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Skibbereen Sewerage Scheme
Project Profile
Skibbereen Sewerage Scheme – Collection System
| Value: | €11,600,000 |
| Client: | Cork County Council, County Hall, Cork Contact: Mr. Noel O’Keeffe Ph.: 021-4276891 |
| Consultant: | RPS Consulting Engineers, Innishmore, Ballincollig, Co Cork. Contact: Mr. Kevin Power Ph.: 021-466 5900 |
| Completion Date: | December 2008 |
| Involvement: | Main Contractor / PSCS, 100% participation |
Project Description:
The Contract provided for the construction of a new Sewerage System consisting of:
- 14Km of 150mm to 1200mm diameter precast concrete storm, foul and combined sewers
- 2Km of 100mm to 1000mm ductile iron rising main
- by ‘open-cut’, microtunnel and auger bore pipelaying methods
- also involving pipeline river crossings
- Construction of pumping stations and storm water holding tanks
The prevailing ground conditions consisting of silts with SPT (N) values of 0 presented particular challenges on the Scheme. The town of Skibbereen is constructed on an encrusted/consolidated layer of silt, approx 1.5m deep. This layer overlies a weaker layer of silt up to 40m in depth. Consequently all properties have shallow foundations and are supported at shallow depths in this encrusted layer. Construction techniques for both pipelines and structures were required to deal with, and minimise, the effects of vibration, groundwater loss, settlement, ground heave and ground movement. This necessitated extensive use of the following construction methods:
Caisson construction techniques for all Pumping Stations:
Caissons were constructed underwater. Dewatering was not required until concrete plugs were completed. The largest structure constructed using this method was the main pumping station, 16m x 16m plan area, 17m deep, and weighing 40,000t. The structure was supported/suspended from piles driven around the structures perimeter which were integrally cast into the overhanging roof slab.
Extensive use of Trenchless Technology for pipeline construction:
-Microtunnelling from 600mm to 1200mm diameter pipeline.
-Augerboring from 450mm to 600mm diameter
-Horizontal Directional Drilling up to 450mm diameter
Ground conditions required extensive pre-grouting [to stiffen surrounding soils] using varying mixes of bentonite/cement in order to control both vertical and horizontal alignments. Tunnel drive lengths of up to 230m [curved] were constructed.
Particular properties were underpinned in advance of pipeline construction using micro-piles cast integrally with existing foundations. Concrete columns were not an option.
An existing masonry arch bridge was also partly demolished and reconstructed using micropiles to provide a foundation. It is located at the confluence of two tidal rivers in Skibbereen town.
All of the works were carried out directly. All of the equipment is owned and operated by Ward & Burke Construction.
