Numerical Modeling of Nonhomogeneous Behavior of Structured Soils during Triaxial Tests

The nonhomogeneous behavior of structured soils during triaxial tests has been studied using a finite element model based on the Structured Cam Clay constitutive model with Biot-type consolidation. The effect of inhomogeneities caused by the end restraint is studied by simulating drained triaxial tests for samples with a height to diameter ratio of 2. It was discovered that with the increase in degree of soil structure with respect to the same soil at the reconstituted state, the inhomogeineities caused by the end restraint will increase. By loading the sample at different strain rates and assuming different hydraulic boundary conditions, inhomogeneities caused by partial drainage were investigated. It was found that if drainage is allowed from all faces of the specimen, fully drained tests can be carried out at strain rates about ten times higher than those required when the drainage is allowed only in the vertical direction at the top and bottom of the specimen, confirming the findings of previous studies. Both end restraint and partial drainage can cause bulging of the triaxial specimen around mid-height. Inhomogeneities due to partial drainage influence the stress–strain behavior during destructuring, a characteristic feature of a structured soil. With an increase in the strain rate, the change in voids ratio during destructuration reduces, but, in contrast, the mean effective stress at which destructuration commences was found to increase. It is shown that the stress–strain behavior of the soil calculated for a triaxial specimen with inhomogeneities, based on global measurements of the triaxial response, does not represent the true constitutive behavior of the soil inside the test specimen. For most soils analyzed, the deviatoric stress based on the global measurements is about 25% less than that for the soil inside the test specimen, when the applied axial strain is about 30%. Therefore it can be concluded that the conventional global measurements of the sample response may not accurately reflect the true stress–strain behavior of a structured soil. This finding has major implications for the interpretation of laboratory triaxial tests on structured soils

Seismic Lateral Response of Piles in Liquefying Soil

Soil liquefaction is one of the major factors affecting the behavior of piles founded in seismically active areas. Although methods are available for seismic analysis of pile foundations, in many of them, the supporting soil is assumed to be an elastic material. Here a numerical model is presented which takes into account the reduction of soil stiffness and strength due to pore pressure generation and subsequent soil liquefaction, in addition to the material nonlinearity. Results obtained from the new method are compared with centrifuge test data and show excellent agreement with the observed pile behavior during these tests. To investigate the effects of soil liquefaction on the internal pile response, a parametric study is carried out for a range of material and geometric properties of the pile and surrounding soil. The effect of the nature of the earthquake on pile performance has been studied using 25 earthquake records scaled to different acceleration levels. It is shown that the “Arias intensity” and the natural frequency of the earthquake ground motion have a significant influence on the pile performance in liquefying soil. Existing elastic analysis methods for kinematic pile loading in layered soil deposits with soft and stiff layers are applied to the soil deposits with liquefying and nonliquefying layers. It is found that these simple design methods, which were derived assuming that the soil is a linear elastic material, do not predict bending moments accurately when nonlinear behavior of soil and effects of pore pressure generation are significant. Also a simplified limit equilibrium method proposed for the evaluation of bending response of single pile foundations subjected to lateral spreading is compared with the bending response obtained from the proposed numerical model. It is found that the limit equilibrium method, which is developed based on the centrifuge test results, does not give accurate results when the pile diameter and the thickness of the liquefied soil layer deviates from the values used for the centrifuge tests

Pseudostatic Approach for Seismic Analysis of Piles in Liquefying Soil

The performance of pile foundations during an earthquake significantly influences the integrity of structures supported by them. Therefore, in the overall seismic design process of the structures, modeling of the soil–pile-superstructure interaction is an essential part. Although finite element based coupled analysis of the soil–pile-superstructure interaction models have the potential to provide accurate results, they are computationally expensive and often complex to utilize. In practice, many geotechnical engineers tend to use simple methods for obtaining the internal response of piles subjected to earthquake loading. Therefore this paper presents a simple pseudostatic approach where a single pile is considered, including the contribution of the superstructure to the pile and the interaction between the pile and the soil. The method involves two main steps. First a nonlinear free-field site response analysis is carried out to obtain the maximum ground displacements along the pile and the degraded soil modulus over the depth of the soil deposit. Next a static load analysis is carried out for the pile, subjected to the maximum free-field ground displacements and the static loading at the pile head based on the maximum ground surface acceleration. The method has been verified using an independent dynamic pile analysis program developed by the writers for the seismic analysis of piles in liquefying soil. It is demonstrated that the new method gives good estimates of pile bending moment, shear force, and displacement, despite its relative simplicity. The method is then used to compute the response of pile foundations during the Kobe 1995 earthquake and some centrifuge tests found in the literature where extensive soil liquefaction has been observed. Very good agreement is observed between computed and recorded pile bending moments

Drained bearing response of shallow foundations on structured soils

This paper examines the drained bearing response of circular footings resting on structured soil deposits. Numerical simulations have been carried out using a finite element formulation of the Structured Cam Clay model. A parametric study was conducted by varying the parameters that govern the behaviour of structured soils and guidelines are given for designers to identify when effects of the soil structure are important. Under fully drained conditions, deformation within the structured soil supporting the footing usually occurs as a local or punching shear failure due to high compressibility of the structured soil and the mobilised bearing pressure increases with the footing movement, without reaching an ultimate value. A novel approximate method is presented to obtain the load–displacement response of a rigid circular footing resting on the surface of a structured soil deposit. This requires the properties of the soil in the reconstituted state and two additional parameters, which govern the natural structure of the soil. The proposed method has been applied to a published case study, where plate load test results are given for rigid circular steel plates resting on structured soil deposits. Fair agreement is observed between the computed and experimental results, suggesting the approximate method may be useful in design studies of foundations on structured soil deposits

Numerical modelling of soft ground improved with cement

This paper examines the undrained bearing capacity of shallow circular foundations on soft ground improved with cement by using a numerical model based on the finite element method. Guidelines are given to identify the importance of the degree of cementation for assessing the bearing capacity of foundations. Using a bearing capacity improvement factor, influence of the extent of the cemented region on bearing capacity has been investigated. Finally the performance of deep mixed cement columns has been investigated using the numerical model. The results indicate that there exists an optimum length to diameter ratio for the deep mixed cement columns and this value depends on the degree of cementation of the soil

Measurement System with Accelerometer Integrated RFID Tag for Infrastructure Health Monitoring

This paper presents a measurement system for measuring dynamic acceleration of infrastructure remotely using semipassive radio-frequency identification (RFID) tag. This measurement is critical to the vibration-based method for infrastructure health monitoring. Design considerations of accelerometer integrated ultrahigh-frequency RFID tag and dynamic acceleration measurements through an RFID wireless link are discussed. Measurement results of the system for a structural specimen have shown that it is capable of acquiring data which provides the information of natural frequency of the structural specimen. Moreover, the system can distinctively identify the state changes of the structural specimen by natural frequency shifts. These results are benchmarked against the results obtained with two commercial systems. It is shown that the standard deviation of the measurement of the natural frequency is ±0.01 Hz which is very close to the standard deviation of the commercial measurement systems

Modelling the behaviour of cemented clay

In this paper, a theoretical study of the behaviour of cemented soft clay is made using a simple predictive constitutive model, the Structured Cam Clay (SCC) model. A simple modification of the original SCC model is proposed so that important effects of cementation on soil behaviour can be represented. The revised model is evaluated based on the comparison of simulations and experimental data