بهسازي زمين به روش تراكم تفضيلي ديناميكي با استفاده از مدل سازي عددي به روش تفاضل محدود در ماسه خشك

Design of Ground Improvement with Dynamic Compaction Method for Granular Soil Using Finite Difference Method

چكيده- در اين مقاله استفاده از تراكم ديناميكي به عنوان يكي از روش هاي بهسازي زمين بررسي شده است . با وجود كاربرد گسترده اين روش، اساس طراحي آن هنوز تجربي يا نيمه تجربي است و فرايندهاي درگير در مسيله به طور كامل مشخص نيست. در اين پژوهش براي روشن كردن ابهامات پروسه تراكم ديناميكي، مدل سازي عددي عمليات تراكم ديناميكي در خاك هاي دانهاي با نرم افزار و با روش تفاضل محدود انجام شده است. مدل رفتاري خاك، مدل پلاستيك موهر كولمب غيرهمراه در نظر گرفته شده است FLAC و براي مدلسازي اثر ضربه بر سطح خاك از روش اعمال سرعت اوليه به گره هاي كوبه استفاده شده است. با استفاده از نتايج مدلسازي ميتوان ميزان بهسازي در اعماق مختلف را تخمين زد و نيز تعداد ضربات مورد نياز براي رسيدن به حد بهينه بهسازي را تعيين كرد. اين مدل مي تواند به عنوان ابزاري براي طراحي پروژ ه هاي بهسازي به روش تراكم ديناميكي به كار رود.

Abstract: Dynamic compaction (DC) method is an effective method in soil improvement, which is widely used in the world. This method includes repeated drops with high energy on the surface of the soil by the tampers weighing 5 to 40 tons. The dampers fall from 10 to 30 m heights. This method is used to increase the density of soil deposits. The degree of density depends on the weight of the hammer, the height from which the hammer is dropped, and the spacing of the locations at which the hammer is dropped. The initial weight dropping has the most impact, and penetrates into a greater depth. The following drops, if spaced closer to one another, compact the shallower layers and the process is completed by compacting the soil at the surface. Nowadays, D.C. method is one of the common improvement methods in Iran because the required equipments and technology of D.C. are simple and available. Since the design of this method is empirical and there are a large number of parameters (variables), so to achieve an efficient D.C. pattern, trial D.C. with before and after compaction tests must be carried out in some areas. Considring the cost of the trial D.C and control tests, numerical D.C. models will increase the efficiency and accuracy of this method and the costs will drop as well. In this study, numerical D.C. has been modeled for granular soil using finite difference method. According to axis symmetric assumption, just half of the soil mass and tamper has been modeled in 2D. To model the drop effect on soil surface, initial velocity method is used on the tamper nodes. Granular soil D.C. has been analyzed with Mohr Coulomb behavior model using Flac 2D 4.0 software. The results of this study have been compared with those of Pan & Selby (2002) studies. Also final settlement of the tamper has been compared with the results in Assaluyeh D.C. project. In both of the above cases, the results of the numerical models and the real measured values are nearly the same. Numerical method can estimate improvement degree in different depths as well as the required number of drops to achieve the ideal improvement degree. Also horizontal extent of the improvement area can be determined at the end of each compaction stage by using relative density contours, Then spacing of impact points can be estimated with reviewing the horizontal extent of the improvement area.