Poroelastic modeling to assess the effect of water injection for land subsidence mitigation

The possible effect of water injection to mitigate land subsidence was studied through numerical simulations based on the theory of poroelasticity. The Kujukuri Plain, Japan, was chosen as a study area. The effect of past injection was evaluated by comparing a model with injection and the one without injection. The calculated results suggested that the past injection played a significant role to reduce land subsidence. For achieving more effective mitigation practices in the future, we proposed to install injection wells in shallower formations. The effect of proposed injection method to mitigate land subsidence from 2014 to 2030 was also investigated. The calculated results show that the proposed method can work similarly by lesser water injection than the past method. The results also indicate that the upper limit of injection rate should be carefully determined to control the pore pressure build-up in the formation to be small enough to avoid formation failure.

Solid-phase bulk modulus and microinhomogeneity parameter from quasistatic compression experiments

Quasistatic deformation experiments in the laboratory are key to determining the poroelastic moduli of rocks. For microinhomogeneous porous rocks, it is a challenge to determine a complete set of poroelastic parameters. This is because an additional parameter is required that quantifies the effect of microinhomogeneities because then the unjacketed bulk and pore moduli are no longer the same as the bulk modulus of the solid phase. We found that measurements for the drained and unjacketed bulk moduli together with Skempton’s pore-pressure build-up coefficient were sufficient to determine the solid-phase bulk modulus and the microinhomogeneity parameter. The latter served as a direct measure for the deviation from Biot-Gassmann prediction for the undrained bulk modulus. We applied the results to a set of measured poroelastic moduli in which microinhomogeneities have been made responsible for a non-Gassmann rock behavior. Accordingly, our estimate for the microinhomogeneity parameter quantified the deviation from the Biot-Gassmann prediction.

Poro-Elasto-Plastic Model for Wave-Induced Liquefaction

In this study, a two-dimensional poro-elasto-plastic model for the wave-induced liquefaction in a porous seabed was presented. Two mechanisms of the wave-induced pore pressures were considered. Both elastic components (for oscillatory) and the plastic components (for residual) were integrated to predict the wave-induced excess pore pressures in marine sediments. The proposed 2D poro-elasto-plastic model allows for the pore pressure build-up process in a sandy seabed. The proposed model overall agreed well with the previous wave experiments and centrifuge tests. Numerical example shows that the pattern of progressive waves -induced liquefaction gradually changed from 2D to 1D.

Pore Pressure Build-up of Silt Soft Clay under Dynamic Loading at Zhoushan Islands

The mechanism of dynamic tri-axial test is introduced in this paper and the dynamic responses of silt soft clay at Zhoushan are studied using a dynamic tri-axial test system. The laws of pore pressure build-up of the silt clay are obtained which are affected by the consolidation pressure and dynamic load. The greater the consolidation pressure and the dynamic loading is, the more the build-up of pore pressure is. However, the dynamic load has minor effect on pore pressure build-up under the anisotropic consolidation.

SLIDING STABILITY OF LANDWARD SLOPE CLAY COVER LAYERS OF SEA DIKES SUBJECT TO WAVE OVERTOPPING

Sea dikes with landward slopes covered by grass and clay cover layers, subject to wave overtopping, can become unstable and slide. Sliding stability of the cover layer is caused by a decrease in shear strength due to an increase in pore pressure in and underneath the clay cover layer. This holds for both clay dikes and sand dikes with a clay cover layer. A method is presented to determine the potential pore pressure build up due to a storm event with wave overtopping. The method combines of the shelf knowledge and is supported by laboratory measurements and field measurements during prototype scale wave overtopping tests and sliding test on Dutch sea dikes. The method contains three steps: 1) determine the infiltration time, depending on the storm duration and the sea state; 2) determine the infiltration capacity of the dike slope, either by choosing a safe value or field measurement and 3) determine the potential pore pressure build up, determined by step 1) and 2) and the dike structure and geometry. The potential pore pressure build up can be used in the standard stability analyses tools published in Dutch guidelines on dike design and dike safety assessment.