What is the unconfined compressive strength test?

The unconfined compressive strength test is a test to measure the undrained shear strength of saturated cohesive soil and the sensitivity of soil. The instrument used is an unconfined compressive tester. The test is characterized by simple structure and easy operation, which can replace the triaxial compression test to determine the undrained strength of saturated cohesive soil.

The soil sample is applied vertically without any lateral pressure, that is, by applying vertical axial pressure, until the soil sample produces shear failure, according to the test results, only one ultimate stress circle can be made, therefore, it is difficult to make a damage envelope for general cohesive soil. For saturated cohesive soils, according to the results of the triaxial unconsolidated and non-drainage test, the failure envelope is close to a horizontal line, i.e., u=0. In this way, if only the undrained shear strength of saturated cohesive soil is measured, the unconfined pressure gauge with a relatively simple structure can be used instead of the triaxial compressor.

In this case, take u=0, then the horizontal tangent of the ultimate stress circle obtained from the unconfined compressive strength test.

It is to destroy the envelope.

The unconfined compressive strength test is a special case of the triaxial test, that is, the pressure test is carried out under the condition that the soil sample is not confined laterally, and the small principal stress of the soil sample is 0, and the limit value of the large principal stress is the unconfined compressive strength.

Instruments and equipment. 1) Strain control unconfined compressor: It is composed of a dynamometer, a pressurized frame, and a lifting equipment.

2) Axial displacement meter: measuring range 10mm, dial gauge of the index value.

3) Balance: weighing 500g, indexing value.

Operation steps: 1) The preparation of undisturbed soil samples is carried out according to the triaxial compression test steps. The diameter of the specimen is 80mm and the height is 80mm.

2) Apply a thin layer of petroleum jelly at both ends of the sample, and when the climate is dry, a thin layer of petroleum jelly should also be smeared around the sample to prevent water evaporation.

3) Place the sample on the base, turn the handwheel, make the base rise slowly, the sample is just in contact with the pressure plate, and adjust the dynamometer reading to zero. According to the hardness of the sample, the dynamometer of different ranges is selected.

4) The axial strain velocity should be 1% 3% of the strain per minute. Turn the handle to make the lifting equipment rise for testing, when the axial strain is less than 3%, every % strain ( or once a reading when the axial strain is equal to or greater than 3%, every 1% strain ( or once reading. The test should be completed within 8 to 10 minutes.

5) When the dynamometer reading peaks, continue to stop the test after 3% 5% strain; When there is no peak in the reading, the test should be carried out until the strain reaches 20%.

6) At the end of the test, the sample is removed and the shape of the sample after destruction is described.

7) When it is necessary to determine the sensitivity, the damaged specimen should be immediately removed from the surface coated with petroleum jelly. Add a little residual soil, wrap it in a plastic film and knead it by hand, destroy its structure, reshape it into a cylindrical shape, put it into a remodeling cylinder, use a metal backing plate, squeeze the sample into a sample equal to the size and density of the original sample, and test it according to the steps of paragraph 1 5 of this article.

Unconfined compressive strength.

The meaning is: the ultimate strength of the specimen to resist the axial pressure in the absence of lateral pressure, which is obtained from the unconfined compression test, and is pronounced wúcèxiànkànɡyāqiánɡdù.

Compressive strength" example sentence:

1. In this paper, the control index is put forward for the quality acceptance of the pavement of the base Zheng, among which the degree of compaction.

and the compressive strength of the bundle which should be the highest.

2. The results show that the cement sand modified by double water-blended emulsion epoxy and slag powder is sold to sell slurry.

It has high flexural strength and compressive strength.

3. Through the experimental study of the compressive strength of rock and ore and the mechanical study of the breaking force of natural ore blocks, the theoretical basis for reducing the size of steel balls in the rough mill is found.

The representative value of unconfined compressive strength refers to the maximum compressive resistance that a soil can withstand without obvious cracks and shear surfaces. In geoengineering, this value is an important parameter used to calculate the stability and bearing capacity of the soil. The following is a formula for calculating the representative value of unconfined compressive strength.

Commonly used formulas. At present, there are three commonly used formulas for calculating the representative value of unconfined compressive strength: simplified triaxial test method, shell method and load transfer ratio method. Among them, the simplified triaxial test method is one of the most commonly used methods, and the calculation formula is as follows:

qu=2σ3/3

where qu represents the representative value of the unconfined compressive strength, and 3 represents the minimum principal stress.

Simplify the application of the triaxial test method.

A series of tests are required to calculate the representative value of unconfined compressive strength using the simplified triaxial test method. These tests include standard penetration tests, cutting tests, triaxial compression tests, etc. After the test is completed, the representative value of the unconfined compressive strength of the soil can be obtained through data processing and analysis.

Application of other methods.

In addition to simplifying the triaxial test method, there are other methods that can be used to calculate a representative value of unconfined compressive strength. For example, the shell method calculates a representative value of unconfined compressive strength by measuring the displacement and stress changes of the soil in the shell test. The load transfer ratio method calculates the representative value of unconfined or compressive strength by measuring the displacement and stress changes of the soil in the load transfer ratio test.

In conclusion, the representative value of unconfined compressive strength is an important parameter in geoengineering, which is used to calculate the stability and bearing capacity of soil. In the calculation process, a variety of methods can be used, such as the simplified triaxial test method, the shell method, and the load transfer ratio method. Different methods are suitable for different soil types and test conditions, and need to be selected according to the specific situation in practical applications.

Refer to the "Highway Engineering Quality Inspection and Evaluation Standards" JTGF80 1-2004, page 131 Appendix G semi-rigid base and subbase material strength assessment:

The strength of semi-rigid base and subbase materials shall be subject to the unconfined compressive strength of moisturizing and maintaining health at the specified temperature for 6 days and 7 days after immersion in water.

Samples are taken at the demolition site according to the specified frequency, and the specimen is prepared according to the compaction degree predetermined to be achieved at the construction site. 1 group of specimens is prepared per 2000m2 or per working shift: 6 specimens can be used when the deviation coefficient CV is 10% for multiple times, regardless of stable fine-grained soil, medium-grained soil or coarse-grained soil; When CV=10 15, it can be 9 specimens; For CV>15, 13 specimens are required.

The average strength r of the specimen should meet the requirements of the following formula:

r≥rd/（1-zacv）

Where: rd—design compressive strength (MPA).

cv—coefficient of deviation of the test result (in decimal);

za—The coefficient that varies with the guaranteed rate in the standard normal distribution table.

High-speed, first-class highway: 95% guarantee rate, za=

Other highways: 90% guarantee rate, za=.

When the strength of the semi-rigid material in the road section is assessed as unqualified, the corresponding sub-project is unqualified.