Changes In Slope Stability Research Articles

Numerical analysis of effective soil porosity and soil thickness effects on slope stability at a hillslope of weathered granitic soil formation

Modeling rainwater infiltration in slopes is vital to the analysis of slope failure induced by heavy rainfall. Amongst the soil hydraulic properties, the hydraulic conductivity K has been frequently analyzed for its effects on slope stability. In contrast, few studies have been published on the effects of water retention characteristics on slope stability. In this study, a numerical model was developed to estimate the extent of rainwater infiltration into an unsaturated slope, the formation of a saturated zone, and the change in slope stability. This model is then used to analyze the effects of the soil porosity parameters (i.e., saturated soil water content θs and effective soil porosity (ESP)) and soil thickness on the occurrence of slope failure. Results showed that when the surface soil of a slope has a relatively large ESP value, it has a greater capacity for holding rainwater, and therefore delays rainwater infiltration into the subsurface layer. Consequently, the increase in pore water pressure in the subsurface layer is also delayed. In this manner, a relatively large surface layer ESP value contributes to delaying slope failure. Under weaker storm conditions, slope failure tends not to occur when the surface soil has a relatively large ESP value. In addition, the thickness of soil is also a significant parameter in slope stability analysis. A shallow soil depth resulted in greater discharge volume and a lower peak pore water pressure during the major rainfall event, and consequently the slope failure tends not to occur. However a deeper soil depth increased the weight of solids and the soil moisture conditions in the slope consequently increasing the pore water pressure causing slope failure.

Effects of forest harvesting on the occurrence of landslides and debris flows in steep terrain of central Japan

AbstractLandslides and debris flows associated with forest harvesting can cause much destruction and the influence of the timing of harvesting on these mass wasting processes therefore needs to be assessed in order to protect aquatic ecosystems and develop improved strategies for disaster prevention. We examined the effects of forest harvesting on the frequency of landslides and debris flows in the Sanko catchment (central Japan) using nine aerial photo periods covering 1964 to 2003. These photographs showed a mosaic of different forest ages attributable to the rotational management in this area since 1912. Geology and slope gradient are rather uniformly distributed in the Sanko catchment, facilitating assessment of forest harvesting effects on mass wasting without complication of other factors. Trends of new landslides and debris flows correspond to changes in slope stability explained by root strength decay and recovery; the direct impact of clearcutting on landslide occurrence was greatest in forest stands that were clearcut 1 to 10 yr earlier with progressively lesser impacts continuing up to 25 yr after harvesting. Sediment supply rate from landslides in forests clearcut 1 to 10 yr earlier was about 10‐fold higher than in control sites. Total landslide volume in forest stands clearcut 0 to 25 yr earlier was 5·8 × 103 m3 km−2 compared with 1·3 × 103 m3 km−2 in clearcuts >25 yr, indicating a fourfold increase compared with control sites during the period when harvesting affected slope stability. Because landslide scars continue to produce sediment after initial failure, sediment supply from landslides continues for 45 yr in the Sanko catchment. To estimate the effect of forest harvesting and subsequent regeneration on the occurrence of mass wasting in other regions, changes in root strength caused by decay and recovery of roots should be investigated for various species and environmental conditions. Copyright © 2007 John Wiley & Sons, Ltd.

Effects of Soil Porosity on Slope Stability and Debris Flow Runout at a Weathered Granitic Hillslope

Accurate models of rainfall infiltration are needed for analysis and prediction of slope failure induced by heavy rainfall. In this study, a numerical model was developed to simulate two‐dimensional rainwater infiltration into an unsaturated hillslope, the formation of a saturated zone, and the resultant changes in slope stability. This model was subsequently used to analyze the effects of soil porosity parameters (i.e., saturated soil water content θs and effective soil porosity [ESP]) on the occurrence of slope failure, the moisture conditions of the displaced material, and the movement of debris flow on weathered granitic hillslopes. We conducted the simulations by imposing various conditions of rainfall, initial wetness of the slope, soil thickness, and slope gradient. Results showed that when the surface soil of a slope has a relatively large ESP value, it has a greater capacity for holding water and therefore delays deeper water infiltration into the subsurface. Consequently, the increase in pore water pressure in the subsurface at a greater depth is also delayed. In this manner, the greater ESP value contributes to delaying slope failure. Under small storm conditions, slope failure tends not to occur when the surface soil has a relatively large ESP value. However, a greater ESP tends to increase the water content of the displaced matter, which results in faster and longer travel distances, and more deposition of debris flow, thus increasing the risk of damage in downstream regions.

The fluvial system response to abrupt climate change during the last cold stage: the Upper Pleistocene River Thames fluvial succession at Ashton Keynes, UK

The last interglacial–glacial cycle (125–10 ka BP) is characterised by numerous rapid shifts in global climate on sub-Milankovitch timescales, recorded in the ocean and ice core records. These climatic fluctuations are clearly recorded in those European terrestrial sedimentary sequences that span this time period without interruption. In the UK, only fragmentary Upper Pleistocene sequences exist, mainly within the fluvial archive of the major river systems such as the Thames. The response of the upper River Thames to abrupt fluctuations in climate is documented in the fluvial sediments beneath the Floodplain Terrace (Northmoor Member of the Upper Thames Formation) at Ashton Keynes, Wiltshire. A number of criteria are set out by which significant changes in the fluvial system may be established from the sedimentological, palaeoecological and geochronological information contained within the succession. The sedimentary succession is divisible into four facies associations, on the basis of their sedimentology and bounding surface characteristics. These represent distinct phases of fluvial activity at the site and allow changes in fluvial style to be inferred. Palaeoecological reconstructions from pollen analysis of peats within the sequence provides an indication of the nature and direction of Late Glacial environmental change and optically stimulated luminescence and radiocarbon dating methods provide chronological control on the sequence. These data suggest that major changes in fluvial style are recorded within the succession, which can be related to the climatic fluctuations that took place on the oxygen isotope stage 5a/4 transition (approximately 70 ka BP) and the Devensian Late Glacial climatic warm–cold–warm oscillation (13–11 ka BP). The changes in fluvial style are a result of variations in sediment supply to the river resulting from changes in slope stability, vegetation cover and cold-climate mass movement processes and variations in discharge regime caused by changes in precipitation patterns, snow cover, permafrost distribution and vegetation cover.

Slope-stability change from late Pleistocene to Holocene in the Ulleung Basin, East Sea (Japan Sea)

A wide variety of mass-movement deposits prevail along the entire margin of Ulleung Basin, overlain by a hemipelagic mud unit (1–2 m thick). The boundary between the lower turbidite sequence and the upper hemipelagic mud on the basin plain is distinct, and approximately coincides with the well-known, basin-wide ash layer (Ulleung ash) of early Holocene age. In the absence of significant terrigenous sediment input, the lithologic change from the turbidite to hemipelagic mud may reflect physical effects of glacio-marine eustatic sea-level change. Sea-level lowering during the glacial period may have generated excess pore pressure within the sediments caused by reducing hydrostatic confining pressure, and facilitated triggering of the deep-water slope failure by earthquakes. Storm waves may also have affected the uppermost slope area to induce small-scale slope instabilities during the lowered sea-level stand. These glacial factors destabilizing the slope appear to have enhanced large-scale slope failures during the late Pleistocene. In contrast, the topmost hemipelagic mud attests to a stable slope phase associated with raised sea-level in the Holocene, consistent with the infinite slope stability analyses. Calculations of the sedimentation rates based on C-14 dates of several ash layers indicate that the pre-Holocene turbidites on the basin plain had accumulated at an average rate of 40 cm/1000 years with a minimum recurrence interval of 50 years during the late Pleistocene. Mass-flux estimations between the slumps and turbidites suggest a predominance of the nondisintegrative mode of slope failures in the Ulleung Basin. In each event of slope failure, most of the failed mass rested on the slope as slide/slump blocks, and only its meager portions were deposited on the basin plain as turbidite layers.

夕張山地南部・日高山脈西部における最終氷期から完新世初頭にかけての斜面安定性の垂直的変遷

This study reconstructed the vertical change of slope stability in the southern part of the Yubari Mountains and the western part of the Hidaka Range, Hokkaido, Japan, by using three eolian pumice layers deposited during the Last Glacial and early Holocene; Spfal, 33, 000-32, 000y.B.P.; En-a, 19, 000-16, 000y.B.P.; Ta-d, about 8, 000y.B.P. A pumice layer overlying a mountain slope indicates that the slope has been stabilized since the pumice fell. In this view, the examination of each pumice layer in about 330 outcrops on the mountain slope suggests a belt of slope stability in the study area, as follows:Spfal; crest slope lower than about 250m high.En-a; crest slope lower than about 500m and side slope lower than about 300m high.Ta-d; crest-side slope lower than about 1, 250m high.On the other hand, the lack of a pumice layer and/or the absence of a pumice layer which is usually incorporated in the colluvium suggest slope instability after the fall of the pumice. Several outcrops on the mountain slope suggest that the removal of the pumice layer was mainly caused by periglacial slope processes, associated with the frost shattering of the underlying bedrock. The belt higher than the belt of slope stability, in which each pumice layer was removed, therefore, corresponds to that of the periglacial belt in the period after the fall of each pumice layer. According to this view, the lower limit of the periglacial belt rose from about 250m during the 33, 000-18, 000y.B.P., to about 500m during 18, 000-8, 000y.B.P., to about 1, 250m after 8, 000y.B.P. This vertical change in slope stability is parallel to that of the timber-line and snowline of the study area which has been indicated by other data.

湿原たい積物中の混入土砂量の変化からみた屋久島における斜面の安定性の変動

Slope stability and its change play important roles in geomorphic development both of mountains and coastal plaines. Changes in slope stability is expected to have been recorded in geological sections at depositional areas close to slopes and/or in mountain areas. Changes in slope stability were investigated on the basis of stratigraphical analysis, especially analysis of variation of inorganic content in peat deposits at moors and a grassland of Yakushima Island (30°20'N, 130°30'E), which is famous as one of the most rainy regions in Japan.Three localities on an alluvial cone, a flood plain and a gentle slope on a ridge were selected as the study areas (Fig. 1). Using the panning method, the inorganic content is calculated as the ratio of inorganic content against dry bulk weight of each horizon and expressed by percent. The change in inorganic content was concordant with those in facies and water content (Fig. 2, 3), and showed good agreement with that of ignition loss of thick humic soil (Fig. 7). From the sequential changes in degree of plant decomposition and the inorganic content, peat and thick humic soil profiles can be divided into four layers, namely, the Lowest layer, the Lower one, the Middle one, and the Upper one in ascending order. The change in the composition of materials from the Lower to the Middle layer is the most conspicuous. The changes in the inorganic content suggest that the yield of detritus was the most intensive at the Lower layer and was reduced towards the surface.These inorganic materials on the alluvial cone and the flood plain were transported by mud flows and floods. Therefore, the high content of inorganic materials can be considered to have been caused by heavy rainfall. Consequently these changes in inorganic content roughly suggest the changes in heavy rainfall occurrences. Initiation of the deposition of the Lower layer is presumed to be after 2000BP from the 14C date (2600-2700BP) of the base of peat layer at Hananoego moor and the thickness of the Lowest layer. Therefore, the slope condition on Yakushima Island is considered to become less stable after 2000BP.