Design organizations in the development of the design part of the documentation using pile foundations calculate the bearing capacity of the pile based on the data of engineering and geological surveys.
To get the actual value bearing capacity piles, before the start of mass driving, the project provides for the so-called. test piles, after receiving the test result of which, the designer decides to confirm the number of piles accepted by the project, or makes changes to the design of the pile field, changing the length or pitch of the piles.
To answer questions and quantify individual causes of growth effects, appropriate large-scale field trials must be conducted in conjunction with computer simulations. Computer simulations are needed to study various causes separately from each other. The following list is recommended to the applicant to clarify the above issues: computer simulations to identify the various causes of escalating effects and their impact on the time-dependent behavior of the piles.
What is a pile foundation
Data from long-term field trials can be used as a basis for the design of large-scale long-term field trials to prove individual causes of growth effects separately. Field trials would make sense to validate computer simulations.
Resting the piles before testing
Before testing, the piles must be settled in order to restore structural bonds in the soil and, accordingly, the pile showed real results. Time so-called. "rest" of the pile before testing in accordance with GOST is:
1 day - if there are coarse soils or dense sands under the point of the pile
Final reports
Laboratory tests for pinpoint verification of computer simulations and validation of the displayed material models of the investigated effects. Based on the experience of long-term laboratory tests, it is necessary to develop an experimental device with which the temporary effects of growth can be demonstrated without the influence of external disturbances. Conclusion of assessment bases, taking into account the impact on the piles. The final reports of the joint project “Comparison of existing substance laws and procedures using model calculations for thermomechanical behavior and rock salt healing” can be found at the link below.
3 days - for sandy soils
6 days - for clay and dissimilar soils
10 days - for water-saturated sands
For Moscow and the Moscow region, in 95% of cases, the pile "rest" time is taken 6 days from the moment of driving.
Dynamic testing of piles
The essence of the dynamic test is to measure the failure (settlement of the pile) when dropping a diesel hammer onto the head of a pile already loaded to the design level. This type of test can be used for any type driven piles... You can also test the load-bearing capacity of the clogged pipes (sheet pile).
Exams will be held on Thursday. The symposium will be held at. However, you can do other analyzes so that the calculator has more elements of judgment about fund behavior. However, other analyzes can be done so that the engineer can integrate more elements into a judgment regarding foundation behavior.
Static test technology
In the geotechnical work of foundations carried out in South America, a lack of technical control and field testing is notorious. In the previous assessment, the lifting capacity of the foundations is the same as multiple load tests are performed.
Diesel hammer, without fuel supply, that is, "idle" is dropped from a certain height (for hammers with a 2.5 ton impact part used by our company, the drop height is 1.8 meters). Next, the laboratory assistant measures the settlement of the pile using a special refusal device, or with an ordinary level. If the draft is too low, the value of the draft (failure) is recorded from 10 blank strokes. Based on the results obtained, the laboratory calculates the bearing capacity of the pile and issues a report in the prescribed form with all graphs and calculations.
Static testing of bored piles
In the area of \u200b\u200bload testing, there are those who prefer dynamic load tests for the lowest cost and speed, and those who prefer static load tests, despite their higher cost, to provide more direct analysis of the results.
For static load tests, both on the plate and on the foundation members, they can be performed slowly or quickly, with the slowest test being the most common. The lifting capacity is usually related to the target settlement level, which is taken as a fraction of the diameter of the foundation element or, taking into account the differential settlement allowed by the superstructure.
The advantages of dynamic testing:
Low cost
The time spent on the test is no more than 20 minutes (you can get a report by the end of the day)
Fairly accurate bearing capacity indicators
Typically, the designer assigns dynamic tests to 1% of the total number of piles.
An example of a dynamic pile test report.
Dynamic tests: in what cases are carried out
Typically, the focus is on determining bandwidthand little is discussed in relation to settlements. In this scenario, this article is based on the results of several slow load tests carried out in the Brazilian Federal District on piles, stacks and slabs, although some of them are only illustrated. No test was conducted on an instrumental basis, which led to some difficulties in interpretation and in the proposal for additional analysis presented in this article.
Static pile testing
The essence of the static pile test is to "load" the driven pile from above and to track its settlement with a stepwise increase in the load. There are several options for how to push the pile. You can build a special stand and, for example, use a crane to add concrete blocks from above. But it is much easier to create a system using anchor piles. This test system is used by our company at every facility. Pay attention to the photo below. In the middle, a small blue device is a hydraulic jack. It stands between the test pile and a large metal beam. With increasing pressure, the jack resting on a beam (or a system of beams) presses on our test pile with a given load. In turn, the beam is attached to the so-called. "anchor" piles, for which the heads of the anchor piles are broken, and the reinforcement of the piles is welded to the beam. Anchor piles, depending on the expected load, can be from 2 to 6 pieces. The jack is equipped with a pressure gauge. The pile is loaded in steps. Settlement indicators at each stage are recorded. If the sediment exceeds a certain value, the tests are stopped and the pressure readings at the previous stage are recorded in the test result.
Testing of permafrost soils with static pressing and pulling loads
In tropical regions, the structural aspects of the soil profile have great importancebecause, although in the foundation element, its side region, which creates lateral friction, occurs mainly in deeply weathered, unsaturated, kaolinite and rich iron and aluminum oxyhydroxides, formed by accumulations, porous and therefore well drained; On the other hand, its tip almost always sits on young, saturated soils with lower porosity, with primary and sometimes expansive minerals.
The main advantage of static testing is that the test simulates the actual operation of the pile in the foundation. But the cost of such a test is approximately 10 times higher in comparison with the "dynamics".
On average, the cost of testing piles with dynamic load is 6 thousand rubles, static - 60 thousand.
Obviously, conditions different from them are found in several regions of South America, but these are soil profiles characteristic of regions of a tropical climate with a large thickness of weathered soil, emphasizing that in mineralogy, quartz, as a primary mineral with high resistance to weathering, is present in the entire profile, while maintaining proper proportions in relation to the composition of the source rock.
They illustrate the microstructure at two depths of a tropical soil profile located in the experimental area of \u200b\u200bthe postgraduate program in geotechnics at the University of Brasilia. It is also a structure characterized by the presence of aggregates forming macropores between them and micropores in their interiors. The structure has a more uniform porosity. It shows the microstructure of sandy latosol of the Brazilian Federal District in its natural state and on the same soil after flooding under 100 kPa vertical tension.
According to the requirements of GOST 5686 “Soils. Field test methods by piles ", SNiP 2.02.03-85" Pile foundations ", SP 50-102-2003" Design and construction of pile foundations "," Pile test programs ", pile testing is carried out by two main methods:
- static pile testing
our company conducts both types trials, although the latter are much cheaper and technically simpler. However, it should be noted that pile testing by the second method is not used on plastic clay soils, as well as on bulk soils in the form of construction waste or household dumps.
The last figure further notes that structural collapse increases the number of contacts between quartz grains. It is obvious that the effect on the mechanical behavior of this increase in the number of contacts in clay soil, as in soil, will not be the same. In the quartz grains present in them, aggregates of clay and other minerals are replaced. Rupture of these contacts, which can be formed by cement bonds or clay bridges, although they lead to loss of adhesion in the soil due to structural collapse, also lead to a decrease in voids, which leads to an increase in frictional resistance by providing an increase in the contact area.
Pile testing: why is it needed
Field tests of piles are carried out:
- to determine the required type and size, as well as the bearing capacity of piles
- to check the real possibility of driving piles to the design depth and assess the degree of soil homogeneity, for which the soil is tested with piles
- to establish the dependence of the settlement of piles in the soil on the applied loads and over time
Thus, the verification of the compliance of the bearing capacity of the piles with the calculated design loads is carried out.
However, due to the mineralogical nature of the grains, quartz in the case of sandy soil and clay in the case of aggregate soil, the increase in frictional resistance will be greater for sandy soil than for clay soil, which ends up indicating more efficient heap behavior in the short term in sandy soil. However, in the long term, the great regenerative capacity of the clay soil and the fact that the absorption of the collapsed soil will be in equilibrium with that of the neighboring material will lead to a change in behavior, depending on the end result, chemical-mineralogical, macroporosity and soil suction.
Static pile testing - what is needed for this
To carry out static tests of piles for indentation and pulling loads, it is necessary special installation, which consists of:
- a device for ensuring the load on the pile (this can be a platform with a calibrated load or using a jack). Vertical static load tests are also called die tests.
- The supporting structure through which the loads will be transmitted to the pile (a system of metal (reinforced concrete) beams or trusses with anchor piles) measuring devices that record the degree of movement of the pile during testing with a measurement accuracy of 0.01 mm.
See also:
Therefore, in the case of clay soil in an unsaturated state, it seems necessary to better investigate this effect by checking the time elapsed between the casting of the foundation and the load test. To illustrate the above, it presents the results of stress tests performed on the same pile, 30 cm in diameter and 7.65 m long, dug with a power drill. Load tests were carried out at different times by Guimarães and Mascarenha. The presence of kaolinite and gibbsite up to 8 m profile is known, and the level of quartz is about 30%.
Static pile testing process
When testing piles static method, it is loaded with steps, the weight of which is calculated depending on the design load on the pile (as a rule, no more than 1/10). In this case, the load by the next stage is carried out after stabilization of the settlement at the previous stage.
Static and dynamic pile testing - theory
Throughout the depth, the sum of the fraction of hematite plus goethite was about 14%. Illite appears at a depth of 6 m, maintaining approximately the same proportion to 9 m and then to 11%. Structurally, the soil profile was porous, with a void index from 1.6 at the top of the profile to 1.0 at the bottom. Heap tests were done after the land was flooded. Comparing the decrease in the load curves after the point of plasticization, it is verified that in the case of sandy soil, the increase in resistance with structural collapse generated in each test is much higher than in the case of clay soil, thus confirming the previous discussion.
Before loading the test pile, a zero mark is set on all measuring devices. Readings are also taken from all devices at each loading stage.
The conditional stabilization of the pile is determined by the following criterion: at a given stage of loading, the rate of settlement of the pile does not exceed 0.1 mm. During the last 30 minutes of observation. 
The results presented here illustrate analyzes that have been developed over the past five years by the authors of this article. It should be noted that in the Aguas Claras region, in the same work, mainly in the period of low precipitation, it is usually necessary to find water in some holes, but not in others, a situation usually associated with the penetration of water through rock fractures.
Testing of thawed soils with static pressing, pulling and horizontal loads
The soil profile of the first group of tests is a very interesting feature of the location because it is located near the artificial lake, Lake Paranoi. Therefore, the water level before the construction of the lake was lower than its current level, which is important because although the soil is deeply weathered, folded in nature, it no longer collapses due to changes in moisture, because it is in a state that changes from very wet until saturated. Obviously, for those who think that collapse is just a problem of high humidity, this, even if it were, would not be folding ground under this vision.
The value of the partial limiting resistance is the load at which further loading of the pile is stopped.
We produce by any method. As we have already mentioned, the price of static tests is much higher, moreover, they require optional equipment... However, as mentioned above, in some cases it is simply impossible to do without static tests.
However, care is required as it is known that from the linear behavior versus load and sedimentation, once the load is plotted on a logarithmic scale, curvature is induced in the indicated graph. The original proposal was based on the idea of \u200b\u200bconstructing the settlement stabilization time depending on the load applied in the static load tests carried out on a soil profile called "Orla", which is predominantly porous, very weathered and saturated with shallow depths.
Although this method usually presented curves with poorly dispersed results, it sometimes showed results with clear dispersion, reflecting read times, which may be slightly higher or lower than the actual settling time due to the time intervals set for the readings and frequent interferences with daily temperature changes in settlement measurement systems. This idea arose from the classical interpretation of consolidation tests in relation to the determination of the secondary consolidation ratio, with the difference that here we are working with a calculated measurement in millimeters, and not in terms of a specific strain or change in the vacuum index when used in this case.
Static testing of bored piles
The static test technology (pictured below) shows the real state of permissible loads on the tested piles.
The test technique is quite simple, but it requires labor, time and special equipment... The principle is simple - you need to press on the test pile with sufficient force so that it sinks down. In this case, it is required to press, and not to drop something from above, as in dynamic tests. In this case, special stands are made over the tested pile.
Installation diagrams for static testing of piles
In the proposed method, the point of collision of the first segment with the second segment of the load curve in relation to the calculated coefficient determines the onset of plastic deformations, the end of the pseudo-elastic mode, and the lateral grounding system. The second inflection point, i.e. the collision of the second segment with the third segment of the curve corresponds to the beginning of plastic deformations of the soil on which the pile tip lies. Due to the linearity of the final part of the calculated curves versus time in a logarithmic scale, we can estimate in this graph the long-term calculations, which can be significant when the initial plasticization points are exceeded.
Stand for static testing of piles and soils
These stands are loaded with a certain weight and the pile settlement is monitored. Testing is terminated after the pile has settled more than the required level. However, there is an easier and cheaper way to use a hydraulic jack. It is necessary to push down, respectively, the issue is solved by installing additional piles near the subject (usually next to in one line).
Such piles are called anchor piles. A metal beam is installed on top of the anchor piles, welded to them. It is in this obstacle that emphasis will be made hydraulic jack with a gauge-manometer.
The test pile is driven slightly below the anchor piles for mounting a jack on it. The pressure is applied gradually in steps under constant control. Testing stops either according to the readings of the pressure gauge (usually 20-30% more than the planned load), or if the pile fails and sinks down. Static tests have a high cost, take about a day, but at the same time they are the most accurate among all similar ones. According to SNiP data, up to 1% of the total number of installed piles are subjected to static tests.
To calculate the cost, an estimate is used and at the end of the tests carried out, a test report is issued.
Test standards, rules and regulations.
The current testing standard is GOST 5682 2012. The standard applies to methods of field testing of soil with piles (reference, full-scale, pile probes), which are carried out during engineering surveys during the construction period. It also applies to proof testing of various piles.
Construction rules and regulations SP No. 50-¬102-¬2003, SNiP No. 2.02.03-¬85 apply to the survey of foundations, survey of foundations and design of pile foundations of reconstructed and erected buildings and structures.
