Use of the observational method and optimising design has improved construction of Ireland's biggest infrastructure project.
The E680M (£477M) Dublin Port Tunnel Project, will channel 9,000 heavy goods vehicles per day from the hinterland around Dublin to the Irish capital's port area. The mid-section of the alignment comprises 12.5m external diameter twinbored tubes built using TBMs launched from a central shaft.
As the alignment rises to the north and south, shallower sections are constructed using cut and cover methods. The project is funded by the National Roads Authority of Ireland and is one of the largest infrastructure developments in the Irish Republic in recent years.
Construction is well advanced, under a modified version of the ICE Design and Construct Contract, with a designer certification, independent checker and employer's representative's consent approval process being used.
Responsibility for design and construction, including the Clause 12 risk for adverse ground conditions, lies with the contractor/designer. The project has been challenging, requiring effective teamwork between all parties. This has allowed optimisation and implementation of some novel geotechnical solutions, illustrated with two examples, a deep diaphragm wall for the TBM launch shaft and soil nailing on a section of the northern cut and cover section.
Optimisation of design and construction of the 56.6m diameter, 30m deep diaphragm wall-supported TBM launch shaft took full benefit of shaft-ground interaction to achieve significant reductions relative to the winning tender design. For example, reinforced concrete volumes were reduced by as much as 9,270m 3.Over a 850m stretch of the northern cut and cover section, advantage of competent ground was taken to meet tight land-take constraints using steep, 12m deep slopes. A motorway was diverted to run at the crest, which together with adjacent properties had to be continuously protected. Slope support was needed for three to six months for horseshoe lining construction and backfilling.
A novel solution was developed, involving controlled use of excavation-induced pore water suctions, working together with soil nails. The basic design required soil nails over the full slope height. However, for further economy and time saving, a novel observational approach was evolved whereby rows of nails were omitted unless required by adverse geology or unsatisfactory monitored performance.
The partly nailed slopes were specifically designed with a limited 'stand-up time' to match construction requirements. Successful implementation married:
lstate of the art design and finite element analyses;
la full-scale field trial;
ldetailed risk assessments;
lsophisticated site procedures and contingency measures.
Good teamwork was required to meet the often conflicting needs of the employer (performance, safety and functionality) and the contractor (cost and programme savings).
Project team Design and site support: The Geotechnical Consulting Group/University College Dublin Design and site support: Charles Haswell and Partners Contractor: Nishimatsu-MowlemIrishenco Joint Venture Employer: Dublin City Council Accredited checker: Mott MacDonald Construction supervisor: KBR