D. S. Liyanapathirana

Comparison of Different Two-Dimensional Idealizations for a Geosynthetic-Reinforced Pile-Supported Embankment

AbstractEmbankment construction on soft ground has increased considerably over recent years as a result of the increase in infrastructure development activities and because of the unavailability of suitable land. Geosynthetic-reinforced pile-supported (GRPS) embankments provide an effective and reliable solution to the problem of constructing embankments over soft ground. The combination of geosynthetic reinforcement and piles can alleviate the uneven surface settlements on the embankment crest while reducing the embankment load transferred to the soft foundation soil. This paper presents a numerical analysis based on the FEM carried out on a GRPS embankment. Analysis was carried out in both a two-dimensional (2D) plane strain condition for different 2D idealizations of piles and in a three-dimensional (3D) condition. The interaction between geosynthetic and soil was taken into consideration during the analysis. The results obtained for the 2D models are compared with the 3D model results. The stress tran...

Numerical Modeling of Geotextile-Reinforced Embankments over Deep Cement Mixed Columns Incorporating Strain-Softening Behavior of Columns

Geotextile-reinforced embankments over deep cement mixed (DCM) columns are widely used for the construction of highway embankments over soft clay with low shear strength and high compressibility. Numerical modeling based on the finite element method (FEM) is widely used to investigate the behavior of these embankments during construction and serviceability, incorporating consolidation of the foundation soil over time. However, not much attention has been paid to the strain-softening behavior of DCM columns beyond yield, which is essential in ultimate limit-state computations to determine the stability of embankments during the failure of columns. This paper presents a constitutive model, which is an extension of the Mohr-Coulomb model, for the simulations of strain-softening behavior of cement admixed clays. The model is validated using triaxial test data found in the literature for cement admixed Singapore and Hong Kong marine clays and Ariake clay. A two-dimensional (2D) coupled mechanical and hydraulic numerical implementation of a geotextile-reinforced DCM column-supported (GRCS) embankment constructed over a very soft soil in Finland is carried out incorporating strain-softening behavior of DCM columns. Even though the isolated columns and overlapped column walls used in this embankment do not yield significantly under the service loads, the model simulations show good agreement with field data, confirming the capability of the 2D plane strain finite-element model in predicting the GRCS embankment behavior. Finally, the finite-element model with strain-softening DCM columns is used to investigate the progressive failure of a typical hypothetical GRCS embankment with isolated columns in a square pattern. Results clearly illustrate the bending failure mode caused by progressive softening of the DCM columns, including the plastic hinge development within the columns.

Numerical Modeling of an Embankment over Soft Ground Improved with Deep Cement Mixed Columns: Case History

AbstractThis paper describes a case history of a deep cement mixed (DCM) column–supported embankment that is part of the Ballina Bypass section of the Pacific Highway Upgrade project in Australia. Measured settlements during and after construction of the embankment were significantly greater than the predicted settlements and suggested that the DCM columns were yielded. The case history was analyzed using a finite-element model based on the coupled theory of nonlinear porous media. Two cases were analyzed: with and without the strain-softening behavior of DCM columns caused by breakage of the soil-cement structure. The computed settlements, excess pore-water pressures, and lateral deformations were compared with field measurements. Results show that there was good agreement between the measured and the computed parameters when the strain-softening behavior of the columns was included. These results clearly show that consideration of the strain softening of DCM columns in the analysis is important if yield...

Numerical simulation of EPS geofoam behaviour in triaxial tests

Purpose – EPS geofoam has been widely used in embankment construction, slope stabilisation, retaining walls, bridge approaches and abutments. Nevertheless, the potential of EPS geofoam as an engineering material in geotechnical applications has not been fully realised yet. The purpose of this paper is to present the finite element formulation of a constitutive model based on the hardening plasticity, which has the ability to simulate short-term behaviour of EPS geofoam, to predict the mechanical behaviour of EPS geofoam and it is implemented in the finite element programme ABAQUS. Design/methodology/approach – Finite element formulation is presented based on the explicit integration scheme. Findings – The finite element formulation is verified using triaxial test data found in the literature (Wong and Leo, 2006 and Chun et al., 2004) for two varieties of EPS geofoam. Performance of the constitute model is compared with four other models found in the literature and results confirm that the constitutive mod...

Investigation of Composite Compacted Ground Using Microtremors

AbstractThis paper presents an interesting and unique case study of a composite compacted site where the upper section of dynamically compacted material achieved in the first stage of compaction was subsequently removed, reinstated, and recompacted in lifts in the second stage using conventional roller compaction. Dynamic compaction was employed initially in this area because of the need to densify deep fill materials. Although a number of mechanical methods are already available for assessing deep compaction, it has been particularly rare to find a cost-effective method that can be applied to a deep and extensive compacted site. Noninvasive techniques based on measurement of the horizontal-to-vertical spectral ratio (HVSR) of ambient vibrations (microtremors) are proposed in this paper to assist in a pilot appraisal of this area, which occupies a part of a deep and laterally extensive compacted site. First, the key features of the measured HVSR curves were interpreted to give a preliminary insight into t...

Shielding effect in pile groups adjacent to deep unbraced and braced excavations

This paper investigates the shielding effect within piles in a group adjacent to deep unbraced and braced excavations. Numerical simulations based on the finite element method are performed on free-head and capped-head piles in three different pile group configurations. The numerical model is validated by simulating a series of centrifuge tests. The problem was modelled considering the three-dimensional geometry, which facilitates to simulate the shielding effect of piles within the group during an excavation. Results show that the presence of front piles near the excavation face reduces the detrimental effects on the rear piles within the group in unbraced excavations. In addition, the provision of a pile cap significantly reduces the deflection of pile group in unbraced excavations due to load transfer to rear piles, which are located away from the excavation. However, in braced excavations, unless the excavation is deep, the shielding effect and the presence of a pile cap are less significant on the pile group behaviour.

Deep Cement Mixed Walls with Steel Inclusions for Excavation Support

Deep cement mixed (DCM) walls are widely used in supporting excavations in many parts of the world. In this paper, a case study of an excavation supported by a DCM wall with steel inclusions is analysed using a three-dimensional finite element model and based on the coupled theory of nonlinear porous media. The DCM wall is constructed with wide flange steel inclusions. The stress–strain behaviour of the DCM wall section is simulated using an extended version of the Mohr–Coulomb model, which considers the strain-softening behaviour of DCM columns beyond yield. The computed lateral deformations are compared with the field measurements to validate the numerical modelling procedure. Using the same case study, the internal stability of the wall against bending and shear failure modes is investigated. In addition, the lateral pressure distribution along the wall length is investigated because in practice design is carried out considering a uniform pressure distribution assuming rigid wall movements. A parametric study was carried out to investigate the viability of DCM walls in supporting excavations by varying the spacing between steel inclusions, wall thickness and initial lateral earth pressure. Based on the results of the parametric study, guidelines are proposed to select the most efficient geometric arrangement of steel inclusions within DCM walls.

A Numerical Model for Seismic Analysis of Piles in Liquefying Soil

Liquefaction of the soil is one of the major factors affecting the behaviour of piles founded on seismically active areas. Although methods are available for dynamic analysis of pile foundations, many of them are purely elastic methods or consider only the non-linearity of the surrounding soil. Here a numerical model is presented which takes into account the reduction in stiffness and strength of the soil due to pore pressure build up and subsequent soil liquefaction, in addition to the non-linearity of the soil and pile material. Soil in the far field is represented by the displacements calculated from a ground response analysis based on the effective stress method. The interaction between pile and the surrounding soil is modelled using the Mindlins fundamental solution with appropriate extensions to account for the soil radiation damping. Interaction coefficients and ultimate lateral pressure of soil at the pile-soil interface are calculated by taking into account the reduction in effective stresses due to build-up of pore water pressures. The non-linearity of the soil at the pile-soil interface is modelled via an iterative procedure which ensures that no where along the pile-soil interface, soil pressure will not exceed the ultimate lateral pressure of the soil. Results given by the numerical model are compared with the data recorded during centrifuge tests and the 1995 Hyogoken-Nambu earthquake. It is demonstrated that the new method gives results in agreement with the recorded data despite its relative simplicity.