The secondary piles are drilled in Guidewall – completed portion (near) and under construction Once the borehole has been drilled and properly cleaned out, reinforcement, if required by the design, is inserted and properly located. The borehole is then concreted using the accepted practice for cast-in-situ bored piles, preferably using the tremie method. Immediately after concreting, the temporary casing, when used, is extracted using the drilling equipment, a casing oscillator or casing extractors. Care must be taken during this operation, to ensure that withdrawal or oscillation of the casing commences before the concrete starts to set. This is particularly important in deep boreholes, those of large diameter or when there is a delay in pouring successive batches of concrete due to breakdown of machinery. Generally, special attention must be given in all those instances where there is a risk that the first concrete poured will start to set before the concreting operation is completed. between two adjacent primaries such that the secondary pile intersects the two primary piles and cuts into their shafts. For this operation to be successful, timing becomes of critical importance as the strength gain of the concrete of the primary piles, which is to be cut to form the interlock, is related to time, temperature and mix design. Ideally, before drilling the secondary piles, the strength of the (primary pile) concrete should be high enough to prevent it from slumping or cracking but should be low enough to offer minimum resistance to the action of the cutting crown. Drilling secondary piles proceeds with care, with special attention paid to maintaining verticality to ensure the minimum designed amount of intersection is obtained throughout the length of the shaft, particularly at depth. Equipment Strategy The project was not rated as a production job due to the required pile lengths and constraints for the supply of concrete. As such, only one completed pile per 10-hour shift was anticipated and budgeted. The contractor’s initial approach was to use machines that were relatively small and more agile in the very constricted work areas. The anticipated class of machines to be used for this job was in the range of 28 to 36 ton-meter (249 to 320 kN-m or 183,653 to 236,020 ft-lb) of torque. Once the design was close to being finished, it became clear that the anticipated size of machines could only do a part of the required work. The ground conditions and the design loads from the roof slab required increasing the diameter BG 39 and casing oscillator In general, jobsite requirements create an increasing demand for the development of technical support systems. For example, systems like the “drilling” assistant should enable a more constant and quality ensured installation of the piles. As part of quality control, real-time installation control of the production process, data transfer and reporting systems have become more and more important. On this project, the system enabled the user/equipment owner to efficiently operate, maintain and admin- ister individual machines, and could be expanded and integrated to the owner’s entire fleet. The assistant helped to securely monitor and precisely evaluate the various work processes. The assistant was an inte- grated system for controlling all operations and visualizing actual operating para- meters in real-time on a large interactive touch-screen monitor. In addition to basic operational data, General secant pile wall construction sequencing 102 • DEEP FOUNDATIONS • NOV/DEC 2018 general machine operating parameters (e.g., engine data) were also acquired and monitored. The display of the machine operating state and error message(s) was a valuable aid for targeted and effective fault finding by service personnel onsite, but also by specialists based in the office, as data and depth of the structural piles 1.2 m (4 ft) and 44 m (144 ft), respectively. Corres- pondingly, the equipment strategy was changed and required an increase in rig size in the range of 36 to 44 ton-meter (320 to 391 kN-m or 236,020 to 288,681 ft-lb) of torque to accommodate the new demands of the project. Later in the project, oscillators were used to extract the casing during the concreting process, which increased the speed of pile construction. Using the combination of drill rig and oscillator, one additional pile was installed every other day.
