BY ALICOR PANAO
A major landslide buried an entire village in Leyte following
heavy rains early last year. Some 2,000 people lost their lives and millions
worth of property were swamped by the flow of mud. Around the world, landslides
kill thousands of people each year and cause extensive property damage or loss,
especially in heavily populated mountainous regions.
Recent studies, however, indicate that it may actually be
possible for rainfall-induced failures to be predicted by carefully monitoring
changes in soil moisture contents and deformations at specific areas within the
slope. In a research article which appeared in the National Engineering Center’s
Philippine Engineering Journal, Yamaguchi University professor Rolando P. Orense
proposed a simple method of predicting the occurrence of landslides caused by
heavy rainfall by installing soil moisture sensors at "critical locations,
possibly in areas where seepage forces will develop." Orense is a civil
engineering graduate of UP Diliman (BSCE, 1984, cum laude; MSCE, 1989) now based
in Japan.
A landslide is a downward movement of rocks and soil,
although sometimes the collapse may involve vegetation, structures, or parts of
roadways. Sometimes the soil can liquefy with the slide becoming a mudflow that
acts much in the same way as a stream of water. Landslides may be set off by
human activities such as cutting roadways and building dwellings or placing
improperly engineered fill on steep slopes. Earthquakes can also trigger
landslides. The most common triggering mechanism, however, is the combination of
heavy rainfall, steep slopes and loose or soft soil.
Orense conducted reconnaissance studies in Quezon province
and laboratory tests at the University of Tokyo’s Geotechnical Engineering
Laboratory to show the correlation between rainfall intensity and the frequency
of landslides. Among Orense’s interesting findings was the fact that numerous
landslides occurred even in areas which were heavily forested. By contrast,
there are areas where very few landslides take place despite of their marginal
forest cover. Landslides were also found to occur more likely in steep slopes
regardless of whether or not they are forested.
In all the affected areas, soil composition is typically made
up of silty sand or sandy silt with very little clay content. Orense believed
the heavy rainfall saturated the slopes causing the soil to lose sheer strength
and eventually triggering failure. And while environmental conditions
particularly land use and logging may have contributed to the impact and scale
of the disaster, as far as Orense’s study is concerned they are not the main
cause of the landslides.
Orense’s observation implies not only that loose soil slopes
would undergo more rapid deformation than dense ones when subjected to heavy
rainfall but that more rapid ground movement would be expected for steeper
slopes than for mild ones. This is consistent with the general idea that "slope
gradient is a significant factor in establishing the instability state as well
as the condition of slopes after the failure."
Slight slope movements and tensile crack formation usually
precede slope failure. Cracks are formed due to the decrease in strength at the
slip surface related to the pore-water pressure increase. This indicates,
according to Orense, that "small scale" slope movements can also serve as
indicator of impending collapse.
If Orense is right, there is no need to install numerous
moisture sensors along the slopes. This is a more economical landslide disaster
detection and mitigation system. Soil moisture sensors, says Orense, may be
installed only at critical locations where seepage pressures will likely
develop. Monitoring devices, on the other hand, may be fixed only at the height
when the estimated travel distance of the failed portion of the slope reaches a
critical level in which a downslide becomes inevitable. Even though the system
will not be able to predict collapse time accurately, alarm can be issued
several minutes before slope failure reaches its critical level to give
residents and disaster workers ample time to respond.
Orense admits, however, that further investigations using numerical analyses
and actual on-site monitoring may still be necessary to validate the
effectiveness of his proposed system. Nevertheless, he maintains that a
carefully laid out system along with accurate information on ground conditions
may just spell the difference between life and death.
