بررسي ظرفيت باربري شمعهاي كوتاه تحت اثر بار جانبي در خاك ماسه‌يي

Evaluation of Short Piles Bearing Capacity Subjected to Lateral Loading in Sandy Soil

در بسياري از سازه ها، بارهاي جانبي بالايي نسبت به بارهاي ثقلي به سازه تحميل ميشوند. شمعهاي اين‌گونه سازه ها را، كه تحت اثر بارهاي جانبي قابل توجهي قرار ميگيرند، مي توان به 2 گروه شمعهاي بلند و كوتاه تقسيم كرد. معمولاً تخمين ظرفيت باربري شمع تحت اثر بار جانبي براي شمعهاي كوتاه براساس بيشينه‌ي ظرفيت باربري و براي شمعهاي بلند براساس تغييرمكان افقي مجاز است. در اين پژوهش، براي بررسي رفتار شمع تحت اثر بار جانبي، از لولههاي فلزي به‌‌منزله‌ي شمع در آزمايشگاه استفاده شده است. جهت تعيين ظرفيت باربري، بار جانبي به شمعهاي فلزي مدفون در خاك ماسه‌يي اعمال شده است. با انجام آزمونهاي آزمايشگاهي رفتار شمعهاي كوتاه به لحاظ تغيير طول و قطر شمع و نيز وزن مخصوص خاك مورد بررسي قرار گرفته است. سپس نتايج آزمونهاي آزمايشگاهي با روشهاي هانسن، برامز، پتراسوويت، ميرهوف و پراساد مورد مقايسه قرار گرفته است. تحليل نتايج نشان ميدهد كه روش پراساد براي تخمين ظرفيت باربري جانبي شمع نسبت به ديگر تيوريهاي ارايه‌شده مناسبتر است.

Almost all types of pile are subjected to lateral loads. In many cases, however, the applied lateral loads are comparable with gravity loads. Lateral loads and moments are generally induced by wind and earthquake. All piles subject to lateral load are usually divided into two categories: long piles and short piles. The general methods to estimate the lateral bearing capacity of piles are based on ultimate bearing capacity and allowable horizontal displacement for short and long piles, respectively. Several theoretical methods, including Hansen, Broms, Petrasovits, Meyerhof, Prasad and Chari, have been proposed to predict the lateral bearing capacity of piles in cohesion-less soils. All these theories are based on a simplified soil pressure distribution assumption along the pile length. In engineering practice, the Broms method is most popular, since it is simple and applicable for both short and long piles. In the present research, steel pipe is used as pile in the laboratory to evaluate the lateral capacity of piles subjected to horizontal loads. Steel model piles, with two different outside diameters of 21.7 mm and 27 mm, wall thickness of 2.4 mm, and lengths of 400, 600, 800 mm, were used for tests. The soil, in which the piles were embedded, was fine sand. The lateral load was applied to the pile using a robe and pulley system. The friction angle of the sand, determined by a Direct Shear device, was 33? and 41.5? for loose (? = 13.8 kN/m3) and medium dense (? = 15 kN/m3) states, respectively. The sand container was cylindrical in shape, 0.7 m in diameter and 1.0 m in height. Thin wires, attached to the pile at different levels, were utilized to measure the horizontal displacement of piles within the soil. According to theories and experimental test results, the behavior of piles with different length and diameter embedded in sand was evaluated. A comparison between experimental test results and different theories reveals that the Prasad and Chari method is more suitable for estimation of the lateral bearing capacity. It is shown that by increasing the length and diameter of the piles and, also, the density of the soil, the lateral bearing capacity increases also, but, soil density is more effective than other parameters. In addition, it is shown that the location of the pile rotation point is not affected by changes in diameter and soil density.