Journal of Geotechnical and Geoenvironmental Engineering

The effects of stochastic hydraulic conductivity on the slope stability of an embankment dam are investigated using a combination of random field simulation, seepage analysis, and slope stability analysis. The hydraulic conductivity distribution is treated as a spatially stationary random field following a lognormal distribution. The turning band method is used to generate the spatial variability of the saturated hydraulic conductivity K_s in the domain. Different standard deviations of log hydraulic conductivity ${\sigma }_{\mathit{lnKs}}$ are investigated. For each value of ${\sigma }_{\mathit{lnKs}}$, various realizations of hydraulic conductivity were generated and combined with a numerical model to simulate water flow in an earth dam with variable K_s. The first-order second-moment reliability index β was employed to characterize the influence of the variability of K_s, and hence, pore-water pressures, on the stability of the downstream slope. A linear relationship between ${\sigma }_{\mathit{lnKs}}$ and the standard deviation of the factor of safety σ_F was obtained from the simulation results. A relationship between β and ${\sigma }_{\mathit{lnKs}}$, in which every 0.1 increment of ${\sigma }_{\mathit{lnKs}}$ results in a decrease of 1.0 in β, is deduced based on the simulation results. Results of a Shapiro-Wilk test for goodness of fit indicate that the factor of safety can be assumed to be normally or lognormally distributed when the saturated hydraulic conductivity follows a lognormal distribution and ${\sigma }_{\mathit{lnKs}}$ is small (≤0.5). When ${\sigma }_{\mathit{lnKs}}$ is large (>0.5), neither normal nor lognormal distributions provide a reasonable approximation of the factor of safety. Simulation results show that neither standard deviation nor coefficient of variation of the factor of safety is constant when only the variability of hydraulic conductivity is considered. While the results presented are directly applicable only to the particular earth dam geometry and boundary conditions studied, the methodology is general and may be extended to embankments with different boundary conditions.

The effect of pore fluid viscosity on the stiffness, damping, and liquefaction characteristics of sands was investigated to assess the potential contributions to a centrifuge model seismic response for soils saturated with high-viscosity pore fluid. Resonant column tests with cyclic loading frequencies in the range of 20–45 Hz were performed on a variety of fluids and sand sizes. At a strain level less than 2 × 10⁻⁴, the damping increased with pore fluid viscosity and shear strain amplitude, and it decreased with sand particle size. However, at shear strain greater than about 2 × 10⁻⁴, the increased skeleton damping tended to mask any effect of additional damping due to fluid viscosity. The liquefaction tests on fine silica sand revealed that the increase in total energy dissipation was not more than 10% for 100 cS oil when compared with water at a driving frequency of 25 Hz. Based on the experimental results, a simple model is proposed to examine the dependency of viscous damping on pore fluid viscosity, loading frequency, particle size, and shear strain amplitude.

Many of the mining operations in Western Australia are located in arid and semiarid regions where there is a severe scarcity of freshwater. Most of the processing is carried out using ground water from paleochannels. Much of this ground water is very saline, with concentrations approaching saturation in many cases. The potential rates of evaporation in the region can be very high (over 3 m/year). With careful management, tailings deposited subaereally in this region can achieve high strengths and densities due to evaporation. However, high salinity results in a severe reduction in the rate of evaporation from the tailings, thereby inhibiting consolidation due to evaporation. This paper presents the results of laboratory evaporation tests carried out to examine the mechanisms by which this reduction occurs. In these tests, all tailings samples with saline water formed salt crusts on the surfaces during evaporation, resulting in evaporation rates that were markedly low compared with those from equivalent freshwater tailings samples. This was the case even if the salinity was low. From measurements made on these samples, it is concluded that the most important mechanisms for reduction due to the salt crust are the increase in surface reflectivity and the increase in the surface resistance to moisture transfer. The vapor pressure reduction in the air due to the salinity of the tailings water is also a factor.

This study describes the results of field investigations and analyses carried out on 61 corrugated metal pipes (CMP) that were shaken by the 1994 Northridge earthquake. These CMPs, which include 29 small-diameter (below 107 cm) CMPs and 32 large-diameter (above 107 cm) CMPs, are located within a 10 km² area encompassing the Van Norman Complex in the Northern San Fernando Valley, in Los Angeles, California. During the Northridge earthquake, ground movements were extensively recorded within the study area. Twenty-eight of the small-diameter CMPs performed well while the 32 large-diameter CMPs underwent performances ranging from no damage to complete collapse. The main cause of damage to the large-diameter CMPs was found to be the large ground strains. Based on this unprecedented data set, the factors controlling the seismic performance of the 32 large-diameter CMPs were identified and framed into a pseudostatic analysis method for evaluating the response of large diameter flexible underground pipes subjected to ground strain. The proposed analysis, which is applicable to transient and permanent strains, is capable of describing the observed performance of large-diameter CMPs during the 1994 Northridge earthquake. It indicates that peak ground velocity is a more reliable parameter for analyzing pipe damage than is peak ground acceleration. Results of this field investigation and analysis are useful for the seismic design and strengthening of flexible buried conduits.

Atterberg limits, free swell, and hydraulic conductivity tests were conducted to assess how wet-dry cycling affects the plasticity and swell of bentonite, and the hydraulic conductivity of geosynthetic clay liners (GCLs) hydrated with deionized (DI) water (pH 6.5), tap water (pH 6.8), and 0.0125-M CaCl₂ solution (pH 6.2). The plasticity of bentonite hydrated with DI water increased during each wetting cycle, whereas the plasticity of bentonite hydrated with tap water and CaCl₂ decreased during each wetting cycle. Wet-dry cycling in DI water and tap water had little effect on swelling of the bentonite, even after seven wet-dry cycles. However, swelling decreased dramatically after two wetting cycles with CaCl₂ solution. Hydraulic conductivity of GCL specimens remained low during the first four wetting cycles (∼1 × 10⁻⁹ cm/s). However, within five to eight cycles, the hydraulic conductivity of all specimens permeated with the 0.0125-M CaCl₂ solution increased dramatically, to as high as 7.6 × 10⁻⁶ cm/s. The hydraulic conductivity increased because cracks, formed during desiccation, did not fully heal when the bentonite rehydrated. In contrast, a specimen continuously permeated for 10 months with the 0.0125-M CaCl₂ solution had low hydraulic conductivity (∼1 × 10⁻⁹ cm/s), even after eight pore volumes of flow.

A procedure for analyzing the mechanical response of an unbound pavement to the repeated loading of traffic is presented. The pavement is modeled as a layered elastic/plastic structure, and its response is described by the concepts of shakedown theory. A critical shakedown load is identified as the key design parameter. Pavements operating at higher loads will eventually fail, and those operating at loads less than critical may initially exhibit some distress but will eventually shakedown to a steady state. Estimates of this critical load, for different types of pavement, are found by studying various types of failure mechanisms, such as rut formation and subsurface slip. Optimization procedures are then used to determine the most likely form of failure for a particular pavement. The effects of self-weight, dual loads, moisture content, relative strengths of the various layers, and nonassociated plastic flow are studied. Some preliminary implications for pavement design are discussed.

A large excavated rectangular pile (barrette) with lateral earth pressure and pore-water pressure cells was successfully constructed and tested in a sequence of marine, alluvial, and weathered granite soils. A “soft” base formed beneath the bottom of the barrette permitted over 100 mm of vertical settlement, completely mobilizing the shaft friction at the barrette-soil interface. During the vertical load tests, an unusual and complex response of pore-water pressures and earth pressures at the barrette-soil interface was measured. During each vertical loading cycle (except the last one) and before interface slippage of the barrette occurred, excess positive pore-water pressures were recorded in all soil layers. Upon the initiation of slip at the barrette-soil interface, a sudden drop in the measured pore pressures as well as a substantial drop in lateral earth pressures generally resulted. Subsequent loading or unloading slippage events did not show the same dramatic behavior unless a period of consolidation/recovery was allowed first. This implies that caution must be used in design of barrettes relying heavily on skin friction when shearing induces contractive soil behavior. The current test results indicated that the empirical uncorrected SPT-N value approach and the effective stress β-method were inconsistent.

A finite-element approach is used to analyze the slope stability problem and to examine the effect of soil dilatancy on the stability of slopes. It is found that soil dilatancy has a significant effect on the stability of slopes, and that higher values of dilation angle lead to larger stability numbers. Therefore, the stability numbers obtained from limit analyses (lower/upper bound solutions) are not conservative for granular soils that exhibit a dilation angle smaller than a soil's friction angle.

The results of experimental research are presented and discussed with focus on the internal well resistance of prefabricated vertical drains (PVD) under vacuum-induced water flow. Measured results included fluid flow rates for two different cross-sectional hydraulic profiles (Types I and II PVDs). Experimental results indicated linear relationships, independent of the PVD widths, between extracted fluid velocity and the applied hydraulic gradient. Data showed a laminar flow regime to predominate for test velocities corresponding to hydraulic gradients <0.5. The larger nominal hydraulic radius of the Type II PVD is credited with providing a flow rate equal to approximately 3.2 times that of the Type I PVD at approximately the same operating total head. There was no apparent dependency of the transmissivity θ on the width or lengths (3, 4, and 5 m) of the PVDs tested. In the case of the 100-mm-wide Type I PVD, θ = 618 mm²/s was estimated from the measured data versus θ = 1,996 mm²/s for Type II PVD with the same dimensions.

This paper examines the influence of variations in matric suction on the collapse behavior of compacted Bangalore clay soil. The ASTM filter paper method measured the matric suctions of the compacted soil specimens. The matric suction of the compacted clay soil specimens ranged between 50 and 8,000 kPa at the as-compacted S_r values of 90 and 35%. Comparison of the matric suction-gravimetric water content relations of various compacted soils showed that the soil with a higher liquid limit has a higher matric suction at a given gravimetric water content. Variations in as-compacted degrees of saturation at a constant relative compaction or variations in relative compaction at a constant as-compacted degree of saturation notably affected the matric suction of the Bangalore clay soil. Experimental results also showed that the influence of matric suction on the collapse behavior of this compacted clay soil greatly depended on the relative compaction of the specimens and the pressure at which they were inundated.

The Deep Foundations Committee of the Geo-Institute commissioned a task force, chaired by Alan Macnab, to evaluate the state of the practice of Nondestructive Evaluation of Drilled Shafts. The task force consisted of: • 2 researchers on the topic—Al Dimillio of the Federal Highway Administration and Dr. Michael O'Neill of the University of Houston • 1 academic—Dr. Richard Finno of Northwestern University • 4 representatives of testing firms—Frank Rausche of Goble Rausche Likins & Assoc., Inc.; Dr. Allen Davis of CTL Construction Technology Laboratories, Inc.; Bernard Hertlein of STS Consultants, Ltd.; and Larry Olson of Olson Engineering, Inc. • 2 owners' representatives—Robert Stott of Caltrans and Barry Berkovitz of the Federal Highway Administration • 3 contractors—Bill Starke of Deep Foundation, Ltd.; Tim Smith of Malcolm Drilling Co., Inc.; and Alan Macnab of Condon Johnson & Assoc., Inc. This report was previously published in the May 1988 issue of Foundation Drilling. It is being republished here to bring this consensus state-of-the-practice information to the attention of the entire geotechnical engineering profession and to invite feedback through the regular discussion section of the Journal.