Preface

Land subsidence, that is, the loss of land elevation, is a major geomechanical process that threatens viability and sustainable development of many millions of people throughout the world, especially in, but not restricted to, coastal and highly urbanized areas. The most severe cases of land subsidence are associated to groundwater overdraft. Unfortunately, the unsustainable use of land and water resources is expected to increase in the next few decades, mainly in developing countries of Asia and Africa. The effects of climate change in terms of sea level rise, variation in the distribution and timing of precipitation, runoff and aquifer recharge will be compounded by an increasing concentration of population in (mega-) cities and elsewhere along the coasts of the world. The expanding need for freshwater resources in more and more concentrated, at-risk, areas will inevitably lead to an increase of sites affected by land subsidence, with a growing number of people involved.

Almost one century has passed since scientists started to investigate land subsidence of anthropogenic origin. Impressive progress has been made in terms of: a) recognizing the basic hydrologic and geomechanical principles; b) measuring aquifer compaction and ground displacements, in both the vertical and horizontal directions; c) modeling historic and simulating predicted future events; and, d) mitigating environmental impact through aquifer recharge and/or surface water injection.

This book addresses anthropogenic land subsidence, the most widespread and studied geomechanical response to groundwater pumping. A historical review is followed by a description of the major areas that have experienced considerable land subsidence illustrating a selection of the major environmental impacts. The main factors controlling the process and the basic principles and equations underlying it are discussed, with reference to the most relevant soil stresses and properties. Pumping from water table, confined and complex aquifer systems is outlined. Compaction of low permeability formations (aquitards) along with time-factor and compaction profiles are analyzed. The most advanced tools for recording and monitoring in situ deformation and surface displacements are mentioned. The occurrence opposite to land settlement, that is, the upheaval of land surface induced by aquifer recharge or water injection into the subsurface, is also reviewed. A few processes are mentioned which are still poorly understood, such as the influence of differential vertical compaction, horizontal displacements, and discontinuity in the bedrock on near-surface ground ruptures, fissure generation and fault reactivation including induced seismicity. Finally, the discussion focuses on the connection between research into groundwater geomechanics and the present challenges to be met in undertaking effective remedial measures aimed at mitigating the associated environmental and socio-economic impact.