Progress in Landslide Science

Landslides cause great disasters and their impact to society is very great. Thus, they are studied in many scientific and engineering fields. However, studies on landslides from various fields have not been conducted in an integrated manner. There was neither international society, nor international journal, and the meaning of landslides was not defined internationally and interdisciplinary. During the United Nations International Decade for Natural Disaster Reduction (IDNDR) in 1990–2000, landslide researchers worldwide agreed the definition of landslides as “the movement of a mass of rock, debris or earth down a slope”. This is a basis for the development of the study of landslides as a scientific field. This paper describes the progress in landslide science as an integrated discipline together with the development of international landslide community and a global cooperation platform as its infrastructure.

An overview is presented of the landslide problems experienced in Britain, and their primary causes. Principally, landslides occur in Britain where the strata are argillaceous and there is sufficient topographic relief. This combination occurs in swathes through the southern and central parts of Britain and along the lengthy coastline. In these parts of Britain, the main rock types are sedimentary, and they often exhibit low angles of dip. This gives rise to the occurrence of compound landslides, often with a bedding-controlled flat basal shear surface. Issues relating to this type of landslide are discussed in the article, which concludes with a discussion of the conflicts between interests of land users and the need for conservation, especially in some areas that are of significant interest in the history of the geological and other sciences, and to research in the present day.

Slow active landslides in clay include slides, mudslides and spreads. Movement is induced by any change of effective stress and by creep; in the very long-time, some role may be played by a change in soil properties. Looking at geological phenomena causing movement, pore pressure fluctuations and erosion have a strong influence on shallow translational slides and mudslides, while creep or erosion and other geological phenomena of stress relief govern movement of deep seated slides and spreads. In several cases, excess pore pressures generated by changes of boundary conditions may play a significant role.

Velocity is the most important parameter determining the destructive potential of landslides. “Catastrophic” velocities of the order of several meters per second are attained only by certain types of landslides. High velocities are the consequence of a range of strength loss mechanisms, reviewed in this chapter. Strength loss can occur instantly during the process of failure, through loss of cohesion, liquefaction of granular material or remoulding of sensitive clay. Further important loss of strength can occur during movement, including rock joint roughness reduction, shearing in clays, sliding surface liquefaction, frictional heating, loss of internal coherence of the sliding body, material entrainment, rapid undrained loading, and entrainment of water. Extremely rapid landslides include rock, debris and earth fall, rock block topple, rock slide, debris slide, flow slide in granular soil or clay, debris avalanche, debris flow and rock avalanche. There is a need to study the post-failure behavior of materials, in order to facilitate predictions of the behavior of extremely rapid landslides for hazard assessment.

The processes of the two major types of debris flow initiation; the bed erosion type and the landslide-induced type, are discussed and the methods to analyze the respective phenomena are introduced. Especially for the landslide-induced type, a new model of liquefaction of the slid earth mass while in motion is introduced. The earth mass is liquefied at the bottom without water supply from the outside and this liquefied layer gets behind the mass as a following debris flow. The reviews of previous investigations on the mechanics of developed debris flow confirms that the two-phase model is influential, in which debris flow consists of the mixture of two continuum media of fluid phase and solid phase. Using this model, developed debris flows are classified into three types from the point of the dominant stresses within flow. The characteristics of the respective types of flow such as the solids concentration distribution, the velocity distribution, the equilibrium sediment transport concentration are given.

Landslides are gravitational mass movements of rock, debris or earth. Shear deformation in landslides before failure conforms to the field of statics. But shear deformation during seismic loading and post-failure motion of landslides conforms to the field of dynamics. Thus, study of the initiation of earthquake-induced landslides and rapid landslide motion needs to develop “Landslide dynamics” involving dynamic loading and dynamic generation/dissipation of excess pore-water pressure during motion. New developments in science can be facilitated by new technological advances. This study aimed to develop a new testing method that can geotechnically simulate the formation of the shear zone and the following long and rapid shear displacement that occurs in high-velocity landslides. Sassa K and his colleagues at DPRI (Disaster Prevention Research Institute), Kyoto University, have worked to develop an undrained stress-controlled dynamic-loading ring-shear apparatus and its testing method for this purpose. This paper describes the development of this testing method, and its application to the study of earthquake-induced landslides and landslide triggered debris flows in Japan.

“Sliding Surface Liquefaction” is a process causing strength loss and consequent rapid motion and long runout of certain landslides. Using a new ring shear apparatus with a transparent shear-box and digital video camera system, shear-speed-controlled tests were conducted on mixed grains (mixture of three different sizes of sand and gravel) and mixed beads to study shear behavior and shear zone development process under the naturally drained condition in which pore pressure is allowed to dissipate through the opened upper drainage valve during shearing. Higher excess pore water pressure and lower minimum apparent friction were observed in the tests where grain crushing was more extensive under higher normal stress and higher shear speed. Along with the diffusion of silty water generated by grain crushing, smaller particles were transported upward and downward from the shear zone. Concentration of larger grains to the central and upper part of the shear zone was confirmed by means of visual observation together with grain size analysis of sliced samples from several layers after the test. On the other hand, smaller particles were accumulated mostly below the layer where larger grains were accumulated. The reason why larger grains were accumulated into the shear zone may be interpreted as follows: grains under shearing are also subjected to vertical movement, the penetration resistance of larger grains into a layer of moving particles is smaller than that into the static layer. Therefore, larger grains tend to move into the layer of moving grains. At the same time, smaller particles can drop into the pores of underlying larger grains downward due to gravity.

The motion of landslides sourced from mostly bedrock (called rockslides) is controlled by the phenomenon of grain flow, and the frictional resistance of the constituent rock grains and their interstitial fluids. Modern understanding of grain-flow dynamics recognises that the important interactions between grains are irregularly distributed within the grain mass, with fortuitous alignments of grains carrying most of the stress in “force chains”, while other grains are only weakly stressed. In rapidly shearing grain flows, under substantial confining stress, force-chain stresses rise high enough to crush grains. Such comminuting grain flows develop a distinctive grain-size distribution that is fractal over many orders of magnitude of grain size down to sub-micron sizes. In the moment of crushing, grains are not solids, and behave as high-pressure fluids. As the grain fragments are injected into lower pressure surroundings, they behave as would any other fluid, lowering the effective stress on other grains, and thereby lowering frictional resistance to flow. We show how this affected the blockslide component of New Zealand’s prehistoric giant Waikaremoana rockslide; New Zealand’s Falling Mountain rock avalanche triggered by an earthquake in March 1929; and a small prehistoric New Zealand rockslide that was too small to be a comminuting grain flow, but which fell on and mobilized a fine, saturated substrate. We use grain-flow dynamics to explain the motion of these rockslides determined through field studies and physical and numerical modeling.

Extensive research done on the residual shear strength of the Tertiary mudstone showed that mudstones are rich in expansive clay minerals. Those clay minerals are responsible for making the mudstone highly weathering susceptible, which is the main cause for excessive landslides in the mudstone formations. Because of high brittleness, residual shear strength is more important in analyzing the landslides occurred in such formations. Research results showed that average liquid limit, plasticity index, activity, and clay sized fractions in the mudstone from the Niigata Prefecture of Japan are 72%, 36%, 1.4, and 26%, respectively. Dominating clay mineral in mudstone is smectite with an average proportion of 15%. Dominating clay mineral oxide of mudstone was aluminum oxide with an average value of 18%. Average residual friction angle was observed to be 13°. At the presence of saline pore water the strength can increase up to 40%, depending on the nature of soil and site condition. If it is hard to get the soil specimens in sufficient quantity from the mudstone area, residual friction angles can be approximately estimated with index properties. However, if the mineralogical composition is known, residual friction angle can be estimated with up to 90% accuracy, using the diagram proposed by Tiwari and Marui (2005). This paper deals with the chemical, mineralogical, and mechanical properties of the Tertiary mudstone from the Niigata Prefecture of Japan, as well as the methods to measure residual shear strength in conventional soil testing devices.

One of the serious problems in mega cities in developing countries is the management of municipal waste. Due to priority of economic development or insufficient attention to waste problems, many mega cities simply dump waste in the field without provision for environmental and mechanical risks. An example of problems of this type occurred in Bandung City of Indonesia in February, 2005, in which a large waste landfill collapsed after rain fall and killed more than one hundred people. Similar accident occurred earlier in the Philippines as well. This text therefore makes a brief report on this event and shows the need for more care for landfill operation and management.

In May 2004, a landslide occurred at the right flank of the Jinnosuke-dani landslide, and transformed into a debris flow after fluidization. By analysis of the monitored video images of the debris flow, field investigation on the source area of the landslide, and a series of simulation tests with a ring-shear apparatus on the initiation of the rainfall-induced landslide and its traveling process, the initiation and traveling mechanisms of the debris flow traveling in the valley were investigated. It is shown that concentrated groundwater flow was the main reason for the landslide initiation, and a rapid decrease of the mobilized shear resistance even under naturally drained condition caused the rapid landslide motion. During the debris motion in the valley, high potential for grain-crushing of deposits in upstream and lower potential for the downstream deposits controlled the traveling and depositing process of the debris flow. Different grain-crushing potential of the valley deposits played an important role in the debris flow traveling and depositing processes.

Using a small flume, a series of tests was conducted to trigger rainfall-induced landslides. Based on monitoring of sliding distance and pore pressures, the process of pore-pressure generation in relation to sliding distance was examined. By performing tests on sands of different grain sizes (silica sand no. 7 (D ₅₀= 0.14 mm) and no. 8 (D ₅₀= 0.057 mm)) at different initial dry densities or different thicknesses, the effects of these factors on pore-pressure generation and failure behavior of a landslide mass were analyzed. Results from tests of different initial densities showed that for each sample there was an optimal density index, at which the pore pressure build-up after failure reached its maximum value. This optimal density index varied with the thickness of sample and the grain size of samples. Moreover, observed failure phenomena showed that the failure mode also depended greatly on the grain size and sample thickness. In fact, flowslides were initiated in the tests on finer silica sand (no. 8), whereas retrogressive sliding occurred in the tests on silica sand no. 7. Results of tests on mixtures of silica sand no. 8 with different contents of loess by weight showed that the existence of fine-particle soil (loess) could significantly change the flow behavior of a landslide mass during motion: the flow behavior of soils with 20 and 30 percent loess was different from these two silica sands and the mixture with 10 percent loess, showing greater velocity without deceleration. This suggests the existence of a mechanism that maintains high pore pressures during motion for these soils. In addition, by rotating saturated samples in a double-cylinder apparatus, a mechanism was examined in which pore pressure in saturated soils during motion was maintained. The results showed that the pore pressure of the saturated mixture increased with velocity because of the “floating” of sand grains that accompanied the movement for each test. In addition, the sample with finer grain sizes or greater fine-particle (loess) contents floated more easily, and high pore pressure could be maintained during motion. The floating ratios of grains reached a high value (>0.85) at a very slow velocity for samples with 20 and 30 percent loess. Based on these test results, it is concluded that grain size and fine-particle contents can have a significant impact on the mobility of rainfall-induced landslides.

Various investigations on tectonically-induced landslides in Shikoku Region of West Japan have been carried out, most of which conclude at tectonic activities through the major tectonic faults and enhanced rock mineral decomposition as being mainly responsible for the landslide occurrence. This paper looks into strength parameters of the landslide clays, as measured in ring shear apparatus, from clay mineralogical point of view. As a result of strength tests and X-ray diffraction analysis, it is found that the drop from peak to residual friction angles for the tested samples reaches as high as 20°, and the residual strength of the landslide clays was found to decrease with higher amount of expansive clay minerals, which was estimated as being relative to chlorite mineral.

Many landslides are triggered by rainfall and many landslides are triggered by earthquakes. The probability that rainstorms (typhoons or hurricanes) and earthquakes attack same area is not high or rare. Combined effects of rainstorms and earthquakes have not been examined. The 2004 Mid-Niigata Prefecture earthquake (M6.8) caused twelve landslides more than one million cubic meters, and many landslide dams were formed by large-scale displaced landslide masses. While, the 1995 Hyogo-ken Nambu earthquake (M7.2) did neither cause any large-scale landslide, nor landslide dam although it has a greater magnitude and a similar depth of earthquake. One major difference is: a typhoon attacked Niigata Prefecture in three days before the earthquake, and the 1995 Hyogo-ken Nambu area was very dry before the earthquake. Combined effects of two triggering factors were examined for two cases which the authors investigated: the 2006 Southern Leyte landslide possibly triggered by a nearby small earthquake (M2.6), and the Higashi- Takezawa landslide triggered by the 2004 Mid-Niigata Prefecture landslide.

An almost real-size slope model was used to study the initiation process of landslide fluidization during torrential rain. Experiments were conducted by filling an inclined flume with loose sand under the rainfall simulator to induce the sand to collapse. Both the movement, volumetric strain and the pore water pressure of the sand were monitored throughout the experiments, from the start of spraying to the cessation of the landslide. These experiments showed: (1) Landslide fluidization caused by undrained rapid loading undergoes three stages: compaction of the sand layer by the sliding mass from upper slope, generation of excess pore water pressure in saturated zone, and induction of fast shearing; (2) Fluidization at the collapse source area undergoes also three stages: destruction and compaction of sand layer skeleton by outbreak of shearing, increase of pore water pressure in saturated zone, and shift to hispeed shearing, these three stages take place almost simultaneously.

An experiment to induce a fluidized landslide by artificial rainfall was conducted on a natural slope at Mt. Kaba-san in Yamato village, Ibaraki Prefecture, Japan. The experimental slope was 30 m long, 5 m wide, and the average slope gradient was 33 degrees. A landslide initiated 24627.5 s (410 min 27.5 s) after the start of sprinkling at a rainfall intensity of 78 mmhr⁻¹. The landslide mass was 14 m long and 1.2 m deep (at maximum). It first slid, then fluidized, and changed into a debris flow. The travel distance was up to 50 m in 17 seconds. The apparent friction angle of the fluidized landslide was 16.7 degrees. Formation of the sliding surface was detected by soil-strain probes. Motion of the surface of the failed landslide mass was determined by stereo photogrammetry.

Progressive failure in short-term slope stability is examined in this study. The relative displacement between a sliding mass and a basement in a landslide was monitored precisely. The onset time of relative displacement at each borehole was determined and the speed of propagation of local relative displacement was calculated. The lowest speed of propagation was 4.6 mhr⁻¹. Stresses inside the specimen were measured in the direct shear apparatus with a 400 × 400 mm shear plane. Results show that the horizontal sliding surface determined by the apparatus does not coincide with the plane in which Mohr’s failure criteria are first fulfilled, but with the plane of the maximum shear stress. Shear stress reductions were observed and the drop position migrated with the shear.

Based on those results, we can explain the kinematic processes of formation of a sliding surface during landslides: First local rupture occurs at a certain position where stress conditions fulfill the rupture criteria. The rupture surface that is mobilized by this rupture, in general, does not coincide with the potential sliding surface. Plastic deformation resulting from local failure increases in terms of its area and amount until plastic deformation along the potential sliding surface is eventually mobilized throughout the potential sliding surface. The enlargement process is dependent on material characteristics, variations in external forces, and boundary conditions. The discrepancy between the orientation of local rupture surface is determined mainly by stress and the orientation of potential sliding surface, which are determined mainly by boundary conditions and heterogeneity of materials, which are essential factors for progressive failure during landslides.

Detailed landform classification is important if effective measures against landslides are to be taken. Conventional techniques can only measure the detailed terrain in vegetated areas with difficulty. Airborne light detection and ranging (LIDAR) is a promising tool to precisely and directly measure a digital elevation model (DEM). Using a two-meter-grid DEM we attempted to understand landslide characteristics, namely, we produced manual and automated landform classification maps in Tomari-no-tai area in Shirakami Mountains, Japan. In advance, 1 : 2500-scale two-meter-interval contour map was newly printed using the LIDAR-DEM. It was found that valleys and other geomorphological features could be seen in better detail in the airborne LIDAR contour map than in the existing photogram-metric contour map. The map and 1 : 8000-scale aerial photographs were interpreted, and manual landform classification map was produced. As a result, 17 classifications were identified in the map.

In producing the automated landform classification map, in advance, three variables such as slope, surface texture (feature frequency, or spacing), and local convexity were calculated from the DEM. The three variables were subdivided into three, two, and two classes, respectively, and 12 classifications, which mean the combination of 3 × 2 × 2 classes, were identified in the map. The manual landform classification map can give useful information and ideas about landform evolution of the study area, but it may not fully extract geomorphological features. The automated landform classification map can objectively describe the surface morphology, but it of itself does not give information about landform evolution. Interpreting and extracting geomorphological features from the automated landform classification map will help us to revise the manual landform classification map and to comprehensively understand landform and landslide processes.

Recent advances in remote sensing techniques have yielded numerous new applications which provide benefits for all stages of landslide management. This paper describes how the data produced from different types of sensors and platforms have been used to estimate, model and mitigate landslide risk in sites in Italy and in other parts of the world. EO data have been utilized to update landslide inventories, with the identification of new landslides and the ratification or modification of landslide boundaries and states of activity, and to improve landslide hazard and risk assessment procedures at regional scales. Radar data provided a global topographic dataset that was used to model lahar inundation hazard while an example of the millimetric resolutions attainable from repeat-pass satellite radar data, with its beneficial implications for landslide monitoring, is also illustrated. Ground-based systems are shown to be innovative early warning systems for slow-moving landslides while coupled techniques involving both optical and radar images can provide support for the management of emergencies. The illustration of these case histories demonstrates the increasing importance of remote sensing in all facets of landslide management, with significant advantages for both policy makers and society.

The article deals with a socially important topic — preservation of cultural heritage in Slovakia. Current results of an engineering geological investigation of five medieval castles, one of them under the patrimony of UNESCO (Spis Castle) located in different geotechnical and engineering geological conditions are discussed. The obtained results are based upon an investigation program sponsored by the Ministry of Environment in which about 40 most prominent medieval castles or castle ruins have been investigated with the aim to recommend the remedial measures and thus to prevent their gradual disintegration. A following project focused on monitoring of the selected castle rocks showing instability problems was later carried out.

The results of engineering geological study and following monitoring showed, that the castles suffer from extremely slow displacement of creep character. This movement is caused mainly by the geologic structure and it is impossible to stop it or even reduce it. Since the monitoring results are influenced by seasonal periodic temerature changes a numerical modeling was used in order to understand better the landslide failure mechanism at Spis Castle site. Further a study of the thermal expansion as one of the potential triggering factors of the displacements observed in the castle’s subgrade was carried out.

The results gained during the study are of great value and are helpful in design of stabilization and preservation works.

This paper introduces ideas for extracting important parameters for landslide risk assessment from real landslide masses. The parameters included velocities estimated from mud spatters and therefore equivalent viscosities of fluidized soil masses. Mud spatters remaining on dwelling walls etc., which often show parabolic patterns, are used to estimate flow velocities. One example shows that the viscosity of a liquefied pyroclastic material is about two to three digits smaller than those obtained from torsional shear tests for sandy soil, indicating high potential for the fluidized volcanic products to travel fast a long distance even on this extremely gentle slope.

Geometric parameters describing a deformed shape of a coherent landslide mass indicate ultimate load capacity that the landslide mass can bear. This capacity is less sensitive to the entire length of the landslide mass, and determines the distal reach of the soil mass.

It was a remarkable aspect that a tremendous number of landslides were triggered by the Mid-Niigata Prefecture Earthqauke. Because the epicenter was located at a depth of 13 km just in the landslide prone area of central part of the Niigata Prefecture, severe damage was caused by the earthqauke-induced landslides. Furtehermore many landslide dams were formed mainly in the watershed of the Imogawa-River by the displaced soil mass of the earthquake-induced landlslides. Some large landslide dams should pose a great threat of flood and debris flow in case of dam collapse to the settlemnt of the Ryuko-District in the downstream area of the watershed. It was urgently needed to arrange the emergency operations to avoid the destructive collapse of the major landslide dams. This contribution illustrates overview of landslide dams and emergency operations against dam collapse.

Shear behavior of clay in slope for pore water pressure increase was investigated with both triaxial compression and ring shear apparatuses. Not only shear strength but also shear deformation was examined. Pore water pressure loading test was conducted with tiaxial compression apparatus. Yield and failure points for pore water pressure increase were obtained. It was shown that yield point was on the critical state line described by internal friction angle of φ’. It means the design method with φ’ indicates the design to prevent slope from instability and deformation occurrence. Shear displacement in pore water pressure loading test revealed time dependent behavior as creep. It was made clear to be caused by water migration and successive progressive failure. Creep behavior of landslide was discussed from the viewpoint of coupling theory in continuum.

Stress controlled ring shear test was implemented for both virgin and re-sliding type landslide conditions. In virgin type landslide test, shear displacement increased with the decrease in normal stress. Yield and failure points were obtained. It was clarified that yield points were on the internal friction angle line obtained by strain controlled ring shear test. On the contrary, shear displacement was found to be fragile and to increase sharply at yield point in case of re-sliding type landslide test. Yield points were found on the residual internal friction angle line. Discussion on consolidation effect on yield and failure points was carried out in detail.

The slope stability is commonly assessed using limit equilibrium methods. However, the limit equilibrium methods cannot calculate displacements of slopes, thus they cannot evaluate correctly the soil-slope stabilization interaction. Here we discuss how to use the finite element analysis to evaluate the stability of slopes reinforced piles, anchors, or drainage pipes under static condition, and to calculate the behavior of slopes under dynamic loads of earthquakes.

In the first half, the finite element method with shear strength reduction technique (SSRFEM) is explained. This method is effective for the prediction of the stability of slope improved with piles, anchors, or drainage pipes in order to consider the three-dimensional effects in slopes. Based on numerical comparisons between the finite element method and limit equilibrium methods, it is suggested that the finite element method with shear strength reduction technique is a reliable, robust, and maybe unique approach to evaluate the slope stability and slope stabilization. In the second half, the dynamic response analysis of a slope based on the elastoplastic finite element method is explained. A simple 3-D cyclic loading model that can be applied to the seismic design of slope is introduced, and it is suggested that the dynamic elasto-plastic FEM makes it possible to evaluate the residual deformation of a slope induced by the earthquake precisely.

The work summarizes information about spatial and temporal occurrence and damage caused by debris flows within last 50 years in the vicinity of the Machu Picchu village. Size and dynamic characteristics of limited number of events reveal, that two main classes of debris flows occurred in the study area. These classes are distinguished by specific triggering conditions, amount of delivered material to the main river valley (Urubamba River), duration time and speed. Incomplete weather information and debris flows allowed only preliminary comparison of relationship between rainfalls and debris flow occurrence. The results suggest coupling between 6-days preceding precipitation index and debris flow occurrence. Several low cost mitigation measures and risk management practices were suggested to reduce risk of the economically important area of the Machu Picchu village.

The present paper describes deformation processes and geomorphological hazards affecting the historical site of Bamiyan (Central Afghanistan). The major cultural heritage of the site were the two standing Buddhas that, carved in rock since the 2^nd-3^rd century A.D., were destroyed by Taliban in March 2001. Slope instablity from both the consequences of the explosion as well as from natural processes are quite evident: rock slides and rock falls have already occurred in the recent past and most areas are prone to collapse.

Under the UNESCO coordination, a global feasibility project for the needed restoration work was developed; field data were collected and mechanisms for potential cliff and niche evolution were provided. In the meantime a first practical consolidation work for the most critical rockfall-prone area, was implemented to avoid any further collapse in the period 2003–2005. This last is also considered essential to allow archaeologists the safe cataloguing and recovering of the Buddha Statue remains, still on the floor of the niches.

Debris flow disasters are usually accompanied by serious loss of lives and properties. However, debris flows are also part of Earth’s natural phenomenon, what is the reasonable budget to be spent on mitigation measures becomes an important issue on the budget allocation processes. This paper utilizes economic concept to propose a reasonable estimation of the hazard damage and the cost of proposed mitigation measures. The proposed method is composed of 4 steps, namely, delineating the area of the disaster with different return periods, itemizing the land use within those area, calculating the hazard lose using official values and computing the expected probable maximum lose with a probability distribution. The comparison between the assessment of hazard and the economic gains of any proposed mitigation measures can be used as a reference for future decision making process.