|Steel Casings||Installing Casings||Crawler Crane||Oscillator|
Bored piles are used as the foundation for the bridge structures in this project. The piles were specified to be founded on sound bedrock.
Prior to the piling work, pre-drilling of a borehole drilled at each proposed pile position was required to determine the founding level of each pile. The exact position of a bored pile was then set out and excavation of the pile shaft was commenced. Temporary steel casings were employed for supporting the excavation. Such steel casings were installed into the ground by oscillating and jacking motion exerted by a hydraulic casing oscillator. The oscillator was connected to either the excavating crane or I-beams driven into the ground to balance the reaction torque. While excavation was carried out within the casing by a hammer grab operated by a crawler crane, the casing was driven simultaneously and its toe level was kept ahead of the excavation level, particularly in soft ground. This was to prevent cavities forming outside the casing, ground collapse and pile movement during the excavation process. A positive water head was maintained inside the casing to keep the excavation at balanced head conditions, prevent ingress of materials from the bottom of the casing. When rock boulders or hard materials were encountered, a rock chisel or reversed circulation drill (RCD) were utilized to remove the obstruction. Verticality of the casing was checked from time to time and rectified by the oscillator as necessary.When the required bedrock was reached, a rock socket was formed using the rock chisel or RCD. Once the founding level was reached, the bottom of the casing and the shaft were cleaned by air-lifting using high pressure air compressors prior to concreting work. Reinforcement cages were prefabricated and fitted with concrete spacers to maintain the concrete cover. They were lowered by the crane into the casing in sections linked by U-bolts. The pile was concreted normally by tremie pipe. Grade 40/20 concrete with 175mm slump was poured into the tremie pipe via a concrete skip. The base of the pipe was kept below the concrete surface during the process. The steel casing was extracted simultaneously and gradually by the oscillator during the concreting but the casing toe was kept below the concrete surface all the time. The concrete level was finished with a sufficient length above the cut-off level of the pile. The pile was then backfilled to the ground level when the concrete was set.