Revised Cam-Clay Model

The Cam-clay models, or any other plasticity-based models, do not make distinction between the mode of stress transfer in coarse- and fine-grained soils. An examination of behavior at micro level in fine-grained soils, from the consideration of load transfer through physico-chemical interactions, suggests that the plastic compressions result from the grouping of particles into larger clusters and that elastic compressions result from the decrease in the spacing between particles. During shearing, these clusters gradually get dismembered, releasing the locked-in energy. The effect of such dismembering of clusters can be easily incorporated into the original Cam-clay model, and better predictions can be obtained with the associated flow rule, itself, for both normally and over consolidated states. The method essentially defines the hardening of yield surfaces with internal changes in the spacing between particles, instead of changes in externally observed plastic strains. The approach describes the behavior of over consolidated soils as yielding along successfively hardening Roscoe surfaces with gradually varying plastic properties.

Estimation of soil hydraulic properties and their uncertainty: comparison between laboratory and field experiment

Often the soil hydraulic parameters are obtained by the inversion of measured data (e.g. soil moisture, pressure head, and cumulative infiltration, etc.). However, the inverse problem in unsaturated zone is ill-posed due to various reasons, and hence the parameters become non-unique. The presence of multiple soil layers brings the additional complexities in the inverse modelling. The generalized likelihood uncertainty estimate (GLUE) is a useful approach to estimate the parameters and their uncertainty when dealing with soil moisture dynamics which is a highly non-linear problem. Because the estimated parameters depend on the modelling scale, inverse modelling carried out on laboratory data and field data may provide independent estimates. The objective of this paper is to compare the parameters and their uncertainty estimated through experiments in the laboratory and in the field and to assess which of the soil hydraulic parameters are independent of the experiment. The first two layers in the field site are characterized by Loamy sand and Loamy. The mean soil moisture and pressure head at three depths are measured with an interval of half hour for a period of 1 week using the evaporation method for the laboratory experiment, whereas soil moisture at three different depths (60, 110, and 200 cm) is measured with an interval of 1 h for 2 years for the field experiment. A one-dimensional soil moisture model on the basis of the finite difference method was used. The calibration and validation are approximately for 1 year each. The model performance was found to be good with root mean square error (RMSE) varying from 2 to 4 cm(3) cm(-3). It is found from the two experiments that mean and uncertainty in the saturated soil moisture (theta(s)) and shape parameter (n) of van Genuchten equations are similar for both the soil types. Copyright (C) 2010 John Wiley & Sons, Ltd.

Stiffness Coefficients of Layered Soil Systems

One of the most important dynamic properties required in the design of machine foundations is the stiffness or spring constant of the supporting soil. For a layered soil system, the stiffness obtained from an idealization of soils underneath as springs in series gives the same value of stiffness regardless of the location and extent of individual soil layers with respect to the base of the foundation. This paper aims to develop the importance of the relative positioning of soil layers and their thickness beneath the foundation. A simple and approximate procedure called the weighted average method has been proposed to obtain the equivalent stiffness of a layered soil system knowing the individual values of the layers, their relative position with respect to foundation base, and their thicknesses. The theoretically estimated values from the weighted average method are compared with those obtained by conducting field vibration tests using a square footing over different two- and three-layered systems and are found to be very good. The tests were conducted over a range of static and dynamic loads using three different materials. The results are also compared with the existing methods available in the literature.

Sediment Volume Tests for Expansive Soil Identification and Classification

Equilibrium sediment volume tests are conducted on field soils to classify them based on their degree of expansivity and/or to predict the liquid limit of soils. The present technical paper examines different equilibrium sediment volume tests, critically evaluating each of them. It discusses the settling behavior of fine-grained soils during the soil sediment formation to evolve a rationale for conducting the latest version of equilibrium sediment volume test. Probable limitations of equilibrium sediment volume test and the possible solution to overcome the same have also been indicated.

A photoelastic approach to framed structure-foundation beam-soil interaction

The interaction of a framed structure with a foundation beam resting on an elastic medium, representing the soil, has been studied using the photoelastic method. The contact pressure distribution, the fibre stress in the foundation beam and frame structure, as well as the stresses in the elastic medium, have been obtained. These have been compared with theoretical results obtained by idealizing the soil as (a) elastic half plane, and (b) elastic half space. It is shown that the photoelastic method can provide an easy solution to this type of problem if the soil can be idealized as an elastic continuum.

Critical Reappraisal of Colloidal Activity of Clays

Soil properties and their behavior, apart from stress history, are influenced markedly by physicochemical characteristics of the constituent clay and nonclay minerals and their relative proportions. Atterberg limits and Skempton’s colloidal activity, which are simple quantitative parameters, reflect the composite effects of the soil constituents and their interactions with pore fluid. Micromechanistic interpretations of these parameters have been provided in this paper. It has been shown that, in general, the liquid limit of fine-grained soils reflects the physicochemical potential and that each of the factors of Skempton’s colloidal activity are interdependent. It has been illustrated that property correlations with colloidal activity, as well as with Atterberg limits, result in involved interrelationships due to the interdependence of the parameters.

Critical Reappraisal of Colloidal Activity of Clays

Soil properties and their behavior, apart from stress history, are influence markedly by physicochemical characteristics of the constituent clay and nonclay minerals and their relative proportions. Atterberg limits and Skempton's colloidal activity, which are simple quantitative parameters, reflect the composite effects of the soil constituents and their interactions with pore fluid. Micromechanistic interpretations of these parameters have been provided in this paper. It has been shown that, in general, the liquid limit of fine-grained soils reflects the physicochemical potential and that each of the factors of Skempton's colloidal activity are interdependent. It has been illustrated that property correlations with colloidal activity, as well as with Atterberg limits, result in involved interrelationships due to the interdependence of the parameters.

Influence of Starch Polysaccharide on the Remolded Properties of 2 Indian clay Samples

The reported presence in marine clays and the recognized role of polysaccharide as a bonding agent provided the motivation to examine the role of starch polysaccharide in the remoulded properties of nonswelling (kaolinite) and swelling (bentonite) groups of clays. The starch polysaccharide belongs to a group of naturally occurring, large-sized organic molecules (termed polymers) and is built up by extensive repetition of simple chemical units called repeat units. The results of the study indicate that the impact of the starch polysaccharide on the remoulded properties of clays is dependent on the mineralogy of the clays. On addition to bentonite clay, the immensely large number of segments (repeat units) of the starch polysaccharide create several polymer segment - clay surface bonds that cause extensive aggregation of the bentonite units layers. The aggregation of the bentonite unit layers greatly curtails the available surface area of the clay mineral for diffuse ion layer formation. The reduction in diffuse ion layer thickness markedly lowers the consistency limits and vane shear strength of the bentonite clay. On addition to kaolinite, the numerous polymer segment - clay surface bonds enhance the tendency of the kaolinite particles to flocculate. The enhanced particle flocculation is responsible apparently for a small to moderate increase in the liquid limit and remoulded undrained strength of the nonswelling clay.

Resistance of Norway spruce (Picea abies) to root and butt rot (Heterobasidion parviporum) in peatland and mineral soil

Root and butt rot is the most harmful fungal disease affecting Norway spruce in southern Finland. In approximately 90 % of cases the causal agent is Heterobasidion parviporum. Root and butt rot infections have not been reported in Finnish peatlands. However, the increase in logging operations in peatlands means there is a risk that the fungus will eventually spread to these areas. The aim of this study was to find out the impact of growing site on the resistance of Norway spruce to Heterobasidion parviporum infections. This was investigated by artificially inoculating H. parviporum to spruce trees in pristine mire, drained peatland and mineral soil and comparing the defence reactions. Additionally, the effect of genotype on resistance was studied by comparing the responses of spruce clones representing different geographic origins. The roots and stems of the trees to be sampled were wounded and inoculated with wood dowels pre-colonised by H. parviporum hyphae. The resulting necrosis around the point of inoculation was observed. It was presumed that increased length of necrosis indicates high susceptibility of the tree to the disease. The relationship between growth rate and host resistance was also studied. The results indicated that growing site does not have a statistically significant effect on host resistance. The average length of necrosis around the point of inoculation was 35 mm in pristine mire, 37 mm in drained peatland and 40 mm in mineral soil. It was observed that growth rate does not affect resistance, but that the genotype of the tree does have an effect. The most resistant spruce clone was the one with Russian origin. The results suggest that the spruce stands in peatlands are not more resistant to root and butt rot infections than those in mineral soil. These findings should be taken into consideration when logging peatland forests.

Theoretical and experimental investigation of framed structure-layered soil interaction problems

The plane stress solution for the interaction analysis of a framed structure, with a foundation beam, resting on a layered soil has been studied using both theoretical and photoelastic methods. The theoretical analysis has been done by using a combined analytical and finite element method. In this, the analytical solution has been used for the semi-infinite layered medium and finite element method for the framed structure. The experimental investigation has been carried out using two-dimensional photoelasticity in which modelling of the layered semi-infinite plane and a method to obtain contact pressure distribution have been discussed. The theoretical and experimental results in respect of contact pressure distribution between the foundation beam and layered soil medium, the fibre stresses in the foundation beam and framed structure have been compared. These results have also been compared with theoretical results obtained by idealizing the layered semi-infinite plane as (a) a Winkler model and (b) an equivalent homogeneous semi-infinite medium

Thermophilic fungi: An assessment of their potential for growth in soil

An attempt has been made to forecast the potential of thermophilic fungi to grow in soil in the laboratory and in the field in the presence of a predominantly mesophilic fungal flora at usual temperature. The respiratory rate of thermophilic fungi was markedly responsive to changes in temperature, but that of mesophilic fungi was relatively independent of such changes. This suggested that in a thermally fluctuating environment, thermophilic fungi may be at a physiological disadvantage compared to mesophilic fungi. In mixed cultures in soil plates, thermophilic fungi outgrew mesophilic fungi under a fluctuating temperature regime only when the amplitude of the fluctuating temperatures was small and approached their temperature optima for growth. An antibody probe was used to detect the activity of native or an introduced strain of a thermophilic fungus, Thermomyces lanuginosus, under field conditions. The results suggest that although widespread, thermophilic fungi are ordinarily not an active component of soil microflora. Their presence in soil most likely may be the result of the aerial dissemination of propagules from composting plant material.

Cement stabilised rammed earth. Part B: compressive strength and stress-strain characteristics

Strength and behaviour of cement stabilised rammed earth (CSRE) is a scantily explored area. The present study is focused on the strength and elastic properties of CSRE. Characteristics of CSRE are influenced by soil composition, density of rammed earth, cement and moisture content. The study is focused on examining (a) role of clay content of the soil on strength of CSRE and arriving at optimum clay fraction of the soil mix, (b) influence of moisture content, cement content and density on strength and (c) stress-strain relationships and elastic properties for CSRE. Major conclusions are (a) there is considerable difference between dry and wet compressive strength of CSRE and the wet to dry strength ratio depends upon the clay fraction of soil mix and cement content, (b) optimum clay fraction yielding maximum compressive strength for CSRE is about 16%, (c) strength of CSRE is highly sensitive to density and for a 20% increase in density the strength increases by 300-500% and (d) in dry state the ultimate strain at failure for CSRE is as high as 1.5%, which is unusual for brittle materials.

Bearing capacity factors of circular foundations for a general c-phi soil using lower bound finite elements limit analysis

By using the lower bound limit analysis in conjunction with finite elements and linear programming, the bearing capacity factors due to cohesion, surcharge and unit weight, respectively, have been computed for a circular footing with different values of phi. The recent axisymmetric formulation proposed by the authors under phi = 0 condition, which is based on the concept that the magnitude of the hoop stress (sigma(theta)) remains closer to the least compressive normal stress (sigma(3)), is extended for a general c-phi soil. The computational results are found to compare quite well with the available numerical results from literature. It is expected that the study will be useful for solving various axisymmetric geotechnical stability problems. Copyright (C) 2010 John Wiley & Sons, Ltd.

Stress-strain response of plastic waste mixed soil

Recycling plastic waste from water bottles has become one of the major challenges worldwide. The present study provides an approach for the use plastic waste as reinforcement material in soil. The experimental results in the form of stress-strain-pore water pressure response are presented. Based on experimental test results, it is observed that the strength of soil is improved and compressibility reduced significantly with addition of a small percentage of plastic waste to the soil. The use of the improvement in strength and compressibility response due to inclusion of plastic waste can be advantageously used in bearing capacity improvement and settlement reduction in the design of shallow foundations. (C) 2010 Elsevier Ltd. All rights reserved.