Integrated sample preparation method of saturated soft clay sample containing structure

Abstract

The invention provides a method for preparing a saturated soft clay sample containing a structure, which comprises the following steps: selecting a pre-consolidation mold, respectively and sequentially adhering filter paper and permeable cotton cloth on the surfaces of a pressure head and a base of a mold cylinder, and pouring a certain mass of slurry into the pre-consolidation mold filled with the structure; placing the pre-consolidation mold in a consolidation apparatus, covering the consolidation apparatus with a cover plate, and applying set pre-consolidation stress in a grading manner until a test stop standard is reached; and pushing the soil body containing the structure out along the bottom to obtain the saturated soft clay sample containing the structure. The initial saturation degree and uniformity of the prepared soil body sample can be guaranteed, the density of the prepared soil body can be relatively accurately controlled, and good preconditions are provided for data stability and scientificity of a subsequent saturated soft clay-structure interface test.

Description

Technical Field

[0001] The present invention belongs to the technical field of soft clay testing, and particularly relates to an integrated specimen preparation method for saturated soft clay specimens containing structures. Background Art

[0002] Saturated soft clay refers to clay with high water content, large pores, high collapsibility, and low permeability. It mostly exists in sedimentary layers in areas such as rivers, lakes, and the ocean. This clay is usually used as the friction material for pile foundations of facilities such as bridges, roads, and marine structures, and has important research significance for ensuring the reasonable budget and safety and stability of large-scale engineering projects.

[0003] In the field of research on the interaction between saturated soft clay and structures, indoor test methods, as key technical means, have accumulated relatively rich research results. In recent years, relying on the technical advantages of triaxial shear testers, a new type of soil-structure interface shear device and measurement method has emerged (CN1 11896395A). Compared with traditional interface meters, this improved device has the characteristics of being convenient for assembly and disassembly, and can accurately measure interface pore pressure and volume change, providing more powerful experimental support for the construction of interface constitutive models and numerical analysis research.

[0004] However, due to the presence of structures, the specimen preparation method for soil-structure interface shear tests is significantly different from that of triaxial specimens. In addition, when the structure is subjected to pulling or penetration forces, it is necessary to ensure a constant contact area between the soil and the structure. Therefore, considering factors such as the morphological characteristics of the specimen, the load form of the structure, and the movement displacement, it is urgent to develop an integrated specimen preparation method for saturated soft clay specimens containing structures suitable for this interface device. Traditional methods use the form of stacking weights for mud preloading, or the form of a lever to apply pre-pressure, both of which apply vertical pressure to the soil, but these traditional methods cannot achieve automatic specimen preparation and have poor repeatability. Summary of the Invention

[0005] To solve the above technical problems, the present invention provides a specimen preparation method for saturated soft clay specimens containing structures, including:

[0006] Step 1: Select a pre-consolidation mold, and sequentially attach filter paper and permeable cotton cloth to the surface of the mold cylinder ram and the base, and pour a certain mass of mud into the pre-consolidation mold containing the structure.

[0007] Step 2: Place the pre-consolidation mold in a consolidometer, cover the cover plate, and apply the set pre-consolidation stress in stages until the test stop standard is reached.

[0008] Step 3: Push out the soil containing the structure along the bottom to obtain a saturated soft clay specimen containing the structure.

[0009] In a preferred embodiment, in step 1, the sample preparation method further includes the preparation of slurry, and the water content of the slurry is controlled to be 1.0 to 2.0 times the liquid limit ω of the soil mass, where the liquid limit of the soil mass refers to the boundary water content value between the plastic state and the flowing state of cohesive soil. Further, the prepared slurry is left standing in a container for 24 h, and the slurry in the container is shaken to exhaust gas every 6 h. L In a preferred embodiment, in step 1, the pre-consolidation mold includes: a hollow punch, a porous base, a hollow mold cylinder, and a friction pile (i.e., the structure). The slurry is poured into the hollow mold cylinder containing the structure.

[0010] In a preferred embodiment, in step 1, the mass of the slurry is calculated by the following formula:

[0011] Assuming that only a very small number of soil particles are lost during the staged loading preloading process and the discharged volume is the volume of water in the slurry, an equation can be established,

[0012] ρ

[0013] ×h ini ×S ini -(h a -h ini -h obj )×S a ×ρ ω =ρ obj ×S a ×h obj (1) In the formula, ρ ini is the initial density of the prepared slurry, with the unit g / cm 3 ; h ini is the initial height of the slurry after being poured into the mold cylinder, with the unit cm; S a is the bottom area of the hollow soil body, with the unit cm 2 , h obj is the ideal soil body height after preloading, with the unit cm, ρ obj is the density of the soil body after preloading, with the unit g / cm 3 ,

[0014] The initial density ρ ini of the slurry and the density ρ obj of the soil body after preloading are both related to the specific gravity of the soil particles and the water content in this state. Referring to soil mechanics, the expression for the density of the soil body is:

[0015]

[0016] In the formula, ρ is the density of the soil body, with the unit g / cm 3 ; w is the water content of the soil body, with the unit %; ρ wis the density of pure water at 4°C, usually denoted as 1 g / cm 3 ;

[0017] The initial mud density ρ ini and the height h of the ideal soil mass obj are known values, so the expression for the initial mud height h ini is:

[0018]

[0019] Let the total mass of the poured mud be m ini , in grams, and its calculation formula is shown in Equation (4):

[0020] m ini = h ini × ρ ini × S a (4).

[0021] In a preferred embodiment, in step 2, vertical loads of different weights are applied in stages. Usually, vertical loads of 3.125 kPa, 6.25 kPa, 12.5 kPa, 25 kPa, 50 kPa, 75 kPa, and 100 kPa are applied in stages, and the loading levels can be appropriately adjusted according to the in-situ burial depth of the soil and the test requirements.

[0022] The calculation formula for the magnitude of the vertical load is shown in (5):

[0023]

[0024] In the formula, p is the magnitude of the vertical load, in kPa; M d is the weight of the applied surcharge, in kN; M g is the weight of the cover plate above the specimen, in kN; S is the total contact area of all specimens and the hollow platen, in m 2 .

[0025] In a preferred embodiment, in step 2, the stopping condition for each stage of load is that the mud drops less than 1 mm per hour.

[0026] In a preferred embodiment, in step 2, the stopping criterion for the preconsolidation test is that the height of the specimen reaches the ideal soil mass height h obj .

[0027] The beneficial effects of the present invention are:

[0028] The specimens prepared by the improved slurry method of the present invention, that is, the soil mass, have regular and uniform surfaces, and no obvious bubbles and fracture phenomena are presented. By means of this improved slurry sample preparation method, not only can the initial saturation degree and uniformity of the prepared soil specimens be ensured, but also the density of the prepared soil can be relatively accurately controlled, providing good preconditions for the data stability and scientificity of subsequent saturated soft clay-structure interface tests.

[0029] Furthermore, through computer programming software and using a control module, automatic hierarchical loading of vertical pressure can be realized, and the true height of the soil mass can be measured in real time, so as to judge whether the specimen reaches the stop standard of hierarchical load or reaches the final ideal specimen height (that is, the termination condition of the preconsolidation test). Given that the loading grading size of the whole test is given, and the hierarchical load and the termination condition of the whole test can also be directly measured, unmanned operation of the whole test can be achieved. Brief Description of the Drawings

[0030] Figure 1 Shows a schematic structural diagram of the preconsolidation mold of the present invention;

[0031] Figure 2 Is the preloading process diagram and sample forming diagram of the improved slurry method of the present invention.

[0032] Detailed Description of the Specific Embodiments

[0033] In order to make the purpose, technical solutions and advantages of the present invention clearer and more understandable, the present invention will be further described in detail below in conjunction with specific embodiments.

[0034] Example 1:

[0035] Use the preconsolidation mold as Figure 1 shown to prepare standard saturated soft clay specimens with structures having a height of 120 mm, a diameter of 61.8 mm and an inner diameter of 20 mm. The soil mass is taken from typical soft clay in the central South China Sea Basin. The preconsolidation mold includes a hollow punch, a porous base, a hollow mold cylinder and a friction pile with a diameter of 20 mm.

[0036] Add water to the dried South China Sea soil to prepare slurry. The initial water content of the slurry is 1.35 times that of the South China Sea soil with a liquid limit ω L = 100%. Let the prepared slurry stand in a container for 24 h, and shake the slurry in the container every 6 h to exhaust air.

[0037] As Figure 2 shown, carry out the integrated preparation of the specimen:

[0038] Step 1: Filter paper and permeable cotton cloth are respectively pasted on the surfaces of the mold cylinder punch and the base in sequence. Their function is to prevent soil particles from flowing out along the water outlet holes after the preconsolidation load is applied. The bottom area S of the hollow soil massa = 26.85 cm 2 , the initial mud height h ini = 17.6 cm, the ideal soil body height h after preloading obj = 12 cm, the initial mud density ρ ini and the soil density ρ after preloading obj are both calculated from Equation (2). Therefore, the mass of the mud poured into each preconsolidation mold is m ini = 647.8 g;

[0039] Step 2: Prepare 6 specimens each time for the same batch. Place the 6 preconsolidation molds filled with mud in the consolidometer, cover them with a cover plate with a mass of 7 kg, add a lever support above the cover plate, and through the computer programming software that controls the magnitude of the vertical load, set the stop condition for each level of load and the stop criterion for the preconsolidation test (that is, the stop condition for each level of load is: the mud drops less than 1 mm per hour. The stop criterion for the preconsolidation test is: the specimen height reaches the ideal soil body height h after preloading obj ). Set the pre-applied vertical loads of 3.125 kPa, 6.25 kPa, 12.5 kPa, 25 kPa, 50 kPa, 75 kPa, and 100 kPa. The total contact area S between the specimen and the hollow punch head is 1.611×10 -2 m 2 . The magnitude of each level of vertical load can be calculated from Equation (5). When the preconsolidation test reaches the stop criterion, the computer control system forcibly stops the preconsolidation vertical load and reminds the tester that the test has ended. The final preconsolidation load of this embodiment's preconsolidation test is 75 kPa.

[0040] Step 3: Push out the soil body containing the structure along the bottom to obtain a saturated soft clay specimen containing the structure, as Figure 2 shown in c.

[0041] It can be seen that by using the improved integrated specimen preparation method for saturated soft clay specimens containing structures of the present invention, the surface of the obtained specimen soil body is regular and uniform, without obvious bubbles and fracture phenomena. Through computer control, automatic hierarchical loading of vertical pressure can be achieved, and the true height of the soil body can be measured in real time, so as to judge whether the specimen reaches the stop criterion for the hierarchical load or reaches the final ideal specimen height (that is, the termination condition of the preconsolidation test). Given that the magnitude of the loading grading in the whole test is given, and the grading load and the termination condition of the whole test can also be directly measured, unmanned operation of the whole test can be achieved.

[0042] The above are only embodiments of the present invention, and do not thus limit the patent scope of the present invention. Any equivalent structure or equivalent process transformation made by using the content of the specification and drawings of the present invention, or directly or indirectly applied in other related technical fields, shall similarly be included within the patent protection scope of the present invention.

Claims

1. A method for preparing a saturated soft clay specimen containing a structure, comprising: Step 1: Select a pre-consolidation mold. Filter paper and permeable cotton cloth are sequentially pasted on the surfaces of the mold cylinder ram and the base. Pour a certain mass of slurry into the pre-consolidation mold containing the structure. Step 2: Place the pre-consolidation mold in a consolidometer, cover the cover plate, and apply the set pre-consolidation stress in stages until the test stop criterion is reached. Step 3: Push out the soil mass containing the structure along the bottom to obtain a saturated soft clay specimen containing the structure.

2. The sample preparation method according to claim 1, wherein, In Step 1, it also includes the preparation of slurry, controlling the water content of the slurry to be 1.0 to 2.0 times the liquid limit ω of the soil mass L of the prepared slurry is left standing in a container for 24 h, and the slurry in the container is shaken to exhaust air every 6 h.

3. The sample preparation method according to claim 1, wherein, In Step 1, the pre-consolidation mold includes: a hollow ram, a porous base, a hollow mold cylinder, and a friction pile. Pour the slurry into the hollow mold cylinder containing the structure.

4. The sample preparation method according to claim 1, wherein, In Step 1, the mass of the slurry is calculated by the following formula: Assume that during the staged loading and preloading process, only a very small number of soil particles are lost, and the drained volume is the volume of water in the slurry. Therefore, an equation can be established. ρ ini ×h ini ×S a -(h ini -h obj )×S a ×ρ ω =ρ obj ×S a ×h obj (1) Where ρ ini is the initial density of the prepared slurry, in g / cm 3 ; h ini is the initial height of the slurry after pouring into the mold cylinder, in cm; S a is the bottom area of the hollow soil body, in cm 2 , h obj is the ideal soil body height after preloading, in cm, ρ obj is the soil body density after preloading, in g / cm 3 , Initial mud density ρ ini and the density of the soil mass ρ obj after preloading are both related to the specific gravity of soil particles and the water content in this state. Referring to soil mechanics, the expression for the density of the soil mass can be obtained as follows: Where ρ is the soil density in g / cm 3 ; w is the water content of the soil in %; ρ w is the density of pure water at 4°C, usually denoted as 1 g / cm 3 ; Initial mud density ρ ini and the height h of the ideal soil mass obj are known values, so the initial mud height h ini has the expression as follows: Let the total mass of the poured slurry be mini, in g, and its calculation formula is shown in Equation (4): m ini = h ini × ρ ini × S a (4).

5. The sample preparation method according to claim 4, wherein, In Step 2, the loading level can be appropriately adjusted according to the in-situ burial depth of the soil mass and the test requirements. The calculation formula for the magnitude of the vertical load is shown in (5): where p is the magnitude of the vertical load in kPa; M d is the weight of the surcharge load applied in kN; M g is the weight of the cover plate above the specimen in kN; S is the total contact area between all specimens and the hollow indenter in m 2 .

6. The sample preparation method according to claim 5, wherein, In Step 2, apply vertical loads of 3.125 kPa, 6.25 kPa, 12.5 kPa, 25 kPa, 50 kPa, 75 kPa, and 100 kPa in stages.

7. The sample preparation method according to claim 4, wherein In Step 2, the stop condition for each stage of load is: the slurry drops less than 1 mm per hour.

8. The sample preparation method according to claim 4, wherein, In Step 2, the stop criterion for the pre-consolidation test is that the height of the specimen reaches the ideal soil height h after preloading. obj .

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