Foundation pit support project
Year: 2014-2015
Anchor used: Temporary Prestressed Ground Anchor

Project Overview
Suzhou Taihu Golden Port Commercial Integrated Project has a total site area of 31000 square meters and building footprint of 170000 square meters including two 22-storey towers, eight 2 to 3-storey podiums and two basements. The project place is surrounded by municipal roads. It has river channel between the road at the north and east side. Besides, the walls of foundation pit are near to the boundary line and thus it is a limiting factor to the construction works. The foundation soil at the project site is mainly composed of miscellaneous fill, clay, silty clay, silty clay, silt mix with silty clay and silty clay mix with silt.

The total area of foundation pit is about 31000 square meters in rectangular shape. Longest distance between the east and west site is about 169.4 meters whereas 197.9 meters between the north and south site. The north excavation depth is 9.35-10.85 meters whereas east excavation depth is 9.60-10.35 meters. The bracing of foundation pit utilizes the bored pile integrated with prestressed hot melt removable anchor support structure whereas its corner adopts the bored pile integrated with concrete support structure. The corner reinforced concrete adopts temporary steel column as the vertical supporting member of the supporting system, and the mid-span anchor adopts the hot-melt compressive stress-dispersive removable underreamed anchor.

Hot-melt compressive stress-dispersive removable underreamed anchor has anchorage with 3 meters long that uses high pressure jet expansion head. In addition, it has built-in 3 unit carrier, the spacing of the carriers are 1m and equipped with 6 Φ15.2 unbonded steel strands. The ultimate bearing capacity is 760KN, 800KN respectively.

The foundation pit project commenced at the end of 2014 and was excavated to the bottom in May 2015, as shown in Figures 6-12. The anchor construction adopts the compressive stress-dispersive underreamed anchor by jet grouting, which is convenient and quick as it was constructed in one-time. The rotary jet grouting adopts the specific rotation speed and the drilling depth control and the anchor has mild dispersion as well. Inspection was made with 16 anchors selected randomly and all of them met the requirements (both with capacity 760KN, 800KN respectively). At the end of the project, the recovery rate reached 96.7% while the normal recovery rate is 95.5%. The rest is recovered via large tonnage.

Suzhou Taihu Golden Port Commercial Integrated Foundation Pit Support Project had used the compressive stress-dispersive underreamed anchor by jet grouting according to the characteristics of the project and achieved excellent results. According to the study, the adoption of bored pile + hot-melt removable anchor as the bracing scheme has saved up to 10million yuan construction cost and shorten the construction period for 6 months, as compared to the bracing scheme of bored pile + reinforced concrete. Furthermore, dust, noise and solid waste pollution was at minimum impact and hence improving the reliability and safety of the support.