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Discussion on some problems of geotechnical engineering test in the new era
2020-03-13 [470]

With the unprecedented scale of China's capital construction, geotechnical engineering and testing are facing such joy and great challenges. This puts forward new requirements for colleagues who attach importance to geotechnical engineering. Therefore, it is an important task for us to deepen reform and innovation.

As we all know, the demand and rapid development of geotechnical engineering in the past 20 years have been in particular. Like the springing up of high-rise buildings, underground engineering and bridge engineering are like hidden dragon and crouching tiger, which are really various and brilliant. But all of this does not mean that the pace of geotechnical engineering has come to an end. On the contrary, we need to deepen reform and innovation to maintain sustainable development.

Therefore, this paper points out that there are still many problems in geotechnical engineering and testing that need further understanding.

First of all, is the soil sample we tested disturbed? What is the degree of disturbance? This will directly affect the reliability of the parameters provided, and the safety and economy of the design and construction of the building. Therefore, as an important aspect, the prevention of soil sample disturbance has been strictly checked and improved continuously. How to reduce disturbance? Those soil samples are unqualified and unqualified, or qualified. How to evaluate the degree of soil disturbance.

Here is a brief introduction to the previous research of foreign scholars (schmertmann, 1955), which proposed to divide the disturbance degree according to the disturbance index ID and.

1、 On the evaluation method of sample disturbance degree

The degree of sample disturbance will directly affect the reliability of parameters provided, which involves the safety and economy of building design and construction. Therefore, from drilling sampling, transportation, storage to soil excavation and test, the disturbed soil samples have been strictly controlled as an important aspect, and constantly studied and improved. However, due to various objective reasons and many influencing factors, the disturbance of soil structure is still inevitable.

In order to reduce the disturbance of soil sampling, in-situ measurement technology has been developed. For example, field vane strength test, static cone penetration test, dynamic cone penetration test, standard penetration test, pressuremeter test, spiral plate load test of deep soil and dynamic characteristics in-situ test, etc. In addition, in order to reduce the impact of transportation and storage on the original structure, the geotechnical test will be carried out on site.

In order to evaluate the disturbance degree of the sample, there are three methods: residual pore water pressure method, undrained modulus comparison method and volume compression method. The first two methods are relatively complex, especially in determining the "reference pore water pressure" or "undrained modulus of the ideal sample". Therefore, at present, there are some difficulties in the test Under the condition of room, it is not easy to realize. The latter method, volume compression method, is relatively simple and has a clear concept, which can be achieved in most laboratories.

The volume compression method is used to average the disturbance degree of the sample. According to the indoor compression test results, the e-1gp compression curve is drawn as shown in figure 2-6-7. The test method is completely in accordance with the method of determining the preconsolidation pressure, and then the PC point is found out from the curve,

Figure 2-6-7 e-1gp curve determination of disturbance index

In the upper part of the extended curve, it is 2 times that of 1 OGPC, and the coordinate is E0. The horizontal line that has done e0o is ab, and point B corresponds to PC abscissa, so ABC curve is theoretical field compression curve, and point E0 as AFC straight line is compression curve of complete disturbance remodeling of the sample. It can be seen that the compression curve that has suffered partial disturbance must be between ABC and AFC curve, and the curvature of compression curve of local disturbed soil will be obvious in PC attachment of pre consolidation pressure Significant change. This is a method of average soil sample disturbance degree proposed by schmertman (1955). Its concept is shown in figure 2-6-7, and the disturbance index ID is determined as follows:

Therefore, according to the disturbance index ID, the classification and evaluation are as follows (see the table on the next page):

According to the data provided by many researchers, it is shown that the influence of disturbance on normal consolidated soil and over consolidated soil will bring many troubles to determine the stress history of soil. It is generally believed that the factors that cause the disturbance, besides the sampling, transportation and soil cutting described above, the loading rate of the test also has a great influence. Therefore, we not only provide a convenience for rating the disturbance degree, but also hope to obtain an ideal original soil sample. The following figure 2-6-8 and figure 2-6-12 provide the e-1gp curve of soil and the settlement effect of soil under different disturbance degrees.

Fig. 2-6-10 influence of load ratio on compression index Fig. 2-6-11 relation curve between void ratio and pressure with different disturbance degree

On the test of shear strength of soil

The shear strength of soil refers to the ultimate capacity of shear stress of soil under external load. When the shear stress produced by the external force reaches the shear strength of a point in the soil, when one part of the soil moves relative to the other part, it is considered that the point has shear failure. Shear failure is an important characteristic of strength. So the problem of strength is one of the important basic contents in soil mechanics and geotechnical test. So far, engineering problems related to strength mainly include the following aspects:

① As a material, soil constitutes the stability of geotechnical structures. Such as earth dam, embankment and other fill slope and natural soil slope (including excavation slope) and other stability issues.

② Soil as an environmental problem of engineering structures. That is, the problem of earth pressure, such as the soil around the retaining wall underground structure, their strength failure will cause too much side pressure on the wall, which may cause the engineering structures of these projects to slide, overturn and other damage accidents.