Abu Dhabi Municipality wanted a number of underpasses to take pedestrians across the busy six or eight lane main roads of the city in safety, without impinging on the architectural integrity of the high rise, modern cityscape.
Local firm Bauer International, working in conjunction with specialists from its German parent, developed an ingenious solution involving chemical injection along the line of the tunnel to produce stable ground conditions.
Access shafts were constructed using a combination of Berlin Walls and secant pile walls. Following excavation and dewatering, between 17 and 27 horizontal boreholes were constructed around the crown of each underpass tunnel by directional drilling using a special unit made up from a Bauer drill rig, model UBW 06D, and a Tracto Technik 10S Grundodrill. Position of the drill head was monitored using Tracto Technik's Digitrak system.
The drill head was then exchanged for a 110mm backreamer which pulled through the injection pipe.
The injection fluid was selected after careful analysis of soil characteristics with two types of grout produced to suit different locations. Where sand is predominant, a cementious grout was used using ultra fine cement. This resulted in a soil strength of 10N/mm 2-12N/mm 2, greatly exceeding the design requirement.
The second formula was Hard Gel; a sodium silicate solution mixed with a special hardner and water. This had excellent penetration in different types of soil, and though it resulted in a lower compressive strength, it still achieved the requirement with an added safety factor.
With the arch stabilised, the civils contractor could proceed with construction of the underpass, with no danger of damage to the road surface above. This method proved to be both cost and time saving, and most importantly, caused no disruption to traffic flow.