Abstract: This paper presents a case history of deep excavation supporting system installed under a very low headroom restraints for minimizing the impact on the adjacent buildings, utilities and other structures. A U-shaped excavation in soft soils was designed under cross a bridge with active traffic. The net clearance under the bridge was only 1.93 meters and the edge of excavation was 2.7 meters to the bridge pile foundation. Under the very low head constraints, the supporting system was planned to be steel H-beam inserted Metro-Jetting-System (MJS) formed columns with one layer of internal support by reinforced concrete struts. The construction sequence for the excavation supporting system was different than the conventional H-beam inserted MJS columns, that is, MJS column formed after the installation of steel H-beams. However, there are zones of un-jetted exist in this method and the local re-jetting is necessary. The field study indicated that the grouted column formed by the MJS had a diameter of 3.1 to 3.2 meters and the dimensions met the design requirement. There are no-gaps between the steel H-beam and the grouts. The MJS formed soilcrete had an unconfined compressive strength greater than 1.5 MPa. The length of the H-beam for the insertion had to be between 1.5 and 2.5 meters and the H-beam insertion needed to be spliced on-site. The full penetration welding with the guide plate can effectively resolve the strength reduction due to the splicing.

Key words: metro jetting system formed columns, hp beams, low headroom constraint, construction sequence, multi-splices