Xianjing Kong

Stress-Dilatancy Relationship of Zipingpu Gravel under Cyclic Loading in Triaxial Stress States

AbstractIn this study, cyclic triaxial tests were performed on Zipingpu gravel, and they were followed by a discrete element study that was designed to investigate the stress-dilatancy relationship of gravelly soils under cyclic loading in triaxial stress states. Several conclusions emerged from the results. (1) A nearly linear relationship was found between the stress ratio η=q/p and the dilatancy ratio Dp=devp/desp under both conventional compression and extension monotonic loading. The slope parameter α, which relates η and Dp, was smaller during triaxial compression; (2) The stress-dilatancy relationship was different during the virgin and cyclic loading. The dilatancy line under cyclic loading was located inside the virgin/monotonic loading dilatancy lines, and the dilatancy relationship was related to the location of the most recent load reversal point; and (3) A nearly parallel linear relationship was found between η and Dp in the dη>0 and dη<0 paths under cyclic loading, with α smaller under cycli...

Three-Dimensional Large Deformation FE Analysis of Square Footings in Two-Layered Clays

In strong over soft two-layered clays, there is a potential for the footing to experience a punch-through failure, where the footing penetrates a large distance at a short time after the initial peak resistance is reached. Three-dimensional 3D large deformation finite-element analyses using 3D RITSS method were conducted to simulate the penetration responses of square footings in strong over soft clays. The effects of surface soil heave and soil layer interface deformation during footing penetration were studied in weightless soils. Fitted equations were proposed to express the footing capacity response against the penetration depth. Based on the fitted equations, formulas to calculate footing peak bearing factor and the corresponding penetration depth were developed. The peak footing capacity factor and the corresponding penetration depth increases with the increasing of soil layer strength ratio, relative top soil layer thickness and soil weight factor, thus the potential of punch-through failure was reduced accordingly. It was also found that the soil weight effect can be a simple surcharge based on the formula developed in the weightless soil. Design charts for the peak footing capacity factor and the corresponding penetration depth were developed. DOI: 10.1061/ASCEGT.1943-5606.0000400 CE Database subject headings: Deformation; Load bearing capacity; Layered soils; Clays. Author keywords: Large deformation; Punch-through; Bearing capacity; Double-layered soils; Square footing.

DISLOCATION OF FACE-SLABS OF ZIPINGPU CONCRETE FACE ROCKFILL DAM DURING WENCHUAN EARTHQUAKE

Zipingpu concrete face rockfill dam (CFRD) is one of the tallest dams that have ever been subjected to strong earthquake shaking and recorded damages. In this paper, finite element method and limit equilibrium method are used to determine the most critical failure surface for Zipingpu CFRD during the 2008 Wenchuan earthquake in China. The dislocation damage of the face-slab joint was then obtained from the sliding block analysis. The major factors affecting dislocation displacement of face-slabs were analyzed. The results showed that the rigid sliding block method is relevant for analyzing the damage of face-slabs of CFRD during earthquake. The peak ground acceleration had a significant influence on the dislocation. As the acceleration of the dam increases with the height while the strength of face-slab joints reduces, the dislocation could easily occur when the upstream slope of the dam started to slide. The water elevation had considerable impact on the dislocation of face-slabs during strong shaking. There might be no dislocation of face-slabs at full reservoir, and the water elevation just above construction joints would induce larger damage.

Aseismic safety analysis of a prestressed concrete containment vessel for CPR1000 nuclear power plant

The containment vessel of a nuclear power plant is the last barrier to prevent nuclear reactor radiation. Aseismic safety analysis is the key to appropriate containment vessel design. A prestressed concrete containment vessel (PCCV) model with a semi-infinite elastic foundation and practical arrangement of tendons has been established to analyze the aseismic ability of the CPR1000 PCCV structure under seismic loads and internal pressure. A method to model the prestressing tendon and its interaction with concrete was proposed and the axial force of the prestressing tendons showed that the simulation was reasonable and accurate. The numerical results show that for the concrete structure, the location of the cylinder wall bottom around the equipment hatch and near the ring beam are critical locations with large principal stress. The concrete cracks occurred at the bottom of the PCCV cylinder wall under the peak earthquake motion of 0.50 g, however the PCCV was still basically in an elastic state. Furthermore, the concrete cracks occurred around the equipment hatch under the design internal pressure of 0.4MPa, but the steel liner was still in the elastic stage and its leak-proof function soundness was verified. The results provide the basis for analysis and design of containment vessels.

PFC Numerical Simulation of Particle Breakage of Rock-Fill Dam

In this paper, a new way of modeling the rock-fill dams is presented. Based upon the principle of particle flow code - PFC, it is possible to model the phenomenon of rock-fill breakages due to the dams self weight and the filling of the reservoir in a simple way. The dams self weight leads to 8.2% breakages and 1.6% additional broken blocks occurred during the reservoir filling. The breakages mainly occur at the bottom and the upstream of the dam. The reason of the particle breakage, the distribution of the contact force and the broken zone can be obtained accurately by this research. The results of the numerical simulation provide a practical basis to quantitatively analyze the rock-fill breakage of rock-fill dam using PFC, and also provide a new way to explain and analyze the particle breakage from the perspective of micro mechanism.

Experimental Study on the Behaviors of Sand-Gravel Composites Liquefaction

By use of medium scale dynamic triaxial apparatus(φ 200×500mm) the development of axial strain and pore water pressure of sand-gravel composites are studied in cyclic loading. Adopting same relative density, a series of substituted material specimens gained by eliminating the oversized (>5mm) gravel particles are studied. The results show that with isotropic consolidation, the development of excess pore water pressure and axial strain in sand-gravel composites differs from that in substituted material. A series of undrained cyclic triaxial tests were performed on sand-gravel composites specimens with relative density of 50%, 55%, and 60%. Test results showed that the increase of relative density may delay the development of pore pressure of sand-gravel composites.

Three-Dimensional Scaled Memory Model for Gravelly Soils Subject to Cyclic Loading

AbstractA comprehensive constitutive model has been formulated to capture the complex behaviors of gravelly soils under irregular cyclic loading. The application of this model aims to analyze the n...

Dynamic responses of concrete-faced rockfill dam due to different seismic motion input methods:

The concrete-faced rockfill dam valley foundation was considered as an open energy system and a reasonable non-uniform seismic motion input method was applied to the dynamic analysis of a concrete-faced rockfill dam based on the generalized plastic model. First, the corresponding program was validated by means of the scattering question of an idealized semicircle valley. Subsequently, the seismic elasto-plastic finite element analyses were performed to compare and investigate the performances of a concrete-faced rockfill dam under different seismic motion input methods. The results show that the dynamic responses of the concrete-faced rockfill dam are decreased by 10%–30% approximately with the use of non-uniform seismic motion input method. As a result, the traditional uniform seismic motion input method would overestimate the responses of the dam. From the perspective of seismic safety evaluation, the overestimations would disturb the reasonable assessment of the aseismic capacity of the dam. Moreover, the slope stability analysis results might be conservative and unreasonable due to overestimating the accelerations during the earthquake.

A Simple Measurement of Membrane Penetration in Gravel Triaxial Tests Based on Eliminating Soil Skeleton Plastic Deformation with Cyclic Confining Pressure Loading

Previous investigations of membrane penetration mainly focused on uniform sandy soils, and few studied gravelly soils. A common problem in most previous investigations was that unverified assumptions were made to evaluate or mitigate the membrane penetration of a cylindrical specimen in triaxial test. In this study, a simple measurement of the membrane penetration of gravelly soils was proposed, based on small-amplitude cyclic confining pressure Δσ3 loading, in which the cyclic membrane penetration amplitude ΔVm is recoverable, and the cyclic soil skeleton volume deformation amplitude ΔVk gradually decreases with increasing number of cycles and becomes elastic. The accuracy in the proposed method is related to the accuracy in measuring ΔVk. Because of the natural limitations in measurements of the radial strain of a cylindrical specimen, ΔVk is calculated based on the isotropic assumption. The proposed method provides sufficiently accurate ΔVm values for gravelly soils because of the small portion of ΔVk in the total cyclic volume change amplitude ΔVc (=ΔVm+ΔVk), and the accuracy increases as the coarse fraction increases. Furthermore, the method can be easily implemented for individual specimens to remove inconsistences in specimen preparation. The test results indicate that ΔVm values corresponding to the same Δσ3 exponentially decrease with the confining pressure σ3. Membrane penetration increases with increases in the void ratio and coarse fraction, but the trend is becoming weak with decreasing coarse fraction and void ratio, respectively. It also demonstrated that there exists a clear difference between the results of Nicholson’s empirical formula and the test results.

Large-deformation finite element analysis of the interaction between concrete cut-off walls and high-plasticity clay in an earth core dam

Purpose A large, uneven settlement that is unfavourable to dam safety can occur between a concrete cut-off wall and the high-plasticity clay of earth core dam built on alluviums. This issue has been often studied using the small-strain finite element (FE) method in previous research. This paper aims to research the interaction behaviour between a concrete cut-off wall and high-plasticity clay using large-deformation FE analyses. Design/methodology/approach The re-meshing and interpolation technique with a small-strain (RITSS) method was performed using an independently developed program and adopted for large-deformation FE analyses, and a suitable element size for the high-plasticity clay region was suggested. The layered construction process of an earth core dam built on thick alluviums was simulated using the RITSS method incorporating a hyperbolic model for soil. Findings The RITSS method is an effective technique for simulating the soil–structure interaction during dam construction. The RITSS analysis predicted a higher maximum principle stress of the concrete cut-off wall and higher stress levels in the high-plasticity clay region than small-strain FE analysis. Originality/value A practical method for large-deformation FE analysis was advised and was used for the first time to study the interaction between a concrete cut-off wall and high-plasticity clay in dam engineering. Large deformation in the high-plasticity clay was handled using the RITSS method. Moreover, the penetration process of the concrete cut-off wall into the high-plasticity clay was captured using a favourable element shape and mesh density.