Soils and Foundations最新文献

A comprehensive study on the stress-dilatancy behavior of cemented sand and its modeling is presented. The effect of confining pressure, relative density, and cement content on stress-dilatancy behavior are studied from the published experimental results and an additional series of experiments performed in this study. To facilitate a contrast and comparison of stress-dilatancy behavior between these datasets, a normalized stress ratio is proposed which removes the effect of mineralogy and morphology of parent sand. A set of key insights were obtained from this comparative study which aided in improving the stress-dilatancy relation; for example, the effect of initial conditions on stress-dilatancy behavior was found to be captured by the ratio of cohesion intercept (or tensile strength) and mean effective stress before shearing. The limitations of stress transformation, often used in modelling of cemented sand, were also systematically studied by a set of carefully designed experiments; it was found to be only applicable before gross yielding of cementation. After gross yielding, it is necessary to take in account of the breakage of bonds/cementation. The gross yield locus was identified from 70 experimental datasets and a cohesion/bond degradation model was formulated to model the stress-dilatancy behavior of cemented sand. The efficacy of stress-dilatancy relations (after including the gross yield locus and bond degradation behavior) is evaluated from the experimental results; the Rowe's stress-dilatancy relation was found to be most suitable with the proposed bond/cohesion degradation model.

Bonding steel plate has been used as a strengthening approach to repair disrupted segmental lining of operational tunnels. This paper introduces numerical investigation into the composite behaviour of the initially deformed segmental tunnel linings strengthened by bonding steel plates using finite element modelling. Cohesive zone modelling was used to simulate the interface bonding behaviour between the segmental linings and steel plates. The full history of the tunnel behaviour before and after strengthening were simulated, where the segmental tunnel lining is initially loaded to create some deformation, then after bonding steel plate, the strengthened tunnel is reloaded until failure occurs. By comparing the results with experimental data from the literature, the proposed model was proved to be capable of simulating the strengthened lining behaviour and able to capture the strengthening failure process in terms of the interface debonding. Subsequently, the segmental lining response and interface shear stress distribution and propagation were analysed to interpret the interaction and failure mechanism of the steel plate strengthened segmental linings. The influence of the initial deformation and the steel plate thickness were investigated and discussed in terms of the strengthened stiffness and capacity. It has been found that the interface shear stress concentration occurred at the positions of the segment joints, where bond damage first initiated. The ultimate failure of the steel plate strengthening happened suddenly once a local debonding zone close to the segmental joint was formed. In addition, the predicted results indicate that a delay in strengthening would result in an increase in strengthened capacity but a decrease in strengthened stiffness. By using thicker steel plates, the strengthened stiffness was improved, while the strengthened capacity could be improved only if the thickness was relatively thin.

The construction process of a cast-in-situ bored pile is too complicated to be described by the cavity expansion theory with a single process. An exact unified semi-analytical solution for both cylindrical and spherical cavities reverse expansion after unloading in drained soil is developed. The non-associated Mohr-Coulomb model and definition of logarithmic strain are adopted in the reverse plastic zone. This model can be used to solve the stress and displacement fields of the soil around a bored pile along both the horizontal and depth directions. Parametric analysis shows that the effect of the unloading phase does not change the ultimate pressure of cavity reverse expansion compared with in situ expansion. The cavity cannot re-expand to its initial radius even though the cavity pressure reloads to the initial value. The increase of internal friction angle, cohesion, and Young's modulus has a positive effect on radius recovery, while the dilatancy angle has a negative effect. A simulation of the construction process of cast-in-situ bored piles is presented, where the role of boring, mud wall protection, and concrete placement is defined. An example that describes the stress and displacement fields around a pile shows that the total radial displacement of soil around the pile is dominated by contraction displacement. And it is closely related to depth and horizontal distance. The results of both parameter analysis and example analysis demonstrate that a low reverse cavity pressure corresponds to a stress-reduction area surrounding the cavity.

On the 30^th of October 2020, a 6.6 magnitude earthquake occurred 14 km north of Samos Island, causing 119 casualties (117 in Izmir, Türkiye, and 2 in Samos, Greece) and significant damage in the 3^rd biggest city of Türkiye, Izmir. Although the city is roughly 70 km far away from the epicenter, the damage was significant and concentrated in the city center settled on alluviums. This paper aims to analyze the distribution of damage in Izmir province, by crosschecking the recorded motions, the subsoil conditions and the evidence of damage as collected by an ad-hoc on-site reconnaissance. The intrinsic behavior of the Samos earthquake was investigated by employing three different ground-motion prediction equations. The results of the analyses revealed that site effects play a significant role in the amplification of ground motions, and valley effects are responsible for the concentration of damage. The damage in buildings was classified in terms of the intensity and structural typologies for the 30 districts of Izmir metropolitan area. In-depth analysis of the distribution of damages revealed that the earthquake caused damage all over the boundaries of Izmir province, and the concentration of damage in Bornova and Karşıyaka districts has a clear correlation with double resonance effects.

The Thilawa Port Development Project is the first large-scale development project to be carried out in the area of sedimented soft clays along the Yangon River in Myanmar. As there is little information about the soft clays sedimented along this river, the mechanical properties of the soft clays in the Thilawa Port area were summarized and compared with the properties of clays in other countries from the perspective of the geotechnical design. It was shown that the high-quality data obtained by the samples taken by a fixed piston sampler were essential to properly determining the design parameters of the soft clays in the Thilawa Project. The relationship between the strength and consolidation history of Thilawa clay was clarified from the results of unconfined compression tests and consolidation tests conducted on the high-quality samples. Furthermore, it was shown that the clay at shallow depths became overconsolidated due to desiccation stress and that its compressibility decreased. The relative characteristics of Thilawa clay were clarified by comparing the geotechnical parameters related to the strength and consolidation-compression properties of the Thilawa clay with those of representative clays of Eastern and Southeastern Asia.

In an attempt to go beyond the conventional laboratory experiments widely reported in literature around the emerging technique of soil bio-cementation, this work addresses key challenges related to its large-scale application. Precisely, a state-of-the-art installation with a draining bottom boundary is introduced and a novel treatment strategy, based on ex-situ hydrolysis within a 1000 L bioreactor, is described. Hydrolyzed solutions are injected in a tank filled with 0–4 mm sand, via a system of eight injection tubes to treat a total surface of 40 m² across a depth of 2 m. A multilevel, spatial and temporal quality control system is used to monitor the injection processes across several cycles via chemical and hydraulic means. In total, 20.8 m³ of reactant solutions are supplied to the targeted zone, equal to one pore volume and over 120 chemical analyses are carried-out. Reaction efficiencies overall exceeded 80%, while by increasing the number of treatment cycles, and thus calcification levels, a gradual increase in the recorded pressure at the injection inlet was captured, that reached up to 75 kPa. Zones where the injection pressure increased the most are found to yield better resistance in the vicinity of the corresponding injection tube. A dynamic penetrometer campaign reveals that increase in the tip resistance, is found to exceed 5 MPa and yields more homogenous response across the bottom 0.5 m of the tank, which is believed to reflect the effect of initial confinement on the deposition of calcite. For the zones with the highest cementation, correlated φ’ values yield a 5° increase, while the oedometric modulus is found to double. The results suggest that ex-situ bio-cementation, where hydrolysis occurs in bioreactors instead of inside the soil mass, is capable of yielding similar precipitation efficiencies and mechanical improvement compared to traditional bio-cementation, where bacteria are injected directly into the soil. Finally, the monitoring of MICP at the scale of typical geotechnical works is discussed along with the problematic of residual ammonium, which in this study is found to reach absorded quantities of 4 mol/L.

A simulation framework based on meshless method has become an alternative numerical tool to model many deformation problems in geotechnical engineering. Large deformations in the failure zone can alter porosity, permeability etc., which in turn can affect the process of the failure. In this paper, a two-phase model in the framework of Smooth Particle Hydrodynamics (SPH) is introduced to model the interaction between water and soil through drag forces according to Darcy’s law. Changes in soil porosity and associated permeability are automatically adjusted within this framework. Firstly, two different problems i.e., flow through porous media and fluidized bed problem, are investigated to examine the suitability, and stability of the proposed SPH method. Then, the stability analysis of a soil slope under different water level conditions is performed with the strength reduction technique, and the groundwater effect of the slope is simulated. It is found that owing to the negative impact of seepage on soil slope, the horizontal displacement of the slope can be significantly larger. Afterwards, the influence of the variable permeabilities on the slope failure is investigated. The simulation results show that the change in permeability has a slight effect on the slope. Although the calculated safety factor does not change, the sliding distance differs by about 10%. The initial porosity has a large negative influence on the stability of the slope. The developed SPH model has been shown to be a valuable and effective tool for modelling complex problems that are challenging to be addressed with traditional approaches.

Recently, environmentally friendly soil reinforcement and stabilization techniques, used to reconstitute weak expansive soils, are on the rise, calling for an in-depth analysis of the consolidation projections on the engineering structures built on them. This study investigated one-dimensional consolidation coefficients by conducting a series of oedometer tests on expansive soils reinforced with basalt fibers of different lengths, stabilized with rice husk ash (RHA) as an environmentally friendly cement-reducing aggregate, and nominal dosages of cement in specified combinations. The correlation between the coefficients of consolidation (c_v), volume change (m_v), and permeability (k) and different basalt fiber lengths and RHA-cement contents in ultimate soil composite material was quantified using equations and graphical forms. Furthermore, scanning electron microscopic imagery (SEM) was conducted to examine the structural modifications within the reinforced and stabilized soil specimens upon one-dimensional consolidation. The results showed that basalt fiber-reinforced specimens, comprised of 5% RHA and 3% cement mixtures, showed the lowest one-dimensional consolidation coefficients with a notably greater reduction at high-stress states than the control specimen. Additionally, the coefficients of volume change (m_v) and permeability (k) decreased with the increased compactive effort, with a clear and significant reduction in the basalt fiber-reinforced stabilized soil composites. This study also proposed the best material combination scheme and analytical equations for evaluating the c_v, m_v, and k considering basalt fiber lengths at different pressure levels. The ultimate soil composites had superior properties, and thus, can be used as fill or subbase material for such engineering structures as embankments, pavements, and foundations.

In practice, post-liquefaction strength is largely determined from case histories, but the case history database is limited and there are gaps in the information available in each case. As a result, laboratory tests have continued to play a key role in determining how various factors influence post-liquefaction strength. This paper presents an investigation into how factors such as particle size, particle size distribution, fines content, and compressibility influence post-liquefaction strength in simple shear tests. Eleven particle size distributions of a natural soil and a tailings material, ranging from silt to fine gravel, were studied. The results were compared with case histories.

Materials of different geologic origins have meaningfully different post-liquefaction strengths. Particle size and particle size distributions were both found to significantly affect post-liquefaction response as well. Post-liquefaction strengths obtained from simple shear tests agreed well with those back calculated from case histories. Laboratory testing can be utilized to determine how the post-liquefaction strength of a given material may compare to the case history database, and can be used to guide design decisions. This is an important practical finding, given the sparse case history database.

In-situ cement-mixing lattice-shaped ground improvement (lattice wall) is one of the effective countermeasures for liquefiable grounds. However, its high cost hinders its wide applicability. This is mainly due to the conventional concept of seismic designs which do not allow any liquefaction of the ground against earthquakes.

The recent seismic design code was revised to comply with the concept of performance-based design, which allows some displacement or slight damage to structures, such as pile foundations, during major earthquakes. In order to apply lattice walls to meet the concept of the recent design standard, especially against major earthquakes, it is necessary to establish a rational design method that considers the quantitative effect of lattice walls.

In this study, therefore, a series of 1-g field shaking table tests was performed with a lattice wall, and the effect of the wall was carefully evaluated through the quantitative measurement of the stress–strain relationship of the liquefiable ground inside the lattice wall and in the free ground (without a lattice wall). It became possible to quantitatively examine the effect of the wall by installing small accelerometers into the ground with precision.

Two major positive effects of the lattice wall were observed through the series of shaking table tests. One was the delay in the onset of liquefaction by the restriction of shear strain, and the other was the recovery of the shear stiffness of the ground even after the onset of liquefaction. These experimental results indicate that lattice walls can be applied as an effective liquefaction countermeasure method, especially when the performance-based design is applied to address large earthquakes.