Abstract:This paper reports on the results of soil-foundation numerical modelling and the seismic response of a cooling tower founded on piles of a petrochemical facility located in the city of Augusta (Sicily, Italy). The city was affected in the past by some destructive earthquakes (1693, 1848, and 1990) that damaged a large territory of Southeastern Sicily, which was characterized by a very high seismic hazard. With this aim, the paper reports the FEM modelling of the seismic behaviour of the coupled soil-structure system. To determine the soil profile and the geotechnical characteristics, laboratory and in situ investigations were carried out in the studied area. The seismic event occurred in January 1693 and has been chosen as a scenario earthquake. Moreover, a parametric study with different input motions has also been carried out. A Mohr-Coulomb model has been adopted for the soil, and structural elements have been simulated by means of an elastic constitutive model. Two different vertical alignments have been analysed, considering both the free-field condition and the soil-structure interaction. The dynamic response has been investigated in terms of accelerations, response spectra, and amplification functions. The results have also been compared with those provided by Italian technical regulations. Finally, the seismic response of the coupled soil-structure system has been further examined in terms of peak bending moments along the pile foundation, emphasizing the possibility of a kinematic interaction on piles induced by the seismic action.Keywords: petrochemical equipment; geotechnical characterization; coupled soil-structure system; FEM 3D analysis
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This study focuses on the possibility of numerical modelling of the most important sealing technology, diaphragm walls as the most major popular reliable option when it comes to engineering construction rehabilitation. It is included how to carry out, interaction with adjacent soil, safety factor evaluation associated with the state of the dam body and foundation; before, during, and after reconstruction, changing of pore water pressure with the time, settlement of dam, cement shrinkage, and sensitivity analysis. This modelling was conducted with the finite element method based on software Plaxis 3D. Diaphragm wall has been used in Karolinka dam for reducing seepage through its body. The results are concluded that the highest value of the displacement during the reconstruction process is the horizontal displacement due to water load and pore water pressure variations with the time. Safety factor is highly influenced by the variation of water level in the reservoir, elasticity modulus, and cohesion of the soils.
In this paper a 3D numerical model using a software based on the Finite Element Method (FEM), was developed and validated using the results obtained in a geotechnical centrifuge model of a piled raft system founded in soft soils undergoing regional subsidence. The piled raft configuration had nine piles distributed in the center of the raft. The kaolin parameters were obtained, calibrated, and validated for the Hardening Soil Model (HSM), based on laboratory triaxial and oedometer test results. Also, a single pile load test was carried out in the centrifuge to get the resistance parameters used in the FEM. The developed numerical model reproduced satisfactorily soil and foundation consolidation displacements due, not only by the structural service load but also by the pore pressure drawdown. For load distribution on piles and raft, the model reproduces with good agreement the foundation behavior only for the structural service load, for pore pressure drawdown some adjustments on the embedded piles elements shaft and base resistance had to be done. The developed model allowed to identify the most sensitive parameters for this type of simulation, to define the types and stages of analysis that had the best fit for the physical model, and to obtain additional results to those measured in the physical model, e.g., the axial load distribution developed along the piles and therefore the magnitude of the negative skin friction, that is an important load that should be considered for the structural safety review of piled foundations subjected to this complex conditions.
In this paper, the case of a piled raft system used on soft soils undergoing regional subsidence was studied. According to Alnuaim et al. (2018), a piled raft is a composite structure with three components: subsoil, raft, and piles. The structural components interact with each other and with the surrounding soil (pile-soil, raft-soil, and pile-raft) to bear vertical, horizontal, and moment loads coming from the superstructure. Luo et al. (2018) refer to this system as an effective foundation due to its efficiency in reducing settlements and improving bearing capacity.
The aim of this work is to develop and validate a three dimensional (3D) numerical model based on the Finite Element Method (FEM, Plaxis 3D) capable of simulating the complex behavior of a piled raft system founded in soft soils undergoing regional subsidence. For this purpose, the results obtained by Rodríguez-Rincón (2016) of a geotechnical centrifuge model were used. This model allows to identify the most sensitive parameters for this type of simulation, to define the types and stages of analysis that had the best fit to the physical model, and to obtain additional results to those measured in the physical model, e.g., the axial load distribution developed along the piles and therefore the magnitude of positive and negative skin fractions and point load. According to Auvinet & Rodríguez-Rebolledo (2017), the effect of the negative skin friction developed on piles shafts should be considered for the structural safety review and for the estimation of the long-term displacements of piled foundations.
The soil profile used was composed of three layers of a mixture of kaolin with water content at 1.5 times the liquid limit, divided by two sand layers that work as a filter and a bottom layer as drainage. This profile is intended to represent a soft clay soil typical of the city of Bogotá. To physically model a piled raft foundation, a 70 g centrifugal acceleration was adopted due to the capacity of the modeling box (boundary conditions), the size of elements sections after scaled and the size and capacity of the available instrumentation. The configuration of the piled raft is a model with nine piles distributed in the center of the raft, with a pile spacing of two diameters. Table 1 summarizes the dimensions and parameters of the piled raft elements.
With the aim of numerically reproduce the behavior of a pile raft foundation system and to take into account the need to determine the mechanical parameters of the HSM, it was necessary to carry out tests on a kaolin soil mixture whose profile represented the one proposed by Rodríguez-Rincón (2016). In this way, it was possible to experimentally determine the behavior of the soil in a different stress state, as well as the value of the axial pile resistance. The procedure described by Rodríguez-Rincón et al. (2020) was used for the fabrication of the soil mixture in the experiments. The results of the oedometer, triaxial tests, and the pile load test in the centrifuge are presented next.
To model the structural components, such as concrete piles and raft, a linear elastic constitutive model was assumed. Regarding the element type used for the design of the piled raft foundation, a plate element for the raft and embedded beams for the piles, were assumed. Plates are structural objects used to model structures in the ground with a significant flexural rigidity that does not allow plastification, only linear elastic behavior. As for an embedded beam element, it is defined as a structural object with special interface elements providing the interaction between the beam and the surrounding soil. The interaction involves a skin friction as well as a base resistance, which is determined by the relative displacement between soil and pile. This element type was chosen instead of the volume elements since with them it is possible to generate a mesh with fewer finite elements, thus decreasing the analysis time (Oliveira, 2018).
Phase 1, construction and loading: in this phase it was simulated the construction of the piled raft and the application of the load along the foundation surface, in accordance with the experimental test. A consolidation calculation was used to analyze the development of pore pressure as a function of time. As it is possible to apply load in this analysis, a value of 38.25 kPa was applied in 5000 hours corresponding to the interval time from tC-tE of the centrifuge test, Figure 11;
The displacement-time curve for the piled raft foundation, under vertical loading and pore pressure drawdown, obtained from the centrifuge test is presented along with the results obtained from FEM. Figure 13 shows the displacements measured at a point over the soil near the raft (Es1) with respect to time. In the first stage of the test, the results in the prototype are reasonably close to those from the centrifuge. Regarding the drawdown pore pressure phase, the results move away slightly, although the tendency is similar.
The developed numerical model reproduced satisfactorily soil and foundation consolidation displacements due, not only by the structural service load but also by the pore pressure drawdown (regional subsidence). For service load the maximum settlements reached up to 6 cm in the raft region. At the end of pore pressure drawdown, the maximum settlement was approximately 50 cm (8 times bigger). These comparative results do evidence the distinct phenomena and resulting engineering behavior that take place on a typical system founded in this type of environment.
For load distribution on piles and raft, the model reproduces with good agreement the foundation behavior only for the structural service load, for pore pressure drawdown some adjustments on the shaft and base resistance of the embedded piles elements had to be done. 2ff7e9595c
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