G. Madhavi Latha

Simulation of excavations in jointed rock masses using a practical equivalent continuum approach

A simple practical equivalent continuum numerical model previously presented by Sitharam et al. (Int. J. Rock Mech. Min. Sci. 38 (2001) 437) for simulating the behaviour of jointed rock mass has been incorporated in the commercial finite difference programme fast Lagrangian analysis of continua (FLAC). This model estimates the properties of jointed rock mass from the properties of intact rock and a joint factor

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

(J_f)

Large Diameter Triaxial Tests on Geosynthetic-Reinforced Granular Subbases

, which is the integration of the properties of joints to take care of the effects of frequency, orientation and strength of joint. A FISH function has been written in FLAC specially for modelling jointed rocks. This paper verifies the validity of this model for three different field case studies, namely two large power station caverns, one in Japan and the other in Himalayas and Kiirunavara mine in Sweden. Sequential excavation was simulated in the analysis by assigning null model available in FLAC to the excavated rock mass in each stage. The settlement and failure observations reported from field studies for these different cases were compared with the predicted observations from the numerical analysis in this study. The results of numerical modelling applied to these different cases are systematically analysed to investigate the efficiency of the numerical model in estimating the deformations and stress distribution around the excavations. Results indicated that the model is capable of predicting the settlements and failure observations made in field fairly well. Results from this study confirmed the effectiveness of the practical equivalent continuum approach and the joint factor model used together for solving various problems involving excavations in jointed rocks.

Shaking table tests to investigate the influence of various factors on the liquefaction resistance of sands

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.

Shaking table studies on geosynthetic reinforced soil slopes

The estimation of strength and stiffness of reinforced aggregates is very important for the design and construction of reinforced unpaved/paved road sections. This paper presents the experimental results from static and cyclic triaxial tests carried out on granular subbase samples reinforced with multiple layers of geogrid reinforcement. Aggregates of different size ranges were mixed in calculated proportions by weight to obtain the gradation specified for rural roads. Triaxial samples of 300 mm diameter and 600 mm height were prepared using this sampled aggregate. The strength and stiffness characteristics of this aggregate reinforced with geogrids at different elevations were determined from static and cyclic triaxial tests. Triaxial tests were also carried out on geocell encased aggregates, and the results are compared. From the experimental results it is observed that reinforced systems carried more stresses than unreinforced systems at the same strain level. The beneficial effect increased with increase in the quantity of reinforcement, whereas for geocell reinforcement, the advantage was evident only at higher strains

Modulus Ratio and Joint Factor Concepts to Predict Rock Mass Response

This paper presents the shaking table studies to investigate the factors that influence the liquefaction resistance of sand. A uniaxial shaking table with a perspex model container was used for the model tests, and saturated sand beds were prepared using wet pluviation method. The models were subjected to horizontal base shaking, and the variation of pore water pressure was measured. Three series of tests varying the acceleration and frequency of base shaking and density of the soil were carried out on sand beds simulating free field condition. Liquefaction was visualized in some model tests, which was also established through pore water pressure ratios. Effective stress was calculated at the point of pore water pressure measurement, and the number of cycles required to liquefy the sand bed were estimated and matched with visual observations. It was observed that there was a gradual variation in pore water pressure with change in base acceleration at a given frequency of shaking. The variation in pore water pressure is not significant for the range of frequency used in the tests. The frequency of base shaking at which the sand starts to liquefy when the sand bed is subjected to any specific base acceleration depends on the density of sand, and it was observed that the sand does not liquefy at any other frequency less than this. A substantial improvement in liquefaction resistance of the sand was observed with the increase in soil density, inferring that soil densification is a simple technique that can be applied to increase the liquefaction resistance.

Effect of overburden stress and surcharge pressure on the liquefaction response of sands

Abstract This paper describes shaking table model studies conducted on unreinforced and reinforced soil slopes. A computer controlled hydraulically driven single degree of freedom shake table was used in these tests. Model soil slopes were constructed using clayey sand (SC) in a laminar box. Two different slope angles were used in tests. Two different types of reinforcement, namely, a woven geotextile and a biaxial geogrid were used in the tests. The models were instrumented with ultrasonic displacement transducers and accelerometers at different locations. The slopes were subjected to horizontal base shaking of known acceleration and frequency and the displacement and acceleration were monitored for time increments. Comparison of displacement and acceleration of tests with different slope angles and with reinforcing layers of different stiffness values resulted in some important observations regarding the response of these soil slopes under seismic shaking conditions. Tensile stiffness of the reinforcement plays an important role in controlling the seismic deformations and the beneficial effects of reinforcement are more pronounced in flatter slopes. Acceleration amplifications did not alter with the inclusion of reinforcing layers in the model studies reported in this paper because of the dimensional limitations. Numerical modeling of the geosynthetic reinforced soil slopes subjected to base shaking is also presented and the models are verified with the experimental results.AbstractThis paper describes shaking table model studies conducted on unreinforced and reinforced soil slopes. A computer controlled hydraulically driven single degree of freedom shake table was used in these tests. Model soil slopes were constructed using clayey sand (SC) in a laminar box. Two different slope angles were used in tests. Two different types of reinforcement, namely, a woven geotextile and a biaxial geogrid were used in the tests. The models were instrumented with ultrasonic displacement transducers and accelerometers at different locations. The slopes were subjected to horizontal base shaking of known acceleration and frequency and the displacement and acceleration were monitored for time increments. Comparison of displacement and acceleration of tests with different slope angles and with reinforcing layers of different stiffness values resulted in some important observations regarding the response of these soil slopes under seismic shaking conditions. Tensile stiffness of the reinforcemen...

Reply to Discussion of “Modulus Ratio and Joint factor Concepts to Predict Rock Mass Response” by T. Ramamurthy, G. Madhavi Latha, T.G. Sitharam, Rock Mech Rock Eng, 2017. 50:353–366. doi:10.1007/s00603-016-1112-z

The commonly adopted rock mass classifications, namely RMR, Q and GSI, are used to estimate compressive strength and modulus of rock masses. These values have been examined as per modulus ratio concept, Mrj, for their reliability. The design parameters adopted in some of the recent case studies based on these classifications indicate that the Mrj values for rock masses are higher than those of the corresponding intact rocks. The joint factor, Jf, which is defined as a weakness coefficient in rock mass suggests that modulus ratio of rock mass (Mrj) has to be less than the modulus ratio of the corresponding intact rock (Mri), on the basis of extensive experimental evidence. With joint factor, compressive strength, elastic modulus, cohesion and friction angle were estimated and applied in the analyses of a few cases. The predictions of deformations with this approach agreed well with the field measurements by adapting equivalent continuum approach. The modulus ratio concept is considered to present a unified classification for intact rocks and rock masses. Soil–rock boundary, standup time in under ground excavations and also penetration rate of TBM estimates have been linked to Mrj.

Effect of sample size and anchorage on the performance of reinforced soil–aggregate systems

Abstract The physics and mechanism of soil liquefaction is understood sufficiently well by researchers now. However, the effect of various external factors such as the weight of the structure and overburden pressure on the liquefaction behavior of cohesionless soils is still under investigation. This paper presents a comprehensive study consisting of shaking table experiments on saturated sand beds to understand the effect of overburden pressure for free field conditions and the effect of surcharge weight for under the structure conditions on the liquefaction response of the sand beds. Pore water pressure was monitored continuously and the loss of effective stress with dynamic time was calculated for all the tests. Liquefaction occurred for shaking under specific ground motion conditions, which was visibly seen in the shaking table tests when the sand bed started to shake like a liquid and sand boils were formed on the bed. The number of cycles of dynamic shaking required for liquefaction and the pore water pressure were compared for different models. It was observed that increase in overburden pressure increases the liquefaction resistance of the soil. For sand beds under building pressure, the liquefaction response was observed to depend a great deal on the dynamic loading parameters. In the case of sand beds under surcharge loads, the response was complicated because of the complex soil–structure interaction under dynamic loading conditions.

Repeated load tests on geosynthetic reinforced unpaved road sections

We thank the discusser for appreciating our work and highlighting his complementary work on joint factor. The discusser raised three important issues in the discussion: influence of infill material on the joint strength parameter used for the calculation of joint factor; relating the joint factor to the RQD, GSI, RMR or Q; and estimation of strength and modulus using joint factor. Our detailed response to these three points is as follows. The joint factor is a combination of joint frequency, joint inclination parameter that depends on the orientation of the critical joint and strength along the joint considered. For calculating joint strength parameter for joints filled with a gouge material and reached the residual shear stage, Ramamurthy (1993) has provided a table, which gives the least strength parameter of 0.18 for joints filled with clay– silt (75% clay) and maximum strength parameter of 1.0 for joints filled with gravelly sand. While calculating the joint strength parameter, only the relevant strength in terms of friction (/j) has to be considered. Because of cementation, the cohesion component has to be converted into an equivalent friction and used. For calculating the strength parameter for clean joints, joint strength parameter is correlated to the uniaxial compressive strength of the intact rock. Clear guidelines are also given for the estimation of other two parameters, namely joint frequency and joint inclination. The joint factor should not be linked to GSI, RQD, RMR or Q classifications. RQD does not consider two of the parameters considered in joint factor calculation, joint inclination and joint strength. The third parameter joint frequency is a realistic parameter, and there is no ambiguity in its use. In the laboratory tests, even a small kink or step considerably influences both strength and modulus. Therefore, upper and lower bound limits were given to the strength and modulus estimations based on joint factor (Ramamurthy 2001). So a near-lower-bound value of strength and modulus from these correlations should be adopted for a known value of Jf. These solutions were validated against large experimental data of rocks and rock-like materials and were successfully applied to a number of problems, like large underground cavities, tunnels and deep mines. Introduction of empirical multiplication factors to Jf so as to get strength and modulus of rock mass may not help in predicting responses of in situ problems. We certainly appreciate the effort and views of the discusser on our paper.