6.0 SITE
EFFECTS
6.1
Soil Types Along Southern Coast of Peru
The predominant soils in the affected region
are of alluvial-colluvial origin. An upper layer of variable thickness of
alluvial-colluvial sand overlies coarse sandy gravels. There are also deposits
of aeolian soils.
Materials of fluvial origin can be found in
the cliffs nearby the coastline. These are typically boulder gravel deposits
(locally called "conglomerate") consisting of boulders up to 40-cm in
size within a silty sand matrix. These deposits were formed during the
Pleistocene deglaciation (quaternary). Because of the extreme relief along the
coast, rivers flowing to the ocean from the Andes are short in length. These
rivers had a steep gradient and considerable flow, and were able to carry a
substantial amount of large-size material into their deltas, forming alluvial
fan-shaped deposits. These deposits of "conglomerate" are generally
very dense and deep, and are alternated with layers of sandy clays or clayey
sands, without any specific order. A typical profile of this soil deposit is
shown in Figure 6.1.
In some areas there are considerable
proportions of gypsum and soluble salts, which have cemented the sands. There
is also a significant presence of soils with volcanic origin.
Figure 6.1. Typical section of "conglomerate"
deposit along the southern coast of Peru.
The concentration of damage in some areas of
several cities (Tacna, Moquegua, Ilo) suggested the influence of site
amplification in the resulting damage levels. Indication by local engineers
pointed to the presence of sand deposits in areas of concentrated damage, while
areas underlain by stiffer gravel deposits suffered less damage. It is
noteworthy that public schools in Peru are constructed to a similar design and
using similar construction practices. Observations of the performance of school
buildings in different locations permit a comparative evaluation of the ground
motions experienced during the earthquake.
The following pictures show two reinforced
concrete frame school buildings in Tacna, one with severe structural damage and
the other with minor damage to non-bearing walls.
Figure 6.2. Wall cracking and damaged column
at Tacna school. The column was partially confined by non-bearing walls,
creating a "short" column effect.
Figure 6.3. School in Tacna with damage
limited to cracking of non-bearing walls. Ing. Adolfo Gonzales Palma, in the
picture, built this school.
Figure 6.4. Three-story house located along
a ridge in Arequipa.
Damage to houses and business was
concentrated at or near the top of several steep ridges in the towns of
Arequipa and Moquegua. The inclinations of the ridge slopes varied, but were on
the order of 35 to 60 degrees. The house in Figure 6.4 above suffered severe
damage in the earthquake.