2b.ce2004 Rock Mass Strength HO

8/8/2019 2b.ce2004 Rock Mass Strength HO

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CE2004: ROCK MASS

STRENGTH

AIM:To understand the features of rock mass

strength for civil engineering.

Definition:Rock mass strength is the overall strength of a

rock mass, and is more complex than rock

strength because rock mass strength involves

structural features. Ref: Waltham 2002.

We have seen previously that rocks have varying

rock strength and rock mass strength --------->


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Gypsum in Late Triassic mudstones, Somerset, UK.


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Views of bedding

surfaces of Wrekin

Quartzite, with wind-

blown ripples (formedin intertidal conditions)

Quartzite is quartz sand cemented by quartz

cement: It is one of the most resistant rocks


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The rock is slate, a

regionally-metamorphosed

mud; here the

original bedding can

be seen in the

quarry wall (thisview is about 100m

high)

Colour differences show slightlithological variation; the lighter

bands are slightly coarser. These

sediments were deposited in the

deep parts of a sedimentary basin,

this one is 500 Ma, in north Wales

SEDIMENTARY ROCKS AND LOW GRADE METAMORPHISM


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Also important to be aware

of the varying geometry ofrock masses and how this

may relate to their strength.

Most rocks on Earths

surface are sedimentaryand deposited in flat layers

on the surface, like here at

Hunstanton in Norfolk.


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. . . But rocks can be folded . . . .


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Normal Fault

. . . . . and faulted


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But some stone is

ideal as building

stone, such as

Portland limestonea major building

stone of Victorian

London


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Ordovician limestones in the Franklin Mountains of southwest Texas;

strong rock formations


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Folded and fractured rock masses,Jurez Mts, Mexico


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Differential rock mass strength causes collapse


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Porous conglomerates have

lower water pore pressures,

and are more stable


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Budleigh Salterton, south Devon, UK

Tafoni

weathering

Early Triassic

conglomerates

and sandstones;porous and are

major aquifers in

southwest

England


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Faulting and folding relate to

cliff collapse

See also Waltham, A. 2002.Foundations of Engineering

Geology, 2nd ed. Spon Press,

London


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Slate (low grade

regionally

metamorphosed

mudstone) hasdifferent strength

depending on

orientation of forces

with respect to its

cleavage

Cleavage

orientation

Bedding

orientation

See also Waltham, A. 2002.Foundations of Engineering

Geology, 2nd ed. Spon Press,

London


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ROCK MASS CLASSIFICATION

Depends on all the features of the rock, and there

are two types of classification:1. Geomechanics Rating (RMR), which ADDS

rating values

2. Norwegian Q system, which MULTIPLIES

rating valuesSee Waltham, A. 2002. Foundations of Engineering Geology, 2nd

ed. Spon Press, London, for a full description of these

classifications; also you can find guidelines for the details of rock

mass classifications and how they are used in applications of rock

descriptions for civil engineering applications.

Walthams book also provides useful information on ground

conditions for building, and gives examples.

2b.ce2004 Rock Mass Strength HO

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