From the magazine, Microtunnelling, Tunnelling

Digging deep for the Airport Tunnel Link

The airport Link project in Brisbane will connect the Brisbane CBD and the North-South Bypass Tunnel to the East-West Arterial Road which leads to the Brisbane Airport. The work under the contract was to install 30 metres of 1,650 mm ID RCJP (reinforced concrete jacking pipes) that is to act as an enveloper pipe for a 1,400 sewer carrier pipe 11 metres under Park Terrace in Kedron.

As well as from the pipe jacking, TBA’s scope of work also included the design, fabrication and construction of the retrieval shaft. The shaft was constructed as a 6 metre diameter, 12 metre deep, steel segmental cased temporary caisson shaft. The retrieval shaft was used not only to retrieve the jacking shield, but also used as a shoring system for the duration of the
manhole construction.

Using reusable segments

The obvious benefit of this system is the ability to retrieve the segments from the ground upon completion and reuse them for another project requiring deep shafts. The shaft was constructed using custom built hydraulic jacks (gallows) shown in figure 1, which provided for the controlled advancement of the shaft segments into the ground and for recovery of the same.

TBA used one of the two telescopic clam deep shaft excavators it owns for the excavation operation; the excavators are capable of soil removal from up to 20 metres of depth. The telescopic clam excavator, as seen in figure 2, can be used in various construction environments that require deep excavation applications. In this particular case in Brisbane’s CBD, the need for a truck ramp was eliminated and significant cost and time savings were achieved.

The shaft was made of nine standard shaft rings and one initial shaft ring. The standard shaft ring is made of four segments, each 1,200 mm high. The initial shaft ring has a different design, which also includes a 200 mm cutting edge.

Once the centre of the shaft was marked on the surface (figure 3), the initial 6 metre diameter shaft ring was assembled and placed in an excavated area 1,200 mm deep with a diameter of approximately 8.5 metres. A concrete beam (figure 4) was then constructed, adequately designed to withstand the forces applied by the four hydraulic jacks (gallows).

Once the concrete beam was poured and reached its strength, the four gallows were then placed over the pre-positioned plates (figure 5). After the initial shaft ring was installed, the first of the nine standard shaft rings was joined and placed on top of the initial shaft ring and the pushing/installation of the initial shaft ring commenced. Excavation of layers of 200-300 mm preceded each push until the full 1,200 mm height of the initial shaft ring was jacked into position. The hydraulic jacks are used to control the plumb of the shaft.

The subsequent rings were positioned in such a way as to secure the vertical
joints between segments to be staggered to in centre of adjacent ring of segments.

Safe and sound

Upon reaching depths greater than
1.5 metres, a removable suspended working platform (false flooring) was introduced (figure 6), which provided access for skilled workers to join and bolt the shaft steel segments. The workers were wearing harnesses and a fall/arrest system was utilised for additional safety which was connected to a davit arm, mounted on one of the four gallows.

During this whole process of excavation and jacking, the annulus between the shaft and the collar (concrete beam) was kept filled up with bentonite suspension, which prevented ground movement and kept a good sliding surface between the outside of the rings and the ground. The cutting edge was at all times kept tight into the strata, which prevented any loss of bentonite.

The process of part excavation, part jacking continued until the initial ring and cutting edge come within 50 mm of the final position, in this case, the top of an existing sewer pipe/tunnel. Once this position was reached, parts of the initial ring were removed.

At the time of writing, TBA has completed the shaft and the pipe jacking, recovered the shield and demobilised from site. The company will return to remove the segments when the manhole has been completed.

Upon completion of all works associated with pulling out TBA’s jacking shield, and the construction of the manhole, the shaft will be pushed out in 300 mm increments. Once the 300 mm push has been achieved, the jacking rams will be withdrawn and the 300 mm will be backfilled. After 1,200 mm of the shaft is exposed above the surface level, the first ring of segments will be dismantled. This process will continue until the full depth of the segmental shaft is retrieved in a safe and controlled manner.

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