An active heating optical fiber and in-situ method for estimating the permeability coefficient of unsaturated soil

Abstract

The present invention discloses an active heating optical fiber and an in-situ method for estimating the permeability coefficient of unsaturated soil. The active heating optical fiber includes: an AHFO cable, a heat shrinkable sleeve, and a PVC pipe; wherein the AHFO cable includes two layers of PE sheaths, two layers of copper meshes, a steel corrugated pipe, and an optical fiber. The in-situ method based on the active heating optical fiber includes selecting the water content profile of the target soil mass and arranging the AHFO measuring pipe; determining the drying or wetting process; according to the functional relationship between the temperature change of the AHFO measuring pipe and the soil volumetric water content, determining the soil volume water content profile diagram, fitting the soil-water characteristic curve, and further obtaining the suction profile diagram; calculating the hydraulic gradient and determining the unsaturated hydraulic conductivity; the present invention reveals the thermal-hydraulic-mechanical coupling behavior, provides support for predicting unsaturated seepage, is crucial for preventing potential disasters such as geotechnical structure failure and harmful substance leakage, and the present invention also helps to study the interaction between deep soil and the atmosphere.

Description

Technical Field

[0001] The present invention relates to the field of geotechnical engineering parameter measurement, and particularly relates to an active heating optical fiber for estimating the permeability coefficient of unsaturated soil, and also relates to an in-situ method for estimating the permeability coefficient of unsaturated soil. Background Art

[0002] The hydraulic conductivity of unsaturated soil (k(ψ), where ψ is suction) plays a crucial role in controlling transient seepage. It can be used to predict water movement in unsaturated soil, which is crucial in agricultural irrigation, soil-atmosphere interaction, and groundwater recharge. In geotechnical engineering, it greatly affects the distribution of pore water pressure and suction in unsaturated soil, which is crucial for evaluating the stability of geotechnical structures (such as compacted embankments and slopes affected by rainfall). In addition, the constitutive relationship or numerical simulation study of the thermo-hydro-mechanical coupling behavior of unsaturated soil requires the relationship between unsaturated hydraulic conductivity and volumetric water content or suction.

[0003] The determination of the hydraulic conductivity of unsaturated soil includes steady or transient methods. When using the steady method, a constant head or flow rate must be maintained, and due to the low hydraulic conductivity of unsaturated soil, the test time is relatively long. The instantaneous profile (IP) method is a typical transient method for determining the hydraulic conductivity of unsaturated soil. In this method, the instantaneous volumetric water content and suction profiles are used, and the hydraulic conductivity of unsaturated soil can be calculated by Darcy's law. Compared with the steady state method, a major advantage of the instantaneous profile method is that the test time is relatively short. This method has been widely used in the study of the hydraulic conductivity of unsaturated soil.

[0004] The instantaneous profile method can also be used for in-situ testing of the hydraulic conductivity of unsaturated soil. Although the in-situ application of the instantaneous profile method is crucial for capturing certain in-situ features such as soil stratification, discontinuity, and heterogeneity, the application of the instantaneous profile method for determining the unsaturated hydraulic conductivity in-situ is very limited. The main reasons are the complex sensor installation process, the increased test cost caused by multiple sensors, and most sensors are limited to shallow layers. Sensors for measuring water content, such as neutron probes and TDR, can only be installed within a few meters. However, the hydraulic conductivity of deep unsaturated soil is very important. For example, long-term studies of soil-atmosphere interaction usually involve depths of dozens of meters, especially in areas where the unsaturated zone or the phreatic zone is very thick. In some projects involving compacted fill, such as land reclamation, the depths involved can reach hundreds of meters.

[0005] Recently, the Active Heating Fiber Optic method (AHFO) has been successfully used to measure soil water content, thermal conductivity, and geothermal flux. According to the fiber optic technology used, this technology can be divided into three categories: AHFO-DTS (Distributed Temperature Sensing), AHFO-FBG (Fiber Bragg Grating), and AHFO-UWFBG (Ultra-weak FBG). The AHFO-DTS method has the advantages of distributed and long-distance measurement. However, due to the complex temperature calibration process and the high cost of the demodulator, the field application of this method is limited. Summary of the Invention

[0006] The object of the present invention is to provide an active heating optical fiber and an in-situ method for estimating the permeability coefficient of unsaturated soil, so as to in-situ estimate the large-depth unsaturated hydraulic conductivity of soil, and overcome the defects of the existing instruments and methods for measuring the water conductivity of unsaturated soil, such as the complex temperature calibration process, the high cost of the demodulator, and the limited field application.

[0007] To achieve the above functions, the present invention designs an active heating optical fiber for estimating the permeability coefficient of unsaturated soil, including: AHFO cable, heat shrinkable sleeve, polyvinyl chloride pipe;

[0008] The AHFO cable is tightly wound around the polyvinyl chloride pipe in a spiral shape, and the outside of the AHFO cable is integrally wrapped with a heat shrinkable sleeve for encapsulation to form an AHFO measuring tube;

[0009] The structure of the AHFO cable arranged from the outermost layer to the innermost layer includes: the first layer of PE sheath, the first layer of copper mesh, the second layer of PE sheath, the second layer of copper mesh, a steel corrugated pipe, and an optical fiber; among them, the first layer of PE sheath is used for protection and insulation, the second layer of PE sheath is used to isolate the two layers of copper mesh, and the two layers of copper mesh are connected to each other at the bottom of the AHFO cable and are used as heating elements;

[0010] A plurality of UWFBG sensors are arranged on the optical fiber, and each UWFBG sensor is periodically arranged at a preset distance.

[0011] The present invention also designs an in-situ method for estimating the permeability coefficient of unsaturated soil. Based on the active heating optical fiber for estimating the permeability coefficient of unsaturated soil, the following steps S1-S5 are executed to complete the estimation of the unsaturated hydraulic conductivity of each depth layer of the target soil mass:

[0012] Step S1: Select the water content profile of the target soil mass. In the selected water content profile, the change in the water content of two adjacent depth layers exceeds a preset value;

[0013] Step S2: Vertically drill holes in the target soil mass, and deploy AHFO sensors. The AHFO sensors include AHFO cables and AHFO piezometers, and pass the AHFO cables through soil layers at various depths; collect soil samples of the target soil mass to determine the basic properties of the target soil mass, including dry density, pore size distribution, and saturated hydraulic conductivity; after the AHFO sensors are deployed, backfill the drill holes.

[0014] Step S3: Determine the drying or wetting process.

[0015] Step S4: Heat the AHFO sensors, and according to the functional relationship between the temperature change of the AHFO sensors and the soil volumetric water content, determine the soil volume water content profile of soil layers at various depths, fit the soil-water characteristic curve, and further obtain the suction profile of soil layers at various depths.

[0016] Step S5: Calculate the hydraulic gradient of soil layers at various depths and determine the unsaturated hydraulic conductivity of soil layers at various depths.

[0017] Advantageous effects: Compared with the prior art, the advantages of the present invention include:

[0018] 1. In the present invention, hundreds to thousands of UWFBG sensors are embedded in one optical fiber, which can realize real-time measurement at different depths. Only one optical cable with multiple sensing points is needed to realize measurement at different depths. The increase in sensing points does not significantly increase the cost, and the installation process is simplified, without complex wiring integration, and extends the instantaneous profile method to deeper depths.

[0019] 2. The present invention provides support for predicting unsaturated seepage and can further study the thermal-hydraulic-mechanical coupling behavior, which is crucial for preventing potential disasters such as geotechnical structure failure and harmful substance leakage. Given the increasing frequency and severity of extreme weather events, this method is also helpful for studying the interaction between deep soil and the atmosphere. Description of the Drawings

[0020] Figure 1 is a structural diagram of an actively heated optical fiber for estimating the permeability coefficient of unsaturated soil according to an embodiment of the present invention;

[0021] Figure 2 is a structural diagram of an AHFO cable according to an embodiment of the present invention;

[0022] Figure 3 is the soil volume water content profile and suction profile of soil layers at various depths according to an embodiment of the present invention;

[0023] Figure 4 is the meteorological condition diagram of the target soil mass according to an embodiment of the present invention;

[0024] Figure 5It is a schematic diagram of sensor layout provided according to an embodiment of the present invention;

[0025] Figure 6 It is a schematic diagram of a calibration equation provided according to an embodiment of the present invention;

[0026] Figure 7 It is a soil volumetric water content map of a 77-meter deep borehole obtained through an AHFO cable from November 1, 2021 to November 1, 2022 according to an embodiment of the present invention;

[0027] Figure 8 It is a soil-water characteristic curve and unsaturated hydraulic conductivity curve predicted according to the pore size distribution provided according to an embodiment of the present invention;

[0028] Figure 9 It is a soil volumetric water content and water head change map at a depth of 0.12 meters provided according to an embodiment of the present invention;

[0029] Figure 10 It is the unsaturated hydraulic conductivity measured by an AHFO cable provided according to an embodiment of the present invention. Detailed implementation manners

[0030] The present invention will be further described below with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present invention and cannot be used to limit the protection scope of the present invention.

[0031] The active heating fiber optic (AHFO) method based on ultra-weak fiber Bragg grating (UWFBG) technology can measure the volumetric water content. When broadband light is injected into the optical fiber, the UWFBG can reflect light of a specific wavelength. The Bragg wavelength shift maintains a linear relationship with the temperature change. A cylindrical sensor encapsulated with UWFBG and a heating wire can be used as an infinite linear heat source. The temperature rise (T t ) obtained by the UWFBG within a certain period of time can be correlated with the volumetric water content (θ) of the soil through a calibration equation. Thousands of UWFBG sensors can be inscribed on one optical fiber, but due to the limitation of detection technology, the distance between two UWFBG sensors is generally 0.5 - 1 meter.

[0032] An active heating fiber optic for estimating the permeability coefficient of unsaturated soil provided by an embodiment of the present invention, referring to Figure 1 , includes: an AHFO cable, a heat shrink sleeve, and a polyvinyl chloride pipe;

[0033] The AHFO cable is tightly wound around the polyvinyl chloride pipe in a spiral shape, and the outside of the AHFO cable is integrally wrapped with a heat shrink sleeve for encapsulation to form an AHFO measuring tube;

[0034] Referring to Figure 2, the AHFO cable has a structure arranged from the outermost layer to the innermost layer including: the first-layer PE sheath, the first-layer copper mesh, the second-layer PE sheath, the second-layer copper mesh, a steel corrugated pipe, and optical fibers; among them, the first-layer PE sheath is used for protection and insulation, the second-layer PE sheath is used to isolate the two layers of copper mesh, and the two layers of copper mesh are connected to each other at the bottom of the AHFO cable and are used for heating elements;

[0035] A plurality of UWFBG sensors are arranged on the optical fiber, and each UWFBG sensor is periodically arranged at a preset distance.

[0036] The diameter of the AHFO measuring tube is 54 mm, the diameter of the PVC tube is 42 mm, the distance between two adjacent UWFBG sensors is 0.0352 m, and the spatial resolution of the AHFO measuring tube can reach 0.04 m.

[0037] An embodiment of the present invention also provides an in-situ method for estimating the permeability coefficient of unsaturated soil. Based on the active heating optical fiber for estimating the permeability coefficient of unsaturated soil, the following steps S1 - S5 are performed to complete the estimation of the unsaturated hydraulic conductivity of each depth layer of the target soil:

[0038] Step S1: Select the water content profile of the target soil. In the selected water content profile, the change in water content between two adjacent depth layers exceeds 0.01 m 3 / m 3 ;

[0039] Step S2: Vertically drill a hole in the target soil and arrange AHFO sensors. The AHFO sensors include AHFO cables and AHFO measuring tubes, and make the AHFO cables pass through each depth layer of soil; collect soil samples of the target soil to measure the basic properties of the target soil, including dry density, pore size distribution, and saturated hydraulic conductivity; after the AHFO sensors are arranged, backfill the hole;

[0040] Step S3: Determine the drying or wetting process;

[0041] Step S4: Connect the AHFO sensors to a demodulator and a DC power supply respectively, heat the AHFO sensors at a constant rate for a preset time, and according to the functional relationship between the temperature change of the AHFO sensors and the soil volumetric water content, determine the soil volumetric water content profile of each depth layer of soil, fit the soil-water characteristic curve, and further obtain the suction profile of each depth layer of soil;

[0042] The functional relationship between the temperature change of the AHFO sensor and the soil volumetric water content is as follows:

[0043]

[0044] In the formula, T t$T$ is the temperature of the AHFO sensor, $\theta$ is the volumetric water content of the soil, and $A$, $B$, and $C$ are constants related to the target type, which are obtained from the fitting results of the calibration experiment.

[0045] Refer to Figure 3 , where Figure 3 (a) are the profiles of soil volumetric water content obtained at two elapsed times ($t_1$ and $t_2$), Figure 3 (b) are the suction profiles of each soil layer at different depths;

[0046] In step S4, the pore size distribution of the soil is obtained by mercury intrusion porosimetry, and the soil-water characteristic curve is fitted based on the cumulative curve of the pore size distribution.

[0047] Step S5: Calculate the hydraulic gradient of each soil layer at different depths and determine the unsaturated hydraulic conductivity of each soil layer.

[0048] The specific steps of step S5 are as follows:

[0049] Step S5.1: Establish the one-dimensional continuity expression of the soil in the vertical direction as follows:

[0050]

[0051] where $v$ is the water flow velocity in the soil, $z$ is the soil depth, $\theta$ is the volumetric water content of the soil, and $t$ is time;

[0052] Step S5.2: Integrate the expression in step S5.1 from depth $z$ B to depth $z$ C . The water flow between the two depths is expressed as follows:

[0053]

[0054] Calculate the change in water flow velocity with time at two different elevations as follows:

[0055]

[0056] where $\Delta V$ represents the change in soil water content from soil depth $z$ B to soil depth $z$ C during the time period from $t_1$ to $t_2$; where $t$ ave represents the average time, and respectively represent the water flow velocities at soil depth $z$ B and soil depth $z$ C at the average time $t$ ave ;

[0057] According to Darcy's law, we have the following equation:

[0058]

[0059] Let k represent the hydraulic conductivity and i represent the hydraulic gradient. In the formula, and respectively represent the hydraulic conductivity at soil depths z B and soil depth z C within the average time t ave ; and respectively represent the hydraulic gradient at soil depths z B and soil depth z C within the average time t ave ;

[0060] Step S5.3: Use to replace and to obtain the following formula:

[0061]

[0062] Step S5.4: Calculate in the

[0063]

[0064] by the central difference method. In the formula, H represents the sum of the total head and the pressure head, and z is the soil depth.

[0065] The following is an application embodiment of the present invention:

[0066] 1. Select the target soil mass: A field test was conducted in the New Area of Yan'an, China (latitude 36°38'51", longitude 109°31'18"). This land was formed by cutting mountains and filling valleys, with both compacted loess and natural loess. Figure 4 Describes the daily average temperature and precipitation at 2 meters above the ground from November 1, 2021, to November 1, 2022. The monitoring location belongs to the temperate monsoon climate. During the monitoring period, the daily average maximum temperature in July was 28°C, and the daily average minimum temperature in December was -12°C. The cumulative precipitation during the monitoring period was 559 mm, and the precipitation was mainly concentrated in July and August. The evaporation and temperature showed the same change trend, reaching the maximum in July and August.

[0067] 2. Sensor layout: Refer to Figure 5, the AHFO cable was installed through a 77-meter-deep borehole. The borehole revealed the interface between compacted loess and natural loess at 63 meters and the groundwater level at 66 meters. Subsequently, the weight guide together with the cable was carefully and slowly lowered into the borehole. Finally, the borehole was backfilled with loess. To install the access tube, a 1.5-meter borehole with a diameter of 60 mm was first drilled. Then, the probe was inserted into the soil. The last step was to backfill the borehole. During the installation process, undisturbed soil samples were collected using a ring knife to determine some basic properties in the laboratory, including dry density, pore size distribution, and saturated permeability. After the installation was completed, the two sensors of the AHFO cable and the AHFO access tube were connected to a demodulator (model: NZS-DDS-A01, NanZee Sensing, Suzhou, China) and a DC power supply (model: PRO-10020D, MegaTrust, Shenzhen, China). Once every three days, the AHFO cable was heated at a rate of 8 W / m for 15 minutes at 22:00. The AHFO access tube was heated once at 22:00 every day at a heating rate of 78.4 W / m for 25 minutes. The sampling interval of different sensors mainly depends on the change frequency of the water content in the target layer. The sampling interval can be shortened to meet different measurement requirements, but to avoid moisture migration caused by temperature, a two-hour interval is maintained between each heating test.

[0068] Two TDR access tubes were installed at a depth of 0.5 meters beside the AHFO sensors, respectively for calibrating the AHFO access tube and the optical cable. The calibration equations are as Figure 6 shown. Due to the different internal structures and heating powers of the sensors, the parameters are also different. It was determined that the root mean square errors of the in-situ calibrations of the AHFO cable and the AHFO access tube are 0.5% and 0.2% respectively, both lower than 1%. When the calibration error is large, it should be noted that it will affect the accuracy of k(ψ) estimation in two aspects. First, it will cause errors in the suction profile, and second, it will affect the estimation of the total water content. The influence on suction is more obvious in the high suction range because a small change in water content will cause a significant change in suction. The error in the total water content estimation is linearly correlated with the calibration error.

[0069] 3. Determine the soil volumetric water content profile: Figure 7 Shows the volumetric water content of a 77-meter-deep borehole obtained through the AHFO cable from November 1, 2021, to November 1, 2022.

[0070] Where Figure 7 (a) is the distribution map of the initial water content (θ), Figure 7 (b) is the change map of the volumetric water content at different depths relative to the initial volumetric water content (Δθ) measured on November 1, 2021, Figure 7 (c)- Figure 7(e) Variation diagrams of the volumetric water content of shallowly compacted loess (SCL), deeply compacted loess (DCL), and natural loess (NL) over time;

[0071] Spatially, the initial water content (θ) is relatively evenly distributed, with a coefficient of variation (ratio of standard deviation to mean) of 0.11. Temporally, the volumetric water content varies significantly within the depth range of 0 to 4 m. During spring and summer, due to water infiltration, the volumetric water content increases, while during autumn and winter, due to water evaporation, the volumetric water content decreases. Some minor changes can be observed at greater depths. Three sections on the profile, namely shallowly compacted loess (SCL) from 0 m to 4 m, deeply compacted loess (DCL) from 57 m to 63 m, and natural loess (NL) from 63 m to 66 m, are used to verify the method proposed in the present invention.

[0072] 4. Fitting the soil-water characteristic curve: To obtain the suction profile, the soil-water characteristic curve for each section should first be determined. Refer to Figure 8 , where Figure 8 (a) is the pore size distribution diagram, Figure 8 (b) is the cumulative pore size distribution curve, Figure 8 (c) is the soil-water characteristic curve, Figure 8 (d) is the unsaturated hydraulic conductivity curve;

[0073] The pore size distribution (PSD) was obtained through mercury intrusion porosimetry (MIP) tests. The PSD cumulative curve was used to describe the soil-water characteristic curve of loess during the drying process. Soils at different depths exhibit different PSDs. The main pore diameters of shallowly compacted loess are larger than those of deeply compacted loess and natural loess. In addition, due to the in-situ compaction process, large pores (>100 μm) are very abundant in compacted loess. Therefore, there are two pore groups in compacted loess, corresponding to small pores and large pores respectively. The pore diameter corresponding to the inflection point is 6 μm, which can be used to divide small pores and large pores. These two pore groups have different water retention characteristics, thus forming their bimodal soil-water characteristic curves. Small pores have a higher water sensitivity to changes in water content. The inflection point of its soil-water characteristic curve (SWRC) is located at 50 kPa. The Van Genuchten model was used to verify the soil-water characteristic curve fitted in the present invention:

[0074]

[0075] where θ s is the saturated volumetric water content; θ r is the residual volumetric water content; ψ is the suction; α, n, θ r are fitting parameters, m = 1 - 1 / n. For compacted loess, the relationships of suction above 50 kPa (θψ2) and suction below 50 kPa (θψ1) are respectively fitted.

[0076] Then, a variable-head permeability test was conducted to obtain the saturated hydraulic conductivity. The unsaturated hydraulic conductivity curve (HCC) was obtained using the van Genuchten-Mualem model:

[0077] k = k s Θ 0.5 [1 - (1 - Θ 1 / m ) m 2

[0078] where k s is the saturated hydraulic conductivity. For compacted loess, according to the superposition principle, k = k ψ1 + k ψ2 .

[0079] 5. Measurement of unsaturated hydraulic conductivity: The natural volumetric water content change was used to determine the unsaturated hydraulic conductivity k(ψ), which involves infiltration and evaporation processes. The volumetric water content and head change diagram measured at a depth of 0.12 m using an AHFO piezometer tube is shown in Figure 9 . Taking the hydraulic gradient at 0.12 m as an example, it can be seen that the hydraulic gradient was positive during the period from November 2021 to December 2021, which means the water flowed upward, proving the evaporation effect. After that, the hydraulic gradient was close to zero, indicating no water flow. Subsequently, during the period from April 2022 to November 2022, the hydraulic gradient was mostly negative, that is, the water flowed downward, indicating that the infiltration process was dominant. In terms of the change in volumetric water content, both the drying and wetting processes can be used for calculation.

[0080] 6. Results: The unsaturated hydraulic conductivity k(ψ) measured by the AHFO cable is as shown in Figure 10 , where Figure 10 (a) is the hydraulic conductivity of shallow compacted loess; Figure 10 (b) is the hydraulic conductivity of deep compacted loess; Figure 10 (c) is the hydraulic conductivity of natural loess; For the Shazhong line, when the suction increased from 3 kPa to 228 kPa, k(ψ) decreased from 7.26×10 -8 m / s to 7.95×10 -10 m / s. By extrapolating the measured k(ψ), the hydraulic conductivity at zero suction can be obtained, which is equal to 9.86×10 -8 , slightly lower than the k s (1.28×10 -7 ​m / s). Compared with the k(ψ) measured during the drying process, the HCC derived from the SWRC slightly overestimates k(ψ). For the DCL, the k(ψ) calculated during the drying process is very little. When the suction increases from 2 kPa to 346 kPa, the k(ψ) value decreases from 2.04×10-8 m / s to 1.67×10 -9 m / s. When the suction is zero, its value is 1.41×10 -8 , which is very close to the measured value of k s (1.38×10 - 8 m / s). In the DCL, the average value at a depth of 60.5 m is 4.48×10 -10 m / s, and this serious underestimation is due to the relatively large dry density at this depth (1.93 Mg / m 3 ). The measured values are in very good agreement with the derived HCC. Overall, the agreement among the in-situ measured values, the falling head method, and the HCC derived from the SWRC is very good, demonstrating the effectiveness of the method of the present invention for large depths.

[0081] The embodiments of the present invention have been described in detail above in conjunction with the accompanying drawings. However, the present invention is not limited to the above embodiments, and various changes can be made without departing from the spirit of the present invention within the scope of knowledge possessed by those of ordinary skill in the art.

Claims

1. A method for estimating the permeability coefficient of unsaturated soil based on actively heated optical fibers, characterized in that The actively heated optical fiber includes: an AHFO cable, a heat shrinkable sleeve, and a PVC pipe; The AHFO cable is tightly wound around the PVC pipe in a spiral shape, and the entire exterior of the AHFO cable is wrapped with a heat shrinkable sleeve for encapsulation to form an AHFO measuring tube; The AHFO cable is structured with components arranged from the outermost layer to the innermost layer as follows: the first layer of PE sheath, the first layer of copper mesh, the second layer of PE sheath, the second layer of copper mesh, a steel corrugated pipe, and an optical fiber; among them, the first layer of PE sheath is used for protection and insulation, the second layer of PE sheath is used to isolate the two layers of copper mesh, and the two layers of copper mesh are connected to each other at the bottom of the AHFO cable and are used as heating elements; A plurality of UWFBG sensors are arranged on the optical fiber, and the UWFBG sensors are periodically arranged at a preset distance; Based on the actively heated optical fiber described above, perform the following steps S1 - S5 to complete the estimation of the unsaturated hydraulic conductivity of each depth layer of the target soil mass: Step S1: Select the water content profile of the target soil mass. In the selected water content profile, the water content change between two adjacent depth layers exceeds a preset value; Step S2: Vertically drill a hole in the target soil mass and install an AHFO sensor. The AHFO sensor includes an AHFO cable and an AHFO measuring tube, and make the AHFO cable pass through each depth layer; collect soil samples of the target soil mass to measure the basic characteristics of the target soil mass, including dry density, pore size distribution, and saturated hydraulic conductivity; after the AHFO sensor is installed, backfill the drill hole; Step S3: Determine the drying or wetting process; Step S4: Heat the AHFO sensor, and based on the functional relationship between the temperature change of the AHFO sensor and the soil volumetric water content, determine the soil volumetric water content profile of each depth layer, fit the soil - water characteristic curve, and further obtain the suction profile of each depth layer; Step S5: Calculate the hydraulic gradient of each depth layer and determine the unsaturated hydraulic conductivity of each depth layer.

2. The method for estimating the permeability coefficient of unsaturated soil based on actively heated optical fiber according to claim 1, wherein The diameter of the AHFO measuring tube is 54 mm, the diameter of the PVC pipe is 42 mm, and the spatial resolution of the AHFO measuring tube is 0.04 m.

3. The method for estimating the permeability coefficient of unsaturated soil based on actively heated optical fiber according to claim 1, wherein In step S1, the water content change between two adjacent deep soil layers exceeds 0.01 m 3 / m 3 .

4. The method for estimating the permeability coefficient of unsaturated soil based on actively heated optical fiber according to claim 1, characterized in that, In step S4, connect the AHFO sensor to a demodulator and a DC power supply respectively, and heat the AHFO sensor at a constant rate for a preset time.

5. The method for estimating the permeability coefficient of unsaturated soil based on actively heated optical fiber according to claim 1, wherein The functional relationship between the temperature change of the AHFO sensor and the soil volumetric water content in step S4 is as follows: ； Where, T t is the temperature of the AHFO sensor, θ is the volumetric soil water content, and A, B, and C are constants related to the target type, obtained from the fitting results of the calibration experiment.

6. The method for estimating the permeability coefficient of unsaturated soil based on actively heated optical fiber according to claim 1, wherein In step S4, the pore size distribution of the soil is obtained by mercury intrusion porosimetry, and the soil - water characteristic curve is fitted based on the cumulative pore size distribution curve.

7. A method for estimating the permeability coefficient of unsaturated soil based on actively heated optical fiber according to claim 1, characterized in that, The specific steps of step S5 are as follows: Step S5.1: Establish the following one - dimensional continuity expression of the soil in the vertical direction: ； Wherein, v is the water flow velocity in the soil, z is the soil depth, θ is the soil volumetric water content, t is the time; Step S5.2: Integrate the expression in Step S5.1 from z B depth to z C depth. The water flow rate between the two depths is expressed as follows: ； Calculate the change of the water flow velocity with time at two different altitudes as follows: ； where, Δ V represents the change in soil water content from soil depth z B to soil depth z C and from time t 1 to time t 2; where t ave represents the average time, , and respectively represent the water flow velocities at soil depths z B and soil depth z C during the average time t ave ; According to Darcy's law, there is the following formula: ； Let k represent the hydraulic conductivity, i represent the hydraulic gradient, where and represent the soil depths z B and z C of the hydraulic conductivity at the average time t ave ; and represent the soil depths z B and z C of the hydraulic gradient at the average time t ave ; Step S5.3: Use to replace and , and we have the following formula: ； Step S5.4: Calculate by the central difference method in the expression , : ； ； In the formula, H represents the sum of the total head and the pressure head, z is the soil depth.

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