as 24 hours for each meter of depth. However, this indicator may vary depending on the composition of the land.

Technology features

Carrying out the field tests themselves, the piles are static, make sure that the vertical load is maximally centered. The precipitation at each stage is measured using hourly indicators with a 0.01 mm dividing point.

When immersing piles in the ground, various kinds of reactive forces inevitably arise. Through the block system, they are transferred to the anchor piles, which are the supports of the installation used to create the load. Monitoring of these elements is also carried out with the help of hourly indicators. According to the standards, the maximum permissible value of their pull-out is 0.2 cm. The indications of all installed instruments are necessarily removed at each stage of testing. Then they are registered in a special journal.

Time of rest of supports according to GOST

As already mentioned, piles must be put up before each stage of testing. The rest time of the supports is determined by GOST. Testing of soils with piles using static technology is carried out taking into account their composition. What exactly are the relationships in this case, you can find out from the table below.

Determine the type of soil at the site of building construction by conducting laboratory tests.

Dynamic tests: in which cases are conducted

The advantages of this method are cheapness and small time costs. However, in terms of the accuracy of the results obtained, dynamic tests are inferior to static ones. They are conducted very often and practically on all significant objects simultaneously with the performance of mathematical calculations of the bearing capacity of piles. Equipment in this case, in most cases, is used the same as in the actual construction of the foundation.

Dynamic testing of piles: general scheme

As the pile sinks into the ground, the latter becomes denser. This is manifested in the fact that simultaneously with the deepening of the point, the pile failure is reduced. In the final analysis, the support stops altogether from entering the ground. The dynamic method of calculation, in fact, is an attempt to relate the amount of pile failure and its bearing capacity. In general form this is expressed in the formula: G * H = Fu ∙ Sa + G * h + α ∙ G ∙ H, where:

• G * H - work of the hammer;

• Fu ∙ Sa - immersion work;

• Α ∙ G ∙ H - work on inelastic deformation;

• G ∙ h - work on elastic deformations;

• Sa - pile failure in meters;

• Fu - maximum resistance of the load support (kN);

• A is the conversion coefficient of a part of the energy into a thermal one.

The parameter Sa is usually defined as the average value of a pile immersion in a series of impacts (from 4 to 10). Measure the failure of the tested supports using various devices (a measuring device, a tape measure, a measuring ruler).

Testing of soils with dynamic piles should be carried out using a sufficiently heavy hammer. The fact is that with increasing its weight, the accuracy of calculation increases.

During the tests, the following are recorded:

• Number of hammer blows;

• The amount of failures after diving to a given mark;

• Coefficient of sucking (ratio of failure when driving to failure when finishing).

Features of dynamic tests on different types of soil

Sand with the use of such technology as the piling is evenly compacted. On clay soils, a completely different picture is observed. First, as in the sand, a gradual decrease in failure occurs. But from a certain depth, this indicator stops changing. Sometimes even an increase in failure is observed. In this regard, it may appear that pile driving does not increase their bearing capacity. However, this, of course, is not so. The resistance of clay decreases primarily because of its impact dilution.

It is because of this effect that field testing of soils with piles in such areas is usually done using a static method. In some cases, you can use dynamic. But at the same time, after liquefying the soil, you should take a break for a week or two.

On sandy areas during dynamic tests, sometimes the opposite effect is observed. After a break in driving the piles for a while, the failures begin to increase. This is primarily due to the re-consolidation of the soil under the pile tip. In this regard, according to the standards, dynamic tests on sandy areas are performed three days after pile driving, and on clay - in 6 days.

What you should know about

Dynamic testing of piles in certain situations can show either too high a result of the bearing capacity, or on the contrary - understated. This is primarily due to the layered structure of the soil. The pile can cut through a dense layer and enter a looser layer or vice versa. On such complex soils, often even inaccurate results show even static tests. That is why before starting work on the site, it is necessary to make a thorough geological survey.

What should be guided by

The piles are tested in compliance with the regulations prescribed in the following documents:

• GOST 5686-96 "Soils. Test methods for piles ";

• SNiP 2.02.03-85 "Pile foundations";

• SNiP 3.02.02-87 "Bases, excavations, foundations";

• SP 50-102-203 "Design and design of pile foundations";

• GOST 25258-82 "Method of field temperature determination";

• Interstate standard for conducting geological surveys (Belarus, Kazakhstan, Russia).

Before carrying out static tests, the terms of reference, the design of the work, and the plan of the construction site and its geological section are drawn up . After the completion of all field work and after conducting mathematical expenses, the optimal diameter and length of the piles are determined.