RU2619383C2 - Determination method of soil strength characteristics in stress relaxation mode - Google Patents

Abstract

FIELD: construction.

SUBSTANCE: invention relates to engineering surveys in construction, and notably it is applied in determining the soils strength characteristics, that are required for the design of buildings foundations. The method for determining the soil strength characteristics as a destruction result of the prelimenary compacted by different loads soil samples in the triaxial comporession devices, single-plane cut or in the bevelling instruments, based on its step loading with futher Coulomb-Mohr diagram making, based on the values of destructive stress, according to which the strength characteristics - internal friction angle ϕ and tractive resistance c are determined graphically. The soil samples loading are carried out by the sample step deformation method by the specified displacement values, at that the next step of deformation application is carried out after the strain drop in the free relaxation mode upto the conventional stabilization, corresponding to the filtration process completion. The loading step completion criterion is the rate of strain drop, corresponding to the point of intersection of two straight lines, drawn to the initial and final portion of the dependence diagram the strain - the voltage drop rate logarithm.

EFFECT: invention provides the significant reduction of the soils tests timing, when determining their strength characteristics - internal friction angle and tractive resistance, improvement of measurement accuracy.

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Description

The invention relates to the determination of the strength characteristics of soils - the angle of internal friction ϕ and adhesion c during engineering surveys in construction when testing soils in shear devices, true shear (mowing) devices and triaxial compression devices.

The known "dynamometric loading method" [2], in relation to uniaxial tests of frozen soils. The method consists in stepwise deformation of the sample with the possibility of its unlimited lateral expansion. The loading of the sample is carried out by means of a screw press through a dynamometer until the conditional stabilization of stress and strain at each stage is achieved.

The disadvantages of the known solution are the limited scope of its application by frozen soils, and the lack of preliminary compaction of the soil during uniaxial tests does not provide for the strength characteristics of the soil - the angle of internal friction and adhesion.

There is also a method of determining the strength characteristics of soils in the loading mode with a given speed of movement [1]. The method consists in testing a sample previously densified with various loads at a given speed of movement. The speed of movement is selected taking into account the guaranteed dispersion of the first pressure during loading. Tests can be carried out both under conditions of axisymmetric loading in triaxial compression devices (Fig. 1), and under conditions of shear along a given plane in shear devices (Fig. 2) or in mowing devices.

According to the test results, a fracture graph is constructed in the coordinates ε = f (σ) for triaxial tests and ε = f (τ) (Fig. 3), where ε is the relative deformation of the sample, σ is the vertical stress (for triaxial tests), and τ is the tangent stress (during shear tests and mowing tests).

The disadvantage of this solution is also the low productivity of the tests due to their high duration.

An object of the invention is a significant reduction in the timing of determining the strength characteristics of soils.

To solve the problem of soil testing to determine the strength characteristics in triaxial compression devices, single-plane shear or in mowing devices, stress relaxation is performed (by analogy with the determination of deformation characteristics of soils [3]). Previously, the soil sample is consolidated by different loads until the conditional stabilization criterion is reached. Then, by stepwise setting the displacements, 3 soil samples are destroyed. In this case, at each stage of loading after application of a given value of displacement, a voltage drop occurs in the free relaxation mode. In this case, the change in stress and additional deformation is recorded by means of displacement sensors and dynamometers, indicated in figures 1, 2 by the letter "D".

The completion of the stage is determined by the completion of the filtration consolidation process in accordance with the criterion GOST 12248-2010 [1] or according to the graph of the dependence of the strain on the logarithm of the strain rate (figure 4). The criterion for the completion of the unloading stage in this case is the strain rate corresponding to the intersection of two straight lines drawn to the initial and final sections of the graph.

You can also check the completion of the filtering consolidation process according to the graph of the voltage dependence on the logarithm of the voltage drop rate. In this case, the criterion for completing the step will be the rate of change of voltage corresponding to the intersection of two lines drawn to the initial and final sections of the graph of the voltage versus the logarithm of the voltage drop rate.

After reaching the stabilization criterion, the next stage of displacement is applied, and so on until destruction.

According to the stabilized values of displacements and stresses, a fracture graph is constructed in the coordinates ε = f (σ) for triaxial tests and ε = f (τ) for shear tests (Fig. 5). The test is completed when the maximum conditional deformation of the sample is achieved by 15% for triaxial tests and 10% for shear tests [1], or in mowing devices.

From the maximum values of the breaking stresses, a Coulomb-Mohr diagram is constructed (Figs. 6, 7), from which the values of c and ϕ are determined.

The test to determine the strength characteristics of soils in the mode of stress relaxation dramatically reduces the duration of the test as a whole.

The duration of soil strength tests using the MRI method (in stress relaxation mode) can be further reduced by forced pressure relief at the relaxation stage.

The greatest time when testing the soil by the MRI method is the process of stress relaxation and the accompanying additional deformation of the sample εΔ due to elastic unloading (BC, 1 ... 3). Moreover, the higher the rigidity of the equipment (load system + dynamometer), the less εΔ and faster relaxation processes. However, it is not always possible to achieve the necessary accuracy in measuring stresses with high rigidity of a dynamometer. The relaxation process can be accelerated by step forced discharge σ. Fig. 8 is a graph of ε versus σ in a forced reset process σ.

Section BD corresponds to the process of free relaxation, and section DE corresponds to forced discharge σ. The σ discharge process is repeated until the adopted stabilization criterion is reached (point C ’), which allows more efficient use of equipment with low stiffness and shortens the test time. From Fig. 9 it can be seen that the time T1 spent on the test with forced discharge of the total voltage σ is much less than the time T2 without voltage relief.

The proposed method was tested on various varieties of soil of natural and disturbed composition and showed full compliance with the results with parallel tests according to GOST [1].

Determination of strength parameters - the angle of internal friction ϕ and adhesion with the proposed method solves the problem of reducing the time of testing several times. The method has industrial applicability due to significant differences from the prototype and other known solutions, therefore, in the opinion of the author, can be protected by a patent.

Information sources

1. GOST 12248-2010. Soils. Laboratory methods for characterizing strength and deformability. M .: Standartinform, 2011

2. Vyalov S.S. Rheological foundations of soil mechanics / M .: Higher school, 1978, 447 p.

3. Patent RU No. 2272101 C1, 08.25.2004. Laboratory method for determining the deformation characteristics of soils. Trufanov A.N.

Claims (1)

A method for determining the strength characteristics of soils from the results of the destruction of soil samples previously compacted by various loads in triaxial compression devices, a single-plane cut or in mowing devices based on their step loading, followed by the construction of the Coulomb-Mohr diagram from the values of the breaking stresses, according to which the strength characteristics are graphically determined - angle of internal friction ϕ and adhesion with, characterized in that the loading of soil samples is carried out by step deformation of the specimen with the given displacement values, and the next strain stage is applied after the voltage drop in the free relaxation mode to conditional stabilization corresponding to the completion of the filtration consolidation process, and the criterion for the completion of the loading stage is the voltage drop rate corresponding to the intersection point of two straight lines drawn to the initial and the final section of the voltage dependence graph is the logarithm of the voltage drop rate.

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