Liquefaction and Shear Banding of Saturated Sand

Liquefaction

It is practical to study the development of liquefaction because either to prevent liquefaction hazard or to consolidate sand foundations needs to understand the mechanism of liquefaction. Research work is mainly in the following:

1. A series of dynamic experiments on mechanical property were carried out and a constitutive relation under confined conditions was presented. This relation was expressed in hyperbolic form which is constituted of loading and unloading part. It can describe the nonlinear and hysteretic characteristics.
2. Formation and evolution of liquefaction under vertical dynamic loading were studied. Effects of main factors on liquefaction were investigated carefully. The evolution of pore pressure, effective stress, displacement and expansion of liquefaction zone under vertical loading were numerically studied.
3. Mechanism and evolution of water film (void redistribution) in saturated sand was analyzed. Water film is a gap formed in saturated soils after liquefaction which can play as a slide surface of landslide. First, a set of quasi three-phase mathematic model considered the erosion and re-deposition of fine grains and the relation between permeability and porosity and content of fine grains in pore fluid was presented. Then the critical condition $U(t)>\varepsilon&space;\sqrt{2\sigma&space;_{e}/\rho&space;_{a}}$  and expansion speed of water film initiation   $W=\frac{U(t)-\varepsilon&space;\sqrt{2\sigma&space;_{e}/\rho&space;_{a}}}{1-\varepsilon&space;}$  were found.
 Pore pressure versus depth Changes of pore pressure with time

Water film observed in experiment (Black strip in horizontal direction)

 (a) Expansion of water film (b) Expansion and shrinking of water film Evolution of water film

Shear banding in saturated soils

Shear band is often observed in saturated soils. This phenomenon is closely dependent on the material characteristics and the local strain. It is related mainly to stability and bifurcation analysis of diffused and localized failure models.  The main advances are as follows:

1. Based on bifurcation theory, the initiation criterions of shear band under simple shear, plane strain shear and anti-plane shear conditions were obtained.
2. Simplified method for evaluating shear band width was obtained.
3. Evolution of post-shear banding was carefully investigated.
 Development of strain ratio Development of pore pressure

Reservoir physics(Low permeability and extra low permeability)

Reservoir with low permeability or extra low permeability is widely distributed in China. This kind of reservoir containing micro-fractures and micro-throat and pores is sensitive to stress. A multi-pore-fracture network was presented which can not only consider the effects of matrix, micro-fracture and micro pore-throat, but also consider the characteristics of fluid flow in micro-fractures and micro-pore-throat. Based on this network, software was programmed under static, quasi-static and dynamic conditions and used to analyze the characteristics of multi-phase seepage in pore-fracture media whit low or extra low permeability.

Considering stress sensitivity, the seepage was investigated. Expressions for one phase and two phase seepage were deducted. Pore pressure distribution, flux and permeability development were numerically and experimentally studied.

A pore-fracture dual percolation model was built to analyze the connectivity and permeability.

Pore pressure distribution considering the stress sensitivity

CT  image of a core with low permeability

Debris flow and landslide

Effects of rainfall intention and soil depth on the transportation of fine grains were mainly investigated. The changes of grain series along the soil depth under rainfall induced seepage were observed.

Rainfall is an important factor to trigger the slope failure such as landslides and debris flows. First the relationship between rainfall duration with the initiation of debris flow and rainfall intensity was mainly studied by the series tests in a box model. Then the rainfall induced responses of slopes and the initiation of slope failure were simulated by using the software FLAC2D. Relation between initiation of debris flow and rainfall duration and intensity has been presented.

 Distribution of shear stress Rainfall duration and rainfall intensity

Distribution of shear stress after rainfall

Distribution of displacement parallel to slope surface after rainfall

CO2 sequestration

Escape of carbon dioxide (CO2 ) from the storage reservoir through the cap-rock may result in severe hazards. In our work, the escape of CO2 after sequestration was first analyzed numerically. Then theoretical method is presented and used to analyze the effects of gas state equation and diffusion. It is shown that the escape of CO2 stops after about fifteen thousand years when the cap-rock is uniformly distributed with permeability of 10-17m2, while it is half of the duration if there exists a well or a crack with high permeability in the cap-rock. Seepage and diffusion are both contributing factors during long-term escape. The study shows that perfect sealing cap-rock must be assured for CO2 storage reservoirs.

 Dissipation under different pressures at upper boundary Escaped CO2 mass under different pressuresat upper boundary