Analysis of bottom failures in Deep Excavations.

The greatest concern during the foundation works, are the failures that may occur at the walls of excavations. Nonetheless, there's is another concern we should pay attention to and that happens at the bottom of the excavation. This is a real danger on soils composed by plastic saturated clay.

Autodesk Robot Structural Analysis Professional is a powerful  tool to plan and analyze this type of excavation failures.

This failure happens during the extraction of the soil, or material, from an excavation, causing the soil to rise or bulge. This failure in the terrain is present on soils composed of soft saturated clay and even when the sides of the excavation are already contained by walls or screens.

Typically, these failures can be recognized as a bulge, or rise, at the center on the excavation and a dejection at the level of the perimeter. This may cause the stabilization system of the slope to collapse.This type of failures may present variations depending of the geothecnical profile, abandoned foundations or structures, waterworks systems or dejections on non-uniform phreatic levels.

Autodesk RSA can create an analysis simulation to prevent these failures. The objective is to plan our work project as a step by step sequence and analyze the loads of the terrain on each construction step. This type of analysis will allow us to locate the critical moments, in which we need to increase the precautions and consider solutions.

Lets see a case...

The bottom failure, caused by shear forces, is presented in a soft and saturated clay. It manifest as an abrupt bulging or rise at the bottom caused by the extraction of material, simultaneously with the sudden sinking of the surrounding terrain near the excavation area which may cause the collapse of the walls of the lateral support, if any.

During deep excavations, it is common to use slurry walls (Milan Walls) made from reinforced concrete (which is a popular technique). This wall is shoring for metallic beams or big steel pipes that are driven into the ground as the excavation gains depth.

The objective of this practice is to increase the security factor to the possibility of a failure. In the case of  slurry walls, it is important to execute a careful shoring to maintain integrity of the walls against the thrust of the soil.

A typical case may present during the excavation process, when the pressure created by a terrain of saturated clay behind the wall presses its weight against the lower part of the wall, even the soil of the excavation, creating the sliding. These slidings are critical when they happen during the shoring steps with the vibration of the heavy machinery operating at the margin of the excavation.

In the illustration, we can see the bottom failure and the subsequent sliding in a Slurry Wall.
Autodesk RSA allows the analysis of the analytical models, for the construction process stage. In this way, we can notice the critical stages of the deep excavation, the shoring process and the permanent ground containment system.

Autodesk Robot Structural permits us design the technical solutions for preventing catastrophes in deep excavations.