Shield mud cake risk evaluation system based on stratum parameters and construction method

Abstract

The invention relates to the technical field of tunnel engineering, in particular to a shield mud cake risk evaluation system based on stratum parameters and a construction method.The risk evaluation system comprises the steps that stratum grain composition is conducted, and stratum grain composition measurement is conducted; judging a critical particle size d: judging whether the detected stratum contains particles smaller than the critical particle size d, and if the detected stratum contains particles smaller than the critical particle size d, judging a critical surface area ratio k; judging a critical surface area ratio k: judging the size relationship between the surface area ratio k0 of the fine particles to the coarse particles of the measured stratum and the critical surface area ratio k, and if the surface area ratio k0 of the fine particles to the coarse particles is greater than the critical surface area ratio k, judging a viscosity index Ic; and judging the viscosity index Ic: calculating the viscosity index Ic of the fine particles, and judging the risk degree of the shield mud cake according to the viscosity index Ic of the fine particles. According to the method, a shield mud cake judgment system based on the stratum parameters is established, and accurate pre-judgment of the shield mud cake risk caused by the viscous stratum is achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of tunnel engineering, and in particular to a shield mud cake risk assessment system and construction method based on stratum parameters. Background Art

[0002] Mud cake formation on shield structures has always been a major problem for tunnel engineers. If not promptly addressed, this can lead to adverse working conditions, such as increased wear of the cutter and cutterhead, blockage of the slurry suction port, increased load on the main drive motor, increased energy consumption, and a significant decrease in tunneling speed. There are many reasons for mud cake formation in shield structures, among which the formation is a major factor beyond human control. Formations prone to mud cake formation are primarily coarse-fine mixed formations containing both coarse particles such as gravel, pebbles, and sand, and fine particles such as silt and clay.

[0003] The factors that affect the shield structure mud cake caused by this coarse-fine particle mixed stratum mainly include the following three points: first, the critical particle size of the stratum that causes shield structure mud cake. Fine particles are prone to cause shield structure mud cake, while coarse particles will not cause shield structure mud cake. Therefore, there should be a boundary between coarse and fine particles, that is, the critical particle size that causes shield structure to easily form mud cake; second, the ratio of fine and coarse particles when causing shield structure mud cake. When fine particles fully wrap coarse particles, the shield structure is prone to form mud cake. The premise for fine particles to wrap coarse particles is that the surface area of ​​fine particles must be greater than the surface area of ​​coarse particles. Therefore, determining the ratio of fine and coarse particles when it is easy to cause shield structure mud cake is essentially to determine the critical surface area ratio of fine and coarse particles; in addition to the content of fine particles in the stratum, the factors that affect shield structure mud cake include the soft and hard state of the soil sample. When the moisture content of the soil sample in the stratum is close to the plastic limit, its adhesion to the cutterhead and tool is stronger, and the easier it is to form mud cake.

[0004] Existing techniques generally evaluate the adhesion of ground formations based on the viscosity index. However, according to the "Standard for Geotechnical Test Methods," only soil samples with a particle size less than 0.5 mm can have their liquid limit and plastic limit determined. Therefore, evaluating adhesion based on the viscosity index is only applicable to fine-grained soils with a maximum particle size less than 0.5 mm and is not applicable to cohesive soils with a maximum particle size greater than 0.5 mm. Therefore, it is urgent to develop a shield mud cake risk assessment system and construction method based on ground parameters. Summary of the Invention

[0005] The present invention provides a shield mud cake risk assessment system and construction method based on stratum parameters. Traditional mud cake risk assessment is mainly based on experience, and the evaluation method is inaccurate. In addition, the quantitative assessment method based on viscosity index is not suitable for strata containing coarse particles. The present invention proposes a critical particle size index and surface area ratio index of stratum debris that causes shield mud cake formation in cohesive strata, establishes a shield mud cake assessment system based on stratum parameters, and realizes accurate risk prediction of shield mud cake caused by cohesive strata.

[0006] The technical solution of the present invention is: a shield mud cake risk assessment system based on formation parameters, the assessment system includes:

[0007] Stratum particle gradation: Determine the stratigraphic particle gradation and moisture content;

[0008] Critical particle size d evaluation: Based on the formation particle grading results, determine whether the measured formation contains particles smaller than the critical particle size d. If the measured formation does not contain particles smaller than the critical particle size d, there is no risk of mud cake formation in the shield. If the measured formation contains particles smaller than the critical particle size d, the critical surface area ratio k is evaluated.

[0009] Critical surface area ratio k evaluation: judge the relationship between the surface area ratio k0 of the measured fine particles and coarse particles and the critical surface area ratio k. If the surface area ratio k0 of the fine particles and coarse particles is less than the critical surface area ratio k, there is no risk of mud cake formation in the shield. If the surface area ratio k0 of the fine particles and coarse particles is greater than the critical surface area ratio k, the viscosity index I is calculated. c judge;

[0010] Viscosity Index I c Evaluation: Calculate the viscosity index I of fine particles c , and according to the viscosity index I of fine particles c Determine the risk level of shield mud cake formation, including:

[0011] When the viscosity index I c When it is less than 0, the measured stratum is in a fluid state and there is no risk of mud cake formation in the shield;

[0012] When the viscosity index I c When it is greater than 0 and less than 0.5, the measured stratum is very soft and the risk of shield mud cake formation is low;

[0013] When the viscosity index I c When it is greater than 0.5 and less than 0.75, the measured stratum is in a soft state and the risk of shield mud cake formation is high;

[0014] When the viscosity index I c When it is greater than 0.75 and less than 1, the measured stratum is hard and the risk of shield mud cake formation is high;

[0015] When the viscosity index I c When it is greater than 1, the debris in the measured stratum is clumped and the risk of mud cake formation in the shield is low.

[0016] According to the present invention, a shield mud cake risk assessment system based on stratum parameters is provided. The stratum particle gradation is measured by a screening method, and the specific experimental steps refer to the specification "Highway Aggregate Testing Procedure"; the moisture content is measured by a moisture method, and the specific experimental steps refer to the specification "Soil Moisture Determination Method".

[0017] According to the present invention, a shield mud cake risk assessment system based on formation parameters is provided, wherein the value of the critical particle size d is determined by a first rotational shear test.

[0018] According to the present invention, a shield mud cake risk assessment system based on formation parameters is provided. The first rotational shear test is conducted using soil samples of different particle sizes to find the maximum particle size at which the interface cohesion c is always 0. The maximum particle size is the critical particle size d.

[0019] According to the present invention, a shield mud cake risk assessment system based on stratum parameters is provided, wherein the fine particles are stratum debris particles with a particle size smaller than the critical particle size d; the coarse particles are stratum debris particles with a particle size larger than the critical particle size d.

[0020] According to the present invention, a shield mud cake risk assessment system based on formation parameters is provided, wherein the surface area ratio k0 is the ratio of the specific surface area of ​​fine particles to that of coarse particles.

[0021] According to the present invention, a shield mud cake risk assessment system based on formation parameters is provided, wherein the value of the critical surface area ratio k is determined by a second rotational shear test.

[0022] According to the present invention, a shield mud cake risk assessment system based on formation parameters is provided. The second rotational shear test is conducted using soil samples with different surface area ratios k0, and the minimum surface area ratio k0 at which the interface cohesion c is always 0 is found. The minimum surface area ratio k0 is the critical surface area ratio k.

[0023] According to the present invention, a shield mud cake risk assessment system based on formation parameters is provided, wherein the viscosity index I c is the viscosity index I of fine particles in stratum soil with particle size smaller than the critical particle size d c .

[0024] Based on the same inventive concept, the present invention also provides a shield construction method, which is a shield construction method based on any of the above-mentioned shield mud cake risk assessment systems based on stratum parameters. When the excavated stratum has the risk of causing shield mud cake, the construction is guided according to the risk assessment system and countermeasures are prepared in advance.

[0025] The above technical solution of the present invention is not a limitation on the time-series steps of the method of the present invention. The steps of the present invention can be but are not limited to the above steps.

[0026] The advantages of the present invention are:

[0027] 1. This invention solves the problems of traditional shield mud cake assessment methods, which are based on experience and are inaccurate, and quantitative assessment methods that are not suitable for coarse-particle strata. It proposes a critical particle size index for stratum debris that causes shield mud cake formation, as well as an index for the critical surface area ratio of debris particles in fine-coarse mixed cohesive strata. It also establishes a shield mud cake assessment system based on stratum parameters, achieving accurate prediction of the risk of mud cake formation in cohesive strata.

[0028] 2. The present invention measures the formation particle size distribution by screening, and the screening is carried out strictly in accordance with the specifications, which makes the determination of the formation particle size distribution more accurate;

[0029] 3. The present invention introduces a critical particle size index to conduct a preliminary screening of formation parameters, which can quickly screen out some formations with significantly larger particle sizes and lower risk of mud cake formation, thus improving the shield mud cake risk assessment system;

[0030] 4. The value of the critical particle size d of the present invention is determined by a large-scale rotational shear test, and the data is accurate and reliable;

[0031] 5. The present invention divides formation particles into fine particles and coarse particles by comparing the formation particle size with the critical particle size d, which facilitates the calculation of the surface area ratio index proposed later;

[0032] 6. The present invention further screens formation parameters by introducing the surface area ratio index, which can quickly screen out some formations with a lower risk of mud cake formation and a significantly higher content of coarse particles, thus improving the shield mud cake risk assessment system;

[0033] 7. The value of the critical particle size d of the present invention is determined by a large-scale rotational shear test, and the data is accurate and reliable;

[0034] 8. Viscosity index of fine particles of the present invention I c The mud cake risk determination was based on the literature [Hollmann FS, Thewes M. Assessment method for clay clogging and disintegration of fines inmechanised tunnelling[J]. Tunnelling and Underground Space Technology, 2013, 37: 96-106.], which provides a reference for the viscosity index I cA careful and rigorous study was conducted to give the viscosity index I c Relationship diagram with shield mud cake risk. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 : Flowchart of the shield mud cake risk assessment system based on formation parameters of the present invention;

[0036] Figure 2 : Criteria diagram of shield mud cake formation at different particle sizes in the large-scale rotating shear test of the present invention;

[0037] Figure 3 : Shield mud cake criterion diagram under different fine-to-coarse particle surface area ratios in the large-scale rotational shear test of the present invention;

[0038] Figure 4 : viscosity index I of the present invention c Classification diagram of the relationship with the risk of shield mud cake formation. DETAILED DESCRIPTION

[0039] The embodiments of the present invention are described in detail below, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to be used to explain the present invention, but are not to be construed as limiting the present invention.

[0040] In the description of the present invention, it should be understood that the terms "longitudinal", "transverse", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside" and the like to indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as limiting the present invention.

[0041] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of the technical features being referred to. Thus, a feature specified as "first" or "second" may explicitly or implicitly include one or more of the features. In the description of the present invention, "plurality" means at least two, such as two, three, etc., unless otherwise specifically defined.

[0042] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0043] The present invention relates to a shield mud cake risk assessment system based on stratum parameters, proposes a critical particle size index and surface area ratio index of stratum debris that causes shield mud cake formation in cohesive strata, establishes a shield mud cake assessment system based on stratum parameters, and realizes accurate risk prediction of shield mud cake formation caused by cohesive strata.

[0044] Specifically, such as Figure 1 As shown, the evaluation system of the present invention includes:

[0045] Stratum particle gradation: Determine the stratigraphic particle gradation and moisture content;

[0046] Critical particle size d evaluation: Based on the formation particle grading results, determine whether the measured formation contains particles smaller than the critical particle size d. If the measured formation does not contain particles smaller than the critical particle size d, there is no risk of mud cake formation in the shield. If the measured formation contains particles smaller than the critical particle size d, the critical surface area ratio k is evaluated.

[0047] Critical surface area ratio k evaluation: judge the relationship between the surface area ratio k0 of the measured fine particles and coarse particles and the critical surface area ratio k. If the surface area ratio k0 of the fine particles and coarse particles is less than the critical surface area ratio k, there is no risk of mud cake formation in the shield. If the surface area ratio k0 of the fine particles and coarse particles is greater than the critical surface area ratio k, the viscosity index I is calculated. c judge;

[0048] Viscosity Index I c Evaluation: Calculate the viscosity index I of fine particles c , and according to the viscosity index I of fine particles c Determine the risk level of shield mud cake formation, including:

[0049] When the viscosity index I c When it is less than 0, the measured stratum is in a fluid state and there is no risk of mud cake formation in the shield;

[0050] When the viscosity index I c When it is greater than 0 and less than 0.5, the measured stratum is very soft and the risk of shield mud cake formation is low;

[0051] When the viscosity index I c When it is greater than 0.5 and less than 0.75, the measured stratum is in a soft state and the risk of shield mud cake formation is high;

[0052] When the viscosity index I c When it is greater than 0.75 and less than 1, the measured stratum is hard and the risk of shield mud cake formation is high;

[0053] When the viscosity index I c When it is greater than 1, the debris in the measured stratum is clumped and the risk of mud cake formation in the shield is low.

[0054] In some embodiments, the method for determining the formation particle size distribution of the present invention is optimized. In this embodiment, the formation particle size distribution is determined by a screening method.

[0055] The specific experimental steps can refer to the specification "Highway Aggregate Test Procedure":

[0056] 1) Prepare the following experimental equipment: test sieve, vibrating screen, balance, soft brush, smooth paper (such as tracing paper), and watch glass;

[0057] 2) Sample preparation: Mix the sample to be analyzed thoroughly, and then weigh 50 grams of the sample.

[0058] 3) Install the sieve: Select the test sieve and put it on in descending order according to the sieve hole size. Place the base plate at the bottom and the sample on the sieve with the largest hole size at the top. Install the cover and fix it on the vibrating screen machine.

[0059] 4) Start screening: start the vibrating screen and vibrate for 10 minutes;

[0060] 5) Weighing the powder: After sieving, weigh the amount of powder on each sieve surface and the bottom plate to the nearest 0.1 g;

[0061] 6) Collect the powder: Take out a sieve from each sieve, pour the powder on a smooth paper, use a soft brush to sweep the powder attached to the sieve and the bottom of the sieve frame into the next adjacent sieve, then turn the sieve upside down on the smooth paper and gently tap the sieve frame to clear all the powder in the sieve;

[0062] 7) Data processing: Calculate the percentage of powder mass on each sieve surface to the total powder mass.

[0063] In particular, in the embodiment of the present invention, only particles having a particle size smaller than the critical particle size d need to be screened out.

[0064] In some embodiments, the method for determining the formation water content of the present invention is optimized. In this embodiment, the water content is determined using a moisture method for testing.

[0065] The specific experimental steps can refer to the standard "Soil Moisture Determination Method":

[0066] 1. Experimental steps

[0067] 1) Take 15-30g of representative sample or 50g of sample from a ring cutter for organic soil, sandy soil and monolithic frozen soil, put it into a weighing box, cover the box, and weigh the mass of the moistened soil in the box to an accuracy of 0.01g.

[0068] 2) Open the lid of the box and place it in an oven. Bake at a constant temperature of 105-110°C until the weight is constant. The drying time should be no less than 8 hours for clay and silt, and no less than 6 hours for sand. For soil containing more than 5% of organic matter by weight, the temperature should be controlled at 65-70°C until the weight is constant.

[0069] 3) Take the weighing box out of the oven, cover it, place it in a dry container and cool it to room temperature, and weigh the mass of the box plus dry soil to an accuracy of 0.01g.

[0070] 2. The moisture content W0 of the test is calculated according to the following formula, accurate to 0.1%;

[0071] W0=(m0 / md-1)×100%

[0072] In the formula: md is the mass of dry soil (g); m0 is the mass of wet soil (g).

[0073] 3. Moisture content testing of frozen soil with layered or reticular structures should be conducted according to the following steps: Use the quartering method to cut a 1000-2000g sample (depending on the uniformity of the frozen soil structure; take less if the structure is uniform, and more if it is not) and place it in an enamel pan. Weigh the pan and sample to the nearest 0.1g. After the frozen soil sample thaws, mix it into a uniform paste (if the soil is too wet, allow excess water to evaporate naturally or use a suction bulb to remove it, but do not remove soil particles. If the soil is too dry, add appropriate amount of water). Weigh the paste and pan to the nearest 0.1g. Sample the soil from the paste to determine its moisture content. The test procedures and calculations shall be in accordance with Sections 2 and 3 of this standard.

[0074] 4. The moisture content W of layered and reticular frozen soil should be calculated according to the following formula, accurate to 0.1%.

[0075] W=[(m1 / m2)(1+0.01Wh)-1]×100%

[0076] Where: W——water content (%);

[0077] m1——mass of frozen soil sample (g);

[0078] m2——mass of paste sample (g);

[0079] Wh——Water content of paste sample (%).

[0080] 5. This test must be conducted on two specimens in parallel. The difference between the measurements is: ±0.5% when the moisture content is ≤5%; ±1% when the moisture content is >5% and ≤40%; ±2% when the moisture content is >40%; and <3% for frozen soil with layered or reticular structures. The average of the two measurements is to be expressed as a percentage.

[0081] In some embodiments, the critical particle size d of the present invention is optimized. In this embodiment, the value of the critical particle size d is determined by a first rotational shear test.

[0082] The first rotary shear test was conducted using a large rotary shear instrument. The equipment consists of a sample chamber, a loading system, a power system, and a control system. A soil sample is placed in the sample chamber, and normal pressure is applied to the sample through the loading system. The power system then drives the disc to rotate and measures the rotational torque in real time. The control system is responsible for controlling parameters such as normal pressure and rotation speed, and recording and storing test data.

[0083] Shield mud cake is the adhesion of stratum debris to the cutterhead or cutting tool. Its essence is the same as that of clay adhering to a metal disc. During large-scale rotary shear tests, it was found that when the cohesion c between the soil sample and the metal interface is 0, no soil sample adheres to the disc after the test is completed; when the cohesion c between the soil sample and the metal is not 0, the soil sample will adhere to the disc after the shear test is completed, and the greater the interface cohesion c, the more soil sample adheres to the disc. Although the soil sample adhered to the disc is disturbed during the process of removing the disc, the adhesion of the soil sample on the disc cannot represent its actual situation, but it can still qualitatively reflect the influence of the interface cohesion c on the adhesion.

[0084] Therefore, the interfacial cohesion c can be used to characterize the adhesion of the soil sample. That is, when the interfacial cohesion c of the soil sample is 0, the adhesion of the soil sample is poor, and there is no possibility of mud cake formation when the shield tunneling in this stratum; when the interfacial cohesion c of the soil sample is not 0, the soil sample has a certain adhesion, and there is a risk of mud cake formation when the shield tunneling in this stratum.

[0085] like Figure 2 As shown in the figure, after repeated experiments, it was found that when the particle size is less than 0.15mm, the soil sample has a certain interface cohesion c, indicating that the soil sample with this particle size may form mud cakes; when the particle size is greater than 0.15mm, the interface cohesion c of the soil sample is always 0, indicating that the soil sample with this particle size may not form mud cakes. Therefore, it can be determined that the critical particle size d of stratum debris that is prone to cause shield mud cakes is 0.15mm.

[0086] In some embodiments, the surface area ratio k0 and the critical surface area ratio k of the present invention are optimized. In this embodiment, the surface area ratio k0 is the ratio of the specific surface area of ​​fine particles to coarse particles; the value of the critical surface area ratio k is determined by a second rotational shear test.

[0087] In order to determine the ratio of the total surface area of ​​coarse particles to the total surface area of ​​fine particles per unit mass of mixed stratum soil with coarse-fine particles of different surface area ratios, it is necessary to obtain the specific surface area of ​​soil samples in each particle size range, and the calculation method is as follows:

[0088] Assuming that the soil particles are spherical, the volume V and surface area S of each particle are:

[0089]

[0090] S=4πR 2

[0091] Where: R is the radius of the particle. For a certain range of particle sizes, the average value is taken. For example, for soil samples with a particle size range of 0.15 to 0.25 mm, 0.2 mm is taken. The same applies to other samples.

[0092] The number of particles n contained in a unit mass of soil sample is:

[0093]

[0094] Where: G s is the specific gravity of soil particles.

[0095] The specific surface area B of the soil sample is:

[0096] B=nS

[0097] The calculated specific surface areas of soil samples in each particle size range are shown in Table 1.

[0098] Then, based on the specific surface area of ​​each particle size, the surface area of ​​particles smaller than the critical particle size of 0.15 mm and the surface area of ​​particles larger than the critical particle size of 0.15 mm in the slag are calculated respectively, and the ratio k0 between the two is obtained.

[0099] Table 1 Specific surface area of ​​soil particles in various particle size ranges

[0100]

[0101] The determination of the critical surface area ratio k requires the introduction of a large second rotational shear test:

[0102] The second rotational shear test is measured using a large-scale rotational shear instrument. The equipment consists of a sample chamber, a loading system, a power system and a control system. A soil sample is placed in the sample chamber, and normal pressure is applied to the sample through the loading system. The power system then drives the disc to rotate and measures the rotational torque in real time. The control system is responsible for controlling parameters such as normal pressure and rotation speed, and recording and storing test data.

[0103] Shield mud cake is the adhesion of stratum debris to the cutterhead or cutting tool. Its essence is the same as that of clay adhering to a metal disc. During large-scale rotary shear tests, it was found that when the cohesion c between the soil sample and the metal interface is 0, no soil sample adheres to the disc after the test is completed; when the cohesion c between the soil sample and the metal is not 0, the soil sample will adhere to the disc after the shear test is completed, and the greater the interface cohesion c, the more soil sample adheres to the disc. Although the soil sample adhered to the disc is disturbed during the process of removing the disc, the adhesion of the soil sample on the disc cannot represent its actual situation, but it can still qualitatively reflect the influence of the interface cohesion c on the adhesion.

[0104] Therefore, the interfacial cohesion c can be used to characterize the adhesion of the soil sample. That is, when the interfacial cohesion c of the soil sample is 0, the adhesion of the soil sample is poor, and there is no possibility of mud cake formation when the shield tunneling in this stratum; when the interfacial cohesion c of the soil sample is not 0, the soil sample has a certain adhesion, and there is a risk of mud cake formation when the shield tunneling in this stratum.

[0105] like Figure 3 As shown in the figure, when the surface area ratio of fine and coarse particles is 10:1, 15:1, 20:1, and 25:1, respectively, the interfacial cohesion c between the soil sample and the metal interface is always 0 under any moisture content; when the surface area ratio of fine and coarse particles is 30:1 and 35:1, the interfacial cohesion c is 0 only when the moisture content is 0. Under other moisture contents, the interfacial cohesion c is not 0, and with the increase of moisture content, the interfacial cohesion c between the soil sample and the metal interface first increases and then decreases.

[0106] Therefore, for coarse-grained mixed clayey soil, the critical surface area ratio, k, of fine to coarse particles, at which shield tunneling is prone to cake formation, can be considered 25:1. If the surface area ratio, k0, of fine to coarse particles in the soil sample is less than the critical surface area ratio, k, there is no risk of cake formation when the shield tunnel is excavating in this stratum. However, if the surface area ratio, k0, of fine to coarse particles in the soil sample is greater than the critical surface area ratio, k, there is a risk of cake formation when the shield tunnel is excavating in this stratum.

[0107] In this embodiment, a liquid-plastic limit combined measuring instrument with a cone mass of 76g is used to measure the sinking depth of the cone and the corresponding moisture content when the soil layer sample is close to the liquid limit, plastic limit and intermediate state. Then, a plot is drawn based on the measured results to find the liquid limit and plastic limit of the soil layer sample, and finally the viscosity index I is calculated based on the results. c .

[0108] Specifically, to calculate the moisture content of fine particles in stratum debris with a particle size smaller than the critical particle size d, it is necessary to know the natural moisture content of the stratum debris and the water absorption rate of coarse particles with a particle size larger than the critical particle size d. The difference between the two is the moisture content of the fine particles.

[0109] Determination of water absorption of coarse particles:

[0110] When determining the ideal state of mixed slag with coarse and fine particles, it is necessary to measure the water absorption rate of particles larger than 0.15mm in the slag. According to the relevant provisions of the "Test Procedures for Highway Engineering Aggregates", the water absorption rate test method for soil particles larger than 2mm is as follows: Place the sample in a container filled with water, so that the water level is about 5mm higher than the sample surface. After 24 hours, remove the sample from the water and gently wipe the moisture on the surface of the particles with a wrung-out wet towel. Immediately measure the moisture content of the sample, which is the water absorption rate of the soil particles.

[0111] The water absorption test method for 0.15-2mm soil particles is as follows:

[0112] (1) Place the sample in a shallow container and add clean water so that the water level is about 20 mm above the sample surface. Stir continuously with a glass rod for 5 minutes to eliminate bubbles and let it stand for 24 hours.

[0113] (2) Pour off the water on the top of the sample, but do not pour out the fine powder, and use a straw to absorb the remaining water.

[0114] (3) Spread the sample on a plate and slowly blow warm air into it with a handheld hair dryer. Stir the sample constantly to allow the water on the surface of the particles to evaporate evenly from all parts, achieving the estimated soil sample state. Be careful not to lose fine powder during the blowing process.

[0115] (4) Then loosely load the sample into the saturated dry test mold at one time, and tamp it lightly 25 times with a tamping rod. The distance between the end of the tamping rod and the surface of the sample should not exceed 10 mm, so that it can fall freely. After tamping, scrape the mold mouth flat. If there is any gap left, there is no need to fill it up again.

[0116] (5) Slowly lift the test mold from the vertical direction. If the sample retains its conical shape and does not collapse, it means that the soil sample still contains surface water. Continue to dry and test with warm air according to the above method until the sample begins to collapse after the test mold is lifted. If the sample collapses too much after the test mold is lifted, it means that the sample has been dried too much. At this time, sprinkle the sample with about 5mL of water evenly, mix it thoroughly, and let it stand in a covered container for 30 minutes. Then continue to test according to the above method until the desired state is achieved.

[0117] (6) After the slump test is completed, immediately take a sample of no less than 20g and measure its moisture content, which is the water absorption rate of the sample.

[0118] Liquid and plastic limit test:

[0119] 1) Sample preparation: In principle, natural soil samples are used to prepare specimens for liquid limit and plastic limit tests. However, air-dried soil samples are also allowed to prepare specimens. When the soil sample is uneven, air-dried samples are used. When the sample contains soil particles and debris with a particle size greater than 0.15 mm, it is passed through a 0.15 mm sieve. Take 200 g of a representative soil sample under the 0.15 mm sieve, divide it into three parts, put them into soil dishes respectively, add pure water, and repeatedly adjust the moisture content of the soil until it reaches close to the liquid limit, intermediate state and plastic limit in turn, and then adjust it into a uniform paste. Samples are loaded in sequence for testing.

[0120] 2) Sample loading: Use a soil knife to fully stir the prepared soil sample in the bowl and fill it into the sample cup densely without leaving any gaps in the soil sample. Then use the soil knife to scrape off the excess soil paste along the rim of the cup and place it on the instrument cup. When scraping off the excess soil, do not use the knife to repeatedly smear the soil surface. Then place the sample cup on the base of the instrument.

[0121] 3) Install the cone gauge: Turn on the power supply to allow the electromagnet to attract the cone gauge.

[0122] 4) Zero adjustment: Adjust the screen alignment so that the initial reading is at the zero mark.

[0123] 5) Cone drop: Adjust the lifting seat nut. When the cone tip just contacts the soil surface, the timing indicator light will light up; the cone meter will fall freely, delay for 5 seconds, and the degree indicator light will light up, and the reading will be taken immediately. If manual operation is required, move the switch to the "manual" side. When the cone tip contacts the soil surface, the contact indicator light will light up, but the cone will not fall. Press the manual button to allow the cone meter to fall freely. After reading, press the reset button on the instrument so that it can be used again next time.

[0124] 6) Determine the moisture content: Take two samples of no less than 10g from the test cup and determine their moisture content.

[0125] 7) Repeat the test: Repeat steps 1 to 6 to measure the cone sinking depth and corresponding moisture content of the sample near the liquid limit, intermediate state and plastic limit respectively.

[0126] The moisture content is calculated according to the following formula:

[0127]

[0128] Where:

[0129] W represents the moisture content of the soil sample, m represents the wet soil weight of the soil sample, m s It represents the dry weight of the soil sample, and the calculation accuracy of W is 0.1%.

[0130] Plot the relationship between water content and cone sinking depth:

[0131] Plot the three moisture contents and the corresponding cone sinking depths on double logarithmic coordinate paper, and connect the three points with a straight line A. If the three points are not on a straight line, connect two straight lines through the point with the higher moisture content and the other two points, and find out the two moisture contents when the cone sinks to a depth of 2 mm under the straight line. If the difference between the two moisture contents does not exceed 2%, draw a straight line B between the average value of the two moisture contents and the point with the higher moisture content. If the difference between the two moisture contents exceeds 2%, an additional test should be performed.

[0132] In the relationship diagram between water content and cone sinking depth, when the cone sinking depth is 17 mm, the corresponding water content on line A or line B is the liquid limit W l When the sinking depth is 2 mm, the corresponding water content on line A or line B is the plastic limit W p .

[0133] Calculation of fine particle viscosity index I c :

[0134]

[0135] Where: W l represents the liquid limit of soil sample fine particles, W represents the moisture content of soil sample fine particles, W p Indicates the plastic limit of fine particles in the soil sample.

[0136] Finally, based on the literature [Hollmann FS, Thewes M. Assessment method for clay clogging and disintegration of fines in mechanised tunnelling [J]. Tunnelling and Underground Space Technology, 2013, 37: 96-106.], the viscosity index I is given. c Determination of the risk of shield mud cake formation, such as Figure 4 As shown:

[0137] When the viscosity index I c When it is less than 0, the shield soil sample is in a fluid state and there is no risk of mud cake formation in the shield;

[0138] When the viscosity index I c When it is greater than 0 and less than 0.5, the shield soil sample is very soft and the risk of shield mud cake formation is low;

[0139] When the viscosity index I c When it is greater than 0.5 and less than 0.75, the shield soil sample is soft and the risk of shield mud cake formation is high;

[0140] When the viscosity index I cWhen it is greater than 0.75 and less than 1.0, the shield soil sample is hard and the risk of shield mud cake formation is high;

[0141] When the viscosity index I c When it is greater than 1.0, the shield slag soil sample is hard and forms blocks, and the risk of shield mud cake formation is low.

[0142] The present invention also relates to a shield construction method, which is a shield construction method based on any of the above-mentioned shield mud cake risk assessment systems based on stratum parameters. When the excavated stratum has the risk of causing shield mud cake, the construction is guided according to the risk assessment system and countermeasures are prepared in advance.

[0143] Specifically: first, the stratum particle gradation of the stratum excavated by the shield is measured, and the measured soil samples are separated according to the particle size; then, it is determined whether the separated soil samples of the measured stratum contain particles smaller than the critical particle size d. If the measured stratum does not contain particles smaller than the critical particle size d, the shield has no risk of forming mud cakes and the shield continues to construct; if the measured stratum contains particles smaller than the critical particle size d, the surface area ratio k0 of fine particles with a particle size smaller than the critical particle size d to coarse particles with a particle size larger than the critical particle size d is calculated; if the surface area ratio k0 of fine particles to coarse particles is smaller than the critical surface area ratio k, there is no risk of forming mud cakes and the shield continues to construct; if the surface area ratio k0 of fine particles to coarse particles is greater than the critical surface area ratio k, the viscosity index I of the fine particles is calculated. c , when the viscosity index I c When the viscosity index I is less than 0, the measured stratum is in a fluid state, the shield has no risk of mud cake, and construction continues; c When the viscosity index I is greater than 0 and less than 0.5, the measured stratum is very soft and the risk of shield mud cake formation is low. Construction can continue after taking appropriate anti-mud cake measures. c When the viscosity index I is greater than 0.5 and less than 0.75, the measured stratum is in a soft state and the risk of shield mud cake formation is high. It is necessary to take appropriate anti-mud cake measures and pay attention to the state of shield mud cake formation at all times. c When the viscosity index is greater than 0.75 and less than 1, the measured stratum is hard and the risk of shield mud cake formation is high. It is necessary to take appropriate anti-mud cake measures and pay attention to the state of shield mud cake formation at all times. c When it is greater than 1, the stratum debris in the measured stratum is clumped and the risk of shield mud cake formation is low. Construction can continue after taking appropriate measures to prevent mud cake formation.

[0144] The basic principles, main features, and advantages of the present invention are shown and described above. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The above embodiments and descriptions are merely illustrative of the principles of the present invention. Various changes and modifications may be made to the present invention without departing from the spirit and scope of the present invention. Such changes and modifications are intended to fall within the scope of the present invention. The scope of protection claimed in the present invention is defined by the appended claims and their equivalents.

Claims

1. A shield mud cake risk assessment system based on formation parameters, characterized in that: The steps include: Formation particle gradation: Determination of formation particle gradation and water content; Critical particle size d judgment: According to the formation particle grading results, it is judged whether the measured formation contains particles smaller than the critical particle size d. If the measured formation contains particles smaller than the critical particle size d, the critical surface area ratio k is judged; Critical surface area ratio k: Determine the relationship between the surface area ratio k0 of the measured fine particles and coarse particles and the critical surface area ratio k. If the surface area ratio k0 of the fine particles and coarse particles is greater than the critical surface area ratio k, the viscosity index I is calculated. c judge; Viscosity Index I c Evaluation: Calculate the viscosity index I of fine particles c , and according to the viscosity index I of the fine particles c Determine the risk level of shield mud cake formation.

2. A shield mud cake risk assessment system based on formation parameters as claimed in claim 1, characterized in that: The stratum particle gradation determination is tested by a screening method; the moisture content determination is tested by a moisture method.

3. A shield mud cake risk assessment system based on formation parameters as claimed in claim 1, characterized in that: The value of the critical particle size d is determined by a first rotational shear test.

4. A shield mud cake risk assessment system based on formation parameters as claimed in claim 1 or 3, characterized in that: The first rotational shear test is conducted using soil samples with different particle sizes, and the maximum particle size at which the interface cohesion c is always 0 is found, and the maximum particle size is the critical particle size d.

5. A shield mud cake risk assessment system based on formation parameters as claimed in claim 1, characterized in that: The fine particles are stratum debris particles whose particle size is smaller than the critical particle size d; the coarse particles are stratum debris particles whose particle size is larger than the critical particle size d.

6. A shield mud cake risk assessment system based on formation parameters as claimed in claim 1, characterized in that: The surface area ratio k0 is the ratio of the specific surface area of ​​fine particles to that of coarse particles.

7. A shield mud cake risk assessment system based on formation parameters as claimed in claim 1, characterized in that: The value of the critical surface area ratio k is determined by a second rotational shear test.

8. A shield mud cake risk assessment system based on formation parameters as claimed in claim 1 or 7, characterized in that: The second rotational shear test is conducted using soil samples with different surface area ratios k0, and the minimum surface area ratio k0 at which the interface cohesion c is always 0 is found. The minimum surface area ratio k0 is the critical surface area ratio k.

9. A shield mud cake risk assessment system based on formation parameters as claimed in claim 1, characterized in that: The viscosity index I c is the viscosity index I of fine particles in the stratum slag with a particle size smaller than the critical particle size d c .

10. A shield construction method, characterized in that: The construction method is a shield construction method based on a shield mud cake risk assessment system based on stratum parameters as described in any one of claims 1 to 9. When there is a risk of shield mud cake forming in the excavated stratum, the construction is guided according to the risk assessment system and countermeasures are prepared in advance.

Images