When drawing up a project for any home, among other things, one should also determine what its foundation will be. There are several varieties of grounds for buildings. At the same time, the most reliable type of foundation is pile. Such structures are usually erected on soils with weak bearing capacity. At the same time, piles are buried in the ground until dense layers are reached. Such a solution allows you to give the building maximum stability and completely eliminates the possibility of movement, and therefore the destruction of load-bearing structures. Before constructing a foundation of this type, a procedure such as a pile test is often performed. Such an event is carried out in compliance with the defined SNiP and GOST standards.
What is a pile foundation
For the first time, the foundation of this type was built under the house in 1838. Such a foundation was initially used only in the construction of buildings on water and marshy areas. Later, 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 design of such a base is based on long supports, actually called piles, connected by a horizontal grillage. The latter can be either a continuous concrete slab or a tape. Supports, depending on their design, are either clogged or screwed into the ground.
Pros and cons of pile foundations
Piles, the field testing methods of which are different, are made only of very durable materials. Usually it is steel or reinforced concrete. The main advantages of pile foundations are:
the ability to use on unstable soils;
the ability to work in all weather conditions;
relatively low cost;
high degree of reliability.
Among other things, the advantages of such foundations include the fact that the buildings erected on them practically do not shrink. The disadvantages of pile foundations are very few. But they are, of course, available. The disadvantages of such foundations are primarily:
Basic technologies for determining the bearing capacity
Before erecting the foundation of the designing organization, among other things, it is necessary to decide on such parameters as the diameter and length of piles. Supports in the first place should, of course, reliably hold the structure. But at the same time, during the construction of the foundation, you should also try to avoid unnecessary expenses.
In order to find out exactly what kind of margin of safety this or that support differs, different methods can be used:
The last two methods are also called soil testing by piles. Such procedures can be carried out at any stage of construction - during surveys, during design, when submerging the supports themselves, etc.
In most cases, when determining the bearing capacity of supports, they are simultaneously calculated both by the formula and tests. Further, the smallest of the values obtained is taken as the basis. This ultimately allows you to build the most reliable foundation. Specific field test methods for piles are usually selected depending on the composition of the soil.
SNiP formula
This methodology for determining the degree of resistance of piles 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 - soil resistance of a particular composition under the sole (determined by the table);
A is the base area of the pile;
fi is the average value of the resistance of the side surface of the support (depends on the material of manufacture);
u is the perimeter of the pile;
hi is the thickness of the soil layer.
For a bored pile, the diameter of which can reach up to 1.5 m, and the length up to 40 m when building a house on clay soil, these parameters will have the following values:
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 the table values fi and hi).
Substituting these data into the formula, we can obtain such a result Fdu = 794 * 0.5 + 2.5 * 222 = 952 kN = 95.2 t.
When static pile testing is needed
This technique allows you to determine the bearing capacity of the supports as accurately as possible. However, static testing is usually quite expensive. Another disadvantage of this technology is that it takes a very long time. From the beginning of the work to a specific result when using this technique usually takes at least a week. But more often this time period is even longer.
Static tests are usually carried out only in case of special need. For example, with the strengthening of foundations in the face of the threat of collapse of old buildings. Also, clay soils are often tested using this technology . Different methods of field testing by piles in such areas can be used, including dynamic ones. But static give the most reliable result. The same applies to bulk soils (from construction waste) or loesses.
How are static tests carried out: general scheme
The bearing capacity of piles by this technology is determined by modeling the mechanism of their work on a specially designated site - with the worst soil conditions for this particular object. This allows you to get the most accurate result. Field tests of soils by piles using static technology in several stages are carried out. First, the supports are driven into the ground to a conditional level. Then they are allowed to stand for 3-20 days, depending on the composition of the land. This is necessary in order to restore structural bonds in the soil.
Next, the test subject pile in stages (steps), also with standing, load, each time measuring the draft using special tools. In thawed soils, the total duration of this procedure is usually about 2 days, in permafrost - about 10. The necessary load when testing piles can be created using the following installations:
In addition, static testing of piles can be carried out using pulling loads. In this case, jacks are used as the main equipment.
Preparatory Activities
To begin work on determining the bearing capacity by a static technique, according to the standards, it is possible only after the strength of concrete piles reaches 75%. Supports are pre-inspected for chips and other defects. Only high-quality piles are allowed for testing. If necessary, their heads are reinforced using an external clip. Chips in some cases may be covered with a cement-sand mixture.
One of the piles is prepared as a reference before testing. Before hammering it, among other things, it is checked for straightness. They also carefully examine the places of its docking for wear. Deviations in a straight line at such supports, according to the standards, should not be higher than 10 mm along the entire length in any of the planes.
In addition to inspection, the test of piles involves such a preparatory work as saturation of the soil with moisture. After all, the land under the house in the subsequent dry will not always be. Consequently, the load on the pile may be higher than under normal conditions. To moisten the soil during tests along the perimeter of the site, trenches of 0.5 m wide and 1-1.5 m deep are excavated. Wells are also sometimes drilled (usually in the amount of 3 pieces). Their diameter is most often 20 cm. Wells are also located on the edge of the site.
During testing, ensure that the water level in the mines and trenches remains constant. The minimum soil soaking time SNiP is defined 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 actual static field tests of piles, they make sure that the vertical load is maximally centered. The draft at each stage is measured using hourly indicators with a division value of 0.01 mm.
When piles are immersed in the soil, various kinds of reactive forces inevitably arise. Through a system of blocks they are transmitted to anchor piles, which are the supports used to create the load of the installation. These elements are also monitored using hourly indicators. According to the standards, the maximum permissible value of their pulling out is 0.2 cm. The readings of all installed devices are mandatory taken at each stage of testing. Then they are registered in a special journal.
Rest time of supports according to GOST
As already mentioned, piles are supposed to stand before each test phase. The rest time of the supports is determined by GOST. Testing soils by piles using static technology is carried out taking into account their composition. Which relationships exist in this particular case can be found in the table below.
Pile rest time during testingSoil type | Rest time (day) |
Coarse or coarse sand | 1 |
Ordinary sand | 3 |
Clay soil and dissimilar soils | 6 |
Water saturated sands | 10 |
Determine the type of soil at the place of construction of the building by conducting laboratory tests.
Dynamic tests: in which cases
The advantages of this method are low cost and low time costs. However, in terms of the accuracy of the results obtained, dynamic tests are inferior to static ones. They are carried out very often and practically at all significant objects simultaneously with the implementation of mathematical calculations of the bearing capacity of piles. In most cases, the equipment used is the same as that used in the course of foundation work.
Pile Dynamic Testing: General Layout
As the pile is immersed in the soil, the latter is compacted. This is manifested in the fact that simultaneously with the deepening of the tip, the failure of the pile decreases. Ultimately, the support completely ceases to enter the ground. The dynamic calculation method, in fact, is an attempt to relate the pile failure value and its bearing capacity. In general terms, this is expressed in the formula: G * H = Fu ∙ Sa + G * h + α ∙ G ∙ H, where:
G * H - hammer work;
Fu ∙ Sa - work for immersion;
α ∙ G ∙ H - work on inelastic deformations;
G ∙ h - work on elastic deformations;
Sa - pile failure in meters;
Fu - ultimate support resistance to load (kN);
A is the coefficient of conversion of part of the energy into heat.
The parameter Sa is usually defined as the average value of the pile sinking during a series of impacts (from 4 to 10). The failure of the test supports is measured using various devices (failure meter, tape measure, ruler).
Dynamic soil testing by piles should be carried out using a sufficiently heavy hammer. The fact is that with an increase in its weight, the accuracy of the calculation also increases.
During the tests are recorded:
Features of dynamic testing on different types of soil
Sand with the use of such technology as the piles are driven in uniformly compacts. On clay soils, a completely different picture is observed. At first, as in the sand, a gradual decrease in failures occurs. But from a certain depth, this indicator stops changing. Sometimes there is even an increase in failures. In this regard, it may seem that driving piles does not increase their bearing capacity. However, this, of course, is not so. Clay resistance decreases primarily due to its impact liquefaction.
It is because of this effect that field tests of soils by piles in such areas are usually carried out by the static method. In some cases, you can use dynamic. But at the same time, after liquefying the soil, a break in work for a week or two should be made.
On sandy areas during dynamic tests, the opposite effect is sometimes observed. After a break in driving piles for a while, failures begin to increase. This is due primarily to soil compaction under the tip of the pile. In this regard, according to the standards, dynamic tests on sandy areas are carried out three days after driving piles, and on clay - after 6 days.
What you should know about
Dynamic testing of piles in certain situations can show either too high a result of bearing capacity, or vice versa - underestimated. This is due primarily to the layered structure of the soil. A pile can cut through a dense layer and enter a looser or vice versa. On such complex soils, often inaccurate results even show static tests. That is why, before starting work on the site, thorough geological surveys must be carried out.
What should be guided
Piles are tested in compliance with the standards prescribed by the following documents:
GOST 5686-96 "Soils. Pile test procedures ";
SNiP 2.02.03-85 "Pile foundations";
SNiP 3.02.02-87 “Foundations, earthworks, foundations”;
SP 50-102-203 “Design and construction of pile foundations”;
GOST 25258-82 "Method for the field determination of temperature";
Interstate standard for geological surveys (Belarus, Kazakhstan, Russia).
Before conducting static tests, a technical task, a project for the execution of work are drawn up, as well as a plan of the construction site and its geological section are drawn. At the end of all field work and after mathematical expenses, the optimal diameter and length of piles are determined.