Disappearing act

by Andrew Lees, on October 01, 2020

Ever wonder why water disappears from wet beach sand when you step on it? It’s all about dilatancy – a phenomenon with a significant influence on the engineering behaviour of dense granular materials and highly overconsolidated clays. You would expect that applying pressure to saturated sand would cause the sand grains to move towards one another, and to squeeze the water out like a sponge, but in fact, the opposite happens.

The reason is dilatancy: the sand particles want to move a little under the weight of your foot but, to do that in a dense state, they need to move apart or dilate first; this creates space for water to flow in – hence the apparently dry sand around your foot. Once you step off the sand, the grains move back together and the water flows out, leaving wet footprints in your path.

Ground Coffee Episode 25 - Filmed pre COVID-19, Andrew Lees and Brian take time out from their holidays to discuss dilatancy on a beach in Cyprus

The importance of dilatancy in geotechnical engineering

Dilatancy is a significant contributor to shear strength in dense, coarse granular materials, which is why proper compaction of engineered fill is so important.

Increasing the density of the aggregate by compaction increases both dilatancy and internal friction and therefore the overall peak shear strength. If shear stresses are greater than the peak shear strength, shearing and failure can occur. The soil then has a lower post-peak strength because dilatancy has been exhausted. This can happen at quite low strain levels, so peak soil strengths should only be used in design with care. If in doubt, use the lower post-peak strength.

Highly overconsolidated clays also dilate with increasing strain, which can result in the progressive and delayed failure of slopes. This is why Victorian cuttings and embankments on railways in London Clay (which is overconsolidated) suddenly fail when they have been standing for years: dilatancy and shearing of the clay particles over a long period of time leads to a progressive weakening (strain-softening) of the soil to beyond the peak shear strength, which eventually results in failure.

It happens slowly in clays because of their very low permeability, whereas it occurs instantly in the high permeability beach sand.

It is therefore important to consider dilatancy in geotechnical design – and not just in slope stability analysis – it can also influence the stability of tunnels and shallow and deep foundations in these materials.

Controlling dilatancy using stabilising geogrids

In subgrade stabilisation – for roads and platforms – particle-geogrid interlock inhibits particle movement and dilatancy, increasing the energy needed to shear and dilate the aggregate and hence shear strength is enhanced significantly.

Additionally, the ductility of the geogrid means that the strain level needed to overcome the enhanced peak strength is very much higher than in non-stabilised, or chemically-stabilised materials. So the higher peak strength can be used safely in design, delivering significant savings in the amount of fill needed in working platforms, for example.