Differential uplift and settlement between inner column and diaphragm wall in top-down excavation

Abstract

Top structure and basement will confront the risk of being damaged on account of large stress and strain fields incurred by differential uplift and settlement between inner column and diaphragm wall in top-down method. Top-down excavation of the Metro Line 10 in Shanghai was modeled with finite element analysis software ABAQUS and parameters of subsoil were obtained by inverse analysis. Based on the finite element model and parameters, changes in the following factors were made to find more effective methods to restrain differential uplift and settlement: length of diaphragm wall, thickness of jet-grouting reinforcement layer, ways of subsoil reinforcement, sequence of pit excavation, connection between slabs and diaphragm wall or column and width of pit. Several significant results are acquired. The longer the diaphragm wall is, the greater the differential uplift between column and diaphragm wall is. Rigidity of roof slab is in general not strong enough to keep diaphragm wall and column undergoing the same uplift during excavation; Uplift at head of column and differential uplift between column and diaphragm wall decrease when subsoil from −16.6 to −43 m in pit is reinforced through jet-grouting. But, as excavation proceeds to a lower level, benefit from soil reinforcement diminishes. During the process applying vertical load, the larger the depth of diaphragm wall is, the smaller the settlement is at head of column and diaphragm wall, and the greater the differential settlement is between column and diaphragm wall. When friction connection is implemented between column, diaphragm wall and floor slabs, uplifts at head of column and diaphragm wall are larger than those of the case when tie connection is implemented, and so does differential uplift between column and diaphragm wall. The maximum deflection of diaphragm wall decreases by 58% on account of soil reinforcement in pit. The maximum deflection of diaphragm wall decreases by 61.2% when friction connection is implemented instead of tie connection.

Keywords

top-down method / differential uplift / differential settlement / jump-layer excavation / diaphragm wall with outriggers

References

[1] LiuY, LiJ-p, ChenW. Key construction techniques for oversized excavation pits using top-down method [J]. Chinese Journal of Geotechnical Engineering, 2013, 35: 489-494

[2] FanQ-g, ZhaoX-h. Theory and computation of up-down construction method for deep excavation engineerings [J]. Rock and Soil Mechanics, 2006, 27(12): 2169-2176

[3] ZhangJ-x, ZhongX-m, ZhangS-c, WuD-y, NiuL. Bearing and deformation characteristics of erect column piles in deep foundation pits using top-down method [J]. Chinese Journal of Geotechnical Engineering, 2008, 30: 400-403

[4] ZhangJ-x, LiuS-j, ZhouJ-b. Analysis of influence of foundation pits excavation unloading by top-down method on engineering structures [J]. Rock and Soil Mechanics, 2010, 31: 218-223

[5] YangX-l, ZhouP-h. Some key problems in design of basements using top-down method [J]. Chinese Journal of Geotechnical Engineering, 2010, 32: 238-244

[6] YangX-l. Application and development of new technology for support of deep excavations in coastal areas of Zhejiang [J]. Chinese Journal of Geotechnical Engineering, 2012, 34: 33-39

[7] WangW-d, WengQ-p, HuY-y. Application and design of new top-down excavation method [J]. Chinese Journal of Geotechnical Engineering, 2006, 28: 1546-1551

[8] WengQ-p, WangW-d, XuZ-h. Design and practice of a super large excavation using top-down method in soft deposit [J]. Chinese Journal of Underground Space and Engineering, 2005, 1(4): 587-590

[9] ZhongZ, XuL, WangQ-g, FengH-t. Design and practice of top-down method in deep foundation pits very close to protected buildings [J]. Chinese Journal of Geotechnical Engineering, 2010, 32: 249-255

[10] DaiB, WangW-d, XuZ-h. Design & practice of deep foundation pit constructed by top-down method in dense-constructed area [J]. Chinese Journal of Underground Space and Engineering, 2005, 1(4): 579-583

[11] ZhuX-h, WangG-p, GuQ-y. Design and practices of deep foundation pits of towering buildings constructed by top-down method [J]. Chinese Journal of Geotechnical Engineering, 2006, 28: 1565-1568

[12] JiaJ. Deflection controlling measures and practices of deep foundation pits by use of top-down excavation method [J]. Chinese Journal of Geotechnical Engineering, 2007, 29(2): 304-308

[13] LiuZ, LuoZ-q. Design and construction of ultra-large deep excavations [J]. Chinese Journal of Geotechnical Engineering, 2012, 34: 507-511

[14] WangW-d, ZhuW-l, ChenZ, WengQ-p, WuJ-b. Design, study and practice of deep cylindrical excavation of Shanghai World Expo 500 kV Underground Transmission Substation project [J]. Chinese Journal of Geotechnical Engineering, 2008, 30: 564-576

[15] ZhengG, LiuQ-c, DengX, ZhangL-m. Analysis on monitoring of top-down method of section 2 of Tianjin railway station underground transfer centre [J]. Journal of Tianjin University, 2012, 45(10): 930-937

[16] TanY, LiM-w. Measured performance of a 26 m deep top-down excavation in downtown Shanghai [J]. Canadian Geotechnical Journal, 2011, 48(5): 704-719

[17] TanY, WangD-l. Characteristics of a large-scale deep foundation pit excavated by the central-island technique in Shanghai soft clay. II: Top-down construction of the peripheral rectangular pit [J]. Journal of Geotechnical and Geoenvironmental Engineering, 2013, 139(11): 1894-1910

[18] LiuG B, JiangR J, NgC W W, HongY. Deformation characteristics of a 38 m deep excavation in soft clay [J]. Canadian Geotechnical Journal, 2011, 48(11): 1817-1828

[19] OuC-y, LiaoJ-t, LinH-d. Performance of diaphragm wall constructed using top-down method [J]. Journal of Geotechnical and Geoenvironmental Engineering, 1998, 124(9): 798-808