CIPA 2003 XIX th International Symposium, 30 September - 04 October, 2003, Antalya, Turkey
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eased the building of a landfill which raised the level of the lake’s
surface artificially by resorting to man-made islets, chinampas
and the like. In the Cortez Map, the Tenochtitlan islet can be seen
at the center, completely surrounded by water. Nevertheless, 50
years later, in the Uppsala Map, the land ranging between Tacuba
and Azcapotzalco is depicted as dry land. Churubusco, after being
a lake settlement, it becomes a lakeshore village in the Uppsala
Map. In the following half-century, the entire western region of
the city gradually became dry land which during the rainy season
turned into quagmires. From 1520 to 1620 the lake level could
have diminished by some meters, thus reducing the body of water
between the eastern parts of the dyke to the Texcoco lakeshore.
For the reasons so explained, the western zone of the basin favored
the founding of most settlements depicted on the ancient maps.
However, historic cartographic limitations produced inaccurate
maps that make pinpointing areas on present day urban maps
especially difficult. It’s evident that for this and other reasons we
cannot to assure the location of islets in a determined zone, by
resorting to this information alone.
We can consider that the group of islets in the lake should have
been formed by regularly spread chinampas and canals.
Nonetheless, the ruling centers of each community should have
contained larger islets whose better foundations supported heavier
structures. This would cause different soil pre-compaction
conditions, lighter on the farming chinampas and heavier on the
ritual and administrative zones. The other extreme would be
constituted by the ruins of the Main Temple, wherein the soil is
mostly pre-compacted. Succeeding building phases gradually
compacted the subsoil further, which, now freed from the load of
colonial buildings and having lost most of its original volume
and weight, is now recovering itself and rising over the street
level that used to cover it (Mazari et al 1985).
3. THE SINKING OF THE CITY
The sinking of Mexico City is one of the great problems this
major urban center is facing. The problem is originated in its
inhabitants’ huge water supply needs and the consequences are
manifold. For the purposes of this research, the most relevant
consequence is structural affectation caused by differential
sinking. In many places within the city it is noticeable the crinkles
and the caving in of architectural structures whose walls thus
show tilted cracks. This causes huge maintenance costs, specially
in structures classified as part of the nation’s architectural heritage.
A recent example is the great effort undertaken to save the
Metropolitan Cathedral, deformed by differential sinking and by
the existence of pre-Hispanic structures beneath it, for which a
huge amount of economic resources are being spent every year
(Matos 1992). The proposed solution has to do with the
importance of the structure being preserved, yet as the features
of the damaged structures are so varied and so many, it’s however
crucial to understand the phenomenon that affects them so to
prevent aspects which are later hard to correct.
Studies complied by Marsal and Mazari (1969) and Kumate and
Mazari (1990) on the city’s problems, have helped to understand
the phenomenon causing the difference in performance of pre
compacted soils. Ovando and Manzanilla ( 1997:65), describe that
after applying a charge to the clays that form the city’s subsoil,
they expel water and diminish its volume, thus causing the
material to harden and become less compressible. In this case,
most of the studied pre-compaction cases appear to be a result of
the accumulation of building materials throughout some centuries.
In the case of a larger structure that would have required a
foundation with wooden beams, the performance of the land
would be even more contrasting. This type of foundation would
support remains of a pre-Hispanic structure with its floors and
the toppling of its walls and roofs, which would sink at a different
speed than in the less altered terrain that surrounds it. These
differences can currently be seen as sinking and topographic relief
at the streets of Mexico City. Yet more still, these differences in
building techniques are in stark contrast with the soil that was
once the bottom of the lake, which lack human labor and
consequently pre-compaction. Together with the overall sinking
of the city, differential sinking occurs daily in which the sinking
of the less consolidated soil is faster than that of the pre-compacted
land, then the presence of mounds over the surface is a
phenomenon every day more evident.
Summing up, in ascending order we can tentatively sort out the
precompaction process in the bottom of the unaltered basin, the
less-altered farming plots, and the landfills used to built platforms
and floors. At the highest level, we would have the ceremonial
and administrative stone structures with wooden foundations
which represent the utmost alteration level. Currently, the latter
would be plots of land that would show outstanding differences
on the topographic relief and these would be what we might record
in this study.
As it’s well known, there is an average rate of sinking for the
city. However, every zone has its own particular rate. Registers
collected in the past century (Kumate and Mazari 1990) enable
us to know how some areas of the city have been sinking. Average
sinking varies from 7 to 9 meters, yet there are specific zones in
which the rate is just 6 or 7 meters, which are revealed as mounds
measuring 1 or 2 meters.
Figure 1. Table of readings of height above sea level through
time in four points at downtown Mexico City
Due to the presence of big architectural structures now in danger
of collapsing, one of the locations under study from a geotechnical
standpoint is downtown Mexico City. However, throughout the
lake basin there are other sites sharing the same features that, at a
lesser scale, harbor similar problems. It’s quite likely that the
currently visible mounds do correspond to an artificial islet formed
on the lake basin, due to the accumulation of layers of earth and
building materials from pre-Hispanic times.
Sinking Readings at Mexico City
