# Design Study of Steel Fibre Reinforced Concrete Shaft Lining for Swelling Ground in Toronto, Canada
## 1 Introduction  
Underground spaces have been developed to accommodate various types of infrastructure in congested urban areas. Underground structures are significantly different from superstructures because they are constantly subjected to various vertical/horizontal loading from surrounding ground, depending on geotechnical factors such as density, porosity, permeability, elastic modulus, Poisson's ratio, rock strength, joint sets, depth, and groundwater [1; 2; 3]. In addition, accidental events, such as earthquakes, can cause structural problems to underground structures [4; 5; 6]. Vertical shafts are essential in order to access the tunnel level necessary for construction, ventilation, and maintenance purposes from the ground level. In general, shaft structures have been constructed in a circular shape to sustain the lateral pressure using the arching effect [7; 8; 9]. Generally, these shaft linings are predominantly loaded in compression with low flexural loading due to their circular shape. Therefore, in these cases, plain concrete linings are structurally adequate; however, most underground structures are designed with steel reinforcements to withstand the high bending moment from the varied lateral loads in different directions. Reinforced concrete (RC) has many advantages in terms of strength, durability, fire, and weather resistance, that make it ideal for usagein lining structures, especially in increasing cracking resistance, which is a weakness of concrete material [10; 11]. However, the installation of RC lining in underground structures generally accounts for the largest portion of costs; it requires a longer construction time and generally exhibits low resistance to corrosion, and it can cause spalling and the associated loss of the structural capacity of the concrete [12; 13; 14; 15]. Moreover, southern Ontario is known for its cold climates in winter; thus, there are many restrictions on construction works in winter in terms of labour cost, working efficiencies, and construction period [16]. Recently, many types of fibre reinforced concrete (FRC), including steel and synthetic materials, have been investigated to serve as a material that can partially or fully substitute for RC in structures [17; 18; 19; 20; 21; 22]. In particular, steel fibre reinforced concrete (SFRC) is becoming increasingly used in structures because it has advantages over RC in terms of increasing cracking resistance and residual strength of cracked concrete due to the fibres bridging effect [23; 24; 25; 26; 27; 28; 29]. In general, the addition of steel fibres does not affect the compressive strength of the concrete [30], but provides a tensile capacity after the first crack occurs. In tunnelling works, steel fibres distributed throughout the lining strengthens the corner for the segmental lining in the TBM (tunnel boring machine) tunnel, resulting in enhanced durability [31; 32]. Therefore, SFRC could replace the steel bars utilised in the traditional RC solution because the use of SFRC can lead to substantially increased productivity, while also reducing the construction cost and time [33; 34; 35; 36; 37; 38]. The Hydro One Midtown Tunnel is an electric utility tunnel in Toronto, Canada that was designed 60 m below the ground with a length of 2.3 km. The vertical shafts were designed in Georgian Bay shale, which generally exhibits long-term rock stress release behaviour (swelling) [39]. The swelling behaviour of the bedrock can result in significant deformation of the linings over time, after the linings are installed. Thus, it was essential to estimate a substantial increase in the lining forces in the long-term, through experimental or numerical analysis. Geological Baseline Reports recommended that the shaft lining must be installed after permitting ground displacement for at least three months after shaft excavation [40]. The original design consists of a conventional RC lining with steel bars for the vertical shafts to accommodate for the swelling behaviour of the bedrock, based on the geological reports.