كليدواژه :
آبشستگي موضعي , پايه پل , خاك چسبنده، , بنتونيت و كائولين
چكيده فارسي :
از دلايل اصلي تخريب پل ها آبشستگي موضعي اطراف پايه مي باشد كه با گذشت زمان و عدم كنترل آن پايداري سازه را به مخاطره مي اندازد. تاكنون محققين بسياري آبشستگي موضعي اطراف پايه هاي پل را موردبررسي قرار داده اند و روش هاي مناسبي در جهت كنترل و كاهش آبشستگي ارائه داده اند كه شامل روش هاي اصلاح الگوي جريان در اين نواحي و همچنين روش هاي مقاوم سازي بستر مي باشد. در اين پژوهش به بررسي آبشستگي موضعي پايه پل در خاك هاي چسبنده پرداخته شده است و استفاده از خاك چسبنده رسي از نوع رس بنتونيت به صورت اختلاط با خاك بستر، به عنوان راهكاري در جهت مقاوم سازي بستر در برابر تنش هاي واردشده و كنترل آبشستگي مورد بررسي قرارگرفته است. اهداف دنبال شده در اين پژوهش شامل تعيين بهترين شرايط تراكمي (ميزان رطوبت و درجه تراكم)، ميزان بهينه رس مورد استفاده و همچنين تاثير مقاومت برشي بستر در ميزان آبشستگي مي باشد. نتايج نشان داد كه استفاده از 10 درصد رس بنتونيت به صورت اختلاط با خاك ماسه اي بستر در درجه تراكم 90 درصد و رطوبت بهينه تراكم، باعث كاهش آبشستگي به ميزان 100 درصد نسبت به نمونه شاهد مي شود. همچنين نتايج نشان داد كه رس بنتونيت تاثير بيشتري در كاهش آبشستگي نسبت به رس كائولين دارد.
چكيده لاتين :
The main reasons of bridge failure are local scour around the piers. In fact, they will be appeared as a scour hole in the river bed. Moreover, lack of control during the time, local scouring will threaten stability of structure. Therefore, determining of depth and dimension of the scour hole, also finding effective factors on scouring are important. Lots of researchers have studied the local scour around the bridge piers. Actually, they have proposed lots of appropriate techniques to control and to reduce scour around piers and bridge abutments. These proposed approaches are divided into two methods which are altering the flow and bed-armoring. In this research, the local scour around the bridge piers in a cohesive soils have studied. Particularly, the cohesive soils in form of mixture of Bentonite (montmorillonitic clay from 0 to 15%) and bed sediment (fine sand) used. In fact, this approach is a solution to challenge bed-armoring against existence shear stress. As a result, the proposed method will be used for controlling the scour. The experiments were carried out at the hydraulic laboratory of the Water Engineering Department, at Shiraz University, Iran. The laboratory flume was a rectangular cross section with 18 m length, 1.2 m wide and 0.4 m deep. According to channel geometry, the discharge and the depth of flow were determined to be 0.034 m^3⁄( s ) and 13 cm, respectively. In all experiments, the discharge and flow depth were constant. This study focused on the best compaction conditions, optimum clay content and the bed shear strength. The undrained shear strength of the soil was measured using an in situ miniature van shear apparatus. The best compaction conditions including optimum moisture and compaction energy. From standard Proctor test, the optimum initial water content Wopt and the optimum dry density ρdopt was determined. The compaction was expressed as the ratio of dry density ρd to maximum dry density ρdopt of the mixture. Therefore three relative compaction equal to70%, 80% and 90% used. Also, three water content optimum initial water content, optimum initial water content – 3% and optimum initial water content + 3% are used. The results indicate that for smaller clay content (5%) the shape of the scour hole was similar to that in sand sediment and is regular and symmetrical. For clay content equal to10%, scour hole is geometrically much irregular compared to that observed in sand bed and it is observed that a steeper slope of the scour hole in sediment mixture. The slope became steeper whit an increase in relative compaction. In fact, the higher relative compaction and the higher clay content increases shear strength and decreases the maximum scour depth. The samples compacted at optimum water content creates a structure with the most resistant to scour. The erodibility of samples compacted dry of optimum water content is less than samples compacted on the wet side of optimum. The results showed that under conditions which the amount of Bentonite equals at least 10% of dry weight in the mixture (Bentonite and sand sediment), relative compaction equal s to 90% and and water content equals to optimum moisture, simultaneously the local scour reduces by 100% in single pier. In addition, the influence of the compaction and type of clay mineral was investigated. The results show that Bentonite was more effective than Kaolin. If the 15% of dry weight Kaolin clay mixed with bed sand sediment to be used and relative compaction equal to 90% and water ontent equals to optimum moisture, simultaneously, the maximum scour hole was reduced only by 34%.
