Effect of non-plastic fines on cyclic behaviour of sandy soils

This paper presents the results of the detailed studies on stress - controlled cyclic triaxial tests on sandy soils from Ahmedabad, Gujarat, India subjected to a loading frequency of 0.1 Hz in cyclic triaxial equipment. Undrained stress controlled cyclic triaxial tests were carried out on cylindrical samples of size 50 mm diameter and height 100 mm with different cyclic stress ratios. Laboratory evaluations were carried out to compare the cyclic resistance of clean sand to that of sand with various fines contents at a constant gross void ratio. The gross void ratio considers the voids formed by sand particles and fines. The effects of gross void ratio with and without fines on pore water pressure build up and liquefaction potential of sandy soils in stress controlled tests are presented. The results obtained from this study provide direct evidence that the limiting silt content plays an important role in the cyclic resistance of sandy soils. Below the limiting silt content the cyclic resistance decreases until the limiting silt content is reached and then the cyclic resistance increases.

Impact of Sulphate counter ion in the migration of sodium ion through soils

Laboratory advection-diffusion tests are performed on two regional soils-Brown Earth and Red Earth-in order to assess their capacity to control contaminant migration with synthetic contaminant solution of sodium sulphate with sodium concentration of 1000 mg/L. The test was designed to study the transport/attenuation behaviour of sodium in the presence of sulphate. Effective diffusion coefficient (De) that takes into consideration of attenuation processes is used. Cation exchange capacity is an important factor for the attenuation of cationic species. Monovalent sodium ion cannot usually replace other cations and the retention of sodium ion is very less. This is particularly true when chloride is anion is solution. However, sulphate is likely to play a role in the attenuation of sodium. Cation exchange capacity and type of exchangeable ions of soils are likely to play an important role. The effect of sulphate ions on the effective diffusion coefficient of sodium, in two different types of soils, of different cation exchange capacity has been studied. The effective diffusion coefficients of sodium ion for both the soils were calculated using Ogata Bank’s equation. It was shown that effective diffusion coefficient of sodium in the presence of sulphate is lower for Brown Earth than for Red Earth due to exchange of sodium with calcium ions from the exchangeable complex of clay. The soil with the higher cation exchange retained more sodium. Consequently, the breakthrough times and the number of pore volumes of sodium ion increase with the cation exchange capacity of soil.

Contolling Alkali Induced Heave in soil with fly ash

The properties of the soils can change drastically due to the presence of contaminants leading to several geotechnical failures founded on them. One important pollutant that can have considerable effect is the alkali released from varies industries. It is known that alkali solutions can increase the swelling of soil containing both expansive and non-expansive minerals.Many attempts to control this alkali-induced heave in soils through chemical agents were not successful. With a view to study the use of fly ash to stabilize alkali contaminated soil, the behavior of soils containing 25% and 50% of fly ash has been studied in the presence of 2N-alkali solution. Results of volume change behavior of non-expansive soil containing kaolinite clay mineral in the presence of fly ash showed that it is effective to control the alkali induced swelling in the soil. The effectiveness increases with an increase in the percentage of fly ash in soils. Detailed X-ray diffraction and SEM studies showed that the mineralogical changes that occur in soil due to alkali interaction are inhibited in the presence of fly ash.

Enhancement of Strength of Soft Soils with Fly ash and Lime

Disposal of large quantities of fly ash poses a major environmental problem. To enhance its utilization, fly ash is considered for stabilizing of expansive soft soils. Improving the strength of soil, which is of major importance, depends on the pozzolanic nature of fly ash. Fly ashes with high pozzolanic reactivity are widely used but those with less pozzolanic reactivity are greatly inhibited. As the strength development in natural expansive soil considered in this investigation is very less with different percentages of fly ash, an attempt is made to increase the same by addition of lime along with fly ash. Based on several tests conducted, the optimum lime contents for fly ash and soils are 5% and 8% respectively. The strength of compacted soil with different fly ash contents of 10 to 40% with lime contents of 5% and 8% are determined after curing for different periods. The strength improvement for any soil-fly ash mixture, which is substantial with 5% of lime, is further improved with 8% of lime. The strength of soil-fly ash mixtures with any lime content increases with curing period.

Volume change behaviour of expansive soils

Understanding the volume change behaviour of expansive soils/clays becomes a dire necessity to obtain engineering solutions to structures founded on these soils. Behaviour of expansive soils does not conform to the natural behaviour of fine grained soils. Most of the cases, the permissible heave/settlement forms the design criteria. The paper discusses the basic properties, the role of effective stress concept, basic mechanism in controlling the volume change behaviour, the role of double layer repulsion and its validity and certain basic considerations of footing resting on an expansive soil with respect to heave or settlement and the soil reinforcement as a possible engineering solution.

Control of shrinkage and swelling of expansive soils

Fly ashes are used to improve the properties of expansive soils. The paper brings out the effect of two different fly ashes containig different lime contents on shrinkage and swelling behaviour of expansive Indian Black cotton soil. Since the specific gravities of the fly ashes are considerably different,Void ratio at shrinkage limit and % of swelling are used to describe the shrinkage and swell behaviour of soils. Both fly ashes increase the shrinkage void ratio and decrease the % swell of the soil. While high lime fly ash is more effective in increasing the shrinkage void ratio, low lime flyash is more effective in reducing the swelling. Lime content which causes floculation of soil particle, is responsible for the differences.

A rational approach to model the behavior of bonded soils

The paper presents a rational approach to model the behavior of bonded soils within the frame work of hardening plasticity. The approach is based on the premise that the resistance of bonded materials is a superposition of the two components of cement bond strength and soil frictional strength and that the deformation of the soil is associated with the frictional component of stresses just as in the case of a remoulded soil, the bonds offering additional resistance at any given strain level. This concept is similar to two stiffnesses acting in parallel for the same strain response. The proposed model considers the constitutive laws separately for the two components (bond and frictional) and adds the two to get the overall response. The unbonded soil component is described by the well known 'modified Cam clay' model. The response of the bond component is also described by a strain softening elasto-plastic model, considering the behavior to be elastic up to the yield surface and elasto-plastic beyond yield surface. To illustrate the capability of the proposed, model some laboratory test results of both compression and-extension shear tests are predicted. Despite the model being simple, several typical features of the behavior of bonded materials are well reproduced. The model parameters are well defined and easily determinable.

Effect of pore medium chemistry on hydraulic conductivity of fine grained soils

Hydraulic conductivity of fine-grained soils has assumed greater importance in waste disposal facilities. It is necessary to understand better the factors controlling hydraulic conductivity of fine-grained soils which are used as liners in waste disposal facilities. Hydraulic Conductivity study with ten soils with two fluids having extreme dielectric constants(epsilon) namely water and CCl4 has shown that intrinsic permeability (K) increases drastically with decrease in epsilon. These changes are attributed to the significant reduction in the thickness of diffuse double layer which in turn mainly dependent on the epsilon of the permeant. Hydraulic Conductivity with water of each pair of soils having nearly same liquid limit but different plasticity properties is found to be vastly different, but found to correlate well with shrinkage index, defined as difference between the liquid and the shrinkage limits. Also the ratio Kccl(4)/K-w is found to significantly increase with the increase in the shrinkage index.

Stability of a long unsupported circular tunnel in soils with seismic forces

By using the lower-bound finite element limit analysis, the stability of a long unsupported circular tunnel has been examined with an inclusion of seismic body forces. The numerical results have been presented in terms of a non-dimensional stability number (gamma H/c) which is plotted as a function of horizontal seismic earth pressure coefficient (k (h)) for different combinations of H/D and I center dot; where (1) H is the depth of the crest of the tunnel from ground surface, (2) D is the diameter of the tunnel, (3) k (h) is the earthquake acceleration coefficient and (4) gamma, c and I center dot define unit weight, cohesion and internal friction angle of soil mass, respectively. The stability numbers have been found to decrease continuously with an increase in k (h). With an inclusion of k (h), the plastic zone around the periphery of the tunnel becomes asymmetric. As compared to the results reported in the literature, the present analysis provides a little lower estimate of the stability numbers. The numerical results obtained would be useful for examining the stability of unsupported tunnel under seismic forces.

Impact of heavy metals on bacterial communities from mangrove soils of the Mahanadi Delta (India)

This study aimed to assess soil nutrient status and heavy metal content and their impact on the predominant soil bacterial communities of mangroves of the Mahanadi Delta. Mangrove soil of the Mahanadi Delta is slightly acidic and the levels of soil nutrients such as carbon, nitrogen, phosphorous and potash vary with season and site. The seasonal average concentrations (g/g) of various heavy metals were in the range: 14810-63370 (Fe), 2.8-32.6 (Cu), 13.4-55.7 (Ni), 1.8-7.9 (Cd), 16.6-54.7 (Pb), 24.4-132.5 (Zn) and 13.3-48.2 (Co). Among the different heavy metals analysed, Co, Cu and Cd were above their permissible limits, as prescribed by Indian Standards (Co=17g/g, Cu=30 g/g, Cd=3-6 g/g), indicating pollution in the mangrove soil. A viable plate count revealed the presence of different groups of bacteria in the mangrove soil, i.e. heterotrophs, free-living N-2 fixers, nitrifyers, denitrifyers, phosphate solubilisers, cellulose degraders and sulfur oxidisers. Principal component analysis performed using multivariate statistical methods showed a positive relationship between soil nutrients and microbial load. Whereas metal content such as Cu, Co and Ni showed a negative impact on some of the studied soil bacteria.