Research on a Critical Hydraulic Gradient of Piping in Noncohesive Soils

The critical hydraulic gradient of cohesive soil is an important condition for judging soil piping. For force analysis of movable particles in pore channels of soil, this study proposes to consider the influence of surrounding particles on the drag force of movable particles by water flow. According to the principle of relative motion, considering the interaction force between moving objects in still water, the value of the drag force of water flow that is affected by surrounding particles is calculated, to derive the method of the critical hydraulic gradient. This calculation method is suitable for the results of previous piping tests, and the method is accurate and concise.

Evaluation of the internal stability of well-graded silty sand through the long-term seepage test

Suffusion is the phenomenon responsible for internal erosion, and is the process by which finer soil particles are moved through the constrictions between the larger soil particles by seepage forces. Generally, gap-graded soil is known to be susceptible to suffusion. Meanwhile, suffusion of well-graded silty sand and the resulting soil behavior are not well understood. Moreover, the previous researches on laboratory suffusion tests focused on the study of the critical hydraulic gradient, which triggers the internal instability of the soils within a short period of time. Therefore, in this study, long-term suffusion tests were conducted on well-graded silty sand under a hydraulic gradient lower than the critical value. As a result, abrupt increases in permeability and amount of soil discharged were observed due to the progressive migration of the soil particles, resulting in suffusion even at a relatively low hydraulic gradient.

Effects of Contact Area and Contact Shape on Nonlinear Fluid Flow Properties of Fractures by Solving Navier-Stokes Equations

The influences of contact shape and contact area on nonlinear fluid flow properties through fractures are investigated by solving Navier-Stokes equations. The evolutions of nonlinear relationships between flow rate and hydraulic pressure drop, Forchheimer coefficients, nonlinear factor, critical hydraulic gradient, distributions of flow streamlines, and tracer flow paths at different times are systematically estimated. The results show that the nonlinear relationships between flow rate and hydraulic pressure drop can be well described by Forchheimer’s law, in which the nonlinear term coefficient b is approximately three orders of magnitude larger than the linear term coefficient a. The smaller contact area corresponds to smaller variations in many aspects such as flow rate, critical hydraulic gradient, flow streamlines, and tracer flow paths. The critical hydraulic gradient decreases with the increasing degree of contact shape variations while the contacts have the same mean area. The increase in hydraulic pressure drop can induce significant eddies and decrease the permeability and/or conductivity of fractures. However, the distributions of streamlines and tracer flow paths are not dramatically disturbed under a large hydraulic pressure drop.

Study on Critical Hydraulic Gradient Theory of Flow Soil Failure in Cohesive Soil Foundation

The current formula of critical hydraulic gradient is not adapted to solve critical hydraulic gradient of cohesive soil. Assume that the seepage failure mode of the cohesive soil foundation was cylindrical or inverted circular truncated cone, based on the calculation formula of the critical hydraulic gradient of Terzaghi, the analytical formula of the critical hydraulic gradient considering the influence of the shear strength of the soil was derived. Then, the seepage failure process of the clay layer was simulated numerically, and the effects of the clay layer thickness, failure radius, and shear strength indexes on the critical hydraulic slope were analyzed. The comparison results show that the numerical test results are in good agreement with the calculated results of the new formula. In addition, the critical hydraulic gradient of sandy loam and loess under different working conditions was studied severally by a self-made permeation failure instrument. The results show that the critical hydraulic gradient decreases with the increase of soil thickness and failure radius, and the maximum error between the test and the corresponding formula results is no more than 16%.

Impacts of Consolidation Time on the Critical Hydraulic Gradient of Newly Deposited Silty Seabed in the Yellow River Delta

The silty seabed in the Yellow River Delta (YRD) is exposed to deposition, liquefaction, and reconsolidation repeatedly, during which seepage flows are crucial to the seabed strength. In extreme cases, seepage flows could cause seepage failure (SF) in the seabed, endangering the offshore structures. A critical condition exists for the occurrence of SF, i.e., the critical hydraulic gradient (icr). Compared with cohesionless sands, the icr of cohesive sediments is more complex, and no universal evaluation theory is available yet. The present work first improved a self-designed annular flume to avoid SF along the sidewall, then simulated the SF process of the seabed with different consolidation times in order to explore the icr of newly deposited silty seabed in the YRD. It is found that the theoretical formula for icr of cohesionless soil grossly underestimated the icr of cohesive soil. The icr range of silty seabed in the YRD was 8–16, which was significantly affected by the cohesion and was inversely proportional to the seabed fluidization degree. SF could “pump” the sediments vertically from the interior of the seabed with a contribution to sediment resuspension of up to 93.2–96.8%. The higher the consolidation degree, the smaller the contribution will be.

Evaluation of Water Retention Capacity of Bulkheads in Underground Coal Mines

The mechanism of water flow in and around the bulkheads and the surrounding rock is studied for Panel No. 21102 in the Sanhejian coal mine in Xuzhou, China. Based on an analysis of the properties of the bulkheads and the surrounding rock, three types of water conducting pathways are identified: (1) a water conducting pathway at the interface between the bulkhead and the surrounding rock; (2) a water conducting pathway in the faults of the rock; and (3) a water conducting pathway in the fractures of the rock. The possibility of these three pathways for water flow at Panel No. 21102 is analyzed, and the connectivity coefficient of the water conducting pathway is determined. The expression for calculating the critical hydraulic gradient of the rock by using the connectivity coefficient in the water conducting pathways is presented which is based on the permeability and integrity of the rock. The ratios of the critical hydraulic gradient to the steady state hydraulic gradient are calculated for 13 mines in China. An acceptable safety factor in controlling the water flow for the bulkhead is found to be 1.68. When the safety factor is less than 1.68, water leakage has occurred in a number of cases. Finally, changes in the water pressure in the bulkheads with time and changes in the seepage flow with time in the surrounding rock are analyzed. It is found that there is a good correlation between the rate of water flow and water pressure which confirms that water pressure plays a decisive role in controlling seepage from the rock in and around the bulkheads.

Effect of Clay on Internal Erosion of Clay-Sand-Gravel Mixture

In this study, a one-dimensional seepage test apparatus was used to investigate the effect of clay on the critical hydraulic gradient, hydraulic conductivity, migration of fine particles in soil, and percentage of fine particle loss during the internal erosion of clay-sand-gravel mixture, compared with clean gravel. The critical hydraulic gradient and fine sand loss percentage of the clay-sand-gravel mixture decreased, and critical flow velocity and the hydraulic conductivity increased. Six clay-sand-gravel mixture samples with different clay contents were used to evaluate the effect of different clay contents on internal erosion. As the percentage of clay mass to fine particle mass increases from 0% to 25%, the critical hydraulic gradient of soil samples decreases by nearly half and the fine sand loss percentage decreases from 13.73% to 3.48%. Overall, clay has a significant effect on the development of internal erosion of clay-sand-gravel mixture. And attention should be paid in engineering project; clay-sand-gravel mixture with a small amount of clay is more likely to be damaged than clean gravel.

In situ seepage testing method for fractured zones of rock mass

The permeability characteristics of rock mass discontinuities are important in the stability of hydropower station projects. We propose a large-scale in situ seepage testing method and use this method to test gently dipping bedding faults (C3 zone) and steep faults (F14) in a hydropower station construction field in China. The in situ test results are compared with those of both undisturbed and reconstituted specimens. The comparison indicates that the largest critical hydraulic gradient and the smallest seepage permeability coefficient are obtained via in situ tests because they are performed under stress states that simulate the natural stress of the surrounding rock mass. The natural stress of the surrounding rock mass cannot be reflected in tests of undisturbed and reconstituted specimens.