Drammen City Bridge: how soft clay shaped the foundations

A new 286 m steel-concrete composite bridge in Drammen, Norway, shows how ground conditions can drive the whole design of a river crossing.

In an in-depth report, New Civil Engineer describes how the designers and contractor dealt with soft clay, an old timber-pile “forest” and a demanding river environment.

Key points

The bridge replaces a 1936 reinforced concrete structure and crosses 170 m of river in spans of 46 m, 86 m and 42 m, with a shorter 84 m section over a railway. Beneath the site lie soft to medium stiff silty clay, sensitive clay and quick clay towards one bank; good bearing soil is found 40 m to 70 m below water level.

The foundations rely on steel core piles drilled into bedrock. Some are inclined to share the horizontal forces from long spans and ice loads, and an outer steel pile filled with reinforced concrete adds moment capacity and stiffness. Piles were installed from a floating barge, then a sheet pile cofferdam was built to form the pile caps, the largest about 14 m by 14 m and 4 m high. The old timber piles, limited to about 20 m, could not reach good soil and were not reused.

An early analysis reduced the number of river piers from three to two because of the complex ground. The rail and river sections were built separately and joined only when nearly complete, which avoided transferring forces between them and removed the need for an expansion joint.

Why this matters for design and verification

The case illustrates a point familiar to any bridge team: foundation conditions often decide the structural layout, not the other way round. Pier count, span arrangement, bearing strategy and construction sequence were all tied to what the ground allowed. For an independent checker, the ground model, the load path from deck to pile cap, and the assumptions on horizontal actions are among the first items worth examining.

Practical points

• Treat legacy foundations, such as old timber piles, as a design input from the start.
• Check horizontal stiffness and ice or flow actions, not only vertical capacity.
• Keep construction stages in the analysis model when sections are built separately.

These remarks are general technical observations and not a statement about any specific project.

Source: New Civil Engineer – Drammen City Bridge shows how smart engineering decisions can facilitate foundation construction

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