Effective Stress Analysis Research Articles

Site effect determination at liquefiable sites

Soft surface sedimentary layers can considerably increase damages during earthquakes. This soil amplification can be computed with different analytical methods. At loose saturated sandy layers, where pore pressure increase and liquefaction are probable, the effective stress method is the appropriate tool to compute site response. Some examples of computations are presented herein for the site of the Paks NPP, showing the effects of pore pressure increase to shear stress, strain and surface acceleration. The peak ground accelerations (PGA) computed with different methods has also been compared for different bedrock acceleration levels and it is shown that the results diverge with growing acceleration levels. For this soil profile differences appear first between linear and nonlinear methods at about 0.1 g, then between linear and equivalent linear methods at about 0.15 g and at last, between nonlinear total and effective stress methods at about 0.3 g. But application of the effective stress analysis may be necessary at lower acceleration levels under unfavourable site conditions. To delineate these areas, a preliminary liquefaction hazard map of Hungary has been constructed.

Scaling of the piezoelectric response in ferroelectric nanostructures: An effective clamping stress model

It is well understood that due to substrate induced clamping the piezoresponse in thin ferroelectric films is lower than bulk. Recent studies on the effect of lateral scaling have shown that by creating nanostructures one can achieve high piezoelectric constants even in thin films. In this letter we present a simple analytical model, to describe the evolution of the out-of-plane piezoresponse as a function of the device size in 100-nm-thick PbZrxTi1−xO3 thin films. It is shown that by an effective clamping stress analysis, one can successfully map out the size dependence of the piezoelectric constants in thin ferroelectric films. The results of the model are also compared to the predictions made by finite element analysis.

Practical applications of a nonlinear approach to analysis of earthquake-induced liquefaction and deformation of earth structures

Seismic stability, liquefaction, and deformation of earth structures are critical issues in geotechnical earthquake engineering practice. At present, the equivalent linear approach is considered the ‘state of practice’ in common use. More recently, dynamic analyses incorporating nonlinear, effective-stress-based soil models have been used more frequently in engineering applications. This paper describes a bounding surface hypoplasticity model for sand [Wang ZL. Bounding surface hypoplasticity model for granular soils and its applications. PhD Dissertation for the University of California at Davis, U.M.I. Dissertation Information Service, Order No. 9110679; 1990; Wang ZL, Dafalias YF, Shen CK. Bounding surface hypoplasticity model for sand. ASCE, J Eng Mech 1990;116(5):983–1001; Wang ZL, Makdisi FI. Implementing a bounding surface hypoplasticity model for sand into the FLAC program. In: Proceedings of the international symposium on numerical modeling in geomechanics. Minnesota, USA; 1999. p. 483–90] incorporated into a two-dimensional finite difference analysis program [Itasca Consulting Group, Inc. FLAC (Fast Lagrangian Analysis of Continua), Version 4. Minneapolis, MN; 2000] to perform nonlinear, effective-stress analyses of soil structures. The soil properties needed to support such analyses are generally similar to those currently used for equivalent linear and approximate effective-stress analyses. The advantages of using a nonlinear approach are illustrated by comparison with results from the equivalent linear approach for a rockfill dam. The earthquake performance of a waterfront slope and an earth dam were evaluated to demonstrate the model's ability to simulate pore-pressure generation and liquefaction in cohesionless soils.

サンドコンパクション工法による改良地盤の地盤物性評価手法の研究 -(その2)3次元有効応力解析によるSCP改良地盤の地震時挙動-

サンドコンパクション工法による改良地盤の地盤物性評価手法の研究 -(その2)3次元有効応力解析によるSCP改良地盤の地震時挙動-

非排水有効応力解析によるドレーン工法改良地盤のモデル化に関する検討(その3)ドレーン材の剛性の影響

非排水有効応力解析によるドレーン工法改良地盤のモデル化に関する検討(その3)ドレーン材の剛性の影響

Morphology and geotechnique of active-layer detachment failures in discontinuous and continuous permafrost, northern Canada

Fifty active-layer detachment failures triggered after forest fire in the discontinuous permafrost zone (central Mackenzie Valley, 65° N.) are compared to several hundred others caused by summer meteorological triggers in continuous permafrost (Fosheim Peninsula, Ellesmere Island, 80°N). Most failures fall into compact or elongated morphological categories. The compact type occur next to stream channels and have little internal disturbance of the displaced block, whereas the elongated types can develop on any part of the slope and exhibit greater internal deformation. Frequency distributions of length-to-width and length-to-depth ratios are similar at all sites. Positive pore pressures, expected theoretically, were measured in the field at the base of the thawing layer. Effective stress analysis could predict the instability of slopes in both areas, providing cohesion across the thaw plane was set to zero and/or residual strength parameters were employed. The location of the shear planes or zones in relation to the permafrost table and the degree of post-failure secondary movements (including headwall recession and thermokarst development within the failure track) differed between the localities, reflecting dissimilarity in the environmental triggers and in the degree of ground thermal disturbance.

CHARACTERISTICS OF GROUND MOTION IN LIQUEFIED DEPOSITS DURING EARTHQUAKES

Two well-documented case histories on liquefaction of reclaimed fills during the 1995 Kobe earthquake are discussed in this paper. The two sites, Vertical Array site (VA-site) and Packing House site (PH-site) are in proximity to each other and have practically identical stratification with an 18 m thick fill layer overlying an alluvial clay layer. At the VA-site, no ground improvement had been implemented and the reclaimed deposit of gravelly sand is loose with a low SPT blow count of about 5 to 10. The fill layer at the PH-site, on the other hand, had been densified prior to the earthquake by means of the rod compaction technique resulting in an increase in the SPT resistance to approximately 20 to 30 blow counts. To investigate the effects of ground treatment and evaluate the difference in the ground response between the densified fills and undensified loose fills during the Kobe earthquake, detailed studies were conducted for the two sites including field investigations, laboratory tests and effective stress analyses. It was found that both the extent of liquefaction and ground deformation were significantly different at the two sites. The undensified loose fills at the VA-site completely liquefied below the water table resulting in maximum shear strains of about 4% and settlement of the ground of about 25-30 cm. On the other hand, the analyses reveal that only the deep part of the densified fills at the PH-site liquefied where the maximum shear strains are estimated to have reached 2% and settlement of the ground was about 8 cm. Thus, the densification of the fills was found to be effective in preventing liquefaction from developing at shallow depths of the deposit and in limiting the overall deformation and settlement of the ground.

Seismic Effective-Stress Analysis of Caisson Quay Walls: Application to Kobe

ABSTRACT Deformation-based seismic design of gravity quay-walls requires realistic computation of residual deformations. This article presents an effective-stress analysis method, which is based on an elasto-plastic constitutive model formulated into a finite-difference algorithm. The model is applicable to cohesionless soils, for a wide range of relative densities and confining pressures. The formulation is applied first to re-analyze one of the failed caisson-type quay-walls of Rokko Island during the 1995 Kobe (Hyogoken-nambu) earthquake (Case 1). Subsequently, it is applied to analyse three closely related case studies of quay-walls, subjected to the same earthquake excitation, to demonstrate the effects of ground improvement on the wall performance. Case 2 considers a quay-wall in which both the foundation and backfill consist of improved, non-liquefiable soils. Case 3 considers a quay-wall in which the backfill soil remains liquefiable, whereas the foundation soil has been improved. Finally, in Case 4 the foundation soil is liquefiable, and the backfill soil improved. The results are consistent with both field observations and earlier independent computer simulations by Iai et al. 1998 which were based on the finite-element method and a different constitutive model.

525 レベルセットX-FEMの破壊力学への適用(S03-3 有限要素法/メッシュフリー法の最近の動向,S03 有限要素法の最近の動向/メッシュフリー法)

The X-FEM in conjunction with the level set method can be used to simplify the modeling of continua containing several cracks and hence perform effective stress analyses related to fracture mechanics. This paper describes the two-dimensional stress analysis of structures containing cracks. Stress intensity factors having mixed modes coupled with mode-I and mode-II are evaluated by M_1-integral method. This method is examined in numerical examples of elastic analyses. The numerical results show that X-FEM is an effective method for performing stress analyses and evaluating stress intensity factors.

The origin of overpressure in the Carnarvon Basin, Western Australia: implications for pore pressure prediction

Porosity–effective stress analysis of 37 wells in the Carnarvon Basin, Australia, has identified 12 wells where the overpressure was generated by disequilibrium compaction and four wells where the overpressure was generated by fluid expansion. Disequilibrium compaction was identified as the dominant overpressure-generating mechanism in wells along the Rankin Trend as far south as Gorgon 1 and Spar 1. Fluid expansion was identified as the dominant mechanism of overpressure generation in wells along the Barrow Trend and around the Alpha Arch. Disequilibrium compaction-generated overpressures occur, as would be expected, where the Tertiary sediment thickness is greatest and fluid expansion overpressures where the Tertiary is thinnest. Indeed, where the N-1 (35 Ma) reflector is greater than c . 1500 m below seabed, disequilibrium compaction overpressures are observed and where itis shallower than c . 1500 m, fluid expansion overpressures are observed. Acoustic log-based pore pressure detection using Eaton's (1975) method yielded accurate estimates with an exponent of three in Wilcox 1 where the overpressure was generated by disequilibrium compaction, and an exponent of six in Bambra 1 where the overpressure was generated by fluid expansion. If the Eaton (1975) technique is to be applied to seismic processing velocities for pore pressure prediction, then an exponent of three can be used where the N-1 horizon is greater than c . 1500 m and an exponent of six where the N-1 horizon is less than c . 1500 m.

二次元有効応力解析法の解析精度向上に関する検討

The influences of many factors on the accuracy of the damage prediction have been evaluated for a two-dimensional effective stress analysis program code: FLIP. Although many improved numerical models for more precise prediction have been proposed, the influences of those improved models varied with the difference of structural type and situation. This study was carried out to verify th accuracy ofthe seismic damage estimation for various types of quay wall on the liquefiable ground by the FLIP code with all these improved models simultaneously.Through the seismic damage analyses of sheet pilequaywall, gravity quaywall and piled pier constructed on the liquefiable ground, it was observed that the improved models work well not only for the structure in which the conventional model had worked well, but also for the structure in which the conventional model cannot work.

Two-dimensional Thermal Stress Analysis using X-FEM

Conventional FEM requires that the boundary of the element just matches the crack line to perform stress analyses of structures containing cracks in the field of fracture mechanics. These modeling processes are rather complicated and cannot be performed easily even if the automatic mesh generation technique is utilized. Recently, Belytschko et al. proposed the extended finite element method (X-FEM) based on the concept of partition of unity, and applied this method to the evaluation of the stress intensity factors and perfoming the crack extension simulation. The X-FEM can be used to simplify the modeling of continua containing several cracks and hence perform effective stress analyses related to fracture mechanics. This paper describes the two-dimensional thermal stress analysis of structures containing cracks. Stress intensity factors having mixed modes coupled with mode-I and mode-II are evaluated by M1-integral method in conjunction with X-FEM. This method is examined in numerical examples of elastic and thermo elastic analyses. The numerical results show that X-FEM is an effective method for performing thermal stress analyses and evaluating stress intensity factors.

Effective Utilization and Cost Reduction of Temporary Cutoff Walls as Preventive Measures against Liquefaction

In this paper, we discussed the effectiveness and cost reduction of using temporary cutoff walls as preventive measures against liquefaction. The U-type retaining walls studied here are based on the liquefiable soil strata. We carried out a comparative study for the effectiveness and cost of some preventive measures against liquefaction. We concluded that making use of cutoff walls is most effective in terms of preventive effect and cost reduction. Effective stress analyses revealed that the stresses of cutoff walls were almost within the allowable stresses under seismic loading. We found that the rigidity between cutoff walls and underground structure plays an important role for the safety of cutoff walls. A quantitative estimation method was verified to be valid for the principle of this countermeasure that “The liquefiable soil just below the underground structure is made as a closed space”. As the result, it is concluded that the practical design method of cutoff walls against liquefaction is shown here based upon the results calculated by a beam-spring model taking the decrease in rigidity of non-liquefiable layer, liquefied soil-water pressure acting against the wall, and so on, into consideration.

SEISMIC DESIGN OF PORT STRUCTURES

As a result of the Kobe disaster in 1995, new analysis methods including the effective-stress analysis for quay walls and pushover analysis for pile-supported wharves are introduced in the technical standards for port and harbour facilities in Japan to assess seismic performance of structures beyond the limit of force balance. In this article, design practice of port structures in Japan is described, with focuses on these new analysis methods. Some of the problems to be solved are also briefly described.

Backstop geometry and accretionary mechanics of the Sunda margin

The convergent Sunda margin off Indonesia displays all geological features characteristic of an accretion‐dominated subduction zone. A combined interpretation of prestack depth‐migrated seismic reflection data and velocity information gained from refraction studies is supplemented by high‐resolution bathymetric data and for the first time allows the exact mapping of backstop regimes. Initially, the outer high evolved as material was pushed against a static rigid arc framework backstop underlying a forearc basin. Increasing material strength of the outer high due to lithification formed a dynamic backstop, which controls accretion today. An out‐of‐sequence thrust marks the transition from the recent active frontal accretionary prism to the outer high and may be traced in the seismic and bathymetric data over the whole extent of the study area. The existence of a static as well as a dynamic backstop controls the forearc geometry and is associated with the segmentation of the forearc, which is observed in regimes of frontal as well as of oblique subduction. Mass balance calculations, which account for porosity changes and metamorphism, indicate a subduction history dominated by accretionary processes since the late Eocene. Accretion is associated with the low values of basal friction inferred for the Sunda margin. Structural investigations of conjugate fault planes indicate a very weak basal detachment. Effective stress analyses reveal that intrinsically weak material causes the high strength ratio of the detachment to the overlying sediments, whereas overpressuring within the frontal accretionary prism is negligible.

Geometrically nonlinear stress–strain behavior of hyperelastic solid foams

The present study is concerned with an effective stress analysis of cellular solids in the finite strain regime. The homogenization of the microstructure is performed by means of a strain energy based RVE-procedure which assumes macroscopic equivalence of a representative volume element for the given microstructure and a similar volume element consisting of the effective medium, if the average strain energy density in both volume elements is equal provided that the deformation gradient with respect to both elements is equal in a volume average sense. Disordered microstructures are considered by means of a randomized periodic model in conjunction with a stochastic approach. The model is applied to an analysis of the effective stress–strain behavior of two-dimensional model foams with periodic and disordered microstructure. Special interest is directed to effects of the geometric nonlinearity.

Correlation between local site effects and structural damage distributions of Kobe earthquake

This study aims to investigate the local concentration characteristics of ground responses due to nonlinear wave propagation in the Kobe earthquake (1995 Hyogoken nanbu earthquake) and the correlation between analytical distributions of maximum responses and actual responses of structural damage. Eight cross sections of Kobe ground are chosen as the surface layer models and the 2D-FE effective stress analysis program which is available for unbounded grounds and also applicable to liquefaction is used. To investigate the nonlinear effects of acceleration and strain responses of surface layers, waves with variable maximum acceleration amplitudes are input at the base rock surface. Results show that actual damage distributions of buildings and buried pipelines along the selected cross sections mostly agree well with those of response accelerations and strains near the ground surface, respectively.

Modeling uncertainty in stability analysis for design of embankment dams on difficult foundations

A probabilistic 3-D slope stability analysis model (PTDSSAM) is developed to evaluate the stability of embankment dams and their foundations under conditions of staged construction taking into consideration uncertainty, spatial variabilities and correlations of shear strength parameters, as well as the uncertainties in pore water pressure. The model has the following capabilities: (1) conducting undrained shear strength analysis (USA) and effective stress analysis (ESA) slope stability analysis of staged construction, (2) incorporation of field monitored data of pore water pressure, and (3) incorporation of increase of undrained shear strength with depth, effective stress, and pore water pressure dissipation. The PTDSSAM model is incorporated in a computer program that can analyze slopes located in multilayered deposits, considering the total slope width. The main outputs of the program are the geometric parameters of the most critical sliding surface (i.e., center of rotation/radius of rotation and critical width of failure), mean 2-D safety factor, mean 3-D safety factor, squared coefficient of variation of resisting moment, and the probability of slope failure. The program is applied to a case study, Karameh dam in Jordan. Monitored data of induced pore water pressure in the dam embankment and soft foundation were gathered during dam construction. The stability of Karameh dam embankment and foundation was evaluated during staged construction using deterministic and probabilistic analysis. Foundation stability was evaluated based on the monitored data of pore water pressure. The study showed that the mean values of the corrective factors which account for the discrepancies between the in situ and laboratory-measured values of soil properties and for the modeling errors have significant influence on the 2-D safety factor, 3-D safety factor, slope probability of failure, and on the expected failure width. The degree of spatial correlation associated with shear strength parameters within a soil deposit also influences the probability of slope failure and the expected failure width. This correlation is quantified by scale of fluctuation. It is found that a larger scale of fluctuation gives an increase in the probability of slope failure and a reduction in the critical failure width.

The Origin of Overpressure in the Cooper Basin, Australia

The most commonly cited mechanisms of overpressure generation are burial-driven disequilibrium compaction and hydrocarbon generation. However, it is unlikely that these mechanisms generated the overpressure in the Cooper Basin. The last significant burial event in the Cooper Basin was the Cretaceous deposition of the Winton Formation (~90 Ma). Maximum temperature was attained in the Cretaceous, with cooling beginning prior to 75 Ma. Hence, overpressure related to rapid burial or paleo-maximum temperatures (e.g. hydrocarbon generation) must be at least 75 million years old. To hold overpressure in sedimentary basins over this time scale requires average pressure seal permeabilities at or below the lowest published shale permeability measurements. Moreover, the Cooper succession has cooled since the Late Cretaceous, and cooling is an underpressure generating mechanism. Therefore, it is unlikely that burial or temperature driven processes generated the overpressure witnessed in the Cooper Basin. The Cooper Basin has been subjected to an increase in horizontal compressive stress during the Tertiary, and thus an increase in mean stress. The presence of polygonal faulting the Late Cretaceous Eromanga sequence enabled the paleo-stress to be estimated. Layer bound polygonal faulting occurs in a normal fault regime with low differential stress (sv > sH = sh). The contemporary stress regime in the Cooper Basin, measured using the density log, well tests and wellbore deformation modeling, is on the boundary between reverse and strike-slip (sH > sh = sv). The largest measured overpressure in the Cooper Basin is approximately 14.5 MPa in excess of hydrostatic pressure at 3780 m in the Kirby 1 borehole. Mean stress has increased by 46 MPa at 3780 m in the Cooper Basin since the Late Cretaceous. The maximum increase in pressure that can be generated via disequilibrium compaction is equal to the increase in applied stress. Therefore, the increase in mean stress in the Cooper Basin is sufficient to explain the magnitude of the observed overpressure. Additionally, velocity / effective stress analyses on 9 wells indicate that the overpressure was generated by disequilibrium compaction related to increases in mean stress.

Generalised Effective Stress Analysis of Strength and Small Strains Behaviour of a Silty Sand, from Dry to Saturated State

Very small to large strains properties of a silty sand were measured at several water contents ranging from a few percents to saturation, using three triaxial devices with different types of transducers, In the analysis of the data, attention was focused on the influence of water content on the maximum (Young's) modulus, the decay of modulus with strain, the effect of the confining stress, etc. Isotropic and triaxial tests, with measurement of negative pore water pressure, were performed on the same material. The effective sterss concept, validated by a recent theoretical analysis by Coussy and Dangla (2002), was used to interpret the results. The predictions of a micromechanical model are compared with the experimental data in the elastic domain and on the failure criterion. The method highlights a simple way to relate the strength and stiffness of the soil to its negative pore water pressure.