The Influence of Seasonal Moisture Cycles on Clay Slopes

Andy Take, Cambridge University
Geotechnical Engineering Group


Most of the railway infrastructure in the United Kingdom was built by Victorian engineers. During this era of rapid railway expansion, earthworks were predominantly constructed by end or side-tipping, with little consideration for compaction. The legacy of these poor construction practices is infrastructure embankments which suffer from significant annual displacements and are prone to failure in the long-term. Research conducted over the past four decades on stiff clay embankments and cuttings has indicated that many of these slopes are in the process of progressive failure. More recently, the climatic effect of seasonal moisture cycles has been hypothesised to be the force driving failure in embankment slopes. However, little experimental or field data exists which documents this long-term failure mechanism.

An experimental investigation has been conducted using the accelerated time-scaling for seepage flow provided by centrifuge modelling to observe the behaviour of overconsolidated clay embankments during many "years" of seasonal pore pressure cycles. In this investigation, seasons are created using an atmospheric chamber in which the relative humidity boundary condition above the surface of the model embankment is controlled. This boundary condition is translated by the soil into seasonal pore water pressure variations - predominately negative during the dry season and positive (or nearly positive) during the rainfall infiltration of the wet season. The reliable measurement of negative pore water pressures is demanding as the growth of pre-existing bubbles, air-entry, or nucleation will all result in erroneous observations of soil suction. These difficulties have been overcome through the development of a new miniature sensor, a high air-entry filter element, and a quality-assured method of tensiometer saturation. Observations of the transient distributions of seasonal pore water pressure are interpolated within the network of up to eleven instruments buried throughout the cross-section of the soil model.

These seasonal variations in effective stress have been observed to cause the soil to undergo strain cycles. In the dry season, the embankment shrinks under the action of soil suction, whereas swelling is driven by the elevated pore water pressures of the wet season. These seasonal displacements are accurately measured at thousands of locations distributed throughout the model profile using a new image-based displacement measurement system which combines the technologies of digital imaging, particle image velocimetry, close-range photogrammetry, and multi-threshold centroiding. The development of this system is discussed, and its precision evaluated in both image-space and real-space.

Finally, an episodic account of the soil behaviour observed the response to seasons of varying lengths and severities in five model embankments is presented. These results provide detailed experimental evidence that seasonally pore pressure cycles produce irrecoverable downslope movements. Furthermore, average stress paths calculated by a modified Spencer analysis indicate that super-critical strengths are always accompanied by irrecoverable damage to the slope during the wet portion of both seasons. Repeated excursions within this region of super-critical effective stress ratios cause the peak strength envelope to reduce until catastrophic failure occurs.

Keywords: progressive failure, seasons, embankments, slopes, particle image velocimetry