A main objective of this paper is to demonstrate, by examples, the vast applicability of simple. direct formulae and graphs to solve a variety of slope stability problems in practice. The basic developments of these tools took place between 1920 and 1955, rougnly speaking, although a number of refinements have been added since. Only overall average conditions were considered in these approaches. Herein, three alternatives are presented, named Simple Equilibrium Analyses, SEA, Resistance Envelopes, RE, and Stability Charts, SC.
The practical exploration of the approximate stress distribution within a soil mass of a potential slide was based on limit equilibrium principles and the method of slices using non-circular shear surfaces. This approach was called the Generalized Procedure of Slices, GPS. Janbu (1957, 1973, 1996). A spread sheet version of GPS will be presented and used in an analysis of the initial slopes at the Finneidfjord Slide in June 1996. The numerical GPS-results compare well with the SEA, RE and SC analyses.

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A brief review is given of stability and deformation analysis of slopes, focused on the limit equilibrium method (Part 1) and the finite element method (Part 2). In part 1, historical development of slice and column methods is first summarized. Common characteristics as a single value factor of safety analysis for the conventional methods are then made clear. These are followed by summaries of the central ideas of five typical factor of safety equations. In Part 2, principle of finite element method and how to apply it to slope problems are briefly presented, emphasizing that FEM can handle deformation behavior which LEM cannot. Future subjects in this field are also indicated.

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Two different approaches, i. e. iterative procedure and load-incremental procedure are developed for the analysis of progressive failure of slopes, based on the limit equilibrium frameworks. In the former approach, self-weight of slope material together with surface load, if any, are simultaneously applied and iterative calculations are used until convergent solution is reached. On the other hand, total load including the self-weight is divided into many incremental steps, in the latter approach, and with each load-incremental step, analysis is repeated up to the final loading condition. Both procedures define local safety factor at the base of each slice to consider progressive, local failure and its evolution. Application shows that either of the procedures captures the behavior of progressive failure of slopes.

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A finite element model for the mixed type of failure in landslides is presented with special reference to: (1) the progressive failure in strain softening materials and (2) the renewed movement with residual strength along the base layer. Strain softening elasto-plastic model in which the effect of shear band formation can be taken into cosideration is incorporated for first-time slides. Seepage analysis is performed using boundary condition simple and suitable for a long slope in order to evaluate the seepage force. Solution of collapse analysis of actual landslides based on the experiment result of undisturbed soil samples is presented.

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A method of the slope stability analysis using a spring attached to the inter-slice planes (hereafter called SSM) determines the inclination angles of inter-slice forces by using the displacements of slices. This study will show that the SSM can account for the displacements of the slices by using the characteristics of each component utilized in the SSM directly. Examination of the static equilibrium condition by using analytical examples clarified that besides this condition, the conservation of work is satisfied in the SSM. In general, it is difficult to satisfy the necessary conditions in the displacement field and stress field at the same time. Though the SSM can derive accurate results, this method does not satisfy the stress-strain relationship of the shear force acting on the inter-slice planes which is one of the conditions required in the failure phenomena of structure. The reason why the SSM does not satisfiy all of the necessary conditions is that the inclination angles concerning the inter-slice forces due to the displacement field are converted into those due to the stress field by a value of k included in the assumed equation concerning the interslice forces.

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In order to examine the utility of slope surface displacement measurement for the investigation of slope instability, effect of slope angle, soil layer depth, rainfall intensity, and initial suction in the slope to the proceeding of shear deformation of the slope and surface displacement were examined by the numerical simulation. The simulation used the shear deformation model of infinite slope due to rainfall, which had been already proposed by the author. As results, following facts were made clear.
(1) With the increase of slope angle, rainfall intensity and decrease of soil layer depth, initial suction, remaining time until slope collapse becomes shorter. The increase of remaining time until slope collapse is proportional to the increase of soil layer depth, but the rate of decrease of the remaining time becomes larger in accompany with the increase of slope angle, rainfall intensity and the decrease of initial suction.
(2) Local shear deformation in the slope develops largely in the high water content layer near the bottom which is formed by the arrival of the vertical unsaturated seepage water, although local deformation at the upper part with small water content is very small. The local deformation around the bottom becomes larger in accompany with slope angle and soil layer depth. In the case of large soil layer depth, shear deformation develops at every depth without high water content at each depth.
(3) With the increase of slope angle and soil layer depth, surface displacement grows relatively large at early stage of displacement so that detecting slope instability in early stage can be easier. On the other hand, change of rainfall intensity doesn't affect to the shape of surface displacement curve. On the initial suction of the slope, surface displacement can grow large at early stage in the case with very low suction condition (wet condition), while initial suction doesn't affect to the shape of the surface displacement curve in a case of relatively high suction condition (dry condition).

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Two dimensional stability analysis along mid-section by limit equilibrium method with back analysis is popularly used in the world for stability analysis of landslides. However, the residual shear strength of the soil is mainly dominated by the clay mineralogy and may give the different shear strength than the calculated value by back analysis. The residual shear strength of 18 soil samples collected from shallow and deep sliding zone of Okimi landslide in Niigata prefecture, Japan have shown less than 15% variation in shear strength. The average of those shear strengths have been utilized to conduct the 3-D stability analysis of each landslide blocks. The sliding surface contour map has been prepared according to the check boring information to estimate the shearing surface depth. Likewise, available piezometer data and ground topography has been used to estimate the water level throughout the landslide area. New coefficient called stability correction factor (Cf) has been introduced to compare the ratio of calculated factor of safety with unit value by 2-D stability analysis along various sections and 3-D stability analysis. The value of Cf by all methods is found to be consistent when the sliding block has uniform width throughout and primary data are abundant. However, the value of Cf has considerably varied in the block having nonuniform width and less primary data. As the 3-D stabil-ity analysis proposed in this paper is simple 3-D form of simplified Bishop's 2-D method, the calculation process is very simple and can give better solution for the stability analysis of landslide than the method in common practice.

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The safety factor of 2-dimensional analysis used by countermeasure plan of a landslide serves as a value which included the error with various indefinite elements about the calculation error and stability analysis by the stability analysis formula to adopt. Although the error of asfety factor has influenced on effect presumption of countermeasure greatly, there is almost no verification example with the general-purpose but only with individual on-site example. On the other hand, though the designed safety factor in countermeasure plan is an important value which determines the scale of countermeasure, the validity of the value owes a lot to experience, and it is not verified scientifically yet. By verifying the influence of the indefinite element on effect of countermeasure generally, I think in the future, it will be also useful to verification of the validity of designed safety factor. In this paper, I took notice of 3 main indefinite elements, the unit weight of move layer, strength parameters of slip surface, slip surface depth. Then verified influence of these 3 elements on effect of couner measurements, groundwater drainage works, soil removal works, prevention works by using the 133 spot and 243 section of on-site data all over the country. I also verified about the superposition effect of the presumed error by two or more indefinite elements, or the presumed error by the difference in a stability analysis formula.
As s result, it became clear change of effect presumption of countermeasure by the presumed error of indefinite elements becomes larger as a scale of landslide is small, the influence of the indefinite elements increases by using countermeasure together, and the landslides which's evaluation value of countermeasure effect using the Fellenius method are bigger than using the simple Janbu method are more than 60% of whole.

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Rigid plastic finite element method was formulated for slope stability assessment by introducing the shear strength reduction coefficient to define the factor of safety used in slope stability. The framework of the proposed method was clarified in comparison with the limit equilibrium method based on slice method. The limit equilibrium method was shown to coincide with the upper bound theory in limit analysis and the process of minimizing the factor of safety for possible slip line was clarified to be important from the viewpoint of limit analysis. Rigid plastic FEM analysis has the advantages of (1) no need to assume the failure mode prior to analysis, (2) mathematical minimization of factor of safety, (3) use of constitutive equation and (4) taking account of dilatancy property of geo-material. The factor of safety was examined with simple slope for various conditions. It shows the good agreement with conventional methods and the applicability of proposed method was clarified. Case studies shows non-linear slip lines were well obtained corresponding to geometry and geology of slopes in the analysis. In some cases the sliding mass included several shearing zones inside and this fact suggests the existence of complicated failure mode not to be covered in the conventional methods.

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A field loading test on a natural slope was conducted in order to study suitable slope stability evaluation technique. Safety factors by Fellenius method and simplified Janbu's method were calculated based on the results obtained from the field loading test. The differences of the analysis and the relation between the displacement of the slope and the change of safety factor in each loading step were considered. The results are summarized as follows.
(1) The Fellenius method and the simplified Janbu's method gave the almost same value to the test slope.
(2) When safety factor by the Fellenius method decreased about 5% from the time that monitoring equipment detected dis-placement, the rate of displacement reached a steady state or had an acceleration tendency, which suggested that the slope changed into a limit equilibrium.

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