A Study on the Effect of Pile Penetration Length in the Soil Layer on End Bearing Capacity

In geotechnical engineering practice، piles are employed usually where، because of soil conditions، economic or constructional considerations، it is desirable to transmit loads to strata beyond the practicable reach of spread foundations. Many factors affect the behavior of piles such as soil-pile interaction، compressibility of soil، pile construction technique، shape and also type of the pile. Given the many uncertainties inherent in the design and construction of piles، it is difficult to predict with accuracy the performance of a pile. Therefore، several approaches have been developed to overcome the uncertainty in the prediction. These approaches include static analysis، dynamic analysis، dynamic testing، in-situ testing and pile load test. Among these methods، pile load test is the best option to investigate a load-settlement relationship for a specific site and pile. There are different types of piles that cast-in-situ piles are a common type of them. Cast-in-situ piles were successfully applied in different geotechnical conditions and have gained wide acceptance in recent years because of their many advantages over other types of piles. For example، the construction of cast-in-situ piles generates less noise and vibration or in many cases، a large diameter cast-in-situ pile can replace a group of piles which in turn eliminates the need for a pile cap. The axial capacity of a cast-in-situ pile can be estimated by summing the skin friction capacity and the bearing capacity of the pile base. The skin friction develops between the shaft concrete and the surrounding soil. The skin friction is transmitted to the soil along the length of drilled shaft. However، the end bearing is analogous to shallow foundation bearing capacity with a very large depth of footing. The end bearing capacity is transmitted to the base of drilled shaft. The side resistance of piles is، in most cases، fully mobilized well before the maximum base resistance is reached. In other words، the movement of the pile base against the soil necessary to mobilize the ultimate base resistance is considerably larger than that necessary for mobilizing the side resistance. After full mobilization of side resistance، any increment of axial load is transferred fully to the base. As the side resistance is mobilized early in the loading process، the determination of base resistance is a key element in the design of cast-in-situ piles bearing in sands. The end bearing capacity of cast-in-situ piles can have an important role in their design so that in some projects، cast-in-situ piles are designed primarily based on the magnitude of their end bearing capacity. In this study، by performing axial compression load tests on 14 small scale cast-in-situ concrete piles in sandy soils of Caspian Sea southern coasts، load-settlement curves and the end bearing capacity of piles were investigated. Then، the values of settlements and end bearing capacities of piles obtained in this study were compared with the results of other methods and finally، the depth of penetration of the piles to fully mobilize the ultimate base resistance of the soil layer was estimated.