Generally, local site effects (landslides, liquefaction,
amplification and faulting systems) play an important role in the intensity of
earthquakes. Thus, Earthquake-resistant design of new structures and evaluating
the seismic vulnerability of existing buildings take into account their
response to site ground motions. Geophysical studies of seismic activity in
Palestine, deep seismic sounding, paleoseismic excavation, and instrumental
earthquake studies of half a century demonstrate that damaging earthquakes
occurred along the Dead Sea Transform fault. The topography,
geomorphology and geology of the West Bank have been the main reasons behind
several sizeable landslides that occurred around ten years ago in different
parts of the West Bank. Also, it has been shown that Palestine suffered from
several landslides during historical earthquakes.
Landslides are rock, earth, or debris flows on slopes due to
gravity, and they can occur on any terrain given the right conditions of soil,
moisture, and the angle of slope. Landslides can be triggered by rains, floods,
earthquakes, and other natural causes as well as human-made causes, such as
grading, terrain cutting and filling, excessive development, etc. Because
the factors affecting landslides can be geophysical or human-made, they can
occur in developed areas, undeveloped areas, or any area where the terrain was
altered for roads, houses, utilities, buildings, and even for lawns in one's
backyard.
To investigate landslides one should do field studies (including geophysical
seismic study), laboratory studies and slope stability analysis. It is
important that the field and laboratory investigations be supplemented by field
measurements so that the behavior of a slope can be checked and corrective
measures be taken in times. The first step in landslides analysis is the
collection of available information geological, hydrological, topographical,
and soil maps.
The
subsurface geology determined by seismic studies is extremely important for the
development of urban areas. The shallow upper part of the rock formation
section is the most significant part for civil infrastructures. The seismic
refraction technique is considered an accurate geophysical method to
investigate the shallow geological structures of an area. During the past
decades, the seismic parameters obtained by a refraction survey have been
widely used in cases of site investigation as indicators of rock mass quality.
These parameters are of great importance in land use management of various
civil engineering purposes (Primary velocity Vp, shear wave velocity Vs and
Soil profiles).
Several landslides occurred in Palestine during 2012 in which
geophysical seismic explorations were used for slope stability analysis and
landslides studies. Four major landslides are focused on, namely, Al Bathan –
Nablus main road landslides, Nablus District, A’arrabeh landslides, Jenin
District, Kufur Qalil landslides, Nablus District and Beit A’oor, Ramallah
District.
In order to better understand the causes of
landslides and slope instability in the above mentioned areas, it is important
to describe soil strata, soil properties and the geotechnical conditions of
these sites. These parameters and conditions at the study areas were found from
the seismic geophysical investigation by using seismograph 24 channels, trial
pits, lab tests and eye observations. Then, slope stability analysis using
software GeoStudio2007 was carried out for several conditions along the
critical sections in the studied areas to figure out the minimum factor of
safety and comparing to the acceptable limits. In general, the factor of safety
for slope required for any temporary structures is 1.3 and that for permanent
structures it is 1.8.
During the presentation in the Workshop, these four major
landslides which occurred in Palestine during 2012 will be focused on and
results of slope stability analysis and landslides studies will be presented.
Based on these analysis and studies the factor of safety found, it is clear
that these sites suffer slope instability and hence, landslides may occur at
any moment. Furthermore, soil properties may be changed at these sites due to
weather conditions, such as, rainfall or snow fall or / and these sites will be
affected by relative strong earthquakes. This will reduce the factor of safety
and landslides may occur.
In general, several recommendations were suggested to
overcome slope instability in sites were landslides occurred. Such
recommendations are to reduce the weights at the top of slopes, increase and
install weights at the bottom (toe) of the slope, flatten the slope, methods to
drain the slope and the site around landslides, increase stability by planting
trees on the slope.
It is found out as planning strategy; landuse policy should
be done for Palestine, especially, for sites of high slopes and those of which
they have previously landslides.