الفهرس 
INTRODUCTIONOnly 14 pages are availabe for public view
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Abstract
Pile foundations are used when the soil has low shear strength or bearing capability. In such cases, piles are used to shift the load to deep, strong strata. A point to be noted is that there are two main types of piles: end load-bearing piles and friction piles. Such piles are classified by their load transmission method. The reduction in a pile’s cross-sectional area (necking), as one of the structural defects, can occur during the casting process of the pile due to local concrete failures, fractured zones, weakened zones, damaged zones in the pile, and geotechnical defects such as debris or a soft layer under the pile toe. A very important point to be noted is that poor designs or poor geotechnical studies were the main causes of geotechnical defects. Moreover, the load-bearing capabilities were lower than expected at the lateral friction and the base load [Poulos, 2005]. Considerable experimental and numerical studies were performed in the past to evaluate the behavior of defective reinforced concrete piles in sand subjected to axial compression loads. Evaluating the effect of existence defects on the behavior of piles, structural defect (necking) and geotechnical defect (soft layer and debris) have been studied. The effect of necking position at 0.25 L from top, at center, and at 0.25 L from bottom has been tested. Geotechnical defect as the soft layer under the toe of the pile and vertical distances (0, D/2, D, and 3D/2) have been tested by conducting twenty-four experimental tests in the laboratory. A numerical study was conducted to verify the results of experimental tests and study the failure pattern of necking piles with caps in different cases. The existence of debris as a result of bad cleaning that works as a soft toe and has a significant effect on the bearing capacity of piles was also tested by conducting forty-eight numerical models to study the effect of it on a single pile, pile composite foundation, two-pile group, three-pile group, and four-pile group.