Structural engineers for underground structures should have a basic and general understanding of the ground, as the ground is often a support or hazard to the structure. Maximising efficiency of the structure can only be achieved if the structural engineer has a good understanding of the behaviour of the ground. This blog post kicks off the series with one of the prettiest types of ground – the chalk.
The snow white
These photos are first-hand taken by myself at Seven Sisters near Eastbourne.
Chalk is a bedrock widely spread across the south and east of England. Its snow white colour is aesthetically appealing and very easy to identify visually.
Geologists say chalk is solidified dead bodies of ancient organisms, which is an explanation that I personally find hard to believe – the purity of chalk looks more like all of it almost just appeared with a flick of fingers.
If you classify it as ‘rock’, then it is a ‘weak’ to ‘very weak’ rock, whilst exhibiting some soil behaviour. If you classify it as ‘soil’, then it is a very stiff self-standing soil, able to form vertical cliffs without any support. Chalk’s engineering behaviour goes between rock and soil hence some people call it ‘sock’. Good quality chalk used to be a building material in the old days and there were chalk quarries.
From the photos you can see that from an overall perspective, the entirety of the rock mass consists of two distinctive parts:
· the discrete jointless chalk blocks
· the gaps in between the blocks. Both frequency and the property of the gaps themselves matter.
The factors of chalk behaviour
In civil engineering, we normally rely on the behaviour of the entirety of the rock (mass behaviour), rather than the discrete parts of it. So based on the above observation, we can infer that the engineering properties of the chalk are influenced by the following factors:
· Mechanical properties of the intact rock (without joints)
· Pattern of joints (bedding and discontinuity), including orientation, frequency and style in vertical/horizontal directions. Orientation of the joints are important because the joints create a weak shear plain, so they become a hazard if they coincide with the flow of shear stress. The orientation is particularly important in the context of tunnelling and embankments.
· Geometry and mechanical properties of the joints themselves (‘aperture’)
The first bullet is a ‘material property’ and the other two are ‘mass properties’. Combined, they form the overall behaviour of chalk in its entirety. The 1st bullet point is often derived from lab and in-situ tests. CIRIA C574 provides guidance on a systematic way of describing the 2nd and 3rd bullet points – it takes the form of a letter followed by a number, such as ‘A3’ or ‘B4’. The letter represents the width of the gap (discontinuity aperture) and the number represent how often the gap appears (discontinuity spacing). From A1 to C5 and D, the chalk-joint pattern goes from virtually intact to completely de-structured. There is usually a correlation between the intact properties of chalk and the geometry and pattern of the joints – understandably so, as the weaker the rock is, the worse it tends to crack subject to the same pressure.
Chalk is not alone
You can see in this picture that the white chalk is accompanied by nodes of black bits. They are flint embedded in chalk, typically in the formation of horizontal bands. Flint is typically a very hard rock compared to chalk. In terms of compressive strength, flint is typically several times higher than typical concrete. Flint are normally small enough to be handled, and generally does not weaken the mass behaviour of chalk, although it does give us some practical problems in wear:
· Wear of cutterheads and excavator teeth
· Wear on rubber tyres
Chalk and water
The chalk is well known for its water bearing and high permeability. The ground water network around the London area is connected through chalk. Believe it or not, the intact (jointless) chalk is virtually impermeable like London Clay, because its has the similar granular size as London Clay. However, water leaks through the joints, making chalk a stratum that you should usually expect high to very high permeability. Also due to the orientation of the joints, the horizontal permeability is typically higher than vertical permeability.
Moreover, just because of the fine granularity of chalk, when excavated chalk is ground up and mixed with water, if you are not careful enough, it can become toothpaste like putty, which can jam up your machines such as TBM during tunnelling operation.
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