Sandpile behaviour in Mexico Clay soils

Abstract

The construction of the new airport of Mexico City (NAICM) is challenging, as the subsoil at the site is very weak. The airport, which is under construction at the moment, is located on top of thick layers of Mexico Clay soils. These clayey layers consist of a mixture of both clay particles and pyroclastic volcanic materials. These unique soils are characterised by a high water content and compressibility. Upon loading, the soft soils will settle as a result of the consolidation process. This consolidation is defined as the dissipation of excess pore water as a result of an increase in stress. The rate of consolidation is governed by the hydraulic conductivity of the soil, which is in general very low in soft soils, and the drainage path. In order to speed up this time consuming process, vertical permeable elements can be installed in the soil. Upon installation of these elements, a reduction in drainage path is realised. This reduction in drainage path allows faster dissipation of excess pore water pressure, hence reducing the consolidation time. Due to the conditions at the site, the design of the runways is challenging. The differential- and residual settlements need to be within strict boundaries in order to keep the runways operational. Prior to the design of the runways, trial embankments have been constructed in order to measure the performance of different soil improvement techniques at this specific site. Based upon this field test results, prefabricated vertical drains (PVDs) were selected as the most efficient soil improvement technique. Besides the PVD trial, the sandpile trial also showed promising results. The PVDs and sandpiles both decrease the drainage path length inside the soft soil layers. The main difference is the relative high stiffness of the sandpiles, the stiffness of the PVDs is negligible. By installing stiff elements in the soft soil, the loads applied on the top of the soils are transferred to deeper and stiffer soils. Hence, the total settlements of the top soft soil layers are reduced. The sandpiles could therefore be feasible alternative to PVDs, applied at locations in which the total settlement needs to be limited.
The investigation into the behaviour of sandpiles in Mexico Clay soils is performed in multiple stages. First, analytical models are used for gaining knowledge about the expected sandpile behaviour. In the second stage the FEM software Plaxis 2D is used for the numerical modelling of the sandpiles, using an axisymmetric model setup. The model is constructed in multiple steps, with increasing complexity. The first models simulate the material behaviour with the Mohr-Coulomb model, in the advanced models the Soft Soil Creep and Hardening Soil models are used. The final numerical model is verified by the measurement data obtained from the field trials.
After fitting the numerical model to the field data, the verified model is used in the sandpile sensitivity analysis. In this analysis the sensitivity of the material properties and geometry of the sandpile on the performance are researched. The residual settlements or performance, which is defined as the difference in settlement after construction and over a period of 8 years, is affected by the length, radius and centre to centre (ctc) distance of the sandpile. Adjusting the pile stiffness has minimal effect on the performance of the sandpile. An optimum in performance is found by varying both the pile radius and ctc distance. A combination of a small pile diameter with a small ctc distance results in the best performance. When comparing the performance of both sandpile and PVD, the PVD is found to be more effective in terms of performance. The additional stiffness of the sandpile is not reducing the total settlements, on the contrary, the self-weight of the piles increases the total settlements by providing an additional load to the soft soil layers located underneath the pile tip. In conclusion the sandpiles have no additional benefit over the use of PVDs, therefore the application of sandpiles at the NAICM site is not a feasible alternative to the use of PVDs.