Bi-layer diaphragm walls: Experimental and numerical structural analysis Luis Segura-Castillo a,b,⇑ , Antonio Aguado a , Alejandro Josa c a Technical University of Catalonia–Barcelona Tech (UPC), School of Civil Engineering (ETSECCPB), C/Jordi Girona, 1-3, Modul C1, Barcelona 08034, Spain b Instituto de Estructuras y Transporte, Facultad de Ingeniería, Universidad de la República (UdelaR), J. Herrera y Reissig 565, 11300 Montevideo, Uruguay c Department of Geotechnical Engineering and Geosciences, School of Civil Engineering, Universitat Politècnica de Catalunya–Barcelona Tech (UPC), Barcelona, Spain article info Article history: Received 29 November 2012 Revised 16 March 2013 Accepted 22 April 2013 Keywords: Fibre concrete Sprayed concrete Numerical analysis FEM PLAXIS Watertightness abstract The bi-layer diaphragm wall, a new type of wall, consists of two concrete layers, the first of which is poured and the second sprayed, in different construction stages. A major aim of the research conducted is to maximize the functional attributes of the second layer, enhancing both structural performance and watertightness. The central objective of this study is to corroborate the structural behaviour of these walls in experimental and numerical terms. It follows a three-step methodology: a full-scale experimen- tal campaign; development of a Finite Element Model (FEM) capable of predicting the structural behav- iour of the wall; and, assessment of the second layer contribution. The experimental campaign confirmed the viability of the constructive solution and the FEM model accurately reflected the experimental data. A comparison between the bi-layer wall and other single-layer walls showed that the contribution of the second layer permitted reductions in first-layer reinforcement, adding to its various other functional advantages. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Large cities are encouraged to make efficient use of space, espe- cially below ground level [1]. Expanding fleets of vehicles require the adaptation of their transport systems for circulation and park- ing. Urban metro systems and road tunnels help to reduce traffic congestion and to minimize contamination. The excavation works that these structures require should not adversely affect existing infrastructure and should minimize any interruption to the daily life of the city. In this scenario, the conventional diaphragm wall technique frequently represents a viable solution. Economies in a diaphragm wall project may be achieved at the beginning of the design process, when selecting the method, the construction sequence, and the type of wall, and in the optimiza- tion of the temporal and permanent use of the retaining structure [2]. Accordingly, material consumption, the final dimensions of the wall, maintenance requirements, and construction complexity should all be evaluated before the adoption of any one solution [2]. A widespread problem associated with this construction tech- nique is leakage whenever the walls are erected in water-bearing ground. As there are no existing techniques to make diaphragm walls fully watertight, a variety of alternatives have been devel- oped to cope with the leakage problem [3]. A common technique is repairing locally with a waterproof mor- tar render over areas where leakage is detected. However, leakage usually only appears over long time periods, at different times, and in different areas of a wall, causing problems for both owners and contractors. A less widely applied solution consists of casting a sec- ond layer of waterproof mortar (or concrete) over the inner face of the walls. Since the whole surface is covered, this is an effective al- beit expensive solution [4]. Finally, another common practice, al- ready standardized in British construction codes [5,6], is to construct an inner wall separated by a cavity [3], at the bottom of which the water is left to accumulate before it is pumped out. Although dry inner walls are still constructed, this solution presents some drawbacks: the inner wall loses significant volume in view of the cavity and construction tolerances and it may, at worst, conceal dangerous leakages and even structural problems. The major aim of this research project is to maximize the func- tional attributes of the second layer of concrete, based on the sec- ond lining solution described above, by allowing it to play a structural role, in addition to its initial intended purpose (water- proofing). In accordance with the structural role of the second layer, the thickness and reinforcement of the first layer may there- fore be reduced. The dimensions of this bi-layer diaphragm wall and its improved watertightness suggest that it could be a feasible structural solution. 0141-0296/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.engstruct.2013.04.018 ⇑ Corresponding author. Address: Universitat Politecnica de Catalunya, C/Jordi Girona Salgado, 1-3, Módulo C-1, 2ª planta, Despacho C-8, Barcelona 08034, Spain. Tel.: +34 93 401 07 95. E-mail address: lsegura@fing.edu.uy (L. Segura-Castillo). Engineering Structures 56 (2013) 154–164 Contents lists available at SciVerse ScienceDirect Engineering Structures journal homepage: www.elsevier.com/locate/engstruct