5th April 2017.
By Nick O’Riordan, Arup Fellow
The site of NAICM covers a hitherto undeveloped 10 km square area immediately to the east of Mexico City, a conurbation with a population in excess of 20M. The airport is located on the former Lake Texcoco, to the east of the Aztec dam that separated the ‘freshwater’ lake, upon which Tenochtitlan was founded and where much of the city later developed, from the ‘salt water’ lake deposits.
Lake Texcoco deposits comprise up to 500m of soft clays that become progressively stiffer below about 30m depth, and are interbedded with silts and sands of variable mineralogy, diatom content, density, stiffness and permeability. Groundwater is extracted for the public water supply from wells below 120m depth.
Since 1930 there has been regional surface subsidence of about 8 to 10m, with current settlement rates being about 20 cm/year in the vicinity of the site. The softest soils have a void ratio of about 6, a water content around 250%, a plasticity index of abou 200% and an effective stress friction angle of over 40° in compression. Engineering properties of the soil in this area of continuing consolidation and subsidence present something of a moving target.
The presentation describes the complex depositional and drainage history of the site, ground investigations, the use of compensated foundation design principles for the 1.6km x 0.5km passenger terminal building, and the shallow piled mat foundation for the 89m high air traffic control tower.
Newly developed soil properties together with back-analyses of some case histories of instrumented excavations bridges and structures enable predictions of foundation behaviour during large magnitude earthquakes to be made. The choice of operational strengths for excavation and pile design in these materials is somewhat challenging and some results from a full scale testing program will be presented.
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