Hydraulic Conductivity of Soils from Grain-Size Distribution: New Models

This paper presents new developments of regression-based models to predict the saturated hydraulic conductivity of compacted soils from grain-size distribution. The models incorporate parameter values that adequately represent the distribution of grain sizes. Alternative representations of the grain-size distribution, the fractal dimension and entropy of the distributions, as well as porosity, soil density, and fines content are used in the models to estimate the hydraulic conductivity. These parameters that characterize the textural and hydraulic properties of the soil are combined and used in a multidimensional analysis to estimate the hydraulic conductivity. The predictions of the developed models are compared with those of existing models and laboratory measurements of hydraulic conductivity. The results suggest that the newly developed models outperform the existing models in predicting hydraulic conductivity using information from grain-size distribution. The presented models are suggested as alternatives to, for example, laboratory measurements of the hydraulic conductivity of certain soils that may be difficult to prepare or that may take several days or perhaps weeks to perform. In certain circumstances it may also be used to give first-hand information about the hydraulic properties in a field environment.     

Calibration of Water Distribution Hydraulic Models Using a Bayesian-Type Procedure

Estimating model parameters is a difficult, yet critical step in the use of water distribution system models. Most of the optimization-based approaches developed so far concentrate primarily on efficient and effective ways of obtaining optimal calibration parameter values. At the same time, very little effort has been made to determine the uncertainties (i.e., errors) associated with those values (and related model predictions). So far, this has typically been done using the first-order second moment (FOSM) method. Even though reasonably computationally efficient, the FOSM approach relies on several restrictive assumptions and requires computationally demanding calculation of derivatives. To overcome these limitations, the recently developed shuffled complex evolution metropolis (SCEM-UA) global optimization algorithm is linked to the Epanet2 hydraulic model and used to solve a least-squares-type calibration problem. The methodology is tested and verified on the Anytown literature case study. The main advantage of the SCEM-UA algorithm over existing approaches is that both calibration parameter values and associated uncertainties can be determined in a single optimization model run. In addition, no model linearity or parameter normality assumptions have to be made nor any derivatives calculated. The main drawback of the SCEM-UA methodology is that it could, potentially, be computationally demanding, although this is not envisaged as a major problem with current computers.     

Hydraulic Conductivity Measurement from On-the-Fly uCPT Sounding and from VisCPT

Detailed profiles of hydraulic conductivity are recovered from the deployment of direct-push permeameters at the Geohydrologic Experimental and Monitoring Site, Kansas. Measurements with thin tapered tips, and with standard cone penetration test (uCPT) tips, show only minor differences, suggesting that tip-local disturbance effects are small, and that routine uCPT measurements are therefore representative of pristine conditions. Permeameter measurements are correlated against closely deployed uCPT measurements, estimates of hydraulic conductivity from uCPT sounding correlations, and from grain size correlations derived from both vision CPT (VisCPT) and from cone metrics. On-the-fly evaluations of hydraulic conductivity require that the tip-local pressure field is both steady and partially drained. Continuous penetration is shown to yield pore pressures sufficiently close to steady to enable conductivities to be directly determined. Cone metrics of cone resistance, sleeve friction, and pore pressure ratio are shown to be sufficient to discriminate between partially drained and undrained behavior, and therefore to define the permissible regime where conductivities may be determined from uCPT sounding data. Estimates of hydraulic conductivities from uCPT sounding data are shown to correlate with independently measured magnitudes of hydraulic conductivity recovered using the permeameter tests. However, most of hydraulic conductivities from the permeameter tests (4.5?cm length screen) are underpredicted, suggesting that storage effects, the inability to reach a steady state, or the effects of dilation may influence the response. Profiles of hydraulic conductivities evaluated from the on-the-fly method also correlate well with the permeameter measurements. Predictions from soil classification and from VisCPT methods are also capable of estimating conductivities, with soil classifications giving the closest correlations of these two for this particular suite of data.     

Measurement of Subsurface Unsaturated Hydraulic Conductivity

Measurement of unsaturated hydraulic conductivity is needed for precise control of water and solutes in the vadose zone. Because of the spatial variation of soils, a large number of surface and subsurface measurements are needed to characterize a field. In this work, permeameters were developed and tested for estimating subsurface unsaturated hydraulic conductivity. The permeameters apply water under tension; they are easy to use and have adequate accuracy. Unsaturated hydraulic conductivity was determined by measuring the steady flow rates for various values of negative pressure. Tests using a soil of known hydraulic conductivity showed that the permeameters provided valid measurements. Two types were used, a porous cloth model that was inflated against the soil and a porous ceramic cup that was rigid. The field testing determined that a rigid design using a ceramic cup coupled to the soil by a layer of fine sand was easier to use, was reliable, and provided good results.     

Hydraulic Conductivity of MSW in Landfills

This paper presents a laboratory investigation of hydraulic conductivity of municipal solid waste (MSW) in landfills and provides a comparative assessment of measured hydraulic conductivity values with those reported in the literature based on laboratory and field studies. A series of laboratory tests was conducted using shredded fresh and landfilled MSW from the Orchard Hills landfill (Illinois, United States) using two different small-scale and large-scale rigid-wall permeameters and a small-scale triaxial permeameter. Fresh waste was collected from the working phase, while the landfilled waste was exhumed from a borehole in a landfill cell subjected to leachate recirculation for approximately 1.5 years. The hydraulic conductivity tests conducted on fresh MSW using small-scale rigid-wall permeameter resulted in a range of hydraulic conductivity 2.8×10?3–11.8×10?3?cm/s with dry unit weight varied in a narrow range between 3.9–5.1?kN/m3. The landfilled MSW tested using the same permeameter produced results between 0.6×10?3–3.0×10?3?cm/s for 4.5–5.5?kN/m3 dry unit weights. The hydraulic conductivity obtained from large-scale rigid-wall permeameter tests decreased with the increase in normal stress for both fresh and landfilled waste. The hydraulic conductivity for fresh MSW ranged from 0.2 cm/s for 4.1?kN/m3 dry unit weight (under zero vertical stress) and then decreased to 4.9×10?5?cm/s for 13.3?kN/m3 dry unit weight (under the maximum applied normal stress of 276 kPa). The hydraulic conductivity of the landfilled MSW decreased from 0.2 cm/s to 7.8×10?5?cm/s when the dry unit weight increased from 3.2 to 9.6?kN/m3. The results clearly demonstrated that the hydraulic conductivity of MSW can be significantly influenced by vertical stress and it is mainly attributed to the increase in density leading to low void ratio. In small-scale triaxial permeameter, when the confining pressure was increased from 69 to 276 kPa the hydraulic conductivity decreased from approximately 10?4?to?10?6?cm/s, which is much lower than those determined from rigid-wall permeameter tests. The published field MSW hydraulic conductivities are found to be higher than the laboratory results. Landfilled MSW possesses lower hydraulic conductivity than fresh MSW due to increased finer particles resulting from degradation. The decreasing hydraulic conductivity with increasing dry unit weight is expressed by an exponential decay function.     

Hydraulic Conductivity of Geosynthetic Clay Liners Exhumed from Landfill Final Covers

Samples of geosynthetic clay liners (GCLs) from four landfill covers were tested for water content, swell index, hydraulic conductivity, and exchangeable cations. Exchange of Ca and Mg for Na occurred in all of the exhumed GCLs, and the bentonite had a swell index similar to that for Ca or Mg bentonite. Hydraulic conductivities of the GCLs varied over 5 orders of magnitude regardless of cover soil thickness or presence of a geomembrane. Hydraulic conductivity was strongly related to the water content at the time of sampling. Controlled desiccation and rehydration of exhumed GCLs that had low hydraulic conductivity (10?9?to?10?7?cm/s) resulted in increases in hydraulic conductivity of 1.5–4 orders of magnitude, even with overburden pressure simulating a 1-m-thick cover. Comparison of these data with other data from the United States and Europe indicates that exchange of Ca and/or Mg for Na is likely to occur in the field unless the overlying cover soil is sodic (sodium rich). The comparison also shows that hydraulic conductivities on the order of 10?6?to?10?4?cm/s should be expected if exchange occurs coincidently with dehydration, and the effects of dehydration are permanent once the water content of the GCL drops below approximately 100%. Evaluation of the field data also shows that covering a GCL with a soil layer 750–1,000?mm thick or with a geomembrane overlain by soil does not ensure protection against ion exchange or large increases in hydraulic conductivity.     

Hydraulic Conductivity of Bentonite Slurry Mixed Sands

The hydraulic conductivity (k) of specimens from columns containing initially dry sands mixed with bentonite slurries was measured. The mixed specimens represented a range in void ratios (0.672 ≤ e ≤ 3.94) and bentonite contents (0.61% ≤ BC ≤ 7.65%, by dry weight). The measured k values, which ranged from 2.4×10?7?cm/s to 6.8×10?4?cm/s, correlated poorly with the total void ratio (e) of the specimens, due to the complicating effect of the bentonite in the sand-bentonite slurry mixtures. However, the measured k values correlated better with the void ratio of the bentonite (eb), which is consistent with the results of previous studies involving permeation of compacted bentonite and sand-bentonite specimens, even though the range in values of eb in this study (42.5 ≤ eb ≤ 127) was much higher than that previously reported. The relatively large range in eb values for the sand-bentonite slurry mixtures was also consistent with the relatively large range in measured k values, which are about one to seven orders of magnitude higher than values of k commonly reported for compacted sand-bentonite mixtures, despite similar bentonite contents. In terms of bentonite content, addition of more than 3% bentonite via slurry injection and mixing with the sands was successful in reducing the k of the unmixed sands (9.4×10?3?cm/s ≤ k ≤ 5.4×10?2?cm/s) by as much as four orders of magnitude to values less than 1.0×10?6?cm/s.     

Determination of Hydraulic Conductivity Using Piezocone Penetration Test

An elastoplastic, finite-strain, coupled theory of mixtures in an updated Lagrangian reference frame is applied to the piezocone penetration test to estimate the hydraulic conductivity of the soil via analysis of the steady-state excess pore pressure generated during piezocone penetration. The results of this approach were compared with piezocone penetration test data. It showed that reliable hydraulic conductivities can be estimated conveniently without performing pore pressure dissipation tests. This study also shows that the change in the dimensionless excess pore pressure (excess pore pressure is normalized by the effective overburden pressure) at the cone tip is almost constant when the dimensionless hydraulic conductivity (hydraulic conductivity is normalized by the penetration speed and cone radius, hereafter called DLHC) is less than 10?7 or greater than 10?4. It is also shown that the drainage condition around the cone tip is close to a fully undrained condition when the DLHC of the soil is less than 10?7, while it is close to a fully drained condition when the DLHC of the soil is greater than 10?4.     

Study of Variation of Saturated Hydraulic Conductivity with Time

The variation of the saturated hydraulic conductivity with time, as a function of temperature, has been studied involving field measurement at the College of Aboureyhan Research farm. The College of Aboureyhan is a part of the University of Tehran and the above-mentioned farm is located in the lowland of the southeast of Tehran, Iran. For the purposes of this research study it was planned to measure and record the field data in a plot of 18?m2. The hydraulic conductivity data were measured in 18 test points inside the study area using the inverse auger-hole method. Experiments were carried out from August 16, 2005 to June 14, 2006. The recorded filed data were then used to calculate the saturated hydraulic conductivity data using Excel software. The relationship between soil temperature, water temperature, and also water’s viscosity with hydraulic conductivity, respectively, were determined. The results of the statistical analysis involving SAS software demonstrated that the variation of temperature can considerably affect the saturated hydraulic conductivity values. The results showed that the lowest values of Ks were obtained in the winter when the water, soil, and air temperature were minimal and these values increased when the temperature increased. The effect of the means of daily K values (values measured in each sample date Kd) and their corresponding soil temperature adjusted values on drain spacing was determined compared with the average of total K values.?The maximum over- and underestimation of drain spacing was 19.1 and 23.3%, respectively, for measured values. These estimations were 9 and 16% for adjusted ones. Also, using the average values of K measured in the soil temperature range of 17–23°C resulted in a lower over- or underestimation of drain spacing.     

Hydraulic Conductivity and Leachate Characteristics of Stabilized Fly Ash

Disposal of fly ash on land amounts to sacrificing precious land space. Recycling of fly ash is one of the methods of solving the disposal problem. Stabilization of a low lime fly ash with lime and gypsum was studied through large scale tests on the stabilized material designed to simulate field recycling conditions as closely as possible, and found to be a very effective means to control hydraulic conductivity and leachate characteristics. The effects of moulding water content, lime content, gypsum content, curing period, and flow period on hydraulic conductivity, and on leachate of metals flowing out of the stabilized fly ash are reported herein. With proper proportioning of the mix, and adequate curing, the values of hydraulic conductivity on the order of 10?7 cm∕s were achieved. The concentrations of As, Cd, Cr, Cu, Fe, Hg, Mg, Ni, Pb, and Zn in the effluent emanating from the hydraulic conductivity specimens of mixes with higher proportions of lime or lime and gypsum were below threshold limits acceptable for contaminants flowing into ground water.     

Measuring the Hydraulic Conductivity of Soil–Bentonite Backfill

The hydraulic conductivity of soil–bentonite backfill in three pilot-scale cutoff walls was measured using laboratory tests on disturbed samples, laboratory tests on undisturbed samples, piezocone dissipation tests, and piezometer tests (also known as slug tests or single-well tests). In addition, a global measurement of the average hydraulic conductivity of the soil–bentonite backfill in one of the cutoff walls was made using the pilot-scale test facility. Two main factors distinguish these five different methods of measuring hydraulic conductivity: remolding and sample size. Remolding of samples tested in American Petroleum Institute filter press equipment significantly reduced their hydraulic conductivity compared to the hydraulic conductivity of undisturbed samples, which were of similar size. For the other tests, where the degree and extent of remolding were less significant, hydraulic conductivity was found to increase as sample size increased, with the global measurement producing the highest value. The existence of bentonite filter cakes on trench walls reduces the influence of sample size on the equivalent hydraulic conductivity of the barrier. Findings regarding locating defects with a piezocone and hydraulic fracture in piezometer tests are also presented.     

Characterization of Models for Time-Dependent Behavior of Soils

Different classes of constitutive models have been developed to capture the time-dependent viscous phenomena (creep, stress relaxation, and rate effects) observed in soils. Models based on empirical, rheological, and general stress-strain-time concepts have been studied. The first part is a review of the empirical relations, which apply only to problems of specific boundary conditions and frequently involve natural time alone. The second part deals with different rheological models used for describing the viscous effects in the field of solid mechanics. The rheological models are typically developed for metals and steel but are, to some extent, used to characterize time effects in geomaterials. The third part is a review of constitutive laws that describe not only viscous effects but also the inviscid (rate-independent) behavior of soils, in principle, under any possible loading condition. Special attention is paid to elastoviscoplastic models that combine inviscid elastic and time-dependent plastic behavior. Various general elastoviscoplastic models can roughly be divided into two categories: Models based on the concept of overstress and models based on nonstationary flow surface theory. Although general in structure, both have shortcomings when used for modeling of soils.     

Hydraulic Models of the Flow Distribution in a Four Branch Open Channel Junction with Supercritical Flow

Intense rainfall on urban areas can generate severe flooding in the city, and if the conditions are right, the flow in the streets can be supercritical. The redistribution of the flow in street intersections determines the flow rates and water levels in the street network. We have investigated the flow that occurs when two supercritical flows collide in a 90° junction formed by streets of identical cross section. Several flow configurations within the intersection are possible, depending on the position of the hydraulic jumps that form in and upstream of the intersection. Previous work has identified three flow types, with Type II flows being further classified into three subregimes. Hydraulic models have been developed, based on the principles of the conservation of flow and momentum flux in the intersection, which predict the angles at which the jumps will form. These models can be used to determine the flow type that will occur. Moreover, additional models have been developed for computing the outflow discharge distribution. For Type I flows, it has not been possible to develop such a hydraulic model for the discharge distribution, but some data are provided for one configuration to indicate the influence of different parameters. For Type II and Type III flows, such models are developed, and their predictions agree with data obtained from the channel intersection facility at the Laboratory of Fluid Mechanics and Acoustics in Lyon.     

Influence of Freezing Temperature on Hydraulic Conductivity of Silty Clay

Hydraulic conductivity of thawed consolidated slurries of a silty clay from Lachute, Quebec, Canada, subjected to closed-system freezing at different temperatures ranging from ?2 to ?12°C were determined from constant-head permeability tests. The permeability index defined as the slope of the relation between log k and void ratio was found to increase with decreasing temperature. It was also established that the ultimate permeability index was related to the temperature at which no further change in unfrozen water content occurs. For the silty clay studied, the permeability index increased from 1.4 for the unfrozen soil prior to freezing to a maximum value of 8 at a temperature of ?12°C.     

Hydraulic Damping of Saturated Poroelastic Soils During Steady-State Vibration

A theoretical study of the steady-state response of a saturated poroelastic soil column during compressional and rotational harmonic vibrations is presented. Hydraulic damping due to Biot flow is evaluated for top-drained and double-drained boundary conditions and for compressional and rotational motions using the theory of a damped single-degree-of-freedom system. For compressional motions, the dynamic response of gravels and sands is highly influenced by the compressibility of the pore fluid. More hydraulic damping occurs as soil hydraulic conductivity increases and as the column boundary conditions change from top drained to double drained. On the other hand, hydraulic damping for rotational motions is significantly less than that for compressional motions and is dependent on a dimensionless hydraulic conductivity parameter Ks. For Ks within the range of 10?3–100, hydraulic damping may have an important contribution to total soil damping, especially at small strain levels.     

Influence of the Grain-Size Distribution Curve of Quartz Sand on the Small Strain Shear Modulus Gmax

The paper presents a study of the influence of grain-size distribution curve on the small strain shear modulus Gmax of quartz sand with subangular grain shape. The results of 163 resonant column tests on 25 different grain-size distribution curves are presented. It is demonstrated for a constant void ratio that while Gmax is not influenced by variations in the mean grain-size d50 in the investigated range, it significantly decreases with increasing coefficient of uniformity Cu = d60/d10 of the grain-size distribution curve. Well-known empirical formulas (e.g., Hardin’s equation with its commonly used constants) may strongly overestimate the stiffness of well-graded soils. Based on the RC test results, correlations of the constants of Hardin’s equation with Cu have been developed. The predictions using Hardin’s equation and these correlations are in good accordance with the test data. Correlations of the frequently used shear modulus coefficient K2,max with Cu and empirical equations formulated in terms of relative density, are also given in the paper. A comparison of the predictions by the proposed empirical formulas with Gmax-data from the literature and a micromechanical explanation of the experimental results are provided. Correction factors for an application of the laboratory data to in situ conditions are also discussed.     

Short-Term Electrical Conductivity and Strength Development of Lime Kiln Dust Modified Soils

Lime kiln dust (LKD) is used for modifying pavement subgrades to expedite construction on wet clayey soils. This paper describes the short-term development (typically, over the first 3?to?7?days) of electrical conductivity and penetration resistance of LKD-modified soils. The normalized net change of electrical conductivity is solely related to the LKD dosage. The decrease of electrical conductivity with time coincides with the increase of penetration resistance with time. The correlations of electrical conductivity with strength gain in LKD and lime-modified soils suggest that electrical conductivity measurements can potentially be useful for quality control in field applications.     

Regression Models for Dynamic Properties of Highly Organic Soils

Regression models are presented for the dynamic properties of highly organic soils. The models are based on a database of triaxial and resonant-column/torsional-shear cyclic loading tests on thin walled tube samples mainly retrieved from the Sacramento-San Joaquin Delta. The soils in this database range from highly fibrous peat to amorphous organic clays with organic contents (OC) ranging from 14–81%, water contents ranging from 88–495%, total densities (ρ) ranging from 1.056–1.450?Mg/m3, and effective consolidation stresses (σvc′) ranging from 11–135?kPa. The secant shear modulus (G) and equivalent damping ratio (ξ) were modeled as variables dependent on the shear strain amplitude (γc), consolidation stress (σvc′), and OC. The residuals of the regression models were analyzed against other predictor variables including undisturbed density (ρ), loading frequency (f), and number of loading cycles (N). A regression model for ρ was developed, and conditional probabilities were used to improve the estimation of G and ξ when ρ measurements were available. The database of in situ measurements of shear wave velocity (Vs) was used to adjust the regression model for in situ conditions. Variances and correlations in the regression models are presented.     

Experimental Investigation of the Hydraulic Erosion of Noncohesive Compacted Soils

This paper presents the results of a laboratory investigation whose purpose was to evaluate the effects of compaction on the erodibility of cohesionless soils. By means of a recently developed flume experiment, sediment erosion rates and incipient motion, as a function of shear stress, average velocity, and dry density, have been determined for three compacted sand and gravel mixtures. A preliminary comparison of the incipient motion values shows that granular soils compacted at the Proctor optimum have a higher resistance to free surface flow erosion than those compacted at lower and higher densities. This leads one to infer that the Proctor optimum, generally used as a standard for construction, might also be an optimum for hydraulic resistance and stability. Additional comparison of the experimental data with two commonly used incipient motion criteria also suggests that Yang’s criterion is a better predictor of soil detachment than the Shields-Yalin criterion.