Seepage Velocity Research Articles

Influence of frost heave with confining pressure under seepage on compressive strength of frozen silt in artificial ground freezing

Artificial ground freezing (AGF) is used for constructing subway cross passages in coastal areas. The continuous construction of complex projects (cross-sea or cross-river tunnels) has resulted in freezing environments with high groundwater seepage velocities. Seepage cannot provide an open system in soil freezing for moisture migration, resulting in large amount of frost heave owing to segregated ice compared to close freezing system. The influence of frost heave under seepage on the compressive strength of frozen silt during artificial ground freezing must be studied to ensure AGF construction safety. Thus, a unidirectional freezing device that can control seepage and confining pressure conditions during freezing was designed to prepare frozen silt samples under different seepage velocities with confining pressures and to measure the relevant segregation frost heave. Subsequently, uniaxial compression tests were performed. The compressive strength of frozen silt under the influence of frost heave and seepage conditions was analyzed. The frost heave rate, uniaxial compressive strength, and elastic modulus of frozen silt in an open freezing system were measured. The seepage velocity during freezing increased the frost heave rate of the silt; the uniaxial compressive strength and elastic modulus of the frozen silt increased subsequently. The effective confining pressure suppressed frost heave of silt and reduced the uniaxial compressive strength and elastic modulus of the frozen silt. The test results were analyzed considering the segregation frost heave formation mechanism, and a theoretical improvement model of the uniaxial compressive strength and elastic modulus of frozen silt related to the content of segregated ice was proposed. This research is significant for ensuring the AGF construction safety in cross-river or ocean tunnel engineering under seepage conditions.

Study on the influence of water supply and drainage pipeline damage on urban silt ground collapse

Underground drainage pipelines play crucial roles in urban construction and development. Over time, these pipelines may incur damage and leakage due to aging and changes in surrounding loads. Ruptures and leaks in pipelines can lead to water seepage through cracks, resulting in erosion of surrounding soil layers and the formation of underground cavities. The loss of support in the soil above these cavities can lead to structural instability and eventual ground collapse. Such collapses can disrupt urban transportation systems, leading to significant social and economic consequences. This study specifically investigates ground collapse phenomena resulting from damage to drainage pipelines in silty soil within the Yellow River flood area in Zhengzhou City. By considering the influence of seepage velocity at the damaged pipeline section, the study differentiates between seepage erosion and scour erosion, conducting theoretical analyses and calculations for each scenario. The results show that ① when the surrounding area of the pipeline is damaged and the slow seepage rate causes a change in the critical groundwater level ΔH ≥ (Lσt )/((1 – n)γw), the soil around the pipeline will be damaged by seep erosion; ② when the water flow velocity at the damaged area of the pipeline is reached V 0 = 2λ(γs – γw )a 2 B 2/(9μ(B 2 – b 2)), the water flow impacts the soil and causes erosion damage. The critical pressure within the pipeline can be derived from the stress state where the pipeline is damaged P1=ρ2[2λ(γs−γw)a2B2/(9μ(B2−b2))] ; ③ the underground cavity formed by water flow erosion is assumed to be an “soil arch.” The stress analysis on it obtains the critical span of the “soil arch” when the “soil arch” is damaged 2b = [2c(H + h) + Kaγ(H + h)2 tan φ – P]/(H + h/3). This study provides a theoretical basis and technical support for the management of silty ground collapse in the Yellow River flood area.

Model test study on the formation of basin-shaped freezing range in sandy gravel stratum under seepage conditions

The basin-shaped freezing method has a wide range of engineering application prospects because it is environmentally friendly and not sensitive to the depth of the phreatic layer. Based on the similarity theory, multiple sets of model tests were performed to study the development of the basin-shaped freezing range and analyse the influence of seepage velocity, the distance between the freeze pipes and the refrigerant temperature. The test results revealed that, under the influence of group pipe effect, the cooling rate in the middle of the basin bottom was the fastest, and the formation time was the shortest. A single factor had no noticeable impact on the formation time of the closed basin bottom. However, the formation speed of the top surface of basin wall was the slowest under the influence of boundary effect and was significantly influenced by various factors. When the basin wall freeze pipes were arranged with the same spacing along and perpendicular to the water flow direction, the formation time of the top surface of the basin wall perpendicular to the water flow direction was the longest. When the spacing of the freeze pipes along the water flow direction was greater than that perpendicular to the water flow direction, attention should be given to the temperature of the top surface of the basin wall in both directions. The empirical relationships between the formation time of the basin wall and the distance between freeze pipes and the seepage velocity were obtained through regression analysis.

Impact of groundwater seepage on thermal performance of capillary heat exchangers in subway tunnel lining

The metro tunnels are currently facing challenges associated with thermal pollution, which significantly affects their operational safety and energy consumption. The subway source heat pump system (SSHPS) with front-end tunnel lining capillary heat exchangers (CHEs) is an effective technology to address this problem. The heat transfer characteristics of SSHPS have been previously investigated; however, there is a lack of research on the thermal performance of tunnel lining CHEs under groundwater seepage. This study established a numerical simulation model of tunnel lining CHEs based on a demonstration project of SSHPS. Subsequently, the influence of groundwater seepage on the heat transfer characteristics of tunnel lining CHEs was comprehensively analyzed. The findings suggest that an increase in groundwater seepage velocity leads to the initial expansion and subsequent contraction of the thermal influence range caused by CHEs. Simultaneously, the performance of CHEs is enhanced with increasing flow velocity. When the seepage velocity increases from 0 to 1 × 10−4 m/s, the heat exchange capacity per meter and efficiency of the CHEs during the cooling season increase by 450 % and 446 %, respectively. During the heating season, they experience a growth of 320 % and 310 %, respectively. In addition, the heat exchange performance of CHEs is also influenced by the relative flow direction between the fluid within CHEs and the groundwater. The counter flow arrangement exhibits a 7.7 % higher efficiency of heat exchange during the cooling season compared to the parallel flow arrangement, and a 3.9 % higher efficiency of heat exchange during the heating season. Therefore, it is highly recommended to evaluate groundwater seepage conditions during the design phase of SSHPS in order to enhance the performance of tunnel lining CHEs. This study serves as a valuable theoretical reference for the design of tunnel lining CHEs.

Prevention of Water Seepage Impact on the Soluble Rocks Using Colloidal Silica

Water seepage flow can dissolve soluble minerals that exist in rock formations. With the development of the excavated area due to dissolution, the water seepage velocity (discharge) into the dissolved rock will also increase. Therefore, water seepage and dissolution propagation are two interrelated processes. Mosul Dam foundation has experienced these processes since its construction, resulting in karstification in the reservoir and foundation of the dam. The present seepage-dissolution measure to minimize this phenomenon relies on traditional cementitious grouts. However, this measure has not been able to address the issue effectively. Currently, there are a few studies on the chemical remediation of soluble rocks under the influence of high-velocity water flow and water pressure. Therefore, the first part of the current study focuses on the impact of high-velocity water flow and water pressure on the dissolution acceleration of gypsum/anhydrite rocks. In the second part, the waterproof capacity of silica colloidal and its impact on the solubility reduction of the rocks is evaluated. Two distinct laboratory models were designed to simulate rock dissolution in the dam abutments and under the dam. Two sets of experiments were conducted on untreated and silica-treated samples. The experiments were executed on the samples extracted from Fatha Formation outcrop and problematic layers of brecciated gypsum situated at varying depths of the Mosul Dam foundation. The obtained findings reveal that the colloidal silica grout markedly prevents the water seepage impact on the soluble rock and that it can be very useful as an alternative to cement-based grouts.

Numerical Study on Permeability of Reconstructed Porous Concrete Based on Lattice Boltzmann Method

The reconstruction of the porous media model is crucial for researching the mesoscopic seepage characteristics of porous concrete. Based on a self-compiled MATLAB program, a porous concrete model was modeled by controlling four parameters (distribution probability, growth probability, probability density, and porosity) with clear physical meanings using a quartet structure generation set (QSGS) along with the lattice Boltzmann method (LBM) to investigate permeability. The rationality of the numerical model was verified through Poiseuille flow theory. The results showed that the QSGS model exhibited varied pore shapes and disordered distributions, resembling real porous concrete. Seepage velocity distribution showed higher values in larger pores, with flow rates reaching up to 0.012 lattice point velocity. The permeability–porosity relationship demonstrated high linearity (the Pearson correlation coefficient is 0.92), consistent with real porous concrete behavior. The integration of QSGS-LBM represents a novel approach, and the research results can provide new ideas and new means for subsequent research on the permeability of porous concrete or similar porous medium materials.

Research on the effect of water-cooling steel pipe on preventing spontaneous combustion of coal pile and its thermal migration behavior

During the storage and transportation process after mining, coal piles are placed in open environments, making them prone to self-heating and spontaneous combustion due to the nature of coal and factors like natural wind flow. In recent years, there have been frequent spontaneous combustion incidents involving coal piles, posing significant safety risks. To effectively prevent and control spontaneous combustion disasters in open-air coal storage piles, we propose a method involving the arrangement of water-cooling steel pipes within the coal piles. This method applies theories of coal spontaneous combustion mechanisms, porous media heat transfer, and non-isothermal pipeline heat transfer. The multi-physics coupling model of COMSOL numerical simulation software is used to analyze the spontaneous ignition process and prevention effect of open pit coal pile. In the model, the thin material transfer of porous media is taken as the oxygen concentration field, the heat transfer of porous media is taken as the temperature field, and the free and porous media flow is taken as the air seepage velocity field. The simulation results of the spontaneous combustion process in the coal pile indicate that the high-temperature zone of spontaneous combustion is situated within the range of 0.5 ~ 1.5 m inside the wind-facing surface and extends 0.5 m above the ground level. These findings serve as a basis for determining the optimal placement of water-cooling steel pipes within the coal pile. The simulation results of a single water-cooling steel pipe demonstrate a positive correlation between the cooling effect on the coal pile and the water cool flow, and a negative correlation with the water cool temperature. Additionally, the cooling radius of the water-cooling steel pipe is determined by the circumference of the pipe and remains unaffected by the water cool flow. Finally, simulations were conducted to evaluate the cooling effect of multiple rows of steel pipes, and optimal arrangement parameters were determined: a center distance between steel pipes of 1 m and a water cool flow rate of 1500 L/min. As a result, the onset of the self-heating period in the coal pile was delayed by 11 days, and the spontaneous combustion period was extended by 56 days. The arrangement of water-cooling steel pipes in the coal pile has demonstrated significant efficacy in preventing and controlling spontaneous combustion.

Effects of the hydrologic process and geochemistry on dissolved carbon in shallow groundwater surrounding Qinghai Lake

A high concentration of dissolved carbon in groundwater increases drinking water health risks and carbon transport. Understanding the comprehensive impact of hydrologic processes and geochemistry on dissolved carbon in shallow groundwater also is a fundamental prerequisite for estimating the carbon budget of lakes in the Tibetan Plateau. This study investigated the spatial–temporal characteristics of the hydrologic process, geochemistry and dissolved carbon in shallow groundwater by the stable isotope tracer method, Piper diagram and Boomerang envelope model. The driving factors of dissolved carbon in shallow groundwater were explored by correlation analysis and redundancy analysis. The results showed low dissolved inorganic carbon (DIC) concentrations and high dissolved organic carbon (DOC) concentrations during the thawing period and rainy season and high DIC concentrations and low DOC concentrations during the freezing period. The seepage velocity, soil carbon dioxide dissolution and gypsum dissolution were the main factors influencing DIC concentrations during the thawing period. The mineralization and decomposition of DOC and dissolution of carbonate rocks were the main factors influencing DIC concentrations during the freezing period. The concentrations of DOC were mainly controlled by the adsorption of Ca2+ and Mg2+, microbial activity and pollutants produced by human activities. Our results are useful for ecological sustainable development, human health, and research on the carbon transport in groundwater in the Tibetan Plateau.

Migration trajectories and blocking effect of the fine particles in porous media based on particle flow simulation

The particle flow code method based on the discrete element method was used to establish the seepage migration model of fine particles [fine particles (FPs), i.e., suspended particles] in a porous medium. A series of numerical simulations were carried out by changing the particle size, seepage velocity, particle injection number, and wide particle size gradation. The research showed that large FPs play a major role in blocking porous medium channels when the injected FPs have a wide size gradation. Due to the blocking effect, small FPs that would not otherwise have deposited also deposit. Moreover, by increasing the number of large FPs in the mixed particles, the total number of particles deposited and the number of smaller FPs deposited will also increase. The distribution of FPs in porous mediums can be divided into three types: surface deposition, internal deposition, and non-deposition. When the seepage velocity increases and reaches a seepage threshold, which is the critical seepage velocity, the deposited FPs will once again be in a suspended state and undergo migration. On the contrary, the FPs will continue to maintain their sedimentary state, and the critical seepage velocity will also increase correspondingly with increasing particle size.

ASSESSING LOCAL PORE WATER VELOCITY ALONG PREFERENTIAL FLOW IN EARTHEN DAM USING SALT TRACER

The local pore water velocity along the preferential flow path signifies the hydraulic parameter responsible for erosion within an earthen dam. This study introduces an empirical approach to ascertain the local pore water velocity within the earth dam's leakage zones by monitoring the travel time of the salt tracer through the corresponding electric potential anomalies in the ground. The alignment of electric potential anomalies with the movement of the salt tracer plume over time was confirmed through experiments on a physical model coupled with numerical simulations. The pore water velocity, calculated based on the location of the maximum electric potential anomaly, demonstrated excellent agreement with the experimental value, with an error of under 6%. For illustrative purposes, a field-scale salt tracer test was conducted at a leaking earthen dam in Vietnam. The tracer breakthrough curve originating from the leakage point revealed that the seepage water's travel time is approximately 40 days. The results of electric potential anomalies over time indicate that the pathway of seepage flow from upstream to the leakage point forms a horizontal V-shape, with the local pore water velocity ranging from 1.7 to 9.9x10-5 m/s. These local pore water velocities are subsequently compared with the critical seepage velocity to assess in-situ information regarding the internal erosion status of the target dam.

Convection-Enhanced Drug Delivery: Experimental and Analytical Studies of Infusion Behavior in an In Vitro Brain Surrogate.

Convection-enhanced drug delivery (CED) directly infuses drugs with a large molecular weight toward target cells as a therapeutic strategy for neurodegenerative diseases and brain cancers. Despite the success of many previous in vitro experiments on CED, challenges still remain. In particular, a theoretical predictive model is needed to form a basis for treatment planning, and developing such a model requires well-controlled injection tests that can rigorously capture the convective (advective) and diffusive transport of an infusate. For this purpose, we investigated the advection-diffusion transport of an infusate (bromophenol blue solution) in the brain surrogate (0.2% w/w agarose gel) at different injection rates, ranging from 0.25 to 4 μL/min, by closely monitoring changes in the color intensity, propagation distance, and injection pressures. One dimensional closed-form solution was examined with two variable sets, such as the mathematically calculated coefficient of molecular diffusion and average velocity, and the hydraulic dispersion coefficient and seepage velocity by the least squared method. As a result, the seepage velocity was greater than the average velocity to some extent, particularly for the later infusion times. The poroelastic deformation in the brain surrogate might lead to changes in porosity, and consequently, slight increases in the actual flow velocity as infusion continues. The limitation of efficiency of the single catheter was analyzed by dimensionless analysis. Lastly, this study suggests a simple but robust approach that can properly capture the convective (advective) and diffusive transport of an infusate in an in vitro brain surrogate via well-controlled injection tests.

Effects of Clay Content on Non-Linear Seepage Behaviors in the Sand–Clay Porous Media Based on Low-Field Nuclear Magnetic Resonance

Clay is widely encountered in nature and directly influences seepage behaviors, exerting a crucial impact on engineering applications. Under low hydraulic gradients, seepage behaviors have been observed to deviate from Darcy’s law, displaying a non-linear trend. However, the impacts of clay content on non-linear seepage behavior and its pore-scale mechanisms to date remain unclear. In this study, constant-head seepage experiments were conducted in sand–clay porous media under various hydraulic gradients. Low-field nuclear magnetic resonance (LF-NMR) technology was utilized to monitor the bound-water and free-water contents of sand–clay porous media under different seepage states. The results show a threshold hydraulic gradient (i0) below which there is no flow, and a critical hydraulic gradient (icr) below which the relationship between the hydraulic gradient (i) and seepage velocity (v) is non-linear. Both hydraulic gradients increased with clay content. Moreover, the transformation between bound water and free water was observed during the seepage-state evolution (no flow to pre-Darcy or pre-Darcy to Darcy). As the hydraulic gradient reached the i0, the pore water pressure gradually overcame the adsorption force of the bound-water film, reducing the thickness of the bound-water film, and causing non-linear seepage behavior. When i0 < i < icr, the enlarging hydraulic gradient triggers the thinning of bound water and enhances the fluidity of pore water. Moreover, the increasing clay content augments the bound-water content required for the seepage state’s change.

Experimental study on the optimal spur dike shape under downward seepage

ABSTRACT The successful stabilization of riverbanks and the construction of sustainable foundations, extensive knowledge of the scour process and accurate assessment of scour depth around spur dikes are required. This study focuses on optimizing the spur dike shape to manage flow turbulence effectively and minimize local scour while considering the impact of downward seepage. The investigation was conducted within an experimental flume featuring a mobile bed under a clear water regime and the provision for downward seepage. The investigation concentrated on three distinct spur dike shapes (T, L, and rectangular) under different seepage velocities (V S1 and V S2) and a no-seepage condition. The current study also illustrates changes in bed morphology, temporal evolution and longitudinal profile of scour depth with and without downward seepage. The results reveal that the downward seepage intensifies the motion of sediment particles, and more seepage leads to an escalated particle detachment, resulting in deeper scour depressions. The T-shaped spur dikes exhibited the lowest scour depths compared to the other shapes, both with and without seepage. The negative impact on velocity and RSS magnitude was observed at the near channels bed where maximum scour depth was achieved. This phenomenon is closely linked to horseshoe vortices’ formation and current generation. These factors play a pivotal role in the detachment of particles from the base of the structure.

Biomimetic hybrid porous microspheres with plant membrane-wall structure for evaluating multiscale mechanisms of ultrasound-assisted mass transfer

Determination of mass transfer during ultrasound-assisted extraction (UAE) has always been a challenge, not to mention the novel breakthrough of mechanisms for its positive effects. Herein, versatile porous microspheres (PM) with membrane-wall structure like plant tissues, which were made from sodium alginate/polyacrylamide dual crosslinking gels loaded with liposomes, were prepared to reveal the micro mass transfer effects during UAE. Results showed that prepared PM kept stable microstructure during extraction and ultrasound significantly accelerated the pore diffusion rather than surface diffusion. This effect resulted in higher UAE efficiency of PM with 60 % porosity than solid microspheres (143.72 to 155.47 mg/L), and the finding can be further explained as the accelerated internal diffusion (14.25 %) under ultrasound treatment. Moreover, multi-physics coupling simulations were performed to elucidate the enhanced pore flow and UAE efficiency. Acoustic streaming induced by cavitation effect increased the seepage velocity within the PM, and the pore structure would undergo compression and expansion deformation with alternated acoustic pressure due to its elastic characteristic. Both microscale effects improved the mass transfer and increase the UAE efficiency without structural damage from cavitation. This study fills the UAE mechanisms from micro view and provides a novel method for the study of vital micro processes.

Transport of graphene oxide in the capillary fringe: Insights from sandbox experiments and numerical simulation

Studying contaminant transport in the capillary fringe (CF), a crucial part of the vadose zone, offers insights into the mechanisms controlling pollution in soils and groundwater aquifers. This paper investigated contaminant transport in the CF by continuously injecting a conservative tracer (NaCl) and graphene oxide nanoparticle (GONP), an adsorptive contaminant, into a sandbox. After entering the CF from the unsaturated zone, both NaCl and GONP underwent lateral transport. The breakthrough curves (BTCs) for NaCl and GONP were derived from water samples collected at predetermined sampling holes. Subsequently, contaminant transport in the CF was modeled using a one-dimensional–two-dimensional (1D-2D) coupled hydrodynamic model. This model incorporated lateral dispersivity (αL = 1.198 cm) and longitudinal dispersivity (αT = 0.286 cm), calculated using a point-by-point method. The hydrodynamic dispersion coefficients obtained were then applied to the Brooks and Corey (BC) and the van Genuchten (VG) parametric models. The BC model more accurately simulated the NaCl migration compared to the VG model, leading to its application in simulating GONP transport in the CF. However, the simulated BTCs for GONP showed a lag behind the measured data, especially at high ionic strengths. This discrepancy was attributed to the variable adsorption partition coefficient of GONP under different ionic conditions. During the experiment, GONP adsorption onto the porous media's surface altered the capillary dynamics, notably increasing capillary rise height, decreasing seepage velocity, and reducing GONP dispersion. Therefore, it is necessary to consider the adsorption capacity of the contaminants in order to accurately assess their transport within the vadose zone.

Fractal mechanical model of variable mass seepage in karst collapse column of mine

Currently, the study of karst collapse column water inrush mechanism often ignores the effect of pore structure, and the traditional fractal seepage model ignores the effect of nonlinear seepage velocity field on model permeability. In order to solve this problem, the fractal seepage theory is combined with variable mass seepage theory, and the influence of seepage velocity field on model permeability is considered. The fractal seepage model of the third flow field of the settling column is established and improved, and the influence of different pore structure parameters and nonlinear seepage parameters on the macroscopic seepage of the settling column is analyzed.

The impact of soil layering and groundwater flow on energy pile thermal performance

Shallow geothermal energy pile systems have emerged as cost-effective and low-carbon alternatives for heating and cooling buildings, compared to traditional air-conditioning systems. Geothermal applications have been researched extensively in recent years under the assumption of ground homogeneity, and the effect of ground stratification remains mostly unexplored. To investigate this, a 3D finite element numerical model is developed and validated against laboratory-scale experimental data, to study the transient diffusion-convection heat transfer linked with Darcy groundwater flow around energy piles in multi-layered lithology. The model is used to undertake long-term assessments under balanced and unbalanced thermal loads to evaluate the thermal effects of soil layering and discrepancies against commonly assumed equivalent homogeneous stratum, for soil profiles with different thermal conductivity distributions. The groundwater flow effect at various depths and seepage velocities on the thermal performance of the energy pile is investigated as well. Results demonstrate the need to account for the spatial variability in thermal properties, particularly for unbalanced thermal loading scenarios. The thermal yield can be underestimated by up to 19.6 % due to an inaccuracy of 48.2 % in the accumulated temperature after 25-year of operation with respect to an equivalent homogeneous ground with depth-weighted average thermal conductivity. This discrepancy grows as the contrast between layers increases. An empirical derived formula is presented and tested, presenting a correction to the effective thermal conductivity of the layered systems in this study that considers the thermal contribution of the ground beneath the pile. Groundwater seepage is shown to have a positive impact on the heat exchanger efficiency, and in the layered geology the efficiency under-estimation becomes more critical at low to moderate Darcy velocities, if neglected or inaccurately measured. These findings contribute to a broader understanding of energy piles and can assist engineers and practitioners in optimising energy geo-structure design, boosting the technology’s viability.

Transport and retention of micro-polystyrene in coarse riverbed sediments: effects of flow velocity, particle and sediment sizes

Riverbed sediments have recently been found to be an important reservoir for microplastics. But the hydrogeological factors that control the abundance of microplastics are complex and conceptual frameworks priorising the parameters affecting their transport and retention during deep riverbed filtration are still missing. In this study a series of saturated column experiments was conducted to investigate the vertical distribution patterns of secondary polystyrene fragments (100–2000 μm) in dependence on their particle size, grain size of the sediment, seepage velocity and duration of infiltration flow. The columns with a length of 50 cm were operated with flow velocities between 1.8 m d−1 and 27 m d−1. Invasive samples obtained after the experiments were density separated and then depth profiles of microplastic concentrations were retrieved using fluorescence imaging analysis. Most polystyrene particles were retained in the upper 20 cm and 15 cm of the medium gravel and coarse sand sediments, respectively. Through the high particle retention riverbed sediments can act as a temporary sink or long term retention site for the transport of microplastic particles (MPPs) from streams to oceans. A small fraction of particles ranging from 100 to 500 μm in size was observed down to infiltration depths of 50 cm suggesting that MPPs at the pore scale have the potential to be advectively transferred via hyporheic exchange or induced bank filtration into coarse riverbed sediments and alluvial aquifers. MPP abundance over column depth follows an exponential relationship with a filter coefficient that was found to depend significantly on the flow rate, MPP and sediment grain size, as indicated by multiple linear regression (R2 = 0.92). The experimentally derived empirical relation allows to estimate particle abundances of initially negatively buoyant MPP in riverbed sediments by surface water infiltration.

Investigation of measurement methods for soil thermal conductivity based on optical frequency domain reflection technology

Thermal conductivity is an important parameter reflecting the heat transfer capacity of soil. In engineering applications, the thermal conductivity of soil varies with influencing factors such as particle size, porosity, water content, saturation, and seepage velocity of soil. At present, the thermal probe method and the traditional thermal response test method are mainly used to determine the thermal conductivity of soil. However, these methods can only perform point temperature measurements and require an inversion calculation based on a particular heat transfer model to obtain the value of soil thermal conductivity. Compared with this and other temperature monitoring methods, optical frequency domain reflectometry (OFDR) offers the advantages of distributed measurement, high measurement accuracy, high spatial resolution, and fast speed. In this paper, an OFDR-based measurement of soil thermal conductivity is designed. Taking sand as an example, the influence of different influencing factors on the measured value of thermal conductivity of soil was studied through the measurement test of thermal conductivity of soil, and the results are compared with the calculation results of the theoretical model of thermal conductivity and the results measured by the thermal probe method. The test results showed that when there was no seepage in the soil, the thermal conductivity of the soil exhibited an “S"-shaped growth trend with increasing water content; when the soil had seepage, the thermal conductivity of the soil was larger than that without seepage, and the thermal conductivity showed a positive linear increase with the increase in seepage velocity; With the increase of heating power, the measurement results tend to be stable. By comparing with the results obtained by other methods, the measurement results of the thermal conductivity measurement method proposed in this study have high accuracy.

Inorganic-Organic Antioxidant Gel for Preventing the Spread of Forest Fires

ABSTRACT It is easier and cheaper to prevent forest fires than to extinguish them, therefore, the development of long-lasting and environmentally friendly fire-retarding materials is highly desirable to prevent the spread of forest fires. In this study, soluble carboxymethyl starch replaced insoluble carboxymethyl cellulose to improve the compressive strength of inorganic gel and overcome the weakness of fragility. When the concentration of polymer carboxymethyl starch and crosslinking agent aluminum citrate were 0.5% and 0.3%, respectively, the inorganic/organic composite gel showed the best comprehensive performance (including gelation time, water retention, seepage velocity and strength). Additionally, the antioxidant tannin was introduced into the inorganic/organic gel, which effectively inhibited the oxidation of wood flour, making its inhibition rate reached 23.1%, about twice that of pure inorganic gel. This is mainly attributed to the ability of tannin to inhibit the oxidation process of active groups, including carbon-oxygen, hydroxyl, and methyl of the wood powder, thereby enhancing the flame resistance of the composite gel. Probable mechanism of forest fire suppression by the antioxidant composite gel is proposed, which involves oxygen isolation, evaporative cooling, and chain reaction inhibition.