Nagaratnam Sivakugan

Active earth pressure on retaining wall for c-phi soil backfill under seismic loading condition

This technical note describes the derivation of an analytical expression for the total active force on the retaining wall for c-phi soil backfill considering both the horizontal and vertical seismic coefficients. The results based on this expression are compared with those obtained from earlier analytical expressions for the active force for c-phi soil backfill under seismic conditions, and found to have a similar trend of variation. The parametric study shows that the inclination of the critical failure plane with the horizontal plane decreases with the increase in values of seismic coefficients; the decrease being more for their higher values. The total active force increases with the increase in value of horizontal seismic coefficient; while it decreases with the increase in value of vertical seismic coefficient except for a very high value of horizontal seismic coefficient. Design charts are presented for various combinations of horizontal and vertical seismic coefficients (kh and kv), and values of cohesion and angle of shearing resistance for estimating the total active force on the retaining wall for c-phi soil backfill for practical applications.

Inflection Point Method to Estimate ch From Radial Consolidation Tests with Peripheral Drain

Horizontal coefficient of consolidation ch is a key parameter in the design of vertical drains and the following consolidation analysis of a soil layer. There are graphical and non-graphical methods available to estimate ch from laboratory radial consolidation tests with a central drain. Currently, the consolidation tests with peripheral drains have to be analysed through a curve fitting method for determining ch. In this technical note, a non-graphical inflection point method is proposed for determining ch for an oedometer test with peripheral drainage, based on the characteristic feature observed when the gradient of the theoretical Ur –log Tr relationship was plotted against Tr. The proposed method is validated through a series of consolidation tests on two reconstituted dredged clay specimens, tested in an oedometer subjected to radial drainage with peripheral drains. The consolidation settlements predicted from the proposed method, for the two different clays, were in excellent agreement with those measured in the oedometer. The proposed method will be a very valuable tool in the analysis of radial consolidation data when the drains are peripheral.

Arching in Soils Applied to Inclined Mine Stopes

Determination of stress distribution, giving due consideration to arching mechanism within minefill stopes, is of great importance because of its influence on the ground stability, ore recovery, and cost effectiveness. Most of the past studies on the stress determination have been applied to vertical stopes, and there is a lack of research study, especially analytical work, on inclined stopes. This study provides an analytical expression for the vertical stress at any depth within the inclined backfilled stopes by incorporating an arching effect within the backfill. Comparatively, the results obtained from this study agree well with other limited analytical and numerical results reported in the literature. A parametric study is undertaken to investigate the effect of various parameters involved in the proposed analytical expression. The results obtained reveal that stope geometry, fill properties, and stope inclination are critical factors in predicting the stress distribution in mine stopes.

Can soil arching be insensitive to ϕ

Arching is a common phenomenon that is encountered in backfilling behind retaining walls, trenches, or underground voids in the mine. Marston’s equation has been widely used and modified for computing stresses within backfills, duly accounting for the reduction in stresses due to arching. A critical appraisal of Marston’s equation and its improved forms reveal that the average vertical stress within the soil backfill at any depth is governed by the product of earth pressure coefficient K and wall-backfill frictional coefficient μ , which does not vary significantly with variation in friction angle ϕ of the granular soil backfill. The average normal stress depends on the value assumed for δ/ϕ and whether the lateral earth pressure coefficient has been assumed as Ka , K0 , or KKrynine . Therefore, it is not necessary to direct any effort toward determining the friction angle of the backfill precisely. Rather, attention should be paid to the value of δ/ϕ and the appropriate expression for K . Thus, it can be...

A Laboratory Model to Study Arching within a Hydraulic Fill Stope

A simple laboratory apparatus, to study the effects of arching within a hydraulic fill mine stope, was developed. Four different model stopes, two circular and two square, made from Perspex, were used in the study. A dry hydraulic fill obtained from a local mine was used in the study. The model was filled in 100-mm layers, and the fractions of the fill weight, carried by the bottom and the wall of the stope, were separately measured. From these measurements, the variation of average vertical stress with depth could be computed. These experimental values were compared against those obtained from numerical modeling using FLAC and FLAC3D, and the agreement was excellent, thus validating the numerical model.

Vertical and Radial Consolidation Analysis of Multilayered Soil Using the Spectral Method

A new, easy to implement, solution to the consolidation of multilayered soil based on the spectral method is presented. Combined vertical and radial drainage under instantaneous or single ramp loading is considered, ignoring well resistance. Flow in the vertical direction is based on the average hydraulic gradient at a particular depth which allows smear effects to be included. The excess pore-water pressure profile across all soil layers is described by a single expression calculated with common matrix operations. Average excess pore pressures within or across any number of layers are easily calculated from the single expression. The new model is verified against other solutions from the current literature indicating that the more general spectral method model can replace the separate solutions developed for specific problems.

A state-of-the-art review of geosynthetic-reinforced slopes

Abstract Geosynthetic-reinforced slopes are generally compacted fill embankments that incorporate geosynthetic layers as tensile reinforcement to enhance stability. The reinforcement holds together the soil mass from both sides of the failure surface, thus increasing the factor of safety of the existing slope. Several analytical, numerical and experimental research works, and many case studies on geosynthetic-reinforced slopes have been reported in literature; however, no attempt has been made in recent years to give an insight into such slopes and to present an overview of these developments. This paper presents a comprehensive overview of geosynthetic-reinforced slopes, including suitability of geosynthetics, modes of failure, methods of slope stability analysis and design, model studies, and typical slope stabilization methods and some specific recommendations. The readers, especially students and practicing engineers, will find the concepts presented in this paper very useful.

Prediction of ultimate bearing capacity of eccentrically inclined loaded strip footing by ANN, part I

Abstract Extensive laboratory model tests were conducted on a strip foundation lying over sand bed subjected to an eccentrically inclined load to determine the ultimate bearing capacity. Based on the model test results, a neural network model was developed to predict the reduction factor. This reduction factor (RF) is the ratio of the ultimate bearing capacity of the foundation subjected to an eccentrically inclined load to the ultimate bearing capacity of the foundation subjected to a centric vertical load. Different sensitivity analysis was carried out to evaluate the parameters affecting the reduction factor. Emphasis is given on the construction of neural interpretation diagram, based on the weights of the developed neural network model, to find out direct or inverse effect of input parameters on the output. A prediction model equation is established using the trained weights of the neural network model. The predictions from artificial neural network (ANN), and those from two other approaches, were compared with the laboratory model test results. The ANN model results found to be more accurate and well matched with other results.

Laboratory Simulation of the Stresses Within Inclined Stopes

In the process of mining for earth resources, large underground voids called stopes are created that are later backfilled. For stability analysis of the backfilled stopes, it is necessary to understand the stress developments within the stope while the filling is in progress. Due to an arching effect, a substantial fraction of the fill weight is carried by the stope walls, depending on the physical characteristics of the walls. This paper describes the development of a laboratory model that simulates mine backfilling in an inclined stope and enables determination of the average vertical stress at any depth within the fill. The experimental results are validated against numerical models and stresses determined from an analytical expression. The effect of arching is the least when the stope is inclined at about 80 degrees to the horizontal, giving highest vertical stresses at any depth. This fact is not captured in both the mathematical and numerical models developed in the past and the ones discussed herein. The model tests show that the lateral earth pressure coefficient is closer to K-0 for vertical stopes and k(a) for inclined stopes. In the case of walls with dissimilar frictional characteristics, the analytical expression can still be used with an average value of the wall-fill friction angle.

Consolidation Behavior of Soils Subjected to Asymmetric Initial Excess Pore Pressure Distributions

Although the consolidation settlements beneath foundations and embankments are rarely one dimensional, Terzaghis one-dimensional consolidation theory is often applied to these situations to approximate consolidation behavior. This paper investigates the consolidation behavior of a soil stratum subjected to various initial excess pore pressure distributions which occur under one-dimensional loading. The results show that analysis in terms of average degree of consolidation provides an incomplete representation of the consolidation behavior. While the average degree of consolidation curves for all uniform and linearly varying initial distributions are identical, the degree of consolidation isochrones for each distribution are unique. Furthermore, the application of a bottom-skewed initial excess pore pressure distribution results in a redistribution of pore pressures toward the skewed region so that an increase in excess pore pressure occurs at some depth after consolidation has already commenced. As a result, conventional consolidation relationships are considered inappropriate for these cases, and an alternative method of consolidation analysis in terms of normalized pore pressures is proposed.