4 standard hollow auger piles were set up under the terms of the contract and we added, on our own initiative and for the purposes of the business’s Research & Development, 2 piles set up according to FrankiStar’s specifications so as to add new sizing values to them.
The aim of this test was to characterise Toulouse molasse, which has the distinctive characteristic of being in some cases clayey and in others clayey and sandy, but also include sandy or sandstone and even occasionally limestone sections.
In accordance with the standard, molasse is classified as a clay because its calcite is too low, meaning it cannot be classified as marl despite the pressure limit values and high moduli. It’s for this reason that the test was put in place: to assess the actual bearing capacities of molasse with the goal of optimising pile sizing in this particular geological horizon.
There were three challenges to successfully carry out this test: the geotechnical characteristics of molasse, the restrictive regulatory framework, and the cramped on-site environment.
Firstly, the geotechnical and geological characteristics posed two challenges for the setup of the piles, but we successfully solved both issues:
- The drilling was possible thanks to the power of the Liebherr LRB 355 (30 tonnes.metre of torque and 40 tonnes of crowd force) to traverse 18m of molasse characterised by the following geotechnical surveys:
- The static penetrometers were at the points of refusal on the roof of the molasse (2m under the platform)
- The pressure limits were between 4 and more than 8.3MPa
- The pressuremeter moduli ranged from 80 to 290MPa
- The concreting of the piles in the sandy horizons traversed by the piles (Ø 1,020mm - depth 20m) and reinforced with reinforcement cages connected to 4 full-height drawbars went perfectly thanks to:
- Work done by our concrete specialist upstream from the worksite (by checking the suitability of and inspecting the concrete producer’s formulation) as well as at the start of the worksite (inspecting and modifying the formula of the concrete delivered to optimise fluidity): their expertise allowed us to reinforce the piles with total peace of mind over 20m without using a rebar shaker in this C45 concrete.
- Precise adjustment of piston stroke volume on the parameter recorder: this essential aspect of hollow auger concreting (and by extension any pile concreted by raising the tool via a concrete pump) was perfectly managed by the driller. This adjustment provides reliable information on the volume pumped into the pile, pile concrete overconsumption, and therefore pile concreting quality.
Furthermore, the regulatory framework for load testing changed recently, requiring compliance with a significant centre-to-centre distance between the test pile and the reaction pile: minimum 4m when the tested pile is shorter than the reaction pile (which is practically always the case in load tests).
To take up this challenge, we designed and arranged for sizing and manufacturing of a new reaction mass made up of 3 beams that we sized based on the largest jack used for testing in France: 1,500 tonnes.
The main 13t, 7m-long beam spreads its two “wings”, the 19t, 12m-long secondary beams, which in turn carry 4 chain links to the reaction piles.
The fully assembled mass weighs nearly 60 tonnes but has the advantage of being able to be used in several different ways to adapt to worksite environments:
- 1-beam, I-shaped setup: the most economical, only two reaction piles are required, but the lowest in terms of maximum load: 750t (but this lower load covers 100% of the static load tests that we had been able to carry out to date).
- 2-beam, X-shaped setup: solution requiring four reaction piles for occasional testing with a heavy load (up to 1,500t).
- 3-beam, H-shaped setup: this is the chosen option in Toulouse, which has the most interesting geometric formation in cases involving not only a heavy load (up to 1,500t) but also a high number of tests to carry out one after the other (6 in the case of Toulouse).
Final challenge, worksite environment.
Located in a zone occupied by a business whose access road had to remain open at all times, on the edge of a working railway reducing possible movements of drilling rigs and in a physically restricted space, work on the site was complex but the issues not insurmountable.
The worksite’s organisation was examined upstream to maximally optimise the 600m of available spaces:
- A procedure for work by railway lines produced by the major works department and approved by the SNCF (French railway operator) allowed us to expand our authorised range of movement for the drilling rig.
- A temporary reinforcement cage and drawbar assembly workshop was designed so that:
- The twelve 870mm-diameter, 20m-long cages could be manufactured at the same time as the piles were set up.
- The temporary workshop then made way for the setup of the piles: the final piles were set up in in the same place where the final welds were completed just a few hours earlier.
- A cage handling procedure was drawn up and specific tools were manufactured to switch the reinforcements from a vertical position to a horizontal position in complete safety without damaging the welders’ work.
There’s no need to underline it, this worksite was no exception to the rule: the engineers’ theoretical plans could not have been implemented without the works teams who provided practical solutions to successfully complete the structure based on their in-field experience.
The multiple challenges we overcame allowed us to set ourselves apart in the eyes of the project owner, who congratulated us on the high-quality work done within the contractual turnaround times.
This places us in a great position for the future response to the invitation to tender for the metro construction contract.
Even though the mass’s theoretical maximum load was not achieved, we still pushed to 1,200 tonnes (30MPa of stress in the concrete!) to achieve geotechnical fracture: a real benchmark in the world of special foundations where this load had been achieved only once over the last decade in France on concrete piles.
This worksite’s success demonstrates Franki Fondation’s technical ability and skills to achieve a static loading test of up to 1,500 tonnes.
Finally and above all, the research carried out based on analysis of the instrumentation of the 6 test piles, which featured no fewer than 180 vibrating wires and 12 optical fibres submerged in the concrete, will constitute a new development in our FrankiStar specifications, which will be updated with characteristics regarding Toulouse molasse.
Congratulations and thank you for this outstanding team success: sales, design & engineering, operations and equipment pooled their respective skills for Research & Development.