Bangkok metro - Blue Line

THAILAND, 1997 - 2000

Construction of nine stations and associated works for Bangkok's first underground railway line.

As part of construction of the first section of the first underground railway line in Bangkok, the Blue Line project involves construction of 20 kilometres of tunnel with 18 stations, 1 depot and a number of aeration shafts and other works.

The project is split into two routes, known as the North and South Routes.

As part of the foundation works, SBB JV (in which Soletanche Bachy had a 66% share) was awarded the works for nine stations and associated works.

The geology (water table 3 metres below ground, soft clays and sands below the water table) called for the following works:
- Construction of perimeter diaphragm walls of variable thickness and depth, acting as permanent retaining walls, seepage cut-offs and structural underpinning for vertical loads.
- Prefounded columns and/or barrette piles on station centrelines as added support for vertical loads.
- Jet grouting of the plugs for the TBM break-in and break-out at the stations.

The special construction conditions surrounding these underground engineering works in the city centre without interrupting street traffic meant that the stations had to be built in two halves, backed up by special works such as:
- At Silom station, composite concrete and steel diaphragm wall, 45m deep. The technique consists of using interconnected steel beams over the whole length of the structure, instead of the usual steel bar cages. At Silom, the steel beams were 900mm wide (the trench was 1200mm wide) and installed to a depth of 45m.
- The diaphragm walling had to be carried out with a headroom of only 6 metres.
- Barrette piles were built to take the loads from the motorway flyovers.
- Circular diaphragm-walls were installed to construct shafts 25m deep with an inside diameter of 7 metres, with very little ground level space for the work.

Together, the severe geological, scheduling and quality constraints required six diaphragm wall teams, two piling teams and three jet grouting teams to operate simultaneously with a maximum workforce of 700 people and
• three KL 1000 and 1200 diaphragm wall trenching rigs
• three KS 3000 rigs with 1000 and 1200 hydraulic grab buckets
• two 1500mm and 1800mm piling rigs
• three CMV jet grouting units with Techniwell pumps
• concrete plant with a peak capacity of 2000 m3 of concrete per day.

All the KS 3000 hydraulic rigs were equipped with a real time data acquisition system monitoring grab deviation, in order to meet the design tolerance of 0.5% on verticality.

Special trenching grabs had to be made to suit the non-standard geometry of diaphragm wall panels, with a maximum 4.5m length. The KS 3000s had a grab bucket opening of 4.20m to ensure trench verticality and successfully meet the productivity requirement for completing a panel every 24 hours.

Construction of Silom station and the underpinning for the motorway flyovers had to be performed with a headroom of only 6.00m. A "Short KS 3000" was built especially for the job, capable of:
- digging diaphragm wall and barrette pile trenches with a working headroom of only 6.00m
- performing this work to within 0.5% of the vertical to depths of 60m.



Diaphragm wall

A diaphragm wall is a reinforced concrete wall that is made in situ. The trench is prevented from collapsing during excavation, reinforcing and casting by the use of supporting bentonite slurry. The slurry forms a thick deposit (the cake) on the walls of the trench which balances the inward hydraulic forces and prevents water flow into the trench. A slurry made of polymers can also be used.

Jet grouting

Jet grouting is a construction process that uses a high-pressure jet of fluid (generally 20 – 40 MPa) to break up and loosen the soil at depth in a borehole and to mix it with a self-hardening grout to form columns, panels and other structures in the ground. The parameters for the jet-grouting process and the desired final strength of the treated soil depend on a number of characteristics, such as the soil type, the technique used and the objective to be reached. In granular soils, the high-pressure jet breaks up the grains through erosion, while in a cohesive soil, such as clay, the jet breaks the mass up into small particles. High pressure is needed to produce the kinetic energy required for the jet through a small-diameter nozzle. Waste material from the process (a mix of soil, water and binder) is recovered at the surface before being taken away for disposal.

Piles and micropiles

A pile is a structural element driven into the soil for transferring loads and prevent deformation. Its slenderness ratio is not limited.
Pile shafts can be uniform and rectilinear, telescopic and belled out.
Piles can be installed either separately or in groups. They can also form a retaining wall, a mixed curtain wall, contiguous piles, secant piles and composite curtain walls, such as Berlin walls and similar. Piles are also used as precast beams to be placed in the structure of the building they support.



Soletanche Bachy has helped build most of the world's major metro systems, performing soil consolidation grouting and constructing cut-and-cover tunnels with cast-in-situ diaphragm sidewalls, bored tunnels and major underground openings.

Railway works

Soletanche Bachy has developed a number of techniques for use in railway works; cut and cover tunnels, highways, underground stations, viaduct etc.


Circular shafts excavated inside a pre-formed cylindrical diaphragm wall only began to develop in a major way when modern plant became capable of digging almost perfectly vertical walls such that the cylinder can be considered self-stable without having to be tied back with ground anchors, or strengthened with struts or reinforced concrete rings.

Voyage au coeur de l'ingénierie de pointe

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