Finite Element Program PLAXIS 3D Research Articles

Experimental and numerical settlement analysis of railway track over geogrid reinforced ballast

PurposeThe main objective of the present research is to investigate the benefits of using geogrid reinforcement in minimizing the rate of deterioration of ballasted rail track geometry resting on soft clay and to explore the effect of load amplitude, load frequency, presence of geogrid layer in ballast layer and ballast layer thickness on the behavior of track system. These variables are studied both experimentally and numerically. This paper examines the effect of geogrid reinforced ballast laying on a layer of clayey soil as a subgrade layer, where a half full scale railway tests are conducted as well as a theoretical analysis is performed.Design/methodology/approachThe experimental tests work consists of laboratory model tests to investigate the reduction in the compressibility and stress distribution induced in soft clay under a ballast railway reinforced by geogrid reinforcement subjected to dynamic load. Experimental model based on an approximate half scale for general rail track engineering practice is adopted in this study which is used in Iraqi railways. The investigated parameters are load amplitude, load frequency and presence of geogrid reinforcement layer. A half full-scale railway was constructed for carrying out the tests, which consists of two rails 800 mm in length with three wooden sleepers (900 mm × 90 mm × 90 mm). The ballast was overlying 500 mm thick clay layer. The tests were carried out with and without geogrid reinforcement, the tests were carried out in a well tied steel box of 1.5 m length × 1 m width × 1 m height. A series of laboratory tests were conducted to investigate the response of the ballast and the clay layers where the ballast was reinforced by a geogrid. Settlement in ballast and clay, was measured in reinforced and unreinforced ballast cases. In addition to the laboratory tests, the application of numerical analysis was made by using the finite element program PLAXIS 3D 2013.FindingsIt was concluded that the settlement increased with increasing the simulated train load amplitude, there is a sharp increase in settlement up to the cycle 500 and after that, there is a gradual increase to level out between, 2,500 and 4,500 cycles depending on the load frequency. There is a little increase in the induced settlement when the load amplitude increased from 0.5 to 1 ton, but it is higher when the load amplitude increased to 2 ton, the increase in settlement depends on the geogrid existence and the other studied parameters. Both experimental and numerical results showed the same behavior. The effect of load frequency on the settlement ratio is almost constant after 500 cycles. In general, for reinforced cases, the effect of load frequency on the settlement ratio is very small ranging between 0.5 and 2% compared with the unreinforced case.Originality/valueIncreasing the ballast layer thickness from 20 cm to 30 cm leads to decrease the settlement by about 50%. This ascertains the efficiency of ballast in spreading the waves induced by the track.

Effect of damper mass on soil-structure displacement in dynamic response to earthquake vibration with finite element modelling

Earthquake is a natural disaster that always occur inside the region of pacific ring of fire. Many structures such as high-rise buildings, bridges, dams and many more failed and take severe damage due to the impact of the strong vibration of the earthquake. A pile foundation is a type of deep footing which is suitable for high-rise buildings founded on clay soils where its function to prevent potential failures such as bearing capacity and collapse. In addition, the problem of tall buildings is prone to sway and damaged when earthquake strike and such vibrations cause the dynamic behaviour of structure to change significantly in terms of excessive lateral displacements and accelerations. Thus, the effect of damper mass on soil-structure displacement in dynamic response to earthquake vibration was determined. The effectiveness of vibration absorber to the single pile structured in clay soil was modelled and simulated in finite element program PLAXIS 3D version 20 with and without the presence of the damper where the inputs were taken from the Sumatera and Chi – chi earthquakes. The problem was modelled and simulated by drawing a geometry, assigning the soil and pile materials input, loading and boundary conditions, meshing generated and staged construction for calculations of displacement structure output in response to dynamic due to earthquake vibration. The displacement-time with the effect of mass of the vibration with and without absorber are presented. The results show a good performance of damper on the single pile structured on soft soil with different types of earthquakes.

Analysis and Design of Piled Raft Foundation Taking into Account Interaction Factors

In a combined piled raft foundation (CPRF) both raft and piles take their share of the total load applied. However, in practice, the contribution of a raft in taking load is usually ignored and the load is assumed to be supported on piles. This way of CPRF becomes excessively conservative and uneconomical. To economize the design, relative load sharing of raft and piles in CPRF has to be found. In this connection, different simplified methods have been developed, each one with some limitations. In this study, three simplified methods have been applied to two cases of pile‐raft systems. The methods include Randolph, Poulos‐Davis‐Randolph, and modified Poulos‐Davis‐Randolph. The first case is a hypothetical case consisting of a 12 m × 12 m raft supported on a square group of nine piles. The second case study is an actual eight‐story building to be constructed in Peshawar, Pakistan. The building is supported on a pile‐raft system, with raft resting on very soft clay underlain by dense sand. The two case studies are also modelled in the finite element program PLAXIS 3D for comparison. The results of all the simplified methods are comparable with PLAXIS 3D. However, the Randolph method is much closer to PLAXIS 3D for the two cases studied. Furthermore, it is also shown that piles in a piled raft system can be used as “stress reducers” as well as a “settlement reducers.” Additionally, the effect of interaction factors is also evaluated with the s/d ratio as well as with varying soil stiffness. It was concluded that ignoring these factors leads to a very unsafe design of the pile‐raft system.

Predicting the bearing capacity of pile installed into cohesive soil concerning the spatial variability of SPT data (A case study)

Nowadays, in situ tests have played a viable role in geotechnical engineering and construction technology. Besides lab tests conducted on undisturbed soil samples, many different kinds of in-situ tests were used and proved to be more efficient in foundation design such as pressuremeter PMT, cone penetration test CPT, standard SPT, etc. Among them, a standard penetration test (SPT for short) is easy to carry out at the site. For decades, it has proved reliable to sandy soil, but many viewpoints and opinions argued that the test was not appropriately applicable to cohesive soil because of scattered and dispersed data of SPT blow counts through different layers. This paper firstly studies how reliable the SPT data can predict the physical and mechanical properties; secondly, the soil strength is determined in terms of corrected N-SPT values, and finally the bearing capacity of a pile penetrating cohesion soil. By analyzing data from 40 boreholes located in 18 projects in Ho Chi Minh City, South VietNam, coefficients of determination between SPT numbers and physical and mechanical properties of different soil kinds are not the same: R2 = 0.623 for sand, =0.363 for sandy clay and =0.189 for clay. The spatial variability of soil properties is taken into account by calculating the scale of fluctuation θ=4.65m beside the statistically-based data in horizontal directions. Finally, the results from two theoretical approaches of predicting pile bearing capacity were compared to those of finite element program Plaxis 3D and static load test at site. Correlation between the capacity computed by using corrected N-values instead of soil strength and results of static load test has proved to be well suitable in evaluating the bearing capacity of driven and jack-in piles, particularly installing in the cohesive soil using the SPT blows.

Behavior of pile embedded in different soil types under the effect of earthquake

In the seismic areas, piles are often passed through shallow loose or soft soil deposits and rest on end bearing soils. Damages due to earthquakes in piles built in layered soils occurred close to interfaces separating layers with very different shear moduli. In this study 3D finite element analyses are performed to simulate a soil-pile system under earthquake excitation in different soil types. As an example, the soil profiles of active seismic zones in Iraq will be used. For this a data base was prepared for static and dynamic parameters of different Iraq soils which were gathered and typical profiles for north, middle and south of Iraq were established. The properties of soils were then used as input dynamic data for finite element program PLAXIS 3D 2013 to study the response of single pile foundation. As earthquake input data the recording one obtained from the April 20, 2012 earthquake which hit Ali Al-Gharbi in Missan Province in Iraq was used, because this was one of the influential earthquakes. As a result the maximum bending moment occurs at the influence of different soil layers, also according to the soil-pile interaction, the deflected shape have the same behavior of pile embedded in sand and clayey soil as well as the pile that embedded in two clayey soil layers with thin sandy layers in between.

A numerical parametric study on the efficiency of prestressed geogrid reinforced soil

Prestressing geosynthetics offers a rapid and safe method of improving the poor ground conditions. This paper aims to find out the effect of prestressing the geogrid layer on load bearing and settlement performance. This study also takes into account the impact of the size, depth of placement and the adjacency of footing, for unreinforced (UR), geogrid reinforced (GR) and prestressed geogrid reinforced (PGR) soil on the load-bearing and the settlement characteristics by using the finite element program Plaxis 3D. Based on numerical simulation, it appears that PGR soil can enhance the bearing pressure of the UR soil by almost 500% and reduced the settlement by nearly 88 % by reducing the energy consumption. The footing placed at higher depths for PGR soil gives better performance as compared to GR soil. Moreover, placing two adjacent square footing increases the interference zone of PGR soil by 67% as compared to UR soil. This method can be instrumental in reducing the total input energy requirement to achieve a certain settlement during placement of shallow foundation for various important structures while being economic simultaneously.

Finite Element Analysis of the Geogrid-Pile Foundation System under Earthquake Loading

The finite element method is one of the important methods in analyzing geotechnical engineering problems; its main advantage is the ability to apply for the materials exhibiting non-linear stress-strain behavior. In this study the finite element program PLAXIS 3D 2013 is used to study the behavior of the piles under the influence of seismic waves in saturated sandy soil and the effect of adding geogrid with the pile foundation. The program has been used to facilitate the representation of the real model, input the required soil parameters and implementation of seismic data. Seismic wave, the soil geometry and the pile dimensions were fixed in all models, while dimension and depth of the geogrid used were varied to study the influence of different depth and dimension in reducing the pile displacements and the pore water pressure of soil. The results show that The reduction in settlement ratio (the difference between settlement of pile without and with using geogrid to the settlement without using geogrid) for ( ×L/2), (L×L) and (2L×2L) are 10.6%, 17% and 21.3% respectively. And the settlement ratio for geogrid at depths 8.33% and 12.5% of pile length are 9.6% and 17% respectively.

Effect of soil–pile–structure interaction on seismic behaviour of RC building frames

Pile foundations are generally used to support important industrial buildings, especially located in coastal regions where the major portion is covered by loose soil deposits. They are commonly subjected to lateral dynamic loads mainly due to earthquakes, wind, machineries, and sea waves. Thus, proper analysis and design of pile foundations should be carried out considering soil–pile–structure interaction. In general, the effects of soil–structure interaction are ignored during seismic analysis and design of super-structures. But in reality, the soil-foundation system affects the structural behaviour detrimentally. The main aim of this study was to investigate the effects of soil–pile–structure interaction on the earthquake behaviour of reinforced concrete building frames. Equivalent static analysis was carried out using direct method of approach. A geotechnical finite element program PLAXIS 3D was used for this numerical investigation. Building types ranging from low-rise to high-rise (5 storeys, 10 storeys and 15 storeys), relative densities of sandy soil ranging from loose to dense (30%, 50% and 70%) and spacing of piles ranging from close to large (2D, 4D and 6D) were the parameters considered for this study. The results of pile lateral displacement, rocking of raft foundation, storey lateral displacement and inter-storey drift were obtained for pile supported structure and compared with rigid base structural response. It is observed that soil–pile–structure interaction influences in particular the high-rise structural behaviour significantly. The storey lateral displacements and inter-storey drift values are amplified because of increase in pile lateral displacement and rocking of raft foundation due to increase in number of storeys, reduction in relative density of soil and reduction in spacing of piles. The major effect of soil–pile–structure interaction on storey displacement and inter-storey drift is observed in lower and upper storey levels more particularly in the case of high-rise structures built on loose soil deposit with closely spaced pile foundations.

Excavation Analysis using Stress Path Dependent Soil Parameters

This paper presents numerical analyses of an excavation using stress path dependent soil parameters, where soil elements in a region of an excavation are represented by specific soil parameters that correspond to their specific stress paths. The performance of the M1 excavation pit in Berlin sand was selected as the analysed case. This excavation pit was supported by diaphragm-wall with a single row of pre-stressed anchors. The numerical analyses of the excavation were performed using finite element program PLAXIS 3D. Mohr-Coulomb model and Hardening Soil model were used as the soil constitutive models. The analyses were performed using two approaches, which are: (i) analysis using axial compression soil parameters, and (ii) analysis using stress path dependent soil parameters. A set of conversion ratios were employed to convert the general soil parameters (i.e. axial compression stress path) to the soil parameters of the other stress paths. These conversion ratios were obtained from an experimental program of true triaxial tests conducted on Bangka sand. The comparison of the field records and the analysis results were discussed. The results show that the stress path dependent approach produced better prediction of diaphragm-wall deformation compare to the general approach using axial compression soil parameters.

Lateral Response of Contiguous Pile Wall Subjected to Staged Excavation: Physical and Numerical Investigations

In this paper, the lateral response of a contiguous pile wall retaining system under staged excavation is investigated using physical modelling. The effect of pile stiffness on pile wall deformation is studied by changing the diameter of pile. Numerical simulation of the physical response of the pile wall is studied using the finite element program PLAXIS-3D. Results of the finite element (FE) analysis are in good agreement with the experimental results. Sensitivity analysis is carried out to evaluate the effect of unit weight and friction angle of sand on the lateral response of the pile wall. It is found that with increase in the unit weight of sand, the deformation of pile wall increases and with increase in the friction angle of sand, the deformation of pile wall reduces. Experiments were performed on the sand to evaluate the unit weight, friction angle, Young’s modulus and Poisson’s ratio of sand corresponding to the four different values of the relative density (i.e. Dr = 15, 35, 65 and 85%). The response of the pile wall under stage excavation with change in the relative density of sand is studied by performing FE analysis. It is found that with increase in the relative density of sand, the deformation of pile wall reduces. The interpretation of modulus of elasticity of sand from the modulus of subgrade reaction evaluated from the plate load test is also presented. Based on the results of the FE analysis, it is noted that the finite element method (FEM) can be used for the real-time simulation of the lateral response of the pile wall systems.

Stress-strain States Differences in Specimens during Triaxial Compression and Direct Shear Tests

This article compares the soil strength parameters obtained from data of triaxial compression and direct shear tests, both with dense samples of fine sand. Direct shear device and triaxial device have significant differences in stress – strain state, which is developed in the specimen during test. In order to evaluate the stress-strain state, the direct shear and triaxial tests were simulated with finite element program PLAXIS 3D.

3-D predictions of installation and group effect on driven piles in sands

The installation of individual piles driven in sands increases the soil density and the soil strength around the pile, providing generally a beneficial effect toward the pile capacity. The simultaneous effect of pile installation and pile group interaction is generally difficult to predict by the classical pile analysis procedures, although more recent advances in academic and commercial programs for numerical analyses have the potential for providing a better understanding of this complex interaction. In this paper, the simultaneous effects of pile installation and pile group are investigated by 3-D FE analysis, by taking into account the soil densification around an individual driven pile. The finite element program Plaxis 3D Foundation is used to model a documented case study of a small-scale piling group in sands, taking into consideration the initial state of the soil before installation and the soil improvement after driving. The revised and improved parameters of the sand after the installation are used in this paper to model a 9-pile group with different pile spacing, to account for the simultaneous effect of sand densification and pile group. The comparison of the experimental values with those simulated numerically shows reasonably good agreement.

Experimental and numerical analyses of circular footing on geogrid-reinforced granular fill underlain by soft clay

Experimental and numerical investigations into the bearing capacity of circular footing on geogrid-reinforced compacted granular fill layer overlying on natural clay deposit have been conducted in this study. A total of 8 field tests were carried out using circular model rigid footing with a diameter of 0.30 m. 3D numerical analyses were performed to simulate soil behavior using finite element program Plaxis 3D Foundation. The results from the FE analysis are in very good agreement with the experimental observations. It is shown that the degree of improvement depends on thickness of granular fill layer and properties and configuration of geogrid layers. Parameters of the experimental and numerical analyses include depth of first reinforcement, vertical spacing of reinforcement layers. The results indicate that the use of geogrid-reinforced granular fill layers over natural clay soils has considerable effects on the bearing capacity and significantly reduces the lateral displacement and vertical displacement of the footing.