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Talking Point with Len Threadgold

Last year’s record-breaking rainfall has been blamed for many slope failures but more careful consideration of groundwater is vital in most cases

Increases in water pressures or flows are often the trigger for slope failures which threaten individuals, the buildings which they occupy and the infrastructure on which we all rely. Hence, they are of growing significance at times of climate change.

In investigating slopes which have failed, I have always been relieved if water is evident since the introduction of groundwater control measures can have a significant part to play in the remediation. However, it is vital that such control is applied to the area or zones of the slip which the investigation and instrumentation has shown to be most critical, and that water pressures and flows are intercepted or reduced before reaching them.

In designing drainage measures it is commonly assumed that they should address water pressures above the level of the slip surface, but this intrinsically assumes that this is the only source of such pressures and this may not be so.

“The acquisition of knowledge costs money but ignorance costs both money and lives.”

This was graphically illustrated for me when I dealt with a slip which threatened houses near Derby. Remedial works design had proceeded on the basis that both a berm at the slope toe and counterfort or slope drainage would be installed to achieve the design improvement. However, data acquisition was limited by local perception that the slope was “ancient woodland”.

One nearby borehole gave a hint that a small artesian pressure may be present in the rock below the site and this led to the incorporation of a smallnumber of relief wells into the design. When woodland access was at last granted, a borehole through the slip revealed that an artesian head approaching 4m above slope surface was present in the rock below the lower part of the 10m high slope. At this point the trigger to the slip became much clearer and this helped to explain both the form of the slip and its link to precedent summer rainfall in the land behind the slope. As a result, manymore relief wells were bored into the rock and the resulting flows taken into the granular trench at the slope toe.

Water pressures from below a slip can be far greater than those which would be assumed if a maximum pressure equivalent to water at local ground surface (ru= 0.5) were assumed. These higher pressures have often been the trigger for past periglacial slips, and consequent residual shear planes, as well as more recent failures and tragedies.

It caused me to reflect on the importance of knowledge of the hydrogeology of a slope when considering the causes of landslip and their solution. Hydrogeological studies coupled with geotechnical investigation, instrumentation and design could lead to the introduction of measures to prevent the pressures which could trigger failures.

The costs of such studies and measures pale into insignificance in comparison to the financial and human costs which result from ignorance, un-calibrated observation or hope that the slope would not become unstable. The acquisition of knowledge costs money but ignorance costs both money and lives.

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