The blog of Alvaro Boiero

"Live as if you were going to die tomorrow. Learn as if you were going to live forever" Mahatma Gandhi

The blog of Alvaro Boiero

geo

"Live as if you were going to die tomorrow. Learn as if you were going to live forever" Mahatma Gandhi

Geotechnical soil characterization

Civil Engineering and Environmental Engineering include the conception, analysis, design, construction, operation and maintenance of a great diversity of structures, facilities and systems, which are built on, in, or with soil and/or rock (Mitchell & Soga, 2005). Hence, the geotechnical characterization of a given land is a fundamental step for any civil and/or environmental project.

Soils and rocks

For Civil Engineers, the materials that make up the earth's crust are arbitrarily classified into two categories: soil and rock. But, how are they defined? What are the main differences between them?

 

The first thing to note is that any soil is the product of the physical and chemical disintegration of a rock, regardless of the subsequent processes to which this product is subjected (such as transport in the case of sedimentary soils, or in situ weathering in the case of residual soils).

 

Considering the above, from the point of view of Geotechnical Engineering, we can say that soil is an accumulation of sediments and other unconsolidated solid items, produced by the mechanical and chemical disintegration of the rocks mentioned above, regardless of whether or not they contain a mixture of organic components (Terzaghi, 1943). Rock, on the other hand, is a material with strong internal cohesion and molecular forces that hold together the mineral grains that compose it (Holtz et al, 2011).

Figure 1 Soils (left) and rocks (right).

For the design of civil facilities, it is essential to understand the behavior of soil, since it will undergo greater deformations than rocks under external loads. Likewise, in the case of structures supported on rock massifs, they may suffer deformations depending on the presence or not of discontinuities in the massif. That is why geotechnical characterization is essential for any project.

The geotechnical characterization process

Now that we know the nature of soils and rocks, we can talk about the geotechnical characterization process.

 

Geotechnical characterization is, above all, a complex process. Why is it complex? Because it is the result of a series of activities developed in the field, laboratory and office, following rigorous procedures and standards for its execution, which involves making technical decisions based on the variable conditions of the ground under study.

 

This characterization process begins at the subsurface exploration stage, continues with field and laboratory tests performed in order to estimate certain soil properties, and culminates with the interpretation of the available data and the delimitation of sectors of the studied land, considering the strength characteristics of the detected strata, and the expected response of these strata to external loads.

 

Thus, one of the fundamental features of geotechnical characterization includes the zonation of the studied terrain, in order to identify geologically "homogeneous" units, which may cover different geological ages. Figure 2 shows schematically the geotechnical characterization process.

Figure 2 Geotechnical characterization process.

It is important to note that soil strata are never truly homogeneous. Soil properties at a site may show large local variations, but there may be no general trends in the variations, and the average properties may be essentially the same in all parts of the site (Taylor, 1948). This fact is very important, since the analysis of any geotechnical problem requires the adoption of a soil behavior model, which is based on geotechnical parameters that describe the behavior of the soil in terms of its load-deformation conditions. These soil parameters are not known in advance, so the geotechnical engineer must measure these parameters under controlled laboratory or field conditions, or estimate them from other data (Kulhawy & Mayne, 1990).

 

In this way, the geotechnical characterization is completed by assigning representative geomechanical parameters to each sector delimited in the geotechnical zoning, which will be used later in the geotechnical design of foundations, walls, pavements and site preparation, and for the evaluation of potential soil-related problems that could affect the functionality of the project during its useful life.

Geomechanical parameters

The assignment of geomechanical parameters is one of the most important activities in the geotechnical characterization of a terrain, mainly because they will be the input data for the application of theoretical models of soil behavior. This behavior is extremely complex, as described by Ladd et al (1977): "A generalized model of stress-strain behavior of soils should ideally consider nonlinearity, variable dilatancy (volume change due to shear stresses), and anisotropy, in addition to stress path dependent behavior, stress system (orientation of σ1 and relative magnitude of σ2), and stress history (both initial and those resulting from a consolidation process)". This complexity is illustrated by the information presented in Table 1, which summarizes the main characteristics of the analytical models available to represent soil behavior.

Table 1 Categories of analytical methods for soil modeling (Jamiolkowski et al, 1985).

Category

Main aspects of the model

Determination of geomechanical parameters

I

Very advanced models that employ laws of time-dependent nonlinear elasto-plastic behavior, incorporating anisotropic behavior.

Only from sophisticated laboratory tests, with the exception of variables that must be obtained from in situ tests.

II

Advanced models using constitutive laws with incremental and nonlinear elastic relationships.

Laboratory tests that are slightly more sophisticated than conventional tests; in situ tests may also be required.

III

Simple models, such as continuous isotropic-elastic, including stratification and theoretical models.

Conventional laboratory tests and in situ tests.

As can be seen from the table above, the models range from complex (I), to advanced (II), to simple (III) in terms of soil description. It is also evident that the constitutive models for soil behavior require input data in the form of soil properties and in situ parameters. In most practical cases, category III is usually the most appropriate for use in design.

 

Jamiolkowski et al (1985) discuss the availability of laboratory tests and in situ tests to be used in soil characterization, focusing on the wide range of aspects linked to soil behavior, and suggesting that soil modeling is an extremely difficult task. In a future post we will deal with this topic, which is fundamental for the development of any geotechnical study.

 

References

  • Ladd, C.; Foot, R.; Ishihara, K.; Schlosser, F. & Poulos, H. (1977) “Stress-Deformation and Strenght Characteristics”. Proceedings 9th International Conference on Soil Mechanics and Foundation Engineering, Vol. 2, Tokio, Japan.
  • Holtz, R., Kovacs, W. & Sheahan, T. (2011) “An Introduction to Geotechnical Engineering”, 2nd ed., Pearson, USA.
  • Jamiolkowski, M.; Ladd, C.; Germaine, J. & Lancellotta, R. (1985) “New Developments in Field and Laboratory Testing of Soils”. Proceedings 11th International Conference on Soil Mechanics and Foundation Engineering, Vol. 1, San Francisco, USA.
  • Kulhawy, F. & Mayne, P. (1990) “Manual on Estimating Soil Properties for Foundation Design”, prepared by Cornell University Geotechnical Engineering Group, Hollister Hall, New York, USA.
  • Mitchell, J. & Soga, K. (2005) «Fundamentals of Soil Behavior”, 3rd ed., John Wiley & Sons, New Jersey, USA.
  • Taylor, D. (1948) “Fundamentals of Soil Mechanics”, 1st ed., John Wiley & Sons, New York, USA.
  • Terzaghi, K. (1943) “Theoretical Soil Mechanics”, 1st ed., John Wiley & Sons, New York, USA.