Clay Slopes Research Articles

Three-dimensional numerical investigation on lateral movement and factor of safety of slopes stabilized with deep cement mixing column rows

Deep cement mixing (DCM) column rows have been widely used to increase the stability of excavated slopes in soft clay. However, large lateral movements are encountered frequently and lead to the loss of slope serviceability. This paper investigates the parameters that affect the lateral movements and factors of safety (FS) of an excavated slope stabilized by DCM columns in soft Bangkok clay. The considered parameters are the penetration depth (Pr), the spacing (Sr), improvement area ratio (ar), and the elastic modulus (Er) of the DCM column row. The three dimensional finite element method (FEM) was used for the analyses, and the initial calibration was based on the results of full-scale field tests. The DCM column rows behave as long piles with a fixation condition when Pr/H>3.5, where H is the excavation depth. For a close spacing of DCM column rows of Sr/D≤2 where D is the column diameter, the DCM column rows and soil in between move by approximately the same amount, which is considered effective in terms of soil arching. The improvement area ratio (ar) was found to be the dominant parameter, controlling the lateral movement and FS. FS of greater than 2 was found when ar was greater than 0.19. For ar greater than 0.19, the variation in stiffness of DCM column, possibly due to non-uniformity of mixing cement and in-situ clay, insignificantly affects FS of the excavated slopes. This ascertains the stability of the excavated slopes at various sections. A unique relationship between maximum lateral movement and FS of the excavated slope in soft Bangkok clay exists for all parametric studies. This relationship can be used to examine the stability of the excavated slope from the measured field lateral movement during and after the end of construction.

Reliability-based design and its complementary role to Eurocode 7 design approach

Reliability-based design (RBD) can play a useful complementary role to overcome some limitations in the Eurocode 7 (EC7) design approach, for example in situations with parameters not covered in EC7, different parametric sensitivities across different problems, cross-correlated or spatially correlated parameters, design aiming at a target reliability or failure probability, or when uncertainty in unit weight of soil is modeled. The complementary role played by RBD under these circumstances is illustrated and discussed for a shallow foundation, a reinforced rock slope, a Norwegian clay slope with spatial variability, a laterally loaded pile requiring implicit numerical analysis, and an anchored sheet pile wall. A pragmatic RBD approach involving first-order reliability method (FORM) only and a more rigorous RBD approach involving both first-order and second-order reliability method (SORM) are offered. Both approaches are implementable using either spreadsheet-based FORM and SORM procedures, or using various commercially available FORM/SORM packages.

Interpretation of landslide mechanisms based on numerical modelling: two case-histories

Numerical modelling represents a powerful technique to develop a quantitative assessment of the stress–strain mechanisms leading to either first-time slope failures or evolution of slopes already failed in the past. In this perspective, a valid interpretation of the landslide behaviour and an adequate strategy of risk mitigation can be achieved from a numerical validation of both the causative factors and the evolution mechanism that have been previously assumed according to detailed phenomenological or simple analytical approaches. This paper presents two case histories of slow landslides in clay slopes, both located in Puglia (Southern Italy), for which detailed phenomenological studies have been firstly carried out to infer assumptions on the slope failure mechanisms that have been later on verified by means of numerical analyses accounting for soil mechanical behaviour and slope hydraulic processes. The first case study concerns the first-time failure of a stiff clay slope in Lucera, which has been induced by the slow dissipation of negative excess pore water pressures generated by previous quarry excavation at the slope toe. The second case history is represented by the analysis of the stress–strain evolution of the ancient Volturino landslide, which is observed to reactivate mainly in wet seasons.

Effects of Thixotropy and Cement Content on the Sensitivity of Soft Remolded Clay

Abstract Strength loss of sensitive clay slopes is a common concern for soft soil sites. A useful method for studying this problem is through physical model tests. However, a problem that deters more widespread use of such tests is the difficulty of obtaining the large samples of natural sensitive soil that would be required. The work described in this paper was motivated by the need to develop practical procedures to produce artificial sensitive clay for use in model tests. To reach this goal, some fundamental experiments were performed to explore the effects of cement content, water content, aging, and remolding after aging on the undrained shear strength. Two soils [San Francisco Bay mud (SFBM) and Yolo loam (YL)] with several different cement and water contents were studied. After mixing and aging and measurement of the aged strength, the soils were again remolded to determine the aged-remolded undrained strength. The sensitivity of the aged clay could then be calculated by dividing the aged strength ...

MODELING OF LEVEE EROSION UNDER IRREGULAR WAVES

A levee erosion model is developed to predict the temporal and cross-shore variations of vertical erosion depth under irregular wave action. The product of the erosion rate and the turf resistance force is related to the wave energy dissipation rates due to wave breaking and bottom friction. The turf resistance force is expressed using the turf thickness and the surface and underneath resistance parameters. The empirical parameters are calibrated using available data. The calibrated model has been shown to reproduce the relation between the limiting velocity and steady flow duration, the erosion rate on a seaward grassed slope, and the eroded profile evolution of a seaward clay slope. The levee erosion model is also compared with field tests for erosion on the landward slope caused by wave overtopping. It is found to be difficult to reproduce the observed erosion initiation and progression because of the wide variations of the grass cover and clay resistance. The turf resistance parameters will need to be calibrated for specific levees.

Artificial Rainfall Failure Experiments for a Red Clay Slope

The influence of rainfall on slope stability problem that highway, railway,water conservancy and energy engineering production must consider and study, the rainfall is one of the major factors causing the soil slope failure。Establishing model test of red clay slope, rainfalling the model, the paper analyzing the moist frontal edge, power water pressure, slope displacement under the rainfall. The result has some reference value for fundamental mechanism of rainfall infiltration induced landslides in a red clay slope.

Erosion of a Seaward Dike Slope by Wave Action

AbstractA dike erosion model is developed to predict the temporal and cross-shore variations of vertical erosion depth under irregular wave action. The product of the erosion rate and the turf resistance force is related to the rate of erosion work by turbulence generated by wave energy dissipation caused by wave breaking and bottom friction. The turf resistance force is expressed using the turf thickness and the surface and underneath resistance parameters. The empirical parameters are calibrated using available data. The calibrated model is shown to reproduce the relation between the limiting velocity and steady flow duration, the erosion rate on a seaward grassed slope, and the eroded profile evolution of a seaward clay slope. The turf resistance parameters are uncertain and will need to be calibrated for specific dikes.

Seasonal slope surface deformation measured with TLS

In temperate European climates, soil water removal due to vegetation transpiration peaks in summer and soil rewetting from higher levels of precipitation occurs in winter. In clays of high plasticity, the seasonal cycles of drying and wetting cause the soil to experience a volumetric change, resulting in seasonal shrinking and swelling. For a clay slope exhibiting volume change, such behaviour can lead to excessive deformation and could contribute to strain-softening and progressive slope failure. This can in turn cause traffic disruption and loss of life if roads and railways are founded on or surrounded by such slopes. This paper discusses the driving forces of seasonal surface movement, in particular the role of vegetation, and presents the use of Terrestrial Laser Scanning (TLS) to measure the surface movement of a lightly vegetated London Clay slope near Newbury, UK. Two TLS scans were carried out in early and late summer respectively, representing relative wet and dry conditions of the slope. Continuous field measurements of soil water content in upper layers of the slope were obtained from TDR ThetaProbes already installed at the site. The water content data are used to support the results obtained from TLS by indicating the likely volumetric change in the soil due to loss of water.

Assessment of the Effectiveness of Local Building Materials used for Building Construction in Maikunkele Local Government Area of Niger State

The study was carried out to assess the effectiveness of the use of Local building materials used for building construction in Meikunkele Local Government Area of Niger State. To achieve this, two research questions and two hypotheses were formulated to guide the study. A survey research design was used for the study. A total of 50 respondents comprising 30 residents and 20 masons were used as the population for the study. A structured questionnaire which was developed by the researchers was used for data collection. The instrument was validated by three lecturers in the department of industrial technology education, Federal University of Technology, Minna. The reliability of the instrument was found to be 0.77 using Cronbach Alpha Formular. The data collected was analyzed using mean and standard deviation for the research questions while, the t-test statistic was used to test the hypotheses at 0.05 level of significance. The findings of the study revealed that local materials such as clay and timber easily absorb water when used in water lodged areas; stones used for local buildings are not usually cut to workable size before they are used and stones are not easily accessible. Consequently, it was recommended among others that people should avoid areas with clay soil, torrential flooding or slopes in order to minimize costs for special foundations and damp proofing elements.Key words: Assessment, effectiveness, local building materials and building construction

Assessment of the Effectiveness of Local Building Materials used for Building Construction in Maikunkele Local Government Area of Niger State

The study was carried out to assess the effectiveness of the use of Local building materials used for building construction in Meikunkele Local Government Area of Niger State. To achieve this, two research questions and two hypotheses were formulated to guide the study. A survey research design was used for the study. A total of 50 respondents comprising 30 residents and 20 masons were used as the population for the study. A structured questionnaire which was developed by the researchers was used for data collection. The instrument was validated by three lecturers in the department of industrial technology education, Federal University of Technology, Minna. The reliability of the instrument was found to be 0.77 using Cronbach Alpha Formular. The data collected was analyzed using mean and standard deviation for the research questions while, the t-test statistic was used to test the hypotheses at 0.05 level of significance. The findings of the study revealed that local materials such as clay and timber easily absorb water when used in water lodged areas; stones used for local buildings are not usually cut to workable size before they are used and stones are not easily accessible. Consequently, it was recommended among others that people should avoid areas with clay soil, torrential flooding or slopes in order to minimize costs for special foundations and damp proofing elements.Key words: Assessment, effectiveness, local building materials and building construction

Statistics for the calculated safety factors of undrained failure slopes

Due to the uncertainty and variability involved in ground conditions and analytical methods, the calculated factor of safety (FS) of a slope may not be exact. To know whether the calculated FS is unbiased and also the magnitude of its variability, this paper re-analyzes 43 real cases of undrained slope failure reported in the literature. The FS is re-calculated using two-dimensional (2-D) limit equilibrium methods (LEM) (i.e., the simplified Bishop's method and the Spencer's method) to minimize the human factor in FS calculation. Since all cases failed, the conditions (such as geometries) right before failures were adopted to simulate the nearly failure condition in the LEM. Based on the statistical results of the calculated FS obtained from hypothesis testing, it can be concluded that (a) the 2-D LEM seems to give unbiased FS estimates; (b) the FS variability for natural slopes is significantly larger than that for man-made slopes; (c) the conversion of the undrained shear strength (su) to its field value is the most crucial, and the modeling of su spatial variability is also crucial; and (d) the standard deviation of the human error in the logarithm of the calculated FS is about 0.25, and the human error appears to be uncorrelated to the logarithm of the calculated FS. Finally, the relationship between the failure probability of a man-made undrained clay slope and its calculated FS is developed to facilitate reliability analysis and reliability-based design.

Transitions in Structure of Clay Fills due to Suction Oscillations

In North-western Bohemia the excavated overburden from open-cast coal mines is end-dumped in fills. Clay-type soil prevails among the filled soils. As a consequence of the character and state of the soil, together with the mining and dumping technology, the clay fill comprises a typical lumpy structure. Large and open macrovoids between the single lumps are a characteristic feature of the clay fills. The filled overburden forms spoil heaps under high angles of repose (as high as 40°) which is governed by size and shape of the lumps. However these slopes cannot be considered for design of clay fill final slopes. The lumpy clay fill structure is metastable. Structure transitions influencing the mechanical behaviour and thus slope inclination of the heaps are presented in this contribution. Three main agents are responsible for the transitions of the lumpy structure: Plastic straining of relatively weak lumps as a consequence of overburden load; collapse on wetting of the open metastable structure mainly in theearly stages after landfilling; and oscillations of matric suction. Long-term monitoring of matric suction in years 2004 - 2008 atdepths up to 2 metres in the low-gradient slope of the reclaimed clay fill in the Bílina mine show clear seasonal suction oscillations.

Effect of spatially variable shear strength parameters with linearly increasing mean trend on reliability of infinite slopes

This paper studies the reliability of infinite slopes in the presence of spatially variable shear strength parameters that increase linearly with depth. The mean trend of the shear strength parameters increasing with depth is highlighted. The spatial variability in the undrained shear strength and the friction angle is modeled using random field theory. Infinite slope examples are presented to investigate the effect of spatial variability on the depth of critical slip line and the probability of failure. The results indicate that the mean trend of the shear strength parameters has a significant influence on clay slope reliability. The probability of failure will be overestimated if a linearly increasing trend underlying the shear strength parameters is ignored. The possibility of critical slip lines occurring at the bottom of the slope decreases considerably when the mean trend of undrained shear strength is considered. The linearly increasing mean trend of the friction angle has a considerable effect on the distribution of the critical failure depths of sandy slopes. The most likely critical slip line only lies at the bottom of the sandy slope under the special case of a constant mean trend.

Progressive failure behavior of pile-reinforced clay slopes under surface load conditions

Stabilizing piles are widely used to improve stability level of slopes to avoid landslides. In this paper, the full failure process of clay slopes reinforced using stabilizing piles was produced using serialized centrifuge model tests under surface loading conditions. The strength of pile material and the pile location closer to the slope top were both indicated to have a positive influence on the limit bearing capacity of the reinforced slope on the basis of test observations. The displacement field over the reinforced slope was measured during loading and used to capture a significant progressive failure caused by the surface loading. Local failure started near the inner edge of the load plate and expanded in a downward direction to produce a final slip surface. The pile exhibited a bending failure coupled with the progressive failure of the slope. The failure mechanism could be illustrated with the deformation localization in the slope that developed prior to the slip surface. The surface load exhibited a significant spreading behavior within the slope according to the displacement distribution of the slope.

Stability Evaluation for Soft Clay Slope Based on 2-D and 3-D Finite Element Method Analysis

Slope stability for soft clay is a hard problem in practice. In this paper, 2D and 3D finite element method (FEM) based on elastoplastic model are used to calculate the stress and displacement distribution in the soft clay slope under gravity and uniform load at the slope top. Stability analyses indicate that 3D boundary effect varies with the stress level of the slope. When the slope is stable, end effect of 3D space is not remarkable. When the stability decreases, end effect occurs; when the slope is at limiting state, end effect reaches maximum. The failure mode will not be the same if the slope is under different stress states; furthermore, a standard to evaluate slope failure with FEM calculation is established. The slope failure form caused by accumulative load is different from that caused by slope strength insufficiency. The energy causing slope failure spreads preferentially along Y-Z section. When the failure resistance capability reaches the limiting state, the energy can extend along X-axis direction. The 3D effect of the slope under uniform load on the top is related to the ratio of load influence width to slope height, and the effect is remarkable with the decrease of the ratio.

Stability Analysis on Surface Layer of an Expansive Clay Slope with Consideration of Expansive Pressure

Weathering effects, expansive pressure and pore water pressure are investigated to evaluate their effect on expansive clay slope stability. In particular, the effect of expansive pressure is analyzed to consider its effect on the stability of surface layer of the slope. In this paper, the distribution of expansive pressure in a slope is evaluated and used to analyze the stability of the surface layer of a clay slope. The proposed analysis method is applied in the third ring road project in Chengdu, China, where there are expansive clay slopes. Reasonable results are achieved.

Stability of a strip foundation on a sand embankment over mine tailings

Earthmoving equipment working on a progressively placed cover layer over mine tailings often experiences catastrophic ‘rotational' (or ‘punch-through') failure, with potential for loss of the equipment and harm to the operator. Although the performance of foundations on a homogeneous sand or clay slope is routinely calculated and investigated, comprehensive investigation of a thin, stronger slope overlying a softer layer is scarce. This paper reports the results of centrifuge model tests undertaken to provide insight into strip foundation behaviour during penetration, with freedom in horizontal displacement and rotation, adjacent to a sand embankment (cover layer), into a weaker clay layer (representing mine tailings). Variables were the set-back of the edge of the foundation from the crest of the slope, the height of the slope relative to the foundation size, and the normalised strength of the lower clay layer. Soil movement was captured continuously by a digital camera, and subsequently quantified through particle image velocimetry (PIV) analysis. The load–penetration responses were separately recorded. The effect of normalised set-back ratio (λ = b/B), slope height (η = H/B) and clay strength (sus/γcB) on the evolving soil flow mechanisms and the penetration resistance profile is discussed in the context of the likelihood and severity of failure. Rotational failure, with a peak in penetration resistance followed by some reduction, occurred for all cases investigated except for a higher set-back of λ = 1·5. The severity of failure was greater the closer the proximity of the footing to the slope crest, and the greater the height of the slope, whereas it reduced as the normalised strength of the lower layer increased. Typical critical failure occurred in clear shear planes pushing a (nominally) rigid block of soil, with the shape of a hemisphere followed by a wedge, towards the slope.

Variaciones espaciales de la textura en las laderas de exposición topográfica contrastada en el semiárido aragonés

Textural sediment variations between top-bottom slope position have been studied in a semiarid environment (La Violada, Huesca, NE Spain). Differences are observed according slope aspect. In south aspect the covariability between coarse sand and fine sand with distance from divide is positive (>0.95), and negative with slope angle (>0.95). The same relationship is found with coarse silt, but with negative value. There are not been observed significant relationships between fine silt and clay subfraction witb distance and slope value. In north aspect slope tbere are not found any relationships between sand, silt and clay fraction with distance and slope value. Results are discussed according to erosion processes.

Lateral bearing capacity of rigid piles near clay slopes

Analytical equations were derived to determine the undrained lateral bearing capacity of rigid piles in cohesive soil. Piles in level ground and piles placed at a distance from the crest of a slope were examined, taking account of the effect of the adhesion at the pile–soil interface. The derived analytical solutions were used to develop charts relating the lateral pile capacity to the pile length/diameter ratio, the pile–soil adhesion, the distance of the point of load application from the ground to the pile diameter ratio, the inclination of the slope and the distance of the pile from the crest of the slope to the pile diameter ratio. They were also used to derive a reduction factor which, when multiplied by the lateral bearing capacity for level ground, gives the bearing capacity of the same pile near a slope. In addition, a critical non-dimensional distance between the pile and the crest of the slope, at which the bearing capacity approaches that for a level ground, was determined. The bearing capacity charts obtained for level ground were compared to the classic Broms' charts and to others derived using several different lateral earth pressure distributions along the pile. Comparisons were also made between the results of the proposed method for piles near slopes and those obtained from charts based on upper bound calculations. Finally, the proposed new method was validated through a comparison with the results of a large number of pile load tests, in which a remarkable agreement was observed between the analytical results and the measurements.

Influence of infiltration on the periodic re-activation of slow movements in an overconsolidated clay slope

In Orvieto (central Italy), overconsolidated clay slopes are affected by intermittent slow movements at the top of the clay formation and within the landslide debris cover. Monthly data from inclinometers, Casagrande piezometers, and rainfall gauges show that velocity, pore pressure, and rainfall are closely related. A relationship is suggested to predict slope re-activation using rainfall history alone, once a pore pressure threshold has been reached and response of pore pressures to rainfall is understood. Pore pressures have been continuously monitored through vibrating wire cells. The threshold for shallow movements, critical for infrastructures and buildings, was identified by comparing displacement histories of a shallow movement, representative of many other ones recognized over the slope, and pore pressure, both measured at the centre of the sliding mass. The impact of infiltrated rainfall on groundwater flow was investigated through transient seepage analyses. Seepage analyses performed using hydraulic properties from laboratory and in situ tests do not fully depict the observed pore pressures because field data miss some structural characters and lithologic variations. The hydraulic properties of the shallower model layers were refined, based on field observations and interpretation of monitoring data, to have a good match between computed and measured pore pressures. Once the model is tested at different locations along a slope, it could be used to predict movement re-activation using only rainfall data.