Behaviour of expanded piles in clay under uplift and compressive loading

Existing soil at construction site may not always be suitable for supporting structures. Hence, various techniques can be utilized to improve the shear strength, increase the bearing capacity, increase the factor of safety, and reduce the settlement, shrinkage and swelling of soft soils. Among the improvement techniques, preloading and radial preloading using prefabricated vertical drainage are two popular methods in order to improve soft soils. Recently, a new concept of radial preloading has been presented under the name of expanded piers or expanded piles. In this method, an expandable membrane is expanded by means of an injection of air pressure to make an expanded cylindrical cavity, and is filled with a suitable material such as concrete or sand. Thus, the expanded element and the surrounding soil provide a stiffer component compared to the untreated soil. The main focus of this study was an evaluation of the effect of the diameter ratio (DR= final diameter of membrane after expansion / initial diameter of membrane before expansion) on the load capacity of the pile under upward and downward loading. To achieve these aims, 18 physical modelling tests on white Kaolinite were carried out to determine the pile pull out capacity and bearing capacity of the piles. Hence, the diameter ratios of 1.5, 2 and 2.5 times the initial diameter of the cylindrical cavity were selected. In this study, two methods were used to perform the expanded piles, radial expansion and radial expansion with surcharge. A series of physical modelling was designed to assess the different behavior of these two methods. In addition, a series of numerical modelling, based on the soft soil and Mohr-coulomb model, were conducted to simulate the pile behaviour and verification of the laboratory results. Based on the obtained results in the pull out tests, a significant increase was observed in the load capacity equal to 86%, 132% and 153%, for diameter ratios equal to 1.5, 2, and 2.5, respectively, in the soft clay for expansion method. The increase in load capacity were equal to 170%, 175% and 183% for the same diameter ratios, performed by means of expansion with surcharge method. Similarly, in the cases of compressive loading, the load ratios were increased equal to 40%, 47% and 53%, for diameter ratios equal to 1.5, 2, and 2.5, respectively, for expansion method. The increase in load capacity were 99%, 82% and 69% for the same diameter ratios, performed by means of expansion with surcharge method. Moreover, results showed that with increase in the piles diameter equal to 33% and 66%, the load ratios were increased up to 46% and 86%, for expansion method in case of pull out tests. Meanwhile, the load capacities were increased up to 63% and 144% for the expansion method in case of compressive tests. Furthermore, the soft soil model can be considered to have good agreement to simulate pile behaviour under vertical loading with the effect of radial preloading

Behaviour of expanded piles under upward loading due to radial preloading in soft clay

A two-dimensional numerical analysis was employed to evaluate the radial preloading effects on the bored piles bearing capacity under upward loading. Verification of the numerical model was conducted by comparing the calculated allowable load capacity based on a series of physical models in the small scale. Interpretations of the finite elements models were focused on the load-displacement behaviour, the effect of the increase in the pile diameter on the pile load capacity, deformation of the surrounding soil due to radial preloading and comparison of different constitutive models. The results revealed that the bearing capacity is increased due to impose of radial preloading for bored piles, meaningfully. In addition, the effect of the radial preloading is increased on the bearing capacity with increase in the pile diameter. In another aspect, the simulation of the pile behaviours based on the different constitutive models demonstrated that soft soil model can properly simulate pile behaviour due to radial preloading in comparison with Mohr-Coulomb model

Foundation size effect on modulus of subgrade reaction on sandy soils

Winkler model is one of the most popular models in determining the modulus of sub grade reaction. In this model the sub grade soil is assumed to behave like infinite number of linear elastic springs. The stiffness of these springs is named as the modulus of sub grade reaction. This modulus is dependent to some parameters like soil type, size, shape, depth and type of foundation. The direct method for estimating the modulus of sub grade reaction is plate load test that is done with 30-100 cm diameter circular plate or equivalent rectangular plate. Afterward, we have to extrapolate the test value for exact foundation. In the practical design procedure, Terzaghi's equation is usually used to determine the modulus of sub grade reaction for actual foundation, but there are some uncertainties in utilizing such equation. In this paper the size effect of foundation on sandy sub grade with use of finite element software (Plaxis) is proposed to investigate the validation of Terzaghi's formula on determination of sub grade reaction modulus. Also the comparison between Vesic's equation, Terzaghi's one and obtained results are presented