Prediction of ground vibrations induced by impact driving of dolphin piles in Caland Canal

Abstract

Modeling pile driving is a complex process, including vibrations generated from the driving hammer, propagating further through the pile and transferred to the surroundings at the pile-soil interface. The vibrations propagating through the ground interact with structures, potentially causing damage to both underground and structures at surface level or potentially disturbing their inhabitants. Prediction models of ground vibrations focus on estimating the vibrations at ground surface level. The prediction of vibration amplitude at different depths can be more useful at sites with sensitive ground conditions. A model predicting these subsurface vibrations, easily applicable in engineering practice is currently not available or not validated. One of the impediments in developing a model for predicting ground vibrations at different depths is the lack of field data for validation. The purpose of this thesis is to find a prediction model for ground vibrations at different depths below the surface level. The study is focused on vibrations induced by impact driving of open ended steel tubular piles. A literature study was conducted to gain theoretical knowledge on pile driving and ground vibrations. The model developed by Massarsch and Fellenius [15] was found in the literature and although it predicts ground vibrations at surface level, the adaptation of the model for predicting vibrations at different depths was studied. Vibration measurements from the field test performed in the Caland Canal have been analyzed. The soil profile at the site consists of sandy clay and clay layers overlying a dense sand layer. First, large diameter dolphin piles were driven by a vibratory hammer. When the pile toe reached the dense sand layer, the driving process was changed to impact hammering in order to change the characteristics of the vibrations transmitted through the soil. The pile driving was extensively monitored and particle velocities were recorded by 3D geophones placed up to depths of NAP -22m (9.50m embedment depth). The prediction model of Massarsch and Fellenius [15] makes a distinction between the velocities of different wave types emitted from piles during impact driving. These waves are the shear waves propagating with a cylindrical wave front along the pile shaft and compression waves emitted at the pile toe, propagating with a spherical wave front. Surface waves resulting from wave reflection at ground surface are not considered in this thesis, as this research focuses on vibrations at different depths. Wave velocities are calculated from the hammer input energy, taking into account ground conditions and considering the vibration transmission efficacy which defines the amount of energy transmitted at the pile-soil interface. The model is adapted for open ended tubular piles. Due to the large diameter of the dolphin piles and the present soil conditions, the soil plugging effect and its contribution to dynamic soil resistance at the pile toe are ignored. Dynamic soil resistance at the pile toe is considered the source of spherical waves, while the soil resistance along the pile shaft is the source of cylindrical waves. The ratio between the pile cross section and the total area delimited by the external diameter is taken into account when calculating the dynamic soil resistance at the pile toe, resulting particle velocities of spherical waves smaller than unity. Therefore the cylindrical waves propagating from the pile shaft are considered the main source of ground vibrations. The results of the prediction model have been compared to vibration values measured in the field test. The predicted vibration amplitudes are a good approximation of the vibration levels measured in the sandy clay and clay layers. The predicted vibrations are of higher magnitude compared to the amplitude of vibrations recorded by the sensors placed in the dense sand layer.