Shallow Failure Research Articles

The mechanisms behind shallow failures in slopes comprised of landslide deposits

In areas hit by the 2008 Wenchuan earthquake that occurred in Sichuan Province, China, debris flows are often generated from landslide deposits during heavy rainstorms. The broadly graded and unconsolidated landslide deposits respond to rainfall in very complex mechanical and hydraulic manners. An artificial rainfall test was conducted to simulate the rainfall infiltration and surface runoff processes occurring on the landslide deposit slope at the Wenjiagou Gully, China, with heavy rainfall rates (140mm/h) incorporated. An innovative flume was designed to collect the slope interflow and surface runoff separately. Sensors to monitor the pore water content (PWP) and volumetric water content (VWC) were deployed. The results indicated that there were four stages in the hydrological response of landslide deposits during the artificial rain event: infiltration, a slow increase in interflow (surface runoff begins), a rapid increase in interflow (surface runoff slowly increases) and a steady state. Bed gradient increase will lead to PWP rapidly ascending and regressive failure happening. Concomitant with the observed increases in PWP and VWC, the shear strength of the landslide deposits decreased and led to the occurrence of small-scale shallow failures. Surface runoff, interflow and fine particle migration effects are presented to interpret the process of shallow failure. And although shallow slope failure is the result of interaction with the above three factors induced by rainfall, the key underlying factor is the characteristically loose structure of landslide deposits.

유한요소 연계해석을 이용한 불포화 토사사면 안전성 평가

Limit equilibrium methods of slope stability analysis have been widely adopted mainly due to their simplicity and applicability. However, the conventional methods may not give reliable and convincing results for various geological conditions such as nonhomogeneous and anisotropic soils. Also, they do not take into account soil slope history nor the initial state of stress, for example excavation or fill placement. In contrast to the limit equilibrium analysis, the analysis of deformation and stress distribution by finite element method can deal with the complex loading sequence and the growth of inelastic zone with time. This paper proposes a technique to determine the critical slip surface as well as to calculate the factor of safety for shallow failure on partially saturated soil slope. Based on the effective stress field in finite element analysis, all stresses are estimated at each Gaussian point of elements. The search strategy for a noncircular critical slip surface along weak points is appropriate for rainfall-induced shallow slope failure. The change of unit weight by seepage force has an effect on the horizontal and vertical displacements on the soil slope. The Drucker-Prager failure criterion was adopted for stress-strain relation to calculate coupling hydraulic and mechanical behavior of the partially saturated soil slope.

English

- This paper examines how rainfall patterns affect the mechanisms of shallow slope failure. Numerical modelling, utilising the commercial software SVFlux and SVSlope, was carried out for a coupled analysis of rainfall-induced slope seepage and instability, with reference to a shallow landslide took place in Jabiru, Northern Territory (NT) Australia in 2007. Rainfall events were varied in terms of pattern in this analysis. The results revealed that slopes are sensitive to rainfall pattern when the rainfall intensity has a high degree of fluctuation at around the same value as that of saturated hydraulic conductivity. Average rainfall intensity at the beginning of a rainfall period plays a primary role in determining the rate of decrease in initial factor of safety (F i ) towards minimum factor of safety (F min ). The effect of rainfall events on the slope instability is attributed to the amount of rainwater infiltration into slope associated with rainfall pattern. Keywords: Rainfall pattern; Fluctuating intensity, Shallow failure mechanism; Slope failure; Rainwater infiltration.

The Effect of Rainfall Patterns on the Mechanisms of Shallow Slope Failure

Abstract - This paper examines how rainfall patterns affect the mechanisms of shallow slope failure. Numerical modelling, utilising the commercial software SVFlux and SVSlope, was carried out for a coupled analysis of rainfall-induced slope seepage and instability, with reference to a shallow landslide took place in Jabiru, Northern Territory (NT) Australia in 2007. Rainfall events were varied in terms of pattern in this analysis. The results revealed that slopes are sensitive to rainfall pattern when the rainfall intensity has a high degree of fluctuation at around the same value as that of saturated hydraulic conductivity. Average rainfall intensity at the beginning of a rainfall period plays a primary role in determining the rate of decrease in initial factor of safety (Fi) towards minimum factor of safety (Fmin). The effect of rainfall events on the slope instability is attributed to the amount of rainwater infiltration into slope associated with rainfall pattern. Keywords: Rainfall pattern; Fluctuating intensity, Shallow failure mechanism; Slope failure; Rainwater infiltration.

Probabilistic stability analysis of rainfall-induced landslides considering spatial variability of permeability

Many regions around the world are vulnerable to rainfall-induced landslides. A variety of methods have been proposed for revealing the mechanism of landslide initiation. Current analysis methods, however, do not consider the effects of non-homogeneous soil profiles and variable hydraulic responses on rainfall-induced slope failures. In Korea, where the depth of weathering is very shallow, many slope failures occur in the layer of weathered residual soil that overlays the bedrock. These failures are characterized by shallow failure surfaces located near the interface between the weathered soil and the underlying bedrock. In this study, probabilistic stability analyses were conducted for a weathered residual soil slope with shallow impermeable bedrock to study the failure mechanism of rainfall-related landslides. A series of seepage and stability analyses of an infinite slope based on one-dimensional random fields were performed to study the effects of uncertainty due to the spatial heterogeneity of hydraulic conductivity on the failure of unsaturated slopes due to rainfall infiltration. The results showed that a probabilistic framework can be used to efficiently consider various failure patterns caused by spatial variability of hydraulic conductivity in rainfall infiltration assessment for a shallow infinite slope.

Landslides in a loess platform, North-West China

Frequent occurrence of landslides in the Chinese Loess Plateau has seriously threatened the urban and rural development. Heifangtai loess platform was selected as a case study for the current research. The platform surface is being largely irrigated since 1968. About 70 landslides of different types have occurred in the region including loess slides, loess flows, combined loess, and bedrock landslides along/across bedding planes. The loess slides occurred in the unsaturated soil and have a shallow failure depth. The loess flows initiated at the contact between loess and underlying clay and can travel a distance of more than 300 m at a speed of up to 5 m/s. Controlled by the topography of the underlying clay layer, the loess flows mainly occurred along the eastern margins of Heifangtai. The combined loess and bedrock landslides along bedding planes typically travel a distance of 30–60 m at a speed of 0.002–0.005 m/s with a volume of up to several million cubic meters. This type of landslide commonly occurred along the southern margins of Heifangtai, where the slope aspects are close to the inclination direction of the bedding planes of underlying bedrocks. Monitoring of perched water table in loess layer at the northern margin of Heifangtai shows the water table is rising at a speed of about 18 cm/year.

강우패턴을 고려한 화강풍화토 사면의 침투 및 안정성 분석

전 세계에서 우기 시 내린 유발강우로 인해 사면파괴가 발생하고, 이러한 사면파괴로 재산 및 인명에 상당한 피해를 주고 있다. 강우로 인한 사면파괴는 불포화 지반에서 모관흡수력의 감소로 인해 얕은 파괴형태로 발생한다. 본 연구에서는 실제 사면파괴 당시의 강우패턴을 산정하여 강우패턴이 화강풍화토 사면의 안정성에 미치는 영향을 분석하고자 하였다. 1999부터 2011년까지 사면파괴가 발생한 화강풍화토 지반을 대상으로 붕괴 유발 강우량을 분석하였다. 취합된 사면붕괴 사례는 총 106개이다. 국내 화강풍화토 사면에서 유발된 사면파괴와 유발강우에 대한 분석을 통해 2개의 사면파괴 유발 강우패턴을 도출하였다: 당일 강우량이 사면파괴에 큰 영향을 미치는 Strong major rainfall(SMR) 패턴과 파괴 하루 전 선행강우가 사면안정성에 영향을 미치는 Strong antecedent rainfall(SAR) 패턴. 또한 비교분석을 위하여 일정강우(Constant rainfall, CR) 패턴을 산정하여 불포화 사면의 침투 및 안정해석을 수행하였다. 해석결과, 사면의 안전율은 강우패턴, 강우강도 및 투수계수에 의존하는 것을 알 수 있다. Many slope disasters occur in many parts of the world during rainy season. Rainfall is one of the significant factors related to the landslides which cause a considerable damage to society. The rainfall-induced landslides in unsaturated slopes generally have shallow failure and are triggered by decrease in matric suction during rainfall. In this paper, effects of rainfall patterns on slope stability of the unsaturated weathered soil were investigated. Landslides inventory consisted of 106 landslides data from 1999 to 2011 was used to induce two rainfall patterns: strong major rainfall(SMR) pattern and strong antecedent rainfall(SAR) pattern. SMR pattern is a rainfall type in which the major rainfall of the landslide event day provides a key role on the landslide occurrence. SAR pattern is that the antecedent rainfall of the 1-antecedent day gives an important role on the landslide occurrence. Constant rainfall pattern was also used for comparison in numerical analysis. Combined seepage and stability analyses were performed to study the stability of slope subjected to various rainfall patterns. The results of numerical analysis indicated that variation of factor of safety of weathered granite slope depended on rainfall pattern, rainfall intensity and soil permeability.

Comparison of failure modes below footings on carbonate and silica sands

To investigate the differences in failure mechanisms beneath vertically loaded shallow foundations on silica and carbonate sands by way of particle image velocimetry analysis, 1g and centrifuge model tests have been performed. A comparison of silica and carbonate sand behaviour at low stresses and low initial soil density was made possible by 1g scaling conditions, while centrifuge conditions focused on high stresses and high soil density. This strategy provided separate insights into the effects of the compressibility and crushability of both sands. Results indicated a distinct difference in failure mode between the two sands. The footing on silica sand exhibited a shallow failure mode with large localised strains close to the soil surface. In contrast, the footing on carbonate sand mobilised strains lower in magnitude over a greater depth. The increased stress level in the centrifuge reduced the surface heave and resulted in deeper failure mechanisms in both sands. The bearing capacity factor for the experimental results could be back-calculated with simple limit analysis solutions, provided that different friction angles were used to account for the variation of stress levels. However, such an approach masks the influence of compressibility on the failure mechanism in the loose and compressible cases by requiring a lower friction angle than is realistically possible.

쏘일네일링의 세 가지 파괴모드를 고려한 설계 최적화에 대한 연구

쏘일네일링 공법은 흙막이 또는 사면안정을 위해서 가장 많이 사용되는 공법이다. 일반적으로 쏘일네일링 공법의 설계에서는 인발에 의한 파괴와 전단에 의한 파괴를 고려한다. 쏘일네일링의 파괴거동은 인발파괴와 전단파괴와 같이 파괴면을 가지면서 사면이 무너지는 경우도 발생하지만 굴착에 의해서 사면 표면의 수평응력이 감소함에 따라 점점 표면이 쓸려가는 얕은 파괴에 의해서 파괴에 이르는 경우가 실제 현장에서 자주 발생하게 된다. 따라서 쏘일네일링의 파괴거동을 크게 인발파괴, 전단파괴, 그리고 얕은파괴로 나누어 정의하였다. 본 논문에서는 각각의 파괴모드에 대한 제약조건을 이론적으로 산정하였다. 또한 각각의 파괴를 막기 위한 설계 최적화를 실시하였으며, 네일링의 정착길이, 개수, 그리고 얕은파괴를 막기 위한 전면에서의 최소 구속압을 설계변수로 두어 최적화 과정을 진행하였다. 최적화 과정은 먼저 네일링의 정착길이와 인장력을 설계변수로 하여 인발파괴 및 전단파괴에 대하여 최적화를 실시한다. 다음으로 각 굴착단계별 사면의 표면에서 얕은파괴를 막기 위한 최소의 구속압을 산정한 후 최적화를 반복수행하여 각각의 설계 변수를 산정하게 된다. 이와 같은 설계 최적화 프로그램을 통해서 인발파괴와 전단파괴만을 고려하는 기존의 설계 시스템에서 프리스트레스까지 산정할 수 있게 되었다. Soil-nailing is the most popular method of reinforcing for slope stability. In general, two factors are considered as failure modes during the soil-nailing design stages: pullout failure mode and shear failure mode that will occur on the most probable failure plane. In many cases, however, shallow failure can also occur when the ground near the slope face is swept away by the horizontal stress release during the staged top-down excavation. In this paper, an optimized soil-nailing design methodology is proposed by considering the three failure modes mentioned above: pullout failure; shear failure; and shallow failure. The variables to be optimized include the bonded length and number of soil-nailings, and the confining pressure that should be applied at the slope face. The procedure to obtain the optimized design variables is as follows: at first, optimization of soil-nailings, i.e. bonded length and number, against pullout and shear failure modes; and then, optimization of confining pressure at each excavation stage that is needed to prevent shallow failure. Since the two processes are linked with each other, they are repeated until the optimized design variables can be obtained satisfying all the constrained design requirements in both of the two processes.

Tree Induce Suction for Slope Sustainability

Many slope areas in Malaysia whether it is man made or natural slope are particularly vulnerable to soil erosion and shallow failures due to the increasing of soil moisture and porewater pressure. This study will determined the active root zone of the tree near the toe of the slope which suctions have been generated within this section. Hence, the effect it only focusing on hydrological aspect with soil moisture pattern near vicinity of the tree. The mechanical aspect of tree root such as tensile strength and bonding between root and soil which can led to increasing soil strength are assumed negligible in this study. From preliminary result of field monitoring show significant increase of total suction at near vicinity of tree which can lead to stability analysis on slope. The comparison of FOS will be presented between at the toe of slope with and without the tree. Furthermore, for slope studies area the reresult indicates that tree induced suction can be related to cause the factor of safety against slope failure improve up to 33.07%.

Stability analysis and supporting system design of a high-steep cut soil slope on an ancient landslide during highway construction of Tehran–Chalus

The stability of both natural and cut slopes in mountainous areas is a great challenge to highway constructions and operations. This paper presents a successful case study of stability analyses and protection treatments for high-steep cut soil slopes in an ancient landslide zone which was located at Km12+700 to Km15+000 along the Tehran–Chalus highway. This report has three parts. First, geotechnical investigations of in situ direct shear test, SPT tests and laboratory tests were implemented to get the subsurface profiles and the mechanical properties of the soil mass. Second, finite difference analysis was carried out to evaluate the stability of both the natural and cut slopes. Minimum safety factors and potential failure modes of cut slopes were obtained under both static and dynamic conditions. These results indicated that the ancient landslide could not be reactivated under the present climatic and morphological conditions, but there were some potential shallow failures in some cut soil slopes (failure actually occurred during excavation). Protection treatments and reinforcements were thus necessary. Third, the stability of the cut slopes was re-assessed by simplified Bishop limit equilibrium analysis (using Slide 5.0). Some potential failure zones were designed to be protected by back-anchored concrete retaining wall at the slope toe, rock bolts and frame beams on the slope face and planting grass on the slope face. Numerical analysis indicated that these protection measures could stabilize this remedial slope. These practical experiences may be of benefit for similar highway construction projects.

Predicting shallow surficial failures in the Mississippi River levee system using airborne hyperspectral imagery

Shallow surficial failures or levee slides in the Mississippi River levee system are very common. There is currently no system to identify or predict the location of these slides before they occur. Studies of slide occurrence mechanisms suggest that probable slide-affected areas are characterized by anomalous vegetation. Compact Airborne Spectrographic Imager II (CASI II) imagery was analysed for selected levee sites in association with slide inventory data and field observations. Normalized Difference Vegetation Index (NDVI), Red edge Vegetation Stress Index (RVSI) and Red Edge Position Index (REP) were calculated from the acquired CASI II imagery. The vegetation indices were used to locate the stressed or anomalous vegetation and predict levee slides. The statistical significance of the predictors was determined by logistic regression. All predictors were found statistically significant for developing a slide prediction model. The slide prediction model was developed by combining the single predictors, categorized vegetation indices, into a model based on all three predictors. Percentage of Search Area Reduction (PSAR) and Failure Index (FI) were used to evaluate the performance of the slide prediction model. Evaluation of the model performance shows that it achieves a maximum FI of 0.43 and PSAR of 99.5.

Two and three-dimensional slope stability reanalyses of Bukit Batok slope

A residual soil slope in Singapore that failed on two occasions, in 1989 and 1991, was reanalysed using two (2D) and three-dimensional (3D) slope stability analyses. The geometry of the slope in 1989 differed from that in 1991. The 2D and 3D slope stability analyses on the slope that failed in 1989 showed that the average shear strength parameters of the residual soils are representative of the slope and the factors of safety obtained from 2D slope stability analyses are not necessarily more conservative than 3D slope stability analyses. The analyses also showed that the slope failed in 1989 due to rising groundwater table. The analyses of the slope in 1991 showed that the slope experienced shallow failure due to the high groundwater table. Analyses of the slope after lowering of groundwater table by horizontal drains showed that the factor of safety of the slope has improved tremendously. The differences in factors of safety for 2D and 3D slope stability analyses are greater for low groundwater table as compared with those for high groundwater table. The results illustrated how 3D slope stability analyses have become less daunting to perform and can be incorporated into routine slope designs.

Deep cavity flow mechanism of pipe penetration in clay

The evolution of penetration resistance as a function of penetration depth of a pipe into a cohesive seabed is of practical importance, particularly in the areas of pipeline on-bottom stability assessment and T-bar penetrometer data interpretation. In the past, this subject was addressed primarily in a discontinuous manner by separating the penetration response into two broad regimes of shallow and deep penetrations followed by deriving plasticity solutions assuming a simplified “wished-in-place” configuration. In this manner, the effects of evolving seabed topology and the progressive transition from a shallow failure mechanism to a deep failure mechanism are neglected. This paper aims to provide greater insights into the transition zone, which is especially important for the interpretation of T-bar test data at shallow depths. In this study, the penetration response of a smooth pipe over a wide range of normalized clay strengths is numerically simulated. A deep cavity flow mechanism where the bearing capacity factor is 12% less than the conventional full-flow mechanism is identified and found to be operative up to a depth of 10 pipe diameters under a certain combination of material properties. An analysis method is proposed to predict the load–penetration response for a given set of clay strengths and pipe diameters.

Failure Mechanics of the Nile Valley Landslide, Yakima County, Washington

Research Article| November 01, 2011 Failure Mechanics of the Nile Valley Landslide, Yakima County, Washington THOMAS C BADGER; THOMAS C BADGER Washington State Department of Transportation, Geotechnical Division, P.O. Box 47365, Olympia, WA 98504-7365 Search for other works by this author on: GSW Google Scholar ERIC L SMITH; ERIC L SMITH Washington State Department of Transportation, Geotechnical Division, P.O. Box 47365, Olympia, WA 98504-7365 Search for other works by this author on: GSW Google Scholar STEVE M LOWELL STEVE M LOWELL Washington State Department of Transportation, Geotechnical Division, P.O. Box 47365, Olympia, WA 98504-7365 Search for other works by this author on: GSW Google Scholar Environmental & Engineering Geoscience (2011) 17 (4): 353–376. https://doi.org/10.2113/gseegeosci.17.4.353 Article history first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation THOMAS C BADGER, ERIC L SMITH, STEVE M LOWELL; Failure Mechanics of the Nile Valley Landslide, Yakima County, Washington. Environmental & Engineering Geoscience 2011;; 17 (4): 353–376. doi: https://doi.org/10.2113/gseegeosci.17.4.353 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyEnvironmental & Engineering Geoscience Search Advanced Search Abstract On October 11, 2009, a very large translational landslide, referred to as the Nile Valley Landslide (NVL), destroyed more than 2500 ft (750 m) of State Route 410 near the community of Nile, located about 25 mi northwest of Yakima, Washington. Based on eyewitness accounts, Pacific Northwest Seismic Network records, and an intensive subsurface investigation, we infer two translational failure mechanisms: an initial shallow failure within the unconsolidated surficial deposits, followed by failure within a deep, adversely dipping, claystone interbed between two Grand Ronde Basalt flows. The underlying basalt flow contains a highly pressurized aquifer that significantly reduces the effective stress within the inferred deep failure zone and its stability. Recent inclinometer data showing slope movement within both the shallow and deep failure zones support this interpretation. The NVL is located at the base of Cleman Mountain, an anticlinal ridge that hosts the enormous and currently dormant Sanford Pasture landslide complex (SPLC). Geochemical analyses of basalts encountered in the numerous borings and outcrops within and around the NVL suggest that the basal failure zones of the NVL and SPLC are stratigraphically different, and that the NVL is likely not a reactivation of the SPLC. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Geo- and Hydro-Mechanical Evaluation of Slope Failure Induced by Torrential Rains in Northern-Kyushu Area, July 2009

ABSTRACT Torrential rainfall in mid-July 2009 triggered numerous geodisasters such as slope failure and debris flow in Chugoku and Northern Kyushu areas of Japan. A number of slope failures and debris flows occurred in Yamaguchi and Fukuoka prefectures resulting in extensive damage to human life and infrastructure. One of the most serious geodisasters included a slope failure followed by debris flow at Sasaguri-machi and Fukuchi-machi, Fukuoka prefecture, Japan. This paper summarizes the results of geotechnical investigations on the geodisaster sites in Fukuoka prefecture. The geotechnical investigation included determining a series of grain size distributions, consistency limits and conducting direct box shear tests for collapsed soils collected at six disaster sites. The generation mechanisms of slope failure followed by debris flow were also investigated by analyzing the precipitation, topography, geology, and strength properties of the collapsed soils. Moreover, slope deformation and stability analyses were coupled with an unsaturated-saturated seepage analysis to investigate the slope failure mechanism. The main findings from the study are summarized as: The physical properties, such as the grain size distribution, the plastic limit and liquid limit of collapsed soils, are summarized and compared with the results of other failure slopes in the literature. The collapsed soil was characterized as being a well grained soil (the uniformity coefficient >50) and highly weathered (the ignition loss >5%), however, with regard to the liquid limit and plastic index, there were no remarkable findings. The original shear strength for collapsed soils with natural water content is relatively large and slope failure doesn't occur because the cohesion in the shear strength is induced by a suction force between the soil particles under unsaturated condition. However, water seepage into the soil induces a drastic decrease in the shear strength, which is mainly caused by a decrease in cohesion (losing suction) resulting from soil saturation. In addition, the drained/undrained condition in the shear process is also sensitive to shear strength. For example, both water seepage and the shear process with constant volume cause an approximate 30% reduction in shear strength for Fukuchi-machi and Sasaguri-machi soil samples. Therefore, the reduction of cohesive strength due to water seepage and the low permeability of the slope are the parameters which trigger geodisaster. Based on the results of slope deformation and a stability analyses which took the change in water pressure and cohesive strength into account, the geodisaster at Fukuchi-machi was simulated, it is reasonable to assume that the shallow failure near the top of slope occurred due to torrential precipitation of about 100 mm per hour which triggered a debris flow.

Another potential source of destructive earthquakes and tsunami offshore of Sumatra

[1] We link geodetic data from the Sumatran GPS Array (SuGAr) and earthquake focal mechanisms to show that a 900-km long backthrust, arising from the Sunda megathrust offshore of Sumatra, has recently become active following the series of great megathrust earthquakes of this past decade. Shallow failure of the Mentawai backthrust explains coseismic displacements during moderate-earthquake clusters in 2005 and 2009. These two clusters represent the first activity on the backthrust in more than 30 years. Existing paleogeodetic evidence of vertical deformation in past centuries is too sparse to characterize earlier major activity, but leaves open the possibility of historic great backthrust earthquakes. Our geodetic evidence for rupture of the Mentawai backthrust during the two recent earthquake clusters suggests that this large fault may well pose an additional seismic and tsunami hazard to the coastal communities of central Sumatra.

Effects of soil spatial variability on rainfall-induced landslides

This paper presents a probabilistic framework to assess the stability of unsaturated slope under rainfall. The effects of soil spatial variability on the probability of rainfall-induced slope failure (landslides) are investigated. Soil spatial variability is considered by modeling the saturated hydraulic conductivity of the soil ( k s ) as a stationary lognormal random field. Subset simulation with a modified Metropolis–Hastings algorithm is used to estimate the probability of slope failure. It is demonstrated numerically that probabilistic analysis accounting for spatial variability of k s can reproduce a shallow failure mechanism widely observed in real rainfall-induced landslides. This shallow failure is attributed to positive pore-water pressures developed in layers near the ground surface. In contrast, analysis assuming a homogeneous profile cannot reproduce a shallow failure except for the extreme case of infiltration flux being almost equal to k s . Therefore, ignoring spatial variability leads to unconservative estimates of failure probability. The correlation length of k s affects the probability of slope failure significantly. The applicability of subset simulation with a modified Metropolis–Hastings algorithm to assess the reliability of problems involving spatial variability is highlighted.

Interpretation of T-bar penetrometer tests at shallow embedment and in very soft soils

The cylindrical T-bar penetrometer was developed for profiling the undrained strength of soft soils in the centrifuge and is now a widely-used offshore site investigation tool. The conventional interpretation of the T-bar test is to convert the measured penetration resistance to soil strength using a single bearing factor associated with steady flow of soil around the bar. This paper describes a new analysis for the interpretation of T-bar penetrometer tests at shallow embedment and in soft soils, which is an increasingly significant consideration in the design of seabed infrastructure, including pipelines. The analysis captures two mechanisms that are usually neglected: (i) soil buoyancy and (ii) the reduced bearing factor arising from the shallow failure mechanism mobilized prior to the full flow of soil around the bar. The framework derives from theoretical considerations and is calibrated using large deformation finite element analyses. The depth at which the steady deep penetration condition is reached is shown to depend on the normalized soil strength, su/γ′D, and may be up to several diameters deep. The effect of this new procedure on the inferred soil strength compared with the conventional approach is illustrated through T-bar tests in three different centrifuge samples, spanning a range of strength ratios.

강우침투에 따른 화강풍화토 사면의 얕은파괴 특성

In order to examine the characteristic of shallow failure in cut slopes composed of weathered granite soil, this study calculated critical permeability coefficient according to rainfall characteristic in Korea, performed stability analysis according to the representative physical properties of weathered granite soil distributed in Korea such as horizontal distance to the failure surface of cut slope, slope inclination, slope height, and the depth of wetting by rainfall, and analyzed the results. In the results of analyzing critical permeability coefficient, when the local rainfall characteristic was considered, the maximum critical permeability coefficient was 7.16×10 -4 cm/sec. We judged that shallow failure according to wetting depth should be considered when rainfall below the critical rainfall intensity lasts longer than the minimum rainfall duration in cut slopes composed of weathered granite soil, which had a critical permeability coefficient lower than the maximum critical permeability coefficient. Furthermore, using simulated failure surface, this study could understand the characteristic of shallow failure in cut slopes based on the change in slope safety factor according to horizontal distance, wetting depth, and strength parameter.