بررسي اصطكاك منفي در شمع قايم و مايل منفرد با استفاده از مدل‌سازي عددي

An Investigation of Negative Skin Friction on Single Vertical and Batter Pile using Numerical Modeling

در اين نوشتار اصطكاك منفي در شمع قايم و مايل منفرد بررسي شده است. ابتدا به منظور صحت‌سنجي نتايج، يك شمع قايم اتكايي و اصطكاكي مدل شده و نتايج به دست‌آمده از آن با نتايج ارايه‌شده توسط ساير پژوهشگران مقايسه و تاييد شده است، سپس يك شمع منفرد قايم اتكايي و يك شمع قايم اصطكاكي تحت سربارهاي سطحي متفاوت مدل‌سازي شده و نيروي فروكش آنها مورد بررسي قرار گرفته است. در ادامه، رفتار شمع منفرد مايل اتكايي و اصطكاكي با زواياي تمايل بين 0 تا 30 درجه نسبت به قايم تحت شرايط مشابه شمع قايم مورد ارزيابي و مقايسه قرار گرفته است. نتايج تحليل ها نشان مي دهد كه نيروي فروكش در شمع قايم و مايل كاملاً وابسته به مقدار بارگذاري سطحي و نوع شمع به لحاظ اتكايي و اصطكاكي است. در حالت شمع مايل متناسب با نحوه‌ي توزيع تنش در خاك اطراف شمع بين حالت‌هاي محرك، سكون و مقاوم عمل مي‌كند و مقادير نيروي فروكش متناسب با آن ايجاد مي شود.

A common reason for uneven settlement of existing structures, founded on pile foundations, is negative skin friction (NSF) developed along the piles. As a result of fill placement, the soil surrounding the piles in soft ground settles more than the piles. In that case, negative skin friction occurs, producing pile settlement (down drag) and additional compressive force (down drag), due to the hanging effect around the pile. (NSF) is usually mobilized up to the shear strength of soil in most cases, except at very close proximity to the neutral plane. The relative displacement between the settling subsoil and the pile shaft surface should not be more than a few millimeters to have even total settlement. Disregarding the effect of down drag forces on piles, due to consolidation of the surrounding soil, leads to damage in buildings and foundations. Many methods, especially numerical simulation methods, have been proposed to calculate the drag load on single piles. However, much less work has been done on drag load on a batter pile. Batter piles are commonly used to support marine structures and deep foundations. Based on the experimental data from Hanna & Nguyen (1986), shaft resistance decreases by the increase of angle of inclination for both compression and tension forces. On the other hand, it was found from full scale results reported by Mayerhof (1973) that the shaft resistance capacity for the pile increases due to an increase in the inclination angle. In this study, the effect of negative skin friction on vertical piles and batter piles is investigated. The analysis was undertaken using Abacus software, which is based on the Finite Element Method (FEM). A single vertical end-bearing and one skin friction pile under different earth surface loadings were analyzed and the drag load force was studied. Subsequently, the performance of the single batter end-bearing and friction pile, with different inclination angles between 0-300 ,was analyzed. After different combinations of the existed loadings in both surrounded areas of pile drag loading, considering different cases .the vertical pile was modeled and its results were compared to those of previous studies for model validation. Then, single batter piles with different angles were modeled using the results.