Developments in the design of tunnels and caverns in the Triassic rocks of the Sydney region

Effects of tunnelling on existing support systems of perpendicularly crossing tunnels

“…13). Moreover, this phenomenon is observed in tunnelling practice in the Sydney region; the excavation of the access tunnel to Opera House Carpark cavern with a span of 10 m induced only approximately 0.5 mm of settlement at the crown due to the relatively high horizontal stress compared with the vertical stress [14]. Similar phenomena were also found in the field observations in the Northside Storage Tunnel Project in Sydney [20].…”

“…3 before Step 42. In actual tunnelling in the Hawkesbury Sandstone, Pells [14] also noted the compressive stress concentrations at the crown due to the high horizontal stress field in the Sydney region.…”

“…According to Pells's study [14], in the metropolitan area of the Sydney region, most of tunnels are constructed in the Hawkesbury sandstone, a benign medium with intact unconfined compressive strength of 25-45 MPa, Brazilian tensile strength of 2-3 MPa, and Young's modulus of 2.5-8 GPa. However, the intensive weaknesses (bedding discontinuities, joints and faults) in the Sydney region make the substance parameters of the rock mass at the ''tunnel scale" very low and there is no agreements on the field scale parameters of the rock mass to be adopted in practice.…”

“…Moreover, according to Pells's study [14], the regional stresses in the metropolitan area of the Sydney region can be approximately represented using the following equations:…”

“…It should be noted that not all of the exposed tunnel surfaces are supported in the current step and the unsupported length is 4 m. The unsupported surfaces near the advancing face of the tunnel allow some deformations to occur before the installation of the support system. Moreover, according to Pells's study [14], rock bolts are normally used as supports in combination with the shotcrete lining in underground construction in the metropolitan area of the Sydney region. Thus, the linear elements representing the rock bolts are also reactivated behind the tunnel face in every other step in order to simulate the support provided by the rock bolts, as shown in Fig.…”