Stabilization/solidification (S/S) is commonly applied to treat heavy metal-contaminated soils through the use of lime and ordinary Portland cement (OPC). Recently, reactive magnesia (MgO) has ...emerged as a novel binder for S/S of heavy metal-contaminated soils; however, a comprehensive comparison between MgO, lime (CaO), and OPC for S/S application is still missing. This study compares the S/S efficiency of MgO, CaO, and OPC for soils contaminated by six individual heavy metals (Pb, Cu, Zn, Ni, Cd, and Mn) through unconfined compressive strength (UCS) test, one stage batch leaching test, and microstructural analysis. The addition of binders can transform soluble heavy metal salts to insoluble hydroxides and their complexes, and hence the leachability of heavy metals decreases. However, the level, to which the leachability can be reduced, is highly pH dependent. Contaminated soils treated with MgO have pH of 9–10.5, at which the leachability of Pb and Zn is much lower than that of OPC- or CaO-treated soils with pH of 10.5–13; for example, the leached Pb and Zn from MgO-treated soils are only 0.1%–3.3% and 0.1%–9.4% of those from OPC-treated soils, respectively. On the other hand, the leached Cd and Mn from OPC-treated soils are 0.1%–28.5% and 0.1–10.7% of those from MgO-treated soils, respectively, due to the high pH and the formation of calcium silicate hydrate (CSH) in OPC-treated soils. OPC and CaO are more effective than MgO in decreasing the Ni leachability at high original concentrations, but less effective at low original concentrations. For all soils except those contaminated by Zn, the OPC generally produces a much higher UCS, up to two orders of magnitude, than the CaO and MgO. The results of study indicate that no single binder can treat all types of heavy metal-contaminated soils perfectly, and the selection of binder is a site-specific problem.
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•Comparison of MgO, CaO, and cement for treating heavy metal-contaminated soils.•pH of treated heavy metal-contaminated soils: MgO < cement<CaO.•Leachability of treated Pb- and Zn-contaminated soils: MgO < cement<CaO.•Leachability of treated Cd- and Mn-contaminated soils: cement<CaO < MgO.•Strength of treated Pb-, Cu- Cd-, and Mn-contaminated soils: MgO, CaO < cement.
For expansive soil slopes, the primary failure mode is surface- or shallow-layer slide, which can occur easily under repeated rainfalls. In this investigation, a field testing program is implemented ...on five expansive soil slopes with different inclination subjected to artificial rainfalls, against which a numerical model is calibrated. Hydro-mechanical analysis is then conducted to estimate the factor of safety for expansive soil slopes with different pattern of fissures. A high slope inclination can increase the surface runoff, although the factor of safety decreases. It is of significance to minimize the amount of rainfall infiltration through desiccation cracks, since the increase in volumetric water content from rainfall events is more detrimental to reduce the unsaturated shear strength of soil. A conceptual framework is proposed to apply a drip irrigation system for expansive soil slopes. The volumetric water content can be maintained above a critical level to avoid the initiation of desiccation cracks. In the end, design parameters for the drip irrigation system are optimized.
•Expansive soil slope fails due to rainfall infiltration through desiccation cracks on the slope surface.•Correlation between rainfall infiltration, slope inclination and slope height is established.•There is a trade-off between slope inclination, rainfall infiltration and factor of safety.•Steep slope is suggested to maximize the surface runoff.•Drip irrigation can control the water content of expansive soils to avoid the initiation of desiccation cracks.
Summary
Traditional consolidation theories cannot provide good predictions of consolidation settlement in land reclamation because of their assumptions that the influence of soil's self‐weight is ...often neglected, and the drainage boundary is considered as fully pervious/impervious. In view of these limitations, an analytical solution is derived for one‐dimensional self‐weight consolidation problems with a continuous drainage boundary using the finite Fourier sine transform method. Following the classical Terzaghi's small strain theory, the soil's self‐weight is considered to produce consolidation settlement in dredged materials with a constant coefficient of consolidation. The continuous drainage boundary can essentially describe the time‐dependent variation of drainage capacity at the interface between two adjacent soil layers. By reducing the interface parameters, the effectiveness of the calculation is demonstrated against the Terzaghi's solution. The influence of interface parameters and soil's self‐weight stress coefficient on self‐weight consolidation is discussed. As expected, the rate of consolidation considering the self‐weight stress is faster, although the dependency of consolidation rate on the material property of void ratio is neglected. Moreover, the plane of maximum excess pore‐water pressure is estimated as a function of time factor, based on which a design chart is developed to optimize the layout of horizontal drains in land reclamation.
A simplified three-dimensional finite element model of bell-spigot joints in vitrified clay (VC) pipelines is introduced, which allows the simulation of rotation, axial translation, and ...moment-release behavior at joints. Then the kinematic responses of VC pipelines subjected to ground subsidence with two different joint-ground dislocation configurations are studied. Results show that axial extension and contraction can occur in the adjacent joints of the rotated pipe segment. The maximum kinematic response occurs when the fault offset passes through the midpoint of a pipe segment with a relatively shallow burial depth. Based on the numerical results, an empirical kinematic solution considering the rigid body rotation and dimensions of the pipe segment is proposed, which can provide a straightforward estimation on the axial translation of bell-spigot jointed VC pipelines subject to ground subsidence.
The combined vacuum and surcharge preloading technique is extensively used to accelerate the consolidation process of subsoils. The effect of vacuum pressure is often considered as a ...loading/unloading cycle of mean effective stress, such that elastic rebound occurs after vacuum removal, which cannot explain the observed postconstruction settlement in the field. In this study, the stress state of subsoils subject to vacuum and surcharge preloading is analyzed and decomposed into two components: (a) geostatic consolidation at a different depth, and (b) loading/unloading in the minor principal stress direction. A series of consolidated drained triaxial tests is conducted to simulate the soil behaviour after vacuum removal. Results show that the contribution of unloading in the minor principal stress direction outweighs the magnitude of elastic rebound after vacuum removal, and hence continued settlement dominates. A field case for highways is provided to further demonstrate the proposed mechanism.
To investigate the impact of relative humidity on the mechanical properties and microscopic pore structure of air lime-stabilized compressed earth, unconfined compression strength (UCS), mercury ...intrusion porosimetry (MIP) and scanning electron microscopy (SEM) tests were conducted by varying the air lime content and relative humidity (RH) in compressed earth. The results revealed three typical failure modes in unconfined compression strength tests of lime-stabilized compressed earth. Both the unconfined compression strength and characteristic parameters of pore structure in lime-stabilized compressed earth exhibited a trend of initial increase, following by a decrease as the air lime content and relative humidity increased. At the microscopic level, the relative humidity and air lime content interacted with the changes in macro-level unconfined compression strength, and the increase of both could promote the lime hydration reactions, inhibiting the crack development. Considering the influence of relative humidity, mechanical performance, and economic benefit improvement, the recommended air lime content for high humidity and low humidity were 0–28% and 0–26%, respectively, offering valuable insights for the optimization and application of lime-stabilized compressed earth as a modern construction material for structural walls.
Subgrade soils experience repeated traffic loads, such that dynamic stress and displacement are accumulated to jeopardize the stability of pavement foundations. Laboratory element testing is often ...conducted to apply cyclic loads to the soil. In fact, the stress state of subgrade soil is complex due to the constant rotation of principal stress induced by moving vehicles. It is hard to select constitutive relations for unsaturated subgrade soils in numerical or analytical calculation, since most models are developed without proper consideration of principal stress rotation. Furthermore, most available earth pressure sensors cannot provide stress measurements in three orthogonal directions for a soil element at the same time. A novel orthogonal earth pressure transducer is developed to measure dynamic stresses of subgrade soils in three directions under a roadway at Chencun, China. The cumulative effect of dynamic stress with load cycles is observed explicitly. Results indicate that the dynamic stress becomes negligible at 2 m from the roadway, and stabilization is suggested to be limited within this width. A series of constitutive relations is proposed to correlate the dynamic stress with the resulted displacement in different directions, which can be used to guide the design of unsaturated subgrade soils in South China.
•A novel orthogonal earth pressure transducer is developed to measure stresses in three directions.•A field test is conducted on a roadway over unsaturated subgrade soils under moving vehicles.•Cumulative effect of dynamic stress with load cycles is observed.•Dynamic stresses become negligible at 2 m from the roadway.•Constitutive relations are proposed to correlate dynamic stress with resulted displacement.
Geological discontinuities govern the strength and deformation of rock mass and play an important role in the stability of geotechnical excavations, such as tunnels, caverns, slopes, and mines. In ...this paper, the failure mechanism of a tunnel in the vicinity of a fault zone is investigated by physical model tests with dimensions of 3 m × 2.4 m × 0.4 m, in which the non-contact Digital Image Correlation (DIC) technique is employed to provide full-field displacement measurements. Results show that the fault zone near the tunnel shoulder has a significant influence on the stability of the surrounding rock, in terms of strain localization and failure initiation. With the increase of surcharge loading, cracks initiate at the two sidewalls, following by the initiation of cracks at the left shoulder and the crown. Cracks propagate to the fault zone eventually, leading to asymmetrical collapse. The stress loosening zone (SLZ) in the surrounding rock after tunnel excavation is significantly influenced by the fault zone near the left shoulder of the tunnel, showing the occurrence of stress redistribution. It is recommended to implement reinforcement in the rock mass between the tunnel and the fault to improve the stability.
Rammed earth is a common construction material for thousand years throughout the history of human civilization. The strength degradation of rammed earth under salts attack and drying-wetting cycles ...needs to be assessed. In this study, chemical analyses were conducted on rammed earth materials sampled at different wall heights from a demolished Fujian Tulou for the measurements of salinity. The degree of salinity was found to increase from the top to the bottom of the wall. To better understand the salt effect, sodium chloride (NaCl), sodium sulfate (Na2SO4), and calcium chloride (CaCl2) were mixed in various contents with desalinated soils to produce remoulded specimens for strength tests. The unconfined compressive strength (qu) and the cohesion (c) were reduced with the increase of the salinity degree. The influence of single salt attack was not apparent on the variation of qu or friction angle φ, but it was significant on the cohesion variation. The soil attacked by Na2SO4:CaCl2 = 1:1 mixed salt had the lowest qu and c values. The Ca2+ cations from CaCl2 salt reacted with the SO42− anions from Na2SO4 to form crystals with increasing volume, leading to strength deterioration in rammed earth materials.
•Degree of salinity for ancient rammed earth walls decreases with the wall height.•Cohesion of remoulded rammed earth reduces with the degree of salinity.•Influence of single salt attack is significant on the variation of qu and cohesion.•Attack of mixed salt of Na2SO4:CaCl2 = 1:1 causes the lowest qu and c.•Crystallization between Ca2+ cations and SO42− anions causes strength deterioration.
Estimation of representative elementary volume (REV) is significant to analyze fractured rock mass in the framework of continuum mechanics. Engineers can therefore simplify the analysis by using an ...equivalent rock block with an average property, and the influence of fractures can be neglected in finite element modelling. The indicators to determine the REV size based on the joint geometrical parameters include the volumetric fracture intensity (
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) and the fracture tensor, but this type of calculation generally provides a lower bound evaluation. A novel conceptual framework of damage coefficient is introduced in this paper to consider the mechanical properties of fractures, such as joint aperture and roughness. A parametric study has been performed to establish the correlation between the proposed dimensionless damage coefficient and the traditional derived
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value. The effectiveness of the developed method is demonstrated by a case study, where a larger mechanical REV size is indeed calculated based on the damage coefficient.
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