الفهرس 
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Abstract
Recently, the use of deep foundations has increased as a result of the expansion in the construction of high-rise buildings, train tracks, and port berths. As a result of this expansion, it was necessary to use deep foundations that have low cost, high bearing loads, low settlement, low construction time, and are subjected to different types of loads such as lateral, vertical compression, and tension loads. This thesis will present one of the most important types of deep foundations that are aptly used in such structures and the most important factors affecting their bearing capacity, settlement, and failure mechanisms that occur with this type of foundation, and also study these factors in two types of clay soil (medium and stiff). This type of deep foundation is called an underreamed pile, and the factors affecting the bearing load are such as bulb diameter, number, and spacing. To investigate the effects of these parameters and obtain optimal results, the PLAXIS 3D was used. The total number of series is 244, divided into five groups for each soil cohesion. In all cases, two piles with a length to diameter ratio of (L/D = 30 and 60) and a constant stem pile diameter (D = 0.5 m) are used to investigate the effect of different parameters of the underreamed pile on the bearing load and settlement. The parameters are used in the analysis, such as bulb diameter (Du/D = 1.5, 2, 2.25, and 2.5), spacing (S/D= 2, 3, 4, 5, 6, 7, and 8), and number of bulbs (N= 1, 2, and3) in two types of soil cohesion (c= 38 and 75 kN/m2) and compared with conventional pile (PWB). The analysis shows that, the increase in bulb diameter has a great effect on the bearing load. Bulb spacing controls the failure mechanisms, whether cylindrical shear failure or individual failure. When the number of bulbs increases, the bearing capacity doubles, especially if each bulb works individually.