تاثير درصد خاك رس و انرژي تراكم بر آبشستگي پايه پل در بستر رسوبي و در حضور تكيه گاه

Effect of Clay Content Percentage and Compaction Energy on Pier Bridge Scour in Erosive Bed and in the vicinity of abutment

مهمترين دليل نگراني، در خصوص پايداري پل ها، وقوع آبشستگي موضعي در اطراف آن هاست.مساله آبشستگي موضعي در اطراف پايه و تكيه گاه پل در سال هاي اخير به طور گسترده توسط محققين مختلف مورد بررسي قرار گرفته است كه نتيجه آن ارائه روش هاي مختلفي براي كنترل و كاهش آبشستگي بوده است. يكي از اين روش ها تراكم خاك بستر مي باشد. تراكم بستر رسوبي اطراف فونداسيون پل، مقاومت آن را در مقابل آبشستگي افزايش داده، موجب كاهش آبشستگي و به تاخير افتادن پيشرفت آن مي شود. در تحقيق حاضر به منظور كاهش آبشستگي در اطراف پايه و تكيه گاه پل 5%، 10% و 15% رس به بستر غير چسبنده اضافه و سپس متراكم گرديد. با توجه به نتايج آزمايشات مشاهده شد هنگامي كه 15% رس به رسوبات غير چسبنده اضافه شود، در شرايط اشباع بستر و تراكم 70%، بعد از گذشت 24 ساعت از شروع آزمايش، آبشستگي پيرامون پايه به طو كامل حذف مي گردد.

The most dominant concern about the stability of bridges is the occurrence of local scour around piers and abutments. The local scour around bridge foundations has been widely studied by different researchers in recent years. Different methods have been proposed to control the scour depth and occurrence of scouring around bridge piers and abutments. Construction of bridge foundations in elevations deeper than predicted values is a usual method, in which, proper estimation of scour depth is very crucial. There are other available methods to countermeasure the scour, among which: installation of an amour layer around bridge piers and abutments. In this method heavy particles of stone or concrete blocks are placed around the bridge foundations, enhancing resistance of the bed to erosion and scour. Deviation of the flow from the bridge foundation is another way which usually transfers the scour to the midway channel at downstream of the structure. Installation of collars and spur dikes are the examples of this approach. An innovative method to mitigate scour around hydraulic structures is “soil compaction”. Compaction increases the relative density and soil resistance which mitigates scour and produce a time delay in scour hole development procedure. Effects of clay content percentage and compaction ratio on scour reduction around bridge piers are investigated in this paper. Experimental programs are conducted in a flume with 45 m length, 1.2 m width, 0.45 m depth and with a bed slope of 0.001. Uniform sand with median size of 0.73 mm and standard deviation of 1.27 is selected as non-cohesive material. The clay content liquid limit (LL) is 15.9, plastic limit (PL) is 49.2 and plasticity index (PI) is 10.9, which categorize the clay content as CH according to Unified Classification Standard. The experiments are performed in clear water conditions (vc/v=0.9) with a constant depth of 15 cm and a flow rate equal to 40 L/s (with a subcritical Froud Number of 0.31 and mean velocity of 0.31 m/s). In order to investigate the effect of clay content percentage and compaction energy on local scour, 5%, 10%, and 15% of clay were added to the soil. Thereafter, the specimens were compacted by different ratios. According to the experimental results, compaction leaves no effect on non-cohesive soil scouring rate and magnitude. However if the clay content percentage rises up to 10% -while the compaction ratio is 100%- the scour is completely eliminated. In order to assess the effect of compaction ratio on scour mitigation, the same mixture (with 10% clay content) was compacted to 70% of ultimate compaction energy. The results showed a 50% decrease in scour depth which is comparable to the previous case in the same compaction rate (70%). The percentage of the clay content was raised to 15% for another test. The results showed that the scour was completely captured. In a specimen with 15% clay content mixed with non-cohesive sediment materials -in saturated bed conditions- and with a relative density of 70%, the scour process around the bridge pier was completely controlled after 24 hours. The results prove that adding cohesive materials can postpone the local scour process significantly.