Balfour Beatty Civil Engineering Ltd

Sewage Sea Outfall, Aberdeen

This contract was the first phase of an improved sewage disposal scheme for the City of Aberdeen, involving the partial treatment of raw sewage at the headworks and then discharging to the North Sea through a new outfall shaft and tunnel.

The sea outfall contract comprised the construction of a main shaft 66m deep and 7.32m in diameter, adjacent to the headworks, and a tunnel 2.5km long by 2.5m diameter. At the seaward end the final 270m of tunnel consists of a tapered diffuser section connecting to ten vertical diffuser shafts located on alternative sides of the tunnel.

The prefabricated diffuser shafts are approximately 30m long and were placed and grouted into 1.2m diameter holes drilled below sea bed from a jack-up platform standing in 31m of water. This section of the work was carried out before the main tunnel drive commenced.

The shafts were capped at sea level with reinforced concrete diffuser heads, each containing four diffuser ports. The diffuser discharge area lies in the main tidal stream parallel to the coastline, with surface currents reaching 1 m/sec in a water depth of 31m.

A precise survey was necessary to establish the position of the diffusers and equally precise setting out was required to locate them and drive the short asdits for the connection to the tunnel.

The main shaft was sunk through 33m depth of glacial deposits and into a similar depth of rock below. Ground treatment and ground freezing techniques were required for shaft sinking.

The majority of the ground was taken out using drill and blast methods. In the softer areas precast concrete rings were used to support the sides of the shaft and hard areas were supported by rock bolts and mesh. The whole of the shaft was lined with in-situ slipformed concrete followed by cavity grouting with cement.

The tunnel was driven through varied, fractures and faulted mica schists, gneiss and granites, with temporary support being provided by colliery arches and laggings.

Final lining to the tunnel was constructed in in-situ concrete using steel shutters.

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