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Engineering Geology

Engineering Geology - geomuseu

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E n g i n e e r i n g G e o l o g y<br />

or<br />

E<br />

V 2<br />

2<br />

È3( V / V ) - 4 ˘<br />

S<br />

p s<br />

= r Í<br />

˙<br />

2<br />

g<br />

ÎÍ<br />

( V / V ) - 1<br />

p s<br />

˚˙<br />

(7.7)<br />

or<br />

2<br />

05 . ( V / V ) - 1<br />

P S<br />

u =<br />

2<br />

( V / V ) - 1<br />

P<br />

S<br />

(7.8)<br />

where g is the acceleration due to gravity.<br />

These dynamic moduli correspond to the initial tangent moduli of the stress–strain curve for<br />

an instantaneously applied load and are usually higher than those obtained in static tests.<br />

The frequency and nature of discontinuities within a rock mass affect its deformability.<br />

In other words, a highly discontinuous rock mass exhibits a lower compressional wave<br />

velocity than a massive rock mass of the same type. The influence of discontinuities on the<br />

deformability of a rock mass can be estimated from a comparison of its in situ compressional<br />

velocity, V pf , and the laboratory sonic velocity, V pl , determined from an intact specimen taken<br />

from the rock mass. The velocity ratio, V pf /V pl , reflects the deformability and so can be used<br />

as a quality index. A comparison of the velocity ratio with other rock quality indices is given<br />

in Table 2.7.<br />

Resistivity Methods<br />

The resistivity of rocks and soils varies within a wide range. Since most of the principal rock<br />

forming minerals are practically insulators, the resistivity of rocks and soils is determined by<br />

the amount of conducting mineral constituents and the content of mineralized water in the<br />

pores. The latter condition is by far the dominant factor, and in fact, most rocks and soils conduct<br />

an electric current only because they contain water. The widely differing resistivity<br />

values of the various types of impregnating water can cause variations in the resistivity of<br />

rocks ranging from a few tenths of an ohm-metre to hundreds of ohm-metres (W m) as can<br />

be seen from Table 7.5.<br />

In the resistivity method, an electric current is introduced into the ground by means of two<br />

current electrodes and the potential difference between two potential electrodes is measured.<br />

It is preferable to measure the potential drop or apparent resistance directly in ohms rather<br />

than observe both current and voltage. The ohms value is converted to apparent<br />

resistivity by use of a factor that depends on the particular electrode configuration in use<br />

(see below).<br />

352

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