Numerical Modeling of Excavations in the Soft Soils of Bogota

Abstract

Excavations in soft soils supported by diaphragm walls have been a source of study since the beginning of modern geotechnical engineering. Saturated clay deposits are common throughout the world. During the last century, analytical methodologies with closed formulas were developed to evaluate the stability of excavations in soft soils using cantilevered and/or anchored diaphragm walls. These equations consider apparent thrusts on the retaining structure and generally operate on total stresses. The envelopes of these apparent thrusts in the past were found by field or laboratory measurements. But nowadays it is possible to perform studies using sophisticated numerical tools such as the Finite Element Method, which allows addressing the excavation problem by taking into account the non-synchronous mobilization of the shear resistance and a spatial distribution of the excess pore pressure during consolidation. This study presents a comparison of the analysis of an excavation in the soft soils of the city of Bogota, using classical methods and finite elements, considering recently published average geotechnical parameters. The finite element analysis was performed considering both total and effective stresses, as well as the effect of consolidation with an already lowered water table. The results show that, for the case analyzed, where a concrete diaphragm wall is implemented, the bending moments found by means of the classical methodologies are much higher (up to 10 times approximately) than those found by means of finite elements, the difference in these models can be noticed due to the different forms of analysis on which each of them is based, as mentioned above. Although both methodologies are currently used, the use of finite elements is gaining momentum as it generates more accurate results, thanks to its detailed analysis capability.