CN118345886A - Deep sea sediment mechanical property evaluation method based on indoor static sounding test - Google Patents

Abstract

The invention provides a deep sea sediment mechanical property evaluation method based on an indoor static sounding test, and belongs to the field of geotechnical engineering investigation. The invention obtains the optimal soil sample size, the probe diameter, the optimal penetration depth and the optimal penetration rate based on an indoor static sounding device, develops an indoor static sounding test and a triaxial test on the basis, combines the indoor static sounding test data with the triaxial test data to obtain a plurality of mechanical characteristic parameters of a natural gas hydrate reservoir and an upper earth covering layer thereof, and establishes a set of deep sea sediment mechanical characteristic evaluation method. The method has the characteristics of rapidness, convenience, large data acquisition amount, small interference, low cost and the like, and can be used for repeatability, sample uniformity, application of known/controlled boundary conditions and application of specific stress history. Through an indoor static cone penetration test, establishing a corresponding empirical relationship between physical and mechanical properties of sediment in each region of the deep sea and penetration data, and inverting the properties of soil body strength, seepage, deformation and the like.

Description

A method for evaluating the mechanical properties of deep-sea sediments based on indoor static penetration tests

Technical Field

The invention belongs to the technical field of geotechnical engineering investigation, and in particular relates to a method for evaluating the mechanical properties of deep-sea sediments based on an indoor static penetration test.

Background technique

More than half of the world's major oil and gas discoveries come from the deep sea, and the deep sea will become one of the important successor areas for oil and gas resources. The hydrate mining process will cause the destruction of the reservoir structure and reduce the bearing capacity, which may cause the deformation and settlement of the hydrate reservoir and its overlying soil layer, and finally induce engineering geological disasters such as wellbore instability and submarine landslides. Therefore, before realizing the commercial exploitation of natural gas hydrate resources, it is necessary to conduct in-depth analysis and evaluation of the reservoir type, the boundary of the soil layer and various physical and mechanical parameters, and fully understand the mechanical properties of the natural gas hydrate reservoir and its overlying soil layer. Ensuring mining safety is a prerequisite for the development of deep-sea resources. Moreover, with the development and utilization of marine resources, there are more and more offshore structures. In order to ensure the safety and stability of offshore structures, it is necessary to determine the engineering properties of the structure foundation-marine soil.

Cone penetration testing (CPT) is an in-situ exploration technology that obtains data such as cone tip resistance, side wall friction resistance and pore pressure at different positions of the soil by pressing a cone probe equipped with sensors into the soil at a uniform speed. After data analysis, it can accurately analyze the soil type, soil layer boundary and various physical and mechanical indicators of deep-sea sediments and hydrate reservoirs. It has the advantages of fast and convenient, no sampling, large amount of data collection, continuous data, good reproducibility, low interference and low cost. Indoor static penetration testing has the ability to be repeatable, ensure sample uniformity, apply known/controlled boundary conditions and specific stress history. CPT data can not only be used for soil layer division and soil classification (based on friction ratio), but also for estimating the undrained shear strength, overconsolidation ratio, sensitivity of clay soil, relative density of sand, internal friction angle, soil compression modulus, deformation modulus, saturated clay undrained modulus, initial tangent elastic modulus and initial tangent shear modulus of sand soil, foundation bearing capacity, single pile bearing capacity and sand liquefaction judgment. Research has shown that CPT technology has significant advantages in deep-sea soil testing, is being used more and more widely in the field of marine engineering at home and abroad, and plays an important role in engineering geological surveys.

At present, there are few cases of application of static penetration in deep-sea sediments in my country, and there are few studies on soil layer judgment criteria of reservoirs based on static penetration and empirical formulas for inferring mechanical properties of reservoirs published in China, which have extremely high scientific research value and engineering application value. However, since CPT technology is based on electrical measurement technology, the interpretation and analysis method of CPT data is based on experience, and the empirical relationship used for interpretation has regional limitations. The existing various empirical formulas and judgment criteria have limitations and cannot be widely applied to soils of various types and regions. Only by constantly exploring and studying the interpretation methods of test data, and combining drilling data to further verify and continuously improve the interpretation methods, can we summarize the regional empirical values suitable for the characteristics of my country's marine soil. If you want to use CPT to analyze the mechanical properties of sediments, you need to establish the relationship between CPT data and sample mechanical test data through experiments; therefore, facing the difficulty of interpreting in-situ test data of deep-sea static penetration (CPT), the present invention uses a set of indoor static penetration devices to carry out the optimization of indoor static penetration test methods for deep-sea sediments. The present invention can adopt soil samples and probes of different sizes to carry out static penetration tests, and optimize the indoor CPT test method and equipment of deep-sea sediments based on indoor test data; at the same time, triaxial tests can be carried out to establish the corresponding empirical relationship between various physical and mechanical properties of sediments in various deep-sea regions and CPT penetration data, and summarize a set of deep-sea sediment mechanical property interpretation methods based on static penetration, so as to provide technical support for the major needs of my country's deep-sea resource development.

Summary of the invention

In order to solve the above problems in the prior art, the present invention provides a method for evaluating the mechanical properties of deep-sea sediments based on indoor static penetration tests to ensure the safe development of deep-sea resources.

The purpose of the present invention can be achieved by the following technical solutions:

A method for evaluating the mechanical properties of deep-sea sediments based on an indoor static penetration test. The method is implemented based on an indoor static penetration device, which includes an axial loading system, a base, a sample moving table, a calibration tank, a temperature and pressure control module, and a program control system.

The sample moving platform is connected to the side of the base, the sample moving platform and the surface of the base are on the same horizontal plane, and moving guide rails are installed on the upper surfaces of both.

The calibration tank has a large temperature range and is resistant to high pressure, and can simulate the pressure and temperature conditions under different formation conditions and restore the original state of the soil. The calibration tank is installed on a movable guide rail and can be moved along the movable guide rail, which is convenient for installing and disassembling test soil samples.

The axial loading system includes an upper loading component, a load frame, a lower loading component and a probe; wherein the upper loading component is installed above the base through the load frame, the probe is installed on the upper loading component and is located above the calibration tank, and the probe is driven by the upper loading component to penetrate into the soil sample in the calibration tank to complete the soil penetration test; the lower loading component is installed at the lower part of the base, and the position of the soil sample in the calibration tank is adjusted by driving the lower loading component.

The temperature and pressure control module provides a temperature and pressure environment for the test soil sample in the calibration tank.

The program control system is connected with the upper loading component, the lower loading component, the temperature and pressure control module and the probe to realize real-time monitoring and control of the test process. The loading parameters can be adjusted in real time according to the probe data, and data records can be provided to optimize the test process and improve data accuracy.

The lower loading assembly includes a lower servo motor, a lower reducer, a lower synchronous pulley, a lower synchronous belt, a lower ball screw, a lower loading beam, a lower beam and a lower loading rod. The lower reducer is installed on the lower beam and connected to the lower servo motor; three lower synchronous pulleys are installed at the bottom of the lower beam, and the lower synchronous belt is installed between the lower synchronous pulleys; one of the lower synchronous pulleys is connected to the lower reducer, and the other two lower synchronous pulleys are respectively connected to one end of the two lower ball screws located above the lower beam; the other end of the lower ball screw is installed on the base through a bearing; the lower loading beam is connected to the two lower ball screws through threads, and a lower loading rod is provided at the center of the lower loading beam; the lower reducer controls the rotation of the lower synchronous pulley under the drive of the lower servo motor, and drives the lower ball screw to rotate through the lower synchronous belt transmission, thereby driving the lower loading beam to move up and down, so that the lower loading rod pushes the soil sample to a suitable position.

The upper loading assembly includes an upper synchronous pulley, an upper synchronous belt, an upper reducer, an upper servo motor, an upper ball screw, an upper crossbeam and an upper loading crossbeam. The upper crossbeam is installed with an upper reducer through a reducer mounting plate; two upper synchronous pulleys are symmetrically installed at both ends of the upper crossbeam, and another upper synchronous pulley is installed on the reducer mounting plate and connected to the upper reducer, and the upper reducer is also connected to the upper servo motor; the upper synchronous belt is installed between the three upper synchronous pulleys; two upper ball screws are symmetrically arranged below the upper crossbeam, one end is connected to the two upper synchronous pulleys on the upper crossbeam, and the other end is installed on the load frame through a bearing. The upper loading crossbeam is connected to the two upper ball screws through threads, and a probe is installed at the center of the upper loading crossbeam; the upper reducer controls the rotation of the upper synchronous pulley under the drive of the upper servo motor, and drives the upper ball screw to rotate through the upper synchronous belt transmission, thereby driving the upper loading crossbeam to move up and down, and penetrate the probe into the soil sample.

The load frame is composed of a support arm and a shaft collar; wherein the two support arms are symmetrically fixed at the two ends of the upper surface of the base, and the two ends of the shaft collar are respectively connected to the upper crossbeam and the support arm, and the shaft collar ensures that the relative positions of the various components remain stable during the test.

The calibration tank includes a calibration tank base, a calibration tank shell, a sample base, a soil sample, a sample cap and a rubber membrane. Among them, the calibration tank base is a cylindrical structure with a groove on the top, which is installed on a movable guide rail; the calibration tank shell is a cylindrical shell structure with an opening at the bottom, which is connected to the calibration tank base to form a cavity between the two; the soil sample is wrapped by a rubber mold, and the upper and lower ends of the soil sample are respectively connected to the sample cap and the sample base through the rubber mold as a whole, and placed together in the cavity. A guide hole is provided at the center of the top of the calibration tank shell. During the upward movement of the soil sample, the protrusion of the sample cap is embedded in the guide hole to ensure that the soil sample is raised accurately. A cylindrical hole is opened in the center of the sample cap, and the probe penetrates the soil sample through the cylindrical hole. A through hole is provided at the center of the bottom of the calibration tank base to facilitate the lower loading component to act on the sample base.

The upper part of the calibration tank shell is provided with a lifting lug, which is connected with the lifting bolt and the lifting nut connected to the upper loading beam. The calibration tank shell is lifted by the upper loading assembly to facilitate the installation and removal of the soil sample.

A water injection hole is also provided on the top of the calibration tank shell, through which water is injected into the soil sample wrapped with the rubber membrane, thereby saving the available volume of the confining pressure pump.

A reserved interface is provided on the base of the calibration tank for completing more tests.

The sample base and the sample cap are provided with water inlet holes and drainage holes on their sides, which are connected to the reserved interfaces on the calibration tank base through stainless steel pipes, and water saturation and consolidation drainage tests are performed on soil samples through the water inlet holes and drainage holes.

The temperature and pressure control module consists of a circulating water pump and three pressure pumps that control pore pressure, back pressure and confining pressure respectively, so as to simulate the mechanical behavior of soil under different conditions. Among them, the circulating water pump is connected to the outside of the calibration tank shell, or to the reserved interface of the calibration tank base; the pressure pump is connected to the reserved interface of the calibration tank base through a pipeline.

The probe adopts a compression-designed probe structure and has two sets of built-in weighing sensors; it can independently measure the cone tip resistance and the side wall friction resistance without interfering with each other, and there is no need to consider the friction problem of the sealing ring; the diameter of the probe is consistent with the diameter of the cylindrical hole on the sample cap.

The method for evaluating the mechanical properties of deep-sea sediments comprises the following steps:

S1: calibration probe;

Probes with different diameters are installed on the calibration frame respectively, and a group of known forces are applied to the side wall and cone tip of the probe respectively. The known forces are fitted into a relationship with the cone tip resistance and side wall friction resistance measured before calibration to obtain a coefficient and deviation, and the measured cone tip resistance and side wall friction resistance are corrected to accurate values.

S2: Preparation work before optimizing the indoor static penetration test method for deep-sea sediments;

The calibration tank shell is lifted up by the upper loading assembly, and the calibration tank base is pushed along the moving guide rail to the sample moving platform to start making soil samples; when making soil samples, the sample base is first installed in the calibration tank base, and then the mold is fixed in the sample base. The soil sample is made by compaction method in a mold with a rubber mold of matching size; the above operations are repeated to prepare soil samples of different sizes, including soil samples of various diameters and various heights corresponding to each diameter; after the soil sample is made, the sample cap is placed on the soil sample, the calibration tank base is pushed onto the base, and then the calibration tank shell is lowered to the position connected to the calibration tank base; the probe is installed and the pipeline of the temperature and pressure control module is connected, and the preparation work before the test has been completed.

S3: Determine the optimal penetration rate;

Select the probe with the smallest diameter in S1 and the soil sample with the maximum height corresponding to the largest diameter prepared in S2, and conduct static penetration tests with different penetration rates under the same working conditions. The penetration depth of the probe is two-thirds of the soil sample height, and the cone tip resistance and side wall friction resistance are obtained; the obtained cone tip resistance and side wall friction resistance are compared with the cone tip resistance and side wall friction resistance results of the known soil samples. The penetration rate corresponding to the static penetration test with consistent results is the optimal penetration rate. Among them, the maximum soil sample size and the minimum probe size are selected to eliminate the boundary effect, and the penetration of two-thirds of the soil sample height is to avoid the influence of the bottom rigid boundary.

S4: Determine the optimal diameter ratio;

The optimal diameter ratio is the ratio of the soil sample diameter to the probe diameter. Determining the optimal diameter ratio is a prerequisite for simulating the infinite boundary environment of the real in-situ soil. The specific method is as follows:

At the best penetration rate determined by S3 and under the same working conditions, the test soil samples with different diameters at the maximum height obtained by S2 are subjected to static penetration tests in sequence using probes of different diameters calibrated in S1; the different diameter ratios are arranged from low to high, and the test results are compared. The small diameter ratio will be affected by the boundary effect, and when the diameter ratio exceeds the critical diameter ratio, it will not be affected by the boundary effect. The minimum diameter ratio that is not affected by the boundary effect is selected as the best diameter ratio. Using the soil sample diameter corresponding to the best diameter ratio and the corresponding probe for testing can reduce sample preparation time and improve efficiency.

S5: Determine the optimal soil sample height;

Determining the optimal soil sample height is one of the keys to determining the soil sample size. Under the premise of using the soil sample diameter corresponding to the optimal diameter ratio, the corresponding probe and the optimal penetration rate, static penetration tests are carried out under the same working conditions using test soil samples of different heights. During the test, the probe penetrates two-thirds of the soil sample height to observe whether the cone tip resistance and side wall friction resistance data can tend to be stable, and the stable distance is greater than 30 cm. The minimum soil sample height that meets the requirements is selected as the optimal soil sample height. After determining the optimal soil sample height, the optimal soil sample size is also determined.

S6: Determine the optimal penetration depth;

The purpose of determining the optimal penetration depth is to obtain more effective data. Under the premise of using the optimal soil sample size, the probe corresponding to the optimal diameter ratio and the optimal penetration rate, a static penetration test is carried out under the same working conditions. Since the bottom of the soil sample is a rigid boundary, as the penetration depth of the probe increases, the cone tip resistance will be affected by the bottom rigid boundary, and the cone tip resistance will no longer tend to be stable and will produce a changing trend. The penetration depth corresponding to the inflection point where the cone tip resistance data begins to change is the critical penetration depth, that is, the optimal penetration depth.

S7: Based on the optimal soil sample size, probe diameter, optimal penetration depth and optimal penetration rate, indoor static penetration tests are carried out on deep-sea sediment samples under different working conditions, so that the obtained data is accurate and reliable and the test process is more efficient.

S8: Replace the sample cap of the device and carry out triaxial tests on deep-sea sediment samples under the same working conditions to obtain various mechanical property parameters of the samples, including axial strain, deviatoric stress, pore water pressure and effective average principal stress;

The sample cap used in the triaxial test is different from the sample cap used in the static penetration test. The sample cap used in the triaxial test does not have a cylindrical hole in the center, which ensures the sealing of the test process; the triaxial test only needs to use the lower loading assembly to drive the lower loading rod to push the sample base upward to compress and shear the soil sample, and the remaining operation steps are consistent with the static penetration test; the triaxial test can obtain stress-strain curves, strength characteristics, stiffness characteristics and strength criteria.

S9: Establish a formula for evaluating the mechanical properties of deep-sea sediments based on static penetration sounding;

Combine the indoor static penetration test data with the triaxial test data to establish the relationship between the static penetration test data and the triaxial test data. For example, use CPT to obtain the cone tip resistance as the horizontal coordinate, and use the triaxial test to obtain the undrained shear strength of the sample as the vertical coordinate to draw a fitting relationship. The mechanical properties of deep-sea sediments can also be evaluated based on previous empirical formulas or a set of independently fitted formulas.

Beneficial effects of the present invention: After calibrating the probe and optimizing the indoor static penetration test method for deep-sea sediments, the present invention conducts indoor static penetration tests based on the obtained optimal soil sample size, probe diameter, optimal penetration depth and optimal penetration rate, and conducts triaxial tests based on the indoor static penetration device, combines the indoor static penetration test data with the triaxial test data, obtains multiple mechanical property parameters of the natural gas hydrate reservoir and its overlying soil layer, and establishes a set of deep-sea sediment mechanical property evaluation methods. The method has the characteristics of fast and convenient, large amount of data collected, low interference and low cost, as well as repeatability, sample uniformity, application of known/controlled boundary conditions and the ability to apply specific stress history. The present invention uses large-sized soil samples, so that the soil samples during indoor static penetration tests and triaxial tests are close to the real in-situ state, and the data obtained are more real and accurate. CPT is used to analyze the mechanical properties of sediments. Through a large number of indoor CPT calibration tests, a corresponding empirical relationship between the various physical and mechanical properties of sediments in various deep-sea regions and CPT penetration data is established, so as to more accurately invert the strength, seepage, deformation and other characteristics of the soil, providing technical support for my country's major needs in deep-sea resource development.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a schematic flow chart of a method for evaluating mechanical properties of deep-sea sediments based on an indoor static penetration test in an embodiment of the present invention.

FIG. 2 is a schematic structural diagram of an indoor static penetration device according to the present invention.

FIG. 3 is a schematic diagram of the overall structure of the penetration system of the present invention.

FIG. 4 is a schematic diagram of the structure of the lower loading assembly of the present invention.

FIG. 5 is a cross-sectional view of the calibration tank of the present invention.

Among them: 1 upper reducer; 2 hanging bolt; 3 hanging nut; 4 upper servo motor; 5 calibration tank shell; 6 support arm; 7 calibration tank base; 8 upper synchronous belt; 9 upper beam; 10 upper loading beam; 11 shaft ring; 12 upper ball screw; 13 probe; 14 base; 15 circulating water pump; 16 pressure pump; 17 program control system; 18 reducer mounting plate; 19 upper synchronous pulley; 20 lifting ear; 21 water injection hole; 22 hexagon socket head screw; 23 moving guide rail; 24 cushion block; 25 storage table; 26 lower servo motor; 27 lower reducer; 28 lower synchronous pulley; 29 lower loading rod; 30 lower ball screw; 31 lower loading beam; 32 lower beam; 33 lower synchronous belt; 34 guide hole; 35 sample cap; 36 soil sample; 37 reserved interface; 38 rubber mold; 39 sample base.

Detailed ways

In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the specific implementation modes of the present invention will be described in detail below with reference to the accompanying drawings.

The present invention provides a method for evaluating the mechanical properties of deep-sea sediments based on an indoor static penetration test. The method is implemented based on an indoor static penetration device, which includes an axial loading system, a base 14, a sample moving platform, a calibration tank, a temperature and pressure control module and a program control system 17.

The base 14 is a frame structure, which is used to support and fix the entire axial loading system to ensure the stability and safety of the device during use.

The sample moving platform is connected to the side of the base 14. The sample moving platform and the surface of the base 14 are on the same horizontal plane, and moving guide rails 23 are installed on the upper surfaces of both. Pads 24 are provided at both ends of the moving guide rails 23. A multi-layer storage table 25 is provided on the sample moving platform, which is placed in the bracket of the sample moving platform in a pull-out manner to facilitate the storage of test tools.

The calibration tank includes a calibration tank base 7, a calibration tank shell 5, a sample base 39, a soil sample 36, a sample cap 35 and a rubber membrane 38. The calibration tank base 7 is a cylindrical structure with a groove on the top, which is installed on the movable guide rail 23; the calibration tank shell 5 is a cylindrical shell structure with an opening at the bottom, which is connected to the calibration tank base 7 by a hexagon socket head screw 22 and sealed by an O-ring, so that a cavity is formed between the two; the soil sample 36 is wrapped by a rubber mold 38, and the upper and lower ends of the soil sample 36 are respectively connected to the sample cap 35 and the sample base 39 by the rubber mold 38 as a whole, and placed together in the cavity. A guide hole 34 is provided at the center of the top of the calibration tank shell 5. During the upward movement of the soil sample, the protrusion of the sample cap 35 is embedded in the guide hole 34 to ensure that the soil sample is accurately raised.

The axial loading system includes a lower loading component, a load frame, an upper loading component and a probe 13. The lower loading component has the same working principle as the upper loading component, the upper loading component has a maximum loading force of 200KN, and the penetration displacement accuracy is 0.001mm/s; the lower loading component has a maximum loading force of 150KN, and can restore the original stress condition of the soil sample; the upper loading component drives the probe to penetrate the soil sample in the calibration tank to complete the soil penetration test, and the position of the soil sample in the calibration tank is adjusted by driving the lower loading component, and the triaxial shear test can also be completed by the lower loading component.

As shown in Fig. 4, the lower loading assembly includes a lower synchronous pulley 28, a lower synchronous belt 33, a lower reducer 27, a lower servo motor 26, a lower ball screw 30, a lower loading beam 31, a lower beam 32 and a lower loading rod 29. Among them, the lower reducer 27 is installed on the lower beam 32, and the lower reducer 27 is also connected to the lower servo motor 26; three lower synchronous pulleys 28 are installed at the bottom of the lower beam 32, and the lower synchronous belt 33 is installed between the lower synchronous pulleys 28; one of the lower synchronous pulleys 28 is connected to the lower reducer 27, and the other two lower synchronous pulleys 28 are respectively connected to one end of the two lower ball screws 30 located above the lower beam 32; the other end of the lower ball screw 30 is installed on the base 14 through a bearing; the lower loading beam 31 is connected to the two lower ball screws through a thread. 30, a lower loading rod 29 is provided at the center of the lower loading beam 31; the lower reducer 27 controls the rotation of the lower synchronous pulley 28 under the drive of the lower servo motor 26, and drives the lower ball screw 30 to rotate through the lower synchronous belt transmission, thereby driving the lower loading beam 31 to move up and down, so that the lower loading rod 29 passes through the through hole in the center of the calibration tank base 7 and acts on the sample base 39, pushing the entire soil sample on the sample base 39 to move upward, so that the sample cap 35 enters the guide hole 34 of the calibration tank shell 5, so that the entire soil sample is in a suitable position.

As shown in Figures 2 and 3, the upper loading assembly includes an upper synchronous pulley 19, an upper synchronous belt 8, an upper reducer 1, an upper servo motor 4, an upper ball screw 12, an upper beam 9 and an upper loading beam 10. Among them, the upper beam 9 is mounted with the upper reducer 1 through the reducer mounting plate 18; two upper synchronous pulleys 19 are symmetrically mounted at both ends of the upper beam 9, and another upper synchronous pulley 19 is mounted on the reducer mounting plate 18 and connected to the upper reducer 1, and the upper reducer 1 is also connected to the upper servo motor 4; the upper synchronous belt 8 is mounted between the three upper synchronous pulleys 19; the two upper ball screws 12 are symmetrically arranged below the upper beam 9, one end of which is respectively connected to the two upper synchronous pulleys 19 on the upper beam 9, and the other end is mounted on the load frame through a bearing. The upper loading beam 10 is connected to the two upper ball screws 12 by threads. A groove is opened at the center of the upper loading beam 10, and the probe 13 is fixed in the groove by screws; the upper reducer 1 controls the rotation of the upper synchronous pulley 19 under the drive of the upper servo motor 4, and drives the upper ball screw 12 to rotate through the upper synchronous belt transmission, thereby driving the upper loading beam 10 to move up and down, pushing the probe 13 to pass through the cylindrical hole in the center of the sample cap 35 and penetrate into the soil sample.

As shown in FIG3 , the load frame is composed of two support arms 6 and four shaft rings 11; wherein, the two support arms 6 are respectively connected to the two ends of the upper surface of the base 14 through locking nuts; two holes are opened at the upper end of the support arm 6 for connecting the shaft ring, and the shaft ring 11 is fixed between the support arm 6 and the upper beam 9 through locking nuts, and the two shaft rings on each side ensure that the relative positions of the components remain stable during the test.

The probe 13 adopts a compression-type probe structure with two sets of built-in weighing sensors; it can independently measure the cone tip resistance and the side wall friction resistance without interfering with each other, and there is no need to consider the friction problem of the sealing ring; three CPT probes with diameters of 6mm, 16mm, and 25mm are used for static penetration testing.

The temperature and pressure control module is used to complete tests under different working conditions, including a circulating water pump 15 and three pressure pumps 16 for controlling pore pressure, back pressure and confining pressure respectively; wherein the circulating water pump 15 is connected to the outside of the calibration tank shell and can provide high and low temperatures; the pressure pump 16 is connected to the reserved interface on the base of the calibration tank through a stainless steel pipeline and can provide a pressure of up to 25MPa.

The program control system adopts Labview modular design, which is connected with the upper servo motor 4 of the upper loading component, the lower servo motor 26 of the lower loading component, the temperature and pressure control module and the probe 13, and can control the loading speed of the upper loading component and the lower loading component, customize the penetration depth, and record the pore pressure, confining pressure, back pressure, side wall friction and cone tip resistance data in real time during the test.

The method for evaluating the mechanical properties of deep-sea sediments comprises the following steps:

S1: calibration probe;

Probes with three diameters (6mm, 16mm, and 25mm) were installed on the calibration frame respectively, and a group of known forces were applied to the side wall and cone tip of the probe respectively. There was a certain deviation between the cone tip resistance and side wall friction measured before calibration and the actual cone tip resistance and side wall friction. The known force was fitted into a relationship with the cone tip resistance and side wall friction measured before calibration to obtain a coefficient and deviation. The measured cone tip resistance and side wall friction were corrected to accurate values. The probe was calibrated to ensure the accuracy and reliability of the measurement.

S2: Preparation work before optimizing the indoor static penetration test method for deep-sea sediments;

The optimization of the indoor static penetration test method for deep-sea sediments is to restore the real in-situ environment of the soil sample, and to efficiently complete the test and obtain the most accurate measurement data while avoiding boundary effects and size effects. Before the test, screw the bolt 2 onto the upper loading beam 10, and move the upper loading beam 10 downward to allow the bolt 2 to enter the lifting ear 20. Then screw the nut 3 onto the bolt 2, unscrew all the hexagon socket head screws 22 connecting the calibration tank shell 5 and the calibration tank base 7, lift the upper loading beam 10, and raise the calibration tank shell 5. Push the calibration tank base 7 along the moving guide rail 23 to the sample moving platform to start making soil samples. When making soil samples, first install the sample base 39 into the calibration tank base 7, and then fix the mold to the sample bottom with a stainless steel rolling strip. In the seat 39, there are 9 kinds of molds available for use, with diameters of 100mm, 200mm, and 300mm respectively, and each diameter includes three heights of 200mm, 400mm, and 600mm; the soil sample 8 is made by compaction in a mold with a matching size rubber mold 38; after the soil sample is made, the sample cap 35 is placed on the soil sample, the calibration tank base 7 is pushed onto the base 14, the calibration tank shell 5 is lowered to the position connected with the calibration tank base 7, the probe is installed and the pipeline of the temperature and pressure control module is connected, and the preparation work before the test has been completed. After a set of tests is completed, the probe 13 is first pulled out of the soil sample 8 through the upper loading assembly, and then the calibration tank shell 5 is raised, and the calibration tank base 7 with the soil sample is pushed to the sample moving platform to unload the soil sample and make the next set of soil samples. When optimizing the static penetration test method of deep-sea sediments, it is necessary to ensure that the porosity of each soil sample is consistent.

S3: Determine the optimal penetration rate;

Several factors need to be considered to determine the optimal penetration rate, including soil type, test purpose, equipment performance, etc. Different types of soils have different sensitivities to penetration speed. The optimal penetration rate is when the probe 13 penetrates two-thirds of the soil sample height under the conditions of a maximum soil sample diameter of 300 mm, a maximum soil sample height of 600 mm, and a minimum probe diameter of 6 mm. Static penetration tests with different penetration rates are performed under the same working conditions, and the obtained cone tip resistance and side wall friction resistance are compared with the cone tip resistance and side wall friction resistance results of known soil samples. The penetration rate corresponding to the static penetration test with consistent results is the optimal penetration rate.

S4: Determine the optimal diameter ratio;

The optimal diameter ratio is the ratio of the soil sample diameter to the probe diameter. Determining the optimal diameter ratio is a prerequisite for simulating the infinite boundary environment of the real in-situ soil. The specific method is as follows:

At the optimal penetration rate determined in S3, the maximum soil sample height of 600mm and the same working conditions, static penetration tests were carried out in turn on the soil samples of three diameters (100mm, 200mm, 300mm) prepared in S2 using three probes of three diameters (6mm, 16mm, 25mm); the different diameter ratios were arranged from low to high, and the test results were compared. The small diameter ratio would be affected by the boundary effect, and when the diameter ratio exceeded the critical diameter ratio, it would not be affected by the boundary effect. The minimum diameter ratio that was not affected by the boundary effect was selected as the optimal diameter ratio.

S5: Determine the optimal soil sample height;

Under the premise of using the soil sample diameter corresponding to the optimal diameter ratio, the corresponding probe and the optimal penetration rate, static penetration tests were carried out under the same working conditions using test soil samples of three heights (200mm, 400mm, and 600mm). During the test, the probe was penetrated into two-thirds of the soil sample height to observe whether the cone tip resistance and side wall friction resistance data can tend to be stable, and the stable distance is greater than 30cm. The minimum soil sample height that meets the requirements is the optimal soil sample height. Once the optimal soil sample height is determined, the optimal soil sample size is also determined.

S6: Determine the optimal penetration depth;

Under the premise of using the optimal soil sample size, the probe diameter corresponding to the optimal diameter ratio and the optimal penetration rate, a static penetration test is carried out under the same working conditions. Since the bottom of the soil sample is a rigid boundary, as the penetration depth of the probe increases, the cone tip resistance will be affected by the bottom rigid boundary, and the cone tip resistance will no longer tend to be stable and will produce a changing trend. The penetration depth corresponding to the inflection point where the cone tip resistance data begins to show a changing trend is the critical penetration depth, which is used as the optimal penetration depth.

S7: Based on the optimal sample size, probe diameter, optimal penetration depth and optimal penetration rate, indoor static penetration tests are carried out on deep-sea sediment samples under different working conditions to make the obtained data accurate and reliable and the test process more efficient. The pore pressure, cone tip resistance and side wall friction resistance data can be obtained through static penetration tests.

S8: Replace the sample cap of the device, and carry out triaxial test on deep-sea sediment samples under the same working conditions to obtain various mechanical property parameters of the soil samples;

The triaxial test of deep-sea sediment samples under the same working conditions is also based on this device. The triaxial test can be carried out by replacing the sample cap in the calibration tank to obtain various mechanical property parameters of the sample (axial strain, deviatoric stress, pore water pressure, effective average principal stress) and stress-strain curve, strength characteristics, stiffness characteristics and strength criterion; through the stress-strain curve, when the stress-strain curve does not have a peak, the strength at the axial strain of 15% is taken as the undrained shear strength of the soil sample; the sample cap used in the triaxial test is different from the sample cap used in the static penetration test. There is no cylindrical hole in the center of the sample cap used in the triaxial test, which ensures the sealing of the test process; the triaxial test only needs to use the lower loading assembly to drive the lower loading rod 29 to push the sample base 39 to move upward to compress and shear the soil sample, and the remaining operating steps are consistent with the static penetration test.

S9: Establishment of a mechanical property evaluation formula for deep-sea sediments based on static penetration

Combine the indoor static penetration test data (cone tip resistance, side wall friction, pore pressure) with the triaxial test data (axial strain, deviatoric stress, pore water pressure, effective average principal stress) to establish the relationship between the static penetration test data and the triaxial test data; for example, determine the empirical coefficients _Nc and _Ns according to the empirical formula:

_Su =( _qc - _σmean )/ _Nc

_St = _Ns / _Rf

Where: _Su is the undrained shear strength of the soil sample, _St is the sensitivity of the soil, _qc is the cone tip resistance, _σmean is the average effective stress, and _Rf is the friction resistance ratio (the ratio of the side wall friction resistance _fs to the cone tip resistance _qc ).

The water content, density and porosity of deep-sea sediments were selected as independent variables, and the cone tip resistance was selected as the dependent variable. Multivariate regression analysis was performed and the empirical formula was obtained:

q _c = aω+bρ+ce+d

Among them, ω is the water content, ρ is the density, e is the porosity, and a, b, c, and d are correction coefficients.

According to the moisture content, density and porosity of the sediment, the cone tip resistance can be well predicted, and the undrained shear strength of the soil can be calculated based on the cone tip resistance. The cone tip resistance obtained by CPT can also be used as the horizontal coordinate, and the undrained shear strength of the sample obtained by triaxial test can be used as the vertical coordinate to draw a fitting relationship. The mechanical properties of deep-sea sediments can also be evaluated based on previous empirical formulas or a set of self-fitting formulas. The obtained data can not only be used for soil layer division and soil classification (based on friction ratio), but also for estimating the undrained shear strength of clay soil, overconsolidation ratio, sensitivity, relative density of sand, internal friction angle, compression modulus of soil, deformation modulus, undrained modulus of saturated clay, initial tangent elastic modulus and initial tangent modulus of sand soil, foundation bearing capacity, single pile bearing capacity and sand liquefaction discrimination, so as to more accurately invert the strength, seepage and deformation characteristics of the soil, and provide technical support for the major needs of my country's deep-sea resource development.

Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some or all of the technical features therein. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope defined by the claims of the present invention.

Claims (10)

1. A method for evaluating the mechanical properties of deep-sea sediments based on an indoor static penetration test, characterized in that the method is implemented based on an indoor static penetration device, which includes an axial loading system, a base, a sample moving platform, a calibration tank, a temperature and pressure control module and a program control system; The sample moving platform is connected to the side of the base, the sample moving platform and the surface of the base are on the same horizontal plane, and the upper surfaces of the two are installed with moving guide rails; The calibration tank is installed on a movable guide rail and can move along the movable guide rail; The axial loading system comprises an upper loading assembly, a load frame, a lower loading assembly and a probe; wherein the upper loading assembly is installed above the base through the load frame, the probe is installed on the upper loading assembly and is located above the calibration tank, and the probe is driven by the upper loading assembly to penetrate into the soil sample in the calibration tank to complete the soil penetration test; the lower loading assembly is installed at the lower part of the base, and the position of the soil sample in the calibration tank is adjusted by driving the lower loading assembly; The temperature and pressure control module provides a temperature and pressure environment for the test soil sample in the calibration tank; The program control system is connected with the upper loading component, the lower loading component, the temperature and pressure control module and the probe to realize real-time monitoring and control of the test process; The method for evaluating the mechanical properties of deep-sea sediments comprises the following steps: S1: Calibrate probes of different diameters; S2: Prepare soil samples of different sizes in the calibration tank, including soil samples of various diameters and various heights corresponding to each diameter; install the probe on the upper loading assembly and connect the pipeline of the temperature and pressure control module; S3: Determine the optimal penetration rate; Select the probe with the smallest diameter in S1 and the soil sample with the largest diameter and corresponding maximum height prepared in S2, and conduct static penetration tests with different penetration rates under the same working conditions. The penetration depth of the probe is two-thirds of the height of the soil sample, and the cone tip resistance and side wall friction resistance are obtained; the obtained cone tip resistance and side wall friction resistance are compared with the cone tip resistance and side wall friction resistance results of the known soil samples. The penetration rate corresponding to the static penetration test with consistent results is the optimal penetration rate. S4: Determine the optimal diameter ratio; The optimal diameter ratio is the ratio of the soil sample diameter to the probe diameter. Determining the optimal diameter ratio is a prerequisite for simulating the infinite boundary environment of the real in-situ soil. The specific method is as follows: At the optimum penetration rate determined in S3 and under the same working conditions, the test soil samples with different diameters at the maximum height obtained in S2 are subjected to static penetration tests in sequence using probes with different diameters calibrated in S1; the different diameter ratios are arranged from low to high, the test results are compared, and the minimum diameter ratio that is not affected by the boundary effect is selected as the optimum diameter ratio; S5: Determine the optimal soil sample height; Under the premise of using the soil sample diameter corresponding to the optimal diameter ratio, the corresponding probe and the optimal penetration rate, static penetration tests are carried out under the same working condition using test soil samples of different heights. During the test, the probe penetrates two-thirds of the soil sample height to observe whether the cone tip resistance and side wall friction resistance data can tend to be stable, and the stable distance is greater than 30cm; the minimum soil sample height that meets the requirements is selected as the optimal soil sample height. After determining the optimal soil sample height, the optimal soil sample size is also determined; S6: Determine the optimal penetration depth; Under the premise of using the best soil sample size, the probe corresponding to the best diameter ratio and the best penetration rate, the static penetration test is carried out under the same working condition; as the penetration depth of the probe increases, the cone tip resistance will be affected by the bottom rigid boundary, and the cone tip resistance will no longer be stable and will produce a changing trend. The penetration depth corresponding to the inflection point where the cone tip resistance data begins to change is the optimal penetration depth. S7: Based on the optimal soil sample size, probe diameter, optimal penetration depth and optimal penetration rate, indoor static penetration tests were carried out on deep-sea sediment samples under different working conditions to obtain pore pressure, cone tip resistance and side wall friction resistance data; S8: Replace the sample cap of the device, and carry out triaxial test on the deep-sea sediment sample under the same working conditions to obtain various mechanical characteristic parameters and stress-strain curve, strength characteristics, stiffness characteristics and strength criterion of the sample. The mechanical characteristic parameters include axial strain, deviatoric stress, pore water pressure and effective average principal stress; S9: Establish a formula for evaluating the mechanical properties of deep-sea sediments based on static penetration sounding; The indoor static penetration test data are combined with the triaxial test data to establish the relationship between the static penetration test data and the triaxial test data. 2. A method for evaluating the mechanical properties of deep-sea sediments based on indoor static penetration test according to claim 1, characterized in that the calibration tank comprises a calibration tank base, a calibration tank shell, a sample base, a soil sample, a sample cap and a rubber membrane; wherein the calibration tank base is a cylindrical structure with a groove on the top, which is installed on a movable guide rail; the calibration tank shell is a cylindrical shell structure with an opening at the bottom, which is connected to the calibration tank base to form a cavity between the two; the soil sample is wrapped by a rubber mold, and the upper and lower ends of the soil sample are respectively connected to the sample cap and the sample base as a whole through the rubber mold, and are placed together in the cavity; a guide hole is provided at the center of the top of the calibration tank shell, and during the upward movement of the soil sample, the protrusion of the sample cap is embedded in the guide hole to ensure that the soil sample is accurately raised; a cylindrical hole is opened in the center of the sample cap, and the probe penetrates the soil sample through the cylindrical hole; a through hole is provided at the center of the bottom of the calibration tank base to facilitate the lower loading component to act on the sample base. 3. A method for evaluating the mechanical properties of deep-sea sediments based on indoor static penetration test according to claim 2, characterized in that the lower loading assembly comprises a lower servo motor, a lower reducer, a lower synchronous pulley, a lower synchronous belt, a lower ball screw, a lower loading beam, a lower beam and a lower loading rod; wherein the lower reducer is installed on the lower beam, and the lower reducer is connected to the lower servo motor; three lower synchronous pulleys are installed at the bottom of the lower beam, and a lower synchronous belt is installed between the lower synchronous pulleys; wherein a lower synchronous pulley is connected to the lower reducer, and the other two lower synchronous pulleys are respectively connected to one end of two lower ball screws located above the lower beam; the other end of the lower ball screw is installed on the base through a bearing; the lower loading beam is connected to the two lower ball screws by threads, and a lower loading rod is provided at the center of the lower loading beam; the lower reducer controls the rotation of the lower synchronous pulley under the drive of the lower servo motor, and drives the lower ball screw to rotate through the lower synchronous belt transmission, thereby driving the lower loading beam to move up and down, so that the lower loading rod pushes the soil sample to a suitable position; The upper loading assembly includes an upper synchronous pulley, an upper synchronous belt, an upper reducer, an upper servo motor, an upper ball screw, an upper beam and an upper loading beam; wherein the upper beam is mounted with an upper reducer via a reducer mounting plate; two upper synchronous pulleys are symmetrically mounted at both ends of the upper beam, and another upper synchronous pulley is mounted on the reducer mounting plate and connected to the upper reducer, and the upper reducer is also connected to the upper servo motor; the upper synchronous belt is mounted between three upper synchronous pulleys; two upper ball screws are symmetrically arranged below the upper beam, one end of which is respectively connected to the two upper synchronous pulleys on the upper beam, and the other end is mounted on the load frame via a bearing; the upper loading beam is connected to the two upper ball screws via threads, and a probe is mounted at the center of the upper loading beam; the upper reducer controls the rotation of the upper synchronous pulley under the drive of the upper servo motor, and drives the upper ball screw to rotate via the upper synchronous belt transmission, thereby driving the upper loading beam to move up and down, and penetrating the probe into the soil sample. 4. According to a method for evaluating the mechanical properties of deep-sea sediments based on indoor static penetration tests in claim 3, it is characterized in that the load frame consists of a support arm and an axle collar; wherein the two support arms are symmetrically fixed at the two ends of the upper surface of the base, and the two ends of the axle collar are respectively connected to the upper beam and the support arm to ensure that the relative positions of the components remain stable during the test. 5. A method for evaluating the mechanical properties of deep-sea sediments based on indoor static penetration tests according to claim 3 or 4 is characterized in that the upper part of the calibration tank shell is provided with a lifting lug, which is connected to the lifting bolt and the lifting nut connected to the upper loading beam, and the calibration tank shell is lifted by the upper loading assembly to facilitate the installation and disassembly of the soil sample; a water injection hole is opened on the top of the calibration tank shell, and water is injected around the soil sample wrapped with the rubber film through the water injection hole, saving the available volume of the confining pressure pump. 6. According to the method for evaluating the mechanical properties of deep-sea sediments based on indoor static penetration tests in claim 5, it is characterized in that a reserved interface is provided on the base of the calibration tank; water inlet holes and drainage holes are provided on the sides of the sample base and the sample cap, and the water inlet holes and drainage holes are connected to the reserved interface on the base of the calibration tank through pipelines. 7. According to a method for evaluating the mechanical properties of deep-sea sediments based on indoor static penetration tests according to claim 6, it is characterized in that the temperature and pressure control module consists of a circulating water pump and three pressure pumps that respectively control the pore pressure, back pressure and confining pressure; wherein the circulating water pump is connected to the outside of the calibration tank shell, or to the reserved interface of the calibration tank base; the pressure pump is connected to the reserved interface of the calibration tank base through a pipeline. 8. A method for evaluating the mechanical properties of deep-sea sediments based on indoor static penetration tests according to claim 7, characterized in that the process of making soil samples in S2 is as follows: the calibration tank shell is lifted up by the upper loading assembly, and the calibration tank base is pushed along the movable guide rail to the sample moving platform to start making soil samples; when making soil samples, the sample base is first installed in the calibration tank base, and then the mold is fixed in the sample base, and the soil sample is made by compaction method in a mold with a matching size rubber mold; after the soil sample is made, the sample cap is placed on the soil sample, the calibration tank base is pushed onto the base, and then the calibration tank shell is lowered to the position where it is connected to the calibration tank base for connection. 9. A method for evaluating the mechanical properties of deep-sea sediments based on indoor static penetration test according to claim 8, characterized in that, in S8, there is no cylindrical hole in the center of the sample cap used for the triaxial test; the triaxial test only needs to use the lower loading assembly to drive the lower loading rod to push the sample base to move upward, and compress and shear the soil sample, and the remaining operating steps are consistent with the static penetration test. 10. A method for evaluating the mechanical properties of deep-sea sediments based on indoor static penetration tests according to claim 9, characterized in that, in S1, probes of different diameters are respectively installed on calibration frames, and a group of known forces are applied to the side walls and cone tips of the probes respectively, and the known forces are fitted into a relationship with the cone tip resistance and side wall friction resistance measured before calibration to obtain a coefficient and deviation, and the measured cone tip resistance and side wall friction resistance are corrected to accurate values.