عنوان مقاله [English]
نویسندگان [English]چکیده [English]
Levees are man-made embankments built to prevent rivers from overflowing their banks. A levee is a naturally elongated ridge or artificially constructed fill or wall that regulates the water level. It is usually earthen and often parallel to the course of a river in its floodplain or along low-lying coastlines. The levee construction is a common and very old method of river flood control recently used considerably as an engineering operation to reduce or prevent the detrimental effects of floodwaters. It can be used either alone or together with other methods to minimize the floodwater damage. Engineered earthen levees are of three general types: compact, semi-compact, and uncompact; the former are usually and traditionally used in areas of high property values, high populations, high land use, or in controlled compaction, steep-sloped embankments utilized on good foundation conditions during construction. The semi-compact or uncompact levees are generally used in low value, poor foundation areas or those with high rainfall during the construction season. Compared to other floodwater control methods, levees are more efficient because they are not only quite cost-effective, but also provide more factor of safety for slope stability. Artificial levees are aimed at preventing the adjoining countryside flooding and slowing the waterways' natural course changes to provide reliable shipping lanes for maritime commerce over time; they also confine the river flow causing higher and faster water flow.
Levee construction materials should be coarse enough to withstand the erosion caused by the flowing water, their shear strength should be enough to satisfy the levee's slope stability, and they should be adequately impermeable meaning that they should also contain fine aggregates to prevent the extra seepage that has undesirable effects on the levee's slope stability; therefore, making specific arrangements to reduce repercussions deems necessary. Massive constructions that require large amounts of materials need to use the available on-site materials to reduce the transportation distance and hence the project construction cost; using on-site materials is a significant advantage the levee construction enjoys. As mentioned before, since the levee construction needs both coarse- and fine-grained materials to reduce permeability, the best choice can be the coarse-grained soil containing clay; therefore, both GC (clayey gravel) and SC (clayey sand) soil types take priority over any other types. Hence, evaluating the amount of the soil fine aggregates and the effects of compaction percentage on the permeability coefficient and on other strength parameters to meet the economic, speed, and time requirements of the levee construction is quite important. This research studies the effects of compaction percentage and the amount of fine aggregates on the SC soil permeability and on the steady and unsteady seepage flow through levees. Accordingly, three levee samples A, B, and C (from Mehran River borrow area) with respectively 12.8, 20, and 28.4% fine aggregates and 60, 70, 85, and 100% compaction have been selected to carry out the permeability tests.
Results of tests with different compaction and fine aggregate percentages showed that an increase in the percent compaction of each sample (from 60 to 100, 70 to 100, and 85 to 100) decreased the permeability coefficient 25, 14.3, and 8.91 times, respectively. At every compaction percentage, sample A's permeability coefficient was 1.2 times that of sample B and 1.69 times that of sample C and that of sample B was 1.41 times that of sample C. Next, using the numerical modeling, both the flow rate (in the steady state) and phreatic line (in the unsteady state) through the levee were studied with different compaction percentages and the results showed that in simulating the unsteady seepage through the levee the phreatic line did not intersect the downstream slope and was significantly far from it. The results of this research can be used to either design or construction of the levee; furthermore, it can used to analyze the effect of the soil compaction percent on permeability parameter during construction of levees.