A model study on the decay of volcanic aerosol layer and verification with Pinatubo and El Chichon data

The time evolution of stratospheric aerosol layer formed after a volcanic eruption is studied taking into account the aerosol microphysical processes of growth, coagulation and sedimentation. Using a simple model we could explain the observed evolution of the Pinatubo volcanic layer which decayed in about 3 years. The experimental data obtained by Nd:YAG backscatter lidar over Ahmedabad further supports this finding. The data obtained after the El Chichon volcanic eruption also showed that the El Chichon aerosol layer decayed in about 3 years time. Thus, though the amount of SO₂ injected has been higher, in the case of Pinatubo, about two to three times more than El Chichon, it has resulted in the production of larger aerosol particles due to faster growth and coagulation processes, and subsequently a faster removal rate, to give more or less a similar background aerosol amount at the stratosphere in about 3 years time.     

Hydraulic and compaction characteristics of leachate-contaminated lateritic soil

Large quantities of leachate-contaminated lateritic soil results from dump yards in the southwest coast of India. These dump yards receive large quantities of municipal solid waste which includes chemical, industrial and biomedical wastes. Large areas of land are currently being used for this purpose. An extensive laboratory testing program was carried out to determine the compaction characteristics and hydraulic conductivity of clean and contaminated lateritic soil. Batch tests were used to study the immediate effect of leachate contamination on the properties of lateritic soil. Contaminated specimens were prepared by mixing the lateritic soil with leachate in the amount of 5%, 10% and 20% by weight to vary the degree of contamination. The results indicated a small reduction in maximum dry density and an increase in hydraulic conductivity due to leachate-contamination. The change induced by chemical reaction in the microstructure of the soil was studied by scanning electron microscope before and after contamination of soil with leachate. The structure of the leachate contaminated soil sample appeared to be aggregated in scanning electron microscope analysis. The aggregated structure increases the effective pore space and thus increases the hydraulic conductivity. Fifty percent increase in hydraulic conductivity was observed for specimens prepared at standard Proctor density and mixed with 20% leachate. Compaction characteristics did not change much with the presence of leachate up to 10%. With 20% leachate the maximum dry density decreased slightly indicating excess leachate in the soil. However the changes are not significant.     

Effect of Cement and Quarry Dust on Shear Strength and Hydraulic Characteristics of Lithomargic Clay

The lithomargic clay constitutes an important group of residual soils existing under lateritic soils. This soil is found on the western and eastern coasts of India over large areas. This soil is a problematic one and is very sensitive to water and loses a greater part of its strength when becomes saturated. These high silt deposits have invited many problems such as slope failures, foundation failures, embankment failures, uneven settlements etc. In this investigation an attempt is made to study the effect of cement and quarry dust on shear strength and hydraulic characteristics of the lithomargic clay after the stabilization. Microfabric and mineralogical studies were carried out to find out the reason for the strength development of the stabilized soil using SEM and XRD analysis. The results indicated that there is an improvement in the properties of the lithomargic clay with the addition of cement and quarry dust. The XRD results indicated the formation of CSH and CAH, which are responsible for strength development in the stabilized soil.     

A Critical Review of Construction, Analysis and Behaviour of Stone Columns

Stone columns have been used as an effective technique for improving the engineering behaviour of soft clayey grounds and loose silt deposits. The soil improvement via stone columns are achieved from accelerating the consolidation of weak soil due to shortened drainage path, increasing the load carrying capacity and/or settlement reduction due to inclusion of stronger granular material. This paper discusses the techniques, methods of construction of stone columns, mechanisms of stone column behaviour under load and associated design philosophies along with some practical findings from recent research programs.     

A Review on the Design,Applications and Numerical Modeling of Geocell Reinforced Soil

For improving the stability and load carrying capacity of weak subgrade, strengthening methods are to be followed in the field. Among the various approaches, geocells have been identified as an effective soil reinforcement technique for improving soft subgrade behaviour. The three-dimensional honeycomb structure of geocell offers more lateral confinement to the infill soil resulting in improved load carrying capacity. This led to the widespread use of geocells for different geotechnical applications like pavements, foundations, embankments, slope protection, erosion control etc. Many researchers in the past have confirmed the suitability of geocell reinforcement through their experimental, numerical and field studies. In this paper, a comprehensive review of the reinforcement mechanisms, design aspect and numerical modelling techniques of geocell reinforced soil is provided. In addition, this paper highlights the various field application scenarios where different types of geocells have been used and explores the research challenges and scope for further research in this field.

     

Stone Columns with Vertical Circumferential Nails: Laboratory Model Study

This paper presents results from a series of laboratory plate load tests carried out in unit cell tanks to investigate the improvement in stiffness, load carrying capacity and resistance to bulging of stone columns installed in soft soils. A new method of reinforcing the stone columns with vertical nails installed along the circumference of the stone column is suggested for improving the performance of these columns. Tests were carried out with two types of loading (1) the entire area in the unit cell tank loaded, to estimate the stiffness of improved ground and (2) only the stone column loaded, to estimate the limiting axial capacity. It is found that stone columns reinforced with vertical nails along the circumference have much higher load carrying capacity and undergo lesser compression and lesser lateral bulging as compared to conventional stone columns. The benefit of vertical circumferential nails increases with increase in the diameter, number and depth of embedment of the nails. The improvement in the performance of stone column was found to be more significant, even with lower area ratio. It is found that reinforcing stone column with vertical circumferential nails at the top portion to a depth equal to three times the diameter of stone columns, will be adequate to prevent the column from excessive bulging and to improve its load carrying capacity substantially.     

Interactions Between Soils and Laboratory Simulated Electrolyte Solution

To study the impact of salt water intrusion on two types of soils from west coast region of India were investigated in the laboratory. The key characteristics evaluated included Atterberg limits, compaction characteristics, hydraulic conductivity and chemical characteristics of selected soils. The sea at this coast receives effluent from different points and hence the characteristics change with time and locality. Therefore, to maintain uniform composition, 0.5 N sodium chloride solution (NaCl) was prepared in the laboratory and batch tests were used to determine the immediate effect on soils. Soil specimens were prepared by mixing the soils with 0.5 N NaCl in the increments of 0, 5, 10 and 20% by weight to vary the degree of contamination. Experimental results of soils mixed with 0.5 N NaCl showed that the maximum dry density increases and the optimum moisture content (OMC) decreases with increasing sodium chloride concentration. The study also revealed that the hydraulic conductivity of the soils tested increases with increase in sodium chloride concentration. The Atterberg limits of contaminated specimens show a remarkable change when compared with uncontaminated specimens.     

Assessing the utility of 210Pb geochronology for estimating sediment accumulation rates on river floodplains in Fiji

Low‐energy gamma ray spectroscopy has been employed to estimate floodplain sedimentation rates using measurements of ²¹⁰Pb in floodplain alluvium. The utility of the technique is assessed through the analysis of excess (unsupported) ²¹⁰Pb profiles in three sediment cores taken from the floodplain of the Labasa River on Vanua Levu in northern Fiji. A low‐energy germanium spectrometer (LEGe) was used for the nondestructive determination of excess ²¹⁰Pb in a region cultivated intensively with sugarcane. Measured average historical (c. 25 years) vertical accretion rates are between 2.2 and 4.4 cm yr^?1. The findings are broadly comparable with published sedimentation rates from analyses of radionuclide profiles elsewhere in the tropical South Pacific Islands, but the rates are higher than those measured previously at the same Labasa River sites using ¹³⁷Cs profiles. Accelerated soil erosion owing to cane burning and land tillage seems to be largely responsible for sediment production, although flood‐related effects such as channel accretion by coarse bedload and the emplacement of large organic debris also influence floodplain sedimentation. However, application of the ²¹⁰Pb technique in Fiji (and perhaps neighbouring island countries) is found to have serious drawbacks compared to the more robust ¹³⁷Cs method, owing principally to the low ²¹⁰Pb concentrations in the sandy alluvial sediment tested.     

Travel times ofP-waves in North-East India

Summary Based on 90 accurately localized earthquakes in and around North-East India and the local crustal velocity model of Gupta et al. [4], the travel times of P-waves have been determined from the foci of these earthquakes at arbitrarily selected depths of 5, 13, 25, 41 and 50 km to the sites of the seismic stations operated by the Regional Research Laboratory, Jorhat and the National Geophysical Research Institute, Hyderabad, and also to the sites of the Shillong and Tura seismic stations run by the India Meteorological Department, New Delhi. The travel times of P-waves fit a straight line very well with velocities of 5·97 ± 0·31, 6·18 ± 0·01, 6·41 ± 0·03, 7·82 ± 0·07 and 7·95 ± 0·01 km/sec at each of the depths under study. Similar investigations of P* and Pg-waves of 16 earthquakes at a depth of 10 km have revealed velocities of 6·53 ± 0·31 and 5·64 ± 0·34 km/sec, respectively. A simplified two-layered crustal model consisting of an average crustal thickness of 41·5 km with 22·2 and 19·3 km thick layers has been obtained.     

Experimental Studies and its Application using PLAXIS-2D for Lithomargic Clay Stabilized by GBFS and Lime

Geotechnical and Geological Engineering - Lithomargic clay is found along the Eastern and Western coasts of Southern India. Presence of weak soils like lithomargic clay below the foundations, cause...