Modelling and simulation of vibratory driven sheet piles - Development of a stop criterion

Sammanfattning: During excavations, steel sheet piles are often installed through vibratory driving, to establish a retaining wall that resists soil instability and ground water leakage. The method of vibratory driving is that the sheet pile is driven into the soil with a vertical vibratory motion. This is especially effective in soft soils. In parts of Sweden, however, the most common type of soil is the glacial till, which is generally compact and contains a large range of grain sizes, where cobbles and boulders are not unusual. Thus, when installing sheet piles in glacial till, there is a risk of impact with objects of high stiffness, which may result in damage to the sheet piles. Any damage to the sheet piles may result in insufficient soil stability and ground water resistance, requiring additional expensive measures to regain it. Hence, there is a need for a stop criterion that can be used to detect these hazardous situations, and that stops the driving before the sheet pile is severely damaged. The purpose of the dissertation is to investigate how numerical models may be used to simulate a situation where a sheet pile encounters a boulder during vibratory driving, and to investigate the possibility of developing a stop criterion based on these numerical models. Three different numerical models were created to simulate the vibratory driving: a single degree of freedom (SDOF) model, a uniaxial multi degree of freedom (MDOF) model, and a finite element (FE) model. The SDOF model and uniaxial MDOF model was created and simulated using the numeric computing platform MATLAB. The FE model was created and simulated using the FE analysis software Abaqus. The simulations were carried out through explicit time integration in all three models. The external actions on the sheet pile, i.e., the vibrator force, soil resistances and obstacle resistances were estimated with methods found in literature. The models were then calibrated against a field study by using the results from that field study as input in the models. The encounter with a rigid object (boulder), was simulated in different ways. In the SDOF model and uniaxial MDOF model, the encounter was simulated with an elasto-plastic contact force. In the FE model, a solid body with high stiffness representing a boulder was introduced, and the encounter was simulated by driving the sheet pile into the solid body, resulting in repeated impacts. The results of the numerical models show promising resemblance with the results of the field study. Both the global driving speeds and the accelerations of the sheet pile corresponds well with the reference case for all three models. This suggests that rather simple numerical models may be used to simulate vibratory driving of sheet piles. In addition, the simulations indicate that impacts may be detected through abrupt changes in acceleration amplitude along the sheet pile, created from the compression waves ensuing an impact. This suggest that a future stop criterion for vibratory driving could be based on a change in acceleration amplitude. Such a stop criterion could be enforced by attaching accelerometers to the sheet piles, and creating a system that will cease the driving when a significant increase in acceleration amplitude is detected.