DUBLIN PORT TUNNEL CONSTRUCTION

CHRIS PAGE & THOMAS SMITH19/03/2009

H24GGY GEOLOGY FOR CIVIL ENGINEERSDublin Port Tunnel Project

I. Summary of Project Scheme H24GGY GEOLOGY FOR CIVIL ENGINEERSDublin Port Tunnel Project

Located In Dublin, Ireland. Twin Bore Road Traffic Tunnel Part of M50. Direct Link to Dublin Port from M1 Motorway thus Relieves Congestion in City Centre by

Diverting HGVs. Approx. €752 Million to build.

II. Geological Context H24GGY GEOLOGY FOR CIVIL ENGINEERSDublin Port Tunnel Project

The typical geological formations of the route are:

Inter- bedded Carboniferous limestone, shale’s and mudstones, known locally as the Calp. Overlain by glacial deposits of boulder clay. North of the main shaft, the tunnel had a full face of boulder clay. At the Southern end of the alignment ,estuarine and alluvial silts, sands and gravels are underlain by boulder clay.

II. Geological Context H24GGY GEOLOGY FOR CIVIL ENGINEERSDublin Port Tunnel Project

The typical geological formations of the route are:

Inter- bedded Carboniferous limestone, shale’s and mudstones, known locally as the Calp. Overlain by glacial deposits of boulder clay. North of the main shaft, the tunnel had a full face of boulder clay. At the Southern end of the alignment ,estuarine and alluvial silts, sands and gravels are underlain by boulder clay.

II. Geological Context H24GGY GEOLOGY FOR CIVIL ENGINEERSDublin Port Tunnel Project

The typical geological formations of the route are:

Inter- bedded Carboniferous limestone, shale’s and mudstones, known locally as the Calp. Overlain by glacial deposits of boulder clay. North of the main shaft, the tunnel had a full face of boulder clay. At the Southern end of the alignment ,estuarine and alluvial silts, sands and gravels are underlain by boulder clay.

II. Geological Context H24GGY GEOLOGY FOR CIVIL ENGINEERSDublin Port Tunnel Project

The typical geological formations of the route are:

Inter- bedded Carboniferous limestone, shale’s and mudstones, known locally as the Calp. Overlain by glacial deposits of boulder clay. North of the main shaft, the tunnel had a full face of boulder clay. At the Southern end of the alignment ,estuarine and alluvial silts, sands and gravels are underlain by boulder clay.

III. Detailed Geology H24GGY GEOLOGY FOR CIVIL ENGINEERSDublin Port Tunnel Project

The boulder clay comprises stiff clay interspersed with boulders of up to 750mm in size and contains isolated lenses of saturated sands and gravels under confined hydrostatic pressure.

The limestone has a medium strength of 150Mpa. However localised areas of mudstone and shale bands can reduce this to 25Mpa, with regions of pure limestone reaching 250MPa.

Due to the north-south flow of the hydrological regime, along the route artesian water pressures are anticipated in the limestone.

The water table lies 1m below ground surface over the entire tunnel route.

IV. Problems Encountered H24GGY GEOLOGY FOR CIVIL ENGINEERSDublin Port Tunnel Project

Change of geology, plus isolated areas of saturated sands/gravels and localised areas of mudstone, created various problems.

During construction of diaphragm wall limestone rock heads were encountered.

In Boulder clay, melt water deposits were encountered causing over excavation.

V. Alternative Methods H24GGY GEOLOGY FOR CIVIL ENGINEERSDublin Port Tunnel Project

NATM method – originally proposed however concern after Heathrow Express Tunnel Collapse.

Surface road was not feasible as did not meet project aim.

Explosive charges.

VI. Solutions - Overview H24GGY GEOLOGY FOR CIVIL ENGINEERSDublin Port Tunnel Project

Total project length is 5.6km.

Tunnel built in 4 Distinct Sections: Two cut and cover sections.

• Southern section 600m long.• Northern section extends over 1500m.

Two bored sections.• One through 650m of boulder clay.• One through 2600m of hard limestone.

VII. Solutions - TBMs H24GGY GEOLOGY FOR CIVIL ENGINEERSDublin Port Tunnel Project

Due to mixed geology two TBMs were required.

Shielded Hard Rock TBM was used for the Carboniferous Limestone (Hard Rock).

Shielded, with partial face extraction , TBM was used for the Boulder Clay.

VIII. Solutions - Cut and Cover H24GGY GEOLOGY FOR CIVIL ENGINEERSDublin Port Tunnel Project

A typical Cut and Cover section comprises two Reinforced Concrete Tunnels with a width of 26m.

However localised geology dictated different slope support methods in order to prevent collapse of the dig.

Methods of support were: Diaphragm walls (1.2m thick). Soil nails and Shotcrete.

IX. Solutions - Cut and Cover H24GGY GEOLOGY FOR CIVIL ENGINEERSDublin Port Tunnel Project

Diaphragm walls used for deeper parts of the excavation and typically 1.2 - 1.5m thick: Northern section used 300m diaphragm walls and an average depth of 20m. Southern section used 600m diaphragm walls.

Soil nails and Shotcrete: Technique followed the Northern 300m section of diaphragm walls. Retained a 1300m cut using 25mm and 32mm diameter soil nails 8 – 11m long with shotcrete.

Thank you for Listening H24GGY GEOLOGY FOR CIVIL ENGINEERSDublin Port Tunnel Project

References: Dublin Port Tunnel Authority

http://www.dublinporttunnel.ie Dublin City Council

http://www.dublincity.ie Arup

http://www.arup.com Institute of Civil Engineers

http://www.ice.org.uk Nishimatsu Website on the Dublin Port Tunnel Project, tunnelling contractor

http://www.nishimatsu.co.jp/eng/project/dublin.htm Department of Transport, Ireland

http://www.transport.ie Herrenknecht, manufacturers of TBMs, company website

http:// www.herrenknecht.com