Engineering analysis and design software
Civil and Structural engineering

Case Study

Staged Construction Analysis of a Concrete Shaft on the Dublin Port Tunnel Project

  • Staged construction analysis of large diameter concrete shaft
  • Investigation into hole cutting sequence
  • Assessment of wall strengthening options

The Dublin Port Tunnel in Ireland runs from the M1 motorway at Santry to Dublin Port at East Wall. Its primary purpose is to provide good access to Dublin Port, as well as relieving traffic congestion in the city. A 2.4km length of the route requires twin bored tunnels to be cut in stiff Boulder Clay and Limestone. To provide access for the tunnel boring machines (TBM) for this section of the route a 60m diameter, 32m deep shaft with 1.5m thick in-situ concrete diaphragm walls needed to be constructed. Once the TBM's are in place they each cut 13m diameter openings through the shaft wall and bore a length of tunnel before turning around and re-entering the shaft from the other direction. To assess the manner in which the stress re-distribution takes place around the openings and to investigate the potential internal strengthening options Haswell Consulting Engineers used LUSAS Civil & Structural analysis software.

A LUSAS model of the complete shaft was created and geotechnical loadings, provided by a third party, were applied. The shaft was analysed initally in isolation prior to the construction of any tunnel openings. Crane loads were then added and staged construction analyses were carried out by introducing the tunnel openings in a pre-defined order and taking into account all applied loads and the manner and stage of construction at which they are applied. For each opening cut by the boring machines plots of deformation, shear stress, bending moment and hoop stress were produced. By deactivating the elements defining each hole in turn the sequence of creating the holes could be modelled and  the results from the previous analysis were retained by LUSAS as the starting point for the subsequent one. 

Stress re-distribution in cutting holes in shaft

Graphs of results sliced through the holes were particularly useful. One interesting observation was LUSAS showed that an analysis using only a quarter model would not take account of the accumulative influence of the other tunnel openings. In parallel, the Contractor used another software package to analyse one quarter of the shaft with one opening. Comparison between the two sets of results showed an acceptable level of agreement, taking into account the different assumptions made in each model. However, the results of the Contractors analysis were implemented in the design which was enhanced by the results produced by LUSAS. After all four holes had been cut the eventual stresses around each differed. Dr Ala Sainak, formerly of Haswell Consulting Engineers but now Senior Geotechnical Engineer at Halcrow explains: "It was thought that the stress distribution around one cut hole would be the same as for an adjacent hole but the analysis with LUSAS proved otherwise and showed that it was more appropriate to analyse it as a full model rather than a quarter model.

Slice section results through first cut hole

Shear stress values and bending moments in the wall caused by cutting holes through the shaft were of an acceptable value but a greater factor of safety was required. To try and achieve this three wall stiffening options were considered. These all involved casting additional concrete in place prior to any cutting out of holes.

  • Option 1 : The provision of an inner 2m thickening to the lower section of the 1.5m wall. LUSAS analyses showed that this didn't help transmit any significant loading because the outer wall had already been built and hence it retained its initial stresses.
  • Option 2 : The provision of two, 2m x 2m ring beams, one above the crown of the tunnel openings and at the base of the wall. LUSAS showed that the additional ring beam accommodated most of the hoop loads but failed in attracting the locally redistributed stresses, bending moments and shear forces
  • Option 3 : Provide two ring beams and a reinforced section of a similar thickness to the ring beams to the region surrounding the hole. LUSAS showed that this option also failed to attract and reduce the locked-in stresses.

With the aid of LUSAS all of these potential options were rejected for being either too expensive to implement or for failing to achieve the desired result. The eventual solution was to construct one ring beam above the crown of the tunnel openings and in addition use very high yield and large diameter steel reinforcement in the concrete segments either side of the holes as well as in the concrete segments containing the holes to overcome the large shears produced by the cutting of the tunnel holes.

 

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