MINED TUNNELING | TECHNICAL
The support system was supplemented by a series
of temporary, vertical steel props, installed sequentially at every excavation step next to both sidewalls of each gallery, except for Galleries 3 and 4. The props were supported on precast concrete pads, 500mm x 500mm x 250mm, mechanically connected to the 350mm-thick concrete slab. Every set of props was connected (in the transverse direction) with a set of connecting beams.
DESIGN CONSIDERATIONS The gallery excavation and support was simulated using 2D Finite Element analyses assuming plane strain conditions. These analyses were used to model the excavation sequence and calculate the lining stresses. The existing reinforced concrete TTC box was simulated as shown in Figure 5. The four joints between the segments were introduced as physical discontinuities. An interface was additionally introduced between the
TTC box base slab and the underlying soil to capture the soil/structure interaction as realistically as possible. The joint was a frictionless interface with stiffness parameters corresponding to those of the soil and a Mohr Coulomb slip criterion. This interface allowed the normal separation between the structure and the underlying soil. The analysis also accounted for moving train loads
and seismic loads. The calculated displacements of the most conservative analysis are shown in Figure 6.
CONSTRUCTION
Construction Sequence of Galleries Prior to the start of the construction works of the mined galleries, the installation of the structural steel waler beams in front of the existing TTC box had to be completed. The horizontal beams rest on top of deep foundation piles and are structurally bolted against them, providing end support for the needle beams.
The excavation of the first two galleries, 1A and 1B,
commenced on 30 September 2019. To reach the actual ground level below TTC box, access cuts within the beams had to be excavated and supported by 200mm-thick FRS shotcrete. Each gallery was then advanced from both sides to accelerate the overall construction schedule and to allow symmetrical supporting of the TTC box. The excavation works of the Galleries 1A and 1B were finalized by the end of October (see Figure 7). Once all vertical steel props had been jacked in
Galleries 1A and 1B, the structural needle beams were installed, and excavation works of Gallery 2 commenced on 4 November. Gallery 2A was fully excavated and supported on 11 November. Subsequently, the needle beams in Gallery 2A were installed and, on 16 November, works of the extended Gallery 1A commenced. The removal of the south sidewall of Gallery 1A occurred concurrently with the relocation of the inner, vertical props for access purposes, as shown in Figure 8 (left). Extended Gallery 1A excavation and support works were completed on 22 November, as shown in Figure 8 (right). Construction works continued with the installation
of the needle beams in extended Gallery 1A. On 2 December, excavation and support works of Galleries 3 and 4 commenced. The typical advance length was increased from 1.1m to 2.5m since only the excavation of the remaining soil and spraying the invert slab had to be executed. No installation of vertical steel props was necessary for Galleries 3 and 4, as shown in Figure 3 and Figure 6. Galleries 4 and 5 were completed on 6 December
2019. The total construction time of all mined galleries was just over two months. Figure 9 shows the final configuration of the underpinned TTC Spadina subway box.
TTC box (indicative)
Temporary shotcrete 1st layer spray 100mm
Temporary steel props 1
gallery C
L SEM
Temporary shotcrete 2nd layer spray 200mm @ top 50mm @ bottom
15m @ 275 DWLS drill & epoxy grout
elevation varies
Springline Prop hydraulic unit
500mm x 500mm x 250mm precast concrete pad
Local over excavation to place pads
Fibre reinforced shotcrete slab 350mm thick
Connecting beams c100x11 preinstalled in footings
Above, figure 4: Typical cross-section of the mined galleries (4m wide by 2.75m high) Winter 2023 | 15
WP
Theoretical excavation line Protective former indicative
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57
