Stabilization of Angular-Shaped Riprap under Overtopping Flows
Riprap is mostly used to prevent erosion by flows down the steep slopes in river engineering. A total of 53 stability tests performed on angular riprap with a median stone size ranging from 15 to 278 mm and slope ranging from 1 to 40% are used in this study. The existing equations for the prediction of medium size of angular stones are checked for their accuracy using the available data. Predictions of median size using these equations are not satisfactory and results show deviation by more than ±20% from the observed values. A multivariable power regression analysis is performed to propose a new equation relating the median size with unit discharge, bed slope, riprap thickness and coefficient of uniformity. The proposed relationship satisfactorily predicts the median angular stone size with ±20% error. Further, the required size of the rounded stone is more than the angular stone for the same unit discharge and the ratio increases with unit discharge and also with embankment slope of the riprap. Keywords—Angularity, Gradation, Riprap, Stabilization
Construction of Dams by Dumping Stones into Flowing Water: (original title: Постройка плотин наброской камня в текущую воду)
First pdf is translation, second pdf is original Russian text. - The possibility of constructing a dam by dumping stones into flowing water is considered. Experiments are carried out with different construction methods. In total five different methods have been tested. From the hydraulic standpoint, the most rational method of construction appears to be to use only small stones in the beginning and subsequently change to larger sizes. The method gives greatest economy in total volume of the submerged dam, gives the minimum percentage of large size stones, gives a steadily increasing effectiveness of dumping, gives maximum insurance against piping due to percolation flow between the dam and the foundation and also results in the minimum total percolation flow through the body of the dam. With reference to the plan of using only large stones, it can readily be seen that the attendant expense is not justified for the initial triangular stages because the equilibrium of stones situated near the apex is determined by the coefficient of stability against sliding, rather than the coefficient of overturning. Furthermore, the large stones in the lower interior section of the triangle are outside the zone of flow action and are never subjected to the higher velocities of overflow. The method of using alternate portions of small and large stones decreases the resistance of the latter of overturning. Consequently, the large stones simply roll along over the smaller pieces and, not being able to find a hold capable of developing the requisite interlocking resistance, simply roll along the top of the dam and gradually acquiring high kinetic energy, sometimes damage and destroy previously constructed sections of the dam. The corresponding curve of effectiveness of dumping is subject to erratic changes and exhibits a series of negative slopes. At times an increment of stone actually results in a decrease in the height of the dam for the reason given above. The final volume of a submerged dam constructed of alternate portions of small and large stones is greater than the volume of a similar dam section constructed entirely of small stones.
Analysis of the stress wave effect during coal breakage by a high-pressure abrasive air jet
In view of the defects of borehole collapse, inhibition of gas desorption and migration of gas existing in hydraulic fracturing and other hydraulic permeability–increasing measures for soft coal seams with low-permeability technology is proposed for coal breakage by a high-pressure abrasive gas jet for relieving pressure and increasing permeability. The comparative analysis of gas jet flow field structure between convergent nozzle and Laval nozzle has been given by numerical simulation. For Laval nozzle, the expansion wave and compression wave alternate and move forward steadily in gas jet and vanish when potential core length reaches maximum. So, the Laval nozzle can form more stable flow filed structure of gas jet and avoid shock wave in gas jet. Furthermore, a high-speed camera is adopted to analyze the jet structure and verify the conclusion of numerical simulation. Based on thermodynamic theory, this article calculates and analyzes the critical local sound velocity and pressure generated from the stress wave during the process of coal breakage by the gas jet. Furthermore, experimental coal breakage by a high-pressure abrasive gas jet is carried out. The high-pressure abrasive gas jet impacts the coal body as a quasi-static load and a dynamic load and forms corrosion pits on the surface of the coal body. Penetrating cracks are formed within the coal in the pattern of the loaded stress wave which leads to coal breakage. The effects of porosity and permeability on the propagation of the stress wave in coal are analyzed by establishing the dispersion equation for the spread of the stress wave in coal. The results show that porosity has a significant effect on wave velocity and that the attenuation of the stress wave is intensified with an increase in porosity. Moreover, the stress wave attenuation is more obvious at high frequency. The effect of permeability on the wave velocity is not significant at low frequencies. In contrast, at high frequency and relatively low permeability, the wave velocity increases with the permeability, and the attenuation of the wave velocity initially increases and then decreases. When the permeability is greater than 10−11 m2, the wave velocity is not affected by the permeability. However, the stress wave is not attenuated.