What are the conditions for doing small strain testing of pile foundations?

The detection of piles generally detects the following two aspects:

The bearing capacity of the pile.

The inspection of pile integrity will then look at the large strain test and small strain.

Definition of large strain:

It is a detection method to impact the top of the pile with a hammer, measure the speed and force time history curve of the top of the pile, and judge the vertical compressive bearing capacity and pile integrity of a single pile through the analysis of wave theory.

Small Strain Definition:

It is a detection method that uses low-energy transient or steady-state excitation to excite at the top of the pile, and the velocity time history curve or velocity admittance curve of the top of the pile is measured, and the integrity of the pile is judged by wave theory analysis or frequency domain analysis.

Other testing methods: vertical compressive static load test of single pile, vertical uplift static load test of single pile, horizontal static load test of single pile, core drilling method. Acoustic transmission method.

Answer: There is no hard and fast rule to do a large strain test on pile foundations. If it is the integrity of the pile, the large strain can be replaced with a small strain.

There are rigid regulations that the bearing capacity of the pile and the integrity of the pile must be tested. There is no rule as to which method to use.

The concrete strength of the pile foundation meets the requirements of the design or acceptance specifications, the pile head has been cut, and the pile foundation can be made with a small strain.

The small strain of the cast-in-place pile should be broken after the concrete strength of the pile body reaches 70% and the floating slurry section of the pile head should be broken. The prefabricated piles can be done after they are driven.

The small strain of the pile foundation can be done when the concrete strength of the pile body reaches 70%, but the floating slurry section of the pile head should be broken and then done.

The small strain detection of foundation pile (also known as low strain change bucket search method) is to use a small rubber hammer to hit the top of the pile, receive the stress wave signal from the pile through the sensor bonded to the pile top, use the stress wave theory to study the dynamic response of the pile soil system, and invert and analyze the measured speed signal and frequency signal, so as to obtain the integrity of the pile.

This method is simple to detect and fast to detect, but how to obtain a good waveform and how to analyze the integrity of the pile is the key to the detection work.

1. Large strain detection is a detection method that uses a heavy hammer to impact the top of the pile, measures the speed and force time history curve of the top of the pile, and determines the vertical compressive bearing capacity and pile integrity of a single pile through the analysis of wave theory.

2. Small strain testing, also known as low strain dynamic testing, is relatively for large strain dynamic testing.

Low strain testing is engaged in geotechnical engineering testing, structural testing, engineering geophysical prospecting, engineering surveying and mapping, housing quality testing, indoor environmental quality testing, environmental chemical testing, environmental engineering, safety assessment, water design and construction industry, water conservancy and hydropower industry, railway, highway transportation industry, chemical industry, municipal and other industries geotechnical engineering, geological hazards, environmental protection related technical services, consulting, development work, as well as extended business related to the above business.

<>1. The low strain dynamic detection of pile foundation is mainly to measure the stiffness of the pile with low strain, and then convert the strength of the pile according to the stiffness. The main purpose is to detect the concrete strength of the pile, and then convert the bearing capacity of the pile itself according to the concrete strength of the pile.

2. Implement low-energy transient or steady-state excitation on the top surface of the pile, so that the pile can do elastic vibration within the elastic range, and thus produce the longitudinal propagation of stress waves, and at the same time use the wave and vibration theory to evaluate the integrity of the pile A detection method, mainly including the reflected wave method, the mechanical impedance method, the hydroelectric effect method, etc., in which the reflected wave method has clear physical significance, the test equipment is light and simple, the detection speed is fast, and the cost is low, which is a good means for the quality (integrity) survey of the pile.

3. Low strain is a measurement method for pile integrity. The third and fourth types of piles are judged to be this type of piles because of the defects of the pile, and the diameter reduction, diameter expansion, mud inclusion, segregation, etc. are the written names of the reasons for the location of the defects of the pile.

Summary. The foundation design grade is grade B and grade C pile foundation, and the integrity of concrete cast-in-place pile is evaluated.

When the strain is low, the number of sampling should not be less than 50% of the total number of piles under the same conditions, and not less than 20, and the number of piles for each cushion cap shall not be less than 1: for the project of four piles or more than four piles under the column, the number of sampling should not be less than 50% of the corresponding number of piles. When evaluating the integrity of prefabricated piles with low strain, the number of sampling should not be less than 30% of the total number of piles under the same conditions, and not less than 20, and the number of piles for each cushion cap should not be less than 1

Root: For the project of four piles or more than four piles under the column, the number of sampling inspections should not be less than 30% of the corresponding number of piles.

Is there a specific specification for the number of low strain tests for pile foundations?

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For the pile foundation with the design grade of grade B and grade C, when evaluating the integrity of the concrete cast-in-place pile with low strain, the number of sampling should not be less than 50% of the total number of piles under the same conditions, and not less than 20, and the number of piles for each cushion cap shall not be less than 1: for the project of four piles or more than four piles under the column, the number of sampling should not be less than 50% of the corresponding number of piles. When evaluating the integrity of the prefabricated pile, the number of random piles should not be less than 30% of the total number of piles under the same conditions, and not less than 20, and the number of piles for each cushion cap shall not be less than 1

For the project of four piles or more than four piles under the column, the number of sampling inspections should not be less than 30% of the corresponding number of piles.

First, the reference data that can be obtained from the test are different: large strain (also called high strain) can measure the integrity and bearing capacity of the engineering pile, while small strain (also called low strain) can only measure the integrity of the pile.

Second, the methods of testing are different. The basic principle of large strain test pile: use a heavy hammer to impact the strong top, so that the pile-soil produces enough relative displacement, so as to fully stimulate the soil resistance around the pile and the supporting force at the end of the pile, through the acceleration sensor installed on both sides of the pile below the pile top and the acceleration sensor installed on the heavy Qi Hu hammer to receive the stress wave signal of the pile and the hammer, and the stress wave theory is used to analyze the time history curve of the processing force and velocity, so as to determine the bearing capacity of the pile and evaluate the quality integrity of the pile.

The basic principle of measuring the structural integrity of the pile with small strain is that the stress wave is generated by the excitation signal on the top of the pile, and the stress wave will be generated when it encounters discontinuous interfaces (such as honeycomb, mud inclusion, fracture, hole and other defects) and the bottom surface of the pile during the propagation process of the pile, and the integrity of the pile can be judged by detecting and analyzing the propagation time, amplitude and waveform characteristics of the reflected wave.

Third, the number of tests is different. Generally, the low strain test should detect all engineering piles, and the range of high strain detection is 10% of the total engineering quantity random sampling.

Fourth, the concept is different. Low strain integrity testing is a detection method that uses low-energy transient or steady-state excitation to excite at the top of the pile, measured the velocity time history curve or velocity admittance curve at the top of the pile, and judges the integrity of the pile through wave theory analysis or frequency domain analysis. The high strain dynamic testing method uses a heavy hammer to impact the top of the pile, the speed and force time history curve of the top of the pile are measured, and the vertical compressive bearing capacity and pile integrity of a single pile are judged through the analysis of wave theory.