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Discussion on soil sample and height area correction in triaxial test of geotechnical engineering
2020-03-13 [84]

Relevant instructions of triaxial test and its height and area correction

1. Undisturbed structure of soil sample

As we all know, in order to obtain ideal undisturbed soil samples, first of all, in the process of soil sampling on site, to prevent the structure disturbance of soil samples, which may be caused by improper undisturbed soil sampler. Therefore, it is recommended to use a large-diameter weak soil sampler. During the sampling process, the static pressure method should be used as far as possible for the soft clay. When the soil sample is taken, it should be sealed as soon as possible and transported and stored under the low temperature and humidity conditions. In addition, the soil test should be carried out in time. During the preparation process of soil cutting, the manual disturbance should be avoided, and even the soil loading process should be very careful. It is clearly recognized that the natural strength of saturated soft clay will decrease exponentially due to different disturbance. It is recognized from the sensitivity test of unconfined compressive strength that the sensitivity of soft clay is very high. Therefore, in order to reflect the objective situation of soil and protect the undisturbed soil structure, in addition to field investigation, the test operation is also an important link.

2. Influence of soil sample heterogeneity

The non-uniformity of soil sample is also the objective condition of foundation soil. If a group of triaxial samples with non-uniformity of soil, then the strength index of this group is difficult to determine. For soft clay foundation, because most of them are sedimentary facies soil layers, most of them are layered structures. Therefore, in the process of cutting soil and opening soil, it is recommended that more units adopt the three-part method to divide samples, that is, to carry out tests at the same elevation, so that the dispersion of soil samples does not appear in the same group of tests as far as possible.

3. Determination of pore water pressure

The determination of pore water is not only to calculate the effective strength index, but also to better obtain the pore water pressure coefficient of soil. These indexes are very important for the determination of strength and the change of strength. Therefore, in order to obtain reliable pore water pressure in the test, the air in the pipeline system should be drained as much as possible before soil loading. At present, most of the methods are to use bubble free water for circulating exhaust, that is, the drainage pipes at the bottom and upper part of the soil sample, and the pipes of the pore water measurement system should not be too long, and attention should be paid to the influence of volume change factors. In recent years, more and more hydraulic sensors are used at home and abroad. We think that this is the direction. It can not only shorten the measurement pipeline system, but also has a small lag effect. Therefore, the research and application of electrical measurement of pore water pressure should be an important aspect of reforming triaxial instruments and improving test methods.

4. Volume change and drainage of soil

The volume change of the soil sample mainly refers to the consolidation drainage of the soil under the lateral pressure or under the unequal pressure of K0. For saturated soil, the change of displacement is the change of soil volume. Therefore, to reflect the change of soil volume objectively, the displacement in soil must be measured correctly. At present, in order to accelerate the drainage consolidation, the general method is to use filter paper to drain consolidation. However, if the water content of the filter paper is larger than that of the soil, under the action of σ 3, first the filter paper is drained and consolidated, and then the soil sample is drained. In this way, the drainage amount will be larger. On the contrary, the filter paper will absorb the water content of the soil sample first when it is not immersed and pasted around the soil sample, which results in the smaller drainage amount. Second, the water leakage of rubber film, the tightness of wrapping rubber film and the sealing of pipeline joints will affect the correct drainage, which should be paid attention to in operation and instrument inspection and calibration.

5. Stress-strain curve and initial contact stress of soil

The stress-strain curve of soil is a kind of character of soil, but if we don't pay attention to the strictness in the operation, it will bring the result that is not in line with the reality. The ideal stress-strain curve should be carried out under the condition of unequal consolidation of K0. However, at present, the commonly used method is isotropic consolidation, which sometimes results in larger initial strain and smaller initial stress due to improper contact of vertical load, so it will have a greater impact on the calculation of the initial tangent modulus. Therefore, before the test, it is necessary to pay attention to the vertical stress contact. In general, it is advisable to add a small contact stress in advance.