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The Hong Kong–Zhuhai–Macao Bridge has a total length of 55 km. One of the greatest technical challenges is an immersed tunnel section stretching over 5.664 km and consisting of 33 prefabricated units (tunnel elements). The tunnel is buried 22 m below the seabed, and is therefore subjected to a significant overburden pressure (at a level of ∼100 kPa) during its whole service life. Conventional tunnel elements are either monolithic or segmented. Monolithic elements act like rigid beams, whereas segmented elements behave like a flexible chain. Under this high level of overburden pressure, the tunnel is liable to problems such as concrete cracking, joint opening and leaking. A novel solution for the semi-rigid (structural) system was proposed and adopted. The central idea is to use compressive force and frictional force at the vertical faces of the segment joints to improve the integrity of the longitudinal structure of the tunnel elements. This paper reviews the concept, design and in situ post-construction observations of this semi-rigid system. [ABSTRACT FROM AUTHOR]

• This paper presents novel techniques in Hong Kong-Zhuhai-Macao Bridge project. • Method of deep piling cylinders and cells helps form artificial islands rapidly. • Semi-rigid system developed from segmented element and increases tunnel performance. • Composite foundation layer eliminates uncertainty making settlement pattern instant. • Element built with negative buoyancy is of great interests for long immersed tunnels. The Islands and Tunnel Project of the Hong Kong–Zhuhai–Macao Bridge is a design–build contract involving the construction of two artificial islands, each with an area of 100000 m2, that are connected by a 6.7-km-long underwater tunnel. The immersed section of the tunnel consists of 33 elements. This paper examines the novel techniques used during the design and construction of this project. To eliminate its environmental impact, deep piles consisting of steel cylinders and auxiliary cells were developed to build the artificial islands. Novel structural solutions the semi-rigid system were also developed to address the challenges of a high surcharge load and unfavorable layers of soft soil. Moreover, a foundation scheme consisting of a composite foundation layer and a combined foundation was developed to control settlement and prevent leakage. The factory method was implemented to prefabricate the 33 tunnel elements, including five plane-curved elements, where the mass of a typical element was 78000 t. Severe sediment-related problems were encountered during the immersion of elements of the tunnel that necessitated special measures as well as the development of a sediment warning and forecasting system. The closure joints were constructed by using a novel method involving deployable elements. The novelties introduced during the construction of this project are geared toward simplicity, which is conducive to green construction and sustainable solutions. The authors also provide suggestions for improvements if this kind of project were to be undertaken again. [ABSTRACT FROM AUTHOR]