Understanding Ground Subsidence Caused by Dewatering: A Theoretical Approach
Geotechnical engineers have long known that ground subsidence—or the sinking or settling of the ground surface—is a real possibility when removing groundwater from an underground layer of soil (i.e., an aquifer) at a construction site to provide workable conditions for excavation. This dewatering is often accomplished by pumping the water from one or more wells installed for that purpose.
Nearly a century ago, Karl von Terzaghi presented his one-dimensional consolidation theory that explained the relationship between pore water dissipation and subsidence. Pore water pressure is the pressure of groundwater held within soil in the gaps between particles (pores). Terzaghi’s theory states that the expulsion of pore water (i.e., dewatering) reduces pore water pressure, and the reduced pore water pressure is then transferred into effective stress in the soil structure. Effective stress is a weight and pressure that keeps a collection of soil particles rigid. The increased effective stress from dewatering compresses all the particles making up the soil structure and causes subsidence throughout the soil column.
Bottom line, dewatering (or removing water from an aquifer) may cause subsurface soil compaction and, consequently, sinking or settling of the ground above. This subsidence may damage buildings, roads, utilities, and other infrastructure and can increase flood risk in floodplain areas.
Conversely, restoring water into an aquifer through aquifer recovery techniques increases the water pressure underground, which may lead to aquifer-soil system expansion and consequent land heaving. So controlling aquifer recovery can help us manage land heaving and the damage it causes to infrastructure.
Recently, EBA investigated the effects of these hydraulic forces and developed a model to predict ground movement resulting from aquifer pumping and recovery. The model simulates how changing hydraulic forces compact the soil skeleton when pumping and expand the soil skeleton during recovery.
EBA performed pumping and recovery tests in an Atlantic Coastal Plain physiographic province aquifer. The pumping test program included one pumping well and three observation wells. Following an assigned testing schedule, the team pumped the test well at variable or steady rates and stopped intermittently to allow recovery. Based on the data collected from the test and observation wells, we determined the aquifer transmissivity, rate of flow, and storage coefficient.
As mentioned, EBA’s model predicts ground movements in response to aquifer pumping and recovery. Using the model predictions and in-place instrumentation readings, EBA can evaluate cyclic ground movement resulting from aquifer pumping and recovery. The model may provide future guidelines to monitor the response to pumping and recovery and minimize damage to nearby buildings and infrastructure during dewatering.
Nick Roles and Duowen Ding have outlined the basis of the model in a paper titled “Granular Cyclic Deformation in Response to Pumping and Recovery of an Aquifer.” The abstract is presented below. If you would like to explore the model further, the results will be published in the proceedings of the 2017 World Environmental and Water Resources (EWRI) Congress, which began May 21 in Sacramento, California.
Title: Granular Cyclic Deformation in Response to Pumping and Recovery of an Aquifer
Abstract: The pumping of an aquifer may cause ground surface subsidence and earth fissures. Earth fissures are the result of the radial motion of the solid matrix of an aquifer, driven by hydraulic forces due to well pumping. Ground subsidence may be caused by vertical settlement of an aquifer’s solid matrix. The vertical settlement is the result of soil consolidation and compaction due to effective stress changes. The earth fissures are explained by the radial motion due to well pumping. The differential settlement is explained by the compaction and motion of the aquifer’s solid matrix. The bulk mass flow, conservation of water and solid mass, the Darcy-Gersevanov law, and the drawdown of transient flow are employed to analyze the radial motion of an aquifer. This paper only investigates the granular cyclic deformation in response to pumping and recovery of an aquifer.