Seung-Rae Lee

INSTABILITY OF UNSATURATED SOIL SLOPES DUE TO INFILTRATION

Abstract This paper presents a procedure for calculating the safety factor for an unsaturated slope suffering from rainfall infiltration. The process of infiltration into a slope due to rainfall and its effect on soil slope behavior were examined using a two-dimensional finite element flow–deformation coupled analysis program. The modified Mohr–Coulomb failure criterion was adopted for unsaturated soil strength to consider variations in strength due to the presence of matric suction. A safety factor was calculated based on the smoothed stress field obtained from finite element analysis, and an optimization technique was used to search for a critical slip surface. Some numerical examples are analyzed. The results illustrate a typical process of infiltration into unsaturated soil slopes and the effect of hydraulic conductivity on slope stability during rain-induced infiltration.

An approach to estimate unsaturated shear strength using artificial neural network and hyperbolic formulation

Abstract Since most soils exist above the ground water table, negative pore water pressures develop in unsaturated soils. This negative pore water pressure, known as matric suction, causes increased shear strength. Therefore, it is required that the effect of the increase in shear strength should be included in geotechnical analyses. However, experimental studies on unsaturated soils are generally costly, time-consuming, and difficult to conduct. Therefore, it is better to have an empirical method that is able to predict the unsaturated shear strength with respect to the matric suction in a more convenient way. For that purpose, we formulated a nonlinear unsaturated shear strength relationship with the matric suction in a hyperbolic form. In the formulation, conventional saturated soil parameters (c′, φ′) and an ultimate increment of apparent cohesion (Cmax) are required. A method is also developed wherein Cmax can be predicted using an ANN (artificial neural network) in reference to data obtained from tests conducted in this study and published in references.

AN IMPROVED SEARCH STRATEGY FOR THE CRITICAL SLIP SURFACE USING FINITE ELEMENT STRESS FIELDS

Abstract The finite element method can be used to advantage in slope stability problems. This paper proposes a technique to search for the critical slip surface as well as to define and calculate the factor of safety for the slope, when the finite element method is used to model its formation. First, stresses are estimated at each Gaussian point from the finite element analysis. Then, the global stress smoothing method is applied to get a continuous stress field. Based on this stress field, the factor of safety is calculated for a specified slip surface by a stress integration scheme. An improved search strategy is proposed for a noncircular critical surface which starts with a search method for a circular critical surface. During the search process, points defining a trial slip surface can freely move in the finite element mesh subject to some kinematical constraints. This method can be applied to both the limit equilibrium method and the finite element method. Effects of the slope stress history and soil parameters on the resulting critical surface are investigated.

Behavior of soil plugs in open‐ended model piles driven into sands

Calibration chamber tests were conducted on open‐ended model piles driven into dried siliceous sands with different soil conditions in order to clarify the effect of soil conditions on load transfer mechanism in the soil plug. The model pile used in the test series was devised so that the bearing capacity of an open‐ended pile could be measured as three components: outside shaft resistance, plug resistance, and tip resistance. Under the assumption that the unit shaft resistance due to pile‐soil plug interaction varies linearly near the pile tip, the plug resistance was estimated. The plug capacity, which was defined as the plug resistance at ultimate condition, is mainly dependent on the ambient lateral pressure and relative density. The length of wedged plug that transfers the load decreases with the decrease of relative density, but it is independent of the ambient pressure and penetration depth. Under several assumptions, the value of earth pressure coefficient in the soil plug can be calculated. It gr...

Relationship between the Soil-Water Characteristic Curve and the Suction Stress Characteristic Curve: Experimental Evidence from Residual Soils

The part of effective stress resulting from soil moisture or soil suction variation can be defined by the suction stress characteristic curve (SSCC). For a given soil, the SSCC can be experimentally determined from shear-strength tests. Recent work shows that the SSCC can be uniquely linked to the soil-water characteristic curve (SWCC). The uniqueness of the SSCC determined from both shear strength and soil moisture retention tests is examined for several residual soils in Korea. The validity of the effective stress principle is demonstrated by showing that effective stress-based on the SSCC describes the same unique failure criterion as that for the saturated failure criterion. The measured SSCCs are also shown to predict the soil-water retention curves within a few percentage. The SWCCs of these residual soils, determined directly from soil moisture retention tests, also accord well with the SSCCs determined directly from triaxial shear-strength tests with the difference within several tens of kPa. Therefore, we show that the suction stress characteristic curve or soil-water retention curve alone can be used to describe both the effective stress and soil-water retention characteristics of variably saturated soils. DOI: 10.1061/ (ASCE)GT.1943-5606.0000564.

AN EQUIVALENT MODEL AND BACK-ANALYSIS TECHNIQUE FOR MODELLING IN SITU CONSOLIDATION BEHAVIOR OF DRAINAGE-INSTALLED SOFT DEPOSITS

Abstract The predicted consolidation behavior of in situ soft clay is quite different from that measured, mainly due to the approximate numerical modelling techniques used, as well as the uncertainties involved in soil properties and geological configuration. In order to improve the prediction in two dimensional analysis, this paper takes these into consideration as follows: (1) an equivalent and efficient model is considered to incorporate three dimensional effects and (2) a back-analysis scheme is adopted for characterizing in situ properties by measurements. An approximate model is developed so that the results of the two dimensional analysis are equivalent to the three dimensional behavior, which is estimated by the ABAQUS program. To simulate the effect of three dimensional characteristics, an equivalent permeability concept and an adjusted width of drainage are applied to the drainage-installed soil deposits. The soil parameters used in the modified Cam-clay model, which have an effect on the process of consolidation, are back-analysed by a BFGS scheme on the basis of settlements and pore pressures measured in real sites. The developed equivalent model and the back-analysis scheme have been implemented in a numerical method of the finite element analysis. It has been found that one may be able to appropriately predict the consolidation behavior of drainage-installed soft deposits.

Analysis of soil nailed earth slope by discrete element method

Abstract Soil nailing has been widely used during the last two decades, to stabilize steep excavated slopes in several countries. In this study, the discrete element method (DEM) has been applied to evaluate the stability of reinforced slopes. This method is capable of not only estimating tensile and shear stresses mobilized in nails but also providing individual safety factors for soil and nails. It has been assumed that the nailed slope is composed of slices connected together with elastoplastic Winkler springs. In replacing nails by spring elements, a function is proposed for evaluating shear and tensile forces in nails which are induced by relative displacements between nail and adjacent soil. Taking into account for the sequence of construction, the developed method can predict the measured tensile forces in nails and hence it properly evaluates both local and overall stabilities of the reinforced slope.

Prediction of Leachate Level in Kimpo Metropolitan Landfill Site by Total Water Balance

Kimpo metropolitan landfill has received various kinds of wastessince January 1992. The leachate level was measured to be 10.3 m in May 1995 and the level increased to 12.2 m in August 1996. Therefore, to prove the reason for the increasing leachate level, we calibrated hydraulic conductivity of each waste andintermediate layer using the HELP (Hydrologic Evaluation ofLandfill Performance) model. The leachate generation data measured from February 1993 to October 1995 was used in the model calibration. As a result of a model calibration, we obtained anaverage infiltration ratio and used this in analysis of the total water balance to predict elevation of leachate level. Main causes of the elevation of the leachate level were the high water content of the waste and the degradation of the leachate-drainage system caused by the subsidence of a naturalbarrier layer.

Field Infiltration Characteristics of Natural Rainfall in Compacted Roadside Slopes

Wetting depth in a slope is an important indicator to properly evaluate the rainfall-induced slope instability. This rainfall infiltration has potential to induce shallow slope failures. It is necessary to characterize the field infiltration and movement of the wetting front due to a natural rainfall. To monitor important infiltration characteristics of a field slope, a compacted roadside slope in an express highway (South Korea) was instrumented to measure variations of matric suction and water content. The monitored variations of matric suction and water content in vegetated and nonvegetated areas are discussed. The pattern of field infiltration by severe rainfall storms is also compared with the estimation results obtained by widely used 1D infiltration models. The Chu model, by considering the ponding and run-off at each time interval, showed a good agreement with the field measurements.

Prediction of Cryo-SWCC during Freezing Based on Pore-Size Distribution

AbstractThe freezing process in soils is generally recognized as a coupled problem of heat and moisture transfer. Especially in the process of transfer, it is essential to understand the status of unfrozen water. In this paper, the freezing process in unsaturated soils has been studied with the hypothesis that initially unsaturated soil maintains the apparent unsaturated condition during freezing. Accordingly, the concept of the soil-water characteristic curve (SWCC) of unfrozen soil could be also applied to soil in the freezing stage. Therefore, a methodology for estimating a correlation between the suction and the amount of unfrozen water content, named cryo-SWCC, which considers the volume change of soil pores owing to ice formation, is proposed with the assumption that any deformation in unsaturated frozen soils owing to freezing can be negligible. On the basis of a relationship between a statistical pore-size distribution and the SWCC represented by the Mualem model, the variation of characteristic p...