A catastrophic displacement of a waste dump occurred on December 20, 2015, in Guangming New District, Shenzhen, China. During this event, approximately 2.51 × 106 m3 of construction waste slid out of ...the dump site affecting approximately 0.38 km2, destroying 33 buildings and killing 77 people. This study tries to reconstruct the dump structure prior to the failure event, including evolution of dump geometry, geotechnical profile, hydrogeological conditions, and physical properties of fill materials. The dumpsite had been an abandoned quarry before receiving construction wastes beginning in February 2014. Within 22 months, 5.83 × 106 m3 of construction wastes, some of which had high water content, filled the dumpsite forming a 110-m-high loose fill. The dominant component of the fill was completely decomposed granite (CDG) excavated from the adjacent underground construction. The dumping operation at the site was characterized by rapid filling, inadequate compaction and poor water drainage. Poor compaction on the fill materials resulted in low compactness. The measurements of soil density showed that over 80% of the collected CDG samples had a relative density (Dr) from 4% to 67%. The dynamic penetration tests conducted on the landslide bed indicated that the fill materials were loose to slightly dense at most of the penetration depth. No effective drainage facilities were provided at the dump site. A large quantity of water from a catchment area of 0.51 km2 readily poured into the dump, resulting in a significant rise in the phreatic surface. The rapid filling rate of 5–7 m/month was very likely to have caused a build-up of excessive pore-water pressure in the lower poorly drained fill, which was verified by borehole exposure of the confined groundwater and the evaluation of consolidation degree based on the measurements of soil density. The fill materials within the landslide bed were found to have a consolidation degree of 33% on average. A deep-seated translational failure occurred on the lower poorly consolidated fill layer, forming a very-fast-moving flow slide.
•The evolution of dump geometry and the geotechnical profile were reconstructed.•The dumping operation was revealed to be fast with inadequate compaction.•Most of the fill materials samples had a relative density less than 67%.•Excessive pore-water pressure in the lower poorly-drained fill was verified.•A deep-seated translational failure occurred on the poorly consolidated fill layer.
•Shield tunneling-induced soil settlement is obtained by an FBG-based sensing system.•A time-series dataset is set up for ANN-based dynamic prediction of soil settlement.•A novel settlement ...prediction method is proposed by integrating newly predicted data.•The prediction results by the proposed method meet well with field monitoring data.
The subway systems have greatly released the pressure of ground traffic, but shield construction will cause considerable disturbance to the surrounding soil. Consequently, uneven settlement on the adjacent ground surface will be induced, which will affect the surrounding buildings. Thus, the tunneling-induced soil settlement should be obtained in time for safety assessment. In this study, the soil settlement induced by shield construction was monitored using the fiber Bragg grating (FBG) sensors which had the advantage of small transmission loss, multiplexing capability, and capacity for long-term monitoring. The time-history of the soil settlement of multiple points along a monitoring section was obtained. Furthermore, the back propagation neural network based on time series (TS-BPNN) was adopted to predict the soil settlement in later periods. The influence of the model structures and the number of training data on the prediction accuracy was investigated. The forecasting of a five-day settlement during rapid settlement and slow settlement stages were carried out and verified respectively to evaluate the performance of the proposed approach. The maximum relative error of the forecasted results was 1.35%, reflecting the favorable consistence with the monitored results. This study indicates that the method proposed could provide a valuable reference for the displacement control of ground settlement during the shield tunneling construction.
•A large-scale (5m×5m×7.5m) bioreactor experiment was conducted on unprocessed high food waste content (HFWC) MSW.•A large quantity of self-released leachate was collected within two months after ...waste filling.•A rapid build-up of high leachate level was observed after waste filling.•A linear increase of compression strain with an increase of leachate draining rate was found.
A large-scale bioreactor experiment lasting for 2years was presented in this paper to investigate the biochemical, hydrological and mechanical behaviors of high food waste content (HFWC) MSW. The experimental cell was 5m in length, 5m in width and 7.5m in depth, filled with unprocessed HFWC-MSWs of 91.3 tons. In the experiment, a surcharge loading of 33.4kPa was applied on waste surface, mature leachate refilling and warm leachate recirculation were performed to improve the degradation process. In this paper, the measurements of leachate quantity, leachate level, leachate biochemistry, gas composition, waste temperature, earth pressure and waste settlement were presented, and the following observations were made: (1) 26.8m3 leachate collected from the 91.3 tons HFWC-MSW within the first two months, being 96% of the total amount collected in one year. (2) The leachate level was 88% of the waste thickness after waste filling in a close system, and reached to over 100% after a surcharge loading of 33.4kPa. (3) The self-weight effective stress of waste was observed to be close to zero under the condition of high leachate mound. Leachate drawdown led to a gain of self-weight effective stress. (4) A rapid development of waste settlement took place within the first two months, with compression strains of 0.38–0.47, being over 95% of the strain recorded in one year. The compression strain tended to increase linearly with an increase of leachate draining rate during that two months.
Geotechnical engineers face several challenges when constructing embankments over soft soils. These include potential bearing failure, intolerable settlement, and global or local instability. Piled ...embankments provide an economic and effective method to construct roads on soft soils. Soil arching developed within such embankments has significant influence on its behavior. A total of 15 model tests were conducted to evaluate the effects of pile–subsoil relative displacement, embankment height, cap beam width and clear spacing, and geosynthetics with different tensile strengths on stress concentration ratios and settlements in the embankments. The test results indicate that stress concentration ratio varies with pile–subsoil relative displacement and has upper and lower bounds. A higher ratio of embankment height to cap beam clear spacing, as well as a higher ratio of cap beam width to clear spacing, would result in a higher stress concentration ratio. The inclusion of a geosynthetic membrane can increase the stress concentration ratio. When the embankment height to the cap beam clear spacing ratio,
h/
s, is less than 1.4, apparent differential settlements may occur on the surface of embankment. When
h/
s is greater than 1.6, however, no apparent differential settlements will occur on the embankment surface. In addition, experimental results were also compared to several current design methods.
The role of wettability, often characterized by contact angle (
θ
), in two-phase immiscible phases displacement is not well understood. In this study, the color gradient lattice Boltzmann method ...(LBM), capable of maintaining the prescribed
θ
(from 0° to 180° at intervals of 10°) throughout the numerical simulations, was used to investigate the displacement patterns and displacement efficiency in a 2D porous medium. The capillary numbers (
Ca
) used were 0.01, 1, and 100, and the viscosity ratios (
M
) used were 0.1, 1, and 10. At
M
=10, the saturation (
S
) had a bilinear relationship with
θ
, while for
M
=0.1 and 1, the
S-θ
relationships were complicated by
Ca
. A saturation contour in the
M-Ca-θ
space was proposed to demonstrate the movement of a traditional 2D
M-Ca
phase diagram with
θ
increments. The value of
S
continued to increase after the breakthrough, and the final saturation (0.997) for the hydrophilic condition (
θ
=10°) was higher than that (0.673) for the hydrophobic condition (
θ
=170°).
This paper presents a computational sand model based on the well-known pressure dependent multi-surface constitutive model to solve the necessity of employing a separate set of model parameters for ...each soil relative density change. The proposed model correlates the original model parameters with the soil relative density through critical-state-based soil mechanics formulations to provide a single set of model constants that adapt to different soil states. Model formulation updates are performed for the flow rules, material moduli calculations, and the computation of stress ratios at the phase transformation and failure stages. The model parameters are calibrated for Ottawa F-65 sand against cyclic soil element tests with different stress levels and various soil densities. Thereafter, numerical simulations are conducted for centrifuge experiments of gently sloped grounds to validate the proposed model. Throughout numerical simulations, the proposed model accurately replicates the sand cyclic undrained behavior as similar to laboratory-measured responses for different soil relative densities with a single calibrated set of model parameters and provides reliable numerical predictions in finite element simulations of the engaged centrifuge experiments. Overall, the proposed model robustly simulates the saturated sand seismic response, which can improve the numerical prediction accuracy of liquefaction-induced damage in different engineering applications.
This paper aims to develop an improved CRR-Vs1 characterization model for soil liquefaction evaluation for binary mixtures via a series of DEM simulations of drained monotonic tests, undrained cyclic ...tests and shear wave velocity measurements. The contributions of different contact types to the mean effective stress of binary mixtures depend mainly on the coarse content and to a negligible extent on the confining pressure and particle size ratio. The threshold coarse content, denoting the mechanical behavior transition of binary mixtures, can be identified either by the macroscopic evolutions of critical state lines or the distributions of microscopic contact forces. The skeleton reinforcement factor m decreases approximately linearly with the increase of coarse content, while the skeleton supporting factor b exhibits a nonlinear decrease relationship with coarse content, both of which are influenced by particle size ratio. Under the condition of the same equivalent skeleton void ratio, a Vs-correction function and a CRR-correction function are introduced to consistently describe the nonlinear coarse content effects on the shear wave velocities and liquefaction resistances of binary mixtures. The improved CRR-Vs1 characterization model with consideration of coarse content is established by relating the shear wave velocity and liquefaction resistance of binary mixtures to those of the base fine or coarse matrix using the concept of equivalent skeleton void ratio. Application of the proposed characterization model to real geotechnical materials requires laboratory element tests to determine the Vs-correction function and the CRR-correction function, together with the CRR-Vs1 benchmark curve of the base fine or coarse matrix.
•The threshold coarse content of binary mixtures is identified by DEM simulation.•The equivalent skeleton void ratio depends on coarse content and particle size ratio.•The CRR-Nl curves for binary mixtures rotate clockwise due to the reinforcement effect of coarse particles.•The improved CRR-Vs1 characterization model for liquefaction resistance evaluation of binary mixtures is proposed.
An innovative cycled column test with supporting batch equilibrium and kinetic analysis for adsorption or desorption were developed for evaluation of adsorption behavior of soils. Non-equilibrium ...adsorption was observed in the cycled column tests as the traditional testing methods. The isotherm of local equilibrium of the soil was conducted based on the testing results within a relatively short duration with simple analysis. The concentration curves of influent and effluent of the cycled column tests were simulated by dual-porosity (DP) model with a modified inlet boundary. Based on the modeling results, the isotherm of local equilibrium is close to that of the mobile phase adsorption capacity, whereas the immobile phase of the soil is nearly inactive in the retardation of the contaminants. The testing results from cycled column tests are hardly interfered by desorption or the sorption rate according to the modeling for corresponding scenarios. The cycled column test can be used as an alternative or supplementary method to the traditional column test for the determination of local equilibrium isotherm, with advantages of shorter testing duration and easier data analysis.
Display omitted
•Cycled column test was firstly performed for the determination of adsorption behavior of the soils.•The test results from cycled column tests are hardly interfered by desorption or the sorption rate.•The adsorption capacity of immobile phase of the soil is hard to be fully activated in contaminant retardations.•The cycled column test is effective to gain the adsorption isotherm of local equilibrium within a relative short duration.
Summary
Shield tunneling is an efficient method of the tunnel construction for the system of underground transportation, but it will disturb the surrounding soil and affect the security of nearby ...structures. The monitoring for deformation of the structures influenced by the tunneling construction is crucial for structural safety evaluation, especially for the ancient structures. The computer vision‐based structural deformation monitoring approach is a newly developed method that has advantages such as, long‐distance, noncontact, and so on, which provides a promising tool for deformation monitoring of ancient structures. This study applied a computer vision‐based method for field monitoring of three‐dimensional (3‐D) deformation of an ancient tower under the influence of shield tunneling. The target‐free strategy was adopted to eliminate the need for installation of artificial makers on the surface of the tower. The environmental factors during the long‐term monitoring were discussed, especially the influence of temperature variation. Background modification was employed to reduce the error induced by the microchange of camera position. The long‐term 3‐D deformations of the ancient tower including vertical settlement and horizontal deformation in two directions during the shield tunneling construction were obtained. The results indicate that the proposed computer vision‐based method offers an efficient and cost‐effective approach to monitor the 3‐D deformation of the ancient structure and further for structural safety evaluation.
Deep soil mixing (DSM) grids are widely used as effective countermeasures against liquefaction hazards. A simplified two-dimensional, nonlinear, finite element model involving DSM grid-improved ...ground is proposed to investigate the shear stress responses of the enclosed soil. A numerical model with several approximations of longitudinal walls was carefully validated by a centrifuge model test. The waist-shaped depth distribution of the horizontal cyclic shear stress acting on the enclosed soil (i.e., the “waist effect”) was revealed and well explained according to the characteristics of dynamic soil-grid interactions. Attention should be given to the increasing shear load sustained by the underlain layer, which may lead to an unexpected failure. A mathematical model for the shear stress reduction ratio was proposed based on a comprehensive numerical parametric analysis considering varying geometry and shear modulus of the DSM grid with a reasonable range. The proposed evaluation method, along with conventional liquefaction-triggering analysis, can be utilized for the seismic design of DSM grids in liquefaction mitigation. The previous design methods without considering the “waist effect” underestimate the overall horizontal cyclic shear stress acting on enclosed soil.
