Hydraulic Conductivity Measurements Research Articles

DOC and nitrate fluxes from farmland; impact on a dolostone aquifer KCZ

DOC and nitrate in farmland represent key chemical species that determine the water quality in the Karst Critical Zone (KCZ). The work reported here focuses on quantifying fluxes of these species in an experimental farm site (University of Leeds Farm, UK) overlying a dolomitic karst aquifer of Permian age. In this research, the Transect Method was applied for the first time to farmland by combining hydrochemical data from soil and groundwater for computation of mass fluxes. The Transect Method, developed for management of industrially contaminated sites, was applied to a farm source due to the presence of localised contamination from application of pig slurry.Required inputs for our approach include concentrations of nitrate and DOC in soil water and groundwater, net recharge flux (here derived from a MODFLOW-2005 model) and local hydraulic gradient and conductivity measurements. Key outputs are fluxes and downstream groundwater concentrations of DOC and nitrate. Downstream concentrations were validated against direct groundwater measurements, demonstrating the veracity of the approach. The approach shows that the localised contamination has a significant impact on both concentrations of nitrate and DOC in groundwater, although the DOC impact is greater, because the upstream land uses also produce nitrate as a result of agricultural practices that are widespread in the region.The results of the study also constrain the zone vulnerable to contamination to the upper ∼40 m below the ground surface. Future modelling efforts on solute contaminant transport should focus on this shallow vulnerability zone (0–40 mBGL) and the Transect Method applied in this work can be used to define boundary conditions.Hence, following this research, we envisage to export a generic approach that combines physical flow parameters and hydrochemical analyses for computation of subsurface mass fluxes using the Transect Method, to identify the degree of impact of specific point sources and to support conceptualization and modelling of contaminant transport in the KCZ of farm areas.

Estimation of hydraulic conductivity in fractured crystalline aquifers using remote sensing and field data analyses: An example from Wadi Nasab area, South Sinai, Egypt

Fractured crystalline aquifer is considered as the main source of water supply in most of the arid and semi-arid regions. Fractures play an essential role as pathways through which groundwater flows from high catchments to supply the productive wells in lowlands. The hydraulic conductivity measurements for highly fractured crystalline aquifers are a must for optimal management of groundwater extraction from these aquifers. The current work aims at estimating the hydraulic conductivity of the crystalline aquifer by using an unconventional approach based entirely on the processing of remotely-sensed lineament data and field observations. Extracted lineaments at four sites near the productive wells along Wadi Nasab area (WNA) were traced from Landsat (ETM+7) and other ancillary geological data by using image processing and GIS tools. Hence, four discrete fracture networks (DFN) were designed and practiced for the selected test sites to model the groundwater flow through the fracture system in order to estimate the hydraulic conductivity of the investigated crystalline aquifer. The achieved results indicated that the hydraulic conductivity of fractured basement aquifer of WNA varied between 2.3 × 10−7 and 6.1 × 10−7 m/s. The estimated hydraulic conductivity values are close to the values of previous field infiltration tests in the nearby area at Sinai. The previously estimated infiltration rates (expressing the vertical hydraulic conductivity) on the fractured crystalline outcrops ranged between 2.6 × 10−6 and 6.5 × 10−7 m/s, which are concordant with achieved results. Moreover, the achieved values match the previously published values of hydraulic conductivity for similar crystalline aquifers around the world, reflecting the high validity of the used unconventional, low-cost approach to estimate the hydraulic conductivity of fractured crystalline aquifers. The designed approach can be potentially practiced on similar basins in Sinai peninsula and other comparable basins having similar geological and climatic environments around the world.

Electrical resistivity imaging for monitoring soil water motion patterns under different drip irrigation scenarios

The use of hydrogeophysical methods provides insights for supporting optimal irrigation design and management. In the present study, the electrical resistivity imaging (ERI) was applied for monitoring the soil water motion patterns resulting from the adoption of water deficit scenarios in a micro-irrigated orange orchard (Eastern Sicily, Italy). The relationship of ERI with independent ancillary data of soil water content (SWC), plant transpiration (T) and in situ measurements of hydraulic conductivity at saturation (Ks, i.e., using the falling head method, FH) was evaluated. The soil water motion patterns and the maximum wet depths in the soil profile identified by ERI were quite dependent on SWC (R2 = 0.79 and 0.82, respectively). Moreover, ERI was able to detect T in the severe deficit irrigation treatment (electrical resistivity increases of about 20%), whereas this phenomenon was masked at higher SWC conditions. Ks rates derived from ERI and FH approaches revealed different patterns and magnitudes among the irrigation treatments, as consequence of their different measurement scales and the methodological specificity. Finally, ERI has been proved suitable for identifying the soil wetting/drying patterns and the geometrical characteristics of wet bulbs, which represent some of the most influential variables for the optimal design and management of micro-irrigation systems.

Simulation of vadose zone flow processes via inverse modeling of modified multistep outflow for fine‐grained soils

AbstractCharacterization and measurement of the hydraulic properties of unsaturated porous media is still a challenge in natural environments, although exact knowledge of the soil's hydraulic properties [unsaturated soil hydraulic conductivity (K), volumetric water content (θ), soil matric head (h)] is crucial for solving many soil, hydrological, and environmental issues. The main purpose of this study was to establish a modified multistep outflow technique that facilitates laboratory operations and reduces time and costs. We also focused on inverse parameter optimization of the Mualem–van Genuchten (MVG) model with laboratory‐provided data for three fine‐grained soils. This modified version was assessed by inverse modeling of the MVG model and the generalized reduced gradient algorithm. The results represent the best estimate of the adjusted parameters and the very low uncertainty associated with the fitted values. The soil hydraulic functions were simulated well up to pressures of 1,500 kPa and their uncertainties were extremely low. The R2 values were 88 to 97 and 81 to 96% for the soil water characteristics and the unsaturated hydraulic conductivity functions, respectively. The simultaneous optimization of the nonlinear hydraulic functions from a single transient flow experiment agreed well with their observed data for each soil examined. Besides, by eliminating the use of tensiometers and having no independent measurements of saturated soil hydraulic conductivity and saturated volumetric water content, the modified version was faster and easier than the conventional method. The proposed protocol appears to be suited for the vadose zone flow simulation of fine‐grained soils.

Can Hydraulic Conductivity of Fluvial Sediments be Informed by Spectral Reflectance?

This study explores the statistical relationship between spectral reflectance and hydraulic conductivity (K) of fluvial sediments in two Nebraska rivers. The spectral reflectance curves of sediments are obtained through hyperspectral instruments under controlled conditions. The K values are determined by three different methodologies, grain size analysis, an in-situ permeameter test, and a lab permeameter test. The in-situ permeameter tests calculate vertical K values (Kv), whereas grain size analysis and lab tests and grain size analysis generate non-directional K values. The results show that the lab permeameter tests of repacked sediments present greater hydraulic conductivity values than in-situ tests. The non-directional K values derived from 7 empirical equations, Hazen, Slicher, Terzaghi, Beyer, USBR, Kozeny, and Sauerbrei, correlate well with the in-situ Kv values. Site specific coefficients in 7 equations are developed for the study sites. Correlation analysis is conducted aiming to establish the connection between hydraulic conductivity and spectral reflectance. Inverse trends are found between the reflectance and K values determined by Hazen, Beyer, USBR, and Sauerbrei formulae where particle size distribution is considered to be a key factor. Furthermore, four linear models are developed based on the relationship between grain size derived K and reflectance. The models are used on dried surface channel sediments in the Platte River for predicting K values as a pilot test and proved to be applicable. As direct measurement of hydraulic conductivity can be costly and time-consuming, remote sensing informed hydraulic conductivity of streambed sediments in droughts can be a promising application with further study.

Drought-Induced Xylem Embolism Limits the Recovery of Leaf Gas Exchange in Scots Pine.

Climate change increases the occurrence of prolonged drought periods with large implications for forest functioning. Scots pine (Pinus sylvestris) is one of the most abundant conifers worldwide, and evidence is rising that its resilience to severe drought is limited. However, we know little about its ability to recover from drought-induced embolism. To analyze postdrought hydraulic recovery, we investigated stress and recovery dynamics of leaf gas exchange, nonstructural carbohydrates, and hydraulic properties in 2.5-year-old Scots pine seedlings. We quantified the degree of xylem embolism by combining in vivo x-ray microtomography with intrusive techniques including measurements of hydraulic conductivity and dye staining during drought progression and short-term (2 d) and long-term (4 weeks) recovery. Seedlings were grown under controlled conditions, and irrigation was withheld until stomata closed and xylem water potential declined to -3.2 MPa on average, causing a 46% loss of stem hydraulic conductivity. Following drought release, we found a gradual recovery of leaf gas exchange to 50% to 60% of control values. This partial recovery indicates hydraulic limitations due to drought-induced damage. Whereas xylem water potential recovered close to control values within 2 d, both x-ray microtomography and intrusive measurements revealed no recovery of stem hydraulic conductivity. Moreover, we did not find indications for nonstructural carbohydrate reserves limiting hydraulic recovery. Our findings demonstrate that Scots pine is able to survive severe drought and to partially recover, although we assume that xylem development during the next growing season might compensate for some of the hydraulic impairment. Such drought-induced legacy effects are important when considering vegetation responses to extreme events.

The importance of unsaturated hydraulic conductivity measurements for green roof detention modelling

Characterising the unsaturated hydraulic conductivity of a green roof substrate is essential for accurately modelling runoff detention in response to rainfall events. In this paper, the unsaturated hydraulic conductivities for four representative green roof substrates were determined in an infiltration column using steady state and transient techniques. The conventional Durner-Mualem Hydraulic Conductivity Function (HCF) model, for which parameters were calibrated based on the measured Soil Water Release Curve (SWRC) data, was shown to provide a poor fit to the experimental data. A new three-segment HCF was, therefore, proposed to fit measured unsaturated hydraulic conductivity data. Detention tests were carried out on 100 mm and 200 mm deep substrates using four simulated storm events. The runoff and moisture content data collected during the detention tests was used to validate the HCFs using the Richards Equation. The new three-segment HCF resulted in simulated runoff and moisture content profiles that closely matched the measured data (with mean Rt2 = 0.754 for modelled runoff), in contrast to predictions made using the conventional Durner-Mualem model (with mean Rt2 = 0.409 for modelled runoff). It was also demonstrated that further simplification of the HCF to a function defined by moisture content at just two points – the saturated hydraulic conductivity and at an unsaturated hydraulic conductivity of 0.1 cm/min – provides a model that is fit-for-purpose for green roof runoff estimation (with mean Rt2 = 0.629 for modelled runoff).

Water Flow Behavior and Storage Potential of the Semi-Arid Ephemeral River System in the Mara Basin of Kenya

Alluvial corridors of ephemeral river systems provide viable opportunities for natural water storage in dry lands. Whilst alluvial corridors are widely recognized as water buffers, particularly for areas experiencing constant water scarcity, little research has been undertaken in Sub-Saharan Africa to explore their hydrological variability and water resource potential as alternative water sources for nearby communities. This study investigated the water flow behavior and storage potential of an ephemeral river system in the Mara Basin of Kenya for purposes of supporting water resources development and ecological sustainability. The water flow processes - including the recharge rates and water loss processes - from existing sand storage systems were established through monitoring of ground and surface water levels. Water samples along the alluvial corridor were collected and analyzed for major ions and isotopic signatures required to establish the water storage dynamics. The storage potential was estimated through Probing and Electrical Resistivity Tomography techniques, augmented with in-situ measurements of hydraulic conductivities and channel bed porosities. The mean annual storage volume in the alluvium of the study reach was estimated at 1.1 Mm3, potentially capable of providing for the annual domestic and livestock water demands of the area. Transmission losses into the alluvium beneath the ephemeral channel-bed were noted to attenuate the flood peak discharges, depending on the level of saturation of the alluvial bed. However, water storage in the alluvium was subject to losses through evapotranspiration and seepage through fractured bedrocks. The study demonstrated the potential of alluvial corridors as water storage buffers providing alternative water sources to communities within the dry land regions with water scarcity, thereby to supporting ecosystem sustainability.

Design, Fabrication, Evaluation and Calibration of an Improvised Soil Permeameter for Measurement of Soil Hydraulic Conductivity

This study aimed at the design, fabrication, evaluation and calibration of an improvised soil permeameter for measuring saturated soil hydraulic conductivities. A metal support frame with a permissible strength of 50e-4 N/mm2 was made using steel galvanized pipes of 6 cmØ (internal diameter) and metal flat bars of 8cm long and 0.4cm thick. The water tank (thickness 0.6 cm, height 28 cm, internal diameter 19 cmØ) and permeameter cylinder (thickness 0.6 cm, height 23 cm, internal diameter 19 cmØ) were both made of Perspex glass material. An electromechanical measuring device which was programmed and operated wirelessly was designed and constructed to measure head-in and head-out parameters. To calibrate the improvised soil permeameter, sampled and particle analysed soils from different locations within Zaria metropolis, were a test run on both foreign and improvised soil permeameters and their respective Ksat readings were subjected to regression analysis. According to the R2 values, the generated 3rd order polynomial regression equation from the Ksat readings of both foreign and improvised soil permeameter had a satisfactory R2 value of 0.985.
 Keywords: Permeameter, Hydraulic Conductivity, Electromechanical, Saturated, Automation

Comparison of Laboratory and Field Measurements of Backfill Hydraulic Conductivity for a Large-Scale Soil-Bentonite Cutoff Wall

AbstractSoil-bentonite (SB) cutoff walls are commonly employed to control groundwater flow and contaminant migration, and the hydraulic conductivity (k) of the SB backfill is a critical parameter i...

Hydraulic Conductivity and Compressive Strength of Cemented Soils

The addition of cementing agents is a well-known way of stabilizing an unsatisfactory soil for design parameters. This research evaluates three soils, namely pink kaolin silty soil, Botucatu weathered sandstone residual soil and Osorio uniform sand, stabilized with Portland cement type III. Hydraulic conductivity measurements were performed with a flexible wall permeameter, following the recommendations of ASTM D5084; unconfined compressive strength tests were also carried out, in accordance with ABNT NBR 12025. The result of this research observed a gain in compressive strength, best described as a linear gain with the increase in cement content, and as a power function as porosity decreases. The measured hydraulic conductivity observed a general reduction, similarly, with a linear decrease in regards to increasing cement content and decreased as a power function in regards to decreasing porosity. The possible correlation of the output variables investigated suggests the existence of a grid-like relationship, which can be potentially useful in earthworks and more general applications.

The measurements of saturated hydraulic conductivity of the forest floor under deciduous forest

The measurements of saturated hydraulic conductivity of the forest floor under deciduous forest

Stratigraphic flux—A method for determining preferential pathways for complex sites

AbstractExperience with groundwater remediation over several decades has demonstrated that successful outcomes depend on quantitative conceptual site models (CSMs). Over the last 30 years, we have progressed from groundwater pump‐and‐treat remedies, which were largely designed based on a water supply perspective, to in situ and combined remedy strategies, which are only beginning to benefit from understanding the aquifer architecture and distribution of contaminant mass to assess plume maturity, mass flux, and more reliable means of fate and transport assessment. The U.S. Air Force funded the development of the Stratigraphic Flux approach to provide a framework for understanding contaminant transport pathways at its complex sites and enable more reliable and cost‐effective remediation. Stratigraphic Flux enables the development of quantitative, flux‐based CSMs that are founded in sequence stratigraphy, and high‐resolution hydraulic conductivity and contaminant distribution measurements. The result is a three‐dimensional graphical mapping of relative contaminant flux and classification of transport potential that is easy for all stakeholders to understand. The Stratigraphic Flux graphical model is based on a hydrofacies classification system that describes transport potential in three segments of the aquifer: transport zones—where the majority of groundwater flow occurs and transport rates are measured in feet per day; slow advection zones—where transport rates are measured in feet per year; and storage zones—where typically less than 1% of flow occurs, and diffusion dominates contaminant transport. The hydrofacies architectures are based on stratigraphy and transport potential is defined by grouping facies by orders of magnitude classes in hydraulic conductivity. By combining the hydrofacies architecture with contaminant concentration distributions, one can map relative contaminant flux to define and target the complex pathways that control contaminant transport and cleanup behavior. In this article, we describe the Stratigraphic Flux framework, focusing on the key information needed and the methods of analysis. We illustrate the results of its application to evaluate migration pathways for trichlorethylene and chromium at a former chrome pit at Air Force Plant 4 in Fort Worth, Texas. A comprehensive guidance document that describes the approach with a broad spectrum of tools and several site examples can be requested from the authors.

Characterization of the hydraulic conductivity of glacial till aquitards

Pleistocene-aged glacial sediments are found in many parts the Northern Hemisphere and are often composed of clay-rich tills which form aquitards that can control drainage and influence groundwater movement and contaminant transport. Site-scale investigations have characterized the hydraulic properties of till aquitards; however, the hydraulic conductivity of these units has not been quantitatively described at a regional scale of tens of kilometers. This study constrains regionally representative hydraulic conductivity estimates and characterizes the hydrogeological properties of Pleistocene-aged till aquitards from data collected at 15 sites compiled from 21 studies. The data quantify the scale dependence of hydraulic conductivity measurements in till aquitards and further define the relationship between hydraulic conductivity and depth. Data from centimeter-scale laboratory tests remained generally constant with depth, with a geometric mean hydraulic conductivity of 7.0 × 10−11 m/s and a standard deviation of 0.4 orders of magnitude, while the meter-scale in-situ tests had a geometric mean of 4.9 × 10−9 m/s and a standard deviation of 1.0 orders of magnitude at depths less than 10 m, and 3.7 × 10−11 m/s and 0.2 order of magnitude at depths greater than 23 m. The results support the existence of a shallow fractured zone of higher hydraulic conductivity and a deeper zone characterized by matrix permeability. The observed data variability occurred primarily at the site scale, while the central tendency and variability of the data were consistent between sites separated by hundreds of kilometers suggesting that statistically derived, depth-defined regional hydraulic conductivity estimates can be meaningful.

Micropipette Aspiration-Based Assessment of Single Channel Water Permeability.

Measurements of the unitary hydraulic conductivity of membrane channels, pf , may be hampered by difficulties in producing sufficient quantities of purified and reconstituted proteins. Low yield expression, the purely empiric choice of detergents, as well as protein aggregation and misfolding during reconstitution may result in an average of less than one reconstituted channel per large unilamellar vesicle. This limits their applicability for pf measurements, independent of whether light scattering or fluorescence quenching of encapsulated dyes is monitored. Here the micropipette aspiration technique is adopted because its superb sensitivity allows resolving pf values for one order of magnitude smaller protein densities in sphingomyelin and cholesterol rich giant unilamellar vesicles (GUVs). Protein density is derived from intensity fluctuations that fluorescently labeled channels in the aspirated GUV induce by diffusing through the diffraction limited spot. A perfusion system minimizes unstirred layers in the immediate membrane vicinity as demonstrated by the distribution of both encapsulated and extravesicular aqueous dyes. pf amounted to 2.4±0.1×10-13 cm³ s-1 for aquaporin-1 that served as a test case. The new assay paves the way for directly monitoring the effect that interaction of aquaporins with other proteins or inhibitors may have on pf on a single sample.

Physics‐Informed Deep Neural Networks for Learning Parameters and Constitutive Relationships in Subsurface Flow Problems

AbstractWe present a physics‐informed deep neural network (DNN) method for estimating hydraulic conductivity in saturated and unsaturated flows governed by Darcy's law. For saturated flow, we approximate hydraulic conductivity and head with two DNNs and use Darcy's law in addition to measurements of hydraulic conductivity and head to train these DNNs. For unsaturated flow, we approximate unsaturated conductivity function and capillary pressure with DNNs and train these DNNs using measurements of capillary pressure and the Richards equation. Because it is difficult to measure unsaturated conductivity in the field, we assume that no measurements of unsaturated conductivity are available. The proposed approach enforces the partial differential equation (PDE) (Darcy or Richards equation) constraints by minimizing the PDE residual at select points in the simulation domain. We demonstrate that physics constraints increase the accuracy of DNN approximations of sparsely observed functions and allow for training DNNs when no direct measurements of the functions of interest are available. For the saturated conductivity estimation problem, we show that the physics‐informed DNN method is more accurate than the state‐of‐the‐art maximum a posteriori probability method. For the unsaturated flow in homogeneous porous media, we find that the proposed method can accurately estimate the pressure‐conductivity relationship based on the capillary pressure measurements only, even in the presence of measurement noise.

Differences in grapevine rootstock sensitivity and recovery from drought are linked to fine root cortical lacunae and root tip function.

Structural changes during severe drought stress greatly modify the hydraulic properties of fine roots. Yet, the physiological basis behind the restoration of fine root water uptake capacity during water recovery remains unknown. Using neutron radiography (NR), X-ray micro-computed tomography (micro-CT), fluorescence microscopy, and fine root hydraulic conductivity measurements (Lpr ), we examined how drought-induced changes in anatomy and hydraulic properties of contrasting grapevine rootstocks are coupled with fine root growth dynamics during drought and return of soil moisture. Lacunae formation in drought-stressed fine roots was associated with a significant decrease in fine root Lpr for both rootstocks. However, lacunae formation occurred under milder stress in the drought-resistant rootstock, 110R. Suberin was deposited at an earlier developmental stage in fine roots of 101-14Mgt (i.e. drought susceptible), probably limiting cortical lacunae formation during mild stress. During recovery, we found that only 110R fine roots showed rapid re-establishment of elongation and water uptake capacity and we found that soil water status surrounding root tips differed between rootstocks as imaged with NR. These data suggest that drought resistance in grapevine rootstocks is associated with rapid re-establishment of growth and Lpr near the root tip upon re-watering by limiting competing sites along the root cylinder.

Fabrication of Permeameter for Determination of Hydraulic Conductivity of Earth Materials

Laboratory investigation is one of the major ways of assessing soil hydraulic conductivity. Determination of hydraulic conductivity aids in engineering design of well pumping, prediction concerning spread of polluting fluids, embankment of canal bank affected by seepage, flooding solutions and stability of earth dams. However, different studies have shown that there are alternative models to Darcy’s law which governs the widely use of laboratory measurement of hydraulic conductivity. The deficiencies accustomed to the conventional permeameter such as the time wastage and cost-intensivehas led to different research modification. A low-cost permeameter was fabricated using a plastic column, hose pipe, to serve as water inlet and outlet connected to two manometer tubes to measure the pressure head difference. The hydraulic conductivities measured using the low-cost were 4.31 cm/s, 8.14 cm/s, 6.12 cm/s, 5.86 cm/s for 0.3 mm coarse sand, 0.85-1 mm fine sand, sandy clay and silty clay respectively. In comparison of the fabricated permeameter with conventional permeameter and other fabricated laboratory permeameters, it was observed that the hydraulic conductivity obtained is consistent with the typical permeability range for each soil type.

Determination of soil hydraulic properties and its implications for mechanistic simulations and irrigation management

Water retention θ(h) and hydraulic conductivity K(h) are mandatory soil hydraulic properties (SHP) for consistent hydrological modeling and for an efficient irrigation management. Most commonly, SHP are determined by conventional methods (CM), based on hydrostatic equilibrium and the independent measurement of saturated hydraulic conductivity, which is used as a matching point for K(h) function. Alternatively, inverse-modeling experiments allow simultaneous parameter estimation using data from transient water flow conditions. This study aims to investigate the implications of these two protocols on simulations of soil water content (θ) and crop evapotranspiration (ET), and how they affect irrigation management and scheduling for different irrigation systems and crops. The SHP obtained from CM and IM were used in simulations with the Richards equation-based SWAP hydrological model. ET and θ were simulated for passion fruit under high-frequency drip irrigation and for pasture under conventional sprinkler irrigation. The simulation performance was evaluated using measured θ and ET obtained with passion fruit under drip irrigation. Both methods (CM and IM) gave similar results in the wetter range, while in the drier soil, CM estimated higher θ than IM. These differences affected the simulated ET and irrigation scheduling. Regarding the ET and θ simulations, for the drip irrigation scenario, in which the water content in the root zone remains near saturation, both SHP determination methods produced similar results. On the other hand, for scenarios with larger irrigation intervals such as sprinkler irrigation, simulations were affected significantly, with CM likely biasing irrigation frequency and depth.

Physical modeling of train-induced mud pumping in substructure beneath ballastless slab track

Abstract This paper identifies the fundamental driving factors for the mobilization of mud pumping in railway substructure beneath ballastless slab track. Physical model tests on a compacted gravel layer overlying a permeable layer were performed under different conditions in terms of fines content, moisture (i.e., unsaturated, saturated with and without surface ponding), load patterns, and slit width between load plate and gravel surface. The loading unit primarily consists of a load plate and electric actuator for reproducing the train induced loads under normal operation. The model test system was equipped with various devices and sensors allowing the measurements of vertical displacement, stress, moisture content, and hydraulic conductivity. Visual observations were also made to evaluate the extent of mobilized mud pumping, including the particles migration on the gravel surface. It was shown that the permeability of compacted gravelly soils decreases rapidly with the addition of fines. Both the dynamic loading and hydraulic condition of the gravel layer are essential in the mobilization of mud pumping, and the slit caused by the detachment of concrete base from subgrade surface is recognized as the most fundamental factor for this phenomenon. The substructure becomes more susceptible to mud pumping under dynamic loading when slit width and fines content increase and ponding on the surface of gravel layer is available. The repeated erosion and scouring of subgrade surface (compacted gravel layer) driven by dynamic water pressure leads to a fully established mud pumping for ballastless track given enough load cycles.