• Kristine Lee Kaufmann
4. term, Structural and Civil Engineering, Master (Master Programme)
This thesis consists of two individual papers. In the first paper, the tertiary clay at Moesgaard Museum is geologically and geotechnically described. The second paper concerns finite element modelling of a single pile positioned in a swelling clay. The swelling is modelled as an unloading by an excavation.

The subjects of the two papers are summarised in the following.

Swelling soils are highly expansive in combination with water, which could lead to damaging of buildings due to heave of the soil. Thus, it is important to determine at which stress levels swelling is expected to occur. This is represented by the swelling pressure. Additionally, deformation and strength parameters of the clay are necessary for a detailed design of the foundation of new buildings.

The first paper evaluates the strength and deformation properties of the tertiary clay at Moesgaard Museum situated 10~km south of Aarhus. This includes analyses of two consolidation tests and a single triaxial test. The swelling pressure, the preconsolidation stress, the compression index and the constrained modulus are determined based on the consolidation tests. The drained and undrained strength are found by the triaxial test.

Pile foundations designed to resist compressive loads caused by settlements are occasionally situated in swelling soils. Due to the friction between pile and soil, the heave caused by swelling leads to an additional tensile loading. Analysis of this combination of tensile and compressive loading is complex, both theoretically and numerically.

In the second paper, the mechanisms of the combined tensile and compressive loading of a single pile caused by swelling are decoupled. Thus, the problem is simplified to a fixed pile which, because of an excavation, is exposed to tensile loading. This is modelled numerically, and the heave of the new ground surface, the shear stresses at the soil--pile interface and the internal vertical stresses are analysed based on a case study of Little Belt Clay. The analyses show that the presence of the pile creates a weak zone within a radius of 3 pile diameters from the axis of symmetry of the pile. The maximum heave within this zone is polynomially decreasing with increasing interface strength. The swelling of the surrounding soil implies upward shear stresses at the soil--pile interface. This leads to tensile vertical stresses in the pile which in the current case exceed the tensile strength of concrete. The tensile vertical stresses peak after 35-50~years even though the heave of the soil continues for additional 300~years. It appears that the development of plastic interface implies the shrinkage of the pile because of the slip at the pile surface.
Publication date8 Sep 2010
Number of pages141
Publishing institutionAalborg Universitet
ID: 36893988