ANALYSIS AND NUMERICAL MODELING OF DATA FROM PIEZOMETERS AND EXTENSOMETERS REVEAL THE MAIN DRIVERS OF LAND SUBSIDENCE IN A SITE NEAR MEXICO CITY
Water supply to Mexico City relies in about 60% on groundwater from the regional aquifer overlain by highly compressible lacustrine sediments. The intensive pumping, estimated to be almost twice of the magnitude of the estimated natural recharge, has originated land subsidence, threatening water supply and damaging urban infrastructure. A research site instrumented with piezometers, benchmarks and extensometers, was monitored for 10 years to study the influence of highway Circuito Exterior Mexiquense (completed the same year as the instrumentation). Data are analyzed to understand the main drivers and the process of land subsidence. The aquitard at the research site is estimated to reach 100 m in thickness and the typical stratigraphy of the aquitard was encountered: upper clay formation UCF, hard layer HL, lower clay formation LCF, deep deposits DD, deep clay formation DCF and deep stratified formation DSF. Total settlement amounted to 3.661 m, at an average rate of 0.314 m/year. We found that although pore pressure has not changed from 0 to 36 m, there was a deformation of 1.15 m. We infer that the driver of this deformation is the increase in total stress due to the weight of the highway.
Our analysis also shows that the main driver for land subsidence is groundwater pumping from the regional aquifer, as most of the deformation due to consolidation (2.128 m) was registered below a depth of 82 m. Since the thickness of the aquitard at the research site is 100 m, a preliminary numerical simulation shows that most of this deformation most probably takes place at interbedded compressible lenses within the regional aquifer.
Our results highlight the two main drivers of land subsidence at the research site: increase in total stress due to the construction of new infrastructure and, as the main and leading driver, drawdown due to intensive groundwater pumping from the regional aquifer. To our knowledge, this is the first work to offer evidence that deformation is not only restricted to the aquitard but occurs also within the main aquifer.