Increase In Hydraulic Gradient Research Articles

Experiments on internal erosion in sandy gravel foundations containing a suspended cutoff wall under complex stress states

Internal erosion is one of the most common failure modes of embankment dams or foundations, and the simplest and most effective preventive measure is to build a cutoff wall. The soil at the bottom of the cutoff wall is usually under complex stress states. The deeper the cutoff wall, the higher is the stress. In this study, the effects of stress conditions on the evolution of internal erosion were investigated in sandy gravel foundations containing a suspended cutoff wall using a newly developed stress-controlled erosion apparatus. Three series of erosion tests were conducted on gap-graded soil under different confining stresses, different deviatoric stresses, and different confining and deviatoric stresses. The results of these tests are as follows: (1) The discharge and permeability decrease with an increase in the confining stress, but the critical hydraulic gradient increases. (2) In the second series of erosion tests, the specimen is compressed under low deviatoric stress; the specimen undergoes shear expansion under high deviatoric stress. (3) In the third series of erosion tests, the confining and deviatoric stresses synchronously change, and therefore, their combined effect on the evolution of internal erosion is complicated. Under low stress, the soil is compressed in the early stage of the experiment, and its structure may change during internal erosion. When the stress level is high, the specimen also undergoes shear expansion, and the degree of expansion is controlled by both confining and deviatoric stresses.

The effect of base level changes and geological structures on the location of the groundwater divide, as exhibited in the hydrological system between the Dead Sea and the Mediterranean Sea

Summary The effects of base (sea or lake) level changes on the location and elevation of the groundwater divide were examined in the hydrological system between the Mediterranean Sea and the Dead Sea. Steady-state simulations were conducted with a 1-D analytical model and transient conditions were simulated using FEFLOW groundwater modeling software. Two hydrological scenarios were simulated: (a) a transition to a new steady-state, following the expected drop of 150 m of the Dead Sea level; and (b) the time of the precursor of the Dead Sea (Lisan Lake), some 20,000 years ago, when the lake level was about 250 m above the present-day Dead Sea level and the Mediterranean Sea level was 120 m below its present one. The results of the simulations show that the Dead Sea level drop has led to a progressive decline in the groundwater level up to several kilometers inland from the shoreline. The hydraulic gradient increases, and thus the discharge to the lake also increases at the expense of the storage, and also due to a small enlargement of the recharge zone by a ∼600 m shift of the divide. Broadly, the subsurface hydrological system is compartmentalized into several sections, separated by low permeability fold structures bounding the mountainous uplands. Most of the groundwater recharge occurs in the uplands. Given this geometry and the available water level measurements, analytical and numerical modeling results show that only changes in the Dead Sea level and recharge rate result in dramatic changes for the scenarios examined. Limited by the low permeability folds, the effects of the Dead Sea level on the groundwater level are most pronounced in the vicinity of the Judea Desert lowland. The folds also greatly limit migration of the groundwater divide associated with changes in the Dead Sea level. Transient simulations show a peak increase of ∼30% in the Dead Sea groundwater flux in ∼75 years, resulting from drainage of lowland aquifers. After more than 500 years the groundwater flux is expected to return to values no more than 4% higher than at the beginning of the simulation. This small net increase is consistent with the small shift in the groundwater divide.

Effects of sediment transport and seepage direction on hydraulic properties at the sediment–water interface of hyporheic settings

Relations between seepage flux and hydraulic properties are difficult to quantify in fluvial settings because of the difficulty in measuring these variables in situ. Tests conducted in a 1.5-m diameter by 1.5-m tall sediment-filled tank indicate that hydraulic gradient increased and hydraulic conductivity (K) decreased following the onset of downward seepage but both parameters were little changed following the onset of upward seepage. Reductions in K during downward seepage were more pronounced when surface-water current was sufficient to mobilize sediment on the bed. Averaged ratios of K determined during upward seepage to K determined during downward seepage (Kup/Kdown) through a sand-and-gravel bed increased from 1.4 to 1.7 with increasing surface-water velocity, and decreased to slightly greater than 1 when the sediment bed became fully mobile. Kup/Kdown for tests conducted with a silt veneer on the bed surface was greater than 2 for all but the fastest surface-water velocities. Substantial reductions in K also were associated with a silt floc that formed on the bed surface during and following test runs. Although the silt floc was typically less than 0.5 mm in thickness, most of the hydraulic gradient was distributed across this thin layer. K of the thin silt floc was reduced by two to three orders of magnitude relative to the underlying sediment. Directional bias in K and relation between K and surface-water velocity require the presence or absence of a layer of lower-K sediment at or near the bed surface, without which no reduction in K and corresponding increase in hydraulic gradient can occur at the bed surface. The lack of prior observation of the consistent bias in K associated with seepage direction is somewhat surprising given the numerous studies where K has been measured in fluvial settings, but may be explained by the small value of the bias relative to the typical uncertainty associated with field determinations of K. If shown to exist in field settings, this bias and its relation to fluvial processes will be relevant to many studies conducted in hyporheic settings that require determination of fluxes across the sediment–water interface.

Experimental Studies on Hydraulic Lifting of Solid-liquid Two-phase Flow

Experimental studies with 4.3m and enlarged 30m in height have been conducted to investigate the flow characteristics of solid-liquid mixture in a lifting pipe and to acquire the design data for sea tests that will be performed in the future. From the results, it was observed that the more the discharged volume fraction and the solid diameter increase, the more the hydraulic gradient increases. Also, the more the diameter of the lifting pipe increases, the smaller the friction loss, and consequently, the less pressure drop and hydraulic gradient. From the enlarged hydraulic pumping experiments, it was shown that the results of the experiments were matched with those of the numerical model previously developed. On the bases of these studies, we plan to conduct further experiments and validate the hydraulic pumping model.

The feasibility of mudstone material as a natural landfill liner

This paper presents the results of a series of laboratory tests investigating on physical properties and hydraulic conductive behaviors of a mudstone material obtained from southwestern Taiwan. The Atterberg limits and particle-size distribution examinations showed the liquid limit=34.1%, the plasticity index=15.2%, the percentage fines (<No. 200 sieve) ≥95.1%, the percentage of clay (<2 μm) ≥30%, and the activity=0.5. These characteristics perfectly satisfied the essential requirements for soil properties of landfill liner. Based on the column experiments using two different grain sizes of beads to simulate the mudstone liner constructed on the gravel and the sandy soil, the hydraulic conductivity ≤1×10 −7 cm/s could be achieved if the hydraulic gradient is less than 13.9 (gravel) and 17.8 (sand), respectively. However, the hydraulic conductivity rose with an increase in hydraulic gradient. It was attributed to the fine granular mudstone penetrated through the simulated beads and led to the fine granular mudstone that could not fill up the void of beads. Moreover, the results of permeability tests performed using the rigid-wall and flexible-wall permeameters revealed that all of the hydraulic conductivities were less than 1×10 −7 cm/s for the mudstone with water contents from 17% to 37% (rigid-wall) and 14% to 29% (flexible-wall). Thus, it was confidently expected that this mudstone material could be applied on landfill soil liner.

Large lake basins of the southern High Plains: Ground-water control of their origin?

The origin of the ∼40-50 topographically large lake basins on the southern High Plains of Texas and New Mexico has been an enigma. Previous workers have considered deflation or evaporite dissolution at depth and subsequent collapse as the most probable mechanisms. However, the eolian hypotheses have been unable to provide convincing arguments as to how the wind selectively erodes the thick, deflation-resistant calcrete that is persistent over much of the southern High Plains. Furthermore, recent detailed studies on some of the basins show no significant evaporite dissolution at depth, and neither mechanism offers a satisfactory explanation as to why the basins are almost universally associated with subsurface topographic highs, or why they are absent where the High Plains aquifer thickness exceeds 60 m. We address these latter concerns and modify the deflation hypothesis by proposing that the calcrete caprock may never have been deposited in the areas now occupied by the basins. The absence of calcrete deposition is proposed to have resulted from high water tables caused by an increase in hydraulic gradient where aquifers thinned above bedrock highs. A high water table close to and/or intersecting the surface prevents deposition of calcrete, and, thus, the uncemented surface would be more susceptible to deflation than the surrounding calcrete-covered areas after decline of the water table. The rise in water table associated with bedrock highs is documented by numerical simulation using boundary conditions and hydrologic parameters representative of the southern High Plains.