Journal of Geotechnical and Geoenvironmental Engineering

Dynamically compacted sands often exhibit a drop in cone penetration resistance immediately after compaction, but a gradual increase in the resistance occurs in a matter of weeks and months. An explanation of the former is sought in analysis of the stress state immediately after a dynamic disturbance, and a justification for the latter is found in the micromechanics process of static fatigue (or stress corrosion cracking) of the micromorphologic features at the contacts between sand grains. The delayed fracturing of contact asperities leads to grain convergence, followed by an increase in contact stiffness and an increase in elastic modulus of sand at the macroscopic scale. Time-dependent increase in small-strain stiffness of sand under a sustained load is a phenomenon confirmed by earlier experiments. It is argued that the initial drop in the cone penetration resistance after dynamic compaction is caused by a drop in the horizontal stress after the disturbance. The subsequent gradual increase in the penetration resistance is not a result of increasing strength, but it is owed to the time-delayed increase in stiffness of sand, causing increase in horizontal stress under one-dimensional strain conditions. This process is a consequence of static fatigue at contacts between grains. The strength of sand after dynamic compaction increases as soon as the fabric of the compacted sand is formed and is little affected by the process of grain convergence in the time after compaction. Contact stiffness, with its dependence on static fatigue, holds information about the previous loading process, and it is a memory parameter of a kind; this information is lost after a disturbance, such as dynamic compaction, in which new contacts are formed. The scanning electron microscope (SEM) observations, discrete element simulations, and energy considerations are carried out to make the argument for the proposed hypothesis stronger.

Traditional methods for calculations of active loads on retaining structures provide dependable forces, but these methods do not indicate reliably the location of the resultant load on the walls. The Coulomb method does not address the load distribution because it utilizes equilibrium of forces, whereas the Rankine stress distribution provides linear increase of the load with depth. Past experimental studies indicate intricate distributions dependent on the mode of displacement of the wall before reaching the limit state. The discrete element method was used to simulate soil-retaining structure interaction, and force chains characteristic of arching were identified. Arching appears to be the primary cause affecting the load distribution. A differential slice technique was used to mimic the load distributions seen in physical experiments. The outcome indicates that rotation modes of wall movement are associated with uneven mobilization of strength on the surface separating the moving backfill from the soil at rest. Calculations show that the location of the centroid of the active load distribution behind a translating wall is approximately 0.40 of the wall height above the base, but for a wall rotating about its top point, the location of the resultant is at approximately $0.55H$. In the third case, rotation about the base, the location of the calculated centroid of the stress distribution on the wall is slightly below one-third of the wall height.

Drained residual shear strength measured by multiple reversal direct shear or ring shear tests has been successfully used for over four decades for stability analyses of reactivated landslides in stiff clays and clay shales; A body of literature has accumulated in recent decades, claiming that “healing” or “strength regain” is realized in time on preexisting slip surfaces already at residual condition. In other words, the shear stress required to reactivate a landslide is claimed to be larger than the drained residual shear strength determined using laboratory tests. This article presents (1) a comparison of secant residual friction angle determined from laboratory tests and secant mobilized friction angle back-calculated for reactivated landslides; (2) explanations that field evidence used to claim “healing” can be attributed to alternative factors, and the laboratory evidence on “strength regain” upon reshearing is the result of either the testing apparatus or testing procedure, or is inapplicable to stiff clays and shales; and (3) laboratory aging test results, which show no “strength regain” on preexisting shear surfaces at residual condition.

Spatial variability of soil undrained shear strength is usually not rigorously considered in the design of basal heave for deep excavations. In this study, the slip circle method is employed to investigate how the required safety-factor against basal heave was affected by spatial variability in the context of reliability-based design. The nonstationary random field model is adopted to model spatial variability of undrained shear strength. Results show that the required safety-factor obtained with the consideration of spatial variability is much smaller than that without the consideration. Parametric studies show that the vertical scale of fluctuation has a significant influence on the required safety-factor: the longer the scale of fluctuation, the larger the required safety-factor. For target failure probabilities of 0.01 and 0.001, the corresponding required safety factors are in the ranges of 1.4–1.9 and 1.6–2.4, respectively, for the average value of vertical scale of fluctuation of 2.5 m. Design charts are provided for the ease of implementation, and an example of reliability-based design for basal stability is given for demonstration.

Pressurized grouting has been frequently adopted in soil-nailing systems that are widely used to improve slope stability. In most geotechnical applications, soil nailing using pressurized grouting has been empirically performed without theoretical validation because the interaction between the pressurized grout and adjacent soil mass is very complicated. The present paper deals with a series of pilot-scale chamber tests performed on four different granite residual soils to evaluate the effect of pressurized grouting on the soil-nailing system. When grout is injected into a cylindrical cavity in the soil mass, the pressure exerted around the cavity perimeter initially increases with time up to a peak value and then gradually decreases to a residual stress. The pressure reduction may result from the seepage of water originally retained in the grout paste into the adjacent soil formation. With the application of pressurized grouting, in situ stresses can be increased by approximately 20% of the injecting pressures during the experiments. To develop a desirable residual stress in a soil-nailing system, it is necessary to select an appropriate minimum injection time for which the grout pressure should be maintained. The required minimum injection time increases with an increase in either the fine-grain content or the injection pressure. Moreover, a series of in situ pullout experiments has been performed on soil-nailing systems, using both pressurized grouting and common gravitational grouting to compare the pullout loads of both cases and to verify the effectiveness of the pressurized grouting on the soil-nailing system. The pullout load of soil nailing using pressurized grouting is approximately 36% higher than that of soil nailing using gravitational grouting. This is attributed to the additional compaction of soil by cavity expansion and to an increase in the residual stress and in the dilatancy angle by pressurized grouting. The field experimental results have been verified with analytical solutions by estimating the dilatancy angle from the pressurized grouting tests.

This paper describes a methodology for the measurement of the capillary pressure curve of dense nonaqueous phase liquids (DNAPLs) in a water-saturated sandstone induced fracture. Details of specimen preparation, capillary barrier design, and capillary pressure measurement setup, procedure, and results are given. The aperture distribution of the induced fracture was estimated using the X-ray computed tomography (CT) technique. The primary drainage or invasion of DNAPLs in the water-saturated induced fracture was simulated using the invasion percolation (IP) approach. It was found that the invasion process was highly dependent on the spatial distribution of the fracture apertures. The IP model matched the experimentally measured entry pressure very well but deviated from the measured capillary pressure curve at the midrange and high nonwetting phase saturations. The IP model tended to yield an L-shaped function with a fairly flat portion at the midrange saturation and a sharp rise at the high end. The measured capillary pressure curve was observed to follow the well-known Brooks-Corey porous media function. These discrepancies between the measured and predicted results could be attributed to the fact that the IP model did not consider the effects of fracture surface roughness and undulation. For a fracture with small apertures, these two factors could make the fracture behave more as a porous medium than a parallel-plate channel.

Pumice sand particles have a vesicular nature, making them light and crushable. Previous research showed that the in situ relative density of pumice deposits cannot be estimated from conventional cone penetration testing. Because of this, a need exsits for more study of the geotechnical properties of this material. First, to distinguish compression behavior of loose and dense sand, $K_0$ compression tests were performed on pumice specimens at various strain rates, from 0.33% to $1,000\%/\min$, until a final compression of approximately 33% of the original specimen length was achieved. Second, after compression, the maximum displacement was held constant for a period of time during which the relaxation of the axial stress was monitored. After unloading, the particle-size distribution was measured to confirm particle crushing. From these results, the magnitude of stress relaxation of loose sand was found to be slightly larger than that of dense sand. On the other hand, dense sand particles exhibited more crushing during loading and less tendency for stress relaxation and particle rearrangement when the axial deformation is held fixed.

Design for retaining walls uses almost exclusively limit equilibrium methods, although they do not provide information on the wall deformation level as they consider rigid perfectly plastic behavior for all materials. However, deformation is a key issue for the development of the soil-reinforcement interaction; and therefore, its prediction is very important for design. A simple method on the basis of a very simple theoretical model is presented to predict the face deformations of brick-faced retaining walls reinforced with geotextiles. The model is simple, yet very versatile, and the results compared well with laboratory studies.

This study presents an in situ pilot-scale application of attapulgitic clay for stabilization of toxic metals and metalloids in contaminated soil. The selected field for the pilot-scale experiment was heavily contaminated with toxic metals and metalloids in total (Cu: $122\mathrm{mg}/\mathrm{Kg}$, Pb: $6,610\mathrm{mg}/\mathrm{Kg}$, Zn: $3,630\mathrm{mg}/\mathrm{Kg}$, Cd: $26.3\mathrm{mg}/\mathrm{Kg}$, Ag: $9.4\mathrm{mg}/\mathrm{Kg}$, As: $802\mathrm{mg}/\mathrm{Kg}$, Mn: $1,435\mathrm{mg}/\mathrm{Kg}$, Ba: $304\mathrm{mg}/\mathrm{Kg}$, Sb: $95.3\mathrm{mg}/\mathrm{Kg}$) and leachable concentrations. Geochemical and physical properties of treated soil were thoroughly studied before and after mixing with the attapulgitic clay. Soil mineralogy was determined by X-ray diffraction (XRD) and scanning electron microsope (SEM) techniques. On the basis of the site-specific soil geochemical properties, an appropriate proportion of specific grain-size attapulgitic clay was added and mixed in situ with simultaneous adjustment of soil moisture content to reach saturation. Analytical data of amended soil samples collected 1 month after the application showed a significant reduction of water leachable metal fraction (Cu: 17%, Pb: 50%, Zn: 45%, Cd: 41%, Ag: 46%, As: 18%, Mn: 47%, Ba: 45%, Sb: 29%). In addition, soil pH was stabilized at slightly alkaline conditions and remained constant during a 7-month monitoring period after amending the soil. Overall, the use of attapulgitic clay as a binder for immobilizing metals in contaminated land is a promising stabilization method at a competitive cost under present market conditions.