Official edition
STATE COMMITTEE OF THE USSR FOR CONSTRUCTION OF MOSCOW
DEVELOPED by the State Institute for the Design of Foundations and Foundations "Fundamentproekt" of the Ministry of Trade and Construction of the USSR
Director and head of the topic Yu. G. Trofimenkov
Performers: L. G. Mariupolsky, A. A. Obodovsky, A. A. Sherman
WITH
Research Institute of Bases and underground structures (NIIOSP) them. Gersevanov State Construction Committee of the USSR
Director B. S. Fedorov Theme of the topic V. V. Mikheev Executor 6. V. Bakholdin
All-Union Scientific Research Institute of Transport Construction (TSNIIS) Ministry of Transport
Director D. I. Fedorov Head of the topic A. A. Luga Performers: NM Glotov, V. P. Rybchinsky
It was introduced by the Ministry of Assembly and Special construction works the USSR
Deputy. Minister K. K. Lilodat
PREPARED FOR APPROVAL
Department of Technical Regulation and Standardization of USSR Gosstroi
The head of the department V. I. Sychev
Head of Sub-Department of Building Design Standards VI Baiko Chief Specialist L. Ye. Temkin
Department of Scientific Foundations of Standardization TsNIIpromzdany Gosstroy USSR
Head of Department SN Nersesov Art. Research Fellow S. V. Timofeev
APPROVED AND ENTRY INTO ACTION By the Decree of the State Committee of the Council of Ministers of the USSR for Construction Affairs of April 18, 1978 No. 56
4.3.6. The load during the testing of the piles should be brought to a value causing their draft not less than 40 mm, except for cases when the lower ends of the piles penetrate into coarse clastic soils, dense sands, and also clay soils of solid consistency; the load in the latter cases should be brought to the value provided by the test program, but not less than one and a half times the bearing capacity of the pile, determined by the results of static sounding of the soil in accordance with the requirements of Chapter SNiP II-17-77 or to the design load; transferred to the pile.
In the control pile test conducted during the production process, the maximum load should not exceed the load-bearing capacity of the piles, determined from the strength condition of the materials.
Note. In the absence of conditional stabilization during the day, the tests are terminated regardless of the amount of precipitation.
4.3.7. Unloading of the tested piles is performed after reaching the maximum load (Section 4.3.6) in steps equal to twice the values of the loading stages.
4.3.8. Observations of the elastic movement (draft) of the pile should be carried out at each unloading stage for 15 minutes.
After complete unloading (to zero), observations of the elastic movement should be carried out for 0.5 h - with sandy soils under the lower end of the pile, and 1 h - in clay soils, taking samples every 15 minutes.
4.3.9. When carrying out pile tests in winter time, the requirements of paragraph 2.7 should be met.
4.4. Results analysis
4.4.1. During the pile test with static axial pressing forces, a test log shall be maintained (recommended annex 3).
4.4.2. The results of the tests of each pile shall be drawn up in the form of graphs of the dependence of the sedimentation (displacement) on the load S = f (P) and the change in precipitation (displacement) in time over the loading stages S = f (t) (recommended annex 5).
The scale of the graphs is adopted: vertically - 1 cm, equal to 1 mm of displacement; horizontally - 1 cm, equal to 5 tons of load and 1 mm, equal to 10 minutes of the load holding time. It is allowed to reduce the scale of the graphs while maintaining the correlation between the above-mentioned scales of vertical and horizontal coordinates.
ZGDK 624.154.001.4 The HOV Group
STATE STANDARD OF THE UNION OF SSR
Methods of field testing
Sield test methods
guest
5686-78
The Decree of the State Committee of the Council of Ministers of the USSR on Affairs of Construction dated April 18, 1978, No. 56,
This standard is applied to methods of field trials of piles of all kinds, including piles and shells and piles (hereinafter piles), regardless of their material, method of immersion, or a device in the ground (driven, vibro-impregnated, rammed , boronabivnyh, etc.), conducted in a complex of design and survey works, and control tests during construction.
The standard does not apply to pile test methods: in permafrost soils; with soaking of soils; loads simulating seismic and dynamic impacts (including from technological equipment, etc.); in pile bushes, as well as on methods of long-term testing and special tests for research purposes.
1.1. Field tests of piles are carried out by the loads indicated in the table.
from 01/01/1979.
Non-observance of the standard is prosecuted by law
1. GENERAL PROVISIONS
^ Official publication
Reprint prohibited
© Publishing House of Standards, 1978
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Types of loads used for field testing of piles |
Types of tested piles |
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Dynamic rational) |
(drums and vib- |
Submerged all kinds |
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Axial pressing |
Submerged and arranged in the ground of all kinds |
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Static (step- increasing) |
Axial pulling |
Submerged and arranged in the ground of all types, except for concrete and submerged - spaced along the length, as well as pre-stressed reinforced concrete without transverse reinforcement |
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Horizontal |
Submerged and arranged in the ground of all types, except for concrete |
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1.2. Field tests of other types of static loads, not provided for in Section 1.1 (continuously increasing * alternating and successively increasing with full unloading of each load stage to zero) should be carried out according to special programs.
1.3. Field tests of piles should be carried out in a complex of design and survey works in order to obtain data * necessary to justify the selection of the project foundations and determine their (Parameters, including:
check the possibility of piling the piles for the planned depth, as well as a relative assessment of the homogeneity of the soil on their resistance to piling;
determine the dependence of the movement of piles in the soil from the load.
1.4. Field tests of piles during construction should be carried out with the aim of controlling the compliance of their bearing capacity with the design loads specified in the design of the pile foundation.
1.5. Field tests of piles in the complex of design and survey works should be carried out according to the program meeting the requirements given in the mandatory Annex 1.
1.6. The number of piles subject to field tests conducted in the complex of design and survey works (paragraph 1.3) is established and justified by the program.
Tests of piles should be carried out on the site allocated for the construction of the designed buildings and structures, at a distance of no more than 5 and not less than 1 m from the mine workings, from which monoliths of soils are selected for laboratory research.
and where field completions are carried out, as well as from static ground probing.
1.7. The number of piles subject to control tests during construction (and 1.4) is established by the technical specification within the following limits:
when tested dynamic (shock and vibration) load - up to 1% of the total number of piles in this facility, but not less than 5 pieces;
when tested by a static pressing load - up to 0.5% of the total number of piles on this object, but not less than 2 pieces;
when tested by static pulling or horizontal load - the number of piles is established by the test program, but not less than 2 pieces.
1.8. The cost of dividing the pressure gauges used in the field tests of the shafts with static loads is determined depending on the maximum load that is supposed to be transferred to the pile, the number of jacks and the area of the jack piston.
1.9. During the field testing of piles, test logs (recommended annexes 2 and 3) should be maintained, and the test results should be recorded in the form of graphs (recommended annexes 4-7).
1.10. The bearing capacity of piles according to the results of field tests should be determined in accordance with chapter SNiP II-17-77.
2. PREPARATION FOR TESTS
2.1. Pile driving piles designed for field testing must be checked for compliance prior to immersion in the ground:
3.2.2. Pile reinforcement (section 3.3.1) should be made with the same equipment used for driving, with a head restraint having a dented wooden gasket.
3.2.3. Accuracy of measurement of failures should be not less than 1 mm.
In the presence of appropriate measuring instruments, the residual and elastic movements of the piles must be separately fixed.
3.3. Testing
3.3.1. Pile tests with dynamic loads should include:
when driving piles - counting the number of hammer blows for each meter of immersion and total number of impacts, and on the last meter - for every 10 cm of diving;
with vibration pile driving - the time calculations for each meter of immersion and total dive time, and on the last meter - the time for every 10 cm of immersion;
determination of failures of driven piles during finishing after "rest", i.e., after a break between the end of piling and the beginning of the finishing.
3.3.2. The duration of "rest" is determined by the program of field tests, depending on the composition, properties and condition of the cut ground and soil under the lower end of the piles, but not less than:
3 days - with sandy soils, except for water-saturated small and silty soils;
b day - with clay or dissimilar soils.
Notes:
1. When cutting sandy soils and presence of large-fragmented, dense sandy or clayey soils of solid consistency under the pile edge, the duration of "rest" may be reduced to 1 day.
2. A longer period of "rest" (except bridges and transport hydraulic structures) should be installed when cutting water-saturated fine and silty sands - at least 10 days; at a cut-through of clayey soils of soft and fluid-like consistency - not less than 20 days.
3.3.3. Piles should be pierced with successive collateral of 3 and 5 strokes. The height of the impact of the hammer impact part must be the same for all strokes.
For the settlement take the greatest average failure.
Note. If it is necessary to control the failure at the end of the production, it is permissible to perform an additional 30 strokes after the strike. In this case, the average failure of the last 10 strokes is assumed equal to the failure at the end of the production pile.
3.4. Processing test results
3.4.1. During the pile test, a dynamic test (shock or vibration) load shall be carried out in accordance with the recommended annex 2.
3.4.2. The test results for each pile shall be in the form of a graph of the change in depth failure and the dependence of the total number of impacts on the depth of immersion (recommended annex 4).
The scale of the graphs is adopted: vertically - the depth of piling 1: 100; horizontally - failure of 1: 1 and 1 cm, equal to 50 shocks during driving and 1 min during vibration.
4. METHOD OF PULL TESTING BY STATIC AXIAL IMPACT LOADS
4.1. Tests of piles with static axial pressing "loads are carried out to establish their bearing capacity for indentation and the dependence of displacements in the soil from loads.
4.2. Equipment
4.2.1. The equipment used for testing piles with static pressing loads, depending on the loading method of the pile being tested and the reactive forces perception system (recommended annex 8) is divided into:
installations in which the emphasis for the hydraulic jack is a system of beams or trusses fixed to anchor piles;
installations in which the hydraulic platform is supported by a cargo platform;
installations with a calibrated cargo;
combined plants in which the hydraulic jack is supported by a cargo platform and a system of beams or trusses with anchor piles.
The test equipment shall include a system of stops, jacks or a calibrated load, a reference system with measuring instruments.
4.2.2. Load on the pile must be transmitted centrally and coaxially.
Note. When using units with hydraulic jacks, the construction of the jack support on the pile should ensure a strict alignment of the transmitted load with the axis of the pile.
When using the load platform, the weight of each load element, as well as the weight of the platform, must be calibrated and marked with an indelible paint. In order to exclude the possibility of transferring the load to the tested pile prior to the installation of measuring instruments and taking samples from them and to provide safety equipment for the work, the cargo platform must be mounted on special supports.
4.2.3. When testing piles with installations with anchor piles, the number of the latter should be assigned depending on the maximum test load given by the program, the magnitude of the ultimate resistance of anchor piles to pulling (on the ground), and their tensile strength in the material.
4.2.4. The depth of immersion or anchoring of anchor piles must not exceed the depth of immersion or the laying of the test pile.
4.2.5. The distance in the axes from the tested pile to the anchor or to the nearest support of the loading platform, and also to the supports of the bench set, must be at least 5 largest dimensions of the cross section of the pile (up to 800 mm in diameter).
When testing piles, shells, piles with a diameter of more than 800 mm, piles with a widened heel, and screw piles The distance between the tested and anchored piles in the light is allowed to be reduced to 2 tf, where d is the diameter of the pile-sheath, the pile, the broadened heel (for piles with a broadened heel) or the screwed-on blade (for screw piles).
4.2.6. The greatest deflection of the inventory structure, which serves as an emphasis for the jack, should be no more than 0.004 of its estimated span.
4.2.7. Measuring the movement of piles must be made by special instruments (indicators, progibomers, etc.) with an accuracy of at least 0.1 mm; the number of instruments installed symmetrically on equal (no more than 2 m) distances from the tested pile, should be at least two.
The size of the pile's movement is defined as the arithmetic average of the results of the readings of all devices.
4.2.8. When using the deflectors, a steel wire with a diameter of 0.3 mm should be used, which must be pre-stretched for 2 days with a load of 4 kgf before starting the measurement. When testing, the size of the wire on the wire should be 1.0-1.5 kgf.
Measuring instruments should be protected from direct exposure to sunlight, strong wind, sand dust and precipitation.
4.2.9. The reference system for the installation of indicators, deflectors, etc. should be isolated from accidental shocks during operation, and its design - to exclude the possibility of thermal deformations of the system and the effect of deformations of the ground. When carrying out tests in water areas, the reference system should be performed in accordance with the design.
4.2.10. Instruments used to measure the movement of piles, as well as manometers (when used in tests hydraulic jacks) must be harnessed.
Instruments and other equipment should be periodically checked in accordance with the passport requirements. Prior to shipment of equipment and equipment to the test site, an extraordinary inspection must be carried out.
4.3. Testing
4.3.1. Testing of static pile driving piles should be started after their "rest" in accordance with paragraph 3.3.2.
For piles, loaded in other ways, the beginning of tests is appointed by the test program, but not earlier than 1 day after their immersion.
Printed (bored) piles should be tested after the concrete piles have reached the design strength.
4.3.2. Loading of the tested pile should be made evenly, without impacts, with load stages, the value of which is determined by the test program, but not more than the maximum load given to the pile in the program.
When the lower ends of piles are buried into coarse-grained soils, gravelly and dense sands, and also clay soils of solid consistency, it is permissible to take the first 3 load steps as equal to * / s given in the program the greatest load on the pile.
4.3.3. At each load stage, the piles are taken from all instruments: the first reading is immediately after the load is applied, then four consecutive readings at intervals of 15 minutes, two counts at intervals of 30 minutes and then every hour until the decay of the movement (precipitation) stabilization (Section 4.3.4).
Differences in instrument readings should not exceed:
50% - when precipitation is less than 1 mm;
30% - with precipitation from 1 to 5 mm;
20% - with precipitation exceeding 5 mm.
4.3.4. For conditional stabilization, the speed of settling (moving) the pile in the soil is assumed:
no more than 0.1 mm for the last hour of observations if sandy soils or clay soils from hard to turgid consistency lie under the lower end of the test pile;
not more than 0.1 mm in the last two hours of observation if clay soils occur from the soft-plastic to the flowing consistency under the lower end of the pile.
4.3.5. When testing piles of bridge foundations, the sediment at this stage of loading is considered stabilized if it is:
no more than 0.1 mm over the last 30 minutes - when supported on coarse-grained and sandy soils; clay soils of solid consistency;
not more than 0.1 mm in the last hour - when supported on clay soils of semi-solid and turgid plastic consistency.
Note. With the appropriate justification, it is allowed to test the piles without a conditional stabilization of movements.
Before you send piles to the construction market, they need to be tested. For this, dynamic and static tests of piles are used. For this procedure there is GOST 5686-94, which indicates the necessary parameters for the right choice base construction. Such GOST tests allow you to do the following:
- Find the optimal geometry - the technology makes it possible to measure the area, length and other parameters of the section;
- Determine the possibility to the required depth;
- To study the behavior of the support under load;
- Measure the depth to which the rod can be immersed in the ground;
- Determine the degree of homogeneity of the soil;
- Find the bearing capacity.
This result is provided by both static and dynamic checks. The emphasis in this material will be placed on static testing, as they are more often used in production. We will also devote a couple of items to dynamic testing. Such inspections are an advantageous alternative to research on soil characteristics that are conducted in the laboratory. As a result, the same parameters are determined: the depth of the rods, as well as the dimensions. For this reason, the GOST regulatory document contains a whole section of these tests.
The diagram shows the installation with which the pile tests are carried out. It consists of hydraulic jacks, anchorages, and also from beams.
The test begins with the number of rods and the place where they will dive. GOST requires that the piles are clogged on the site with the worst soil conditions that may be in the area. The verification technology provides for a preliminary "rest", the support should be set aside, so that the land connection is fully restored. Only then will the tests yield objective results. GOST sets the time of such sludge, which depends on different conditions:
- 1 day for dense soils saturated with sand or clay, as well as coarse clastic soil types;
- 3 days should be left in case when the pile will be submerged in sandy grounds;
- 6 days - a heterogeneous land, clay;
- 10 days is required to test the strength of piles in sands saturated with moisture.
Most often the support is in the soil for 6 days - this optimal term for soils that prevail in the territory of the Russian Federation and CIS countries. The pile, which is subjected to tests according to GOST, must be loaded with steps. When there is a 100-percent stabilization of the stage, the transition to the next level occurs. It is important to accurately measure the draft. To do this, we use deflectors - there are electronic variants and models of the watch type. Before loading, it is necessary to take zero values for all devices. These parameters should be removed after each step.
Test methods
We described the theory of GOST, now you can go to the methods. To date, there are three main ones:
- With the help of its own mass of piles, this method is suitable for soft soils where a minimum of additional effort is required to dive to the required depth;
- Due to the platform with a load, which is installed on the test support;
- Using hydraulic jacks is the most common way for our areas.
The method with the use of hydraulic jacks is the most profitable, since it is inexpensive and takes a minimum amount of time. Choosing a method, experts are guided by the regulatory documents of GOST, in which there are many items that help to choose the optimal test method. Technology can also be different. Tests of piles with static loads can be carried out not only in the test areas, but also before the construction on the site.
Dynamic testing
In addition to static checking, in some cases dynamic tests are also required. They differ mainly in that the readings are taken during the immersion of the support in the ground. As the entrance to the soil decreases the failure of the support. Here the connection between the load-carrying capacity of the rod and the energy from impact is important - a special hammer is usually used.
Such tests allow you to specify the necessary dimensions (length and radius) and also to find out whether the calculated index corresponds to the actual one. Observations show how the pile reacts to piling. Also, specialists after dynamic tests can find weak areas of the reference zone. The process is accompanied by the creation of graphs, giving the characteristics of the support, depending on different loads.
The main question: are there any advantages to such tests before the static tests? Yes, the pros are as follows:
- Dynamic testing has increased mobility;
- For such checks, less energy and time is spent;
- With dynamic loads, you can test any types of these structures.
There is a significant drawback in dynamic methods - they can give an overestimate of the load-carrying capacity of rods.
How are dynamic tests carried out?
We already know which technology is used for static checks, now it's time to get acquainted with the GOST tests, which are conducted using dynamics. According to GOST it is established that they need to be carried out at least three times. First, a test of the supports before the start of the pile foundation laying is performed. This must be done in order to know the level of heterogeneity of the soil in the area where the future structure will stand.
The second part is performed already when the rods are clogged in the ground. This stage is carried out in order to find out what bearing qualities they possess. After this, the final stage begins. He demonstrates the most accurate indicators, since the supports are already properly "rested". Sludge depends on the conditions. For example, in soil rich in sand piles can rest about 3 days, and in clay soils - up to 6 days.
Such researches allow to define bearing layers, to find out weak points of a basic zone, and also to learn bearing capacity of already immersed supports. As for the equipment, the same mechanisms and devices are used here, as in the case of static tests. After the tests, the specialists have all the necessary indications, and they can begin the basic construction work.
Static Load Testing updated: July 12, 2016 by the author: zoomfund
Read on The content of the articleStatic pile testing in accordance with GOST 5686-94 is used to determine the parameters influencing the procedure for selecting the foundation design. During such tests, it is possible to determine the optimal geometry of the pile (length and sectional area), to measure the depth of the pile immersion, and also the very possibility of penetrating the support to a given depth.
In addition, during the field tests, it is possible to calculate the bearing capacity and the degree of homogeneity of the soil at a given depth of immersion, and also monitor the behavior of the pile under load.
As a result, field testing of piles with static load can be called an acceptable alternative to complex, laboratory studies of soil characteristics, which in effect determine all the same parameters: dimensions and depth of pile placement.
That is why this section of field trials is devoted to a whole section in normative document, regulating the process of studying the characteristics of the soil - GOST 5686-94.
Static and dynamic testing of piles - theory
Testing of piles with static load occurs by axial pressing of the support into the ground. That is, the static vertical load acts on the pile. In the dynamic version, the static (constantly acting) load is replaced by the dynamic (shock) load.
The test itself takes place right on the construction site, before the design of the foundation construction. And in case of revealing of weak horizons of a ground the test is made without fail.
The procedure should be carried out after 72-144 hours from the moment of penetration of the driven pile. Well, a static test of bored piles begins after the final solidification of the body of a monolithic pile, controlled by a control sample on the surface of the soil.
The test is carried out by uniformly or differentially increasing the load on the pile. Forces increase gradually, increasing the "steps" of the load. And the uniform test assumes a 10-percent stage of load increase (from the maximum, calculated value). A differentiated test begins with a 20 percent and ends with a 10 percent step-by-step increase in the load.
Moreover, in either case, 10 or 20 percent load build-up occurs after the stabilization of the support is completely under the weight of the previous stage. That is, the next test of supports by pressing loads starts after stopping the support, provoked by the previous weight, into the ground. Moreover, the support should not move for at least two hours.
What do you need for testing?
Three types of supports are involved in the tests:
- Full-size pile - dimensions, material and method of laying which correspond similar characteristics and the methods used in real construction. That is, the field pile used in testing the bored basement must be equipped according to all the rules for mounting such supports (drilling the shaft, assembling the formwork and reinforcement, pouring concrete into the formwork).
- The reference pile - is an all-metal rod with a diameter of 114 millimeters, simulating a pile-type pile. The length of the standard (consisting of segments) in the assembled position is 12 meters.
- Pile-probe - is a metal rod with a diameter of 127 millimeters, equipped with a conical tip. The length of the segment probe in the assembled position is 16 meters.
In addition, on these supports should be based support structure - the cargo platform, necessary for the perception of reactive forces from gradually increasing 
load. And, of course, the set of equipment must include "sources" of load - tar cargoes, jacks and stuff. In a word, everything that will ensure the influx of reactive forces onto the cargo platform (or the beam system).
And this set of equipment must provide a strictly vertical direction of the load, directed along the central axis of the pile. Well, piles standards and piles-probes should be equipped with a mechanism for extracting the buried support from the ground. After all, leaving such an instrument in the ground will be the height of waste.
To fix the result of tests in this process, instruments are used that measure the depth of immersion of the support in the ground. In addition, an accurate chronometer is needed to fix the time intervals.
As a result, it can be said that the accuracy, as well as the cost of testing bored piles, depend on the number of supports and the accuracy of the measuring instruments. And the higher the accuracy of measurements, the higher the price of the entire procedure, respectively.
How are static pile tests carried out?
The process of static testing consists of three stages:
- Preparation of support.
- Support tests.
- Processing of the results.
This preparation begins with the penetration of the actual or reference pile into the ground building site. And, if research takes place in winter, then the soil must be warmed to the full depth of the soil freezing. And in the dry season, the soil should be moistened to the level of average annual soil moisture.
In addition, at the same stage, the support structure and the set of equipment responsible for the load are installed. Well, the preliminary stage is completed after the end of the rest period of the pile - 3-6 days after installation of the support in the ground.
The test of a support under the action of a static load occurs as follows:
- Initially, a test program is developed in which the maximum load on the support is declared and the load distribution step is calculated equal to 10 percent of the weight of the calibrated cargo. And with the penetration of the support into the dense ground, the first step can be equal to 20 percent of the total weight of the calibrated cargo.
- After this, you can proceed to install the source of the "step" load on the load platform. And each next "step" is done after fixing the consequences of the previous one, performed after the pile stabilizes completely. That is, before installing the next consignment on the platform, the pile should remain without movement for 1.5-2 hours. Of course, all the indicators taken from the measuring instruments are fixed in a special journal, where the process of immersing the support is linked to the weight of the load.
The second stage - the test process - is realized up to the peak load values. And under the peak values understand such a load, under the influence of which the field pile sags by 40 or more millimeters, and the reference samples are buried into the ground by 20 or more millimeters.
The third stage - the processing of results - is to draw test plots based on the log of observations. At the same time, conclusions are drawn regarding the data of interest to the testers. As a result, the architect of the project takes the information necessary to determine the geometry, depth of the deposit and the number of real piles. These data will be used in the process of designing this foundation.
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. In this case, the most reliable is the pile. They erect such structures usually 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 SNiP and GOST standards.
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 swampy 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 is 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, or 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 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 include the fact that the buildings erected on them practically do not shrink. Disadvantages of pile foundations are very few. But they are, of course, there are. The disadvantages of such foundations are first of all:
the need for the use of special equipment during erection;
impossibility of arrangement under the basement.
Basic technologies for determining bearing capacity
Before the construction of the foundation of the project organization, among other things, it is necessary to determine and with such parameters as the diameter and length of the piles. The supports in the first place must, 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 how the strength margin differs in one or another support, different methods can be used:
mathematical calculation, taking into account the requirements of SNiP;
static testing and steel;
dynamic test.
The last two methods are called the pile test with 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 for 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 as follows: 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 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. Also, the disadvantage of this technology is 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, with the threat of collapse of buildings of old buildings. Clayey soils are often checked 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 carried out: the general scheme
Determine the load-bearing capacity of piles for 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 clogged to 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 soil.
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 anchor piles and a beam system;
with a 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, jacks are used as the main equipment.
Preparatory activities
Begin work to determine the load-carrying capacity according to 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. Chipping in some cases is allowed to be closed with 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, testing of piles involves carrying out this kind of preparatory work, as the 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 a quantity 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 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 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.
poles according to GOST
As already mentioned, before each stage of testing, the pile should be raised. The rest time of the supports is determined by GOST. Testing of soils with piles using static technology is performed taking into account their composition. What exactly are the relations in this case, you can find out from the table below.
Determine the type of soil on 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 penetration 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. AT general view this is expressed in the formula: G * H = Fu ∙ Sa + G * h + α ∙ G ∙ H, where:
G * H - the work of the hammer;
Fu ∙ Sa - work on diving;
α ∙ G ∙ H - work on inelastic deformation;
G ∙ h - work on elastic deformations;
Sa - pile failure in meters;
Fu - ultimate load resistance of the support (kN);
A is the conversion coefficient of a part of the energy into a thermal one.
The parameter Sa is usually determined as the average value of the pile dive in a series of impacts (from 4 to 10). Measure the failure of the test support with various devices (a measuring device, a tape measure, a measuring ruler).
Testing of soils with dynamic piles should be performed 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 application of this 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 on such sites is usually done using a static method. In some cases, you can use dynamic. But with the dilution of 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 connection, according to the standards, dynamic tests on sandy areas are performed three days after pile driving, and on clay ones - after 6 days.
What you should know about
Dynamic testing of piles in certain situations may show either too high a result of the bearing capacity, or vice versa - understated. This is primarily due to the layered structure of the soil. The pile can cut through a dense layer and enter into a looser layer or vice versa. On such complex soils, often even inaccurate results show even static tests. That is why before the start of 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. Methods of testing with 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 the static tests, the terms of reference, the design of the work, and the plan of the construction site and it are drawn up. After the completion of all field work and after carrying out mathematical calculations, the optimal diameter and length of the piles are determined.
Static tests of piles are carried out in accordance with the requirements:
- GOST 5686-94 "Soils. Methods of field testing with piles ";
SNiP 2.02.03-85 "Pile foundations";
SP 50-102-2003 "Design and construction of pile foundations"
Before the start of the work, our specialists develop and approve the "Pile Testing Program". The static tests of piles can be carried out at different stages of construction and design - at the research stage, before the working design, during the piling process, when accepting submerged piles.
Depending on the stage, the goals of static pile tests will also vary:
At the exploration stage, static tests of piles are carried out in order to select the length and cross-section of the piles and to estimate their bearing capacity;
During the immersion and during the removal of submerged piles, the goal of static tests will be to determine the correspondence between the actual bearing capacity of the piles and the comparison of it with the calculated bearing capacity assumed in the design.
The data obtained during static testing of piles, as a rule, is significantly more accurate and reliable than in dynamic pile tests.
At the same time, static tests of piles are more complicated, expensive and labor-consuming in comparison with dynamic ones, and in this connection are assigned mainly for the construction of complex and large objects with a large number of piles in the foundation
Technology of static testing of piles
Static testing of piles begins with the design organization determining the number of test piles and the locations of their piling. After this, several test piles are dipped in certain places. Tests during plugging and acceptance are made on piles located in places with the worst soil conditions for the given site or giving the greatest failures during clogging.
Prior to testing, the piles must be stabilized in order to restore structural bonds in the ground and, accordingly, the pile showed real results. Time so-called. "rest" piles before testing in accordance with GOST is:
1 day - if there are coarse clastic soils under the tip of the pile, or dense sands
3 days - for sandy soils
6 days - for clay and dissimilar soils
10 days for water-saturated sands.
In most cases, the time for "resting" of the pile is 6 days from the moment of blockage.
The test pile is loaded with steps, the transition to the next stage of loading is carried out after the conditioned stabilization of the precipitate in the previous stage. To measure the sludge of the pile being tested, hour-type deflectors with a 0.01 mm division or with an electronic dial are installed.
Before loading the piles, zero samples are taken for all instruments. At each stage of pile loading, readings are taken for all instruments.
The criterion for conditional stabilization of deformation is the rate of settling of the pile at a given loading stage, not exceeding 0.1 mm for the last 60 or 120 minutes of observations.
For the particular value of the limiting resistance of the pile under test, a load is adopted, at which the pile loading is stopped.
Schemes of installations for static testing of piles:
Legend:
1 - tested pile
2 - anchor pile
3 - Repeater system with programmers
4 - a dummy with a manometer
5 - the system of stops, beams
8 - grus (stop for the house)
Methods of static testing of piles
The choice of equipment for static testing of the pile depends on the adopted loading method.
Now the following loading methods are known:
Stacking of cargo on the platform, installed on the pile;
Use the force of hydraulic jacks;
Use of self-weight IED.
The method of static testing of piles with the use of hydraulic jacks - the least labor-consuming and the most inexpensive - has prevailed. Specialists of our company often use the weight of the piling machine to load the pile, which allows our customers to save up to 50% of the funds on tests.
Dynamic testing of piles
Works connected with the construction piling foundation do not do without testing the existing piles. In addition to the static testing of the piles, the piles are also subjected to dynamic loading tests. As the pile is immersed, soil resistance to penetration of the pile increases. Externally, this is manifested in the fact that with the penetration of the point into the ground, the pile's failure decreases, that is, the amount of its immersion from a single hammer blow. Dynamic tests of piles are based on the connection between the impact energy of a hammer when driving a pile into the ground and the bearing capacity of a pile.
During trial pile driving, dynamic tests of piles allow you to assign a rational length of piles and check the compliance of the actual and calculated values of pile failures. When driving the working piles, observations of changes in failures make it possible to identify carrier layers of soil, give a relative assessment of the bearing capacity of hammered piles and identify weak sections of the pile field. During the dynamic testing of the piles, graphs are drawn that describe the changes in the pile state, depending on the loads applied to it.
Dynamic testing of piles have some advantages over static testing of piles - they are more mobile, do not require high costs, is applied to any type of pile regardless of their bearing capacity. But at the same time, the dynamic test method of piles can give an increased value of the load-carrying capacity of the piles. This is possible if the pile cuts through the thick ground with a thicker soil and enters the weaker layer, which has a greater compressibility. It should be noted that in this case the static method of testing piles can be misleading. The point is that in such soil conditions, with a prolonged action on the static load pile, due to creep strains, the load is redistributed and its share significantly increases at the tip of the pile, which causes an overload of the weak foundation soil. Therefore, in multilayered strata, it is necessary that the points of the piles enter a more durable underburden of soil.
In clay soils (homogeneous within the foundation of a building), when piles are driven in the same depth, the magnitude of failures, both at the end of the piling and in time, can vary greatly for different piles, which may lead to incorrect conclusions about their very different load-bearing capacity. However, in this case, the results of the dynamic tests of the piles are compared with the results of static tests, which show the same level of pile resistance.
The dynamic method of testing piles is also unsuitable for the construction of pile foundations on loose foundations of sand, construction waste, household dumps, and the like.
Technology of dynamic tests of piles
As a rule, dynamic tests of piles are carried out three times. Initially, dynamic tests of existing piles are carried out before the start of the main pile works and even before the work on the pile foundation project began. This is done to determine the level of heterogeneity of the soil in the place of future construction.
The next stage of dynamic tests is carried out at the moment of piling the main piles into the ground - in order to assess their bearing qualities and capabilities, as well as to determine the bearing layers in the ground and weak areas in the area where the piles are hammered. Upon completion of the pile works, the piles undergo another dynamic test for a more reliable determination of the load-carrying capacity of the piles after they have "rested". The duration of "resting" of piles in cohesive clay soils is close to six days, and in sandy soils it takes at least three days from the end of the piling.
When driving the working piles, observations of changes in failures make it possible to identify carrier layers of soil, give a relative assessment of the bearing capacity of hammered piles and identify weak sections of the pile field. The control paving of the piles reveals changes in the bearing capacity of the piles after "rest". It must be carried out by the same hammer that piles were driven. In clay soils, it should be produced in a short series of impacts in order not to disturb the soil structure again
Dynamic tests of piles are carried out using the same equipment that is used for basic pile work. After all the tests, the amount of pile failure is obtained equal to the degree of sinking the pile into the ground after one stroke. Further, the necessary calculations are made to determine the load-bearing capacity of the clogged pile. At the same time, the accuracy of the obtained fault data depends entirely on the accuracy of calculating the height of the hammer and the weight of its impact part, as well as the weight of the pile itself and the head restraint. Do not also forget about the accuracy of the measurements of the elastic movements of the pile and soil after the impact.
To measure failure in the dynamic testing of piles, the level is mainly used. The accuracy of the fixed elastic movements of the pile and soil by a level is 1 mm. During driving test piles and during test tests (acceptance of clogged piles), dynamic tests of piles are made only after "resting" piles. Legend:
To correctly determine the pile's load-bearing capacity by a dynamic method, it is important to have a sufficiently accurate measurement of the hammer's fall height. To do this, you usually use a strip with clear divisions after 5 cm, attached to the hammer or pile head restraint. In this way, you can visually determine the height of the hammer drop with the required accuracy of up to 2 cm.