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The Abu Dhabi STEP project includes 45 km of bored tunnel

Photo: COWI/Claus Knuth
The Abu Dhabi Strategic Tunnel Enhancement Programme (STEP) is to provide a major improvement in the capacity of Abu Dhabi’s waste water system

The programme includes a new deep gravity sewer tunnel which takes the main sewer flows from Abu Dhabi to a new treatment works outside the city and a series of link sewers connecting into the deep tunnel.

The new deep gravity sewer tunnel is 45 kilometres in length. It varies in diameter from 5 metres to 6-8 metres along its length. The tunnel is being constructed with seven earth pressure balance (EPB)  tunnel boring machines (TBMs) which are launched and operated from seven deep shafts.

COWI designs bored tunnel and shafts

COWI is carrying out the detailed design of two of the major contracts - T-02 and T-03 - for Impregilo S.p.A..

T-02 includes 15 kilometres of deep, bored tunnel, six deep shafts that vary from 50 metres to 60 metres in depth and all ancillary works.

T-03 includes 10 kilometres of deep, bored tunnel, four deep shafts that vary from 60 metres to 80 metres in depth and all ancillary works.

Design of temporary and permanent works  

COWI’s tasks include the design of all the permanent works which includes a vortex drop structure in one of the shafts.

COWI is also carrying out the design of the temporary works that includes deep diaphragm walls for the shafts and the design of the sprayed concrete lining (SCL) caverns for the launch and recovery of the TBMs.

Leading edge durability design

The chloride levels in the ground and groundwater are considerably higher than in seawater. In order to deal with this severe environment, COWI’s leading edge durability design solutions have been used with the concrete mixes being designed to meet the 80-year lifetime required of the structures.

Steel-fibre reinforced concrete tunnel lining

The segmental tunnel lining is being constructed using steel-fibre reinforced concrete (SFRC). This is a modern development in lining design that makes the tunnel lining less susceptible to chloride-induced corrosion and less likely to be damaged during construction.

The bored tunnel will be lined with a corrosion protection lining formed with a HPDE membrane and a secondary concrete lining.


LAST UPDATED: 01.02.2017