Effective strength eccentricity concept for inelastic analysis of asymmetric structures

A large number of parameters affect the inelastic response of an eccentric system. The centre of resistance no longer remains constant due to continuous loading and unloading of various elements of the system in and out of the inelastic region. The objective of this study is to develop the concept of strength eccentricity for asymmetrical structures excited well into the inelastic region in the event of a severe earthquake. A single mass monosymmetric three-element system is selected. The torsional flexibility parameter is varied so as to obtain nine eccentric configurations covering the entire spectrum of such systems. These models were subjected to S00E component of the El Centro earthquake of 1940 which is considered to be a very severe earthquake. The concept of effective strength eccentricity is introduced which is defined as strength eccentricity at the point of intersection of ductility ratio curves of REE and FEE. It is proposed to provide additional strength in the elements of a torsionally unbalanced system independent of the design eccentricity. Empirical equations are proposed to account for design eccentricity and additional strength as well as distribution of the design strength. The proposed formulation is compared with the torsional provisions of UBC 1991, NBCC 1990 and NZC 1992. It is concluded that additional ductility demand on the REE and FEE designed based on the proposed formulation is almost nil as compared to that given by the three codes. The NZC and UBC require maximum ductility on the REE. The increase in overall strength of the system is least for the proposed formulation as compared to that given by the three codes. NBCC consistently requires the maximum total strength ratio.     

Ranchi mega lineament and its correlation with geological and geophysical data

Remotely-sensed satellite data of Landsat 5 (TM) and IRS-1A (LISS II) covering parts of central portion of Chotanagapur plateau have been analysed visually to delineate some prominent lineaments in Ranchi district and its adjoining area. One of the most prominent lineament “Ranchi mega lineament? has been analysed in this paper. This lineament is a potential site for base metal mineralisation at some places, and it acts as good groundwater conduit in the region. No earlier attempt to map this entire structural unit as a separate entity is known. Geophysical bouguer gravity anomaly map, lithological and structural maps have some remarkable correlation with this lineament. In the present paper authors have tried to analyse the lineament with respect to its tectonic, mineralisation and groundwater prospects. Resistivity survey carried in close proximity of this lineament at selected sites, indicate that rocks are well fractured at depth. Available lithological and structural map support this lineament as a shear near Purulia (W.B.) and also having base-metal mineralisation potential in this area. Authors opine that existing gravity anomaly map may be further augmented in the light of present study.     

Groundwater assessment through hydrogeomorphological and geophysical survey — A case study in Godavari sub-watershed, Giridih, Bihar

Groundwater assessment has been carried out through hydrogeomorphological and geophysical survey. Remotely sensed data has been used to delineate the hydrogeomorphological units. Each hydrogeomorphological unit has been further sub-divided based on the tone, texture and pattern. These units its have been categorised based on depth of weathering, local geomorphological association, recharge characteristics etc. The top layer resistivity of hydrogeomorphological units has been analysed by electrical resistivity method. Correlation between different sub-units of same hydrogeomorphic unit and top soil resistivity has been attempted. The correlation and integration of resistivity map with geomorphological map were carried out through. ‘GRAM’ Geographic information System, This has helped in better understanding the surface resistivity pattern. These sub divisions have different geohydrological and recharge characteristics. Resistivity zonation map prepared on the basis of limited field data anfd through interpolation/extrapolation can be modified/corrected with the help of information derived from remotely sensed data to obtain more realistic picture.     

A longitudinal study of malaria associated with deforestation in Sonitpur district of Assam,India

Assam–Arunachal forest fringed foothill area is endemic for malaria incidence. The present study deals with the temporal analysis of malaria incidence and determines its association with deforestation in 24 villages along the Assam–Arunachal forest fringed foothill area of Sonitpur district of Assam. Malaria epidemiological survey has been carried out in the study area from the year 1994 to 2005. Remote sensing (RS) technique has been used to map the areas of forest changes from the year 2000 to 2005. Geographical information system (GIS) was used to map the malaria incidence and forest cover. The study villages are endemic to malaria infections and there was increasing trend of malaria incidence over the years. The slide positivity rate (SPR) ranged from 5.1% in 1997 to 44.4% in 2005. The percentage forest cover decreased significantly from 23.6% during 2000 to 15.4% during 2005, whereas SPR was increased during 2000–2005. The present study is the first attempt to understand the role of deforestation in malaria incidence using RS and GIS in the north-eastern region of India at a micro-geographic level. The study suggests that the area is endemic to malaria transmission. The decrease in forest cover is a serious ecological concern besides its role in elevating the malaria incidence in the study area.     

Numerical modelling-based pillar strength estimation for an increased height of extraction

Underground extraction of total thickness of a thick coal seam in single lift by bord and pillar method increases pillar height during retreat. Field studies found that the increase in pillar height affects the depillaring operation adversely, especially, during caving of the strong/massive roof strata. Dilution in strength due to the increased pillar height caused catastrophic failure of barrier pillars and goaf overriding. This warrants a systematic study of pillar strength variation for the different heights of pillar. A review of different pillar strength estimation approaches for an analysis of the dilution in strength of the heightened pillar suggested that numerical modelling provides a better option for such a systematic study. Accordingly, investigations are conducted on simulated models in laboratory adopting a recognised numerical modelling procedure. The observed nature of variations in pillar strengths with the increase in its height in the numerical models and empirical formula is found to be matched. But a mismatch is found between the strength values of the two approaches with an increase in height of the pillar. Considering validity of the empirical formulation in Indian coalfields, a relationship is developed to incorporate a correction in the strength values of the numerical models. The suggested correction on the basis of this simple study of the pillar strength variation would be helpful for the use of the established simulation tool during the depillaring of a thick coal seam.     

Geochemistry of an ENE–WSW to NE–SW trending ∼2.37 Ga mafic dyke swarm of the eastern Dharwar craton,India: Does it represent a single magmatic event?

A vast tract of ENE–WSW to NE–SW trending mafic dyke swarm transects Archaean basement rocks within the eastern Dharwar craton. Petrographic data reveal their dolerite/olivine dolerite or gabbro/olivine gabbro composition. Geochemical characteristics, particularly HFSEs, indicate that not all these dykes are co-genetic but are probably derived from more than one magma batch and different crystallization trends. In most samples the La^N/Lu^N ratio is at ∼2, whereas others have a La^N/Lu^N ratio >2 and show higher concentrations of high-field strength elements (HFSEs) than the former group. As a consequence, we assume that the ENE–WSW to NE–SE trending mafic dykes of the eastern Dharwar craton do not represent one single magmatic event but were emplaced in two different episodes; one of them dated at about 2.37 Ga and another probably at about 1.89 Ga. Trace element modelling also supports this inference: older mafic dykes are derived from a melt generated through ∼25% melting of a depleted mantle, whereas the younger set of dykes shows its derivation through a lower degree of melting (∼15%) of a comparatively enriched mantle source.     

Interaction Effect of Group of Helical Anchors in Cohesive Soil Using Finite Element Analysis

In this paper, the interaction effect of a group of two and four symmetrical as well as asymmetrical helical anchors resting in homogeneous cohesive soil deposit with different helix configurations is determined using finite element analysis. The anchors were pulled to its ultimate failure controlling the displacement. Eight different types of anchor configuration were considered in the analysis, where mainly the number of helical plates, the depth of upper- and lower-most helical plates and the ratio of spacing between the helical plates to the diameter of the plate were varied. The variation of load–displacement curve for each anchor in the group was obtained and subsequently, the ultimate uplift capacity of each anchor was determined. The soil was assumed to follow Mohr–Coulomb failure criteria. The present theoretical observations are generally found in good agreement with those theoretical and experimental results available in the literature for single isolated helical anchor.     

Stable isotope dynamics of groundwater interactions with Ganges river

Groundwater depletion has been an emerging crisis in recent years, especially in highly urbanized areas as a result of unregulated exploitation, thus leaving behind an insufficient volume of usable freshwater. Presently Ganges river basin, the sixth largest prolific fluvial system and sustaining a huge population in South Asia, is witnessed to face (i) aquifer vulnerability through surface waterborne pollutant and (ii) groundwater stress due to summer drying of river as a result of indiscriminate groundwater abstraction. The present study focuses on a detailed sub-hourly to seasonally varying interaction study and flux quantification between river Ganges and groundwater in the Indian subcontinent which is one of the first documentations done on a drying perennial river system that feeds an enormous population. Contributing parameters to the total discharge of a river at its middle course on both temporal and spatial scale is estimated through three-component hydrograph separation and end-member mixing analysis using high-resolution water isotope (δ¹⁸O and δ²H) and electrical conductivity data. Results from this model report groundwater discharge in river to be the highest in pre-monsoon, that is, 30%, whereas, during post-monsoon the contribution lowers to 25%; on the contrary, during peak monsoon, the flow direction reverses thus recharging the groundwater which is also justified using annual piezometric hydrographs of both river water and groundwater. River water-groundwater interaction also shows quantitative variability depending on river morphometry. The current study also provides insight on aquifer vulnerability as a result of pollutant mixing through interaction and plausible attempts towards groundwater management. The present study is one of the first in South Asian countries that provides temporally and spatially variable detailed quantification of baseflow and estimates contributing parameters to the river for a drying mega fluvial system.     

Enhancing resilience of highway bridges through seismic retrofit

The present study evaluates seismic resilience of highway bridges that are important components of highway transportation systems. To mitigate losses incurred from bridge damage during seismic events, bridge retrofit strategies are selected such that the retrofit not only enhances bridge seismic performance but also improves resilience of the system consisting of these bridges. To obtain results specific to a bridge, a reinforced concrete bridge in the Los Angeles region is analyzed. This bridge was severely damaged during the Northridge earthquake because of shear failure of one bridge pier. Seismic vulnerability model of the bridge is developed through finite element analysis under a suite of time histories that represent regional seismic hazard. Obtained bridge vulnerability model is combined with appropriate loss and recovery models to calculate seismic resilience of the bridge. Impact of retrofit on seismic resilience is observed by applying suitable retrofit strategy to the bridge assuming its undamaged condition prior to the Northridge event. Difference in resilience observed before and after bridge retrofit signified the effectiveness of seismic retrofit. The applied retrofit technique is also found to be cost‐effective through a cost‐benefit analysis. First order second moment reliability analysis is performed, and a tornado diagram is developed to identify major uncertain input parameters to which seismic resilience is most sensitive. Statistical analysis of resilience obtained through random sampling of major uncertain input parameters revealed that the uncertain nature of seismic resilience can be characterized with a normal distribution, the standard deviation of which represents the uncertainty in seismic resilience. Copyright © 2013 John Wiley & Sons, Ltd.