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Ground water replenishment by fractured-porous subsurface structures for reducing land subsidence

Project head
Prof. Dr.-Ing. R. Hinkelmann
Scientific assistant
I. Martinez Noguez Abraham M.Sc.
Project period
01.10.08 - 01.10.11
Funding
Scholarship from CONACYT-DAAD

In the last decades a rapid increase in human population especially in urban areas has occurred. As a consequence, excessive groundwater withdrawal has been produced. In arid and semiarid zones this discharge is bigger than the recharge resulting in the decline of the water table. The soil’s buoyancy is reduced and a strong increase of the grain-to-grain stresses is produced. This increase of these stresses produces a rearrangement of the soil’s particles. In most cases, this compacting of deposits is not uniform. Because of the heterogeneous strata, associated with an uneven course of the underlying bedrock surface, tensional stresses are produced in the soil resulting in fault zone formations in the soil on the surface of the earth. These fault zones can significantly affect water and solute movements in soils because the flow and transport is accelerated.

Since the water table in arid and semi arid zones is found several meters under the surface, the infiltration is occurring in partially saturated soil, i.e. two phases are present (water and air). Hence a model concept for two-phase flow in porous media has been chosen. The results of the infiltration, i.e. saturations, are used for the stress-strain analysis. As soil behaviour is highly non-linear and irreversible, a linear-elastic perfectly-plastic model (Mohr-Coulomb model) has been applied to determine the infiltration induced deformations.

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Motivation

Land subsidence is a complex phenomenon which occurs all around the world. The study and understanding of the phenomenon, its causes and effects, the processes, such as physical processes like flow, deformation and fracturing are very important in a spectrum of approaches for predicting the consequences and future damages. Numerical simulation is an important and powerful tool to analyse land subsidence and predict new hazards. It also serves to understand new processes if the conditions change.

Coupling

In this research the coupling between infiltration and deformation is weak. First, the infiltration is modelled with MUFTE-UG. The obtained saturation results in the soil were applied for the modeling of deformation with PLAXIS. The saturation results are transferred to modeling specific weights of the soils which are the loads for the deformation modeling, i.e. the specific weight for soil in saturated condition γwet was used for the soil where the water saturation was more than 0.8. This weak coupling approach does not take into account feedback effects of deformation on the flow. Therefore, it allows an estimation –not an exact fully coupled- of the infiltration impact on the deformation.

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Fast Water Infiltration into a Single-layer System

The aim of this work is to better understand rainfall-induced fast infiltration of water through fractures into the subsurface as well as to estimate its influence on mechanical deformation, i.e. land subsidence
The results of the numerical study show that infiltration into a system with a horizontal surface and without fracture only leads to vertical deformations. Infiltration into a system with a vertical fracture has nearly no influence on the deformation because the water mainly propagates in the vertical direction due to gravity. Infiltration into a system with a horizontal surface and an inclined fracture result in considerable horizontal and vertical deformations. Such deformations are further increased when the surface is inclined.

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Fast Water Infiltration into a Two-layer System

The results in this research show that a fast infiltration in a partially saturated aquifer lead to land subsidence, extension of pre-existing fault zones and generation of new cracks.

Infiltration through a fault into a two-layered system, consisting of a clay layer on the top and a sand layer, as well as the soil deformation on the surface were numerically investigated. Faults serve as highly conductive waterways through layers with low permeability into permeable layers inducing hydraulic pressure and a rapid increase of the soil’s weight as a result of the changing soil saturations. Also the results of the water infiltration modeling show that the clay layer is like a barrier. The reason is that the water is strongly sucked in the clay layer because of the strong capillarity effect. The aim of this work is to better understand the dominant hydrogeological parameters in the evolution of the subsidence on the surface.

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Numerical Study of the Influence of Structural Discontinuities on the Groundwater Flow: Conceptual Model and Modeling of Natural System

In natural systems, usually faults and fractures can perform as a hydrological barrier in an aquifer and as a result different behaviours on both sides of the discontinuities in the same aquifer are present. This difference could be of the magnitude of several meters, for as an example in the aquifer in Querétaro, Mexico, it presents a different of more than 10 meters. It is easy to understand if the fault has a low permeability, but in this research it is shown that also a fracture zone with higher permeability can perform as hydrological barrier and this behaviour can also be a trigger factor for land subsidence.

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List of Publications

Journals:

Martinez I, Hinkelmann R, Savidis S (2013) Fast water infiltration: a mechanism for fracture formation during land subsidence. Hydrogeology Journal, Volume 21, Issue 4, pp. 761-771. DOI 10.1007/s10040-013-0971-6. Springer-Verlag Berlin Heidelberg 2013

Martinez I, Hinkelmann R, Savidis S (2010) Modelling land subsidence processes induced by fast rainwater infiltration through fractures into unsaturated zone. In: Land subsidence, associated hazards and the role of natural resources development. IAHS Publ 229, IAHS, Wallingford, UK

Pham-Van S. Hinkelmann R, Martinez I, Nehring M (2011) A comparison of model concepts and experiments for seepage processes through a dike with a fault zone. Engineering applications of computational fluid mechanics Vol. 5, No. 1, pp. 1-10

 

Conferences:

Martinez I, Hinkelmann R, Savidis S (2012) Numerical simulation of infiltration and land subsidence in a two-layered system with fault zone. In proceeding: 10th International Conference on Hydroinformatics (HIC 2012). Hamburg, Germany

Martinez I, Hinkelmann R, Savidis S (2010) Modelling land subsidence processes induced by fast rainwater infiltration through fractures into unsaturated zone. In: Eight International Symposium on Land Subsidence (EISOLS). Querétaro, Mexico.

Schankat M, Hinkelmann R, Schlueter M, Martinez I (2011) Numerical modelling of the tidal influence on submarine groundwater discharge rates and the composition of pore waters. In proceeding: 10th International Conference on Hydroinformatics (HIC 2012). Hamburg, Germany

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