CN108414343B - Method for measuring sample rubber membrane embedding amount in coarse-grained soil large-scale triaxial test - Google Patents
Method for measuring sample rubber membrane embedding amount in coarse-grained soil large-scale triaxial test Download PDFInfo
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Abstract
A method for measuring the embedding amount of a test sample rubber membrane in a coarse-grained soil large-scale triaxial test belongs to the technical field of geotechnical tests. The measurement method is characterized in that a small-amplitude confining pressure cyclic loading process is added in a conventional isobaric consolidation test, the volume change quantity of a sample which can be recovered under the condition of small-amplitude confining pressure increment in different confining pressures is measured, and then the measurement of the embedding quantity of the rubber membrane and the rule of the embedding quantity of the rubber membrane along with the confining pressure change is realized based on the characteristic that the embedding of the rubber membrane is recoverable. The measuring method is easy to realize in a conventional triaxial apparatus, has simple operation steps and low difficulty, can be coupled to the conventional consolidation test process, does not influence subsequent tests, avoids the links of additionally developing equipment and carrying out multiple sample preparation and multiple groups of tests in some measuring methods, effectively eliminates the influence of individual discreteness of the sample, can accurately measure the embedded quantity of the rubber film along with the change of confining pressure, provides a reliable technical means for correcting the volume deformation of a coarse-grained soil sample in the triaxial test, promotes the research of the mechanical properties of the coarse-grained soil and the constitutive model, and has good popularization value.
Description
Technical Field
The invention relates to a method for measuring the embedding amount of a test sample rubber membrane in a coarse-grained soil large-scale triaxial test, and belongs to the technical field of geotechnical tests.
Background
In the triaxial test, the confining pressure is transmitted to the test specimen through a rubber membrane. However, due to the fact that the surface of the sample is uneven, the rubber film can be embedded into pores among particles on the surface of the sample after confining pressure is applied, the embedding depth changes along with the change of effective confining pressure, and the phenomenon is embedding of the rubber film. In a conventional triaxial drainage shear test, a method for determining the volume change of a saturated sample is to measure the water volume discharged or absorbed by the sample, but due to the influence of rubber film embedding, the volume change measurement contains rubber film embedding errors, the deformation characteristics of coarse-grained soil materials cannot be accurately reflected, and the method is not favorable for the accurate design and safety evaluation of geotechnical structures (artificial island reefs, high-speed railway foundations, high earth-rock dams and marine structures of nuclear power plants). The current measuring method for rubber film embedding mainly comprises the following steps:
embedding a cylindrical rod: the method is to embed copper rods with different diameters, the same height as the sample, in the center of the sample, and presume that the overall change of the sample is in a linear relation with the diameter of the copper rod, thereby estimating the volume change caused by the embedding of the rubber film. However, the method is difficult to prepare samples, the copper rod can limit axial deformation, and stress concentration of soil bodies near the copper rod is caused, so that the assumption that the overall sample becomes linear with the diameter of the copper rod is inaccurate.
Hollow cylinder sample method: the method is characterized in that the change relation of the rubber film embedding amount along with the confining pressure is calculated according to the difference value of the solidified body change of samples with different inner diameters. The method is only suitable for hollow cylindrical samples and cannot be applied to traditional cylindrical triaxial samples.
Double size sample method: the method is to determine the embedding amount of the rubber film according to the difference of the test body deformation and the rubber film area of two cylindrical samples with different sizes. The method needs two triaxial apparatuses with different sizes, and for coarse-grained soil with larger size, the method needs to research and develop an ultra-large triaxial apparatus, so that the method is difficult to be applied to coarse-grained soil rubber membrane embedding measurement research.
A special device method comprises the following steps: the method obtains the relation between the rubber membrane embedding amount and the rubber membrane area by manufacturing a special device, but the stress state and the boundary condition of the rubber membrane embedding measured by the method are different from those of a triaxial test, and the embedding amount of the rubber membrane in the conventional triaxial test process cannot be completely reflected.
Local strain measurement: for the condition that the size of the sample is large, the space of the pressure chamber is sufficient, axial and radial displacement sensors can be adopted to measure the skeleton body change of the sample, then the embedding amount of the rubber film is calculated according to the drainage body change, but under the condition of confining pressure change, the embedding of the rubber film, the non-uniformity of the radial deformation of the cylindrical sample and the deformation of the rubber film along the thickness direction can influence the measurement precision of the radial deformation of the cylindrical sample.
Isotropy hypothesis method: the method is to assume that the deformation isotropy in the initial isobaric test process obtains the actual deformation of the soil body framework, but the assumption is greatly different from the actual deformation, particularly the framework drainage body of the sample becomes large (comprising two parts of plastic deformation and elastic deformation, mainly the former part) in the initial isobaric loading process, and the error caused by the assumption can be amplified.
In summary, the conventional embedding analysis and measurement method of the sample rubber membrane in the coarse-grained soil triaxial test has many problems.
Disclosure of Invention
The invention aims to overcome the defects and shortcomings of the prior art and the method, and provides the method for measuring the rubber membrane embedding amount of the sample in the coarse-grained soil large-scale triaxial test.
The scheme adopted by the invention is as follows: a method for measuring the embedding amount of a test sample rubber membrane in a coarse-grained soil large triaxial test comprises the following steps:
(a) preparing a saturated coarse-grained soil triaxial sample by adopting large triaxial test equipment according to a conventional test method in geotechnical test regulations (SL 237-1999);
(b) carrying out conventional consolidation test on the sample according to 'geotechnical test regulation', loading consolidation confining pressure to a target value in multiple stages, adding a small-amplitude confining pressure cyclic loading link at each stage of consolidation confining pressure, wherein the cyclic confining pressure amplitude is 50 kPa, the frequency is 5, and the period is 6 min, and eliminating the plastic incremental deformation of the sample by using the cyclic loading process, thereby measuring the recoverable volume deformation amplitude and axial deformation amplitude of the sample in the link; according to the method, the target confining pressure is reached;
(c) based on the principle that rubber membrane embedding is recoverable and the characteristic that sample recoverable deformation only comprises two parts of rubber membrane embedding deformation and coarse-grained soil framework elastic deformation, measuring the rubber membrane embedding increment caused by confining pressure change according to the sample circulating volume deformation amplitude and circulating axial deformation amplitude caused by circulating confining pressure amplitude at each stage of consolidation confining pressure; the elastic volume deformation amplitude of the coarse-grained soil framework is calculated according to the assumed isotropy, the cyclic axial strain amplitude which is equal to 3 times is multiplied by the volume of the sample, and the axial strain is equal to the axial deformation divided by the height of the sample; and the rubber membrane embedding increment is equal to the subtraction of the sample circulation volume deformation amplitude and the coarse-grained soil framework elastic volume deformation amplitude, and the total rubber membrane embedding volume is calculated according to the rubber membrane embedding increments at different confining pressures.
The invention has the beneficial effects that: the method can measure the embedded quantity of the rubber membrane by using a conventional large-scale triaxial apparatus without additionally designing and developing equipment for measuring the embedded quantity of the rubber membrane. The measuring method is simple in operation steps, avoids the discreteness of the sample, can accurately measure the embedding amount of the rubber membrane along with the change of confining pressure, provides a reliable technical means for correcting the volume deformation of coarse-grained soil in a triaxial test, promotes the research of the mechanical characteristics of the coarse-grained soil and a constitutive model, and has good popularization value.
Drawings
FIG. 1 is a coarse soil test grading curve.
Fig. 2 is a confining pressure loading process.
FIG. 3 shows the rubber membrane embedded increment and the deformation amplitude of the coarse-grained soil skeleton volume at the confining pressure of each stage.
Detailed Description
The above-described measurement method is described in detail below with reference to examples.
A method for measuring the embedding amount of a test sample rubber membrane in a coarse-grained soil large triaxial test comprises the following steps:
(a) saturated coarse-grained soil triaxial samples were prepared according to the conventional test method in geotechnical test code (SL 237-1999) using large triaxial test equipment. The sample preparation control index of the coarse-grained soil is that the porosity is 0.20, and the coarse-grained soil grading curve is shown in figure 1.
(b) Carrying out a conventional consolidation test on a sample according to 'geotechnical test regulation', loading consolidation confining pressure to a target value of 1000 kPa in multiple stages (including 50 kPa, 100 kPa, 150 kPa, 250 kPa, 350 kPa, 450 kPa, 550 kPa, 650 kPa, 750 kPa and 950 kPa), adding a small-amplitude confining pressure cyclic loading link at each stage of consolidation confining pressure, for example, the cyclic confining pressure has an amplitude of 50 kPa, the frequency of 5 and the period of 6 min, and eliminating the plastic incremental deformation of the sample by using the cyclic loading process, thereby measuring the recoverable volume deformation amplitude and the axial deformation amplitude of the sample in the link. The method is carried out until the target confining pressure is reached. The confining pressure loading process is shown in figure 2.
(c) Based on the principle that rubber membrane embedding is recoverable and the characteristic that sample recoverable deformation only comprises two parts of rubber membrane embedding deformation and coarse-grained soil framework elastic deformation, the rubber membrane embedding increment caused by confining pressure change is measured according to the circulating volume deformation amplitude and the circulating axial deformation amplitude of the sample in the 5 th period, which are caused by the circulating confining pressure amplitude at each stage of consolidation confining pressure. Wherein, the elastic volume deformation amplitude of the coarse-grained soil framework is calculated according to the assumed isotropy, and the amplitude of the cyclic axial strain (the axial strain is equal to the axial deformation divided by the height of the sample) which is equal to 3 times is multiplied by the volume of the sample. And the rubber membrane embedding increment is equal to the subtraction of the sample circulation volume deformation amplitude and the coarse-grained soil framework elastic volume deformation amplitude, and the total rubber membrane embedding volume is calculated according to the rubber membrane embedding increments at different confining pressures. The deformation of the coarse-grained soil skeleton in the method is recoverable (elastic), and accounts for 11.2% -19.2% of the deformation amplitude of the sample in the circulating volume (see figure 3) (the embedding amount of the rubber membrane is increased along with the increase of the particle size, so the value of the ratio in the coarse-grained soil with larger particles is smaller). The radial deformation error of the sample calculated according to the isotropy assumption is 20% and 40% (caused by soil anisotropy), the deformation error of the skeleton body of the sample calculated is 13.3% and 26.6%, and therefore, the embedding error of the rubber film is only 1.5% -2.6% and 3.0% -5.2%. Therefore, the measuring method has high precision.
Claims (1)
1. A method for measuring the embedding amount of a test sample rubber membrane in a coarse-grained soil large triaxial test is characterized by comprising the following steps:
(a) preparing a saturated coarse-grained soil triaxial sample by adopting large triaxial test equipment according to a conventional test method in soil engineering test specification SL 237-1999;
(b) carrying out a conventional consolidation test on the sample according to the geotechnical test procedure, loading consolidation confining pressure to a target value in multiple stages, adding a small-amplitude confining pressure cyclic loading link at each stage of consolidation confining pressure, wherein the cyclic confining pressure amplitude is 50 kPa, the cycle frequency is 5, and each cycle period is 6 min, and eliminating the plastic incremental deformation of the sample by using the cyclic loading process, so that the recoverable volume deformation amplitude and the axial deformation amplitude of the sample in the link are measured; according to the method, the target confining pressure is reached;
(c) based on the principle that rubber membrane embedding is recoverable and the characteristic that sample recoverable deformation only comprises two parts of rubber membrane embedding deformation and coarse-grained soil framework elastic deformation, measuring the rubber membrane embedding increment caused by confining pressure change according to the sample circulating volume deformation amplitude and circulating axial deformation amplitude caused by circulating confining pressure amplitude at each stage of consolidation confining pressure; the elastic volume deformation amplitude of the coarse-grained soil framework is calculated according to the assumed isotropy, the cyclic axial strain amplitude which is equal to 3 times is multiplied by the volume of the sample, and the axial strain is equal to the axial deformation divided by the height of the sample; and the rubber membrane embedding increment is equal to the subtraction of the sample circulation volume deformation amplitude and the coarse-grained soil framework elastic volume deformation amplitude, and the total rubber membrane embedding volume is calculated according to the rubber membrane embedding increments at different confining pressures.
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| CN109520799B (en) | 2018-11-21 | 2020-05-08 | 中国科学院武汉岩土力学研究所 | Sample preparation method for eliminating embedding effect of strongly weathered rock rubber film |
| CN110441142B (en) * | 2019-07-20 | 2021-04-16 | 大连理工大学 | Method for measuring sand gravel triaxial sample surface film embedding amount by digital image technology |
| CN110441156A (en) * | 2019-07-20 | 2019-11-12 | 大连理工大学 | A method of three axis sample membrane penetrations are measured based on soil stabilization state characteristic |
| CN110987641B (en) * | 2019-12-23 | 2020-06-30 | 水利部交通运输部国家能源局南京水利科学研究院 | Device and method for measuring large triaxial test embedded quantity of coarse particles |
| CN111579377B (en) * | 2020-06-05 | 2021-05-07 | 大连理工大学 | Dynamic and static triaxial test device capable of eliminating influence of membrane embedding effect |
| CN113324866B (en) * | 2021-06-01 | 2022-03-11 | 水利部交通运输部国家能源局南京水利科学研究院 | Correction method for pore pressure change caused by rubber membrane embedding in triaxial non-drainage test |
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