Allowable Tension of the ground: Knowledge of soils

An inescapable truth so far is that everything that we have come to build and continue to build must rest on the ground. This being, a fundamental part of the support of any surface structure: if the ground gives way because it is not able to support the weight, the structure that is supported on it will give way in the same way. Fortunately, there are ways to improve the resistance characteristics of the floors, as well as the attachment of the structures to it, but we must know very well the ground where we are leaning.

In current construction, the soil continues to be an element that escapes the control of the designer, developer, engineer or builder; It is not built on the ground that we want, it is built on the ground that we find ourselves. On the contrary, the designer, the developer, the engineer or the builder must be molded to the characteristics of the soil to ensure that it complies with the minimum parameters of stability and safety.

This indicates to us how important it is to have a deep knowledge of the characteristics of the soil on which it will be built, and therefore, it speaks of the capacity of the land. All the information necessary to know the capacity of the land is collected in a geotechnical report, the result of a previous study of the mechanics of the soils.

Soil mechanics studies

This type of geotechnical studies, beyond being mandatory for civil works in most of the world, are a tool to identify the different characteristics of a soil that determine its bearing capacity. Even so, the studies carried out in each work may be different, and the amount and type of samples taken and tests carried out will depend on the needs of each structure.

We can divide the types of samples and tests into 3 categories:

In-situ: These are those that are carried out directly on the land object of the study. They are characterized by being fast, cheap and fast studies, sacrificing a little their precision.

In the laboratory: They consist of samples that are extracted from the field, and transported to laboratories for study. They are more expensive and complex tests, but they provide greater precision and certainty.

Auscultation: These are monitoring studies in the post-construction phase, and allow monitoring the state of the land, to prevent a sudden change in soil capacities.

These studies provide a very varied amount of information through different types of tests, but to know the bearing capacity of the land to take into account for the design of the foundations of a future building, the most relevant data is the Admissible Stress.

Admissible tension

This data is so important, because it is precisely what quantifies the bearing capacity or resistance of the ground, or what is the same, how much load it is capable of supporting without sinking. It represents the tension (Force per unit area) that the ground can withstand at a certain depth, and its unit of measurement is normally kilograms per square centimeter (Kg / cm2).

The admissible stress values that are usually found in soils range between 0.5 and 3 Kg / cm2. A low allowable stress indicates a problematic soil for the stability of the foundations, and conversely a value close to 3, or higher, indicates a very good terrain to build on. Very relevant factors on the cost or viability of any work.

Even so, knowing only the value of the admissible stress of a terrain may not be enough depending on the case; It is for this reason, that it is a piece of information that is usually accompanied by two supplements that are intended to give more context to the information obtained, and therefore provide a deeper knowledge about the soil and its composition. These two complements are: The Ballast Coefficient and the Firm Level.

Ballast Coefficient

To understand the ballast coefficient, we must know that the admissible stress of a ground is defined as the point at which the "seat" (deformation) of said ground does not exceed one inch, or 2.54 centimeters. The ballast coefficient measures the stiffness of the ground through the relationship between the allowable stress and the settlement that the ground suffers.

Firm Level

It is known as "Firm" to the stratum where the admissible stress necessary to be able to support the loads corresponding to the construction is found, that is, the depth where we can find a suitable stratum to support the building. It is not the same to achieve a firm layer at 1 meter depth than at 7 meters, it differs The gingiva significantly affects when choosing one type of foundation or another.

Importance of knowledge of soils

Starting from the fact that a geotechnical study, seen from a purely economic point of view, represents an insignificant portion of the cost of the work; It is incomprehensible to jeopardize the integrity of the structure and the safety of its users, by not doing, or by half doing, an in-depth study that allows us to truly know the mechanical properties of the soil where we are going to build.

On the other hand, the lack of knowledge at the level of calculation and structural design is counteracted with higher safety coefficients, and therefore, an oversizing of the structural elements, in this case the foundations. This implies that a geotechnical study correctly executed on the ground will provide the necessary information to design a structure adjusted to the needs of said terrain; saving economic resources and materials consumed in an oversized structure.

As well as the admissible stress, other characteristics of the soil can considerably affect various aspects of the design and execution of a work, which shows us the importance of the intervention of professionals specialized in geotechnics and foundations.