Environmental Engineering Reference
In-Depth Information
p3
1
p1
p2
2
Material
propertie s
w (%) PI,
LL
Cut
γ
φ
c
3
Fill
5
4
Legend:
(1) shallow slump in colluvium; (2) total failure of cut in residuum caused by very high-seepage pressures
in saprolite; (3) large-scale failure of slope caused by very high-seepage pressures in the fractured rock
zone; (4) large rotational failure of fill through residuum;(5) slump failure at fill toe through the colluvium.
FIGURE 9.130
Schematic of typical section prepared at scale of 1:1 showing tentative cut and fill imposed on 30° slope in
residual soil profile as basis for analysis. Piezometers P1 and P2 show excess water pressures in saprolite and
fractured rock compared with saturated zone in residuum at P3. Several possible failure conditions requiring
evaluation are shown.
Nuclear probes lowered into boreholes measure density and water content, and have
been used to locate a failure surface by monitoring changes in these properties resulting
from material rupture. In a relatively uniform material, the moisture and density logs will
show an abrupt change in the failure zone from the average values (Cotecchia, 1978).
Dating Relict Slide Movements
Radiometric dating of secondary minerals in a ruptured zone or on slickensided surfaces,
or of organic strata buried beneath colluvium, provides a basis for estimating the age of
previous major movements.
Growth ring counts in trees that are inclined in their lower portion and vertical above
also provide data for estimating the age of previous major slope movements. The date of
the last major movement can be inferred from the younger, vertical-growing segments
(Cotecchia, 1978). Slope failures cause stresses in the tree wood which result in particular
tissues (reaction or compressed wood), which are darker and more opaque than normal
unstressed wood. On the side toward which the tree leans there is an abrupt change from
the growth rings of normal wood to those of compression wood. By taking small cores
from the tree trunk it is possible to count the rings and estimate when the growth changes
occurred and, thus, to date approximately the last major slope movement.
Slope Assessment
Data Presentation
A plan of the slope area is prepared showing contours, drainage paths, seepage emerging
from the slope, outcrops, tension cracks and other failure scars, and other significant
information. Sections are prepared at a 1:1 scale illustrating the stratigraphy and
groundwater conditions as determined from the explorations, as well as any relict failure
surfaces.
 
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