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, and construction time, and such foundations are subjected to different types of loads such as lateral, vertical compression, and tension loads. This research paper 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 and settlement in stiff clay. This type of deep foundation is called an under-reamed pile. The factors used in this study are pile length to diameter ratio L/D = 30, bulb diameter ratio (Du/D = 1.5, 2, 2.25, and 2.5), number of bulbs (N = 1, 2, and 3), and spacing ratio (S/D = 2 to 8). To investigate the effects of these parameters and obtain optimal results, the PLAXIS 3D was used. The analysis shows that the increase in bulb diameter increases the bearing load by 43%. Bulb spacing controls the failure mechanisms, whether cylindrical shear failure or individual failure and increases the capacity by 66% and 99%, respectively, for two and three bulbs when the bulb spacing becomes S/D = 8. When the number of bulbs increases to three, the capacity increases by 90%. If each bulb works individually, the bearing capacity doubles

Concrete piles that were poorly constructed or analyzed in their soil analyses may have structural or geotechnical defects. To examine such defects, an experimental study was conducted to investigate how a defective reinforced concrete pile behaved. These piles were installed and subjected to a compression axial load in the sand that had relative densities of 30%,60%, and 80%. The tests were performed using four concrete model piles: one intact pile and the other three piles had a structural defect (necking) at three different positions of the pile at (0.25 L from the top, center, and 0.25 L bottom). Geotechnical defect (soft layer or debris) was studied using Styrofoam layer at various vertical distances under the pile toe with Y/D = (0, 0.5, 1 and 1.5) D. The test results showed that the bearing capacity of the structural defect was the most in the case of a neck at 0.25 L from the bottom, followed by a neck at the center, and finally a neck at 0.25 L from the top. In the case of a geotechnical defect, the bearing capacity of the pile decreased with the decrease of the vertical distance between the soft layer and the pile toe.