INTRODUCTION-UNDERGROUND STATIONSIn an urban area, the development of rail-based mass transport system is planned on the basis of availability of right- of-way; which is generally at grade, underground tunnel, or elevated railway (IWGN, 2004). A typical underground station has different levels based on utilization and operations; they can be summarized as follows:
1) Roof Slab: This allows passengers to access the station from the surface. It is generally 1-3 m below the ground level forming the top of the station box and is not visible.
2) Concourse Slab: This is the first level which is exposed to passengers when entering the station. It houses many access facilities and is used for ticketing, public interchange movement, equipment storages and other commercial activities.
3) Base Slab: It has two levels: Platform Level: It is used for boarding and un-boarding the train and Under croft level: it is the lowest level of the station where tracks are placed for rail movement. (Paul, Khursheed, & Singh, 2017)
Figure 1 Typical Underground Station Section ( Numbering of elements represents sequence of construction by Top-down method)
CUT AND COVER METHOD
For construction of underground metro station, Cut-and-Cover method is adopted for minimal disturbance to the adjacent structures. It is economical for depth ranging from 40 to 60 feet. In this method, station structure is constructed post excavation and it is covered again when the structure is complete. To minimize the traffic disruption a temporary decking is provided over the excavated trench immediately after the first layer of excavation is completed. Subsequently station can be constructed by either of the two methodologies: Bottom Up and Top-Down. “Bottom Up” is a conventional construction approach. Here, using open cut- unsupported sloped slides or using vertical support systems, a trench is formed. Depending on the site condition, soil type and excavation depth; the type of retaining wall can be temporary or permanent are installed, namely: diaphragm wall, concrete pile wall or steel sheet pile wall. The excavation is carried till the final depth while installing strut supports at various levels depending on the excavation depth. After reaching the formation level or final depth the cast in situ RCC base slab is constructed, further the lowest level strut is removed, and side walls are constructed. The process continues as construction proceeds upwards by removing the struts, constructing walls, and then the slabs. After the roof slab is completed and waterproofing is carried out; the soil is backfilled to the first struct level. Next, the strut is removed followed by completely backfilling the top of station structure and completing the Cut-and-Cover Method.
Figure 2 Bottom-Up Station Construction
TOP DOWN CONSTRUCTION METHODOLOGYReverse to the ‘Bottom-Up’ construction is the unconventional ‘Top-Down’ method where the construction graduates along with gravity. Here, the earth retaining walls which support the excavation are constructed accompanied by composite pile. Before excavation of slabs, Barrettes pile and plunge columns are constructed as load bearing elements and to reduce the span of the slab. The roof slab is constructed next and joined into the earth retaining walls (via couplers left in wall). Then construction of concourse slab and base slab is completed after the top slab. Here, the slabs are constructed as the excavation proceeds. (Refer figure 1) Before starting the station construction by Top down method, the nearby area is surveyed, and proper instrumentation is fixed for monitoring during deep excavations. After area clearance, the traffic is diverted as per the traffic plan, the area is barricaded, and utilities are relocated as per the utility shifting plan
Guide Wall and Diaphragm WallThe first step is the construction of retaining wall. In soils with less cohesion, diaphragm wall can be constructed. Before construction of Diaphragm wall, ‘L’ shaped RCC guide walls of 1.5-2.5 m are constructed along the perimeter of the station for initial depth to prevent soil collapse, guide the grabbing equipment for diaphragm wall (D-Wall and also uphold the verticality of the D-Wall. After casting of the guide wall, the excavation of D-Wall is carried out using a hydraulic grab. The excavation is continuously supported by circulating polymer slurry to prevent soil collapse. The diaphragm walls are constructed in alternative panels classifying into primary and secondary ranging around 5 m. PVC water stoppers are inserted between two panels for waterproofing, which also acts a shear key. Water stopper is fixed in the stop-end groove which is installed after completion of excavation up to 25 m and width of 800-1200 m. While making reinforcement cage for the D-Wall, L-shaped bars with couplers are tightly welded along with at roof slab, concourse slab and base slab level, in order to join the wall and slab when constructing the slab. The reinforcement cage is then lowered with help of cranes. A thin layer of grease is applied on threaded bars before fixing couplers and then couplers are packed and covered with polystyrene, tape, and ply to protect the coupler from cement slurry or soil ingression. Then concreting is done with help of tremie and polymer slurry is pumped out. The stop-end is removed within 3-5 hrs using hydraulic jacks and this process completes the construction of D-Wall. Such diaphragm wall is constructed along the perimeter of the station box
Figure 3 Barrette Pile and Plunge Column sequential construction procedure
Plunge Column and Barrette PileA barrette is a cast-in-situ reinforced concrete rectangular piles that is used as deep foundation to resist vertical and horizontal loads and stabilize deep excavations that can accompany the diaphragm walls for facilitating Top-Down construction. Prefabricated steel columns known as Plunge Column are embedded in bore piles while concreting. These Plunge Columns are vertical steel members that act as a temporary/permanent column to provide support to the slabs during construction stage. In case of temporary column, sand is filled and for permanent column concrete of appropriate grade is filled along with pile concreting. For Barrette pile construction, first shallow boring is done using auger machine and steel casing is lowered to prevent soil collapse and also to act as guide for the Kelly. The drilling then continues and auger is replaced by bucket to bring the soil up and the process continues. Soil is supported by polymer slurry during excavation. The boring is done till depth depending on the toe level of pile. Depth sounding check is done. Pile cage is fabricated and lowered in the excavated bored hole filled with slurry. Lowering of pile cage is followed by attachment of steel plunge column. Permanent steel column has shear connectors welded along with so the column can be connected to slab while reinforcing of slab. After completion of slab construction jacketing will be done around this column to make it a permanent one. The whole assembly is then lowered, and the plunge column is welded with the adopter which acts as a guide for lowering and to prevent twisting of plunge column during or after concreting. Finally, tremie concreting is done for pile and permanent column, and sand is filled if it is a temporary column. the soil up and the process continues. Soil is supported by polymer slurry during excavation. The boring is done till depth depending on the toe level of pile. Depth sounding check is done. Pile cage is fabricated and lowered in the excavated bored hole filled with slurry. Lowering of pile cage is followed by attachment of steel plunge column. Permanent steel column has shear connectors welded along with so the column can be connected to slab while reinforcing of slab. After completion of slab construction jacketing will be done around this column to make it a permanent one. The whole assembly is then lowered, and the plunge column is welded with the adopter which acts as a guide for lowering and to prevent twisting of plunge column during or after concreting. Finally, tremie concreting is done for pile and permanent column, and sand is filled if it is a temporary column
Construction of Roof Slab
Instrumentation and Monitoring: Monitoring of ground settlement in nearby existing building and water table measurement is carried throughout excavation by continuous readings of the installed piezometer, inclinometer, and building and ground settlement marks.
Excavation: Hacking and removal of all inner guide-walls and unsound top concrete of D-Wall is done, and excavation of the roof slab is commenced up to formation level (roof top bottom). Dewatering is carried out continuously, and water level is maintained 1 m below the on-going excavation level.
Preparation of Formation Level: After completing the excavation to the formation level, the ground is levelled and compacted to 95% of Proctored density. Plain cement concreting of 75 mm thickness is laid and levelled. Continuous survey is done to ensure the finishing levels. This is followed by curing for a day. After hardening of binding concrete, it is covered by 3 mm thick plywood and nailed to PCC. At joints, adhesive tape is used to make it watertight. This ply board shall act as a de-bonding agent between roof slab and binding concrete. This will also ensure lesser finishing efforts when exposed.
Reinforcement: On diaphragm wall, three levels are marked which are: roof top level, roof bottom level, and couplers level. This is followed by chipping of D-wall to expose the couplers. The couplers are mapped, and identification of missing couplers, if any, is done. Tolerance checks are carried out. For roof slab, fabrication of bars for slab and beam is done, marked on PCC and erected. Cut out are kept as planned from where equipment will be lowered for excavation of the next level. Like in D-wall, in slab too couplers are kept and protected for further connection when cut outs will be filled. Reinforcement is connected to in between plunge columns, covers and chairs are placed, this completes the reinforcement of roof slab and beam (inverted).
Shuttering and Concreting: Shutters are cleaned, oiled and fixed along sides of cut out and water stopper for avoiding cold joint is fixed at slab part end. Concreting is done using mechanical means followed by de-shuttering and curing by conventional method for 14 days
Figure 5 Top-Down Station sequential construction procedure
1.1 Comparison of Top-Down with Bottom-up Method
- Top-Down method requires highly skilled manpower.
- Top-Down’s cost is lower as minimal staging is used.
- Waterproofing outside external wall is not possible in the Top-Down method (Ozgur, 2015).
- Less width of construction is required in the Top-Down method.
- Faster Speed- In Bottom-up, the whole excavation needs to be done only after which construction is possible, while in Top-Down the earth acts as natural lateral support taking less time for strengthening. Top-Down also allows early restoration of ground surface above station. Ease of overlapping construction activities also shortens the time(Ozgur, 2015).
- The structural slab in Top-Down acts as internal bracing supporting excavation, thus reducing the quantum of struts required.
- Diaphragm wall connection with slab is a crucial activity which shall be done with utmost precision.
- Access to the excavation is limited in case of Top-Down (Ozgur, 2015).