Clay sediments in depositional basins: The geotechnical cycle

The cycle of events and natural processes that affect the geotechnical properties of clay sediments in a depositional basin (the 'Geotechnical Cycle') includes: deposition, consolidation/compaction, diagenesis, tectonic disturbance, weathering and erosion. Some of these processes can occur in parallel; their combined effects control the mechanical behaviour of clay sediments. Processes involved in the Geotechnical Cycle are reviewed, using case studies chosen where possible from the London Basin; examples from other depositional basins emphasize the many common elements that control the behaviour of clay sediments. A simple soil behaviour model (based on the 'sensitivity framework') is described that combines the consolidation (geological compaction) and strength behaviour of natural clays, both during loading (deposition) and unloading (erosion). This model enables the mechanical consequences of soil structure (including the effects of diagenesis) to be quantified. Its merits are that it describes real soils, thus aiding ground characterization, and also allows an assessment of such factors as sample disturbance, while providing a realistic and straightforward geotechnical framework. The effects of tectonic events on the geotechnical behaviour of clays are reviewed in the particular context of flexural slip, and its influence on stability problems. Uplift, then erosion, results in basin sediments becoming overconsolidated; the history of the London Basin is used to emphasize both what is known and, in particular what is not known, of these phases of basin history. Following erosion, weathering processes further modify the mechanical properties of clays. The most obvious indicator of weathering is oxidation, with its associated colour changes. The depth of oxidation in the London Clay Formation is reviewed, and shown to relate to topographic position and former groundwater conditions. Finally, the sensitivity framework is used to examine the effects of weathering on the mechanical behaviour of clay, particularly the reduction of yield stress in one-dimensional compression.