The project
G-Octopus have been involved on a construction project of two water intake tunnels and was brought on board by NewWaves Solutions, part of the DEME Group, to perform the Pile Driving Monitoring services for the installation of the 6m outer diameter casings of the intake tunnels. The casings were installed using Drive-Drill-Drive (DDD) sequences, a methodology that has been selected to drive the casings to target penetration into the rock layers through a pilot hole.
Our challenge
The location of the casings is characterized by the Charmouth mudstone formation, which poses an installation challenge for the driving of the casings. The G-Octopus team was mobilised to provide the instrumentation of the casings and the Pile Driving Monitoring (PDM) services during the piling operations, which consisted of providing real-time relevant parameters during the driving, in particular an indication of the stresses induced in the casings. The objective was to guarantee the casing tip integrity by providing warnings to the project team when stresses were reaching critical levels and driving was needed to be stopped.
The casings were scheduled to install a rig to process to drilling operation for the DDD sequence and for subsequent drilling. The instrumentation allowed to compute the bearing capacity at end of each driving session.
Our solution
G-Octopus performed the SafeTip analysis prior to the commencement of the casing installation operations. The maximum allowable compression stress limits at sensors level were defined for the different rock layers.
The six casings were instrumented by G-Octopus’ team onshore, by installing two levels of sensors on the outer shaft of the casings, each level being composed of four strain transducers and two piezo-resistive accelerometers. Such quantity of sensors enables 100% redundancy. Sensor cables were glued to allow the instrumentation to resist to the strong sea current. All the positions were discussed and agreed with the installation contractor to keep the instrumentation safe during all the operations, from the transport to the end of the installation.
The casing’s installation operations were carried out following the methodology defined in the SafeTip and closed coordination between G-Octopus and installation contractor have been observed to drive all the casing to target penetration. The stress limits were controlled by limiting the hammer Enthru energy in the most sensitive cases, in particular where inclined rock layers were expected, and the risk of pile tip buckling was higher.
G-Octopus team carried out back-analysis and pile axial capacity at end of drive using the signal matching analysis methodology on selected blows with CAPWAP software.
The impact
All the casings were driven until target penetration with respect of the stress limitation. The real-time monitoring of the stresses allowed us to continue the driving despite the anticipated needs of pilot hole during preliminary analysis and to reduce the number of DDD sequences.
Signal matching was performed by the G-Octopus team at end of each driving session for axial capacity assessment.
Services Included:
- SafeTip analysis
- Pile Driving Monitoring
- Back-analysis
- Analysis of the pile axial capacity at the end of driving
Pile Information:
- Open-ended steel casings
- Diameter: 6.1m
- Thickness: 60mm at sensors level
- Length: 36m to 40m
Equipment Used:
- In-house underwater strain gauges
- In-house underwater Piezo-Resistive (PR) accelerometers
- Cables
- Pile Driving Analyser (PDA-S)
- GRLWEAP® software
- OPILE software (edited and manufactured by Cathie Group)
- CAPWAP® software