CN110568151A - Measuring method for measuring unsaturated soil matrix suction in centrifugal model test - Google Patents
Measuring method for measuring unsaturated soil matrix suction in centrifugal model test Download PDFInfo
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- CN110568151A CN110568151A CN201910718640.9A CN201910718640A CN110568151A CN 110568151 A CN110568151 A CN 110568151A CN 201910718640 A CN201910718640 A CN 201910718640A CN 110568151 A CN110568151 A CN 110568151A
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Abstract
The invention discloses a measuring method for measuring unsaturated soil matrix suction in a centrifugal model test. In the method, the micro pore water pressure sensor is used for structural transformation, and the linear relation between the output voltage and the negative pressure of the micro pore water pressure sensor after sealing transformation is obtained, so that the micro pore water pressure sensor can measure the substrate suction force in unsaturated soil. The measuring instrument used by the method has a simple structure and a small size, and can meet the requirements of sensor miniaturization and light weight in the centrifugal machine.
Description
Technical Field
The invention relates to the technical field of soil property detection, in particular to a measuring method for measuring unsaturated soil matrix suction in a centrifugal model test.
Background
The soil in engineering can be divided into saturated soil and unsaturated soil according to saturation. In saturated soils, the pores are completely filled with water and no air exists in the pores, and the water pressure in the soil is positive or equal to atmospheric pressure, and the soils are usually soil bodies below the underground water surface, coastal soft soil and the like. The pores in the unsaturated soil are not completely filled with water, air exists in the pores, the water can exist in a continuous or discontinuous state, and the unsaturated soil is the most extensive form existing in the soil body in nature.
In unsaturated soils, water is present at the contact points of the soil particles due to the capillary action of the micropores, the air-water interface is a shrinking water film that assumes surface tension, and therefore water is in tension in the pores and the water pressure is negative. The pore air pressure is not equal to the pore water pressure, the pore air pressure is greater than the pore water pressure, and the shrink film bears the air pressure greater than the water pressure. In soil mechanics, this pressure difference is called matrix suction (matrix suction). The substrate suction is an important parameter for describing the mechanical property of unsaturated soil, and a water-soil characteristic curve, namely a curve of the relationship between the substrate suction and the soil moisture content, is an important index for describing the substrate suction.
The centrifugal model test technology utilizes a centrifugal force field to simulate the earth gravity field, the acceleration of the earth gravity field is 1g, the simulated gravity field can reach hundreds of g in a centrifugal machine, and the states of large-scale prototypes such as dams, side slopes and foundations under different working conditions can be simulated in the centrifugal field through a geometric model with a small scale. More than 20 large and medium-sized geotechnical centrifuges are built in China, and the geotechnical centrifuges are built in succession from the last 90 th century, and are mainly distributed in scientific research units such as Chinese water conservancy science research institute, Nanjing water conservancy science research institute, Yangtze river science institute, Qinghua university and Tongji university. At present, the centrifugal acceleration simulation of the highest 600g of centrifugal machine of scientific research units in China can be realized, and the geotechnical centrifugal machine exceeding 1000g is also under construction. In the centrifugal model test, a geometric scale is adopted to simulate a prototype structure, for example, a dam model with the height of 50cm is adopted, a dam with the height of 100m can be simulated under the centrifugal acceleration of 200g, and the consolidation deformation speed and seepage speed of the dam body are 200 of the site2and (4) doubling. Because a model with a smaller size is adopted, the measurement sensor is generally required to be miniaturized and refined, for example, the size of a nail cover can be achieved by the soil pressure sensor at present. In the unsaturated soil model test, how to measure the matrix suction of unsaturated soil by using a miniaturized sensor becomes aand (5) difficult problem.
Generally, when the matrix suction force of unsaturated soil is measured in the field, a heat conduction probe, a tensiometer and the like can be adopted to achieve the aim. The sensor has the defects that the size is large, the diameter exceeds 2.5cm, the heat diffusion is time-consuming, about 10 minutes is needed for one-time measurement, and for a 100g high-speed centrifugal field, the field time is 27 hours, and the hysteresis effect is serious. The latter is a direct measurement method, but the sensor size is larger, typically exceeding 5cm, with a length of 30 cm. The above measurement methods do not meet the requirement of miniature lightweight of the sensor in the centrifuge, and the conventional field test method can not be directly applied to a centrifugal model test for measuring the matrix suction of unsaturated soil.
The miniature pore water pressure sensor is used for measuring the water pressure in saturated soil, the diameter of the miniature pore water pressure sensor is 6.5mm, the length of the miniature pore water pressure sensor is 12mm, the miniature pore water pressure sensor comprises a stainless steel round shell, a glass ring is embedded inside the stainless steel round shell, a pressure strain film is attached to the outer end of the glass ring, a ceramic water filter stone is embedded in the outer end of the round tube shell, a miniature water chamber is arranged between the water filter stone and the pressure strain film, the miniature water chamber is filled with no water, and the water pressure in the miniature water chamber is zero relative to the atmospheric pressure in the initial state (namely, the water pressure in the miniature. The principle of measuring the water pressure is as follows: when the miniature pore water pressure sensor is buried in saturated soil or thrown into water, the water pressure outside the filtering stone is greater than that of the miniature water chamber inside the filtering stone, so that external water can enter the miniature water chamber through the filtering stone until the internal pressure and the external pressure are balanced. At the moment, the strain gauge deforms due to the pressure change of the miniature water chamber in the sensor, and the water pressure in the miniature water chamber, namely the water pressure outside the sensor, can be obtained by measuring the deformation of the strain gauge.
The miniature pore water pressure sensor can only be used for measuring the water pressure in saturated soil, the external water pressure is greater than the water pressure of the miniature water chamber of the sensor during measurement, the water pressure difference exists inside and outside, and the external water penetrates through the filter stone and enters the miniature water chamber, so the pressure in the miniature water chamber is always greater than the standard atmospheric pressure. Because the saturated soil does not have air, the risk that the air enters the miniature water chamber to influence the measurement result is avoided. Therefore, the requirement of the sensor on air tightness is not very high, a certain gap is formed between the periphery of a ceramic plate and an outer steel shell of the existing micro pore water pressure sensor, and the ceramic plate only plays a role in filtering water stones. However, due to the existence of gaps (i.e. insufficient air tightness of the micro water chamber), the existing micro pore water pressure sensor cannot measure the substrate suction force in unsaturated soil. This is because: in the process of measuring the substrate suction in unsaturated soil, because the water pressure of the unsaturated soil is a negative value and is lower than the water pressure (atmospheric pressure) of the miniature water chamber, and pressure difference exists between the inside and the outside of the water chamber, water in the miniature water chamber can migrate into the unsaturated soil through the ceramic plate, so that the water pressure in the miniature water chamber is reduced to be lower than the standard atmospheric pressure (negative pressure state), but the unsaturated soil contains a large amount of air, the water in the water chamber of the sensor is actually in a state of being communicated with the atmosphere due to the existence of gaps at the periphery of the ceramic plate, and the measurement result is the atmospheric pressure which is a zero value (all pressure measurements use the atmospheric pressure as a standard zero value point).
Disclosure of Invention
The invention aims to provide a measuring method for measuring the suction force of an unsaturated soil matrix in a centrifugal model test, aiming at the technical defects in the prior art. In the method, a micro pore water pressure sensor is used for structural reconstruction, so that the micro pore water pressure sensor can be used for measuring the matrix suction in unsaturated soil in a centrifugal model test. The measuring instrument used by the method has a simple structure and a small size, and can meet the requirement of miniature and light weight of the sensor in the centrifugal machine.
the technical scheme adopted for realizing the purpose of the invention is as follows:
A measuring method for measuring unsaturated soil matrix suction in a centrifugal model test comprises the following steps:
step 1: carrying out sealing transformation on the micro pore water pressure sensor;
step 2: performing negative pressure calibration on the micro pore water pressure sensor subjected to sealing modification in the step 1 to obtain a linear relation between the output voltage and the negative pressure of the micro pore water pressure sensor subjected to sealing modification;
And step 3: putting the miniature pore water pressure sensor subjected to sealing transformation in the step 1 into a vacuum cylinder, vacuumizing and saturating to fill a water chamber of the sensor with no water, and taking out the sensor after saturation;
and 4, step 4: measuring and recording an initial zero value point (voltage) of the sensor in the step 3;
And 5: embedding the sensor in the step 4 into a prepared centrifugal model test unsaturated soil body model (models of an earth dam, a side slope, a foundation and the like), and connecting the sensor into a test measurement system;
step 6: operating tests (model hoisting, starting a centrifugal machine and the like) according to the requirements of the centrifugal model test, and measuring and recording the reading of a sensor by a measuring system;
And 7: and (4) according to the voltage difference values obtained in the steps (4) and (6), calculating the substrate suction force in the unsaturated soil centrifugal model by using the calibration relation (namely the linear relation between the output voltage and the negative pressure of the sealed and modified micro pore water pressure sensor) obtained in the step (2).
in the technical scheme, in the step 1, the sealing modification mode is that epoxy resin is adopted to fill a gap between the ceramic plate and the shell. Based on the sealing transformation mode, unsaturated soil with the substrate suction force lower than 100kPa can be measured.
In the technical scheme, the sealing modification mode in the step 1 is that the original ceramic plate of the micro pore water pressure sensor is detached, a new ceramic plate with an upper air inlet value not less than 200kPa is replaced, and epoxy resin is adopted to fill a gap between the new ceramic plate and the shell. Based on the sealing transformation mode, unsaturated soil with the substrate suction not less than 200kPa can be measured. The measured value range of the substrate suction force depends on the air inlet value of the ceramic plate, and if the larger substrate suction force is measured, the ceramic plate with the corresponding air inlet value can be adopted.
in the technical scheme, the negative pressure calibration in the step 2 comprises the following steps:
Step A: placing the sealed and transformed micro pore water pressure sensor into a vacuum cylinder;
And B: vacuumizing the vacuum cylinder and controlling the negative pressure to be stable;
And C: measuring output voltage, and calibrating the linear relation between the output voltage and the negative pressure of the sealed and modified micro pore water pressure sensor.
The invention has the beneficial effects that:
The measuring instrument solves the problem of measuring the suction force of the unsaturated soil matrix in a centrifugal model test, has simple structure and small size, and can meet the requirement of miniature and light weight of a sensor in a centrifugal machine.
drawings
FIG. 1 is a schematic structural view of a PDCR-81 model micro-pore water pressure sensor in example 1;
FIG. 2 is a graph showing the suction of a non-saturated expansive soil matrix measured in a certain centrifugal model test;
FIG. 3 shows the results of the negative pressure calibration test in example 2;
FIG. 4a is a graph showing the substrate suction of unsaturated soil measured in example 1;
FIG. 4b is a plot of the substrate suction of other unsaturated soils measured in example 1.
In the figure: 1-stainless steel round tube, 2-glass ring, 3-pressure strain film, 4-ceramic plate, 5-micro water chamber and 6-lead.
For a person skilled in the art, other relevant figures can be obtained from the above figures without inventive effort.
Detailed Description
in order to make the technical solution of the present invention better understood, the technical solution of the present invention is further described below with reference to specific examples.
example 1
A measuring method for measuring unsaturated soil matrix suction in a centrifugal model test comprises the following steps:
Step 1: carrying out sealing transformation on the micro pore water pressure sensor;
Step 2: performing negative pressure calibration on the micro pore water pressure sensor subjected to sealing modification in the step 1 to obtain a linear relation between the output voltage and the negative pressure of the micro pore water pressure sensor subjected to sealing modification;
and step 3: putting the miniature pore water pressure sensor subjected to sealing transformation in the step 1 into a vacuum cylinder, vacuumizing and saturating to fill a water chamber of the sensor with no water, and taking out the sensor after saturation;
and 4, step 4: measuring and recording an initial zero value point (voltage) of the sensor in the step 3;
and 5: embedding the sensor in the step 4 into a prepared centrifugal model test unsaturated soil body model (models of an earth dam, a side slope, a foundation and the like), and connecting the sensor into a test measurement system;
Step 6: operating tests (model hoisting, starting a centrifugal machine and the like) according to the requirements of the centrifugal model test, and measuring and recording the reading of a sensor by a measuring system;
And 7: and (4) according to the voltage difference values obtained in the steps (4) and (6), calculating the substrate suction force in the unsaturated soil centrifugal model by using the calibration relation (namely the linear relation between the output voltage and the negative pressure of the sealed and modified micro pore water pressure sensor) obtained in the step (2).
The micro pore water pressure sensor used in the embodiment is a PDCR-81 micro pore water pressure sensor produced by GE Druck. The structure is as shown in figure 1, the shell is a stainless steel round pipe 1 with the diameter of 6.5mm and the length of 12mm, a glass ring 2 is embedded inside, a pressure strain film 3 is attached to the outer end of the glass ring 2, a ceramic plate 4 is embedded at the bottom end of the round pipe shell, a miniature water chamber 5 is formed between the ceramic plate 4 and the pressure strain film 3, and the pressure strain film 3 is connected with a lead 6 and used for measuring a pressure value.
In the embodiment, epoxy resin is adopted to seal and reform a gap between a ceramic plate 4 and a stainless steel round tube 1 of the PDCR-81 type micro pore water pressure sensor.
The modified micro pore water pressure sensor has the following measurement principle:
After the modified micro pore water pressure sensor is saturated, the ceramic plate 4 and the micro water chamber 5 are filled with airless water, and the pressure is zero (relative to zero point by atmospheric pressure) under the action of atmospheric pressure. Ceramic plate 4 straightThe water pressure of unsaturated soil is negative and lower than that of the miniature water chamber 5, so that water in the miniature water chamber 5 can migrate to the unsaturated soil through the ceramic plate 4, the water pressure in the miniature water chamber is reduced, the water pressure is negative, and the pressure strain membrane 3 is bent and deformed. Meanwhile, because the measure of sealing the gaps at the periphery of the ceramic plate 4 is adopted, air in unsaturated soil can not enter the micro water chamber 5 through the gaps, and therefore the measured water pressure is the water pressure of the unsaturated soil excluding the influence of the air. When the atmospheric pressure and the water pressure in the pressure strain film 3 and the micro water chamber 5 reach balance, the voltage change in the pressure strain film 3 is measured, and the water pressure in the micro water chamber 5, namely the pore water pressure u in the unsaturated soil can be obtainedw. Because the rear end of the micro pore water pressure sensor is communicated with the atmosphere through the lead 6, the measured value is based on the atmospheric pressure, i.e. ua＝0，s＝ua-uw＝-uwi.e. the substrate suction value.
FIG. 2 shows the matrix suction force test result in a centrifugal model test of unsaturated expansive soil slope by using 400g of geotechnical centrifuge in China's Water conservancy and hydropower science research institute, wherein the modified PDCR-81 is used for measuring the matrix suction force of unsaturated expansive soil slope, sensors (PPT) are embedded at different positions of the slope, the centrifugal simulated gravity field is 60g, and the slope surface of the slope is immersed in water in the test to simulate water level rise. Therefore, the suction force and the change of the matrix can be measured by the improved sensor. In the process that the centrifugal machine is accelerated to 60g in the first 10 minutes and the centrifugal gravity field is simulated, the suction force of the matrix is reduced because the relative water content is increased due to the compression of the soil body; after the slope is soaked in water, PPT5 measures that the matrix of the soil body is reduced due to the increase of the water content; the water content of other sensors at the measuring position is unchanged, so the substrate suction force is kept unchanged.
Preferably, the old ceramic plate of the micro-pore water pressure sensor is detached, a new ceramic plate having an air inlet value of not less than 200kPa is attached, and a gap between the new ceramic plate and the housing is filled with epoxy resin. The PDCR-81 type micro pore water pressure sensor modified by the sealing modification mode can measure unsaturated soil with the substrate suction not less than 200 kPa.
example 2
In this embodiment, based on embodiment 1, the negative pressure calibration method is described.
The calibration is to place the modified PDCR-81 type micro pore water pressure sensor under negative air pressure to measure the sensitivity coefficient, and comprises the following steps:
step A: placing the sealed and transformed micro pore water pressure sensor into a vacuum cylinder;
and B: vacuumizing the vacuum cylinder and controlling the negative pressure to be stable;
And C: measuring output voltage, and calibrating the linear relation between the output voltage and the negative pressure of the sealed and modified micro pore water pressure sensor.
the result shows that the output voltage of the improved micro-pore water pressure sensor and the negative pressure in the vacuum cylinder are in a linear relation (as shown in figure 3), and the sensitivity coefficient obtained by calculation is basically consistent with the coefficient of measuring the positive water pressure given in the specification.
the calibration test results show that: when the device is used for measuring the negative pressure value, the measured values in the two states are not different.
Example 3
in the embodiment, based on the embodiment 1 and the embodiment 2, the feasibility of measuring the unsaturated soil matrix suction force by the sensor after modification is verified.
The preparation process of the sample to be tested comprises the following steps:
step a: sieving the selected soil, and generally sieving the soil by a 2mm sieve according to related test procedures;
step b: b, preparing the soil mass sieved in the step a into a certain water content (such as 20%), and standing to uniformly distribute the water content of the soil sample;
step c: according to the designed dry density (such as 1.55 g/cm)3) And d, layering and compacting the sample obtained in the step b in a sample container to obtain the sample to be tested.
specifically, the method comprises the following steps: according to SL237-1999 geotechnical test regulation, particles with the particle size larger than 2mm are screened out; then the water content is prepared to 20 percent to 27 percent, and the mixture is kept for 48 hours to ensure that the water is distributed evenly(the moisture content is measured by drying method in the sample preparation process). At a dry density of 1.550/cm3Samples were prepared. The sample container is an organic glass cylinder with the inner diameter of 96mm and the height of 120mm, the bottom of the cylinder is sealed by organic glass, and the total height of the prepared sample is 60mm or 80 mm.
The unsaturated soil suction measurement is carried out at room temperature, and as can be seen from the test results, the modified micro pore water pressure sensor is obviously characterized in that the time for the matrix suction to be stably balanced is longer, and the value measured by the sensor is not obviously attenuated for 4h until the test is finished (as shown in fig. 4 a). In other tests even 13h (as in fig. 4b), no attenuation occurred until the end of the test, indicating that it is feasible to measure the matrix suction of unsaturated soil using a modified PDCR-81 model micro pore water pressure sensor, and also indicating the effectiveness of the measure of sealing the pores around the ceramic plate 4 with epoxy.
In this embodiment, the ceramic plate of PDCR-81 is used. Since the inlet value of the ceramic plate is 100kPa, only unsaturated soils with a substrate below 100kPa can be measured.
the foregoing is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, various modifications and decorations can be made without departing from the principle of the present invention, and these modifications and decorations should also be regarded as the protection scope of the present invention.
Claims (4)
1. A measuring method for measuring unsaturated soil matrix suction in a centrifugal model test is characterized by comprising the following steps: the method comprises the following steps:
Step 1: carrying out sealing transformation on the micro pore water pressure sensor;
Step 2: performing negative pressure calibration on the micro pore water pressure sensor subjected to sealing modification in the step 1 to obtain a linear relation between the output voltage and the negative pressure of the micro pore water pressure sensor subjected to sealing modification;
And step 3: putting the miniature pore water pressure sensor subjected to sealing transformation in the step 1 into a vacuum cylinder, vacuumizing and saturating to fill a water chamber of the sensor with no water, and taking out the sensor after saturation;
And 4, step 4: measuring and recording an initial zero point of the sensor in the step 3;
and 5: embedding the sensor in the step 4 into the prepared centrifugal model test unsaturated soil body model, and connecting the sensor into a test measurement system;
step 6: operating the test according to the requirements of the centrifugal model test, and measuring and recording the reading of the sensor by a measuring system;
and 7: and (4) calculating the substrate suction force in the unsaturated soil centrifugal model by using the calibration relation obtained in the step (2) according to the voltage difference values obtained in the step (4) and the step (6).
2. the method for measuring the suction force of the unsaturated soil matrix in the centrifugal model test according to claim 1, wherein the method comprises the following steps: in the step 1, the sealing modification mode is that epoxy resin is adopted to fill the gap between the ceramic plate and the shell.
3. The method for measuring the suction force of the unsaturated soil matrix in the centrifugal model test according to claim 1, wherein the method comprises the following steps: in the step 1, the sealing modification mode is that the original ceramic plate of the micro pore water pressure sensor is detached, a new ceramic plate with the upper air inlet value not less than 200kPa is replaced, and epoxy resin is adopted to fill the gap between the new ceramic plate and the shell. Based on the sealing transformation mode, unsaturated soil with the substrate suction not less than 200kPa can be measured.
4. the method for measuring the suction force of the unsaturated soil matrix in the centrifugal model test according to claim 1, wherein the method comprises the following steps: the negative pressure calibration in the step 2 comprises the following steps:
Step A: placing the sealed and transformed micro pore water pressure sensor into a vacuum cylinder;
and B: vacuumizing the vacuum cylinder and controlling the negative pressure to be stable;
And C: measuring output voltage, and calibrating the linear relation between the output voltage and the negative pressure of the sealed and modified micro pore water pressure sensor.
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