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  • Writer's pictureSi Shen

A structural engineer’s understanding of rock engineering

Rock is a natural material and its behaviour isn’t as easy to grasp as an engineered material with well-defined properties such as concrete or steel. This blog post explains the behaviour of natural rock with plain language understandable by structural engineers.

How is rock different from soil?

The previous blogs have mainly touched on the behaviour of soil.

So what about rock?

The boundary between rock and soil is fuzzy – they are just two theorised extremities on a spectrum. Most natural ground conditions are somewhere in between, i.e. they exhibit partially ‘rock’ behaviour and partially ‘soil’ behaviour. We name a ground condition either ‘rock’ or ‘soil’ based on its dominating engineering behaviour:

  • Soil is weak and granular/cohesive and behaves generally as a continuum. The main failure mode is the relative sliding between particles

  • Rock is strong and blocky. It tends to fail along discontinuities (cracks/joints etc). The dominant failure mode is wedge spall-off.

How does a structural engineer see rock?

The rock, when it is not cracked or broken up (i.e. ‘intact’), can be very strong – sometimes even stronger than the strongest concrete human has ever produced (>200MPa, where is concrete is normally only up to 60MPa). Just like the strength of rock is similar to that of concrete, the engineering behaviour of rock is very similar to that of plain unreinforced concrete. Just because rock behaviour is like concrete, the testing of (intact) rock strength is similar to the testing of concrete.

  • Uniaxial compressive strength test at its very core is just the cylindrical test of concrete

  • The 'point load' test is used both for rock and concrete

Unreinforced concrete tends to crack, so does rock. ‘Cracks’ in rock are called ‘discontinuities’, which can be either due to movement of the earth crust or weathering actions and usually take the form of bedding plane, foliation, folds, joints and fractures. On a mass scale, the overall engineering properties of the rock depends on both its intact strength and the characteristics of its discontinuities.

The following properties of the discontinuities come into play in the overall behaviour of rock:

  • Dimensions of discontinuities, including frequency/spacing, width and continuity

  • Conditions of discontinuities, including rough or smooth, dry or wet, clean or infilled. If they are infilled, the properties of the infill material also matter.

A systematic approach towards assessing engineering behaviour of rock

Engineers would normally use the behaviour of rock on a mass scale. Therefore geotechnical engineers pull together all the above characteristics of the rock and use semi-quantitative scoring systems to approach the assessment of the mass scale engineering behaviour of rock. For example, ‘Rock Mass Rating’ is a popular system of such functionality. With such systems, the mass scale rock behaviour can be classified into five tiers, with tier one being the highest and five being the lowest in mass strength and stiffness. Because of how hard it is to find the ‘perfect’ rock, the intact strength of the rock usually only play a small part in the mass scale engineering behaviour of rock, compared to the influence from discontinuities.

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