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  • Writer's pictureSi Shen

Shear (3) – shear in soil

This blog explains the shear behaviour of soil from a whole new angle for you to better understand the underlying reasons, which are often overlooked by textbooks.

How does soil fail?

In geotechnics, we are primarily dealing with compression, as the state of natural in-situ stress condition is primarily compression. Hence in structural engineering, people treat tension as positive values and compression as negative values; whereas in geotechnical engineering, it is the exact opposite.

Does soil fail in compression? Soil can be considered as a composite material, comprises mainly two things – the soil particles and the ground water that fills in the voids (pore water). There are other things such as air bubbles as well, but from structures point of view they are not normally a significant factor. Water obviously do not ‘fail’ – it just gets pushed out and about, and the soil particles are too fine to crumble in compression, so neither of these really fail in compression.

The failure of soil does not appear as straightforward as concrete, which breaks apart, or steel, which yields or snaps. It fails by keeping its particles in an interlocked state as they are, so that they slide relative to each other.

In a word, soil primarily fails in shear.

Properties of the soil particles

Shear by its nature is a combination of friction and cohesion, either on a macro level (such as shear in concrete or in a piece of rock), or on a micro level (such as shear between soil particles).

Soil is made up of trillions of small particles that are often only visible at microscopic levels. The particles on a mass scale have a rough surface, which interlock themselves between each other by an amount of friction proportional to the normal compression. How much friction is generated by the friction under a given compression is represented by a concepted called ‘internal friction angle’, which is part of the Mohr-coulomb model.

The Mohr-coulomb model is a widely used model for assessing shear failure of the ground. It has two components, cohesion and friction. The friction component is represented by the ‘internal friction angle’. Loosely speaking, the better interlocked the particles of a soil is, the stronger shear capacity it has when subject to the same compressive stress. Better shear capacity is reflected in the friction angle. The better interlocked the particles are, the higher internal friction angle is.

Properties of the pore water

To understand the behaviour of the pore water, the following 3 bullet points are very important things often overlooked by textbooks:

· Water can not be compressed. Water is flowy, but is virtually impossible to be compressed when it is not allowed to flow. It takes an astronomical magnitude of compressive force to compress a glass of water by a hairline fraction.

· Water gives no shear. It has no interlocking particles and no shear capacity. Yes, ZERO shear capacity, whatsoever. In fact, a material with zero shear capacity is a very accurate definition of ‘fluid’. Water can flow freely just because it has zero shear capacity. Anything that has a shear capacity greater than zero is less able to flow.

· Water flows. Water flows in to fill the voids between soil particles, and in the same but reverse fashion escapes when subject to external compression. Water in soil behaves exactly the same as that in a sponge. How quickly water can escape the void when compressed depends on how big the voids are. The bigger the soil particles, the bigger the voids. Fissures and cracks in the ground create huge voids compared to those created by particles.

Composite actions between soil particles and water

Combining the soil and the water, we have a composite material called ‘soil’. How soil behaves depends significantly on whether and how quickly water can escape from the void.

If water can not escape (technically called ‘undrained’), then when under compression, the water is incompressible and therefore takes all the compression and props up the entire structure. All the compression goes into water leaving the soil particles uncompressed, hence not able to generate any friction. So under the Mohr-Coulomb model, the friction part is zero, and the shear capacity only relies on the ‘cohesion’ part. This is why under ‘undrained’ condition, no matter how big the confining compressive force is, the shear failure remains as a flat horizontal line.

However, when water can escape (technically called ‘drained’), it is a different story. Compression forces the water out and effectively the compressive capacity of water is zero. Now the soil particles have to step in to take up all the compression, hence generating friction. Therefore the amount of shear capacity is proportionate to the confining compression the soil has.

Drained or Undrained?

So – in what situations can water escape easily and what not? It’s all about time. It mainly depends on two things – how big the voids are and how sustained the loading is.

· The bigger the soil particles are, the bigger the voids are, the quicker water can travel. London Clay, for example, is famous for its super fine particles and as a result a virtually non-permeable stratum for water. Sand and gravels on the other hand has much larger voids for water to travel.

· The longer the load is sustained, the longer the soil remains compressed and therefore more chance for water to escape. From a practical point of view, loadings from temporary structures are often not sustained whereas those from permanent structures often exist long enough for water to escape.

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