Bauer International, the UAE branch of Germany-based geotechnical and foundation expert Bauer Spezialtiefbau, came up with a suitable solution to enable the construction of underground pedestrian crossings across busy streets in Abu Dhabi.

Abu Dhabi Municipality had invited civil contractors and geotechnical specialists to propose, design and build a significant number of underground tunnels to provide safe crossing points across six to eight-lane-wide roads teaming with heavy traffic.

These tunnels were to be constructed without disrupting the traffic flow while preserving the aesthetics of the buildings adjoining them, says a spokesman for Bauer.

Bauer offered a sound proposal involving a chemical injection technique in order to stabilise the soil below the road structure all along the tunnel.

He elaborates: ''The unique technique was developed and designed through the joint efforts of Bauer UAE and the specialist staff of the construction technology and penetration injection departments of Bauer Germany.

''Horizontal boreholes, 17 to 27 in total, were to be constructed using the directional drilling technique, after constructing the access shafts using secant pile walls and the Berlin wall, and following the excavation and dewatering of both shafts of the tunnel. The boreholes had to penetrate from one side through to the other side of the tunnel.

''Construction of such long horizontal boreholes (3060 m) was carried out by developing a very productive combined drilling rig, comprising the Bauer drilling rig type UBW 06D and TractoTechnik drilling device Grundodrill 10S. This special rig was put into operation at the first tunnel in September 2001. In order to perform the borehole precisely, a special monitoring and detection system Digitrak 111 DCI has been used.

''Installation of the injection pipes, consisting of sectional steel pipes equipped with specially-developed valves, was carried out once the horizontal borehole was completed and the borehead emerge from the other side of the tunnel pit. The borehead was exchanged with a 110 mm backreamer, which was connected with the sectional injection pipes and pulled backwards.

''The chemical injection material has been designed taking into consideration the soil characteristics, by analysing its physical and mechanical properties, such as particle size, sieve analysis and void ratio.

''Based on these characteristics, two different types of chemicals have been designed using a mix design suited to the soil at each tunnel location. Ultra fine cement has been used in the areas where a sand layer is predominant.

''The compressive strength of the stabilised soil using ultra fine cement was found to be in the range of 1012 N/sq mm which is much superior to the design requirements. The second type of chemical injection used is the hard gel consisting of a sodium silicate solution mixed with special hardner and water. The hard gel has a better performance in terms of penetration in different types of soil but with a lower value of compressive strength - yet it still achieves the design requirements with an additional safety factor.

''The arch-shaped injected body is finally produced, allowing excavation to take place safely without any damage to the main road structure and hence the remaining works for the tunnel construction can be carried out easily by the civil contractor.''

''This innovative method proved to be efficient, cost and time saving and, most importantly, did not disrupt the busy traffic flow at any time,'' the spokesman concludes.

Meanwhile, Bauer Maschinen of Germany recently held a four-day exhibition showing off its latest innovations and developments in specialist foundation equipment.

Among the 700 visitors from 40 countries around the world were construction managers from China and Japan, Russia, Poland and Mexico, also South Africa and the Sudan.

Every company within the group was represented, from Bauer Maschinen based in Schrobenhausen to Klemm Bohrtechnik, RTG Rammtechnik, MAT Mischan-lagentechnik and Eurodrill.

Customers of its plant and equipment were also able to attend various plant demonstrations at Bauer's test area and see Bauer plant and equipment in action on construction sites throughout southern Germany.

At the end of the exhibition, Erwin Stotzer, Hubert Merkl and Josef Soier, joint managing directors of Bauer Maschinen, were able to confirm that a number of firm orders had been signed during the exhibition and very promising preliminary discussions had also taken place in connection with several other projects.