بررسي اثر مشخصات خاك در سرعت بيشينه ذرات، ناشي از كوبش شمع با استفاده از آناليز اجزا محدود

Effect of Soil Properties on Peak Particle Velocity (PPV) Due to Pile-Driving Using Finite Element Analysis

شمع كوبي يكي از روش هاي رايج در اجراي پي هاي عميق است، كه به طور گسترده در ساختمان هاي مسكوني، صنعتي، پل ها، بزرگراه ها و ... استفاده مي شود. در كنار مزايايي كه استفاده از اين روش دارد، فرآيند شمع كوبي حايز آثار نامطلوبي است كه مهم‌ترين آن‌ها، ارتعاشات ايجادشده در زمين است، كه مي تواند باعت آسيب به سازه هاي مجاور و تاسيسات حساس شود. رايج ترين شاخص براي سنجش ميزان ارتعاشات ناشي از شمع كوبي، سرعت بيشينه‌ي ذرات خاك (PPV) است. در اين نوشتار، با استفاده از نرم افزار اجزا محدود plaxis2D، پارامتر هاي موثر در ارتعاشات ايجادشده در سطح زمين ناشي از كوبش شمع بررسي شده است. نتايج تحليل نشان داده است كه با افزايش زاويه‌ي اصطكاك داخلي و ضريب پواسون خاك، سرعت بيشينه‌ي ذرات خاك افزايش مي يابد؛ در حالي كه افزايش مدول كشساني، وزن مخصوص و ميرايي مصالح خاك، موجب كاهش سرعت بيشينه‌ي ذرات خاك مي شود. همچنين تغييرات چگالي نسبي خاك، تاثير محسوسي در سرعت بيشينه نداشته است.

Pile-driving is a common method of foundation construction where the soil is not strong enough to support the load of the structure through conventional shallow footing. This method is widely used in residential and industrial building, bridges, highways and etc. Along with the benefits of using this method, pile driving is also a source of negative environmental effects. Noise and air pollution are the most commonly expressed concerns, but, ground vibrations originating from the pile driving impact also have important adverse effects. They can cause disturbances to adjacent structures and also disrupt the operation of nearby sensitive equipment and facilities. Permanent settlement, densification and liquefaction may also occur in the soil due to such vibrations. A common factor for evaluating the amount of vibrations is peak particle velocity (PPV), which is maximum velocity that soil particles reach during the pile driving process. PPV is the maximum velocity that a soil particle experiences during the driving of a pile from the ground surface to the desired depth. Ground motion due to pile driving generally depends on (1) the source parameters (method of driving, energy released, and pile depth), (2) the interaction between the driving machine, the pile and the soil, and (3) the propagation of waves through the soil. In this article, a two-dimensional finite element model is validated using a case study of pile-driving data conducted in the Chennai site in India. Then, by modeling a pile in sandy soil with different soil relative densities, Poisson ratio, moduli of elasticity, Unit weight, friction angle and damping ratio, the effects of these parameters on vibrations of the ground surface, and peak particle velocity on the surface, were studied. The results showed that the increase in friction angle and Poisson ratio increases the PPV. However, increase in the modulus of elasticity, damping ratio and soil unit weight, decreases the PPV. Also, scrutiny of PPV occurrence time shows that by variation of the influencing parameters, except the modulus of elasticity, this time is almost constant.