B.4 Aspects of design

B.4.1 Settlement

B.4.1.1 Total settlement

The design related to the soil deformations caused by the loading operation shall be in accordance with EN 1997-1.

The question of whether the soil is normally consolidated or overconsolidated is of great importance for the correctness of the settlement analysis and for whether the use of vertical drainage is adequate or not. A correct determination of the preconsolidation pressure is of paramount importance. The use of vertical drains in a case where the effective stresses induced by the loading operation are below the preconsolidation pressure of the soil is counter-productive since the installation of the drains may cause disturbance effects that result in an increase in settlement. Thus, vertical drainage should only be utilised in cases where the preconsolidation pressure will be exceeded by the stresses induced by the loading operation.

The soil deformations caused by external loading include both vertical and horizontal displacements, whose relative magnitudes depend on the loading condition, the shear strength of the soil and the ratio of the width of loading to the depth of the soil layer. Especially if test areas are used as a basis for design, the widths of which are small in comparison with the depth of the soil layer, horizontal displacements may contribute considerably to the vertical settlement observed. In such cases vertical inclinometers, placed along the borders of the test area, provide information about the influence on the vertical settlement of horizontal deformations.

In the analysis of the total settlement obtained after completed consolidation, the influence on soil deformation properties of possible disturbance effects caused by drain installation should be considered. The disturbance effects depend very much on the method of drain installation, the size and shape of the mandrel and the structural features and un-drained shear strength of the soil. To ensure the accuracy of the settlement analysis it is important that the average unit weight of any fill material used for loading is given in the specification. It is also necessary to take into account the load reduction due to buoyancy effects if part of a surcharge becomes submerged during the consolidation process.

The total primary consolidation settlement can be estimated from the settlement gradually developed during the consolidation process. For example, according to Asaoka [1] [2], the relation established between the settlements observed at equal time intervals ∆t can be used to assess the final primary consolidation settlement.

The settlement achieved by the use of the vacuum method (see Annex A) is governed by the effectiveness of the sealing system. Normally, a maximum of 70 % to 80 % vacuum can be achieved, resulting in an effective stress increase of 70 kPa – 80 kPa. The ratio of effective vertical stress increase to horizontal effective stress increase will differ from the corresponding ratio obtained by external loading. In consequence, the increase in shear strength during the consolidation process will differ from that obtained by external loading.

Temporary overloading can reduce secondary creep settlement following upon the primary consolidation period. The required temporary overloading depends on the deformation characteristics of the soil and on the secondary consolidation settlement requirements. A temporary overload of at least 0,25 to 0,35 times the permanent design load, maintained until termination of primary consolidation, is usually enough to significantly decrease creep settlement after the temporary load is removed [14] [20].