Yuanqiang Cai

Simplified Approach for Reliability-Based Design against Basal-Heave Failure in Braced Excavations Considering Spatial Effect

This paper presents a simplified approach for reliability analysis of basal heave in a braced excavation considering the spatial variability of soil parameters, The first-order reliability method (FORM) with a variance reduction technique is employed to model the spatial variability in lieu of the conventional random field modeling (RFM). The proposed approach yields results that are comparable with those obtained using the conventional RFM approach that relies on Monte Carlo simulation. The proposed approach requires much less computa- tional effort, is easy to use, and has potential as a practical tool for reliability-based design that has to deal with spatial variability of soils. DOI: 10.1061/(ASCE)GT.1943-5606.0000621.

Improved Vacuum Preloading Method for Consolidation of Dredged Clay-Slurry Fill

AbstractAn improved vacuum preloading method was developed to consolidate clay-slurry fill that is hydraulically placed in seawater for land reclamation. This method adopted both a vacuum and an air-pressurizing system. In the vacuum system, a new connector to connect each prefabricated vertical drain (PVD) directly to a horizontal vacuum pipe is used in lieu of a horizontal sand blanket. In the air-pressurizing system, compressed air is injected into the soil to accelerate consolidation. Field pilot tests using both the improved and the existing vacuum preloading systems were carried out at the same site. The testing data showed that better consolidation results were achieved using the improved vacuum preloading method.

One-Way Cyclic Triaxial Behavior of Saturated Clay: Comparison between Constant and Variable Confining Pressure

AbstractThe one-way cyclic triaxial test has been recognized as a useful tool for solving many engineering problems, such as the prediction of deformation in subsoils under traffic loading. Most studies of the one-way cyclic behavior of soils are performed in triaxial tests with constant confining pressure (CCP), which can apply cyclic compressive deviatoric stress only. However, the real stress field in subsoils under traffic loading includes varying normal stresses in three directions. Therefore, it is more desirable to simulate the in situ loading conditions induced by passing vehicles through a combination of the varying deviatoric stress and varying confining pressure. Recognizing this, two series of one-way cyclic triaxial tests on remolded saturated clay were conducted in this study; one in undrained conditions and the other in partially drained conditions. In each series, both tests with the CCP and variable confining pressure (VCP) were carried out. Comparisons between the results of the VCP and ...

Experimental study of drained anisotropy of granular soils involving rotation of principal stress direction

A series of drained tests were conducted on the medium dense Toyoura standard sands using a hollow cylinder apparatus, including both F tests (sheared with a fixed principal stress direction) and R tests (sheared with a constant deviatoric stress but rotating the principal stress directions). The effects of anisotropy on the deformation characteristics were examined, including both the inherent anisotropy that formed in the process of sand sample preparation and the anisotropy which induced due to the ‘inclined’ consolidation. The results of F tests showed that the effect of the inherent anisotropy was significant on the stress–strain behaviour and failure strength of sand. It was also seen that the deformation characteristics of R tests, including the development of strain components, the nominal strain increments and the degree of non-coaxiality, were considerably dependent on both the ‘inclined’ consolidation directions and the deviatoric stress level. It was also noted that the development of nominal strain increments and volumetric strain was affected by the number of cycles due to the densification of the sand under cyclic rotation of principal stress axes under drained conditions.

Drained Instability in Loose Granular Material

AbstractThe constant shear drained (CSD) test is considered an appropriate test to simulate the instability behavior of soils in slopes under water infiltration conditions or lateral stress relief. However, contradiction of the drained instability in loose granular material exists in the reported test results, which is considered because of the condition of constant deviatoric stress (q) not strictly satisfied in the literature. Because the variation of q during the CSD test has significant effects on the test results, more data from tests under a true constant q condition are needed to clarify this subject. In this paper, CSD and conventional undrained triaxial tests in both compressive and extensive loading were performed to investigate the instability behavior of loose sand. All the CSD tests were conducted by advanced variable confining pressure triaxial apparatus, in which a true constant q was maintained during the test; thus, more reliable results were obtained. It is observed experimentally that w...

Influence of Soluble Salt on Electro-Osmotic Consolidation of Soft Clay

A series of electro-osmotic consolidation laboratory tests was conducted to investigate the influence of soluble salt on the electro-osmotic consolidation of soft clay. Remolded soil samples with different salinities of KCl, NaCl, and CaCl2 were tested in the modified electro-osmosis oedometer experimental system. Final water content, vane shear strength, and pH value of test soil samples were analyzed by monitoring the changes in the electric current, volume of drainage, and vertical settlement during the tests. The results showed that addition of soluble salts can improve the electro-osmotic consolidation of soft clay. However, excessive salinity will reduce the efficiency of electro-osmotic consolidation. The optimal salinity of the soluble salt was determined for each type of salt. Moreover, an improved consolidation efficiency can be achieved by using CaCl2 rather than KCl or NaCl.

Error Assessment for the Coherency Matrix-Based Spectral Representation Method in Multivariate Random Processes Simulation

AbstractMultivariate random processes are usually simulated by the spectral representation method (SRM). According to the matrix for decomposition, the SRM has two main types, that is, the SRM based on the decomposition of the power spectral density (PSD) matrix denoting the PSD matrix-based SRM, and the SRM based on the decomposition of the coherency matrix denoting the coherency matrix based-SRM. The stochastic errors of the PSD for the PSD matrix-based SRM have been given. This paper presents the stochastic errors of the PSD for the coherency matrix-based SRM, and makes a comparison of these errors for the PSD matrix-based SRM. For the random amplitudes formulas and random phase formula and Cholesky decomposition method, the stochastic errors of the PSDs for the PSD matrix-based SRM are the same as or the coherency matrix-based SRM, whereas for the random phases formula and eigendecomposition method and random phases formula and root decomposition method, they are different. However, the differences ar...

Field Behavior of Driven Prestressed High-Strength Concrete Piles in Sandy Soils

AbstractDriven piles are used widely both offshore and onshore. However, accurate axial capacity and load-displacement prediction is difficult at sand-dominated sites, and offshore practice is moving towards cone penetration test (CPT) based design methods developed from instrumented pile research and database studies. However, onshore use of these methods remains limited; there is a paucity of high quality case histories to assess their potential benefits clearly, and application in layered profiles may be uncertain. This paper presents new tests on prestressed concrete (PHC) pipe piles driven in sands for a major new Yangtze River bridge project in China, assessing the performance of the ‘new CPT’ and conventional capacity approaches, considering the influence of weak sublayers on base resistance and noting the marked changes in shaft capacity that apply over time.

Experimental Comparison of Electro-Osmotic Consolidation of Wenzhou Dredged Clay Sediment Using Intermittent Current and Polarity Reversal

ABSTRACT The electroosmosis method is regarded especially suitable for the disposal of dredged clay sediment, but its excessive power consumption and electrode corrosion has limited its widespread application. Thus, intermittent current and polarity reversal are applied to offset its drawbacks. This study aims to examine the consolidation effect and internal relationship between the two approaches and optimize their time settings. Laboratory tests were performed with the same intermittent ratio and different reversal ratios. The test results demonstrated that intermittent current with comparatively greater intermittent times and a higher reversal ratio achieved a better drainage effect during the electroosmosis process and soil uniformity after test and the drainage in the reverse direction was lesser than that in the forward direction. The drainage effect during the power-off mode for 1 h was equivalent to that in the reverse electrifying mode for 0.2–0.25 h. Sharp increases in the current were induced by each intermittent current and polarity reversal. Although the polarity reversal approach improved the drainage effect, it partly increased the energy consumption. Compared with the intermittent current approach, reducing the reverse electrifying period to improve soil consolidation is feasible for application to engineering practice.

Mitigation of Ground Vibration Generated by High-Speed Trains on Saturated Poroelastic Ground with Under-Sleeper Pads

Train-induced track and ground vibrations accelerate the deterioration of track, cause nuisance to nearby residents, and even damage adjacent structures. Past research has shown that under-sleeper pads (USPs) are effective at improving track performance and insulating ground vibrations generated by moving trains. The present study was the first to investigate the effectiveness of USP at reducing train-induced ground vibrations at both subcritical and supercritical train speeds (critical speed is defined as the Rayleigh wave speed of the ground surface) and was the first to study the effectiveness of USP at reducing ground vibrations with a two-phase poroelastic soil model. A vehicle-track-ground model containing a fully saturated poroelastic half-space was established, and the whole system was solved by a semianalytical method using Fourier transforms and Fourier series. The differences in the performance of the USPs on saturated poroelastic ground and single-phase elastic ground were examined. It was found that the USPs effectively reduced free-field ground vibrations at subcritical train speeds, but the USP insulation effectiveness was minimized at supercritical train speeds. USPs might also reduce excess pore water pressure in the ground during the passage of a train.