Construction methodologies

With the Thomson Line running through a variety of locations, different types of construction methodologies will be applied in the construction of the stations. These can be classified into two major groups - bottom-up construction and top-down construction.

A summary of the Thomson Line station construction methodology.

Both the bottom-up and top-down methods take their name directly from the sequence in which the underground structure is constructed. In the bottom-up method, the base of the structure is constructed and subsequently moves up to the surface. The top-down method sees the top slab being constructed and then construction moves downwards to the base slab.



Bottom-Up Methodology

The bottom-up methodology, as explained earlier, refers to the technique of constructing underground structures starting from the bottom and working upwards.

The first step is obtaining the initial parcel of land on which construction of the structure will take place. Clearing of the ground for the construction site commences and leveling is sometimes required in uneven terrain.

Following that, the construction of diaphragm walls or contiguous bored piles are possible. These will help to form the perimeter walls of the excavation site and help to push the surrounding soil back, preventing collapse of the excavation site. For diaphragm walls, usually a hydrofraise machine is used. This machine has a special cutter attached to the boom arm that allows a betonite slurry to be pumped down to the cutters, preventing collapse of the excavated area for the wall.

The cutter works its way down to the intended level and the slurry mixture helps to keep the shaft intact.

Rebar cages are then lowered into the shaft to provide reinforcement for the wall that is being constructed.

While one wall is completed by pouring in concrete and pumping out the betonite slurry, the process commences on the opposite side. Sometimes it is possible to carry out the construction of diaphragm walls on both sides concurrently, depending on the contractor.

Once again, rebar is lowered into the completed shaft for reinforcement of the wall.

Once all the walls are completed, this provides the rigid vertical structure to contain the excavation site and prevent the surrounding soil from moving and collapsing into the excavation area.

Excavation commences and king posts are installed. King posts are metal piles that are driven into the ground and will help to act is pillar supports for the earth retaining system.

Subsequently, the next component of the earth retaining system is installed - the struts. These form the horizontal braces that push the diaphragm walls apart, preventing them from moving inwards.

Excavation commences downwards, with struts being installed at regular intervals to hold the diaphragm walls apart. The base slab is then cast once the desired level is attained.

Work now commences on casting the intermediate level slabs and struts are removed as the structure forms. As the structure now takes over the job of the struts, holding the walls apart, the struts can be dismantled as the structure progresses upwards.

Slab casting continues upwards and the strut removal continues as well.

The roof slab is cast and preparations for the construction of entry points begins with the final layer of struts soon to be removed.

With the walls of the entry points up and the roof slab completed, all struts are removed.

The king posts are cut out from the support system as the final structure takes over to support the surrounding earth unaided. Back-filling of excavated earth over the completed structure begins while work on the entry points commences in earnest.

The structure is capped off and the diaphragm walls demolished from the top of the construction level previously. Soil is filled over the diaphragm wall caps and the station is subsequently fitted out with its architectural finishings (like ceiling panels, wall panels) before testing and subsequent commissioning as a transit point.

The bottom-up method is typically used when there is sufficient land to divert any obstructions such as utility lines and roadways away from the construction site. This is a quick system that allows the contractor to focus on one job at a time.

Top-Down Methodology

For areas that are heavily built up, with tall buildings around and limited space to divert roadways or utilities, the top-down method is a safer option. Here, soil movement is limited as the permanent slabs are constructed to act as the rigid earth retaining system.

The site is handed over to the relevant contractors who clear the area and level it if needed.

Diaphragm walls or contiguous bored piles are to be built. Here, the hydrofraise machine is brought in to construct the diaphragm wall on the cleared site.

The machine's cutter digs the first shaft where one panel of the wall will be installed.

The rebar cage is lowered in to form the reinforcement for the wall. The hydrofraise machine shifts over to construct the parallel wall.

Concrete is poured into the first panel as the parallel panel shaft is readied.

Rebar is installed in the parallel wall to be cast.

Concrete is poured and the wall panels are completed. From here on, the process differs from the bottom-up method.

Excavation commences with the king posts and struts being installed.

Then, the roof slab is cast before excavation continues. Access shafts are created in the roof slab to allow machinery and materials to be delivered below for further excavation of the lower floors.

A second layer of struts is installed below the roof slab and excavation continues down through the access shafts.

The next slab is cast - in this case, the concourse slab. Once again, access shafts are provided for in the slab allowing further access downwards.

Struts are installed following further excavation and slabs cast once sufficient depth is reached in this manner till the final slab layer is reached.

The base slab is then cast. From here on, the struts can be removed and the slabs closed up appropriately. Occasionally, struts are removed after every slab is cast as the concrete structure is able to withstand the pressures of holding the diaphragm walls apart.

As the struts at the lower levels are removed, the slabs can have the temporary access shafts closed up.

At the final level, the entry point is created.

Soil is back-filled above the completed roof slab.

The entrance continues to take shape above ground at street level.

The entry point is topped off and the major structural works are completed.

The diaphragm walls are demolished to below street level and facilities are reinstated above the station. Architectural fit-out works continue before systems testing and final commissioning of the station.

The top-down method allows excavation to continue in densely built up areas while allowing minimal building acquisition or demolition during the duration of construction. Slightly more tedious than the bottom-up method as the contractor needs to keep switching between various processes, this allows stations in densely populated areas to be built (for instance the Holland Village station along the Circle Line). Another factor that slows down the progress as compared to the bottom-up method is the restriction of space within the excavation confines. There is limited space when working in such areas due to the slabs being cast as excavation progresses downwards. Supply of fresh air is important in order for work to continue as well as the provision of sufficient lighting.

Hope that this has given you a better understanding of the construction methods to be used at the different station locations along the Thomson Line and provide a better appreciation for the hard work undertaken by all involved in the construction of the stations.