Soil & Rock Physical Parameters and Levee Design

I have already raised the question of whether the Corps of Engineers did soil borings (cores sampling) to determing the elastic parameters of the soils below the levees. The independent study by the Engineering committee headed by Dr. Seed criticizes the analysis and decision making based upon those samples. In the previous blog entry I have illustrated the hydrostatic effects of the storm surge on the strenght of the soils. Now I will raise the issue of anisotropy. Soils and rocks are not homogeneous bodies. Their heterogeneity is not usually random but rather consists of thin quasi homogeneous layers of differing properties. These layers can be characterized by the terms, Coarse, Medium, and Fine Sand, Silt, Clay, Calcareous (shell), and Organic (peat). Each of these layers has a different set of elastic parameters (strength). The layers are often very thin (less than a mm). The effect of this layering is to introduce anisotropy into the elastic parameters. Anisotropy causes the strength of the soils or rocks to vary with direction of measurement. The usual directions of interest are parallel and perpendicular to the bedding planes. Differences on the order of 20% between the parallel and perpendicular velocities have been reported in the Geophysical Literature. The orientation of the elastic parameters is such that the weakest direction is parallel to the bedding and thus in the horizontal direction. I propose that the Corps of Engineers familiarize themselve with the effects of anisotropy as well as overpressure and make sure that their lab measurements on soil borings measure all of the effects and further, that their levee models include these effects. I believe that the Katrina failure of some of the canals was predictable if correct and complete modeling based on detailed soil boring analysis was done during design.

EXAMPLE FROM THE LITERATURE

The Corps of Engineers Soil Strength Analysis taken from the Independent Analysis headed by Dr. Seed is below. The analysis and decisions made based on the analysis have been criticized in the report. Errors of under estimation of the weakest soil strength of the order of 100% are visible on the graph. The decisions made from this analysis undoubtably contributed to the disaster. All Engineers know that a chain is only as strong as its weakest link. Their graph is below.



I have contributed some addition considerations. My analysis of the hydrostatic effects of the surge imply a 20-30% weakening of these measurements at the depth of failure . Anisotropy considerations add a 5-25% weakening of the shear parameter in the horizontal direction parallel to the bedding planes of the soil. Most core measurements are made perpendicular to the bedding planes unless additional core preparation is done to get a sample that can be measured parallel to the bedding planes. My conclusion is that the U.S. Corps of Engineers modeled the levy response to a storm surge using soil strengths a least 150% higher than reality and that this was a major factor in the levee failure.