A. Murali Krishna

Modeling the Dynamic Response of Wrap-Faced Reinforced Soil Retaining Walls

This paper describes the development of a numerical model for simulating the shaking table tests on wrap-faced reinforced soil retaining walls. Some of the physical model tests carried out on reinforced soil retaining walls subjected to dynamic excitation through uniaxial shaking tests are briefly discussed. Models of retaining walls are constructed in a perspex box with geotextile reinforcement using the wraparound technique with dry sand backfill and instrumented with displacement sensors, accelerometers, and soil pressure sensors. Results showed that the displacements decrease with the increase in number of reinforcement layers, whereas acceleration amplifications were not affected significantly. Numerical modeling of these shaking table tests is carried out using the Fast Lagrangian Analysis of Continua program. The numerical model is validated by comparing the results with experiments on physical models. Responses of wrap-faced walls with varying numbers of reinforcement layers are compared. Sensitivity analysis performed on the numerical models showed that the friction and dilation angle of backfill material and stiffness properties of the geotextile-soil interface are the most affecting parameters for the model response.

Evaluation of the Optimum Mixing Ratio of a Sand-Tire Chips Mixture for Geoengineering Applications

AbstractThis paper presents the evaluation of the optimum mixing ratio of sand-tire chips (STC) mixtures based on void ratio and shear-strength properties. Locally available sand and tire chips of 20×10  mm size are adopted. Different STC mixtures, in the range 10–70% by weight of tire chips (TC), are considered, along with pure sand (0% TC) and pure tire chips (100% TC). Specific gravity, density, and large direct shear tests were conducted for various samples. The volume of voids and weight-volume relations were determined from the dry unit weight and specific gravity values obtained for various mixtures. The results showed that the addition of tire chips up to 40% by weight showed significant decrease of void ratio by 43%. Shear-strength properties, like angle of internal friction values, are increased with TC contents up to 30%. Based on experimental results, the optimum percentage of tire chips of the selected size is in the range of 30–40% by weight, which is the equivalent of 50–60% by volume. Furt...

Behavior of Geosynthetic Reinforced Soil Foundation Systems Supported on Stiff Clay Subgrade

A series of laboratory model tests was performed to investigate the behavior of geosynthetic reinforced stiff clay foundation systems under circular loading. The footing consisted of a rigid circular steel plate with a diameter of 150 mm. Five different series of tests were performed in both homogeneous (clay or sand) and layered configurations. The tests used planar geogrid and three-dimensional geocell reinforcements. Test results indicate that both types of reinforcements substantially improve the performance of the stiff clay foundation bed. A maximum threefold improvement was observed in bearing pressure, depending on the reinforcement type. However, geocell was found to be the most advantageous soil reinforcement technique, giving maximum performance improvement.

Liquefaction Mitigation of Sand Deposits by Granular Piles- an Overview

Soil liquefaction and associated ground failures have been a major source of damage during the past earthquakes. The risk of liquefaction and associated ground deformation can be reduced by various ground-improvement methods including the stone column (gravel drain) technique. Ground improvement by rammed granular piles (RGP) is considered one of the most reliable of these methods. This paper summarizes different mechanisms involved in the effective function of granular piles as a ground treatment method for liquefaction mitigation. Various mechanisms like: Drainage, Reinforcement, Storage, Dilation and Densification effects are briefly discussed. Generation and dissipation of the pore pressures in the granular pile reinforced ground under different earthquake conditions are quantified considering these different mechanisms. Granular piles are proved to be very effective for liquefaction mitigation.

Sand–Tire Chip Mixtures for Sustainable Geoengineering Applications

The applications of scrap tire-derived recycled materials in civil engineering applications have been increasing largely because of their potential economic and environmental benefits. This paper first evaluates sand–tire chip (STC) mixture properties and then discusses about the application of STC mixtures in geoengineering applications through laboratory model studies. Locally available sand and tire chips of 20 mm long with 10 mm square cross section are adopted for preparing the STC mixtures. Tire chips are mixed with sand in various percentages, ranging from 10 to 70 % with an increment of 10 %. Index and mechanical properties of sand–tire chip mixtures are determined for different proportions. Based on large direct shear tests, it was found that internal friction angle values are increased with TC contents up to 30 %. The study indicates that the optimum percentage of tire chips of the selected size is in the range of 30–40 % by weight, which is the equivalent of 50–60 % by volume. Model tests on retaining wall models by using various STC mixtures ranging from 10 to 50 % were discussed. The paper concluded that the STC mixture at the optimum ratio results in lightweight material with 20 % less unit weight with better strength parameters and compressibility behavior, which can effectively be used for geoengineering applications.

Feasibility Study of Retaining Walls Backfilled with Sand-Tire Chip Mixtures

Proper management of waste tires is a major problem in many regions of the world. Scrapped tires in different forms are being used in various civil engineering applications. This paper presents the feasibility study on the use of recycled tire chips mixed with sand as lightweight backfill material for retaining wall applications. Properties of sand and sand-tire chip (STC) mixtures and model tests conducted on retaining wall models with these materials are briefly summarized. Investigations on use of different STC mixtures (different proportions) indicated that STC30 mixture (30% of tire chips by weight) is most efficient in improving wall behavior in terms of displacements and pressures. To evaluate the financial benefits in the construction of cantilever retaining wall for three different sizes (3, 6, and 9 m height) with STC0 (sand alone) and STC30 mixture as the backfill material, wall designs including the structural designs and cost analyses are presented. Based on the feasibility studies, it is indicated that STC30 mixtures have lower bending moment and shear force on stem, heel, and toe. Thus, the STC30 shows the better sustainable backfill material for retaining wall structures providing a financial benefit of about 30%.

Behavior of Rigid-Faced Reinforced Soil-Retaining Walls Subjected to Different Earthquake Ground Motions

AbstractA study of the seismic behavior of rigid-faced reinforced soil-retaining walls subjected to scaled earthquake ground motions is presented in this paper. A numerical model was developed and validated to simulate the shaking-table test of a rigid-faced reinforced soil-retaining wall. The calibrated numerical model was extended to develop a full-scale model to study the behavior of five scaled earthquake (EQ) ground motion excitations with different predominant frequencies ranging from 0.673 Hz for the Loma Prieta EQ to 5.437 Hz for the Parkfield EQ. Analyses of the input ground motions at the base of the wall model and their responses at the top show that amplitudes close to the fundamental frequency of the wall were amplified the most. Two deformation zones formed: a high-strain zone very close to the facing and a constant-strain zone extending beyond the reinforced zone. The extent of these zones varied with different ground motion parameters. The minimum and maximum lengths of the shear deformati...

Behaviour of circular footing resting on layered foundation: sand overlying clay of varying strengths

Abstract A series of laboratory model tests was performed on rigid circular footing, 150 mm in diameter (D), resting on surfaces of different layered foundations. The experimental programme considered different foundation configurations by varying the thicknesses (H) of dense sand layers, in the range of 0.63–2.19D, overlying clay subgrades of different strengths, ranging from very soft (c u = 7 kPa) to stiff (c u = 60 kPa). The responses of homogeneous foundations (configured with the sand and clays) were also obtained in order to compare the performances. The pressure–settlement responses of homogeneous and layered foundations depicted non-linear variations within the range of footing settlement tested (s = 0–24% of D). The layered foundations indicated significant improvements in bearing capacities for the softer clay subgrades (c u ≤ 30 kPa) as compared to corresponding homogeneous clay beds; while, reduced bearing pressures were noted for stiff clay subgrade having c u = 60 kPa. In general, foundation responses depicted higher bearing pressures for stiffer subgrades (c u ), thicker sand-layers (H/D) and at greater footing settlement levels (s/D); however, in terms of bearing pressure ratios (BPR), a decreasing trend was observed with increase in stiffness of the clay subgrades (c u ). In the testing programme maximum of about 5.34-fold improvement in bearing pressure was obtained for layered foundations having c u = 7 kPa; while, it was about 0.9 for the stiff clay subgrade (c u = 60 kPa) as compared to the corresponding homogeneous clay beds. Responses of the model tests were in significantly good agreement with that of the theoretical analyses reported earlier.

Stability Analysis of Non-homogeneous Soil Slopes Using Numerical Techniques

This paper presents the stability analyses of non-homogeneous slopes under different loading conditions. A non-homogeneous soil slope with two different soil layers is considered. Special cases are created, varying the height of the layers, to account for the non-homogeneity of the earth slope. A water table is considered in this study to account for the seepage forces. Pseudo-static earthquake force is considered, taking into account both the forces in the horizontal and vertical directions. A rigorous limit equilibrium method of slices, i.e. Morgenstern-Price method is used to analyze the stability of the slope. Finite element shear strength reduction technique is also used for displacement calculations and comparison with limit equilibrium method. Safety factor, critical slip surfaces and displacements with different loading conditions are studied and compared.

Mitigation of Dynamic Loading Effects on Retaining Walls Using Recycled Tire Chips

This paper presents the results of shaking table tests on retaining wall models using recycled tire chips as compressible inclusions. Scrap tire derived tire chips of 10 × 10 mm size and about 20 mm length have been used in the study. The 600-mm-height model wall is constructed in a Perspex container and instrumented with pressure sensors and LVDTs at different locations. Sinusoidal dynamic excitations were applied on the model walls. The dynamic response of the retaining walls with the variation in the acceleration and frequency of base shaking has been monitored and discussed. It is observed from these tests results that the horizontal displacement and incremental lateral earth pressures are significantly reduced by the inclusion of tire chips as cushion in between the wall and backfill. Reduction of the lateral earth pressure and displacement implies a lower design requirement which implies lesser dimensions of the retaining wall with reduced material cost.