Modelling of piled raft foundations under static vertical loads

In this study, bearing capacity, settlements and stress distributions in different depths of soil mass obtained for steel and fiber reinforced plastic (FRP) piled raft foundations with model experiments in laboratory. After laboratory modeling experiments numerical modeling was carried out by using PLAXIS finite element analysis program. These obtained laboratory experimet results were compared with numerical and theoretical results. Pile foundations have been used since Roman times. Roman buildings featuring timber foundations are still in use today. Timbers have enormous lives especially in water, but ecologic and economic factors have restricted the use of timber piles. Nowadays steel piles are generally used as driven piles. However, steel is an expensive material and it is sensitive to underground and seawater conditions, mainly in sandy soils where they are easily vulnerable to corrosion depending on oxygen saturation of water. It is a fact that natural resources will come to end unless used consciously. Solid waste storage is a major problem today, and recycling has become one of the most vital measures to safeguard environmental and economical safety. One of the prominent recyclable materials is plastic; it is as well cheap and durable especially at harsh environmental conditions such as when exposed to seawater. Although ductile, plastics have the disadvantage of having their strength generally low. Thus, plastics in recent years have started to get reinforced with other materials such as glass fibers, allowing FRP to have a higher strength than ordinary plastics. Recently, plastics, which are cheaper than steel, have been used to deal with soil problems. Geogrids, sheet piles, anchorages and piles are examples of the usage of plastics in such issues. Additionally, FRP piles can be used as end bearing piles due to their high strength. Since they have a higher friction coefficient than steel, they can also be used as friction piles. Steel pipes are generally used as steel piles. However, the friction coefficient between soil and steel is very low, which encouraged the development and implementation of steel piles coated with bituminous materials. Since it is easy to produce FRP profiles with different surface textures, FRP pile surface friction can be enhanced. When top soil layer has insufficient bearing capacity, pile foundations used for transmit loads to the stiff soil layers. In this study, loose top layer and stiff bottom layer are modeling in a sand box. Driven piles were inserted along loose layer and socketed into the stiff layer. Influence of relative density of loose layer on the bearing capacity and settlements of piled raft foundations were determined. Also, effect of number of piles and space between piles were investigated. Behavior of FRP and steel piles will be compared. Stress distributions were obtained with transducers in different depths of soil mass. The experiments were repeated for steel and FRP piled raft foundations. The results of experimental studies have shown that as the number of piles increased (from 9 to 25) and the space between the piles decreased (from 6D to 3D), the group pile bearing capacity for steel and FRP piles increased by 133% and 148%, respectively. As the relative stiffness of the soil increased (from Dr=10% to Dr=65%), the group bearing capacity for steel and FRP piles increased by 56% and 68%, respectively. The bearing capacity of the pile groups made of FRP were determined to be 10-30% higher than that of the pile groups made of steel. Decreases were seen in settlements as the space between the piles decreased and the relative stiffness of the soil increased. In addition, given the stresses measured from different points in the soil mass, it was seen that the stresses decreased when it is moved away from the pile tip and the foundation center. Laboratory experiment results were analyzed by using finite element program and numerical, theorical and laboratory results were compared.