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Vacuum preloading has been widely used to improve soft soils in coastal areas of China. An increasing amount of evidence from field operations has shown that conventional vacuum preloading is prone to clogging in prefabricated vertical drains (PVDs) and demands a large volume of sand fills. In recent years, air-boosted vacuum preloading has been developed to overcome these limitations; however, this method still requires more data to verify its performance. In this study, a field test for air-boosted vacuum preloading was conducted, and a large-strain two-dimensional (2D) finite element (FE) model was developed and validated against the field test data. Then, a series of FE parametric analyses was performed to assess key factors, i.e. the air injection pressure, the injection spacing, and the characteristics of cyclic injection, which affect the performance of the air-boosted vacuum preloading. The results showed that the ground settlement and lateral displacement of the soils increased due to an increase in the injection pressure, a decrease in the injection spacing, or increases in the number and duration of the injection cycles. Based on the parametric analyses, an empirical formula for ground settlement prediction was proposed and compared with a case history reported in the literature, showing good agreement.

Many uncertain factors in the excavation process may lead to excessive lateral displacement or overlimited internal force of the piles, as well as inordinate settlement of soil surrounding the existing bridge foundation. Safety control is pivotal to ensuring the safety of adjacent structures. In this paper, an innovative method is proposed that combines an analytic hierarchy process (AHP) with a finite element method (FEM) to reveal the potential impact risk of uncertain factors on the surrounding environment. The AHP was adopted to determine key influencing factors based on the weight of each influencing factor. The FEM was used to quantify the impact of the key influencing factors on the surrounding environment. In terms of the AHP, the index system of uncertain factors was established based on an engineering investigation. A matrix comparing the lower index layer to the upper index layer, and the weight of each influencing factor, were calculated. It was found that the excavation depth and the distance between the foundation pit and the bridge foundation were fundamental factors. For the FEM, the FE baseline model was calibrated based on the case of no bridge surrounding the foundation pit. The consistency between the monitoring data and the numerical simulation data for a ground settlement was analyzed. FE simulations were then conducted to quantitatively analyze the degree of influence of the key influencing factors on the bridge foundation. Furthermore, the lateral displacement of the bridge pile foundation, the internal force of the piles, and the settlement of the soil surrounding the pile foundation were emphatically analyzed. The most hazardous construction condition was also determined. Finally, two safety control measures for increasing the numbers of support levels and the rooted depths of the enclosure structure were suggested. A novel method for combining AHP with FEM can be used to determine the key influencing aspects among many uncertain factors during a construction, which can provide some beneficial references for engineering design and construction.

Twin curved tunnels are often encountered in shield tunnelling, where significant complexities in densely exploited underground space are observed. In this study, the ground settlement and tunnel deformation due to twin-curved shield tunnelling in soft ground were investigated using numerical simulation and field monitoring. Different curvature radii of twin curved tunnels and subsequent effects of tunnel construction were considered to reveal the tunnelling effect on ground surface settlement and tunnel deformation. The results show that the settlement trough yields one offset towards inside of curved shield tunnelling. The location of settlement trough and maximum settlement were affected by curvature radius but except for the shape and width of settlement trough. Adjacent parallel twin-curved shield tunnelling could increase the offset of existing settlement trough and maximum settlement. Then, an empirical prediction of surface settlement trough due to twin-curved shield tunnelling with same tunnel diameters in soft clay was proposed, which was applicable to curvature radius less than 800 m. Finally, a minimum radius of 600 m of curvature tunnel was proposed in terms of allowable convergence deformation of tunnel. The result could provide guidance on safety evaluation for twin curved shield tunnelling construction.

The stability of the shield tunneling face is an extremely important factor affecting the safety of tunnel construction. In this study, a transparent clay with properties similar to those of Tianjin clay is prepared and a new transparent clay model test apparatus is developed to overcome the “black box” problem in the traditional model test. The stability of the shield tunneling face (failure mode, influence range, support force, and surface settlement) is investigated in transparent clay under active failure. A series of transparent clay model tests is performed to investigate the active failure mode, influence range, and support force of the shield tunneling face under different burial depth conditions, whereas particle flow code three-dimensional numerical simulations are conducted to verify the failure mode of the shield tunneling face and surface settlement along the transverse section under different burial depth conditions. The results show that the engineering characteristics of transparent clay are similar to those of soft clay in Binhai, Tianjin and satisfy visibility requirements. Two types of failure modes are obtained: the overall failure mode (cover/diameter: C/D⩽ 1.0) and local failure mode (C/D⩾ 2.0). The influence range of the transverse section is wider than that of the longitudinal section when C/D⩾ 2.0. Additionally, the normalized thresholds of the relative displacement and support force ratio are 3%–6% and 0.2–0.4, respectively. Owing to the cushioning effect of the clay layer, the surface settlement is significantly reduced as the tunnel burial depth increases.

AbstractThere are many tunneling engineering practices all over the world, in which the tunnel boring machine (TBM) is a main construction way due to its serial advantages such as safety, high efficiency and environment-friendly. TBM cutters are the crucial parts, whose interaction is one of main factors to affect the efficiency of TBM. In this article, a two-dimensional (2D) distinct element method (DEM) code, PFC2D, incorporating an improved microscopic bond contact model proposed for rock, is used to investigate the macro- and micro-mechanical responses during the invasion process of cutter. The influences of the cutter’s geometrical parameters, that is, cutter width and cutter angle, and confining pressure, are investigated in study. The DEM results show that: (1) the whole cutter invasion process can be described into three stages: loading stage, unloading stage and residual leaping broken stage; (2) cutter width has more influence on the capability and efficiency of TBM than cutter angle, and both the capability and efficiency of TBM are the best when confining pressure ratio equals to 0.1; (3) consistent with microscopic information, the analyses on specimen failure modes show that cutter with large width and small angle is helpful to rock fragmentation.

Problems continue to be encountered concerning the traditional vacuum preloading method in field during the treatment of newly deposited dredger fills. In this paper, an improved multiple-vacuum preloading method was developed to consolidate newly dredger fills that are hydraulically placed in seawater for land reclamation in Lingang Industrial Zone of Tianjin City, China. With this multiple-vacuum preloading method, the newly deposited dredger fills could be treated effectively by adopting a novel moisture separator and a rapid improvement technique without sand cushion. A series of model tests was conducted in the laboratory for comparing the results from the multiple-vacuum preloading method and the traditional one. Ten piezometers and settlement plates were installed to measure the variations in excess pore water pressures and moisture content, and vane shear strength was measured at different positions. The testing results indicate that water discharge–time curves obtained by the traditional vacuum preloading method can be divided into three phases: rapid growth phase, slow growth phase, and steady phase. According to the process of fluid flow concentrated along tiny ripples and building of larger channels inside soils during the whole vacuum loading process, the fluctuations of pore water pressure during each loading step are divided into three phases: steady phase, rapid dissipation phase, and slow dissipation phase. An optimal loading pattern which could have a best treatment effect was proposed for calculating the water discharge and pore water pressure of soil using the improved multiple-vacuum preloading method. For the newly deposited dredger fills at Lingang Industrial Zone of Tianjin City, the best loading step was 20 kPa and the loading of 40–50 kPa produced the highest drainage consolidation. The measured moisture content and vane shear strength were discussed in terms of the effect of reinforcement, both of which indicate that the multiple-vacuum preloading method has a better treatment effect not only in decreasing the moisture content and increasing the bearing capacity, but also in increasing the process uniformity at different depths of foundation.

In this paper, the effects of the number of load cycles and combined loading frequencies on the dynamic properties of reclaimed soft soil are studied by experimental tests. In these tests, the reclaimed soft soil samples after vacuum preloading are tested using a GCTS hollow cylinder testing device in the laboratory. The results show that the deformation of reclaimed soft soil under combined low/higher frequency is larger than that under combined high/lower frequency. The pore water pressure is found to increase dramatically and exceed 40% in the second stage of cyclic loading, and has significant difference between the combined loading frequencies. The cyclic loading of the first stage has a significant effect on the tendency and stable value of the degradation index under combined loading frequencies. This study shows that the stress–strain hysteresis curves are closer to the longitudinal axis at different vibration frequencies, and most hysteresis loops are not closed with the number of load cycles increasing at a vibration period. Meanwhile, the stress–strain hysteresis curves related with the number of load cycles are discussed at different combined frequencies.

Artificial structural soil is the best medium for simulating the structural characteristics of undisturbed soil, which is a unique property of undisturbed soil. To figure out the macroscopic and microscopic structural properties of artificially structured soil, this study proposed a method of disposing artificial structural soil to simulate in situ marine clay. In addition, a series of one-dimensional compression tests considering three influencing factors—the initial void ratio, interparticle cementation strength (determined by cement content) and pore size distribution (determined by initial salt content)—were conducted to explore the macroscopic structural properties of artificially structured marine clay. Then, SEM and MIP tests were performed to investigate the microstructure properties of artificially structured marine clay. With respect to the macro-scale experiment, the results show as the initial void ratio increases, the yield stress decreases while the compressibility increases in postyield stage. The pore size distribution mainly affects deformation, which increases with the quantity of relatively large pores. In terms of the micro-scale test, we found that the general microstructure is composed of aggregates of small clay platelets, some of which are either attached to the aggregates or linked to the surrounding aggregates with cemented bridges crossing the interaggregate pores. The most and least common pore space types feature entrance pore diameters of 1 ∼ 17 μm and larger than 17 μm, respectively. An increase in cement content causes increases in intra-aggregate pore space and interparticle cementation strength. Initial salt particle content exerts an influence predominantly in the pores with entrance diameters of 0.3 ∼ 1 μm and 1 ∼ 17 μm.

This study tried to explore the ground movement induced by triple stacked tunneling (TST) with different construction sequences. A case study in Tianjin, China was used to investigate the ground movement during the TST (upper tunneling (UT)). For this, a modified Peck formula was proposed to predict the surface settlement induced by TST. Next, three sets of finite element analyses (FEA) were used to compare the effects of construction sequences (i.e. UT, middle tunneling (MT), and lower tunneling (LT)) on vertical and lateral ground displacements. The results of Tianjin case and UT reveal that compared to a Gaussian distribution for a single tunnel, the surface settlement curve of triple stacked tunnels is a bimodal distribution. It seems that the proposed modified Peck formula can effectively predict the surface settlement induced by TST. The results of the three sets of FEA demonstrate that the construction sequence has a significant influence on the ground movement. Among the three construction sequences, the largest lateral displacement is observed in the MT and the smallest one in UT. The existing tunnel has an inhibitory effect on the vertical displacement. The maximum value of the lateral displacement occurs at the depth of the new tunnel in each construction sequence.

The unprecedented rate of metro construction has led to a highly complex network of metro lines. Tunnels are being overlapped to an ever-increasing degree. This paper investigates the deformation response of double-track overlapped tunnels in Tianjin, China using finite element analysis (FEA) and field monitoring, considering the attributes of different tunneling forms. With respect to the upper tunneling, the results of the FEA and field monitoring showed that the maximum vertical displacements of the ground surface during the tail passage were 2.06 mm, 2.25 mm and 2.39 mm obtained by the FEA, field monitoring and Peck calculation, respectively; the heaves on the vertical displacement curve were observed at 8 m (1.25D, where Dis the diameter of the tunnel) away from the center of the tunnel and the curve at both sides was asymmetrical. Furthermore, the crown and bottom produce approximately 0.38 mm and 1.26 mm of contraction, respectively. The results of the FEA of the upper and lower sections demonstrated that the tunneling form has an obvious influence on the deformation response of the double-track overlapped tunnel. Compared with the upper tunneling, the lower tunneling exerted significantly less influence on the deformation response, which manifested as a smaller displacement of the strata and deformation of the existing tunnel. The results of this study on overlapped tunnels can provide a reference for similar projects in the future.

Studies of the effect of clay structure on piezocone penetration test (CPTU) results are beneficial for interpreting CPTU data in naturally or artificially cemented structured clays. The structured Cam clay (SCC) model was implemented into Abaqus, and the validity of the user-defined material (UMAT) subroutine of SCC was verified by predicting the experimental data for the laboratory oedometer test and drained and undrained triaxial shear tests of structured clays. A numerical model of piezocone penetration into structured clay was established, and numerical simulations showed that, compared with those for unstructured clay, the values of the mean effective stress near the cone tip were smaller, and the values of the excess pore water pressure (u) were larger for structured clay due to the additional destruction-associated plastic volumetric strain. The maximum deviator stress did not appear near the cone tip but appeared at a certain distance below and to the right of the cone tip, which manifested the strain softening characteristic of structured clays. The cone resistance (qc) values increased linearly with increasing structural strength (Sσ). The larger the value of Sσwas, the higher the value of u, but Sσdid not influence the patterns in dissipation curves of u. The value of the empirical cone factor (Nkt) decreased with increasing Sσ, since the clay structure caused a smaller increase in qcthan in the undrained shear strength, and a formula for estimating the Nktof structured clays was proposed. Compared with those for overconsolidated clay, the values of mean effective stress and deviator stress around the cone tip were smaller for the structured clay due to structural damage occurring at large strain, which resulted in a smaller value of qc, and the “nonstandard” dissipation phenomenon was less pronounced for the structured clay due to the shear-induced relatively large positive u.

The air-booster vacuum preloading (AVP) method has been adopted for the treatment of ultra-soft soil foundations, especially in land reclamation projects, but the air injection mode has not yet been studied. In this paper, the authors investigated the difference in improvement effect in laboratory tests of two different air injection modes, including variable pressure and constant pressure. The results show that compared with the constant pressure AVP (CAVP) method with constant air injection pressure of 75 kPa, the variable pressure AVP (VAVP) method with variable air injection pressure from 25 to 150 kPa has a better effect on ultra-soft soil improvement. Besides, if the final air injection pressure increases properly, and the air injection pressure increases with the times of air injections slowly, the soil improvement effect is significantly improved. Compared to CAVP method, VAVP method can fully utilize the air injection effect of pressurization by forming closed spaces and enhancing permeability by crack growth.