Low Bearing Capacity and Highly Compressible Subsoil

The highly compressible subsoil which is the subject of this book occurs in large parts of the Western and Northern Netherlands. It consists of recently sedimented clay and peat layers, mainly deposited by rivers or the sea.

In this region, the layers extend to depths of 10-20 m below ground level; the ground-water level is high and in many cases comes almost up to ground level. Due to the manner in which they were formed, these layers have been found to contain a relatively large percentage of pores. Because of the weight of the layers lying above them, this porosity has gradually decreased over the years, but the layers in question are still much less dense than deeper and older deposits. The reason for this is the structure and composition of such deposits and the fact that the high groundwater level also results in a reduction in weight. When subjected to a load, by a few metres thickness of sand fill, for example, the porosity of these layers is further reduced, resulting in a noticeable compression of the layers and/or a considerable settlement of the ground level.

On the other hand, the removal of a load from these layers, for instance by excavating or raising the groundwater level, causes them to swell, though always to a lesser degree than the compression resulting from the load.

The large settlements resulting from normal stresses therefore means that this material is classified as ‘highly compressible’. It is however known that the consolidation behaviour of soil is closely related to the ultimate failure behaviour which occurs as a result of shear stresses. Highly compressible soil is generally also of low bearing capacity and is commonly referred to as ‘soft’, which means that it has poor resistance to deformation and has low bearing capacity.

Yet another characteristic of layers of this kind is their relatively low permeability. After a load has been applied, therefore, excess pore water pressure decreases only gradually; after removal of the load, there is a reduction in water pressure, which dissipates much more quickly. As the excess water pressure decrease with the dissipation of the water, the density of the material rises, which means that its resistance to deformation and failure also increases; the material becomes firmer, less soft. Removing the load gives the opposite result: the soil becomes softer. The process of drainage and consolidation under the effect of loading is known as the primary consolidation process.

Even after the excess pore water pressures have disappeared, in the layers in question deformation continues as a function of time, without the load being increased as a result of the ‘creep affect’. In the case of the types of soil under consideration, this secondary compression is certainly not negligible; in the case of sand, however, it can be disregarded.

The behaviour of soil layers of low bearing capacity and high compressibility as a basis for the building of earth structures is so important for the design and construction of these structures that it fully justifies the issuing of a manual such as the present publication.