Environmental Engineering Reference
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Figure 2.4.
Variation in ratio of average horizontal stress to vertical stress with depth below surface
(from Brown & Hoek, 1978, by permission of Pergamon Press).
locked into mineral crystals or grains by cementation and interlocking. An analogous
model would be compressed springs embedded in plastic. As the vertical load on high
stressed rock is slowly lowered by erosion, vertical stresses are relieved progressively by
upward expansion. However, because the rock remains confined laterally, the horizontal
stresses decrease in accord with Poisson's Ratio, i.e., at about one-third of the rate of the
vertical stresses. This results in the recorded near-surface imbalance.
2.5.2
Stress relief effects in natural rock exposures
Field evidence of the existence of the high horizontal stresses at shallow depths is seen most
clearly in areas of massive igneous rocks which contain very few tectonically induced frac-
tures. Most of Hast's measurements plotted on Figure 2.2 were made in such rocks. These
rocks usually contain “sheet joints” near-parallel to the ground surface, as shown in
Figure 2.5 . The sheet joints show rough, irregular, plumose surfaces indicating that the
rock failed in tension, by buckling or spalling, the tension being induced by the high hor-
izontal compressive stress. Some sheet joints show slickensides near their extremities indi-
cating local shear failure which should be expected here (Figure 2.5). The spacing of sheet
 
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