Pile Driving Monitoring (PDM) during driving and Pile Dynamic Testing (PDT) are widely used in onshore and offshore construction to ensure the structural integrity of the piles during driving and achieve a safe and economical pile installation. PDM helps in assessing the pile behaviour during installation and confirms the assumptions taken during pile design. In addition, it ensures that driving follows the established criteria and provides key elements for calculating the soil resistance during monitoring and system performance assessment.
Pile Driving Monitoring involves attaching strain gauges and accelerometers to the top of the pile. The striking of the pile with the hammer generates stress waves which travel along the pile shaft and mobilise the shaft friction and base pile resistance. Upward waves are then generated, travelling towards the pile head.
The incident and reflected waves are recorded through the sensors installed at the pile top and analysed, converting the strains into stresses and forces in the pile, while accelerations are integrated into particle velocities and displacements. The downward (incident) wave and the upward (reflected) wave can be isolated from the measured total waves at sensors level, knowing its Force and Velocity components. Pile’s ultimate bearing capacity or soil resistance under continuous driving can be estimated by an inversion process on the recorded reflected wave compared to the incident one. This is commonly known as Signal Matching methodology. The recorded Force and Velocity, integrated over time, are also used to compute the actual hammer energy transferred to the pile, providing information on driving system performance.
Development of pile driving instrumentation for the bearing capacity determination of driven or drilled (in the case of dynamic loading tests) piles, started in the 1950s. Existing pile driving instrumentation offers many advantages but also has certain limitations.
By substituting traditional static load tests with dynamic load tests or instrumentation during driving to determine the axial capacity of a pile, it has been possible to considerably reduce the complexity, cost and time of test’s execution. This is particularly true for the control and validation of offshore piles.
However, the standard pile driving monitoring does not allow precise measurement of the distribution of resistance along the shaft and provides only a rough estimate. The stresses generated along the pile shaft during driving are calculated based on measurements made at the pile head and hypotheses of soil dynamic behaviour. The knowledge of these stresses and their distribution along the pile are necessary to determine the fatigue accumulated by the steel during driving, or to control and reduce the risk of damage pile toe.
Recent technological advances in fiber optic sensors, and more particularly in the available acquisition frequency enable their use for successful pile driving instrumentation, pushing some of the limits encountered. Since the beginning of 2018, G-Octopus have made several pile instrumentations using this technology, either along the pile shaft (in combination with conventional instrumentation at pile top), or as partial replacement of standard instrumentation at pile top.
Multiple benefits are now offered using this technology for pile control. Compared to conventional instrumentation, the impact on the structure (and particularly on driven piles) is relatively low due to the size and the weight of the sensors. These do not require fixing holes or welds, which are considered critical for the structural integrity of foundation piles in offshore wind farms. These sensors also do not require the use of protective angles, which have in the recent past caused incidents at the time of installation or have been problematic for data acquisition. The use of optical fiber strain gauges allows instrumentation during driving (in combination with accelerometers), but the same sensors can be used afterwards for long-term structural health monitoring.
In fact, the instrumentation carried out has resulted in very high-resolution measurements and an increase of the survival rate of the optic fiber sensors installed along pile shaft to nearly 100 percent, against 20 – 30 percent for the traditional sensors (or even less in medium and long term). Their use along the pile shaft allows measurements of the stresses during driving over the entire length of the pile. This application is particularly useful when there is a risk of tip damage under hard driving conditions, and determines the fatigue accumulated by the steel during the installation phase. Given their survival rate, the sensors can be reused to monitor deformations and stresses during the lifetime of the pile, meeting the increasing demands of new technical solutions in the offshore wind market.
In the marine environment, optical fiber sensors offer better durability and are less fragile than resistive sensors technology for a comparable cost. The sensitivity to water is reduced with the absence of current, as the oxidation phenomenon, the shock resistance is higher and information loss is lower in case of attenuation of the signal. By using optic fiber sensors during pile installation, its usefulness and effectiveness in not only monitoring the behavior of piles during driving but also in the monitoring of structures during their lifetime can be demonstrated.
Quality measurement and the possibility to increase measuring points due to the size of the sensors can considerably reduce the uncertainties in terms of interpretation. Nevertheless, data interpretation still requires knowledge in structural and geotechnical engineering and competency in numerical modeling to understand the wave propagation and over-imposition during pile driving.
Limitations accounted for using optic fiber strain gauges are proven to be minor. In fact, temperature compensation required for long term structure monitoring can be provided using optic fiber sensor.
The use of optic fiber strain gauges positioned all along pile shaft, above and below the ground level, is currently developing in such a way that it can confirm project hypothesis such as soil/pile interaction and soil dynamic behavior during pile installation, stresses induced along the pile and effects of the pile geometry on the driving process. In addition, it will enable identification of possible degradation of soil resistance with time or damages to foundation itself, due to cyclic solicitations.
Note: The opinions, beliefs, and viewpoints expressed in this article do not necessarily reflect the opinions of Offshore WIND.