2b.ce2004 Rock Mass Strength HO
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Transcript of 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.