Test procedure and sample preparation for large scale soil structure interaction model tests in eitlsc

Abstract

In geotechnical earthquake engineering, soil-structure interaction has been a highly researched area, particularly for improvement of earthquake resistant designs. Earthquake engineering places significant emphasis on studying soil-structure interaction, as it is vital for designing earthquake-resistant structures and safeguarding communities against seismic risks. There are many solutions to avoid these problems. One is laboratories, which provide significant advantages for conducting detailed research under such conditions. Under these issues, it is essential to apply and develop the test procedures correctly in the laboratory and prepare samples in a way that resembles the field. This study has conducted essential research for large-scale laboratory tests to investigate the soil-structure interaction. For this purpose, a novel laminar soil container had been designed and used for the large-scale shaking tables tests. Laboratory tests have been conducted to determine characteristics of the soil sample to be used in the laminar container, which plays a crucial role in experimental studies. Firstly, the index parameters of the sand sample were determined. Then the static shear strength parameters were determined through direct shear and DSS-C test devices in the Soil Mechanics Laboratory at Istanbul Technical University. The differences in the procedures and in the resulting strength parameters were discussed. The strength parameters of the sand sample were carried out with the help of a direct shear test device. The test was conducted in dry conditions and at a 0.12 mm/sec shear rate. The sand sample was taken from an average depth of 5 m. Therefore, the stress values given to the soil were determined as 25, 50, and 100 kPa. 3 different relative densities (loose, medium-dense, and dense) were conducted to determine the shear strength parameters (internal friction angle, cohesion). 27 tests were performed, repeating the procedure 3 times for each stress value. A simple Shear Device with Confining Pressure (DSS-C) in the Soil Dynamics laboratory was also used to investigate the shear strength values. The stress values given to the soil, shear rate, 3 different relative densities, and a total of 27 tests were also performed in DSS-C. In addition, sample preparation in this device was performed under saturated and drained conditions. While the wet pluviation method was used to perform medium-dense and dense soil conditions, the moisture tamping method was applied to the loose sand. Dynamic parameters of the soil sample were determined in the soil dynamics laboratory using a Dynamic Simple Shear Device with Confining Pressure (DSS-C). The sand sample was analyzed in two different ways, stress-controlled and strain-controlled tests, for a total of 31 experiments. To begin with, a strain-controlled test was conducted to determine the shear modulus of sand specimen under the high strain values ranging from 0.2-1. These tests were performed at 2 different relative densities (medium-dense and dense), and 3 different effective stress values (15, 50, and 100 kPa). On the other hand, in the stress-controlled experiments, various densities (medium-dense, dense) and 2 different Cyclic Stress Ratio CSR (0.15 and 0.2) were utilized to assess the liquefaction potential of the sand sample. All stress-controlled experiments were conducted under an effective stress of 100 kPa. All tests were performed in the sinusoidal waveform at 1 Hz. Determining the relative density is crucial to conducting large-scale soil-structure interaction experiments. The research aimed to get medium–dense and dense soil samples with designed plates. To address this, a small-scale innovative technique was developed prior to proceeding with the experiments. This technique involved the preparation of samples with varying relative densities, which aided in identifying the appropriate plate for the large-scale sand pluviation mechanism. A total of 10 plates were designed, each with different aperture sizes and two distinct radii. The dimensions of the plates were set at 0.3 m in width and length. The aperture sizes chosen were 0.3 cm and 0.5 cm. For this experiment, five different heights ranging from 30 cm to 150 cm were considered, with intervals of 30 cm. Throughout the experiment, the relative densities were obtained by varying the plates and radii, ultimately leading to the selection of the most suitable 2 plates for the large-scale design. Large-scale soil-structure interaction model tests were conducted in the Construction and Earthquake Laboratory at Istanbul Technical University. Enhanced Impermeable Transparent Laminar Soil Container (EITLSC) and large-scale sand pluviation mechanism were used to conduct these experiments. EITLSC is a recently developed device that simulates field conditions, allowing for dynamic experiments. Large-scale sand pluviation mechanism was designed with an automated motor that allows the researcher to control sand pluviation height. A total of 5 sets of experiments were conducted, including the following: free field, shallow foundation 1D structure, group pile with D1 and D2 diameter 1D structures at 1 and 2 Hz structure frequencies, and single pile with D1 and D2 diameter 1D structures at 1 and 2 Hz structure frequencies. A total of 62 harmonic and earthquake loads were analyzed using the EITLSC model test to gain insights into the combined behavior of soil and structure. Within the scope of the study, it has been understood that finding the soil strength values obtained from DST and DSS-C presents different outcomes. The experimental results that best stimulate the field conditions were obtained through DSS-C tests performed at a laboratory scale. Furthermore, in order to determine the shear modulus of the sand samples under cyclic loading test, as the relative density and effective stress increase, the shear modulus increases, too. In addition to dynamic research, it has been determined that CSR values and relative densities show a directly proportional behavior under the stress-controlled test. For example, it has been understood that the soil is prone to liquefaction more easily in medium-dense soil and under high CSR conditions. Moreover, it has been determined that altering the aperture and radii size (0.5 and 0.3 cm) of plates used in small-scale novel sample preparation techniques leads to varying outcomes. Lastly, Free-field and shallow foundation 1D structure test results obtained from EITLSC are included in the research. TUBITAK supported this research with Project ID 119M624 and project title 'Development of Lateral Load Displacement Relations of Piles Buried in Sandy Soil under Earthquake Load. It has been proven that the large-scale sand pluviation mechanism has been successfully designed based on the targeted relative density values. Moreover, It is clearly understood that the response spectrum, acceleration, and displacement values of the EITLSC model test are suitable for use in soil- structure interaction analyses. At the same time, the test procedure, especially the pluviation technique, will significantly contribute to the literature under laboratory conditions. Lastly, soil strength values obtained from DST and DSS-C devices present valuable insights that can significantly benefit design engineers in the future.