Consolidation of Clay with a System of Vertical and Horizontal Drains

Consolidation of inhomogeneous clay with a system of horizontal and vertical drains is considered. Horizontal drains in the system are made of multiple thin pervious layers such as sand layers and geotextile sheets, while vertical drains are vertical cylindrical drains. Consolidation behavior with the drain system is formulated using the transfer matrix method. Special care is given to formulation of thin pervious layers for efficient computation. The developed formulation is verified using available numerical and field information. Parametric studies are conducted to study the consolidation characteristics of clay with the drain system. Based on the findings, a design method for an optimum system of horizontal and vertical drains is proposed and design charts are presented for such a design.     

Installation of Suction Caissons in Sand with Silt Layers

Suction caissons are increasingly being used for offshore anchors because of their ease of installation. However, for soil profiles that have sand overlain by layers of silt, there are reasons to believe that it may not be possible to install suction caissons. This may happen because the low permeability of the silt will create a hydraulic blockage, and thus diminish the upward hydraulic gradient required to reduce the penetration resistance in the sand to enable caisson penetration. The tendency of the silt to be sucked up, and its effect on the underlying sand, are not clearly understood. Furthermore, the blockage by the silt layer may be less than if a clay layer is present. This is because scouring of the silt may occur, allowing an upward seepage gradient to develop, and hence causing a reduction in penetration resistance. A series of suction caisson installation tests have been conducted in a geotechnical centrifuge to provide data on the penetration resistance and mechanisms for soil profiles where silt layers are present, either on top, or interbedded between layers of sand. In these tests, the thickness of the silt layer and its depth below the sand surface have also been varied. It has been found that if silt overlies the sand, a larger suction force is required for installation than when installation occurs in homogeneous sand, but that this force is still much smaller than the soil resistance when the caisson is pushed into the ground. When a silt layer is present, the suction pressures are observed to follow a similar trend that is independent of the position and thickness of the silt layer. Scouring of silt was evident in the tests, and significant upward movement of the soil plug inside the caisson was observed in all soil profiles when silt layers were present. The plug height was seen to reduce after the suction was turned off, suggesting that the resulting heave was unstable.     

Dynamic Compaction of Collapsible Soils Based on U.S. Case Histories

Dynamic compaction (DC) is an economical approach for mitigating the hazard posed by collapsible soils particularly when they are deeper than 3–4 m. In this paper, case histories are provided for 15 projects at 10 locations in the United States where collapsible soils were treated with DC. For each site the soil properties, compaction procedures, and subsequent improvement are summarized. Although cohesionless and low-plasticity collapsible soils were successfully compacted, clay layers in the profile appeared to absorb energy and severely reduced compaction effectiveness. Correlations are presented for estimating the maximum depth of improvement, the degree of improvement versus depth, the depth of craters, and the level of vibration based on measurements made at the various sites. The compactive energy per volume was typically higher than for noncollapsible soils because collapsible soils are usually loose but relatively stiff. The maximum depth of improvement was similar to that for noncollapsible soils; however, significant scatter was observed about the best-fit line. Improvement was nonuniform with nearly 80% of the total improvement occurring within the top 60% of the improvement zone. The crater depth was related to a number of factors besides the drop energy including the number of drops, drop spacing, and contact pressure. The peak particle velocities were typically lower than those for noncollapsible soils at shorter distances, but the vibrations attenuated more slowly with distance.     

Online Earthquake Response Test for Stratified Layers of Clay and Sand

Artificial islands often consist of layers of alluvial clay and reclaimed soil of varying order and thickness. Soft clay layers have nonlinear characteristics and can both amplify and attenuate earthquake ground motions. Liquefied ground impedes propagation of shear waves and thus attenuates the earthquake accelerations. Online testing is a method of feeding soil response characteristics directly from soil samples into a modeling algorithm. The effects of the layer thickness, configuration, and degree of consolidation on the earthquake response characteristics of alternating layers of clay and sand have been investigated. The degree of liquefaction and strain generated in sand adjacent to clay layers increased with the degree of consolidation. Clay layers attenuate the motions of sand layers for short period vibrations but amplify the long period motions, increasing the strain in overlying liquefied sand layers. Clay layers which were closer to the ground surface or of greater thickness tended to increase the surface accelerations. Normalized cumulative energy loss was larger in clay than in sand increasing with a decreasing degree of consolidation.     

Flume Tests for Scour in Clay at Circular Piers

Local scour at circular piers founded on clay was studied experimentally in the laboratory to compare the depth of scour in sand and in clay and to investigate the effects of the Reynolds number, Froude number, and approach flow depth on scour depth. The depths of scour in front, at the side, and in the back of the piers were measured as a function of time under steady, gradually varied flow conditions. The measured scour-depth-versus-time curves were fitted with a hyperbola to estimate the equilibrium scour depths. The extrapolated equilibrium scour depths were compared with values predicted by the Federal Highway Administration equation. The results showed that although the rates of scour were much slower in clay than in sand, equilibrium scour in clay was about the same as in sand. It was found that the shape of the scour hole correlates with the pier Reynolds number. At low Reynolds numbers, the depth of scour was about the same all around the piers. At higher Reynolds numbers, the scour holes developed mainly behind the piers with much less scour in front of the piers. It was also found that the extrapolated equilibrium scour depth correlates well with the pier Reynolds number and that the Froude number and relative water depth did not have a significant effect on the scour depth for these experimental conditions.     

酸法地浸采铀多井系统中渗透系数时空演化模拟

为了解酸法地浸采铀过程中含矿含水层渗透系数时空演化规律,通过数值模拟的方法建立了由两组五点型地浸抽注井构成的二维反应动力学模型,模拟酸法地浸采铀中孔隙度-渗透系数时空演化过程,探讨含矿含水层渗透系数时空演化规律。模拟结束时模拟区域内渗透系数区间为[5.37 m/d, 14.7 m/d]。通过渗透系数时间演化结果可知,在渗流作用和化学反应前锋不断向抽液孔推进,渗透系数发生变化的区域随时间不断增大,模拟区域内渗透系数区间值随时间不断增大,模拟结束时6个注液孔周围的渗透系数演化情况差异较小;矿层内渗透系数大于初始值的范围为注液孔中心相距约3 m（占抽注半径1/10）处的圆柱体内,渗透系数最小值出现在与注液孔中心相距约5.66 m处的圆周上,含矿层内大部分区域的渗透系数均小于初始值,根据模拟结果渗透系数最小值可能出现在与注液孔距离抽注半径的1/6~1/5区域内。      

Clear-Water Scour below Underwater Pipelines under Steady Flow

Experiments on clear-water scour below underwater pipelines (initially laid on the sediment bed) in uniform and nonuniform sediments under steady flow were conducted. Equilibrium scour profiles were modeled by a cubic polynomial. The experimental results are examined to describe the influence of various parameters on equilibrium scour depth. The equilibrium scour depth ds increases with increase in approach flow depth h for shallow flow depths, becoming independent of higher flow depths when h/b>5, where b=pipe diameter. However, the curves of scour depth versus sediment size d and Froude number Fb have a maximum value of ds/b = 1.65 at b/d = 27 and Fb = 0.6. The influence of sediment gradation on scour depth is prominent for nonuniform sediments, which reduce scour depth to a large extent due to the formation of armor layer within the scour hole. The influence of different shaped cross sections of pipes on the scour depth was investigated, where the shape factors for circular, 45° (diagonal facing) and 90° (side facing) square pipes obtained as 1, 1.29, and 1.91, respectively. Using the data of scour depths at different times, the time variation of scour depth is scaled by an exponential law, where the nondimensional time scale increases sharply with increase in Froude number characterized by the pipe diameter. In addition, clear-water scour below circular pipelines laid on a thinly armored sand bed (the sand bed is overlain by a thin armor layer of gravels) was experimentally studied. Depending on the pipe diameter, armor gravel, and bed-sand sizes, three cases of scour holes were recognized. The comparison of the experimental data reveals that the scour depth below a pipeline with an armor layer under limiting stability of the surface particles (approach flow velocity nearly equaling critical velocity for surface particles) is greater than that without armor layer for the same sand bed, if the secondary armoring formed within the scour hole is scattered. In contrast, the scour depth with an armor layer is less than that without armor layer for the same sand bed, when the scour hole is shielded by the secondary armor layer.     

Effects of Layer Thickness and Density on Settlement and Lateral Spreading

This paper presents the results of six large-scale centrifuge model tests that were performed to study the effect of relative density and thickness of sand layers on the amount of settlement and lateral spreading. The models included a “river” channel with clay flood banks underlain by layers of loose and dense sand of variable thickness, and a bridge abutment surcharge on one of the banks. The model container was tilted to provide an overall slope to the model. Each model was subjected to three or four significant ground motion events, which were obtained by scaling the amplitude of recordings of the Kobe (1995) and Loma Prieta (1989) earthquakes. Several measurements of acceleration, pore water pressure, settlement, and lateral movement are presented. The liquefaction potential index and a deformation index, which combine the influences of depth, density, and layer thickness, were found to correlate reasonably well with liquefaction induced settlements and lateral deformations for the range of models tested and indicate that centrifuge results are consistent with field observations.     

Postshaking Shear Strain Localization in a Centrifuge Model of a Saturated Sand Slope

This paper presents analyses of a test conducted on a 9-m-radius centrifuge to study the redistribution of pore water during diffusion of earthquake-induced excess pore pressures in a sand slope with embedded silt layers. The centrifuge model developed large postshaking deformations associated with shear strain localization at the interface between the sand and silt layers. Dense arrays of pore pressure transducers provided detailed measurements of pore pressure variations in time and space within the slope. A new data analysis approach is presented in which measured pore-pressures are used to compute flow rates and volumetric strains as a function of time and position throughout the slope. Hydraulic gradients were calculated by numerical differentiation of measured pore-pressure distributions with respect to position. Flow rates that were based on Darcy’s law were then integrated with respect to time to obtain flow quantities, from which volumetric strains were computed. A second data analysis approach that computes volumetric strains on the basis of soil compressibility and changes in pore pressure provided an independent computation of strains in consolidating zones. Results using these data analysis procedures confirm that a dilating (loosening) zone of significant thickness developed in the sand immediately beneath an embedded silt layer that had impeded the drainage of high pore pressures. These results support the hypothesis that the dilating zone corresponds to regions where the mobilized friction angle exceeds the critical state friction angle and that the dilating zone can be initially relatively thick before its size diminishes to the thickness of a thin shear band after the peak friction angle is mobilized. Quantification of the evolution of the size of the dilating zone is a key to understanding the magnitude of deformations associated with void redistribution.     

Evaluation of Alternative Media for Pebble Matrix Filtration Using Clay Balls and Recycled Crushed Glass

Protecting slow sand filters (SSFs) from high-turbidity waters by pretreatment using pebble matrix filtration (PMF) has previously been studied in the laboratory at University College London, followed by pilot field trials in Papua New Guinea and Serbia. The first full-scale PMF plant was completed at a water-treatment plant in Sri Lanka in 2008, and during its construction, problems were encountered in sourcing the required size of pebbles and sand as filter media. Because sourcing of uniform-sized pebbles may be problematic in many countries, the performance of alternative media has been investigated for the sustainability of the PMF system. Hand-formed clay balls made at a 100-year-old brick factory in the United Kingdom appear to have satisfied the role of pebbles, and a laboratory filter column was operated by using these clay balls together with recycled crushed glass as an alternative to sand media in the PMF. Results showed that in countries where uniform-sized pebbles are difficult to obtain, clay balls are an effective and feasible alternative to natural pebbles. Also, recycled crushed glass performed as well as or better than silica sand as an alternative fine media in the clarification process, although cleaning by drainage was more effective with sand media. In the tested filtration velocity range of (0.72–1.33)??m/h and inlet turbidity range of (78–589) NTU, both sand and glass produced above 95% removal efficiencies. The head loss development during clogging was about 30% higher in sand than in glass media.     

Response of Multilayer Foundation System beneath Railway Track under Cyclic Loading

For an efficient and economical design of a railway track system, it is necessary to understand the behavior of each track component with special reference to ballast and subgrade, which play a pivotal role in distributing the large, cyclic wheel loads longitudinally, laterally, and vertically away from the wheel contact area on the rail surface to the underlying soil strata. This paper presents an analytical model of a track-ballast-subgrade system with different formation soils such as dense uniform sand, stiff clay, loose sand, and soft clay modeled by using a mass-spring dashpot system with two degrees of freedom. This represents the varying energy distribution through ballast and subgrade in the vertical direction. Results are presented in the form of time-displacement response profiles for both the ballast and subgrade layers. In addition, the magnification factors for displacements with variation in subgrade soils for cyclic loading frequencies are reported. It is observed that the results obtained from the present analysis follow the experimentally observed trends already available in the literature.     

Undrained Shear Strength of Pleistocene Clay in Osaka Bay

This study presents the undrained shear characteristics of Holocene and Pleistocene clay samples from depths of 20–200 m under the seabed in Osaka Bay. Automated triaxial K0 consolidation tests and anisotropically consolidated-undrained triaxial compression and extension tests are conducted using the recompression method. The average undrained strength ratio (su/σv0′) is 0.33 (SD = 0.03) when the extension strength is defined as the peak strength or the strength at an axial strain of 15%, while su/σv0′ is 0.29 (SD = 0.04) when the extension strength is defined as the shear stress at the axial strain corresponding to the peak compression strength. Circular arc stability analyses are carried out with the modified Fellenius and Bishop methods for the design cross section of the seawall structure of the Kansai International Airport to study the effects of different definitions of shear strength. The seawall is founded on 19 m of soft Holocene clay and 10 m of Pleistocene sand overlying the Pleistocene clay. The stability analyses show that the factor of safety and depth of the critical circle (i.e., above versus below the sand layer) are sharply affected by both the value of su/σv0′ (0.33 versus 0.29) and the method of slices (Fellenius versus Bishop). The marginal stability calls for careful monitoring of construction with field instrumentation.     

Numerical Study of Resistance with Rib Roughness of Various Scales

A dynamically consistent method of locating the rough wall origin y0 is presented that is applicable to large- and small-scale rib roughness elements, where scale is defined relative to the depth of flow. Using the results of Reynolds-averaged computational fluid dynamics (CFD) calculations, a near-standard resistance law is derived in which ks, the equivalent sand roughness parameter, for large-scale roughness is flow-depth dependent. It is postulated that the relationship with depth of both y0 and ks, when scaled appropriately, is independent of rib roughness height and spacing. For ks this is supported by the CFD results.     

Bridge Pier Scour in Clay-Sand Mixed Sediments at Near-Threshold Velocity for Sand

Local scour at circular bridge piers embedded in a clay-sand-mixed bed was investigated in laboratory flume experiments. The effects of clay content, water content, and sand size on maximum equilibrium scour depth, equilibrium scour hole geometry, scouring process, and time variation of scour were studied at velocities close to the threshold velocities for the sand in the clay-sand mixture. It was observed that clay content and water content were the key parameters that effect the scouring process, scour hole geometry, and maximum equilibrium scour depth. The bridge pier scouring process in clay-sand mixtures involved different dominating modes for removal of sediment from scour hole: chunks-of-aggregates, aggregate-by-aggregate, and particle-by-particle. Regression-based equations for estimation of nondimensional maximum scour depth and scour hole diameter for piers embedded in clay-sand mixtures having clay content of <40% and water content of <40% were proposed as functions of pier Froude number, clay content, water content, and bed shear strength.     

An evaluation of a measure of the proportion of the treatment effect explained by a surrogate marker

Time-dependent markers, such as CD4 and viral load, are potential surrogate markers in AIDS clinical trials. A critical issue with surrogate markers is whether changes in these markers explain the beneficial effect of treatment on the real end point of the clinical trial. A statistic to measure the proportion of the treatment effect explained by the surrogate is p(FGS) = 1 - gamma/alpha, where alpha is the treatment effect coefficient in a Cox model and gamma is the treatment effect coefficient from a time-dependent Cox model adjusted for the marker. In this article we evaluate the statistical properties of p(FGS). Using a Monte Carlo study we show that the statistic is not well calibrated, because it can fall outside the range zero to one, even in very large samples. In the simulation study we consider situations where the time-dependent marker is measured with error at a fixed number of times. We show that a method of fitting a time-dependent Cox model involving smoothing the marker reduces the bias in the estimate of p(FGS) compared with the standard method of using the current or last observed marker value. We also show that the estimate of p(FGS) has considerable variability and can have wide confidence intervals. We conclude that p(FGS) is only likely to be useful in large trials with a strong treatment effect. The methods are illustrated using CD4 counts from an AIDS clinical trial of zidovidine versus placebo.     

Closely Spaced Footings on Geogrid-Reinforced Sand

The results discussed in this paper are based on a total of 74 tests performed on closely spaced strip and square footings on geogrid-reinforced sand. The study was carried out to evaluate the effect of spacing between the footings, size of reinforcement, and continuous and discontinuous reinforcement layers on bearing capacity and tilt of closely spaced footings. The interference effects on bearing capacity and settlement of closely spaced square footings on reinforced sand were almost insignificant in comparison to those on isolated footings on reinforced sand; whereas a significant improvement in the tilt of adjacent square footings has been observed by providing continuous reinforcement layers in the foundation soil under the closely spaced footings. A considerable improvement in bearing capacity, settlement, and tilt of adjacent strip footings has been observed by providing continuous reinforcement layers in the foundation soil under the closely spaced strip footings.     

Experimental and Theoretical Studies for the Behavior of Strip Footing on Oil-Contaminated Sand

This paper presents the results of a series of plain-strain model tests carried out on both clean sand and oil-contaminated sand loaded with a rigid strip footing. The objectives of this study are to determine the influence of oil-contaminated sand on the bearing capacity characteristics and the settlement of the footing. Contaminated sand layers were prepared by mixing the sand with an oil content of 0–5% with respect to dry soil to match the field conditions. The investigations are carried out by varying the depth and the length of the contaminated sand layer and the type of oil contamination. A plain-strain elastoplastic theoretical model with an interface gap element between footing and the soil is carried out to verify the test results of the model. It is shown that the load-settlement behavior and ultimate bearing capacity of the footing can be drastically reduced by oil contamination. The bearing capacity is decreased and the settlement of the footing is increased with increasing the depth and the length of the contaminated sand layer. The agreement between observed and computed results is found to be reasonably good in terms of load-settlement behavior and effect of oil contamination on the bearing capacity ratio. A comparison between the model results and the prototype scale (B = 1.0?m) results are also studied.     

Modification of aluminum antifriction alloys by carbon nanomaterials

A procedure is developed for modification of the near-surface layers of AO-20-1 alloy (AlSn20Cu, GOST Standard 14113–78) by carbon nanotubes at a depth of 2 μm. Data on the depth distribution of carbon across the near-surface layer are obtained. Mechanical and tribological tests of the modified layer are performed. It has been found that the hardness of the near-surface layer increases by 10–60% at a depth up to 1 μm and the friction coefficient decreases by ∼25 %.     

Vertical Vibrations of a Rigid Foundation Embedded in a Poroelastic Half Space

This paper considers the steady-state vertical vibrations of a rigid, cylindrical massive foundation embedded in a poroelastic soil. The foundation is subjected to time-harmonic vertical loading and is perfectly bonded to the surrounding soil. The contact surface between the foundation and the soil is assumed to be smooth and fully permeable. Biot’s poroelastodynamic theory is used in the analysis. The soil underlying the foundation base is assumed to be a homogeneous poroelastic half space while the soil along the side of the foundation is assumed to consist of a series of infinitesimally thin layers. The dynamic interaction problem is solved by a simplified analytical method. The accuracy of the present solution is verified by comparisons with existing solutions for both elastodynamic and poroelastodynamic interaction problems. Selected numerical results for the vertical dynamic impedance and response factor of the rigid foundation are presented to demonstrate the influence of nondimensional frequency of excitation, depth ratio, mass ratio, shear modulus of the backfill, and poroelastic material properties on dynamic interaction between an embedded foundation and a poroelastic half space.     

Penetration Resistance of Offshore Skirted Foundations and Anchors in Dense Sand

Penetration of skirts is an essential design issue for offshore skirted foundations and anchors in sand. Skirts may not penetrate far enough into dense sand by the available submerged weight alone. It may therefore be necessary to apply underpressure inside the skirt compartment to produce an increased driving force and to reduce the penetration resistance. This paper recommends procedures to calculate penetration resistance and required underpressure for skirts penetrated in dense sand with and without interbedded clay layers. The recommendations are based on interpretation of skirt penetration data from prototypes, field model tests, and laboratory model tests in dense sand. The paper first presents a model to calculate the penetration resistance of skirts penetrated by weight, or other external vertical load that does not cause flow of water in the sand. Two models are considered; one based on bearing capacity equations with friction angles from laboratory tests, and the other one based on empirical correlations with CPT tip resistance. The bearing capacity model gives more consistent correlations with the empirical data than the CPT model. Thereafter, a model to account for the effect of underpressure applied inside the skirt compartment is proposed. This model is developed based on interpretation of available prototype and model test data from skirts penetrated by underpressure. The results show that underpressure facilitates skirt penetration in sand considerably by providing both an additional penetration force and a reduced penetration resistance. It is also shown that interbedded clay layers can prevent flow of water through the sand and eliminate the beneficial reduction in penetration resistance.