Effect of structural stiffness on the response of portal frames to groundwater extraction-induced ground movements

Abstract

Civil structures are founded on or within soil. Understanding the interaction of a structure with the soil it is founded on or within is vital for designing and constructing structures that will be safe and functional for the required design life. Structures are, furthermore, designed to meet acceptable limiting requirements to achieve the standards of safety and functionality. If, however, the interaction between a structure and founding soil is altered by an external stimulus such as ground movements it can have detrimental effects on the structure that can jeopardise the safety and functionality of the structure. Extraction of groundwater from aquifers causes a reduction in the interstitial pore-water pressure of the aquifer. The reduction in pore-water pressure, in turn, results in an increase in the effective stresses acting on the soil skeleton inducing settlement. Pore-water pressure reduction induced by groundwater extraction, however, is not uniform throughout the aquifer which results in a non-uniform increase in effective stresses resulting in differential settlement of the soil. The magnitude of settlement is greatest closest to the point of extraction, decreasing with increased distance away from the extraction point. Groundwater extraction also increases the seepage forces acting on the soil skeleton resulting in horizontal ground movements. Structures founded within the zone of influence will be affected by the vertical and horizontal ground movements induced by the groundwater extraction. The interaction relationship between the structure and the founding soil will, therefore, be altered resulting in a change in the manner in which the structure resists and transmits loading as well as additional loading being imposed onto the structure. The structure will respond to the altered interaction by deforming. The deformation of the structure, however, needs to remain within acceptable limits. The stiffness resulting from the material and geometrical properties of the structure will have an effect on the response of the structure and the resultant distortion. The response of structures to groundwater extraction-induced ground movements was investigated by means of geotechnical centrifuge models. Four single storey two-bay portal frames instrumented with strain gauges and linear variable differential transducers were used to represent a structure with varying stiffness, rigid and flexible slabs and columns founded within the zone of influence. The groundwater extraction was modelled with a simulated aquifer constructed in a centrifuge strongbox where the surface settlement was measured with linear variable differential transducers and the ground movements tracked by means of digital image correlation. The pore-water pressure was measured with piezometers while the extraction well water level was monitored with a pressure meter. The study found that the resultant ground movements can be successfully modelled in a geotechnical centrifuge at elevated gravitational fields. The modelling results corroborate the response of the aquifer response previously found through numerical and analytical modelling as well as monitoring and inspection of real aquifers during groundwater extraction. The main conclusions show that as the pore-water pressure reduced non-uniformly, the soil settlement increased non-uniformly throughout the aquifer while the increased seepage forces also induced a component of horizontal ground movement. The conclusions deduced from the response of the portal frames, show that rigid frames with a high global stiffness, rigid slabs and columns, deform by rigid body rotation to the imposed differential settlement. Flexible frames depict greater localised deformation concentrated to bays where deformation is imposed. Frames where the vertical stiffness is greater than the horizontal, rigid slabs and flexible columns, show a greater bending type deformation whereas frames with a greater horizontal stiffness, flexible slab and rigid columns, undergo a vertical shear type deformation in response to the imposed differential settlement.