Long-Term Settlement (Creeping) of Soft Soils, and Ground Improvement

Abstract

This keynote paper focuses on the long-term settlement (creeping) of highly compressible soils, considering also soft sludge. Long-term oedometer tests lasted up to 42 years and were performed on silty sand, (organic) clayey silt, peat and (pre-treated) sewage sludge. Secondary consolidation (creep) could be observed in all cases, lasting over many years and occurring widely linear with the logarithm of time. However, temporary acceleration may also be observed, indicating a discontinuous nature of internal deformations due to accelerated rearrangement in the fabric – mainly in very soft soils with peaty components. This long-term phase is followed by tertiary creeping with a long lasting fading out period. In both phases microcrystalline sliding occurs.

In addition to the laboratory tests results of comprehensive field measurements are summarized, showing the influence of different ground improvement methods on the creeping behaviour of highly compressible fine-grained soils (partly organic). Most data were collected from a highway junction with embankments on very soft, heterogeneous ground (locally 15 m deep and with a natural water content up to 1000%), constructed between 1972 and 1974, and monitored since. Different ground improvement methods were compared, disclosing details of primary, secondary and tertiary settlement: Deep dynamic compaction/consolidation (heavy tamping), vibro-flotation (piled embankments), temporary surcharge loading, and local combinations of the previous methods, including also vertical drains.