Saturated Clay Soil Research Articles

Coupled model for electro‐osmosis consolidation and ion transport considering chemical osmosis in saturated clay soils

AbstractThe electro‐osmosis approach efficiently facilitates the rapid dewatering of soil with high water content and contributes to reducing contaminant levels within the clay soil. However, the changes of chemical field caused by ion transport in the clay soil during electro‐osmosis process will also influence the clay soil consolidation effect. Existing theories predominantly tend to disregard this crucial physical process and its resultant effects, thereby restraining a comprehensive analysis of electro‐osmosis consolidation (EOC) behavior under intricate chemical conditions. This study introduces a concise model of EOC and ion transport considering chemical osmosis. The model considers the nonlinear variation of clay soil parameters such as compressibility, permeability, and effective diffusion coefficients, along with the interaction between EOC and ion transport. Meanwhile, the correctness of the model is verified from different aspects such as theoretical derivation and model comparison. Based on the proposed model, the impacts of the variation in electrical field intensity and chemical concentration on the coupled behaviors between EOC and ion transport are systematically investigated, with and without incorporating nonlinear consolidation characteristics. The results show that diffusion and electro‐migration exhibit a more pronounced effect on ion transport during EOC. Simultaneously, with the increase of ion concentration in clay soil pore solution, the effects of chemical osmosis become increasingly apparent, thereby enhancing clay soil settlement.

Analisis Perubahan Volume pada Timbunan Tanah Lempung Berdasarkan Nilai Batas Susut (SNI 3422:2008)

Sedimentation has continuously resulted in siltation of the port of Pelabuhan Perikanan Samudera (PPS) Lampulo located in Kuta Alam District, Banda Aceh City. The dredged soil collected outside the port area visually indicates clay soil. In addition to the low bearing capacity and high plasticity, the effect of water on clay is high swelling and shrinkage. One of the prominent behaviors that cause major damage to structures built on clay is associated with volume decrease i.e., shrinkage in saturated compressible clay soil when water get evaporated. Shrinkage in clay will result in volume changes. The purpose of this research is to calculate the volume changes in mound of clay soil based on shrinkage limit values using SNI 3422:2008. The soil sample used is an undisturbed sample (UDS) originating from the dredging area of the port of PPS Lampulo. The test consists of water content, specific gravity, liquid limit, plastic limit, shrinkage limit and grain size distribution using the SNI method. Based on the results of the index properties test, according to AASHTO the soil sample belongs to group A-7-5 which means it is a clay soil with a GI value greater than 20, so it is considered poor as a subgrade soil. According to USCS, the soil belongs to the CH classification, which means clay soil with high plasticity, so it has a large potential for expansion and shrinkage. Based on SNI 3422:2008, the average shrinkage limit (SL) is 21.21%, the average shrinkage ratio (R) is 2.27% and the average linear shrinkage (LS) is 9.65%. With an average volume change (VC) value of 35.59%, the shrinkage volume of soil embankment of the dredging area of the port of PPS Lampulo is 32,438.85 m3 compared to its initial volume (91,145.97 m3). Therefore, the shrinkage limit is an important parameter for calculating the volume shrinkage in mound of soil due to the soil type is clay with high plasticity.

Simple approach to assessing excess pore water pressure induced by shield tunneling in saturated-unsaturated clay soil

Excess pore water pressure (EPWP) induced by shield tunneling has a significant influence on the stability of the tunnel face, post-construction settlement, and the mechanical behavior of the tunnel lining. However, the three-dimensional and unsaturated property of the soil field is seldom considered in current research. Considering the non-uniform radial convergence model, a modified three-dimensional displacement solution induced by shield tunneling was established first. Then based on unsaturated EPWP elastic theory, a reliable and efficient method is developed to expedite the evaluation of EPWP distribution in three-dimensional saturated and unsaturated clay soil. The validity of the method is confirmed through comparison with field test and numerical outcomes. The analysis examples demonstrate that negative and positive EPWP are generated above the tunnel crown and beneath the tunnel invert, respectively. In the vertical direction, the negative EPWP exhibits a decreasing trend ahead of the heading face and an increasing trend behind it. Along the longitudinal direction, the influence zone of EPWP extends to 1D ahead of and 6D behind the heading face. With the decrease of soil saturation, the EPWP values tend to diminish. The maximum EPWP values observed in saturated conditions can be 16.13 times higher than those under unsaturated conditions.

Localization patterns emerging in CPTu tests in a saturated natural clay soil

In this work, the mechanics of cone penetration during CPTu tests in Osaka clay, a natural structured silty clay, was investigated with the Particle Finite Element Method (PFEM). To describe the behavior of the soil, the FD_MILAN model, recently proposed by the Authors, was adopted. The model, based on the multiplicative decomposition of the deformation gradient, incorporates a scalar internal variable, the bond strength Pt, to quantify the effects of structure. The macroscopic description of mechanical destructuration effects is provided by the hardening law of Pt, according to which the bond strength decreases monotonically with accumulated plastic deformations. The brittle behavior resulting from the softening process associated to destructuration make the soil quite susceptible to the spontaneous development of strain localization in the form of shear bands. In order to deal with strain localization phenomena, the model is equipped with a non-local version of the hardening laws, incorporating a material constant – the characteristic length ℓc – which provides the material with an internal length scale. The objective of this work was to get some insight on the effects of the characteristic length and of the initial degree of structure on: the kinematics of the soil deformation around the advancing cone tip; the evolution of soil structure with accumulated plastic deformations; the excess pore pressure field generated in the soil as a result of the hydro-mechanical coupling between the solid skeleton and the pore water, and the net cone resistance measured at the base of the cone. The results of the PFEM simulations show that the deformations around the piezocone are strongly affected by the characteristic length. For heavily structured soils, when ℓc is relatively small with respect to cone radius, the accumulated plastic deviatoric deformation field is characterized by clearly visible shear bands while, as ℓc increases, the detection of localized deformation regions becomes more difficult, if not impossible. This depends on the fact that, for large values of ℓc, the shear band width may be of comparable size to the cone radius. In all cases, the soil around the advancing piezocone is subjected to a very strong destructuration process, which leads to the complete loss of bond strength in a large region around the cone tip and shaft. As a consequence, the use of conventional Nc values from the literature in the interpretation of the CPTu test performed in heavily structured soils could provide undrained strength values closer to the ultimate undrained shear strength. At the same time, the use of empirical correlation to estimate the yield stress in oedometric conditions would lead to a significant underestimation of the overconsolidation ratio.

Prediction of Damping Ratio and Shear Modulus of Saturated Clay Soil under Rotating Machine

Developing countries' industrial growth necessitates innovative machine foundation design and resilient infrastructure. Meanwhile, soil properties play a significant role in the satisfactory design of machine foundations. The paper presents an experimental work to determine accurately the damping ratio and shear modulus by equations. The paper studies the effect of different parameters (embedded depth of foundation, shape of footing, and changing operating frequency of machine) on the damping ratio and shear modulus of saturated clay soil. Moreover, mathematical expressions are derived from statistical analysis to predict the damping ratio for rectangular and circular footing on saturated clay. The physical model results reveal that the damping ratio inside the soil will be increased with an increase in the depth of the point inside the soil for all cases. The damping ratio tends to decrease with an increase in operating frequency. The maximum damping ratio has occurred below a rectangular foundation with a 16.6 Hz operating frequency in saturated clay. The damping ratio is a function of many factors, such as embedment depth, degree of saturation, shape of foundation, and operation frequency of the machine.

The roles of microstructure and water mobility in pre-Darcy flow formation in saturated clay soil

Water flow in saturated clay soil at low hydraulic gradients usually occurs via pre-Darcy flow. The occurrence of the pre-Darcy flow phenomenon is attributed to the adsorption of water on pore walls via surface forces. However, a clear understanding of the amount of water that is movable in saturated clay soil and the distribution of movable/immovable in the microstructure of clay soil is lacking. Furthermore, it is difficult to accurately evaluate the effect of water mobility on the seepage characteristics of clay soil. To investigate the effects of water mobility and pore microstructure on pre-Darcy flow in clay soil, nine clay samples were used to conduct constant head permeability experiments. The results showed that the seepage of water in clay deviated from that predicted by Darcy's law, and with increasing hydraulic gradient, the apparent water permeability first increased and then tended to remain constant. The porosity ϕ, average pore radius rp, sorting coefficient Sp, mean pore radius Dm, dry density ρ and tortuosity τ had major influences on the stable apparent water permeability ks. In contrast, the influences of maximum pore radius and median pore radius on ks were relatively weak. Macropores (>1.0 μm) and mesopores (100–1000 nm) were the dominant pore types in the clay and accounted for almost 80 % of the overall porosity. The water in saturated clay can be divided into four types according to interfacial forces. The results showed that weakly bound water (in the outer layer) was the main type of water in the clay. Among the four types of water, ks had the strongest correlation with the proportion of free water. The mobility of water was affected by coupling between interfacial forces and hydraulic gradients. The movable water content was closely related to the permeability of the clay.

PLAXIS 3D numerical analysis of complex geotechnical problems of colossal built heritage

The paper aims to contribute to the preservation of high valuable historic masonry structures and historic urban landscapes through the combination of geotechnical, structural engineering. The main objective of the study is to conduct finite element analysis (FEA) of bearing saturated soft clay soil problems and induced structural failure mechanisms. This analysis is based on experimental and numerical studies using coupled PLAXIS 3D FE models. The paper presents a geotechnical analytical model for the measurement of stresses, deformations, and differential settlement of saturated clay soils under colossal stone/brick masonry structures. The study also discusses the behavior of soft clay soils under Qasr Yashbak through numerical analysis, which helps in understanding the studied behavior and the loss of soil-bearing capacity due to moisture content or ground water table (G.W.T) changes. The paper presents valuable insights into the behavior of soft clay soils under colossal stone/ brick masonry structures. The present study summarized specific details about the limitations and potential sources of error in Finite Element Modeling (FEM). Further field research and experimental analysis may be required to address these limitations and enhance the understanding of the studied soft clay soil behavior. The geotechnical problems in historic monuments and structures such as differential settlement are indeed important issues for their conservation since it may induce serious damages. It deserves more in-depth researches.

Analytical solution for one-dimensional consolidation of structured soft soils with continuous drainage boundary

A closed form analytical solution is proposed to analyze the one-dimensional consolidation behavior of structured saturated clay soils under continuous drainage boundary. Firstly, the soil structural properties and arbitrary loading conditions are taken into account in the established mathematical model. Using the finite sine Fourier transform and characteristic function methods, analytical calculation is conducted, and the solution effectiveness is evaluated against the degradation of solutions and the results of finite difference analysis. Finally, the influences of interface parameter, properties of soil structure and structural yield stress on consolidation behaviors are discussed. Results show that the larger the interface parameter is, the easier the soil structure enters the failure stage, and the smaller the consolidation degree is. Furthermore, the time for initiating the failure stage does not change, when the interface parameter and structural yield stress keep unchanged; in the meanwhile, the greater the consolidation coefficient is, the greater the consolidation degree of foundation becomes when the soil structure begins to fail, and the earlier the soil enters the complete failure stage. When the interface parameter is determined, the ratio of structural yield stress to load can accelerate the soil consolidation rate, but all consolidation curves coincide for the same ratio.

A rigorous analytical approach to the pile-soil interaction of torsionally loaded piles in saturated SANICLAY soils

The paper proposes a rigorous analytical approach to predict interaction between torsionally loaded piles and saturated clay soils. The soil deformation mechanism around the pile is determined using the non-associated and anisotropic constitutive model, SANICLAY, along with the axisymmetric condition, equilibrium equation, and boundary conditions. By solving the derived governing equations, the hoop shear stress against rotation curve is obtained, including elastic, elastoplastic, and critical state processes. Taking advantage of the finite-difference method, the torque-rotation behavior of the pile and the stress-strain state of the soil around the pile are figured out. The proposed approach is compared with a FEM simulation and a parametric analysis is conducted to investigate the effect of overconsolidation ratio and in-situ coefficient. Model pile tests and field tests are used to manifest the validity and capacity of the model. The results demonstrate that the present approach can well predict the torque-rotation response of the pile and can reflect some important phenomena observed in pile tests.

Dynamic monitoring of saturated stiff clay soil foundation structure by falling weight deflectometer system under impact loads sensors effect

Experiments research is done to determine how saturated stiff clayey soil responds to a single impulsive load. Models made of saturated, stiff clay were investigated. To supply the single pulse energy, various falling weights from various heights were tested using the falling weight deflectometer (FWD). Dynamic effects can range from the major failure of a sensitive sensor or system to the apparent destruction of structures. This study examines the response of saturated stiff clay soil to a single impulsive load (vertical displacement at the soil surface below and beside the bearing plates). Such reactions consist of displacements, velocities, and accelerations caused by the impact occurring at the surface depth induced by the impact loads and then recorded using the multi-recorder sensors. Using different factors, including soil surface, and the applied energies, the behavior of saturated stiff clay soil was investigated. Under the same impact loads (5 Kg) for both dropping elevations (250.00 mm and 500.00 mm), at soil surface level the average percent of change in the maximum impact forces generated in the contact surface increased by 41%, which resulted in a reduction of 34% in the maximum displacement reaction of the clay soil model.

BEHAVIOR OF RETAINING WALLS CONSTRUCTED IN THE SATURATED CLAY AND WATER-SATURATED SAND SOILS UNDER THE DYNAMIC LOADS

Since the main purpose of building retaining walls is to hold up the slopes, the calculations are important during the design process. Although static loads that affect retaining walls are generally taken into consideration, dynamic effects should also be marked in our country located in the earthquake zone. Within the scope of this study, stability checks were performed by taking the effects to the retaining walls subjected to static and dynamic loads and wall designs were also made.
 In this study, behavior of retaining walls constructed in saturated clay soil and water-saturated sand soil have been determined under static and dynamic loads. Analyses of static and dynamic behavior of gravity and cantilever walls have been done by Plaxis 2D software packet program. The heights of the walls are selected such as 5.0 m, 10.0 m and 15.0 m. Active earth pressures are calculated by using Rankine active earth pressure theory. Factor of safeties such as overturning, sliding and bearing capacity are selected as 2.0, 1.5 and 3.0, respectively. Three different earthquake loads such as Van, Turkey, Petrolia-California, USA and Volcano-Hawaii, USA are chosen to determine the dynamic behavior of walls. Records of earthquake loads were taken from “United States Geological Survey” (USGS) official web site. Format of earthquake records is “strong motion CD” (.smc) due to Plaxis 2D software packet program.
 The results of analyses done in the saturated clay showed that 5.0 m and 10.0 m heights of retaining walls can be safely constructed in the earthquake zones having the magnitude up to 7.0. 15.0 m height of retaining walls cannot be safely constructed due to the insufficient wall dimensions. The results of analyses done in the water-saturated sand soil showed that 5.0 m height of retaining walls can be safely constructed in the earthquake zones having the magnitude up to 7.0. While the 10.0 m height of retaining walls can be safely constructed in the earthquake zones having magnitude of 6.0, it cannot be constructed in the earthquake zones having magnitude of 5.0 due to the earthquake acceleration. While the 15.0 m height of retaining walls can be safely constructed in the earthquake zones having magnitude of 6.0, it cannot be safely constructed in the earthquake zones having magnitude of 5.0 and 7.0.

Experimental Analysis of the Dynamic Response of Saturated Clayey Soil Under Impact Loading

The impact of loads on machine foundations is a typical cause of vibrations in industrial applications. Typically, these foundations will transfer vertical dynamic loads to the surface, which will result in earth vibrations that may cause structural damage to nearby structures. Dynamic impacts can vary from significant failure of sensitive sensors or systems to evident structural damage. The current work investigates the behavior of saturated clay soil under a single impulsive load. Deflectometry via falling weights was conducted to produce single pulse energy by dropping different weights from various elevations. The reactions of soils at various places were investigated (vertical displacement at topsoil surface). Such reactions consist of displacements, velocities, and accelerations caused by the impact occurring at the surface depth. The maximum displacement reaction of stiff soil was reduced by 80% in comparison with soft soil under the same impact load. The average percentage of change for stiff soil was 49% larger than for soft soil, as a result of kinetic energy caused by an increased contact surface. Maximum displacements increased with increasing operational frequency and dynamic load.

Effect of one cycle of heating-cooling on the clay-concrete pile interface shear strength parameters

This study investigated the effect of applying a one heating-cooling cycle on the interface strength parameters of saturated clay soil in contact with concrete, and the potential use of heating to improve/increase the pile capacity. A large direct shear test device with inner dimensions of 300 mm, 300 mm, and 200 mm for width, length, and height, respectively, was modified to perform the soil-concrete interface tests. A concrete block was built and placed in the bottom section of the shear device. Heating rods system were used to heat the circulating water that heated the specimens. The experimental tests were conducted on low plasticity soil with PI = 12, medium plasticity soil with PI = 29, and high plasticity soil with PI = 60. The specimens were first consolidated to a target normal stress prior to shearing. Two tests were performed for each normal stress and clay type with one test performed without heating-cooling cycle at ∼20 °C and the other test performed at ∼20 °C after a heating-cooling cycle up to 70 °C. The specimens were tested under different normal stresses ranging from 30 to 150 kPa. The specimens' temperature was increased gradually during the heating process from room temperature (∼20 °C) to ∼70 °C. The specimens were then gradually cooled back to room temperature. The test results showed significant increase in both the peak and residual interface friction angles by a percent ranges from 13.6% to 35.5%. The specimens subjected to a one heating-cooling cycle during shear experienced significant volumetric reduction by a percent ranges from 11.3% to 96%. The thermally induced “contraction” volumetric strain (TIVS) was found to increase with increasing temperature loading and the plasticity index, PI, but independent of the applied normal stress.

Undrained Deformation Characteristics of Soft Marine Clay Subjected to One-Way Cyclic Loading with Variable Confining Pressure

To gain a better understanding of the undrained deformation characteristic of saturated marine clay soil subjected to vehicle cyclic traffic load, a sophisticated dynamic triaxial was used to conduct a variety of undrained one-way compression cyclic experiments with variable confining pressure (VCP) as well as constant confining pressure (CCP). The results indicate that, compared to CCP test results, VCP is helpful to raise the axial resilient modulus (Mr) and restrain the permanent plastic strain ( ε a p ) development of the specimens. By normalization analysis of the measured data of Mr and ε a p , the virtually unique correlation between normalized average resilient modulus, normalized permanent axial strain after 1,000 loading cycles, and normalized mean normal stress is established, respectively, regardless of the values of CSR. Additionally, the VCP influence on ε a p is quantified and fitted by a power law function, which can be used for subsoil deformation prediction and provides new insights into the mechanics of strain accumulation under undrained cyclic loading conditions.

Laboratory Tests on Open‐Close Pile Jacking and Load Bearing Characteristics in Saturated Clay Soil

The stress characteristics of the pile‐soil interface have an important influence on the effect and long‐term bearing capacity of jacked piles. In order to obtain the stress characteristics of the pile‐soil interface of the jacked pile in clay, the development law of pore water pressure and earth pressure for open‐ and closed‐end jacked piles during the pile jacking process was studied using indoor model tests based on a saturated soft clay foundation. Test results show that, during the pile jacking process, the excess pore water pressure, earth pressure, and effective earth pressure at the pile‐soil interface all increase gradually along penetration depth, and the increasing trend first increases linearly and then increases sharply at the pile end. The excess pore water pressure at the pile‐soil interface of an open‐end pile is smaller than that of a closed‐end pile. The earth pressure and effective earth pressure at the pile‐soil interface of the closed‐end pile show a “lateral pressure degradation,” and the degradation becomes more and more significant with increasing depth. The pile‐soil interface adhesion coefficient increases with increasing penetration depth, and the greater the adhesion coefficient is, the higher the tightness between the pile and soil will be. As a method of determining the interfacial tightness between the pile and soil, the adhesion coefficient is closely related to the excess pore water pressure and earth pressure generated during the pile jacking process and can accurately reflect the mechanical mechanism of pile jacking in saturated viscous soil. Results of this contribution can provide reference for theoretical research on the mechanical pile jacking mechanism in saturated viscous soil.

ОПОЛЗНИ ТЕЧЕНИЯ: РОЛЬ СТРОЕНИЯ МАССИВОВ ГРУНТОВ В ИХ ФОРМИРОВАНИИ, ОСОБЕННОСТИ РАЗВИТИЯ И ТИПИЗАЦИЯ

The flow («dry», plastic, viscoplastic, viscous) is one of the main mechanisms of landslide displacement. The paper discusses the features of the formation and development of landslides, the displacement of which occurs in the form of a flow caused by soils saturated. The flow slides formation is associated with the excess of the creep threshold by the shear stresses acting in the slope massif with the transition of deformations from decaying to irreversible, progressive ones. When classifying slope processes, most researchers classify flow slides (under various names) into a separate class. Flow slides usually refer to the displacement (in subaerial or subaqueous conditions) of the moist mass or saturated clay, sandy-clay or rubble-clay soil, which moves along an inclined surface. It is shown that the formation of flow slides can occur either for a long time, with the gradual development of plastic deformations, or very quickly, with an almost instantaneous transition of soils to a fluid state (liquefaction). The main features with reference to which typical types of the flow slides is carried out are: 1) the capture depth of the massif by landslide deformations and the location of the flow zone; 2) morphometric characteristics, including the shape of the landslide body (in plan); 3) the type of soils involved in displacement and the mechanism of the transition of soils into a «fluid» state; 4) features of the displacement mechanism, including depending on the consistency of the landslide masses, due to the ratio in the composition of the moving mass of the solid (debris, clay) component and the water component. The features of the development and differences of surface landslides and «deep» flow slides, including the features of their displacement, morphology and structure, are characterized. The sequence of the landslide deformations development is described. The existing approaches to the typification of flow slides are considered. Summary typical types of flow slides of a current on the initiation and mechanism of displacement is prepared. It is concluded that it is necessary to take into account the features and differences of flow slides in the course of their study.

Experimental and Numerical Study of the Drying Process of a Consolidated Clay Soil

The clay soil is composed of solid skeleton and porous and can be studied in the framework of porous media. During the drying process, the initial saturated clay soil deforms mainly due to the changes of water content and capillary pressure. With the further increase in capillary pressure, the soil cracks. In fact, the deformation and the stress in clay soil are tightly related with the water content and capillary pressure. Therefore, it is of great interest to understand the changes in capillary pressure and water content within the clay soil during the drying process. However, it is difficult to obtain these field variables by experiments alone. Therefore, the numerical simulation could be good choice for deeper understanding of the dry process in clay soil. In current research, the combined numerical simulation and laboratory experiments research are conducted; the numerical results are compared with the laboratory observation. This research is the basis for the further study of the cracking process of clay soil due to drying.

Experimental investigation on frost heave characteristics of saturated clay soil under different stress levels and temperature gradients

The frost heave of soil has a significant influence on the stability and durability of various infrastructures. It is necessary to investigate the occurrence mechanism and development law of soil frost heaving. In this research, a triaxial frost heave experimental setup was developed, and a series of frost heave experiments were carried out by considering different confining pressures, deviator stresses, temperature gradients, and water supply conditions. The research found that the temperature gradient was the core driving force of soil frost heave. The increase of deviator stress and confining pressure would limit the development of frost heave by inhibiting water absorption. The growth of warm end temperature would increase the temperature of both water supply and soil near the warm end, which restrained the development of soil frost heaving. Finally, an improved prediction model for saturated clay soil's frost heave ratio was developed based on Takashi's one-dimensional frost heave ratio model, considering the effects of stress level and temperature gradient.

PERBANDINGAN HASIL UJI KAPASITAS DUKUNG TIANG PANCANG PADA TANAH LEMPUNG JENUH SKALA LABORATORIUM DENGAN PERHITUNGAN ANALISIS STATIS

Pile bearing capacity, is an important parameter in infrastructure design. The pile driving process, especially in saturated clay soils, results in an increase in pore pressure which affects the bearing strength capacity of the pile foundation immediately after erection. By knowing the com-parison of various ways to calculate the bearing capacity of a pile foundation, especially in clay soils, various influencing parameters can be studied and to be taken in determining the bearing ca-pacity of a pile foundation. The test is carried out by comparing the value of the pile bearing capacity of a laboratory scale pile model with the results of the calculation of the pile bearing capacity using the static analysis method based on the results of the test parameters of the soil. The clay soil con-ditions in the test are limited to saturated conditions. The reading of the change in pore water pres-sure is carried out using a pressure transducer during the pile driving process. Based on the pile loading test results in the laboratory, the pile bearing capacity results were 0.31 kN, while the bearing capacity results based on the static analysis method were 0.75 kN. The difference in pile bearing carrying capacity is caused by excess pore water pressure that appears around the ground during the driving process.

Diffusion of perfluoroalkyl acids through clay-rich soil

Diffusion through a water saturated silty clay soil column was measured for six perfluoroalkyl acids (PFAAs), including perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS). An aqueous pore diffusion model, which incorporated linear adsorption parameters measured independently in batch tests and a tortuosity factor determined independently using a bromide tracer test, was used to describe the experimental diffusion data. The diffusion model substantially underpredicted PFAA diffusion through the soil column for the more strongly sorbing PFAAs (most notably PFOS). Instead, application of a diffusion model that included a surface diffusion-like process provided substantially improved prediction of PFAA diffusion through the soil. The ratio of the observed pore diffusion coefficient to the observed surface diffusion coefficient ranged from 13 (for perfluorohexane sulfonate) to 0.88 for PFOS. These results suggest that surface diffusion serves a potentially important role for strongly sorbing PFAAs in clay-rich soils, and highlights the need for additional studies into the coupled adsorption and diffusion of PFAAs in low permeability media.