Total Stress Approach Research Articles

Investigation of trench effect on fatigue response of steel catenary risers using an effective stress analysis

The design of Steel Catenary Risers (SCR) heavily relies on the interaction between the riser and seabed in the touchdown zone (TDZ). To accurately estimate the SCR-seabed stiffness, the soil behaviour must be assessed in two main phases: (1) during cyclic SCR motions, which generate excess pore pressure, leading to soil softening and remoulding in the TDZ, and (2) during inactivity periods throughout the SCR’s lifespan, causing dissipation of excess pore pressure associated with the consolidation state. The latter was neglected by the existing non-linear hysteretic soil models, which are based on the total stress approach. In the current study, a global riser analysis was conducted and the consolidation effect was incorporated using an effective stress framework. Long-term soil stiffness was determined by capturing both the remoulding and consolidation effects, which are respectively associated with the damage accumulation during the SCR cyclic motions and soil strength recovery during the intervening pause period. The constructed model was then combined with a new methodology named Hybrid Trench Model to investigate how the consolidation effect contributes to the fatigue performance in different trench depths. Stochastic fatigue analysis showed that the consolidation effect might increase the damage by around 10%-40% over the long-term assessment.

Effect of Soil Damping on the Soil–Pile–Structure Interaction Analyses in Cohesionless Soils

Pile foundations in earthquake-prone regions must be analyzed and designed considering the dynamic loads. In the fully nonlinear dynamic analyses, the soil nonlinearity can be considered using the modulus degradation curves in the total-stress approach, and the soil damping is controlled by the unloading/reloading rule. Several researchers have investigated the effect of soil damping on the free-field soil response analyses, but the effect on the soil–pile–structure system response has not been studied thoroughly. In this study, the nonlinear elastic method (hyperbolic model) and the elastoplastic Mohr–Coulomb (MC) models were implemented to investigate the effect of soil damping on the pile and structure response. Dynamic soil–pile–structure interaction analyses were performed by simulating two different centrifuge tests published in the literature, and the analysis results were compared with the centrifuge test results. The analyses with the low-intensity input motions showed that the superstructure accelerations and the bending moments in the single pile were estimated with reasonable accuracy. However, the superstructure accelerations might be underestimated under high-intensity motions, especially in the MC model. Additional analyses were performed under six earthquake records. The constitutive models (MC and hyperbolic) may significantly vary the maximum structural acceleration and pile maximum moments (up to 50%). As a result, the responses of the superstructure and the pile in soil–pile–structure interaction problems are highly dependent on the soil constitutive model that must take the damping into account accurately; in turn, due account must be given to the selection of the constitutive model.

Analytical and numerical methods for prediction of the bearing capacity of shallow foundations in unsaturated soils

Bearing capacity of saturated soils can be estimated using effective or total stress approaches extending the concepts proposed by Terzaghi (1943) and Skempton (1948), respectively. Recent studies have shown that similar approaches (i.e., Modified Effective Stress Approach, MESA and Modified Total Stress Approach, MTSA) can be used for interpretation and prediction of the bearing capacity of unsaturated soils by considering the influence of matric suction. However, comprehensive discussion for the application of the MESA and the MTSA in geotechnical engineering practice applications is lacking in the literature. For this reason, in this state-of-the-art paper, the background associated with the MESA and MTSA is first introduced. The analytical and numerical methods available for the prediction of the bearing capacity of unsaturated soils from the literature are revisited. The various available methods are explained by categorizing them into two groups: MESA and MTSA along with their applications using examples. The focus of this state-of-the-art paper is directed towards not only for providing tools for rational understanding but also for better prediction of the bearing capacity of unsaturated soils for extending them in geotechnical engineering practice applications.

On the Influence of Thermal History on the Martensitic Transformation in Ti-24Nb-4Zr-8Sn (wt%)

Metastable β titanium alloys, such as Ti-24Nb-4Zr-8Sn, have potential for application in vibration damping systems due to their ability to display superelastic behaviour. However, their use is currently limited due to large variations in the temperature range over which this behaviour is stable, which can additionally be shown to depend on the thermal history of the sample. This study demonstrates the sensitivity of the microstructure to thermal history and highlights a possible cause for this variability. Currently held theories of ω formation supressing the martensitic transformation have been called into question and an alternative mechanism based on a total stress approach has been suggested. Understanding this variability enables better design of alloys and processing routes in order to achieve materials with the desired properties required for industrial application.

Characterization of Ground Response and Liquefaction for Kathmandu City Based on 2015 Earthquake Using Total Stress and Effective Stress Approach

This chapter presents the seismic vulnerability of Kathmandu City (Nepal), based on Nepal 2015 earthquake, in terms of the ground response and liquefaction potential. The spatially well-distributed 10-boreholes and ground motions of Mw 7.8 Nepal 2015 earthquake recorded at five different stations were adopted for the analysis. The range of peak ground acceleration and peak spectral acceleration were in the order of 0.21g-0.42g and 0.74g-1.50g, respectively. Liquefaction potential of the sites were computed using both semi-empirical approach and liquefaction potential index (LPI). LPI shows that the 6 sites out of 10 sites are at high risk of liquefaction.

Model Tests on the Adhesion Factor of Full-Scale Polyethylene-Coated Pipes in Soft and Very Soft Clays

AbstractIn subsea pipeline engineering practice, the total stress or alpha approach, which assumes that the axial resistance to pipeline walking in fine-grained soils such as soft and very soft cla...

Parametric Study of One-Dimensional Seismic Site Response Analyses Based on Local Soil Condition of Jakarta

Seismic site response analysis is used to estimate the response of soil deposits during seismic loading at any depth of interest and to interpret time histories as well as response spectra. This type of analysis involves many parameters that can affect the character of ground shaking. It is important to know the effect of these parameters in order to perform reliable seismic hazard evaluation at a site. This paper presents the effects of several parameters toward the characteristics of surface response spectra based on the local soil conditions of Jakarta using a one-dimensional (1-D) site response model with total stress approach. A parametric study was performed on two cohesive soil deposit profiles with a different site class, namely medium clay (site SD) and soft clay (site SE). The bedrock layers of both profiles were located at a depth of 300 m. In this study, the analytical methods implemented were the equivalent-linear method and the non-linear method. Several different dynamics soil models were also implemented. In addition, variation of property parameters, such as depth of bedrock, shear wave velocity of bedrock, layer thickness, etc., were studied. The results of this study indicate that all of the studied parameters have a significant effect on the response spectra at the ground surface.

Modeling the stress versus settlement behavior of shallow foundations in unsaturated cohesive soils extending the modified total stress approach

The mechanical behavior of unsaturated soils can be interpreted using either modified total stress or a modified effective stress approach depending on the type of soils and various scenarios of drainage conditions of pore-water and pore-air. Recent studies suggest that the bearing capacity of unsaturated cohesive soils can be more reliably estimated using the modified total stress approach (MTSA) rather than the modified effective stress approach (MESA). In the present study, a modeling technique (extending Finite Element Analysis, FEA) is proposed to estimate the bearing capacity of shallow foundations in unsaturated cohesive soils by simulating the vertical stress versus surface settlement behaviors of shallow foundations extending the MTSA. The proposed technique is verified with the model footing test results in unsaturated cohesive soils. Commercial finite element software, SIGMA/W (GeoStudio 2012, Geo-Slope Int. Ltd.) is used for this study. Details of estimating the unsaturated soil parameters (i.e. total cohesion, modulus of elasticity and Poisson’s ratio) required for the FEA are also presented taking account of the influence of matric suction. Good agreements were observed between the measured bearing capacity values and those from the FEA extending the MTSA.

A comparative study between simplified and nonlinear dynamic methods for estimating liquefaction potential

This paper estimated the liquefaction potential of a saturated soil deposit subjected to a horizontal seismic excitation at its base using the total stress approach. A comparative analysis between the simplified and the nonlinear dynamic methods was used to verify to what extent the simplified method could be reliable. In order to generalise the reliability of the simplified method for any value of the maximum acceleration for the used earthquakes, a correction for the maximum acceleration less than 0.3g was proposed based on the comparison of safety factor values determined by the dynamic method illustrated by the equivalent linear model with lumped masses and the simplified method for a given profile of soil subjected to 38 earthquakes. The nonlinear behaviour of soil was represented by two hyperbolic models: Hardin and Drnevich, and Masing. To determine the cyclic resistance ratio (CRR), the cone penetration test (CPT) based method, the standard penetration test (SPT) based method, and the shear wave velocity based method were used. The safety factor was calculated as the ratio of CRR/CSR, where CSR represents the cyclic stress ratio. The results of the proposed correction have given smaller values of the safety factor compared to the nonlinear dynamic methods for the maximum acceleration less than 0.3g. In other words, by considering this correction, the most unfavourable case is always given by the modified simplified method.

Applicability of bed load transport models for mixed‐size sediments in steep streams considering macro‐roughness

AbstractIn steep mountain streams, macro‐roughness elements typically increase both flow energy dissipation and the threshold of motion compared to lower‐gradient channels, reducing the part of the flow energy available for bed load transport. Bed load transport models typically take account of these effects either by reducing the acting bed shear stress or by increasing the critical parameters for particle entrainment. Here we evaluate bed load transport models for mixed‐size sediments and models based on a median grain size using a large field data set of fractional bed load transport rates. We derive reference shear stresses and bed load transport relations based on both the total boundary shear stress and a reduced (or “effective”) shear stress that accounts for flow resistance due to macro‐roughness. When reference shear stresses are derived from the total boundary shear stress, they are closely related to channel slope, but when they are derived from the effective shear stress, they are almost invariant with channel slope. The performance of bed load transport models is generally comparable when using the total shear stress and a channel slope‐related reference shear stress, or when using the effective shear stress and a constant reference shear stress. However, dimensionless bed load transport relations are significantly steeper for the total stress approach, whereas they are similar to the commonly used fractional Wilcock and Crowe (WC) transport model for the effective stress approach. This similarity in the relations allows the WC model, developed for lower‐gradient streams, to be used in combination with an effective shear stress approach, in steep mountain streams.

Resilient modulus modelling of unsaturated subgrade soils: laboratory investigation of silty sand subgrade

In flexible pavement structures, stiffness of unbound granular layers and subgrade soil significantly contribute to the overall performance of the pavement system. The stiffness of pavement unbound materials is widely characterised by the resilient modulus, Mr, which is obtained from repeated load triaxial (RLT) tests. Although pavement unbound materials are usually in partially saturated conditions and experience seasonal moisture content and therefore suction variations in the field, their stiffness is conventionally characterised using the total stress approach in which the effect of soil suction is not taken into account. Thus, an enhanced approach in predicting the stiffness of subgrade soils has to account for the partially saturated conditions and incorporate soil suction (i.e. matric suction) in the Mr constitutive models. In this study, several Mr prediction models that take into account the effect of pore suction were investigated. The Mr data set from an experimental investigation of two silty sand subgrade soils that was conducted using a suction-controlled RLT testing apparatus was used to optimise the parameter of these models. The capability of the Mr predictive models in capturing the resilient modulus behaviour of the silty sand subgrade soils and its variation due to seasonal changes in the moisture content (soil suction) were evaluated. It was observed that the Mr models that combine the three fundamental stress state variables (confining stress, deviator stress and matric suction) performed better in capturing the resilient modulus behaviour of the subgrade materials.

Modelling resilient modulus seasonal variation of silty sand subgrade soils with matric suction control

The resilient modulus of unbound materials is an important parameter in the mechanistic design of pavements. Although unbound layers are frequently in a partially saturated state, a total stress approach is conventionally used in modeling the material behaviour, and therefore pore pressure effects are not considered. In fine-grained unbound materials, the saturation state can affect their mechanical behaviour due to pore pressure effects. In this study a modified test procedure and a predictive resilient modulus model that takes into account the subgrade soil matric suction as a stress state variable is presented. Two different silty sand subgrade materials were tested in unsaturated conditions using a series of repeated load triaxial tests under controlled pore suction conditions to study its influence on the resilient modulus. The test data were further used to obtain the resilient modulus model regression parameters that account for moisture content variations through the matric suction parameter. Generally, the prediction model could effectively capture the resilient modulus behaviour of the subgrades with respect to changes in the normal stress state and the matric suction. Given the completeness of this method, this prediction model is recommended as an improved approach in capturing the moisture content effects on the material stiffness properties.

Closure to “Pile Setup in Cohesive Soil. I: Experimental Investigation” by Kam W. Ng, Matthew Roling, Sherif S. AbdelSalam, Muhannad T. Suleiman, and Sri Sritharan

Closure to “Pile Setup in Cohesive Soil. I: Experimental Investigation” by Kam W. Ng, Matthew Roling, Sherif S. AbdelSalam, Muhannad T. Suleiman, and Sri Sritharan

Undrained Response of Clay in Hollow Cylinder Expansion Tests

Abstract This paper presents the results of undrained plane strain expansion tests of hollow cylindrical specimens of normally consolidated clay. The stress-strain curves and the total stress paths are first deduced from the experimental radial pressure-expansion relationships by using Nadai’s total stress approach. The stress-strain curves are then compared with theoretical relations based on the modified Cam clay theory. The effective stress paths, which are obtained by subtracting measured pore water pressure from the total stress paths, are also compared with those derived from the modified Cam clay. Finally, the undrained shear strengths, calculated from both Nadai’s approach and the modified Cam clay, are compared with values determined by using an expression based upon an elastic perfectly plastic model.

Predictions of total deformations in Jebba main dam by finite element analysis technique

This paper examined the deformations of the Jebba Main Dam, Jebba Nigeria using the finite element method. The study also evaluated the predicted deformations and compared them with the actual deformations in the dam to identify possible causes of the observed longitudinal crack at the dam crest. The Jebba dam is a sloping earth core rockfill. The study methodology consists of the modeling of the dam with the utilization of the actual dam section, development of Finite Element mathematical formulations using plain strain conditions and total stress approach, and simulation of such model using a computer program developed in Visual Basic Language. The 3-node isoparametric triangular elements were used to analyze the model dam. The dam was simulated for end of construction (no water) and full reservoir loadings. The results of the study showed that the actual crest settlement as at 2001 was 184 mm and the predicted total settlements were 280 mm and 307 mm for the two models adopted. Generally, the deformations in the dam body were higher than deformations in the dam foundation. This may have resulted from the adequate foundation compaction during construction. The cracks development criteria were based on the criteria developed by Thomas. From these criteria, the dam was found to have developed cracks due to settlements and hydraulic fracturing. Journal of the Nigerian Association of Mathematical Physics Vol. 9 2005: pp. 479-492

Slope failure in underconsolidated soft soils during the development of a port in Tianjin, China. Part 2: Analytical study

This paper presents the analytical study of a major landslide that occurred in a newly dredged slope during port development in the city of Tianjin, northern China. The slope comprised mainly underconsolidated and submerged soft soils. The landslide consisted of a number of individual slides that occurred sequentially and extended retrogressively and laterally into the reclaimed land. The paper proposes a slope stability assessment methodology for the examination of such slope failures, with retrogressive and lateral extension into reclaimed land. Both the total and the effective stress approaches are adopted in the slope stability assessment. Eight different sets of soil shear strength parameters are used in the assessment. They were determined by using different methods in the laboratory and in the field or by back analysis. The assessment results indicate that the total stress approach, together with undrained vane strength values, gives the best result in the stability assessment of the original dredged slope and the subsequent failed slopes. The paper further examines the factors that might have caused or triggered the occurrence of the landslide. Backfilling of a thick general soil layer and a sand cushion for vacuum preloading of the hydraulic fill in the reclaimed land might have rendered the entire slope marginally stable. Lowering of the tidal level might also have had an effect in triggering the landslide. The engineering approach and results presented in this study could be useful in the design and construction of dredged slopes in underconsolidated soft soils.Key words: land reclamation, dredge excavation, slope stability assessment, landslide investigation, marine mud, case studies.

Sustainability Evaluation Frameworks and Alternative Analytical Scenarios of National Economies

In this article a macro-oriented conceptual framework of the conditions necessary for the advancement of sustainability within an economy and society is formulated and analysed. The theoretical framework presented is a set of logical identities, which define relationships between the total environmental stress (TES) and the basic indicators of economic, technological and social development. The framework, called the Total Environmental Stress Approach, provides necessary but not sufficient conditions for advancing sustainability. Scenario alternatives utilizing the TES approach with data from Finland are presented to evaluate the theoretical framework.

Stable Width and Depth of Gravel-Bed Rivers with Cohesive Banks

An analytical model is developed to determine the influence of the bank stability on the stable width and depth of alluvial gravel-bed rivers with cohesive banks. The formulation of the model is based on the assumption that the stable width corresponds with an optimum condition that is equivalent to the maximum bed load transporting capacity. The optimum condition develops when the channel banks are at their limiting stability with respect to either mass failure or fluvial erosion. Two basic channel types are identified: bank-height and bank-shear constrained. Mass failure stability is estimated using a simple total stress approach. A method for estimating the critical bank shear stress based on model calibration is proposed. Analysis of field data indicates that the effect of the bank vegetation on bank stability can be expressed in terms of the critical bank shear stress. The average critical bank shear stress value calculated for riverbanks covered by vegetation with well-developed root networks was fo...

Triaxial Testing of Unsaturated Agricultural Soils

In this paper, triaxial testing procedures and equipment as they have been applied to unsaturated soils are surveyed. Modifications to conventional triaxial apparatus and procedures for testing unsaturated soils based on principles of “unsaturated soil mechanics” are described in some detail. The modifications accommodate the independent measurement or control of pore-air and pore-water pressures and the resolution of both air and water components of volume change. This makes it possible to study the influence of soil suction on soil strength and volume change behaviour. To illustrate this approach, an agricultural soil was tested in a modified triaxial apparatus using independent stress state variables to describe stresses in the specimen. Matric suction, mean net stress and deviatoric stress were monitored or controlled in a series of constant water content tests. Volume change and shear strength behaviour were evaluated based on unsaturated soil mechanics principles. For low stress regimes, matric suction remained essentially constant under constant water content testing; however, as loading was increased matric suction varied significantly. The relevance of using matric suction as an independent stress state variable in fundamental studies of agricultural soil behaviour, as opposed to a total stress approach, is discussed.

Application of total stress approach to open hull strength prediction

The task of the package SANKO is to make forecast of the levels of cyclic total stresses and hatch deformations due to the action of the sea waves on longitudinal hull structure during the ship life. The current version of the package enables also an assessment of fatigue strength. The ship is regarded as a rigid body with certain hydrostatic and hydrodynamic characteristics. The stresses due to hydrostatic, hydrodynamic and inertial forces are determined in the longitudinal structures using a suitable structural model, which in the present case is the thin-wall beam. The functional relationship between ship response and encountered wave characteristics (transfer function) is therefore established. Knowing encounter probability of each sea state the response values relevant to each probability level can be calculated. This procedure leads to obtain groups of cumulative distribution values known as long-term forecast.