Journal of Geotechnical and Geoenvironmental Engineering

Stratigraphic transitions within the tunnel envelope cause a rapid change in tunnel boring machine (TBM) operating parameters and impose a significant challenge to TBM tunnel projects. This paper presents a geostatistical modeling–based probabilistic approach to quantify the stratigraphic transition location uncertainty in the longitudinal direction of tunneling and at the tunnel face. Geotechnical data from a soil tunneling project were used to elucidate the capability of the approach. A deterministic soil profile and TBM operation data from the project were examined to evaluate the results of the approach. The results revealed that the stratigraphic transition location uncertainty (for 90% confidence interval) in the longitudinal direction, for the two stratigraphic transitions within the tunnel envelope, extended over a longitudinal distance of 28 rings and 130 rings. The occurrence of the stratigraphic transitions in the longitudinal direction at Ring 275 and Ring 550, for a 95% occurrence probability ($P_{95}$), matched the locations of the change in rate of dissipation of chamber pressure from recorded TBM data. The stratigraphic transitions ($P_{95}$) occurred 15 rings before and 30 rings after the locations suggested in the ground profiles suggested in the geotechnical baseline report (GBR). At the tunnel face, the probability of encountering stratigraphic transitions was the highest between Ring 200 and Ring 300, and varies between 60% and 90%.

To mitigate foreshore erosion, it is necessary to find effective and environmentally friendly interventions to stabilize slopes. In this study, the microbiota and enzyme mix–induced carbonate precipitation (MEMCP) method was proposed to improve foreshore slopes’ stability and mitigate erosion. In the tests, the volume ratio of bacterial suspension (BS) and urease solution (US) varied to obtain the optimum condition. The angles of slopes, accumulative soil loss weights, surface strengths, and calcium carbonate (${\mathrm{CaCO}}_3$) contents were used to evaluate the treating effect. The results showed that the slopes treated with microbially induced carbonate precipitation still experienced a drastic collapse. The slopes treated with enzyme-induced carbonate precipitation had higher stability at the beginning; however, the slopes were still damaged in subsequent tidal cycles. However, with MEMCP treatment, the slopes’ stability was significantly improved, especially for the slopes with the volume ratio of BS to US equaling $20:20$. The angle of these slopes almost did not change, and a small amount of sand was washed out by tidal cycles. Moreover, these slopes had higher surface strengths and ${\mathrm{CaCO}}_3$ contents. In addition, the increase in ${\mathrm{CaCO}}_3$ content resulted in an exponential increase of surface strength, regardless of volume ratios of BS and US. The MEMCP method applied in this study for foreshore slope stabilization has shown a success. The research lays a solid foundation for the application for foreshore surface erosion mitigation.

Thixotropy is a very important and unique property of soft clay in engineering, which has been drawing more and more attention in recent years. This study investigated the thixotropic strength recovery of representative marine clays in different sites of offshore China. The microstructure changes of clay during thixotropy were observed using scanning electron microscopy, and then related microscopic characteristics were analyzed. Probability entropy, $E_p$, is proposed as a new indicator based on the statistics of the alteration of diameter and orientation of particles or aggregates. The results show that thixotropy had significant effect on the strength increase for all the offshore Chinese clays that were tested, but their strength regain with time varied between the different sites. Some good correlations were developed between the thixotropy strength ratio and index parameters, particularly for water content, plasticity index, liquidity index, and sensitivity. The microstructural evolution of the clay tested during thixotropy was characterized by the change in both the diameter and the arrangement of particles or aggregates. The probability entropy was proved to quantitatively describe and analyze the microstructural evolution during thixotropy of clay, and its value increases with time. This new indicator could help to provide deeper insight into the thixotropic mechanism of clay.

Energy piles, a new type of heat exchanger that serves dual purposes, have gained increasing attention due to the growing energy demand and corresponding carbon emissions. Depending on intended operational requirements or by accident, energy pile groups may be subjected to cyclic and variable thermal loadings. This study presents the results from a series of full-scale field tests of a $2\times 2$ energy pile foundation. The energy pile group was operated partially or fully during the tests to investigate the potential effects of cyclic nonuniform thermal loadings. The results indicated that the cyclic thermal loadings could induce an increase in compressive stress of piles at the end of the experiments. Further comparative analysis showed that the residual compressive stress was attributed mainly to the drag-down effects of the surrounding soil. The compressive stress induced by drag-down effects will overlap with the thermally induced stress, and thus lead to a more significant effect on the energy piles. If the drag-down effects are presented, the stress change may exceed the theoretical upper bound of thermal stress and may be underestimated. In addition, cyclic nonsymmetrical thermal loadings could induce accumulation of differential settlement and group tilt in the energy pile group. It was shown that the group tilt caused by cyclic and variable thermal loadings can be acceptable for general engineering structures. The group tilt will not accumulate further when subjected to symmetrical thermal loadings. A reasonable arrangement of operational piles still can be used to avoid unwanted group tilt generation.

This work introduces a pilot-scale experiment conducted for 18 months to study the soil heating performance of solar collectors in Japan for borehole thermal energy storage and thermal consolidation applications. In this experiment, water was heated using a solar collector and then circulated in three heat exchangers inserted into a $2.63\text{-}{\mathrm{m}}^3$ box of kaolin clay installed in sandy ground. The solar collector had a heat collection area of $4{\mathrm{m}}^2$ and was attached to a 200-L water tank. The tank containing the clay was internally lined by a 6-cm-thick box of polyethylene foam to reduce heat exchange with the environment. The average clay temperature reached a maximum value of 41.1°C in the summer and a minimum of 14.9°C in the winter. The circulation water temperature varied seasonally between a daytime maximum of 77.5°C in summer and a daytime minimum of 22.5°C in winter. The experiment was simulated using a finite-element analysis model based on thermal conduction. The model used the Dittus–Boelter equation to model the heat exchangers’ energy output and adopted an environmental heat exchange boundary condition using ambient temperature, wind speed, and total daily solar flux data. The numerical results accurately predicted the ground and average kaolin temperature within a 2°C–3°C margin of error. The numerical and experimental spatial distributions of kaolin temperature suggest that gravity and temperature-induced moisture transport was sufficient to measurably change the kaolin water content. Additionally, the numerical results of the sandy ground temperature revealed its sensitivity to variation of moisture content induced by evapotranspiration. Last, changing the boundary condition of the kaolin box to thermal insulation revealed that environmental heat loss accounted for a 13.5°C–19.5°C difference in the average kaolin temperature between the numerical and experimental results, emphasizing the importance of adequate thermal insulation in ground heating applications.

This paper presents the computations of vertical bearing capacity factors $N_c$, $N_q$, and $N_{\gamma }$, of the smooth and rough strip, circular and ring footings resting on soil with a friction angle ($\varphi$) ranging from 5° to 45° using a finite element viscoplastic strain method obeying the Mohr-Coulomb yield criterion. The numerical difficulty could be improved to some extent by using the proper magnitude of soil dilation angle ($\psi$) in the analysis of high non-associative flow. The effects of domain and mesh size are presented thoroughly. The factors are calculated individually and found to be in close agreement with the existing solutions. However, differences are also reported and discussed. The magnitude of bearing capacity factors increases with increasing $\varphi$. Moreover, the factors with a rough base are significantly greater than the smooth base at high values of $\varphi$. Moreover, this work also indicates that the shape factors may depend on $\varphi$.

Deep cement mixing (DCM) is an environmentally friendly technique for offshore ground improvement without dredging or much disturbance to the marine ecological system. Several field construction factors can influence the unconfined compressive strength (UCS) of cement-stabilized soil. In this study, key construction factors are evaluated referring to site investigation records, construction records, and quality test results of a large offshore DCM construction project. The key factors were attributed to geological conditions, construction procedures, and curing conditions. Specific field construction factors include fluctuations of tidal level, original soil type, volume fraction of injected water, volume fraction of injected cement slurry, injection rate of cement slurry, penetrating and mixing time per meter, curing age, and moisture content. The importance of these construction factors on the DCM strength has been quantified using a statistical method based on construction records. The injected water volume and original soil type are noted to be the two most dominant factors on the UCS of the treated soils. Longer mixing time improved the strength of the treated soils.

Considerable bridge-ground interaction effects are involved when evaluating the consequences of liquefaction-induced deformations. Due to seismic excitation, liquefied soil layers may result in substantial accumulated permanent deformation due to the sloping ground near the abutments. Ultimately, global response of the bridge is dictated by soil-structure interaction considerations of the entire bridge-ground system. Of particular interest is the scenario of narrow waterways where interaction between downslope deformations at both ends of the bridge takes place. In order to highlight the involved salient mechanisms, this study investigates the longitudinal response of an overall bridge-ground system. For that purpose, a full three-dimensional (3D) finite-element (FE) model is developed, motivated by details of an existing narrow waterway bridge-ground configuration. As such, a realistic multilayer soil profile is considered with interbedded liquefiable/nonliquefiable strata. Specific attention is given to global response of the bridge structure as an integral entity due to ground deformation in the vicinity of the abutments. The overall results indicated that downslope deformations at both ends of the bridge may result in significant interference within the narrow waterway central section. As such, assessment of deformations at each slope separately might lead to unrealistic outcomes. Generally, the analysis technique as well as the derived insights are of significance for the seismic response of bridge systems with downslope ground deformations.

Excavation of a 16-m deep shaft was suspended due to groundwater drawdown of about 4.5 m that led to nearby ground subsidence and settlement of infrastructure. As a remedial measure, a deep-ground recharge system comprising multipoint recharging wells was conceived and then designed to mitigate the detrimental effects caused by the groundwater drawdown on nearby infrastructure. In the three-dimensional (3D) finite-element model, fully coupled flow-deformation analyses have been successfully developed and used. The results of the numerical analyses show that the predicted and measured ground and groundwater responses achieved reasonably good agreement due to (1) successful understanding of the anisotropy of underlying site condition, (2) sound 3D modeling technique, and (3) sound engineering remedial design. The simulation of this case study evidenced the detrimental effect of anisotropy in permeability of organic soil during groundwater drawdown, in which the soil permeability in the horizontal direction is, on average, five times higher than the permeability in the vertical direction.