Golder Associates in Vancouver, BC, is providing geo-technical consulting services on the South Coast Sewerage Project in Barbados. This major infrastructure project includes collection, treatment and disposal of sewage within the entire South Coast resort area, a coastal strip about 10 kilometres long. It involves installing a 40-kilometre long sewer system, treatment plant, several pumping stations and marine outfall about one kilometre long.

About five kilometres of the sewer system are being installed using micro-tunneling, which installs the sewer without having to dig a trench. This method was chosen in light of the subsurface conditions to address environmental and socio-economic concerns.

Firstly, ground conditions are difficult. The site is underlain by coral formations comprised of highly variable carbonate materials ranging from unconsolidated loose sand to cemented sediments and coral rock. Sewers are being installed to depths up to six metres with long segments below the water table within highly permeable strata, which present construction difficulties for conventional trenching. The required dewatering systems would generate significant quantities of water which is of environmental concern because of adjacent tourist beaches.

Secondly, many of the new sewer alignments follow narrow roads in a busy, densely developed resort area. Installing deeper sewer segments using conventional open-cut methods would have been costly and highly disruptive to the tourist economy. Further, there was concern that more extensive dewatering of the system, which would have been required using open-cut methods, could result in settlement damage to existing facilities and infrastructure in some areas.

To minimize the negative impact on the economy and environment, micro-tunneling was selected for a five-kilometre length in areas of greater congestion and more difficult ground conditions ­ despite a moderate cost premium. Pipe sizes range from 300 millimetres to 1,350 millimetres, and some shafts are required to facilitate construction and operation of the sewer system.

Site and subsurface conditions have presented a number of technical challenges. These included dewatering difficulties, requiring use of wet shaft construction techniques and careful attention to sealing details. In addition, natural and manmade features deflected the drill head, particularly for smaller diameters where soil cover was limited. To date, little difficulty has been encountered drilling the relatively weak carbonate materials.

Shaft construction has required a flexible approach which can be adapted to:

• Flowing sands below the water table;
• Hard cemented zones which must be broken prior to installation of the shoring system; and
• Highly porous coral formations which make dewatering difficult until the shoring system has been installed and a base seal constructed.

To date, the contractor has used the following techniques for shaft construction:

• Conventional lagging and timber planking shoring systems for shallow shafts within stable ground above the water table, with the permanent concrete shaft cast-in-place;
• Prefabricated steel shoring systems for shafts in generally unconsolidated ground above or slightly below water table, with the permanent shaft cast-in-place; and
• Shafts constructed using pre-cast concrete segments with non-reinforced sections ("soft eyes") to permit penetration of the micro-tunneled sewers.

Concrete segmental shafts have proven the most versatile system to date. A circular trench is excavated for pouring a guide collar, with an inside diameter slightly larger than the shafts to accommodate an annular bentonite lubricating layer. A steel or concrete protective shoe is then placed, followed by the first layer of shaft segments. Excavation is carried out within the shaft base as the shaft is sunk, either by applying a heavy weight or hydraulic jacking.

In some cases, it was possible to dewater the shafts as they were excavated and sunk. In other cases, the high rate of inflow combined with environmental constraints required sinking the shaft in the water. This further complicated the breaking of harder cemented zones, requiring divers to break up obstructions. Generally, though, the harder materials were broken using an impact hammer mounted on an excavator. Once the design excavation level was reached, a concrete seal was poured and the shaft was subsequently dewatered.

The micro-tunneling technique is proving highly effective for this project, and construction of the sewage collection system continues. The treatment plant and outfall are complete.

The Barbados South Coast Sewerage Project is managed by Canadian consultant Reid Crowther International Ltd., with Jason Consultants S.A., Geneva, Switzerland providing specialist input on trenchless sewer design.