Interference effect of two nearby strip footings on layered soil: theory of elasticity approach

In recent times, rapid urbanisation coupled with scarcity of land forces several structures to come up ever closer to each other, which may sometime cause severe damage to the structures from both strength and serviceability point of view, and therefore, a need is felt to devise simplified methods to capture the effect of footing interference. In the present study, an attempt has been made to model the settlement behaviour of two strip footings placed in close spacing on layered soil deposit consisting of a strong top layer underlying a weak bottom layer. Theory of elasticity is employed to derive the governing differential equations and subsequently solved by the finite difference method. The perfectly rough strip footings are considered to be resting on the surface of two-layer soil system, and the soil is assumed to behave as linear elastic material under a range of static foundation load. The effect of various parameters such as the elastic moduli and thickness of two layers, clear spacing between the footings and footing load on the settlement behaviour of closely spaced footings has been determined. The variation of vertical normal stress at the interface of two different soil layers as well as at the base of the failure domain also forms an important part of this study. The results are presented in terms of settlement ratio (ξ_δ), and their variation is obtained with the change in clear spacing between two footings. The present theoretical investigation indicates that the settlement of closely spaced footings is found to be higher than that of single isolated footing, which further reduces with increase in the spacing between the footings.     

HIDDEN LAYER PROBLEM IN SEISMIC REFRACTION WORK*

The hidden layer problem in seismic refraction work has been studied for three velocity configurations – the intermediate layer having (a) lower, (b) intermediate and (c) higher velocity than the underlying and overlying beds. It has been shown that conventional methods fail to locate the presence of the intermediate layer for the cases (a) and (c) and lead to errors in calculating the depth to the bedrock. For the case (b), it is possible to interpret the first arrival travel time analytically as an alternative to Green's graphical approach. It has been suggested that the hidden layer may be detected in all the three cases if converted S waves are also recorded in the seismogram.     

Seismic stability analysis of a hunchbacked retaining wall under passive state using method of stress characteristics

Acta Geotechnica - The potential use of a hunchbacked retaining wall over a conventional retaining wall under the seismic passive state is emphasised in this study employing the method of stress...     

Predicting dam-related downstream geomorphic response with widely available stream gauge data: A case study of the Godavari River Basin,India

Dam-related downstream adjustments of riverbeds are normally investigated by analysing the trend in sediment supply and high flow events during the pre- and post-dam periods. The required data for existing predictive models is not measured at river gauges, which limits the application of these tools. We derived the frequency of sediment-transporting streamflow events (T*) and upstream sediment supply (S*) in the pre- and post-dam periods with widely available gauged records and predicted changes in the downstream riverbed by adapting an existing model. Ten gauging stations in the Godavari River Basin, India, located downstream of dams, were chosen as study sites. Annually surveyed cross-sections at each site validated the accuracy of the predicted dam-related downstream changes. Then, a regression equation (R² = 0.75) was established between T*/S* (independent variable) and changes in the downstream bed elevation (dependent variable) for the Godavari Basin. We recommended that similar local empirical equations be formulated for larger river basins. Models of large-scale rainfall-runoff and sediment transport processes that can account for the influence of dams, such as the Soil & Water Assessment Tool, can be paired with the proposed regression equation to estimate dam-related downstream erosion and deposition with globally available data.     

A Petrogenetic Model of Basalts from the Northern Central Indian Ridge:3-11°S

1 Introduction The Northern Central Indian Ridge (NCIR) between 3° and 11°S latitudes is joined to the north with the slow spreading Carlsberg Ridge (CR; ~24–26 mm/a, full spreading rate) and to the south with the intermediate spreading Southern Central Indian Ridge (SCIR; ~50 mm/ a) (Fig. 1). Earlier petrological investigations of the Central Indian Ridge were concentrated either on or along the CR to the north and at the southern end of the CIR up to the Rodriguez Triple Junc…     

Development of limiting soil slope profile under seismic condition using slip line theory

This study embraces the formation of the limiting geometry of finite slopes under the static and seismic conditions within the slip line theory framework coupled with the modified pseudo-dynamic approach. The proposed methodology is expected to achieve a global factor of safety of 1.0 for the obtained slope profile. While analysing the stability of slopes using the limit analysis or the limit equilibrium method, the cognition of the slope geometry and the nature of the slip surface need to be known in advance. Such limitations are ruled out in the present analysis with the aid of the slip line method. Further, by employing the modified pseudo-dynamic approach, the dynamic properties of soil, such as damping ratio and frequency effect, are effectively considered in this stability analysis. The consideration of the slip line theory permits to achieve an adaptive failure mechanism in the analysis. The impact of a set of parameters characterizing the input motion and the dynamic soil properties on the behaviour of a slope explains the relevance of the present modified pseudo-dynamic approach compared to the conventional pseudo-static and the original pseudo-dynamic approaches. The proposed solution serves as a measure of the seismic slope stability in accordance with the geomorphological process generally encountered in nature. Compared with the available literature, the present results propose safe, economical, and efficient design guidelines for finite slopes and intimate the need for preventive measures to enhance the stability of existing slopes.

     

Geochemical and mineralogical studies of chlorine-rich amphibole and biotite from the 2.5 Ga mid-crustal basement beneath the 1993 Killari earthquake region,Maharashtra: Evidence for mantle metasomatism beneath the Deccan Trap

Fluid driven metasomatism and mass transfer from the earth’s mantle have played an important role in the evolution of the lower continental crust in many geodynamically active areas. The epicentral region of the disastrous 1993 Killari earthquake (M 6.2), concealed below a thick suite of Deccan volcanics in central India, appear to be one such region. In connection with the study of seismotectonics of the earthquake prone Deccan volcanic region, we have carried out systematic and detailed geochemical and mineralogical investigation on core samples from the basement, obtained from the 617m deep KLR-1 borehole, drilled in the epicentral region of Killari. Our investigations indicate that the basement, concealed below 338m thick Deccan volcanics, is made up of CO₂, Cl, FeO and CaO-rich, high density (2.82 g/cm³) — high velocity (avg. Vp: 6.2 km/s) moderately retrogressed upper amphibolite to granulite facies mid crustal rocks, which were subjected to pervasive Ca-metasomatism due to infiltration of mantle fluids. Graniticgneissic layer, typical of the upper crust, seems to be totally absent from this earthquake region. Chondrite normalized trace and rare earth elemental patterns display negative Eu anomalies together with LILE enrichment. Similarly, spider diagrams for incompatible elements show depletion in Zr, Hf, Y, Ta and Nb relative to the primitive mantle, indicating possible alterations of such relatively immobile elements at relatively high temperatures. Selective enrichment is also observed in transitional elements like Cu and Zn, indicating the possible role of chlorine in metal transport. The present study suggests that regional metasomatism beneath the Deccan Traps, which apparently alters the basic fabric of the rock during recrystallisation and makes it weak, may have a link with the nucleation of large earthquakes.     

FLAC Based Numerical Studies on Dynamic Interference of Two Nearby Embedded Machine Foundations

This paper emphasizes on the dynamic interaction of two closely spaced embedded square or rectangular foundations under the action of machine vibration. One of the foundations is excited with a known vibration source placed on the top of it, called the active foundation. The objective is to study the effect of dynamic motion of the active foundation on the nearby passive foundation through a layered soil medium. The analysis is performed numerically by using the explicit finite difference code FLAC ^3D . The soil profile is assumed to obey the Mohr–Coulomb yield criteria with non-linear failure envelope. The analysis is performed under sinusoidal dynamic loading with varying amplitude. Under the dynamic excitation, the settlement behavior of the interacting foundations is studied by varying the spacing between the foundations. In addition, the variation of vertical normal and shear stress developed beneath the interacting foundations is also explored. The present theoretical investigation indicates that the settlement and vertical normal stress below the active foundation is generally found to be higher than that obtained for the passive foundation, whereas the shear stress response below the foundations follows the reverse trend.     

Seismic analysis of nailed vertical excavation using pseudo-dynamic approach

An attempt has been made to study the behavior of nailed vertical excavations in medium dense to dense cohesionless soil under seismic conditions using a pseudo-dynamic approach. The effect of several parameters such as angle of internal friction of soil(Φ), horizontal(k_h) and vertical(k_v) earthquake acceleration coefficients, amplification factor(f_a), length of nails(L), angle of nail inclination(α) and vertical spacing of nails(S_v) on the stability of nailed vertical excavations has been explored. The limit equilibrium method along with a planar failure surface is used to derive the formulation involved with the pseudo-dynamic approach, considering axial pullout of the installed nails. A comparison of the pseudo-static and pseudo-dynamic approaches has been established in order to explore the effectiveness of the pseudo-dynamic approach over pseudo-static analysis, since most of the seismic stability studies on nailed vertical excavations are based on the latter. The results are expressed in terms of the global factor of safety(FOS). Seismic stability, i.e., the FOS of nailed vertical excavations is found to decrease with increase in the horizontal and vertical earthquake forces. The present values of FOS are compared with those available in the literature.     

Glass and Mineral Chemistry of Northern Central Indian Ridge Basalts:Compositional Diversity and Petrogenetic Significance

Abstract: The glass and mineral chemistry of basalts examined from the northern central Indian ridge (NCIR) provides an insight into magma genesis around the vicinity of two transform faults: Vityaz (VT) and Vema (VM). The studied mid-ocean ridge basalts (MORBs) from the outer ridge flank (VT area) and a near-ridge seamount (VM area) reveal that they are moderately phyric plagioclase basalts composed of plagioclase (phenocryst [An60–90] and groundmass [An35–79]), olivine (Fo81–88), diopside (Wo45–51, En25–37, Fs14–24), and titanomagnetite (FeOt ~63.75 wt% and TiO2 ~22.69 wt%). The whole-rock composition of these basalts has similar Mg# [mole Mg/mole(Mg+Fe2+)] (VT basalt: ~0.56–0.58; VM basalt: ~0.57), but differ in their total alkali content (VT basalt: ~2.65; VM basalt: ~3.24). The bulk composition of the magma was gradually depleted in MgO and enriched in FeOt, TiO2, P2O5, and Na2O with progressive fractionation, the basalts were gradually enriched in Y and Zr and depleted in Ni and Cr. In addition, the SREE of magma also increased with fractionation, without any change in the (La/Yb)N value. Glass from the VM seamount shows more fractionated characters (Mg#: 0.56–0.57) compared to the outer ridge flank lava of the VT area (Mg#: 0.63–0.65). This study concludes that present basalts experienced low-pressure crystallization at a relatively shallow depth. The geochemical changes in the NCIR magmas resulted from fractional crystallization at a shallow depth. As a consequence, spinel was the first mineral to crystallize at a pressure >10 kbar, followed by Fe-rich olivine at <10 kbar pressure.