木元 小百合

In recent years, many disasters due to floods have occurred around the world. In some cases, the level of river water exceeded the estimated high-water level. It is necessary, therefore, to reconsider the evaluation method for river dike embankments during floods for the safety of the embankments. In this study, a deformation analysis is proposed that can simultaneously consider the unsaturated seepage flow and the overflow. This method is quite different from the conventional method, which considers the seepage flow and the stability separately. An air-soil-water coupled elasto-plastic finite element analysis has been developed to analyze the problem by incorporating unsaturated seepage characteristics. The deformation and the stability of river dike embankments during a flood are investigated. When we consider the overflowing of embankments, the deformation of the embankments becomes larger than the case for which only the seepage flow is considered. From the numerical analysis, it is found that the proposed hydro-mechanically coupled analysis method is an effective tool for estimating the stability of river embankments.

It is known that air can be trapped in some parts of embankments during heavy rain or overflow. In this case, air pressure, as well as water pressure, may change under partially drained conditions. However, most laboratory test programs have been conducted under constant air pressure conditions. In this paper, a numerical model for unsaturated soils based on the mixture theory and an elasto-viscoplastic constitutive model is presented. The collapse behavior, due to a decrease in suction, is expressed by the shrinkage of the overconsolidation boundary surface, the static yield surface, and the viscoplastic potential surface. The theory used in the analysis is a generalization of Biot's two-phase mixture theory for saturated soil. A soil-water-air coupled finite element method is developed in the present study using the governing equations for multiphase soil based on the nonlinear finite deformation theory. Three-dimensional numerical analyses at constant water and constant air content are conducted and the applicability of the proposed method is confirmed. The performance of the model is examined with reference to triaxial compression tests preformed on unsaturated soil at constant water and air content.

The bearing capacity of footing has been studied by both conventional and numerical methods by many researchers. However, degradation of the microstructure of material, that is, a change in the microstructure of the soil, has not been adequately taken into account. Degradation of microstructure causes strain softening of materials and it leads to strain localization such as shear bands and slip bands. From an engineering point of view the strain localization is crucial because it is a precursor of failure. In the present study, finite element analyses of the bearing capacity of a shallow foundation on homogeneous and inhomogeneous saturated clay strata have been conducted using an elasto-viscoplastic soil constitutive model of microstructure change. A series of analyses of footing on clay deposit with different microstructure parameters have been carried out. Numerical results show that strain localization can be predicted during the loading of rigid footing on highly structured soil and strain localization affects the footing-soil interaction. The effects of footing roughness on the failure mechanism are also discussed in the study.

Strain localization is an important geotechnical problem related to large deformations and the onset of failure, such as slope failure. It is necessary, therefore, to clarify the mechanisms of the strain localization of geomaterials in order to predict large deformations of the ground. For the last two decades, many researchers have studied the strain localization of geomaterials through both experimental and numerical works. Most of the works, however, particularly the numerical studies, have been treated as two-dimensional plane strain problems for the sake of simplicity, even though the actual phenomena are generally three dimensional. In order to understand the deformation, the failure, and the strain localization of clay under three-dimensional conditions, triaxial tests on clay and their numerical simulation are performed in the present study. In particular, focus is mainly placed on the effects of sample shape on the localization behaviour of normally consolidated and overconsolidated clays. A series of undrained triaxial compression tests, using rectangular clay specimens with different shapes and strain rates, is conducted. Localized shear strain distributions are successfully observed with an image analysis of digital photographs. It is seen that the shape of a specimen affects the various bifurcation phenomena of clay, e.g. the formation and the progress of various three-dimensional shear bands, failure with buckling, etc. The numerical simulation using the finite element method, with an elasto-viscoplastic model and considering structural changes, can reproduce the generation and the growing process of shear bands well. A comparison between the results of the experiment and the simulation offers new findings regarding the strain localization of clay under three-dimensional conditions. Copyright (c) 2007 John Wiley & Sons, Ltd.

In order to predict ground deformation due to the dissociation of methane hydrates, we have developed a simulation method based on the chemo-thermo-mechanically coupled analysis. With this method, the phase change from hydrates to fluids, the flows of pore water and gas, the mechanical behavior of the solid skeleton, and heat transfer can be simultaneously solved. The numerical method is based on the finite element method using an updated Lagrangian formulation. Applying the proposed method, we have numerically analyzed the dissociation process for the heating method. It has been predicted that ground deformation is caused by the generation of water and gas during the dissociation.

The simulations and numerical analyses of triaxial compression tests on soil samples obtained from seabed ground in deep sea were performed by elasto-viscoplastic constitutive equation. From the results it is proved that simulation can express very well the experimental results. And from the results of three dimensional finite element analyses it is found that the shear strain and volumetric strain are distributed nonuniformly in the test specimen.

In order to predict ground deformations due to the dissociation of methane hydrates, we have developed a simulation method based on a chemo-thermo-mechanically coupled analysis. Within this method, the phase change from hydrates to fluids, the flow of pore water and gas, the mechanical behavior of the solid skeleton, and heat transfer can all be simultaneously solved. The numerical method is based on the finite element method using an updated Lagrangian formulation. Applying the proposed framework, we have numerically analyzed the dissociation process that occurs in the heating and depressurizing methods of natural gas production. It has been predicted that ground deformation is caused by the generation of water and gas during the dissociation process. (c) 2007 Elsevier Ltd. All rights reserved.

We have developed a simulation method to predict the ground deformation due to the dissociation of methane hydrate. In the dissociation process, the phase change from solid to fluids leads to the change in partial stresses in the porous media, which will cause the ground deformation. The simulations are based on the chemo-thermo-mechanical coupled finite element analysis, in which the phase change, the flow of pore fluids, the mechanical behavior of solid skeleton, and heat transfer are simultaneously solved. We treat the ground as unsaturated soils, and apply an elasto-viscoplastic constitutive model to the soil skeleton. Using the proposed method, we have numerically analyzed the dissociation process for heating methods. Ground deformation has been predicted which is caused by water and gas generation during the dissociation.

We propose a thermo-hydro-mechanically coupled finite element analysis method for clay with a thermo-elasto-viscoplastic model. The volume changes in soil particles and pore fluids are introduced into the analysis method. The instability of the problem is studied and a numerical simulation of the thermal consolidation is presented using the newly developed analysis method. it was confirmed that the analysis method can reproduce the thermal consolidation phenomenon well.

Since strain localization is a precursor of failure, it is an important subject to address in the field of geomechanics. Strain localization has been analysed for geomaterials by several researchers. Many of the studies, however, treated the problems brought about by strain localization as two-dimensional problems, although the phenomena are generally three-dimensional. In the present study, undrained triaxial compression tests using rectangular specimens and their numerical simulation are conducted in order to investigate the strain localization behaviour of geomaterials under three-dimensional conditions. In the experiments, both normally consolidated and over-consolidated clay samples are tested with different strain rates. Using the distribution of shear strain obtained by an image analysis of digital photographs taken during deformation, the effects of the strain rates, the dilation, and the overconsolidation on strain localization are studied in detail. The analysis method used in the numerical simulation is a coupled fluid-structure finite element method. The method is based on the finite deformation theory, in which an elasto-viscoplastic model for water-saturated clay, which can consider structural changes, is adopted. The results of the simulation include not only the distribution of shear strain on the surfaces of the specimens, but also the distributions of strain, stress, and pore water pressure inside the specimens. Through a comparison of the experimental results and the simulation results, the mechanisms of strain localization are studied under three-dimensional conditions.

Rate sensitivity is an important characteristic of geomaterials for both saturated and unsaturated soils. However, many constitutive models for unsaturated soil have been constructed within the framework of the rate independent theory. The present study addresses an elasto-viscoplastic constitutive model which considers the effect of suction for unsaturated clayey soil and a soil-water-air three-phase coupled analysis using the elasto-viscoplastic model. The proposed constitutive model adopts the average skeleton stress for the effective stress from the viewpoint of the mixture theory. Hence, it has become possible to construct a model for unsaturated soil starting with a model for saturated soil by substituting the average skeleton stress for the effective stress and introducing the suction effect into the constitutive model. Furthermore, the collapse behavior, which is brought about by a decrease in suction, is described by the shrinkage of the overconsolidation boundary surface, the static yield surface, and the viscoplastic potential surface. A numerical analysis for multiphase materials is conducted within the framework of a continuum mechanics approach through the use of the theory of porous media. The theory is a generalization of Biot's two-phase mixture theory for saturated soil. A soil-water-air three-phase coupled finite element method is developed in the present study using the governing equations for multiphase soil based on the non-linear finite deformation theory. The average skeleton stress is defined as the difference between the total stress and the average pressure of the two fluids and is used in the proposed elasto-viscoplastic constitutive model. A van Genuchten (1980) type of equation is employed as the constitutive equation between the liquid saturation and the suction pressure. Numerical simulations of unexhausted-undrained compression with different strain rates are conducted under plane strain conditions, and the applicability of the proposed method is evaluated with respect to strain localization and the effect of suction. Copyright ASCE 2006.

The present study addresses an elasto-viscoplastic constitutive model which considers the effect of suction in unsaturated clayey soil and a soil-water-air three-phase coupled analysis using the elasto-viscoplastic model. The proposed constitutive model adopts the average skeleton stress for the effective stress from mixture theory. Hence, it has become possible to construct a model for unsaturated soils starting with a model for a saturated soil by substituting the average skeleton stress for the effective stress and introducing for the suction effect into the constitutive model. Furthermore, the collapse behavior, which is brought about by a decrease in suction can be described by the shrinkage of the overconsolidation boundary surface, the static yield surface, and the viscoplastic potential surface. A numerical analysis for multiphase materials is conducted within the framework of a continuum mechanics approach through the use of the theory of porous media. The theory is a generalization of Biot's two-phase mixture theory for saturated soil. A soil-water-air three-phase coupled finite element method has been developed in the present study using the governing equations for multi-phase soil based on the non-linear finite deformation theory. The average skeleton stress is defined as the difference between the total stress and the average pressure of the two fluids and is used in the proposed elasto-viscoplastic constitutive model. A van Genuchten (1980) type of equation is employed as the constitutive equation between the liquid saturation and the suction pressure. Numerical simulations of unexhausted-undrained compression are conducted under plane strain conditions, and the applicability of the proposed method is evaluated with respect to strain localization and the effect of suction. Copyright ASCE 2006.

Instability is usually considered as a problem of shear failure. Unstable behavior is also observed during the consolidation process, whereby the stress paths depart from the failure line. In the present study, an elasto-viscoplastic constitutive model is extended to describe instability of both around the failure state, and away from the failure line. The instability is connected to structural degradation, and formulated as shrinkage of overconsolidation boundary surface and static yield surface in the constitutive model. One-dimensional consolidation process of clay has been simulated to study the effect of structural degradation on behavior during consolidation. The proposed model can effectively reproduce certain types of unstable behavior during consolidation, such as stagnation, or a temporary increase in pore water pressure, and a sudden increase in the settlement rate. Moreover, the distributions of axial strain exhibit apparent strain localization when structural degradation is taken into account. This phenomenon of the compressive strain localization is regarded as compaction bands, which may cause a large displacement.

In the present study, in order to grasp deformation and failure behaviors of clay under 3-D condition as well as large deformation and strain localization, a series of triaxial compression tests using rectangular clay specimens are performed. Various patterns of strain localization are successfully observed by an image analysis. 3-D bifurcation phenomena, e.g. formation and progress of various strain localization patterns, failure with buckling, unstable behavior in the stress-strain relations, can be also observed in the preset tests. Then, a 3-D finite element simulation using an elasto-viscoplastic model for a saturated clay is carried out. The simulated results can well explain the observed strain localization patterns of rectanguler clay sepcimens.

粘性土の3次元的なひずみの局所化挙動を正確に追跡することを目的とし、粘性土の3次元有限要素解析法を開発し、実験のシミュレーションを行った。解析法は有限変形理論に基づいた水―土連成解析法であり、粘性土の構成式には内部構造変化を考慮した弾粘塑性構成式を用いている。実験は、角柱飽和粘土供試体を用いた非排水三軸圧縮試験であり、画像解析により供試体側面のせん断帯発生過程を定量化した。実験結果と解析結果を詳細に比較した結果、本解析法は供試体形状およびひずみ速度の異なる、正規圧密および過圧密粘土双方の3次元的なひずみの局所化挙動を精度良く追跡できることがわかった。

雲母混じりの砂は,雲母の平板状の粒子形状により,強度異方性が顕著に表れることが知られている。本報では,雲母を僅かにシルトに混入することにより,シルト試料の力学挙動がどのように変わるのかについて,三軸圧縮および伸張試験により調べた。具体的には重量比1%, 2.5%,5%の3種類で雲母を混入したところ,わずか1%の混入で圧縮せん断強度は上昇するのに対し,伸張せん断強度は低下することが示された。この強度異方性は,雲母の堆積方向によるものであると考えられるが,SEMによる供試体断面の観察によりその検証を行った。

Computing and analysis methods of bearing capacity conventionally used nowadays do not consider the effect of microstructure change of clay which leads to softening of soil strength and strain localization. The strain localization is generally considered to be important as a sign of failure and therefore the effect of microstructure cannot be neglected. In the present study, the finite element analysis of bearing capacity of surface foundation on homogeneous saturated clay stratum is conducted using an elasto-viscoplastic soil constitutive model considering microstructure change. Finally, the effect of microstructure change on collapse load and load-deformation behavior in bearing capacity problem is evaluated.

不飽和土の力学的挙動はサクション(ua-uw)に強く依存することが明らかになっている。しかし,従来の研究では排気・排水試験が主で非排気条件での研究が少ない。したがって,本研究では低飽和度の不飽和シルトを用いて,非排気・非排水,排気・排水および非排水定体積三軸圧縮試験を行い,その力学的挙動を検討した。試験試料としてはDLクレーを用い,締固めにより供試体を作製している。また,一連の試験では,間隙空気圧(ua)と間隙水圧(uw)を変化させる制御試験によりサクションを与え,それが不飽和土の力学挙動に及ぼす影響について検討した。

不飽和土地盤を解析するためには,サクションの効果をどう取り込むかが重要である。間隙空気圧を基準圧力とし,サクション(ua- uw)による影響を粘着力として考え,構成式の中で表現する方法がある。また,サクションの効果を主に有効拘束圧として考慮する方法がある。これは,気相と液相を1つの流体と見なし,骨格とその間隙を流れる流体の分圧から求められる等価間隙圧に基づいて有効応力として考える方法である。本論文では,不飽和シルトの排気・排水三軸圧縮試験について以上の2つの方法を用いて実験結果を整理し,構成式によるシミュレーションを行った結果を比較する。解析では,内部構造を考慮した弾粘塑性構成式を用いた。

著者らはメタンハイドレートを模擬した氷試料を用いて三軸圧縮試験を行っている。試験は拘束圧とひずみ速度を変えて排水条件で行ったものである。今回、この三軸圧縮試験結果について、岡らの提案する「凍結砂のひずみ軟化型弾粘塑性構成式」を用いてシミュレーションを行い、試験結果との比較を行った。その結果、ひずみ硬化-軟化挙動やひずみ速度効果等の試験結果をよく表現できることが明らかとなった。

地中に存在するメタンハイドレートから抽出されたメタンを新しいエネルギー源として利用する方法が注目されている。本研究では、メタンガスを安定して生産するため、メタンガス生成に伴う地盤環境の影響評価を行う。その評価方法として、地盤環境を再現できるモデルを開発し、有限要素法を用いて加熱・減圧法でのメタンハイドレートの分解、水・メタンガスの流れ、地盤の変形をシミュレートする。解析の結果、加熱、減圧によって地盤の条件を変えることによって、地盤内のメタンハイドレートが分解し、水・メタンガスの流れが生じる。さらに、地盤内の応力状態が変化し、地盤が変形することが確認された。

メタンハイドレートの生産に伴う海底地盤への影響を把握する目的で、有限要素法による弾塑性圧密解析を行った。解析領域としては海底面からメタンハイドレート層下面までを対象とし、外力としては生産に伴うメタンハイドレートの分解を掘削(メタンハイドレートを構成する要素の削除)で模擬し、掘削解放応力を与えることとした。パラメータの設定に際しては、想定される地盤として南海トラフを念頭に置き、基礎試錐などのデータや過去に日本周辺の海底地盤における土質・地質について調査されたデータに基づいて設定し、それらの値が変化することによる海底面での沈下の発生状況,海底地盤内の応力や水圧の分布状況,変形や水圧の経時変化などについて検討を行った。

Strain localization has been numerically analyzed for soils by many researchers. Many of them are, however, treated as two-dimensional problems although the phenomena are in general three-dimensional. In the present study, triaxial compression tests have been carried out using rectangular clay specimens which have relatively fewer axes of symmetry of deformation compared with cylindrical specimens. This feature is convenient for the image analysis of the observed data. Then we have carried out elasto-viscoplastic finite element analysis for water-saturated clay based on the finite deformation theory so-called updated Lagrangian method. From the analysis, it has been elucidated that shape of the specimen, strain rate greatly affects the strain localization phenomena. In addition, the progress of the strain localization such as shear banding has been carefully discussed with comparison of the experimental results.

Natural deposits are affected by chemical bonding or the cementation between soil particles during the sedimentation process. Under the effect called 'aging', aggregations and linkage between assemblages are formed in the soil structure. It is known that the structure affects the characteristics of the strength, the deformation, and the strain localization. Strain localization is generally considered to be important as a presage of failure. It has recently been recognized that strain localization may occur also during compressive deformation, particularly for porous materials with large confining stress, and the localized compressive deformation may cause large displacement. The aim of the present paper is to study the effect of structural degradation on the localization of both shear and compressive strain for water-saturated clay. Firstly, an elasto-viscoplastic constitutive model considering structural changes with the viscoplastic deformation is proposed in the present paper. Structural collapse is expressed as the shrinking of both the overconsolidation boundary surface and the static yield surface with increasing viscoplastic deformation in the model. Secondly, finite element analyses of undrained and partially drained compression tests under plane strain conditions are conducted to study the effect of structural changes on strain localization. It is seen in the results that the localization of both shear and compressive strain is promoted by the degradation of the soil structure. (C) 2004 Elsevier B.V. All rights reserved.

Strain rate sensitivity is one of the typical time-dependent behaviors of soil as well as creep and stress relaxation. In particular, it is well known that a unique stress-strain curve exists for each different strain rate in clayey soil: the isotaches characteristics. Originally, the concept of isotaches was proposed in one-dimensional consolidation of clay; however, we can also observe the isotaches characteristics in the stress-strain relation obtained by the triaxial compression test with various strain rates. The purpose of the present study is to reexamine the concept of isotaches through the detailed results of triaxial tests on reconstituted Fukakusa clay and a constitutive modeling. Undrained triaxial tests both of normally consolidated and overconsolidated clay were performed with various constant strain rates and step-changed strain rates to observe the isotaches characteristics in a wide range of axial strain. The results of the step-changed strain rates test show that the phenomenon of isotaches exists in the range of low level strain. It is also seen, however, that this phenomenon is not observed in a range of high level strain, in particular, around the critical state. The results of step-changed strain rate tests were numerically simulated by an elasto-viscoplastic model for clay. It was found that the model can well simulate the trend of the isotaches behavior in the range of low level strain observed in the experiment. In addition, the trend of stress-overshooting and stress-undershooting in the range of high level strain observed in the experiment can be described adequately using the model except for the stress-overshooting of overconsolidated clay.

本研究では、正規および過圧密粘土の角柱供試体を用いた非排水三軸試験を行い、供試体側面のひずみ分布を画像解析から定量化し、せん断帯の発生・成長について詳細に検討した。また、この実験は境界条件の設定が容易な小型模型載荷実験として行われている。そこで、新たに開発している3次元水-土連成有限要素法により粘土角柱供試体を用いた非排水三軸圧縮試験のシミュレーションを行った。実験結果とシミュレーション結果から、正規圧密および過圧密粘土の3次元的なせん断面の成長過程について比較することにより、ひずみの局所化のメカニズムについて検討した。

現在までひずみの局所化に関する計算工学的研究が行われてきたが、奥行き方向対称を仮定した2次元平面ひずみ条件での解析が主流であった。しかし、斜面崩壊などの実問題はもちろん、要素試験である三軸試験においてもせん断面は3次元的に現れるため、ひずみの局所化のメカニズムを解明するにあたって3次元解析は必須である。そこで本研究では、弾粘塑性構成式を用いた2次元及び3次元水―土連成有限要素法によるひずみの局所化解析を行い、2次元解析結果と3次元解析結果について比較検討を行った。その結果、3次元的なひずみの局所化と共に、供試体内部の応力やひずみの分布が明らかとなった。

内部構造の変化を考慮した弾粘塑性構成式を用いて、粘性土地盤の一次元圧密解析を行った。足立・岡によって提案されている弾粘塑性構成式中の、過圧密境界面および静的降伏関数を、粘塑性ひずみの発生に伴って縮小させることによって内部構造崩壊を表現できるよう拡張したものを用いた。構造変化を考慮した場合、破壊応力付近の軟化のみ考慮した構成式では再現できない、ひずみ速度の一時的な増加および間隙水圧の増加などの不安定現象を再現することができた。また構造変化を考慮しない場合と比較して、圧密中に排水面近くのひずみの集中帯がみられた。内部構造の崩壊が、圧縮ひずみの局所化、すなわち圧縮帯を引き起こすと考えられる。

The aim of the present paper is to study the instability and strain localization of water saturated clay. In particular, effects of dilatancy and strain rate are discussed using an elasto-viscoplastic constitutive model. The model is based on the non-linear kinematic hardening theory and a Chaboche type of viscoplasticity model. The elasto-viscoplastic model for both normally consolidated and overconsolidated clays can address both negative and positive dilatancies. Firstly, the instability of the model under undrained creep conditions is presented in terms of the accelerating creep failure. The analysis shows that clay with positive dilatancy is more unstable than clay with negative dilatancy. Secondly, a finite element analysis of the deformation of water-saturated clay is presented with focus on the numerical results under plane strain conditions. From the present numerical analysis, it is found that both dilatancy and strain rate prominently affect shear strain localization behavior.

In order to investigate the anisotropic behavior of soft sedimentary rock, a series of triaxial compression tests are performed on Tomuro stone, sampled in different directions. From the test results, it is confirmed that the deformation characteristics and strength are strongly dependent on the direction; in other words, Tomuro stone is a transversely isotropic body. The authors have developed an anisotropic elasto-plastic constitutive model which can describe anisotropic behavior through triaxial tests. The model is based on the elasto-plastic constitutive model with strain softening proposed by Adachi and Oka (1995). In the formulation of the model, a generalized Hooke's law is adopted for the elastic strain increment, and the transformation stress concept proposed by Boehler and Sawczuk (1977) is used for the plastic strain. Five independent elastic moduli are used for the transversely isotropic body, and three independent parameters are introduced for the plastic anisotropy. These parameters can be determined from the triaxial compression tests performed with specimens sampled in different directions. Comparisons between the experimental data from triaxial tests and simulated results from tests using the model indicate that the proposed constitutive model can well reproduce the direction dependent behavior of soft sedimentary rock.

It is well known that geomaterials such as soils exhibit an increase in volume during shearing deformation, referred to as dilatancy. Dilatancy is a typical property of such granular materials as soils and is closely related to changes in the microstructure. Normally consolidated clay exhibits negative dilatancy or contractancy, namely, a decrease in volume during shearing. On the other hand, overconsolidated clay shows positive dilatancy, namely, an increase in volume during shearing. The aim of the present paper is to study the effects of the microstructure, such as dilatancy and permeability, on the strain localization of water-saturated clay using an elasto-viscoplastic constitutive model. Based on the non-linear kinematic hardening theory and a Chaboche type of viscoplasticity model, an elasto-viscoplastic model for both normally consolidated and overconsolidated clays is proposed; the model can address both negative and positive dilatancies. Firstly, the instability of the model under undrained creep conditions is analyzed in terms of the accelerating creep failure. The analysis shows that clay with positive dilatancy is more unstable than clay with negative dilatancy. Secondly, a finite element analysis of the deformation of water-saturated clay is presented with focus on the numerical results under plane strain conditions. From the present numerical analysis, it is found that both dilatancy and permeability prominently affect shear strain localization behavior. (C) 2002 Elsevier Science Ltd. All rights reserved.

In order to investigate anisotropic behavior of soft sedimentary rocks, a series of triaxial compression tests are performed for Tomuro stone. Prom the test results, it is confirmed that Tomuro stone is transversely isotropic body. The authors have developed an anisotropic elasto-plastic constitutive model which can describe the anisotropic behavior obtained from triaxial tests. The model is based on Adachi and Oka (1995) ’s elasto-plastic constitutive model with strain softening. In the model, the generalized Hooke’s law is adopted for elastic strain increment, and transformation stress concept is used for plastic strain. Comparisons between experimental data and simulated results, indicate that the proposed constitutive model can well reproduce the direction dependent behavior of soft sedimentary rocks.