Internal fluidisation of granular material

Uncontrolled seepage flow due to defects or imperfect joints in civil engineering structures (such as dams, levees, dry-dock, seepage barriers, and sheet piles interlocking) or from fractured underground pipes is a major concern. The result of such leakage is increased water pressure and hydraulic gradients at the source of leakage making the behaviour of the surrounding material complex. This is because of the interaction of particles with the source of flow, turbulent flow in the surrounding bedding and the potential of developing additional mechanisms at the source of flow. Evidence of the occurrence of internal fluidisation has been reported in a number of fields, and yet not much is known about it.In this fundamental study experimental apparatus and techniques have been developed to investigate the mechanism of internal fluidisation of granular material due to localised flow. A two-dimensional experimental model was created for the study, in which a machined box with variable orifice openings was designed and built to simulate an idealised crack for a localised leak. The box was fitted inside a modified seepage tank to fluidise a bed of granular material. Various parameters were investigated: flow rate, excess pore water pressure in the bed, pressure upstream of the orifice, particle size, particle shape, height of the bed, and orifice size on the observed mechanism. Image analysis techniques based on the Particle Image Velocimetry (PIV) have been developed in this study to monitor the behaviour of the fluidised zone.Results and observations of this study suggest that the mechanism of internal fluidisation in a bed of granular materials is associated with an uplift mechanism of the grains in the active region of the bed. This is attributed to the drag force exerted by seepage flow overcoming the downward bulk weight of the bed. From the results and observations of this study a mathematical model based on the concept of force equilibrium has been proposed to predict the pore pressure at the onset of internal fluidisation. The results show that high pressure heads can be sustained upstream of the orifice without the internally fluidised zone breaking through to the bed surface. They also show that the onset of this mechanism in a bed of granular material is highly dependent on packing properties (grain size, grain shape, height of the bed), seepage velocity and orifice size. <br/

Monotonic direct simple shear tests on sand under multidirectional loading

Stress–strain responses of Leighton Buzzard sand are investigated under bidirectional shear. The tests are conducted by using the variable direction dynamic cyclic simple shear (VDDCSS), which is manufactured by Global Digital Systems (GDS) Instruments Ltd., U.K. Soil samples are anisotropically consolidated under a vertical normal stress and horizontal shear stress and then sheared in undrained conditions by applying a horizontal shear stress acting along a different direction from the consolidation shear stress. The influence of the orientation and magnitude of the consolidation shear stress is investigated in this study. There are only a few previous studies on soil responses under bidirectional shear, of which most studies do not consider the impact of the magnitude of the consolidation shear stress. They are compared with current studies, indicating both similarities and differences. Generally, all test results indicate that a smaller angle between the first and second horizontal shear stress leads to more brittle responses with higher peak strengths, and a larger angle leads to more ductile responses. In addition, the consolidation shear tends to make soil samples denser, and both the magnitude of consolidation shear stress and its direction influence the following stress–strain responses of soil samples

Principal stress rotation under bidirectional simple shear loadings

Previous researches have indicated the non-coaxiality of sand in unidirectional simple shear tests, in which the direction of the principal axes of stresses does not coincide with the corresponding principal axes of strain rate tensors. Due to the limitation of apparatus that most of testing facilities can only add shear stress in one direction, the influence of stress history on the noncoaxiality of sand is not fully considered in previous tests. In this study, the effect of stress history on the non-coaxiality of sand is systematically studied by using the first commercially available Variable Direction Dynamic Cyclic Simple Shear system (VDDCSS). Samples of Leighton Buzzard sand (Fraction B) are first consolidated under a vertical confining stress and consolidation shear stress, and then sheared by a drained monotonic shear stress. Angle (θ) between the consolidation shear stress and the drained monotonic shear stress is varied from 0° to 180°, with an interval of 30°. The change of principal axes of stresses is predicted by well-established equations, and the principal axe of strain rate is calculated using recorded data. Results show that the level of non-coaxiality is increased by the increasing θ, especially at the initial stage of drained shearing

Hydraulic Jump Characteristics Downstream of a Compound Weir consisting of Two Rectangles with a below Semicircular Gate

Weirs are often used in laboratories, industries, and irrigation channels to measure discharge. The discharge capacity of a structure is vital for its safety and plays an important role in the combined gate-weir flow, which is a complicated phenomenon in hydropower. This study carried out experiments on a combined hydraulic structure, which included a compound sharp-crested weir made up of two rectangles along with an inverted semicircular sharp gate. Installed on a straight channel, this structure served as a control instrument. The study aimed to investigate the downstream hydraulic jump characteristics of this combined structure, specifically, the sequent depth ratio (y2/y1), the hydraulic jump height ratio (Hj/y1), the energy loss ratio through the jump (EL/Eu), and the jump length ratio (Lj/y1). The width of the upper rectangle on the weir was set at 20 cm. The width of the lower rectangle (W2) was set at 5, 7, and 9 cm, while its depths (z) were fixed at 6, 9, and 11 cm. The gate's diameters varied between 8, 12, and 15 cm. These measurements were alternated with varying initial Froude numbers (Fn1) ranging between 1.32 and 1.5. The results showed that the dimensions of both the weir and the gate influenced the hydraulic jump characteristics. Empirical formulas were developed to predict y2/y1, Hj/y1, EL/Eu, and Lj/y1 based on the differing dimensions of the combined structure. The findings and analysis of this study are limited to the range of data that were tested

Internal fluidisation of granular material

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Effect of soil on leakage rates in water distribution pipes

This paper describes an experimental and analytical study that examined the effects of soil on leakage rate from defective water pipes. An experimental model was designed and constructed to simulate idealized cracks in defective water pipes discharging into a bed of granular materials. Tests were run on three different materials: LBS-B, LBS-A, and glass ballotini with different characteristics. The results reveal that the presence of bedding material surrounding a leaking pipe causes a significant change in the leakage rate versus free flow conditions. The bedding has a greater impact on larger defect openings. There was a significant drop in the values of the coefficient of discharge (Cd) when the pipe discharged into a soil bed versus free flow conditions; this value was 38% lower for coarse particles and 70% for finer particles. The results also showed that high leakage rates could flow from the defective pipe without fluidization of the bedding materials. An analytical model based on the method of fragments was presented to predict the leakage rate after which the predicted results were compared with values measured experimentally. HIGHLIGHTS An analytical model has been presented to predict the leakage rate, in the presence of soil, that provides reasonable estimates of the leakage rate.; Soil with different characteristics (permeability, particle size, particle shape, and bed height) was tested.; Influence of the surrounding soil on the hydraulic behaviour of leakage was examined.

Internal fluidization in granular soils

Internal fluidization, the process whereby granular soil is transformed into a fluid-like state when liquid or gas is pumped into it, is of importance in a number of civil and geotechnical engineering processes, for example, pipe leakage, flow through sheet piling, pile jetting, and spudcan extraction. This paper draws together the limited literature on the subject and identifies the stages in the process by which fluidization occurs. Using data from small-scale experiments and simple analysis, factors affecting the onset of fluidization and its development are identified

Prediction of the discharge coefficient for the rectangular notch with different hydraulic and geometric properties

This study aims to investigate experimentally the variation of the coefficient of discharge Cd with the rectangular notch hydraulic and geometric parameters such as water head h, notch height p, notch width B, and notch thickness t. The results show that the coefficient of discharge Cd increases with an increase of h and B while it decreases with t. There are no changes in the variation of actual discharge Qact and consequently the discharge coefficient Cd with h for notch height p more than 6 cm. An empirical formula was developed based on the dimensional analysis principle that can be used to predict the coefficient of discharge Cd value for the rectangular notch with known hydraulic and geometric data (h, B, p, and t). HIGHLIGHTS There are no Qact changes and thus the discharge modulus Cd with h for crack height.; The relationship between the theoretical discharge Qth and vertex h to be constant for the thickness of the rectangle Cd increases with increasing h/p because the actual discharge Qact increases with water head h.; The discharge modulus Cd decreases with increasing slit thickness ratio t/p.

Soil fluidisation outside leaks in water distribution pipes - preliminary observations

This paper reports on a preliminary study carried out to investigate the effect of soil, and particularly fluidisation of soil, around leaking water pipes. During the study a bed of particles is fluidised at controlled flow rates, using upward-pointing orifices to simulate holes in a pipe. The extent of the fluidised zone and the head drop within the bed are monitored. The results show that significant head can be sustained in a shallow pipe without the fluidised zone breaking through the bed surface. Head loss from a leaking pipe is divided into three components: through the orifice; in the fluidised/mobile bed zones; and through the static soil. The results show the extent of fluidised and mobile zones appears to be almost independent of the orifice size. Most of the head loss is found to occur within fluidised and mobile zones; significant (but lower) head loss is found to occur through the orifice and only a small fraction of the total losses occurs in the static soil