Hysteresis of Capillary Stress in Unsaturated Granular Soil

Constitutive relationships among water content, matric suction, and capillary stress in unsaturated granular soils are modeled using a theoretical approach based on the changing geometry of interparticle pore water menisci. A series of equations is developed to describe the net force among particles attributable to the combined effects of negative pore water pressure and surface tension for spherical grains arranged in simple-cubic or tetrahedral packing order. The contact angle at the liquid–solid interface is considered as a variable to evaluate hysteretic behavior in the soil–water characteristic curve, the effective stress parameter χ, and capillary stress. Varying the contact angle from 0 to 40° to simulate drying and wetting processes, respectively, is shown to have an appreciable impact on hysteresis in the constitutive behavior of the modeled soils. A boundary between regimes of positive and negative pore water pressure is identified as a function of water content and contact angle. Results from the analysis are of practical importance in understanding the behavior of unsaturated soils undergoing natural wetting and drying processes, such as infiltration, drainage, and evaporation.     

考虑二维降雨入渗的非饱和土边坡瞬态体积含水率分析

降雨是非饱和土边坡失稳的关键因素之一,非饱和土体的强度与体积含水率有密切的关系.考虑二维降雨入渗条件,基于非饱和土达西渗流定理及质量守恒定律建立了非饱和土的二维降雨入渗模型方程.基于交替隐式有限差分法,采用MATLAB编制了计算程序,对方程进行了数值计算,研究了不同降雨条件、边坡内部不同位置的瞬态体积含水率分布.研究表明,由于考虑了水平方向的入渗,边坡内随着深度和水平距离的增加,体积含水率都发生了变化,在距离边坡表面1 m左右时,体积含水率变化率变小,边坡表层的体积含水率变化远远大于边坡深部.初始体积含水率越小,边坡体积含水率变化越大,这对边坡的稳定性不利.      

Drainage Reduction under Land Retirement over Shallow Water Table

The retirement of land from agricultural production as a means of reducing the volume of subsurface drainage generated in the Western San Joaquin Valley, Calif., is under consideration. Reticence among local farmers and water managers renders only willing-seller land retirement feasible. There is an interest to acquire land that will generate the maximum possible drainage reduction relative to a “no retirement” baseline. An investigation was conducted to determine the drainage reduction potential of the retirement of (1) parcels that span land underlain by tile drains and land free of drainage infrastructure; (2) a “downgradient” parcel plagued by shallow ground water and equipped with subsurface tile drains; and (3) an “upgradient” parcel overlying well-aerated soil requiring no drainage. Long-term modeling with a deforming finite-element model suggests that the contiguous retirement yields the greatest drainage reduction. For single parcels, the 31% drainage reduction potential of downgradient retirement appears more attractive than the 16% drainage reduction associated with upgradient retirement.     

Sorbent Wicking Device for Sampling Hydrophobic Organic Compounds in Unsaturated Soil Pore Water. I: Design and Hydraulic Characteristics

The hydraulic characteristics of horizontally installed sorbent wick sampling devices were evaluated through wick tracer studies and laboratory soil column experiments to assess the influence of horizontal wick length and sampler interface design on sampling pore water in unsaturated soils. The nominal sampler design consisted of a cylindrical porous metal interface packed with granular-activated carbon encapsulating the end of a fiberglass wick that extended 100 cm horizontally from the interface before dropping 100 cm vertically to a collection vessel. The maximum sampling rate of horizontally installed wick systems declines exponentially with increasing horizontal wick length, while the vertical length influences the range of soil–water pressures that may be sampled. The nominal design sampled pore water from clay loam laboratory columns at 8 to 14 mL?h?1 under steady-state infiltration conditions and 2 to 5 mL?h?1 under draining conditions across a ?10 to ?45 cm H2O soil–water pressure range. Sampling rates in medium-grained sand under similar flow conditions were less than that of the clay loam due to reduced water content and reduced interface/soil contact area. An analysis of observed sampling velocities versus calculated soil water contents and hydraulic conductivities indicated that the design performs best when the soil water content is greater than 0.15 and unsaturated hydraulic conductivity is greater than 0.2 cm?h?1. A hydraulic model was developed that estimates the sampling velocity of the nominal design based on sampler interface pressure, which was linearly correlated with soil pressure.     

Profiles of Steady-State Suction Stress in Unsaturated Soils

Application of the effective stress principle in unsaturated geotechnical engineering problems often requires explicit knowledge of the stress acting on the soil skeleton due to suction pore water pressure. This stress is defined herein as the suction stress. A theoretical formulation of suction stress profiles, based on the soil water characteristics curve, the soil permeability characteristic curve, and previous shear strength experimental verification, is developed. The theory provides a general quantitative way to calculate vertical suction stress profiles in various unsaturated soils under steady flow rate in the form of infiltration or evaporation.     

FAO-56 Dual Crop Coefficient Method for Estimating Evaporation from Soil and Application Extensions

Crop coefficient curves provide simple, reproducible means to estimate crop evapotranspiration (ET) from weather-based reference ET values. The dual crop coefficient (Kc) method of the Food and Agricultural Organization of the United States (FAO) Irrigation and Drainage Paper No. 56 (FAO-56) is intended to improve daily simulation of crop ET by considering separately the contribution of evaporation from soil. The dual method utilizes “basal” crop coefficients representing ET from crops having a dry soil surface and separately predicts evaporation from bare soil based on a water balance of the soil surface layer. Three extensions to the evaporation calculation procedure are described here that are intended to improve accuracy when applications warrant the extra complexity. The first extension uses parallel water balances representing the portion of the soil surface wetted by irrigation and precipitation together and the portion wetted by precipitation alone. The second extension uses three “stages” for surface drying and provides for application to deep cracking soils. The third extension predicts the extraction of the transpiration component from the soil surface layer. Sensitivity and analyses and illustrations indicate moderate sensitivity of daily calculated ET to application of the extensions. The dual Kc procedure, although relatively simple computationally and structurally, estimates daily ET as measured by lysimeter relatively well for periods of bare soil and partial and full vegetation cover.     

Evaporation Theory for Deformable Soils

Desiccation of a deformable soil is basically the removal of water by evaporation, which is controlled by evaporativity and evaporability. Surface evaporation improves the trafficability, which is essential for the access of construction equipment in areas reclaimed with soft clay. The existing traditional methods for evaluating evaporation cannot account for the deformation of soils during evaporation. Therefore, a theoretical model for predicting the rate of evaporation from the surface of a deformable material is proposed. The model is based on a system of equations for coupled heat and mass transfer in unsaturated soils. The modified pressure plate extractor test and glass desiccator test were carried out to obtain the soil-water characteristic curve for a deformable soil. A column-drying test was conducted to investigate one-dimensional water flow, heat flow, and evaporation in the surface. A finite difference program was developed to solve the coupled nonlinear partial differential equations, which permits the study of liquid, diffusive vapor, and heat flows in the deformable soil. Comparison between measured and simulated values shows good agreement.     

Intrusion within a Simulated Water Distribution System due to Hydraulic Transients. I: Description of Test Rig and Chemical Tracer Method

A pilot-scale test rig was designed and constructed to simulate intrusion behavior associated with hydraulic transients initiated by sudden events such as rapid valve closure or uncontrolled change in on/off pump status (“pump trip”) in a water distribution system. After establishing steady state flow conditions, intrusion volumes were determined for 3.2 (1/8-in.) and 6.4 mm (1/4-in.) diam orifices overlaid with a column of water that provided 91 mm (3 ft) of external head. Average intrusion volumes associated with hydraulic transient events were determined by mass balance calculations using cesium as the tracer chemical. Average intrusion volumes were 11.4 and 71.2 mL for the two diameters, respectively. Given similar conditions in a water distribution system (i.e., an available pathway and favorable pressures), pathogens in the soil and water surrounding a water main potentially can intrude into the pipe during short-term pressure transient events.     

Effective Stress Behavior of Quasi-Saturated Compacted Cohesive Soils

Important geotechnical structures constructed on compacted cohesive soils often involve compaction either around or on the wet side of optimum water content. In general, at these water content values, water voids are continuous and air voids are occluded, and the soil may be assumed to be in a state termed as “quasi-saturated.” This paper evaluates the effective stress behavior of such quasi-saturated compacted specimens of Gangetic silt and Canyon dam clay in the broad framework of the conventional modified Cam-clay model. The initial state of quasi-saturated compacted specimens is shown to lie on the recompression line in w versus ln(p′) space. The actual recompression line on which the specimen state would lie, and the corresponding equivalent past maximum pressure, are found to depend only on the amount of compaction energy and the soil structure, and are independent of the molding water content or initial dry density. It is observed that, at low effective confining stresses, quasi-saturated compacted soils behave like overconsolidated soils and the effective stress paths during undrained shear lie on the Hvorslev surface. However, at confining stresses greater than the past maximum pressure, these soils behave like normally consolidated soils and the effective stress paths move practically along the Roscoe surface toward the critical state line.     

2. Numerical Simulations of Water Flow and Solute Transport Applied to Acid Sulfate Soils

Field investigations of Rassam et al. in 2001 have highlighted the effects of infiltration, drainage, and evapotranspiration on the dynamics of water flow and solute transport in acid sulfate (AS) soils. In this work, HYDRUS-2D is adopted as the modeling tool to elucidate the trends observed in that field experiment. Hypothetical simulations have shown that the relative contribution of drains to lowering the water table is significant only when closely spaced drains are installed in coarse textured soils, evapotranspiration being the main driving force in all other cases. AS soils reaction products that are close to a drain are readily transportable during infiltration and early drainage, but those produced farther away from it near the midpoint between drains are only slowly transported during a prolonged drainage process. Simulating the field trial of Rassam et al. has shown that drain depth and evapotranspiration significantly affect solute fluxes exported to the ecosystem. Managing AS soils should target minimal drain depth and density. Partial or full lining of the drains should be considered as a management option for ameliorating the environmental hazards of AS soils.     

Maximization of Water Storage in Backfilled and Lined Channels and Dimples Subject to Evaporation and Leakage

A channel or axisymmetric dimple filled with a coarse porous material and aimed at a temporary storage of infiltrated water as a perched water table aquifer is studied. The bottom shape is varied based on the criterion of maximal water storage after a certain period of drainage and evaporation. Leakage into the vadose zone through a thin liner occurs with a specific discharge proportional to the pressure drop across the liner. Evaporation through a horizontal shrinking water table is spatially uniform. An ordinary differential equation, which follows from the mass balance condition, is solved either explicitly or numerically. The class of triangular, polynomial, and conical sections is studied. The shape of maximal water retention is calculated for a given initial stored water volume or water table width, evaporation intensity, liner thickness and conductivity, vadose zone pressure beneath the liner, and selected time interval between two sequential infiltration events.     

Identifying Consolidation Coefficient: Linear Excess Pore-Water Pressure

Diagnostic curve methods are developed for simultaneously identifying consolidation coefficient, final settlement, and ratio of top and bottom excess pore-water pressures from observed settlements, in the case of linear excess pore-water pressure. Simple equations are also proposed for estimating consolidation coefficient and final settlement. The proposed methods and equations are applicable for any type of linear loading or drainage condition (one-way or two-way drainage). Solutions for pore-water pressure distribution and transient settlement of clay layers under linear loading of pore-water pressure with one-way drainage are also developed, which are used in the development of the methods. The proposed methods do not require full settlement data for the identification of parameters and the parameters can be identified from only initial but adequate settlement data.     

Pressure-Flow Relationships for DistributionSystem Connections

The suggested practice of using single point test data to construct a “straight line” (i.e., hydraulically scaled) water supply plot is investigated herein. It is shown that this widely used practice may lead to large errors in determining the available pressure at a connection for a design flow condition. It is further shown that using a second test point to construct a quadratic water supply plot can produce far better accuracy. Inaccurate plots of the available pressures can lead to erroneous fire flow predictions and improperly designed fire protection sprinkler systems. This raises safety concerns in addition to poorly designed facilities.     

Assessing Well Drilling Disturbance Effects on Offshore Foundation Piles in Clay

It is widely recognized that hydrocarbon well drilling from offshore platforms affects the surrounding ground. In clays, the level of disturbance can be severe when sections of open well collapse during drilling, impeding fluid return to the seabed (termed “packing-off”), and drilling fluid pressure is increased in order to obtain a breakthrough return path for the fluid. Significant swelling and hydraulic fracturing can take place in the surrounding soil mass. With time the excess pore pressures will dissipate and could cause increases in pore pressures far from the wells, potentially affecting areas occupied by the platform’s foundation piles. There is no established procedure to quantify the impact of such processes on foundation performance. This paper presents a numerical approach that involves a series of finite-element analyses in which drilling disturbance is treated as an idealized fluid injection process. The entry of pressurized drilling water into progressively enlarging fractured disturbed zones, and hence into the surrounding soil mass, is simulated with two- and three-dimensional nonlinear finite-element (FE) models, without modeling the actual hydraulic fracturing processes. The analyses make use of key observations made in the field at a piled offshore platform. The FE analyses predict marginal foundation capacity reductions, as well as significant global vertical and horizontal movements developing around the piles. It is demonstrated that the ground movement predictions are heavily influenced by the degree of geometric idealization.     

Probabilistic Analysis of Coupled Soil Consolidation

Coupled Biot consolidation theory was combined with the random finite-element method to investigate the consolidation behavior of soil deposits with spatially variable properties in one-dimensional (1D) and two-dimensional (2D) spaces. The coefficient of volume compressibility (mv) and the soil permeability (k) are assumed to be lognormally distributed random variables. The random fields of mv and k are generated by the local average subdivision method which fully takes account of spatial correlation, local averaging, and cross correlations. The generated random variables are mapped onto a finite-element mesh and Monte Carlo finite-element simulations follow. The results of parametric studies are presented, which describe the effect of the standard deviation, spatial correlation length, and cross correlation coefficient on output statistics relating to the overall “equivalent” coefficient of consolidation. It is shown that the average degree of consolidation defined by excess pore pressure and settlement are different in heterogeneous soils. The dimensional effect on the soil consolidation behaviors is also investigated by comparing the 1D and 2D results.     

Deformation and Pore-Water Pressure Responses of Elastic Viscoplastic Soil

The deformation and pore-water pressure responses of clayey soils are of great interest to civil engineers. In this paper, displacements and pore-water pressures of a clay subjected to the loading of a strip footing are simulated using a fully coupled finite-element (FE) consolidation method incorporated with a newly developed 3D elastic viscoplastic (EVP) model for the clay. A brief introduction to the 3D EVP model and its implementation in the FE analysis is presented. The 3D EVP model can describe the time-dependent stress-strain behavior of clayey soils, including volumetric creep. The main objective of this paper is to examine how the viscosity (or creep parameter ψ/V) of the clay affects the deformation and pore-water pressure responses of the clay. For this, the value of the creep parameter ψ/V is varied in the FE analysis. When viscous nature is taken into account, the pore-water pressure in the soil is higher than that without consideration of the viscous nature. The phenomenon of pore-water pressure increase due to creep is studied in this paper. It is found that larger creep parameter ψ/V results in higher pore-water pressure and larger deformation in the soil. The difference of the pore-water pressure due to the Mandel-Cryer effects and the creep is investigated using the FE model and discussed in this paper. In addition, a few other parameters (Poisson's ratio ν, permeability k, clay layer thickness h, and thickness h to a half footing width a ratio h∕a) are also varied to investigate their influence on deformation and pore-water pressure of the soil with creep. It is found that, the lower the permeability of soil, the higher is the pore-water pressure and the larger is the local deformation. The thickness of the soil layer also has a great influence on the pore-water pressure induced by the viscous effect. All these increased pore-water pressures result from a balance of the pore-water pressures induced by creep (and the Mandel-Cryer effects or both) and dissipated because of drainage.     

Measured and Estimated Suction Indices for Swelling Potential Classification

McKeen’s expansive soil classification methodology relies on a parameter referred to as the “total suction-water content index” for describing the slope of the soil–water characteristic curve on a semilog plot. The swelling potential of expansive soils is qualitatively classified (e.g., “low” or “high”) based on the magnitude of the total suction-water content index. This study examines the validity of using a “benchmark intercept simplification” for indirectly estimating the total suction-water content index when complete soil–water characteristic measurements are not available or economical. Suction indices estimated using the benchmark intercept simplification are compared with indices measured directly using the noncontact filter paper technique for 80 undisturbed expansive shale specimens from the Colorado Front Range Corridor. The results show that the suction-water content index is consistently overestimated using the benchmark simplification by amounts ranging from negligible to 50%, and averaging 23%. For 49 of the 80 specimens (61%), the estimated indices fall in different swelling potential categories than the measured indices. In 44 of the 49 cases (90%), the estimated indices fall in higher swelling potential categories than the measured indices. These discrepancies reflect potential errors that may arise from the use of the benchmark intercept simplification in classifying expansive soils.     

Performance of a Geogrid-Reinforced and Pile-Supported Highway Embankment over Soft Clay: Case Study

This paper describes a case history of a geogrid-reinforced and pile-supported (GRPS) highway embankment with a low area improvement ratio of 8.7%. Field monitored data from contact pressures acting on the pile and soil surfaces, pore-water pressures, settlements and lateral displacements are reported and discussed. The case history is backanalyzed by carrying out three-dimensional (3D) fully coupled finite-element analysis. The measured and computed results are compared and discussed. Based on the field observations of contact stresses and pore-water pressures and the numerical simulations of the embankment construction, it is clear that there was a significant load transfer from the soil to the piles due to soil arching. The measured contact pressure acting on the pile was about 14 times higher than that acting on the soil located between the piles. This transfer greatly reduced excess positive pore water pressures induced in the soft silty clay. The measured excess pore water pressure ratio max in the soft silty clay was only about 0.3. For embankment higher than 2.5?m, predictions of stress reduction ratio based on two common existing design methods are consistent with the measured values and the 3D numerical simulations. During the construction of the piled embankment, the measured lateral displacement–settlement ratio was only about 0.2. This suggests that the use of GRPS system can reduce lateral displacements and enhance the stability of an embankment significantly.     

Air Binding of Granular Media Filters

Air bubbles that form in water treatment filters create headloss and can form whenever the total dissolved gas pressure exceeds the local solution pressure. The location of potential bubble formation in filters can be predicted based on measurements of the clean bed headloss with depth, flow rate, and the influent total dissolved gas concentration. Bubble formation within filters can be reduced by increasing the pressure within the filter via greater submergence (water head above the media), lower hydraulic flow rate, or using a more porous media. Bubbles trapped in the bed can be released by “burping,” which can reduce the extent of headloss buildup. Burping is more significant at lower flow rates and within a lower density, higher porosity, hydrophobic anthracite layer.