Acta Geotechnica最新文献

Over the past years, ground densification and soil drains have been commonly used as countermeasures against the consequences of seismic soil liquefaction. However, the state-of-practice for designing such mitigation methods is typically for free-field conditions, without considering seismic soil–structure interaction (SSI) or structure–soil–structure interaction (SSSI). In this paper, three-dimensional (3D), fully-coupled, nonlinear finite element analyses are used to evaluate how the mitigation mechanism affects the seismic performance of adjacent, similar and dissimilar, inelastic, shallow-founded structures on liquefiable sites. A combination of densification with enhanced drainage under and around the entire footing is shown as the most effective strategy to notably reduce the mitigated foundation’s permanent settlement and tilt, regardless of building spacing. Enhanced drainage alone may reduce foundation’s average settlement, but it does not necessarily reduce tilt when near another structure. For the conditions evaluated, the presence of mitigation under one structure is shown to notably amplify the permanent tilt and possibly the flexural deflections of its unmitigated neighbor at shorter spacings, while having a minor impact on its settlement. On the other hand, this study utilizes a small suite of numerical simulations to examine the impact of ground motion characteristics on building engineering demands in both unmitigated and mitigated SSSI models at a spacing of 3 m, focusing on combined mitigation (i.e., densification and drainage). In general, increasing shaking intensity notably increases foundation settlements of both unmitigated and mitigated structures in SSSI models, whereas combined mitigation can effectively reduce the foundation tilts of mitigated structures, regardless of shaking intensity. Most importantly, the treated ground tends to amplify the neighboring unmitigated structure’s tilt. These results indicate that mitigation must be designed with extreme care in urban environments, with the goal of improving the overall performance at a systems level for the building as well as its neighbors.

Thermal conductivity (λ) serves as a fundamental physical property governing heat transfer in soil. In practice, the soil can be subjected to wetting and drying that cause variations in λ. The reason behind such phenomenon is still vague, since the effects of suction path and void ratio (e) were not distinguished in previous limited works, especially for clayey soils. In this study, water retention curve and λ are measured over the full moisture range on wetting and drying path, with influence of void ratio being excluded by special experimental design. Results indicate that under the same e condition, the hydraulic hysteresis is obvious for the water retention curves of clayey soils, due to the contact angle and interlayer cation hydration mechanism. For specimens with identical e, λ upon drying is higher than that upon wetting, probably suggesting that s cannot be directly related to changes in λ. In λ-water content (w) plane, there exists a threshold w: Below this value, differences in λ are noticeable for specimens experiencing wetting and drying at identical w; otherwise, λ is independent of suction path. Microstructural observations from mercury intrusion porosimeter and scanning electron microscope confirm that at s = 71.1 MPa, the specimens exhibit comparatively more and/or larger inter-aggregate pores in drying path, accompanied with clear agglomeration of clay aggregates. It implies that comparing to wetting, drying leads to increase in aggregate size and decrease in physical contacts among aggregates, in which the former enables to widen the heat flow path in aggregate region and the latter signifies reduced thermal resistance, thereby enhancing thermal conduction at microscale and resulting higher λ under relatively low moisture condition. By contrast, the hysteresis loop in λ-w curve is comparatively narrower than that in λ-s curve for clays, due to the hydraulic hysteresis mechanism over the full suction range. This research contributes to clarify the role of moisture on λ for clayey soils upon wetting and drying, and also provide a more comprehensive understanding of thermal conduction in variably saturated soils for geotechnical and geoenvironmental applications.

Beach erosion is a major challenge for coastal cities like Singapore. This is increasingly the case with greater impact from climate change and sea level rise. One of the new approaches in improving beach erosion resilience is biocementation using enzyme-induced carbonate precipitation (EICP). A field study was carried out using an EICP method with crude urease enzyme extracted from soybean. A simplified urease enzyme extraction procedure was developed to use seawater as the solvent. The use of seawater and a modified coarse filtration method enabled the enzyme extraction process to be done on site at an accelerated rate. A hybrid biogrout infiltration technique was also adopted on site to overcome the accumulation of unfavourable bioclogging effect. After the treatment, the strength of the treated slope surface increased to a maximum value of 2 MPa. The field tests also indicated that the simplified EICP method not only increased the resistance to wave or rainwater erosion but also facilitates the grass growth on the treated beach. This unique feature enables the proposed method to be an eco-friendly method too.

The spatiotemporal characteristics of a rockburst, such as the distance from the tunnel face, are critical factors related to early warning and control. Therefore, by using the data from over 100 immediate strain rockbursts (ISRs) in three open tunnel boring machine (TBM) granite tunnels, the relationships among their occurrence locations and TBM shield, their impact and control measures are statistically analyzed. The ISRs in open TBM tunnels are categorized into shield area ISRs (SA-ISRs) and main beam area ISRs (MBA-ISRs). The differences in the spatiotemporal characteristics, rockburst pit evolution characteristics and microseismic (MS) activity laws of these two types of ISRs are further analyzed. IS-ISRs occur earlier, and mainly form V-type pits without expansion. MBA-ISRs occur later, with a high proportion of fossa-type pits with more obvious expansion. MS nucleation in the SA-ISR area is high after excavation, with MS activity increasing rapidly and peaking. In contrast, the MS events in the MBA-ISR area are scattered, and MS activity is weak during excavation, gradually increases over time and continues to accumulate in space. On this basis, the influence mechanism of stress level, lithology, structural planes, and construction factors on the spatiotemporal characteristics of ISRs is studied, with the overall stress level identified as the main factor. When the stress level is high, the degree of energy accumulation increases after excavation, making it easier for SA-ISR to occur. Conversely, the degree of energy accumulation after excavation is lower. However, due to the influence of the nearby steep structural plane, energy continues to accumulates during the stress adjustment process in this area, resulting in a delay in MBA-ISR.

This study investigates the thermo-hydro-mechanical (THM) behavior of argillaceous formations, particularly the Callovo–Oxfordian (COx) claystone, over extended timescales to evaluate the long-term safety of radioactive waste repositories. Numerical simulations were performed as part of a benchmark exercise to study the response of the COx formation under heating scenarios representative of high-level radioactive waste disposal. Six modeling teams from various institutions participated in this benchmark, using various numerical codes, providing valuable information on the evolution of temperature, pore water pressure, and stresses within the repository environment, particularly around the disposal cells. The results highlight that the COx formation exhibits significant thermal pressurization and stress relaxation because of its low permeability, whereas the excavation damaged zone (EDZ) remains confined to the near field and does not extend significantly under the thermal load considered. The study demonstrates the robustness of numerical tools for repository safety assessments and emphasizes the importance of validated THM formulations to ensure long-term containment of radioactive waste.

When shield tunnelling in unsaturated sandy ground (USG), the tunnel face is inevitably subjected to the combined effects of unloading and cutterhead vibration (CEUV), which adversely affects ground stability. To investigate the underlying mechanical mechanisms, a tunnel face unloading (TFU) apparatus was developed, and a series of physical model tests were conducted. The results indicate that the longitudinal acceleration amplitude induced by cutterhead vibration attenuates exponentially in the ground due to the energy dissipation. The TFU causes a chimney-like global failure in the dry sandy ground (DSG), but produces a localized failure with a cavity above it in the USG. Under vibration, the loosening zone in DSG expands in width, while it enlarges both in height and width in USG. The apparent cohesion enhances the tunnel face stability in the USG and facilitates the formation of the self-stabilized arch (SSA), as reflected in stress evolution. Nevertheless, the vibration weakens the soil arching effect, causing an increase in the limit support pressure. Via comparison with previous model tests and numerical simulations, the experimental results are well validated. The formation and destruction of the SSA and the increase of vertical earth pressure under CEUV are theoretically explained by the improved multi-arch model. In addition, the hysteretic failure of cavities in USG poses significant risks to tunnel engineering. Non-destructive physical prospecting methods are recommended to assist in shield tunnelling control and ground remediation.

Treating sulfate saline soil with lime or cement can lead to significant ettringite-induced swelling, resulting in the deterioration of subgrade or foundation layers, particularly in the presence of excessive moisture. This study employed reactive MgO and ground granulated blast furnace slag (GGBS) to treat saline loess to mitigate this adverse swelling. Sodium sulfate saline loess with 4% salt content was stabilized by 8%, 10%, and 12% MgO-GGBS binder with MgO to GGBS ratios of 1:9, 2:8, and 3:7. The properties of the treated soil was assessed through linear swelling test, unconfined compressive strength (UCS), X-ray diffraction (XRD), derivative thermogravimetric analysis (DTG), and scanning electron microscopy (SEM) analysis. The results demonstrated that the UCS of saline loess stabilized with the MgO-GGBS binder increased gradually with the GGBS dosage and the curing time, and the MgO content of the binder exceeded 1% favored the hydration of GGBS and the strength development of cured saline soil. In addition, the linear swelling (%) and strength loss (%) of saline soil stabilized by the MgO-GGBS binder under immersion conditions decreased progressively with increasing MgO dosage. Saline loess cured with the MgO-GGBS binder containing 2% MgO exhibited the lowest linear swelling of 0.08%, while the strength of the cured soil with MgO dosages greater than 2% gradually increased under immersion. The optimal formulation for stabilizing saline loess with 4% salt content was 10% MgO-GGBS binder containing 2% MgO. XRD, DTG, and SEM analysis confirmed that the major secondary reaction products formed in the cured saline soil matrix by the MgO-GGBS binder containing more than 2% MgO included low crystallinity ettringite, S-AFm, Mg–Al–SO₄ LDHs, and MSH gel, which primarily contributed to the reduction in linear swelling of the cured soil. In contrast, the swelling of binder treated soil with less than 1.6% MgO was predominantly due to the formation of ettringite and its water-absorption expansion.

The stability of three-dimensional (3D) cylindrical tunnel vaults in soil strata subjected to a compound tension-shear failure is investigated based on the upper bound limit analysis. A failure mechanism for 3D cylindrical tunnel vaults reflecting the transition process from shallow to deep buried conditions, making the deep buried failure mechanisms a special case, is developed from the modified Mohr–Coulomb (MC) criterion with tension cut-off (TC). By deducing the energy balance equation, the expressions for three stability indices, i.e., the stability coefficient, the factor of safety (FoS) and critical required reinforcement strength, which can be used to quantify the tunnel vault stability, are derived. The optimal upper bound solution of stability indices is obtained using compiler optimization algorithms. The influences of 3D geometric characteristics and soil strength parameters with TC on the tunnel vault stability during the transition process from shallow to deep buried conditions of tunnel vault are revealed. Further, the stability charts for the 3D tunnel vault FoS under different parameters and for the critical reinforcement strength required to maintain the tunnel vault stability under different desired FoS are calculated and plotted. In practical engineering, the corresponding data can be acquired quickly by chart query, which provides a theoretical basis for the 3D preliminary design of cylindrical tunnels.

The concentrated solution sludge (CSS) produced by the immersion combustion evaporation process has extremely high soluble salt content. Ground-granulated blast-furnace slag (GGBS) and municipal solid waste incineration (MSWI) by-products geopolymer is adopted for solidification/stabilization (S/S) of CSS. A series of unconfined compressive strength (UCS) test, water immersion tests, microscopic observation tests and leaching toxicity tests were carried out on the solidified sludge to investigate the S/S effect and mechanism of the prepared geopolymer on CSS. Results show that the MSWI by-products can effectively promote the geopolymerization of precursors and improve the compressive strength and water stability of the solidified CSS. The leaching concentration of heavy metal and PCDD/Fs [polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs)] can satisfy the relevant standards of China. The geopolymeric gels generated by GGBS-based geopolymer can effectively cement and wrap the CSS particles, which is the S/S mechanism of the CSS solidified by the synthetic geopolymer. The energy consumption and carbon emissions of using this geopolymer for solidifying the CSS are much lower than those of using cement/MSWI by-products.

This study introduces a new and efficient methodology for the stability analysis of excavations, accounting for soil variability, by integrating an improved limit analysis (iLA) method with the Polynomial Chaos Kriging (PCK) and Monte Carlo Simulation (MCS) technique. First, the iLA method is proposed for deterministic analysis, incorporating factors such as the soil–wall interface, excavation geometry, and wall embedment depth. This approach allows for the estimation of the basal heave safety factor using the strength reduction method in combination with a bisection optimization technique. The accuracy and versatility of the proposed iLA method are demonstrated through comparisons with numerical simulations and four existing analytical methods. Next, the study introduces an active learning method, PCK–MCS, and integrates it with the iLA method to perform probabilistic analyses of excavation stability. The efficiency and effectiveness of the final integrated methodology, iLA–PCK–MCS, are validated through comparisons with established methods, including direct MCS, Subset Simulation, and Kriging- and Sparse Polynomial Chaos Expansion-based MCS. Finally, leveraging the computational efficiency of the iLA–PCK–MCS framework, a parametric study is conducted to provide insights into the effects of soil uncertainties, the soil–wall interface, and wall embedment depth on excavation stability. The methods presented in this study are expected to advance both deterministic and probabilistic analyses in excavation projects by offering a highly efficient and accurate tool for evaluating basal heave stability.