Geotechnical Report from June 23, 2001 Peruvian Earthquake


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.

Typical section of

Figure 6.1: Typical section of "conglomerate" deposit along the southern coast of Peru.

6.2 Site Amplifications

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.

Typical section of

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.

Typical section of 'conglomerate' School in Tacna with damage limited to cracking of non-bearing walls

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.

6.3 Topographic Effects

Three-story house located along a ridge in Arequipa

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.