Pile driveability assessment is a crucial aspect of pile design, as it determines the feasibility of pile installation. Offshore wind farms (OWFs) commonly use piles and monopiles that are driven into the seabed using impact hammers. However, in recent years, several pile driving incidents have been reported, with considerable financial damage to developers and contractors.
Pile driving involves two stages self-weight penetration (SWP), during which the pile is lowered to the seabed and penetrates the soil under its own weight, and impact driving, in which the energy from hammer blows drives the pile further down into the seabed until it reaches the target design depth. Read the blog post and download the full technical article at the end of this blog post. 👇
Two scenarios can prevent successful pile driving and should be avoided: pile run and pile refusal.
Illustration: Azadeh Attari
Mapping soil resistance
To determine the feasibility of pile installation in a given location, a process called pile driveability assessment (PDA) is conducted. The main goal of PDA is to ensure that the piles can be installed safely and successfully to the target design depth using the available impact hammers and installation equipment. Geotechnical designers must ensure that the mobilized soil resistance at any stage of pile driving is such that it can prevent both pile-run and pile refusal incidents. PDA also involves analyzing the pile design, hammer selection, and installation method to ensure they are appropriate for the specific soil conditions and project constraints.
Lack of experience
Reliable and representative pile driveability assessment requires accurate modelling of the pile-soil interactions and prediction of the mobilized soil resistance during pile driving. However, the complexity of soil behaviour and a large number of influential parameters poses several technical challenges. These include but are not limited to difficulty in the characterization of soil behaviour under dynamic pile driving conditions, lack of local offshore pile driving experience in emerging markets, and the effect of geometrical variability of piles. In this article, the most common technical challenges are highlighted and discussed, and some suggestions are provided to improve the reliability of pile driveability assessment.
Soil variability and challenges in soil characterization. Especially in sites with geological complexity, it can be challenging to obtain a detailed and reliable picture of soil variabilities
Lack of local experience. Even though physical boreholes and soil sampling are conducted in all offshore wind farms, the characterization of soil material heavily relies on the measurements from Cone Penetration Testing (CPT)
Geometrical variability. The rapid increase in the capacity of wind turbines has caused an increase in the diameter of piles and monopiles that are designed to support the turbines. This means, most of the existing SRD (Soil Resistance to Driving ) models have been developed and calibrated for piles with significantly smaller diameters than the current piles and monopiles
Long-term vs short-term soil resistance and setup effects. Some of the common SRD formulations (e.g., API) are based on the empirical models used for estimation of axial capacity of piles in service
Friction fatigue. As pile driving continues and pile tip penetrates deeper into the soil, the shaft resistance is gradually lost in a given soil horizon
Quake and damping. Besides the static friction of soil, there is also the dynamic component of soil viscosity which contributes to the SRD
Local densification or liquefaction. The intrusive pile driving process and dynamic effect of multiple hammer blows can temporarily impact the soil structure at the pile tip or along the pile shaft
Strain rate effects. Strain rate effects in soil add another layer of complexity, as the strength and stiffness of soil formation is also impacted by the rate of imposing the external deformation, here the speed of pile penetration
Adjustment of settings during operation. Detailed settings of pile driving, e.g., blow rates and hammer stroke are determined by the installation contractor at the latest stages of the project, and are often adjusted onsite during the real-time monitoring of driving operation
Effect of other features such as pile shoes. Sometimes piles are designed with features such as driving shoes to reduce the driving resistance. Currently, there is no consensus on quantifying the soil resistance in the vicinity of pile tip when pile shoe is used.
Download full 7-page technical article.