N. K. Samadhiya

ROCK MASS STRENGTH PARAMETERS MOBILISED IN TUNNELS

Abstract Experience suggests that there is an enhancement in the strength of rock mass around the tunnels due to constraints in fracture propagation. In this paper, mobilised strength parameters and modulus of deformation have been deduced from back analysis of the field experience for the purpose of realistic non-linear stress analysis of arched underground openings in nearly dry rock masses, and modified correlations have been suggested. It is also inferred from field observations that sympathetic failure of rock mass may take place at all points almost simultaneously within the failure zone if deviatoric strain exceeds a critical value at any point. Block shear tests support Hoek and Browns (1980) criterion, which is recommended for analysis of rock slopes and open cut mines.

A nonlinear criterion for triaxial strength of inherently anisotropic rocks

Rocks encountered at many underground construction sites are laminated and exhibit direction-dependent strength behavior. It is also a well-established fact that the strength varies in a nonlinear manner with confining pressure. There is a need of strength criterion which could capture the nonlinearity as well as the anisotropy in the triaxial strength behavior of the rocks. It is essential that the criterion should be simple and must involve minimum testing to the extent possible. Further, the parameters of the criterion should have wide acceptability among the geotechnical fraternity. In the present study, a nonlinear strength criterion for transversely isotropic rocks is presented. Critical state concept Barton (Int J Rock Mech Mining Sci Geomech Abstr 13(9):255–279, 1976) has been used to define the curvature of the criterion. With a correctly defined curvature and starting from a reference point (UCS), it is possible to accurately assess the triaxial strength for given confining pressure. An experimental study conducted on triaxial strength behavior of three types of anisotropic rocks namely phyllite, slate and orthoquartzite has been discussed. A data base comprising more than 1140 triaxial tests conducted worldwide on anisotropic rocks has been compiled. Statistical evaluation of goodness of fit of the proposed criterion to the data base has been carried out. Further, the predictive capabilities of the proposed criterion have been evaluated by determining the error in estimation of triaxial strength if only few triaxial test data are available for determining the criterion parameters. The data base has also been back analyzed to assess the critical confining pressure for anisotropic rocks. Statistically, the critical confining pressure for anisotropic rocks can be taken nearly equal to 1.25 times the maximum UCS (obtained by applying load either parallel or perpendicular to planes of anisotropy). It is concluded that reasonable estimates of the triaxial strength of anisotropic rock can be made through the proposed criterion using minimum amount of triaxial test data available.

A PSO-ANN hybrid model for predicting factor of safety of slope

Stability analyses of slopes are routinely done to identify potential landslide sections. This is done for applying proper mitigation techniques. The stability of slopes is generally defined by using the term factor of safety (FOS). Various analytical and numerical models for calculating the value of FOS of slopes have been given by researchers. They had identified various independent parameters affecting the stability of slopes. The relation between FOS and influencing parameters is non-linear, which makes it a complex, multivariate problem. Recently intelligent systems have gained acknowledgement in solving such highly complex, multivariate problem. Artificial neural network (ANN), have got high degree success in solving such problem. However, ANN has its own limitations. Various researchers identified slow learning rate and getting trapped in local minima as the potential shortcoming of ANN. These shortcomings are associated with optimisation algorithm in the ANN. A number of metaheuristic optimisation algorithms are coupled with ANN to increase its performance. Particle swarm optimisation (PSO) had recently gained acknowledgement for solving continuous and discrete optimisation problems. The present study discusses in detail the application of PSO-ANN hybrid model for predicting the FOS of the slope. A three layer PSO-ANN hybrid model have been developed for the present study. A comprehensive database of 83 natural slope sections which were analysed for circular failure has been compiled from different literatures. The network has been trained, validated and tested using the database developed. An extensive iterative programme has been carried out to obtain the value of parameters associated with PSO-ANN hybrid model. Six different numbers of neuron in hidden layer, six different number of swarm size and five different value of acceleration factor (c 1 and c 2) have been considered. A total of 900 runs have been made, each having a maximum of 2000 iteration. On the basis of the result of 900 runs, a 6-9-1 network was found to be the most optimum network giving the minimum RMSE and Higher R2 value for both training and testing set of the data. The proposed hybrid model is compared with conventional slope stability methods with the help of a case study.

Wireless hazard communication system

Purpose – Communication of an impending hazard to people in near real time is critical. The purpose of this paper is to develop an internet‐SMS based geo‐hazard warning communication system.Design/methodology/approach – A warning system based on an internet‐resident concept and the available cellular mobile infrastructure is proposed in this study. The functionality of the system is modular in architecture.Findings – The messages have been transmitted in a set of 20 SMSs six times, to locally‐owned mobile numbers, resulting in a total number of 120. It has been found that 58 messages got delivered within ten seconds, the rest within 40 more seconds. The threat messages reached the impending threat areas within acceptable time delay.Originality/value – The paper describes the implementation of a novel and stand‐alone system for dynamic hazard warning. Cellular or mobile phone, a gadget used by common man, is expected to be the best proposition to effectively warn people individually and to propagate hazard...

A semi-empirical method for the design of support systems in underground openings

Abstract A semi-empirical method has been developed for the design of support systems consisting of rock bolts/anchors, shortcrete/fibre-reinforced shortcrete, steel ribs and grouted arches to support tunnels and caverns, especially in poor rock conditions. The empirical relations for mobilizing factors of shortcrete and bolts/anchors have been deduced on the conservative side from extensive data of the Norwegian Geotechnical Institute (NGI) tables and charts for support systems presented by Barton et al. (1974). There is a reasonably acceptable correlation between the empirical approach and the proposed theory of estimating the support capacity, except for abnormal cases. The proposed method is simple to apply in the field. Special designs have been suggested for complex geological conditions for which the assumptions are not valid, such as in thick shear zones, etc.

Experimental and Numerical Analysis of Geosynthetic-Reinforced Floating Granular Piles in Soft Clays

The installation of reinforced granular piles is a commonly adopted technique to improve load carrying capacity and reduce settlements in very soft clayey soils. This paper presents results of a series of laboratory model tests and numerical analysis carried on geosynthetic reinforced granular pile under short term loading. Unit cell concept has been adopted. Laboratory model tests were conducted on unreinforced, vertical encased, reinforced with horizontal strips and combined vertical-horizontal reinforced granular piles. The loading was applied either over the entire cylindrical tank area or only over the area of granular piles. The effects of various parameters such as reinforcement, encasement stiffness, shear strength of clay, length and diameter of granular piles have been studied. Experimental results in the form of vertical load intensity-settlement relationship have been compared with that obtained from PLAXIS 3D. The results of laboratory model tests indicated significant influence of reinforcement on the ultimate load intensity of granular piles and ultimate bearing capacity of treated ground. Lateral bulging in reinforced granular piles has also been controlled by incorporating geosynthetic materials.

Automated Geo-Spatial Hazard Warning System GEOWARNS: Italian Case Study

AbstractHazard warning is an area of research that requires both hazard evaluation and warning dissemination. At present, no such system carrying out both hazard evaluation and warning communication directly to the user community exists. Thus, there has been a need to develop an automated integrated system to categorize hazard and issue warning that reaches users directly. The objective of this paper is to develop an integrated, independent, generalized, and automated geo-hazard warning system, making use of geo-spatial data under popular usage platform. Thus, in this paper, development of GEOWARNS, an automated geo-spatial hazard warning system, has been elaborated. Testing and validation of the developed system has been carried out for landslide hazard evaluation and its warning dissemination pertaining to a comprehensive case study in Italy. The functionality of GEOWARNS is modular in architecture, having input, understanding, rainfall prediction, expert, output, and warning modules. The categories of ...

Strengthening of clay by geogrid reinforced granular pile

Abstract In the present work an attempt has been made to study the behavior of a single pile reinforced with the layers of geogrid and installed in soft clay. Laboratory tests have been conducted to study the response of a geogrid reinforced granular pile under a rigid circular footing. The influence of spacing of geogrid layers and reinforced depth of pile (depth of bottom most geogrid layer), on the performance of reinforced pile has been investigated. Finite element numerical analysis has also been carried out employing rocscience, Phase2, version 6 and the comparison of results has been done with those from the experimental study. In order to validate the general findings from the study, results have also been compared with those from relevant published literature. Geogrid reinforced granular pile has been found to result in significant increase in load carrying capacity. The bulge diameter of pile has been found to reduce due to the incorporation of geogrid layers as reinforcement.

Soft soils improvement by granular piles reinforced with horizontal geogrid strips

The present paper investigates the effect of geogrid, as horizontal strips, on the ultimate load intensity and bulging of floating as well as end bearing granular piles. The laboratory model tests and numerical analyses have been carried out on unreinforced and granular piles reinforced with horizontal geogrid strips embedded in very soft clay. The short-term and displacement controlled laboratory model tests were carried out based on unit cell concept. Numerical analysis has been extended to study the influence of shear strength of clay, number and stiffness of geogrid strips on the granular piles. The results of laboratory model tests in the form of load–settlement relationship were compared with those obtained from FEM software, PLAXIS 3D. The ultimate load intensity for unreinforced floating and end bearing granular piles treated ground has been increased by 185 and 238% as compared to untreated ground. The ultimate load intensity for 25, 50 and 70 mm c/c spaced floating granular piles reinforced with horizontal strips-treated ground increased by 442, 396 and 316% as compared to untreated ground. The results indicated significant improvement in the ultimate load intensity and reduction in bulging of granular piles by incorporating geogrid strips.

An experimental study on random fiber mixed granular pile

Granular pile (GP) is one of the ground improvement techniques. Randomly mixed fiber GP in place of plain GP is a new development. In this paper some parametric studies on randomly mixed fiber GP have been presented in order to evaluate the effect of fiber content, fiber length and depth of sand-random fiber mixture in GP on load carrying capacity and other characteristics of GP. Significant improvement in load carrying capacity of soft clay bed has been observed with the increase in fiber content, fiber length and depth of sand-random fiber mixture in the GP. An equation has been developed by regression analysis to estimate the load carrying capacity of randomly mixed fiber GP. The findings of this model study indicate that the random fiber-reinforced GP may open a new option in the field of ground improvement techniques.