The importance of evaluating the behavior of our soils: the phenomenon of liquefaction in the event of possible earthquakes in eastern Cali

Andrés Felipe García Barrera, Master's student in Civil Engineering with Emphasis in Geotechnics. Credit: Faculty of Engineering Communications Office.

Due to its geographic location, the place where the city of Cali is located represents a high threat to possible earthquakes. According to studies, the eastern area has soils that, in the event of an earthquake with specific characteristics, may suffer from a phenomenon known as liquefaction, which affects the stability of structures, regardless of their composition. An investigation recognizes the importance of studying the behavior of these soils by updating the dynamic measurement instruments located at the Puerto Mallarino Drinking Water Treatment Plant, while proposing a model which helps researchers to perform detailed simulations on the seismic behavior of the city of Cali, in order to foresee future threats and minimize risks and damages to the population. 

Lea el artículo en español aquí.

The need to update seismic measurement systems in the region 

Due to their characteristics (that they are fine sands or not very dense, and that they are saturated, that is to say, their interior empty spaces are occupied by water) there are certain types of soils that are more prone to suffer from a phenomenon known as "liquefaction", which occurs as a consequence of seismic movements.  This phenomenon, which occurs as a result of the increase of water pressure in the pores due to the speed of an earthquake, causes the soil to lose resistance and begin to behave like a viscous fluid, endangering all the structures built on it.  

This type of event has precedents worldwide, such as the one that occurred after the Niigata earthquake in Japan in 1964. At the national level, a similar event was recorded in 1979 in the municipality of Tumaco. At the local level, the city of Cali, located in a high seismic hazard zone, is highly vulnerable to earthquakes and liquefaction, especially in the eastern area, where the Aguablanca district is located, where a large part of the population lives.  

Aware of the vulnerability of its inhabitants, where the socioeconomic strata 1 and 2 predominate, and the importance of buildings such as the Nuevo Latir School, the Jarillón del Río and, with an additional importance due to its nature, drinking water treatment plants as the one of Puerto Mallarino, Andrés Felipe García Barrera, student of the Master in Civil Engineering with Emphasis in Geotechnics, is carrying out a research project that seeks the updating of instruments for measuring seismic events located in Puerto Mallarino and the implementation of a representative model of the area, and through which it is possible to analyze the effects on soil properties such as stiffness, before the application of different seismic input signals. 

His research project, under the direction of researcher and professor Eimar Sandoval, from the School of Civil Engineering and Geomatics, seeks to continue with a work done previously, by upgrading the seismic monitoring center located at the Drinking Water Treatment Plant, so that it is able to obtain all the information received by the acquisition instruments and make it more easily accessible for academic and preventive purposes for the region of Valle del Cauca, especially the district of Aguablanca. 

The state of the monitoring center in Puerto Mallarino and the need to update the measurement systems 

As a result of his undergraduate work, the now researcher Garcia Barrera contributed to the Center for Observation and Analysis of Liquefaction in Aguablanca (COALA), installed in 2019, having at its disposal the instruments for a much more accurate dynamics measurement. This came about through the installation of two piezometers (instruments used to measure water level and pressure in internal soil wells), located at 4.25 and 6.5 meters depth, and the subsequent upgrade of the interface to display the data in the center. COALA currently has two seismometers: one superficial and one at 40 m depth (instrument used to measure soil velocity), in addition to 4 wells, located at 4, 7, 13 and 40 meters depth, depending on the location of the liquefiable stratum of the soil.  

However, the monitoring carried out by the Center is not optimal, due to the power outages that occur in the area, which prevent the continuous measurement of the seismic records presented in the area. In addition to this circumstance, in order to collect the information it is necessary to go directly to the Drinking Water Treatment Plant and then process them at the Universidad del Valle facilities, which implies economic and time costs that reduce the progress of the investigations.  

The proposal of researcher Andrés Felipe García Barrera aims to solve these operational problems in COALA, and use the information collected to feed an algorithmic model for earthquake detection in the area: "The idea is also to implement an earthquake detection algorithm, so that the program that is acquiring the information detects that an earthquake occurred and can save it in a different way," says the researcher, and explains that progress has been made in the characterization of the area in terms of vibration, to separate those produced by the environment where the Center is located and those that correspond to seismic movements. For the development of this model the researcher has relied on information provided by the Colombian Geological Service (SGC).  

The research and methodology used 

The work of the researcher García Barrera has the following objectives: 1) automating the monitoring center, improving operability conditions, which involves the installation of an accelerometer (instrument intended to measure accelerations in the ground), a digitizer and a solar panel of 200 W of power, something that to date has been accomplished; 2) sensitivity analysis of the detection algorithm, since 2019 the COALA Center has acquired about 35 to 40 earthquakes in the city, and is seeking to improve the parameters of the algorithm to suit the area where the Center is located; 3) the creation of a numerical constitutive model with the data acquired at the Center, for which the results of different tests that have been carried out in the geosciences laboratory of the research group in Seismic, Wind, Geotechnical and Structural Engineering (G-7) of the Universidad del Valle, to which researcher Andrés Felipe García Barrera belongs, were compiled. 

"The idea is to feed a constitutive model in the OpenSees program, open source. The parameters of the model are adjusted with the data obtained at the center and its response to dynamic loads is studied. Once the model is verified, it will be possible to introduce an earthquake and analyze how the soil behaves with a shallow foundation, with a pile, how it affects the structure and how the structure affects the soil", says the researcher, and clarifies that the work that is part of his research seeks to be a first step so that later, with new researchers, this study can be carried out on a broader scale. 

With the above in mind, the researcher acquired the information with reference to seismic records, for which he had two important seismogenic sources: La Mesa de los Santos, in the department of Santander, a place known for constant telluric movements, and Mesetas, in the department of Meta. A threshold of relevance was considered for the record: a magnitude greater than 4 Mw. The number of recorded movements was 40. At the same time, the above mentioned equipment was updated. 

For the sensitivity analysis of the detection algorithm, the researcher has established the following parameters: short window duration, long window duration, start threshold and end threshold. The aim of this analysis is to determine the sensitivity within the algorithm based on the modification of any of these values. 

The updating of these measurement systems at the Center for Observation and Analysis of Liquefaction in Aguablanca (COALA) and the development of the detection algorithm is a project that, as a whole, brings positive consequences in terms of seismic recording and analysis of possible liquefaction phenomena for this area of Valle del Cauca. For the researcher Felipe García, his project is, at the same time, a bet to contribute to research on the subject to be carried out in the years to come.  

Technological and social impact 

"The idea is to feed the general knowledge we have about potentially liquefiable soils, how they behave, especially here in Cali", says researcher Andrés Felipe García Barrera regarding the impact his research may have in the technological field. According to him, his bet serves as an input in the search for determining how to reduce the liquefaction potential that exists in the region. 

And, specifically, in the eastern area of the city, where the Drinking Water Treatment Plant is located and it has been evidenced that there is an even greater risk, the update of the measurement systems and the proposed algorithm are intended to instruct about the correct use of the soil and the mitigation of the possible damages that a seismic event may cause in terms of liquefaction. "We know that the eastern area is home to about 40% of the city's population. This is a very vulnerable population, and the construction of buildings has sometimes not been regulated by the standard. The idea is to analyze how it can be improved, how it can be built correctly in this type of soil. And, since we know that houses cannot be demolished, we want to see how to reduce the liquefaction potential without affecting existing buildings", are the words of the researcher. 

 If interested in being in touch with the Master's student or any further information about the investigation, please write the Faculty of Engineering Communications Office: comunicaingenieria@correounivalle.edu.co.