Chemical and geotechnical properties of Mexico City clay

A comprehensive experiment has been done to characterize chemical, mineralogical and geotechnical properties of Mexico City clay. The mineralogical, chemical, and
physical properties of Mexico City clay sediments from depths of 5 to 19 m are presented, and an attempt is made to specify the mineralogy and chemistry responsible for the
unusually high liquid limit of the clay. An X-ray analysis of <2 pm clay fractions indicated the presence of smectite, as a principal clay mineral, and amorphous material.
The contents of poorly crystalline Si, Al, and Fe extracted with oxalate were found to be in the ranges of 35-57, 23-54, and 133-273 g/kg, respectively, and the ferrihydrite
content was as high as 226-464 g/kg. The high observed pH of 9.1 to 10.1 was due to the presence of large amounts of carbonate. Pore water salinity ranged from 51 to 84 g/L
and was 1.4 to 2.4 times higher than seawater salinity. Clay and silt were the dominant grain-size fractions, and the clay fraction (<2 um) varied from 19% to 51%. The specific
surface area varied widely from 26 to 436 m2/g. Because of the high pore water salinity, the original water contents of the clay needed to be modified by taking into account the
weight of the salt present in the oven dried samples. The modified natural water contents, liquid limits, and plastic limits were in the range of 42% to 641%, 45% to 798%,
and 33% to 76%, respectively. The activity of Mexico City clay ranged from 5.6 to 13.9, which was much higher than that for reference samples of bentonite and smectitic
marine clays. Despite smaller clay fractions in Mexico City clay than in bentonite, Mexico City clay exhibited liquid limits as high as 194% to 798%, which were equivalent
to those of bentonite. This is attributed to the predominance of smectite, ferrihydrite, and amorphous Si and Al in the clay and also to the formation of aggregates due to the presence of ferrihydrite. A linear correlation was found between the liquid limit and specific surface area for the combined data of Mexico City clay, bentonite, and marine
clay.
To examine the effects of pore water salinity on liquid limit and the swelling characters of Mexico City clay, liquid limit tests were conducted on Mexico City clay
samples prepared at different poie water salinities. The results demonstrate that the liquid limit decreases almost linearly from 340% to 275% as salinity decreases from 54
to 3 g/L, and then exhibits a sharp decrease for salinities below 3 g/L. X-ray analyses indicated that smectite is the main clay mineral in the clay, and a sediment volume test
on clay fractions confirmed that the smectite is of a high-swelling type. The contents of Al, Si, and ferrihydrite extracted with oxalate were 26.9, 37.8, and 418 g/kg, respectively.
Despite the fact that the smectite in the Mexico clay is of a high swelling type, the change in the liquid limit due to pore water salinity was opposite to that for bentonite
and paddy soil composed of high swelling smectite but was similar to that for marine clays composed of low-swelling clay minerals. This characteristic was ascribed to a
dominant role of poorly crystalline aluminosilicates and ferrihydrite with low-swelling character over the high-swelling smectite in determining the liquid limit of the Mexico clay as the pore water salinity is changed.
Consolidation behavior of Mexico City clay is characterized to be a very compressive, due to high void ratio and chemically contained great amount of noncrystalline minerals
such as Aluminium, Silica, and iron oxide (ferrihydrate) which contributed to the higher water content and liquid limit, but having very low permeability. The pore water
salinity may contribute to the low permeability. The secondary consolidation rate is more than four times greater compared to Ariake marine clay causing continuing to
consolidate after a long period of time.

Shear strength of Mexico City clay is and sensitivity of the clay was examined by using Direct shear and Van Shear test. It is concluded that the peak of shear strengths
is in the range of 7.1 to 7.5 mm horizontal displacements from 10.3 to 22.4 kPa. In addition, the strengths are normally around 35 kPa except in sandy layers which is
reaching the highest strengths more than 150 kPa. The friction angle is in the range of 30 to 53. Shear strength of Mexico City clay is investigated in the Environmental Soil Engineering Laboratory by using Direct shear test apparatus. Undrained shear strength was performed for the undisturbed soil and Van Shear strength tests were
done for remolded samples to determine the sensitivity of the clay.
The results show that Mexico City clay is characterized as having medium strength in comparable with Ariake clay. The friction angle; however; is relatively high. In some
sample is in comparable with sand. Sensitivity of Mexico City clay is quite high. In the depths of 13 and 17 m were found to be quick clay based on the Norwegian scale, which
could generate the damage of infrastructure when the earthquake is happened.