MEXICO CITY ’ S SINKING CATHEDRAL

1. MEXICO CITY’S SINKING CATHEDRAL By Aaron Alarcon Jinghong Li

2. The cathedral has been sinking since the day it was built almost 500 years ago.

3. How much the cathedral has sunk? This pendulum is inside the Cathedral, which should be centred in the middle of the square. It shows us how much the cathedral has sunk. This cathedral was actually worse a few decades ago. Why is Mexico city’s cathedral sinking?

4. Why is Mexico city sinking? • The soft clay subsoil beneath Mexico City • The removal of water from the soil over the past years Irregular and gradual sinking of many buildings, such as cathedral.

5. Geology of Mexico city • Mexico is more than 2000km wide, but narrows to only 210km in the south • The large Basin where Mexico city is located in the central part of the Trans-Mexican Volcanic Belt • The basin, at an altitude of 2,200m above sea level, is the highest valley in the region, and is surrounded by mountains.

6. Mexico City Population Water demand increases as population increase. So much water has been pumped out from the aquifer beneath it to satisfy the metropolitan area's 18 0,000,000 residents that the ground is collapsing underfoot at a stunningrate.

7. Reasons why the Cathedral is sinking • Construction of the Metropolitan Cathedral started in the second third of the 16th century. • The soil where the cathedral was constructed is a land over ancient civilizations constructions. • The layers of human occupation on the natural crust reach down more than 15 meters, and there are pre Hispanic structures more than 11 meters deep. This emphasizes the irregularities in the soil. • The sinking of the ground in the area of the town reaches up to 2.4 meters relative to the point of the apse, while the area of the chambers have sunk up to 1.5 to 1.6 meters from the horizontal.

8. Reasons why the Cathedral is sinking • Other factors that affected the cathedral are the construction of the Mexico City subway, its current operation, the excavations on the Templo Mayor and the effect of a semi-deep drainage ditch that was dug in front of the cathedral. • Also the city has needed more and more underground water and that has made the phreatic horizontal to get down, what accelerated the hardening of the clay. • Many efforts has been made in order to stop the sinking rate. Nevertheless, the sink rate between the northern tower and the apse is more than 2 cm / year. These are the strongest and weakest points respectively.

9. Cathedral Cathedral Water pump from auqifer Water pump from auqifer Foundation Foundation Water table Water table How to Save the Sinking Cathedral

10. Solution Proposed • In 1992 the studies revealed that the cathedral was sinking and the reversal process could not be postponed anymore so they started looking for a way to solve the problem. • Some alternatives they found were to put 1500 piles under the cathedral in order to stop the movement of it or place piles 60 meters below and to reload the aquifer. • The final choice was to make 30 holes about 2.6 meters in diameter under the cathedral. • Those holes were reach down further than all the remains of other constructions, down to the clay that is below the natural crust. • That holes were filled with concrete and were placed 15cm diameter tubes every 6 degrees of the circumference.

11. Water pump from auqifer Cathedral Cathedral Water pump from auqifer Foundation Piles Piles Water table Water table

12. Solution Proposed • The way they make the holes in the soil is by using special machines. Which make the cathedral sink in that area, that why they reinforced it. • The objectives of this are to restore the geometry of 40% of the sunk areas of the cathedral. • The movement of the hole cathedral was measured monthly and there was several studies made after any earthquake registered in more than 3.5 Richter. • The cathedral at the end of the process wont be straight it never was but its verticality will be improved and it will be able to resist the strongest earthquakes.