Urban development on and in clay calls for knowledge and the right approach

15.01.2021 / Leif Jendeby

The steady advance of urbanisation places heavy demands on sustainable urban development the world over. In coastal cities, the subsoil is often made of clay, which poses a particular challenge. In places like Mexico City, Oslo and Singapore, the clay is a constant presence as the cities grow and develop, and it is important to know what to do when building on it.

As cities grow and become more densely populated, access to land is often limited, which drives people to build high or to move buildings and installations underground. All in all, this presents builders with a number of issues that they need to address.

Gothenburg, Sweden’s second largest city, is a coastal metropolis which is growing fast and becoming increasingly built up, and the underlying clay raises particular issues for this process. A new high-rise building will settle significantly (more than a metre) if no preventative steps are taken. The most common way of tackling this is to place the building on piles, which either reach down to the bedrock or “float” in the clay. In Gothenburg, the clay layer around the Göta river is more than 100 metres deep in many places, and there are some buildings resting on more than 1,000 piles, each around 80 metres long.

Of course these piles have to be sized, along with the amount of settlement that may be expected and how this will develop over time. As large parts of Gothenburg have long been backfilled to reclaim areas along the river, there is already some settlement prior to anything being built on this land. This also has to be taken into account, both because the soil will adhere to long piles as subsidence occurs, and because this will affect the settlement of the future building. Most people probably realise that there will be settlement when high-rise buildings go up, but even quite modest backfilling gives rise to subsidence, which is sometimes “forgotten”, and 0.6 metres of backfill is roughly equivalent to the load from a normal concrete building.

The Göta tunnel is one of the deep excavations in Gothenburg. The picture shows piling for the extension of the Göta tunnel and for the Platinan district, in which COWI will have its Gothenburg office from 2022 onwards and for which it has produced both settlement analyses and projections of soil compression from piling.

Well-documented clay with an undeserved reputation

The geology of Gothenburg is well-known and Gothenburg clay is often described as troublesome. In some ways it is, but if handled properly, it is an excellent material to build on and in.

In the media and elsewhere we often hear “that unpredictable Gothenburg clay” offered as an explanation when something goes wrong or a project is delayed.

Gothenburg clay may well be more prone to settlement, and it may be less resistant than many other soils, but one thing is certain –few soils are more well-documented, and it is also extremely homogenous. Apart from all the studies carried out in connection with building projects, numerous PhD theses at Chalmers University of Technology in Gothenburg have looked at the properties of the clay and/or ways of handling it in different situations. Therefore, it is difficult to claim that the clay is unpredictable, or that its properties should surprise anyone. The fact is rather that the issues caused by the clay are not addressed for one reason or another, sometimes through ignorance, sometimes laziness.

Gothenburg clay can be challenging, but there are many cities with similar or even looser subsoil, such as Mexico City, Oslo and Singapore, while several places in Canada have similar conditions.

Based on geotechnical surveys combined with knowledge of the properties of clays, we can make sufficiently confident projections of the amount of settlement that will occur and how this will develop over time, which will enable sustainable urban development.

Build high and dig deep into the clay

One factor that becomes apparent when we erect high buildings on thick clay strata is the way that piling pushes the soil down. If we assume, for example, that 500 piles each 70 metres long are to be installed, that means that some 2,600 m³ of piling will be driven into the ground. Because the clay does not contain any air, only mineral particles and water, the amount of compression will be negligible when the piles are pushed down into the ground, so the 2,600 m³ of clay must go somewhere. This in turn means that there may be large movements – sometimes tens of centimetres – around the piling area, which can damage nearby buildings and buried pipes, for example. This may then influence the choice of piling, but it could also mean that other measures are required.