Description:
Commercial software is used widely in slope stability analyses of reinforced embankments. Almost all of these programs consider the tensile strength of geotextiles and soil-geotextile interface friction. However, currently available commercial software generally does not consider the drainage function of nonwoven geotextile reinforcement. In this paper, a reinforced channel embankment reinforced by a nonwoven geotextile is analyzed using two methods. The first method only considers the tensile strength and soil-geotextile interface friction. The second method also considers the drainage function. In both cases, the reinforced embankment is modeled in rapid drawdown condition since this is one of the most important conditions with regard to stability of channel embankments. It is shown that for this type of application, modeling a nonwoven geotextile reinforced embankment using commercial software which neglects the drainage function of the geotextile may be unrealistic.
Description:
The paper considers two techniques to model the Cone Penetration Test (CPT) end resistance, qc in a dense sand deposit using commercial finite element programmes. In the first approach, Plaxis was used to perform spherical cavity expansion analyses at multiple depths. Two soil models, namely; the Mohr-Coulomb (MC) and Hardening Soil (HS) models were utilized. When calibrated using simple laboratory element tests, the HS model was found to provide good estimates of qc. However, at shallow depths, where the over-consolidation ratio of the sand was highest, the relatively large horizontal stresses developed prevented the full development of the failure zone resulting in under-estimation of the qc value. The second approach involved direct simulation of cone penetration using a large-strain analysis implemented in Abaqus/Explicit. The Arbitrary Lagrangian Eulerian (ALE) technique was used to prevent excessive mesh deformation. Although the Druker-Prager soil model used was not as sophisticated as the HS model, excellent agreement was achieved between the predicted and measured qc profiles.
Description:
The paper considers two techniques to model the Cone Penetration Test (CPT) end resistance, qc in a dense sand deposit using commercial finite element programmes. In the first approach, Plaxis was used to perform spherical cavity expansion analyses at multiple depths. Two soil models, namely; the Mohr-Coulomb (MC) and Hardening Soil (HS) models were utilized. When calibrated using simple laboratory element tests, the HS model was found to provide good estimates of qc. However, at shallow depths, where the over-consolidation ratio of the sand was highest, the relatively large horizontal stresses developed prevented the full development of the failure zone resulting in under-estimation of the qc value. The second approach involved direct simulation of cone penetration using a large-strain analysis implemented in Abaqus/Explicit. The Arbitrary Lagrangian Eulerian (ALE) technique was used to prevent excessive mesh deformation. Although the Druker-Prager soil model used was not as sophisticated as the HS model, excellent agreement was achieved between the predicted and measured qc profiles.
Description:
In the congested urban areas, tunnelling close to existing structures often occurs due to the lack of space. Consequently, tunnelling-induced ground movements may cause serious damage to the adjacent structures. Ground surface settlement caused by tunnel excavation varies in magnitude and trend depending on several factors such as tunnel geometry, ground conditions, etc. There are several empirical, semi-empirical equations and numerical methods available to estimateground surface settlement associated with excavation activities, however among these methods most of empirical and semi-empirical based methods do not simultaneously take all the relevant factors in the complex condition into account. In complex conditions, numerical methods can be used for resulting accurate predictions. In this paper, ground movement in a highly populated region of Iran’s capital city, Tehran, induced by excavation and construction of 7th line of Tehran subway has been investigated. It was aimed to analyse the ground displacements and surface settlement which hasbeen induced by tunnelling using Earth Pressure Balance-Shield Tunnel Boring Machine. The analysis was performed by Finite difference Method (FDM) using FLAC3D FDM package. Results from performed numerical analysis show good agreement with obtained results from analytical and empirical methods. In addition, effect o f some important factors such as tunnel geometry and ground properties on ground surface settlement has been investigated. Obtained results indicate that ground surface settlement is more sensitive to tunnel geometry rather than height of tunnel placement.
Description:
A large portion of the Irish railway network contains earthworks that were poorly constructed mainly because they were built over 100 years ago when scientific understanding of soil behaviour was not nearly as advanced as it is today. Also, engineers involved in the construction of these structures had to work with often unsuitable local materials as sound bulk fill was not able to be transported significant distances. In order to improve the information obtained from site investigation of these poorly constructed earthworks a number of authors have recently suggested complimenting direct geotechnical investigations with a non-invasive geophysical assessment. In addition to providing a review of the factors which cause failure of earthworks, this paper also discusses the capabilities of three geophysical techniques, Ground Penetrating Radar (GPR), Electrical Resistivity Tomography (ERT) and Multichannel Analysis of Surface Waves (MASW) for assessing the stability of railway earthworks. Examples of the use of each technique are provided from forensic investigations into railway earthwork slope failures in Ireland.
Description:
This study describes experimental results achieved on the use of Grand Granulated Blast Furnace Slag (GGBFS) and Lime in stabilising desert silty sand for possible use in geotechnical engineering applications, especially for roadways and railways constructions. The GGBFS and lime were added in percentages of 5, 10 and 15% and 1, 3, and 5% respectively, by dry weight of sand. Different laboratory tests such as mechanical aggregation test, hydrometer analysis, liquid-plastic limit, pH value test, compaction, unconfined compressive strength (UCS), California bearing ratio test CBR , were performed on samples to understand the engineering characteristic of soil and influences of mixtures on the silty sand soil. The study results demonstrate significant improvements in unconfined compressive strength and California bearing ratio strength. Moreover the swelling behaviour of mixtures was decreased effectively. Thus mixture of GGBFS and lime can be suggested to improve engineering characteristic of desert silty sands.
Description:
This study describes experimental results achieved on the use of Grand Granulated Blast Furnace Slag (GGBFS) and Lime in stabilising desert silty sand for possible use in geotechnical engineering applications, especially for roadways and railways constructions. The GGBFS and lime were added in percentages of 5, 10 and 15% and 1, 3, and 5% respectively, by dry weight of sand. Different laboratory tests such as mechanical aggregation test, hydrometer analysis, liquid-plastic limit, pH value test, compaction, unconfined compressive strength (UCS), California bearing ratio test CBR , were performed on samples to understand the engineering characteristic of soil and influences of mixtures on the silty sand soil. The study results demonstrate significant improvements in unconfined compressive strength and California bearing ratio strength. Moreover the swelling behaviour of mixtures was decreased effectively. Thus mixture of GGBFS and lime can be suggested to improve engineering characteristic of desert silty sands.
Description:
This paper presents the results of full-scale load tests performed to investigate the end bearing pressure mobilised by continuous flight auger piles installed in sand. In particular, the tests considered the effects of the footing width and, by varying the load test procedure (from maintained load test to constant rate of penetration), allowed quantification of creep effects. By comparing the load test results with in situ test results from cone penetration tests, correlations between the end bearing pressure mobilised at normalised settlement levels of 10% of the footing width and the cone penetration test qc value were studied. For the maintained load tests, these correlations were found to be similar to those used in routine design practice. When creep effects were reduced using constant rate of penetration load testing, the end bearing pressure mobilised was significantly higher than that assumed in normal practice, and it was in keeping with the results of finite-element analyses performed using a soil model that ignored creep. In the final section, the field test results are compared to database pile load tests performed on non-displacement piles in sand.
Description:
A wide range of correlations have been proposed between cone penetration test end resistance, qc, and the ultimate end bearing pressure developed by bored piles. The qc value is typically related to the pile end bearing resistance at a normalised pile displacement equal to 10% of the pile diameter, qb0·1, through a correlation factor, α. While it is generally accepted that constant α factors can be applied for the design of displacement piles, a combination of field tests and finite-element analyses of bored piles in sand has resulted in a myriad of design approaches, some of which suggest that α varies with pile geometry, stress level and sand density. In this paper the results of finite-element analyses are presented which suggest that a constant α factor can be adopted for the design of deep foundations in sand.
Description:
An analysis of the effects that groundwater flow has on the thermal regime created by a ground source energy system is presented. The change in the development of the sub-surface thermal regime caused by a groundwater flow across a site, relative to a scenario where groundwater flow does not exist, is examined. Analysis is performed using bespoke finite-element formulations of both single- and multi-borehole systems. The results of this work show that even a modest groundwater flow across a site can lead to a significant change in the development of the sub-surface thermal regime.
Description:
An analysis of the effects that groundwater flow has on the thermal regime created by a ground source energy system is presented. The change in the development of the sub-surface thermal regime caused by a groundwater flow across a site, relative to a scenario where groundwater flow does not exist, is examined. Analysis is performed using bespoke finite-element formulations of both single- and multi-borehole systems. The results of this work show that even a modest groundwater flow across a site can lead to a significant change in the development of the sub-surface thermal regime.
Description:
The hydraulic behavior of Victorian brown coal has been studied under low pressure confining conditions. A triaxial constant head permeameter was constructed to carry out permeability tests under alternative confining conditions. Test results from alternative constant head test methods were found to be different indicating higher than expected internal storage due to the possible existence of gas in the coal matrix and/or compressibility of the coal grains. A conceptual model of brown coal with disconnected gas bubbles was proposed to explain the observed variation of hydraulic behavior. In this model, the compressible gas bubbles are assumed to be trapped between compressible coal particles. A numerical model was developed to gain insights into the hydraulic influence of these factors and the model was used to simulate the test process. Constant hydraulic conductivity was assumed. The results from the model and the tests are found to compare well. The model explains most but not all of the observed behavior.
Description:
Skempton's B parameter characterises the effect of fluid compressibility on the time dependent deformation of materials during isotropic loading. The Skempton B-parameter has been determined for a sample of Victorian brown coal for a range of confining pressures, corresponding to the range of conditions which the coal experiences below the ground water table. The relationship between the change in the B-parameter with respect to time and pressure has also been examined. It has been found that a high back pressure (more than 400 kPa) is required in the material to achieve a degree of saturation close to 100+ACU-. The set of data from these tests is able to provide important evidence of the depth and stress dependent behaviour of the coal.
Description:
Understanding all potential slope failure mechanisms is a pre-requisite for predicting the likelihood of batter movements during excavation in open cut mines. The tensile behavior of soils and rocks may be a significant contributor to a slope failure and must be known in order to quantify the risks of slope failure. The contribution can be particularly significant for Intermediate Geotechnical Materials (IGMs) that possess characteristics of both soils and rocks and where the failure mechanisms are complex due to the interplay between ductile and brittle behavior. Brown coal is such an intermediate Geotechnical Material. Recent batter movements in the brown coal mines in the Latrobe Valley, Australia have raised doubts about the current understanding of the mechanisms of slope failure in this material. Research is underway to re-evaluate all properties of the brown coal applicable to slope failure. This paper describes the investigation of brown coal tensile strength. There are alternative test methods available to determine the tensile behavior of materials, including direct tensile tests, beam bending tests and Brazilian compression tests. The applicability of each test method is material dependent, as such, it is necessary to confirm the validity of the methods for each material. Beam bending tests have achieved mixed results for both rocks and IGMs previously. Thus, the present work has explored only the use of Direct tensile and Brazilian test methods. Both methods were implemented using a modified direct shear apparatus and valid test procedures for both test methods were developed. Each test procedure has been verified by Finite Element Modelling (FEM) using ABAQUS 6.12.1 FEM code. The results from the laboratory test methods are in good agreement and show that brown coal is a predominantly brittle material with a peak tensile strength slightly greater than 100 kPa. The finite element analyses confirm that non-uniformity of the tensile stresses during sample loading tends to lead to the underestimation of tensile strength for both tests, but the Brazilian test has less bias for brown coal. It is observed that the rate of loading of low stiffness, low permeability, and saturated samples in the Brazilian test is an important test design parameter for the accurate determination of tensile strength of IGMs in the laboratory.
Description:
Skempton used the B-parameter (see Equation 1) to represent the degree of saturation of a material, such that, B → 0, when the degree of saturation = 0 and, B → 1, when the degree of saturation = 1 Δ Δ Δu B[ (Δ A+ )]σ3 1σ 3 (1) where, Δu is change in pore water pressure, A and B are pore-pressure coefficients, and σ3 and σ1 are minor and major principal stresses.
Description:
Analysis of the thermal regime created by a geothermal borehole heat exchanger is performed using a closed form radial heat flow equation, a geothermal borehole heat exchanger design tool, and a finite-element model. Climatic, heat exchanger construction, and building load data are entered into the heat exchanger design tool to create a theoretical model along with thermal parameters from a number of geological formations. Output data from the design tool model are used in conjunction with the closed form radial heat flow equation to calculate the predicted temperature with respect to time and distance from the heat exchanger for the modeled ground formations. The output data from the design tool are also used to create a number of finite-element method models against which the predictions calculated using the closed form radial heat flow equation can be compared. A good correlation between the temperatures predicted by the finite-element models and the closed form equation calculations is observed. However, when used within its recommended limiting conditions, the closed form equation is shown to slightly underestimate the temperature of the ground when compared with the finite-element model predictions. The limiting conditions associated with the closed form equation are discussed in the context of the output from the finite-element method models.