Tropical landf o rms
557
TYPE A
'~'-~'~3
TYPE C
;,"~~
"
Soi l
~~ [J~'~
.~:~.~:.~:~ ,'~, ~."
- Bedrock
~.'..~
9 .~
Transi ti on ~
l ayer ~ ~
..~i- L
,
TY P E B
~t~ ~
...~., ,,"~
, ~.4 .It',
,,~.~,~ ' ..."'
4~.,'"... ;- :' _~,e.'
~
~ I ~
;' .~' ~.~.
.f "~
TYP~~,x'~x,
Figure 21.24. Slope classification scheme based on root reinforcement and anchoring (Tsukamoto and
Kusakabe, 1984; in Greenway, 1987, Figure 6.14).
sapping and exceptionally by tectonic activity; the increase of topographic gradient
induces a change in the stress state, which can trigger failures.
Humid tropical zones located in alpine orogenic belts are exposed to sporadic
earthquakes, which affect slope stability. Earthquakes produce rock oscillations with
different frequencies and, hence, the sudden stress modification can disturb slope
equilibrium. Liquefaction can take place in silts and loose sands. In Papua-New Guinea,
Simonett (1967) studied slide frequency and pointed out that the number of slides
decreases logarithmically with distance to the epicentre. When earthquake magnitude is
important, the number and size of slides increase.
Investigations on plots and experimental basins indicate that zones bared of vegetation
are exposed to higher erosion than vegetated areas. Deforestation resulting from abusive
tree felling or man-made fires, the latter very frequent in the Amazon rainforest, influence
the rate of mass movement. This anthropic action basically affects saprolites, because in
dry periods desiccation cracks favour water infiltration during rainfall. These processes
have been interpreted as partly responsible for the triggering of numerous slides in the
widely deforested area of Serra do Mar, between Sac Paulo and Rio de Janeiro (Tricart,
1974b). Similarly, the absence of forest mass brings about the lack of root reinforcement in
the regolith, the deprivation of bedrock anchoring and the loss of water absorption. As a
consequence, slopes undergo a shear strength decrease. These circumstances favour slide
development during intense rainfall. On 23 February 1970 a 3-hour rainstorm of 100.7 mm
in the Uluguru mountains (Tanzania) led to the generation of 1000 new superficial failures
and the reactivation of other previous ones (Temple and Rapp, 1972). The work done by
So (1971), however, posed doubt on the role of vegetation as an essential factor in failure
557
TYPE A
'~'-~'~3
TYPE C
;,"~~
"
Soi l
~~ [J~'~
.~:~.~:.~:~ ,'~, ~."
- Bedrock
~.'..~
9 .~
Transi ti on ~
l ayer ~ ~
..~i- L
,
TY P E B
~t~ ~
...~., ,,"~
, ~.4 .It',
,,~.~,~ ' ..."'
4~.,'"... ;- :' _~,e.'
~
~ I ~
;' .~' ~.~.
.f "~
TYP~~,x'~x,
Figure 21.24. Slope classification scheme based on root reinforcement and anchoring (Tsukamoto and
Kusakabe, 1984; in Greenway, 1987, Figure 6.14).
sapping and exceptionally by tectonic activity; the increase of topographic gradient
induces a change in the stress state, which can trigger failures.
Humid tropical zones located in alpine orogenic belts are exposed to sporadic
earthquakes, which affect slope stability. Earthquakes produce rock oscillations with
different frequencies and, hence, the sudden stress modification can disturb slope
equilibrium. Liquefaction can take place in silts and loose sands. In Papua-New Guinea,
Simonett (1967) studied slide frequency and pointed out that the number of slides
decreases logarithmically with distance to the epicentre. When earthquake magnitude is
important, the number and size of slides increase.
Investigations on plots and experimental basins indicate that zones bared of vegetation
are exposed to higher erosion than vegetated areas. Deforestation resulting from abusive
tree felling or man-made fires, the latter very frequent in the Amazon rainforest, influence
the rate of mass movement. This anthropic action basically affects saprolites, because in
dry periods desiccation cracks favour water infiltration during rainfall. These processes
have been interpreted as partly responsible for the triggering of numerous slides in the
widely deforested area of Serra do Mar, between Sac Paulo and Rio de Janeiro (Tricart,
1974b). Similarly, the absence of forest mass brings about the lack of root reinforcement in
the regolith, the deprivation of bedrock anchoring and the loss of water absorption. As a
consequence, slopes undergo a shear strength decrease. These circumstances favour slide
development during intense rainfall. On 23 February 1970 a 3-hour rainstorm of 100.7 mm
in the Uluguru mountains (Tanzania) led to the generation of 1000 new superficial failures
and the reactivation of other previous ones (Temple and Rapp, 1972). The work done by
So (1971), however, posed doubt on the role of vegetation as an essential factor in failure