The relatively low resistance of the clay means that we cannot backfill or dig pits freely, because this can lead to slippage. This is an issue that becomes apparent when we set out to excavate a basement or a tunnel, for example. Deeper pits are always reinforced with some form of “retaining walls”, often made of steel sheets, but concrete walls cast in deep trenches (diaphragm walls) are also used for this. For pits deeper than two basement floors, this is not sufficient, as “the bottom of the pit can rise up before we get that far down”, so further measures are needed; perhaps reinforcing the clay with lime and cement or digging under water.
We saw above that 0.6 metres of backfill is equivalent to the load from a normal concrete building, and the same is true if we lower the pore water pressure by one metre. So preventing unacceptable lowering of the pore water pressure is extremely important in all kinds of ground works. This is especially important for the underlying aquifer – the layer of moraine that is generally present under the clay. The reason why this is particularly sensitive is that if there is subsidence here, the bottom part of the clay will be compressed and the whole mass of clay will go down with it, placing very large friction loads on any piles driven down to the bedrock.
Although the properties of Gothenburg clay are well-known, all ground works carry certain risks. So it is important before starting to check the parameters that are critical in any given case, which might be pore water pressure or movement in a nearby pipe. The measurements should then be constantly compared, both with expected values which show whether the analyses carried out are correct, and with thresholds based on what surrounding structures and installations can bear. In some cases, we may be forced to change the design so as not to exceed established thresholds.
In summary, it is fair to say that building on clay poses a number of different challenges which have to be addressed both in the design and construction phase. However, the properties of the clay as well as the tools that are available to tackle these challenges are familiar to experienced geotechnical engineers. By blaming our failures on the Gothenburg clay, we are simply accusing ourselves of failing to use this knowledge.