The distance through the water column (16 m or 52.5 ft) required that the micropiles be installed through an “empty passage” to the bottom of the proposed excavation (a depth of 17.33 to 18.33 m [56.9 to 60.1 ft]) with high precision to ensure proper location in the established grid. Moreover, the issue of anchoring the head of the micropile to the bottom slab, which would still be underwater, had to be solved. At the same time, it was necessary to ensure that the installation was efficient to be able to meet the tight time constraints as well as to ensure high quality and precision of the work performed. The slab anchoring work was performed by Aarsleff, and required the installation of 914 micropiles in two phases that were located across a square grid with a center-to-center spacing of 4 m (13.1 m): • Phase 1, temporary condition – for a period to not exceed 2 years from the time the water was pumped out of the trench to the completion of all construction works on the building, the tension load on a single micropile would be 2,300 kN (517 kip). • Phase 2, permanent condition – after the completion of all construction works, the tension load would be reduced to 1,690 kN (380 kip) and the system would provide the required lifespan of 100 years. The Ischebeck Titan self-drilling micropiles system was selected for use on this project. Based on the required individual load capacity, type 103/51 (4.06/2.01 in OD/ID) continuously threaded hollow bar micropiles with 220 mm (8.66 in) diameter drill bits were selected. The required length of each micropile was calculated to be 22 m (72.2 ft). Considering that the installation process had to be performed from atop the water surface, the length of empty passage through the 16 m (52.5 ft) height of the water column had to be added to the length needed to resist the uplift pressure. Thus, the working column of the micropile reached a length of nearly 40 m (131.2 ft). Floating vessel with the contractor’s facilities Installation Process Two contractors, Aarsleff and Soley were selected to perform the installation of the micropile works. To ensure high efficiency and precision, it was decided to perform the installation from floating platforms along with constant assistance and support from diving teams. Each contractor had its own floating platform equipped with a drilling rig, a material warehouse (for hollow bars, connectors, drill bits, cement, etc.), facilities for employees and a coordination center for the diving teams. The involvement of the diving teams was crucial. In consultation Micropile installation schematic (credit: Aarsleff) 16 • DEEP FOUNDATIONS • JAN/FEB 2020 with the team on the surface, the diving team was responsible for the activities for the micropile installation, inspection checks, measurements to control the quality of work, and providing information (e.g., continuity of grout or slurry flow or deviations) back to the team on the surface. Continuous cooperation and supervision of the divers was as important as the technical work itself. The surveillance center had a full overview of the underwater conditions, the divers’ work and the installation process, which, in severely limited underwater visibility, was critical. Suitability Load Test Before commencing the installation of the production micropiles, static load tests were performed on two sets of sacrificial micropiles that were installed from the ground level: • Set 1 – 3 micropiles with a total length of 22 m (72.2 ft) measured from the working platform level • Set 2 – 3 micropiles with a free length of 16 m (52.5 ft) to the target bottom of the excavation, while bond length was 22 m below (for a total length of 38 m [124.7 ft]) Therefore, the working conditions for the sacrificial micropiles were very similar to those for the structural micropiles. The reinforcement of the sacrificial micropiles was as a combination of 127/103 (5.00/4.05 in OD/ID) and 73/35 (2.87/1.38 in OD/ID) Titan hollow bars placed one inside the other. The design loads and the maximum load of 3,200 kN (719 kip) was achieved with the satisfactory performance of the work; therefore, the adopted micropile technology was approved for production.
