Soil studies and impact on the construction process of PV plants

One of the soil plant It is ideal for consumers who need a large installation, where high electricity consumption is necessary. That type of enterprise has been widely used in recent years to meet the demand of companies and communities, for example.

To carry out an installation of this type it is necessary to carry out soil studies, after all, any civil structure, of any work, discharges the weight and its tensions into the soil or rock.

Therefore, We have to understand this underground to see if it will withstand the cargo that will be unloaded. We have to carry out an assessment not only of the soil, but also of the with the rock and also of the condition of the rock beneath the ground.

This is the analysis of geologist Fábio Reis, advisor to Crea-SP (Regional Engineering and Agronomy Council of the State of São Paulo) and president of Febrageo (Brazilian Federation of Geologists).

In an exclusive interview to Canal Solar, Reis, who is also a professor in the geology department at UNESP (Universidade Estadual Paulista), Rio Claro (SP) campus, spoke about how soil assessment impacts the civil construction process of photovoltaic plants and how the steps work topography, slope e survey.

Additionally, we spoke with the Paulo Vasconcelos, civil engineer at SSM Structures and auditor, evaluator and engineering expert, who listed seven important points related to the land for deployment of ground solar plants.

How does soil assessment impact the civil construction process?

Fábio Reis: For photovoltaic plants, studies are generally carried out with SPT (Standard Penetration Test or Percussion Sounding Test), which is to check the penetration resistance index of this soil. From this value, we were able to establish the variation in soil resistance with depth until it reaches the rock.

If you are going to place a very heavy weight on this terrain, several characteristics of the rock must also be studied, and then there would be other types of more complex surveys, generally they are rotary surveys in which you remove the sample from the rock to check if it has fractures, such as it is in the process of altering the rock, whether it is an altered rock, that is, what are the rock's resistance characteristics.

Generally, those who do this are geologists, who evaluate and describe this soil profile from the surface to the soil/rock , evaluate the conditions, as well as other characteristics, but, mainly, in the case of photovoltaic plants, they carry out an analysis of resistance.

Another important point is the location of the groundwater level, as this will have an impact if excavation is necessary. For example, you have to lower the water level to check whether it can affect the soil's resistance, whether it is saturated or not.

So, what are the main points for installing soil plants?

Paulo Vasconcelos: Carry out analysis of the geotechnical profile of the soil through survey, preferably SPT type, aiming to identify the type and quality of the soil, to size its load capacity and check whether there is water in the ground (water table).

Carry out a visual analysis. If necessary, carry out a topographic survey to check the slopes and precise limits of the land to be used. It is recommended to look for flat land to avoid earthmoving services that increase installation costs. Land with very steep slopes can restrict the installation of the plant. For imperfections in the terrain in an east-west orientation, consider a maximum slope of 5 degrees.

Clean the land, remove vegetation and preferably remove the organic layer before laying the foundation, if any. Carry out a hydrological study of the area in order to predict possible flooding or points of water accumulation at times of the year due to the elevation of the land or proximity to a lake, stream, river, sea, among others;

In the case of land that received landfill, check whether the landfill was of good quality and whether there was compaction or even technological control was carried out. Provide for demarcation of the plant's implantation area with fencing to ensure security and delimitation of the plant's area;

Finally, when choosing the land, observe how access to the ground plant will be, aiming for better logistics for transporting people and inputs for its installation and maintenance, with adjustments if necessary to make it better viable.

One observation: when we recommend geotechnical soil analysis, it is not to find the depth of the “rock” to the foundation on it, because it is rarely found, especially at low depths.

As photovoltaic plants exert a relatively low load that does not reach 0,5tf/m, shallow (direct) foundations are the most recommended, where we can them on firm soil from the organic layer. If it exists, and according to the ABNT NBR 6122 standard with regard to shallow footing-type foundations, the minimum recommended depth would be 1m.

In general , if a reasonably firm soil is found from 1m up, we can now use it as a base as long as it es the design, which takes into , mainly, the action of the wind (isopleth of the region) and analysis of the soil's SPT for the calculations.

Fábio Reis: Understanding the conditions of the subsoil and the conditions of the water underground is essential to establish what type of work the civil engineer will design according to the conditions of the soil, rock and also the water level, especially in of resistance.

If a project is carried out without such assessments, there is a risk of having a project that will suffer sinking, cracking, slipping, a series of processes that can even bring down the entire structure, in this case the solar plant.

How do the topography, slope and survey stages work? What role do they play in the quality of the work and enterprise?

Fábio Reis: Topography will mainly evaluate the topographic conditions of the land in of elevation and altimetry, and based on these issues and the location of the land, you establish the declivity, which is the slope of the land.

For example, if you have land with large slopes. What would they be? Above 30º, which requires more detailed studies, and 45º, in which there are a series of restrictions. So, the steeper the terrain, the more susceptible the terrain is to suffering movements in the form of landslides, soil movements, which we call gravitational mass movements.

And the sequence, basically of topography, is the initial study to determine the altitude elevations and location of that land. You work with vectors X, Y and Z. X and Y are the latitude and longitude coordinates and Z is the topographic elevation coordinate. Thus, you can, from the typography, have a plan view, which we call a 2D view. With the elevation, you can have a 3D view of the terrain, which is why we call it a planialtimetric survey.

The planimetric is just a question of the plan, working with X and Y. When we put altimetry we are putting the topographic elevation, that is, the altitude in relation to sea level or in relation to some reference level. With the combination of planimetry and altimetry we can understand what the elevations, slope and changes that occur on the surface of the land are.

From the planialtimetric survey, you carry out research on geological and morphological maps to understand what type of soil and rock you expect to find in that location. For example, from maps, bibliographic information and study reports that were carried out by other people in that region, you will have an idea of ​​how deep the rock and soil alteration profile will be, what is the expected depth of soil /rock and whether it can have water or not.

Before carrying out a survey, it is necessary to carry out an investigation plan. So, from the planialtimetric survey, the topography with the bibliographical survey and field visits, reconnaissance, the geologist is able to locate where the surveys should be carried out to better characterize the terrain, and then he will locate these surveys at the points where there is greater doubt regarding the information obtained in the bibliography and visual recognition.

With such surveys you can locate the survey. And the type of survey, there are several such as manual, mechanized, rotary, percussive. Percussive drilling, for example, is generally used to evaluate the soil profile, as most soils cannot drill through rocks, so they need to use rotary drilling. Therefore, the type of test that will be carried out depends on each terrain situation and the geology of that location.

The geologist's job is to follow the survey in the field and, based on the data he receives, make profiles between one survey and another and establish where the soil levels are, the variations in resistance throughout the subsoil, the characteristics of the soil and the rock, whether it has a water level or not and, together with the details of the terrain, it is ed on to the civil engineer to determine based on the characteristics of resistance, permeability, the presence of a groundwater level or not – all of this mapped within the plan.

The geologist does this by mapping the variations that occur in the plan for the civil engineer to establish the engineering project that he will implement, considering the characteristics of the subsoil, mainly resistance.