Design for construction of the Limerick Southern Ring Road Phase II linking between Rossbrien and the existing N18 Limerick-Ennis Road. Consisting of approx 9.75km of 2×2 lane dual carriageway. Includes 3 creek crossings, each in excess of €1m).
ROD are undertaking the bridges/structures, road alignment and earthworks design for the non-tunnel elements
The works are in mostly deep alluvium soils with extensive ground settlement. Significant cofferdams utilised in the construction and permanent foundations of the Creek Bridges.
Roughan & O’Donovan is employed by DirectRoute as the designer of the roadworks portion of this project which includes approximately 10 km of approach roads, four interchanges, two toll plazas and ten major bridges. ROD provided resident site staff to supervise the execution of those works in accordance with the Design and the Works Requirements. About 7.5km of the road is on embankment elevated up to 10m high above floodplains of the River Shannon and its tributaries which are underlain by very soft (typically 10 to 25 kPa shear strength) organic silts up to 12m thick.
Primary geotechnical design services provided include the following:
- Supplemental SI comprising boreholes with piston tube sampling, CPTu probes and complex oedometer, triaxial and direct simple shear strength tests;
- 500m long rock fill causeway built using “wet construction” in Bunlicky Lake;
- Multi-stage embankments on soft ground including prefabricated vertical drains, geosynthetic basal reinforcement;
- Continuous monitoring of performance by means of piezometer, inclinometers, settlement plates and survey monuments.
- Temporary Surcharge Fill (2 to 2.5m typically) to reduce long term secondary creep settlements;
- Variable height surcharge (up to 4m), full or partial excavation and replacement, and short sections of pile supported embankments in transitions to bridge structures;
- 200m long rock cutting including rock bolt strengthening below bankseat foundations for a 46m single span bridge;