Modeling the deformation of faulted volcanosedimentary
sequences associated to groundwater
withdrawal in the Querétaro Valley, Mexico
G.H. Ochoa-González
Western Institute of Technology and Higher Education (ITESO), Jalisco, Mexico
P. Teatini, G. Gambolati
Dept. of Civil, Environmental and Architectural Engineering, University of Padova, Padova, Italy
D. Carreón-Freyre
Centro de Geociencias, National Autonomous University of Mexico, Queretaro, Mexico
ABSTRACT
The City of Querétaro is located on a graben structure that formed a continental basin filled
since the Oligocene time with volcanic and sedimentary materials. In the N-W direction two major normal
faults are dipping to the West and the thicknesses of the filling materials vary many tens of meters in close
distances. The filling materials include lacustrine and alluvial sediments, pyroclastic deposits, and
interbedded fractured basalts. Hence, important differences of hydraulic and mechanical properties
characterize the various units.
Groundwater was been strongly withdrawn over the last three decades in the study area, with a level decline
exceeding than 100 m in some areas. Because of the high variability of the geological deposits, a space
variable decrease of the piezometric levels, and consequently of the effective stress increase, has been
observed. Piezometric variations are also due to faults that strongly impact on groundwater flow dynamics.
The variable distribution of the effective stress increase has caused large differential subsidence causing
ground fracturing that has damaged the urban infrastructures of the City of Querétaro.
The geological heterogeneities of the subsoil were integrated into a flow and geomechanical model to predict
the deformation caused by fluid withdrawal. Initially the hydrodynamics of the pumped aquifer system was
simulated by a 3-D groundwater flow model and then the subsidence was computed with the aid of a 3-D
poro-mechanical model with the pore pressure field specified as an external distributed source of strength
within the porous medium. The model is calibrated using observed groundwater and land settlement records,
with the generated three-dimensional stress field that is compared with the distribution of the major fractures
detected in the city.
The aim of this work is to predict the differential deformation of the faulted volcano-sedimentary sequences.
The simulation is carried out by an advanced three-dimensional finite-element flowdynamic-geomechanical
code. The conceptual model was accurately defined using the correlation of geological logs of extraction
wells, field mapping of faults, fractures, and the integration of major structures reported in previous
geophysical works. The stratigraphic sequences were simplified with seven mayor hydrogeological units with
specific mechanical properties, hydraulic conductivity, and storage capacity. The 30 years records of
piezometric level were used to simulate groundwater depletion and the resulting land subsidence.
The results of the geomechanical simulations show that the areas where large differential subsidence
developed correspond to the portions of the city where earth fissuring have been observed. The spatial
relationship between major withdrawals and the largest simulated subsidence is assessed.
