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