ABSTRACT
Increasing urbanisation, finite land resources and the associated demand for fresh water can combine to produce large regional subsidence troughs. UNESCO Working Group on Land Subsidence has been raising awareness of the issues for many years, however the problem of robust design of foundations under these conditions remains. This article focuses on the particular case of the Mexico City Basin. The presence of deep deposits of highly compressible clays in the Mexico City basin, coupled with regional subsidence that can exceed 350 mm/year due to deep water extraction, presents a unique challenge to foundation design. For structures founded on deep foundations a key concern is the emergence as a result of the ongoing consolidation that varies with depth. The consideration of aspects like down-drag and formation of the gap under the pile cap are also found to be critical for deep foundation design in this area of high seismicity.
This paper presents details of the Finite Element (FE) analysis carried out to assess the short-term and long-term behaviour of the piled foundation solution. The FE analysis was carried out using Hardening Soil model with Small Strain Stiffness (HSSmall) available in commercial FE software. The parameters of the HSSmall model were obtained by carrying out detailed calibrations using the extensive ground investigation data available for the New Mexico City International Airport (Nuevo Aeropuerto Internacional de la Ciudad de México or NAICM) site. The analysis considers modelling of the site history (changes to the piezometric profile in particular) to achieve the current state of stress and future long term conditions based on the continuance of groundwater extraction at current rates. Analytical procedures are described so that the assessment of pile behaviour for a wide range of consolidating soils and water extraction rates can be made.
