STUDIES ON DYNAMIC ELASTIC CONSTANTS FOR REINFORCED SAND AND POND ASH

Abstract

Understanding the dynamic behavior of the foundation-soil system is imperative for analyzing machine foundations and substructures subjected to earthquakes or induced dynamic loadings. Amplitude, frequency, damping and modulii or elastic constants are important factors to be considered in this connection. Mathematical techniques for analyzing dynamic soil structure interaction problems necessitates a realistic determination of the pertinent stiffness properties, which may be quite involved because of the dependence of these properties on a large number of parameters. In the past three or four decades, soil reinforcement has been one of the most popular techniques used for improvement of poor soils and has been extensively used to improve the static strength characteristics of the fill soil. Reinforcement of soil by using metal strips, geosynthetics or natural/ synthetic fibers to increase tensile strength, shear resistance and stiffness of soils is being undertaken. Fiber reinforcement has considerable effect on bearing capacity and dynamic response of the soil similar to that observed when using oriented geogrid layers. In this context, randomly distributed fiber reinforced sand (RDFS) is an emerging technique that has been successfully used in a variety of applications such as slope stabilization, road subgrade and subbase etc. This is a relatively simple technique for ground improvement and may have tremendous potential as a cost effective solution to many geotechnical problems. Evaluation of dynamic response of foundations on the fiber reinforced sand or geogrid reinforced pond ash and their dynamic properties, however, has not received the needed quantum of attention as it deserves. Use of granular piles or stone columns to improve the strength of weak soil deposits is an age old practice. However, its performance under dynamic loading conditions or its influence in modifying the dynamic response has not been studied in detail. The present research work has been carried out to study the dynamic response of foundations on randomly distributed fiber reinforced sand beds and geogrid reinforced pond ash. The effect of foundation embedment in pond ash has also been studied. The efficacy of using the stone columns in pond ash deposits has been experimentally investigated. It is anticipated that the study would help in better understanding of the dynamic behavior of foundations resting on such composite materials. In India, cyclic plate load tests and block vibration tests are generally used for the evaluation of the dynamic stiffness of soils. A large number of reinforcement parameters are likely to influence the dynamic behavior of the soil; the present study, however, has been undertaken to investigate the following aspects: 1) The effect of the percentage of fiber and the area of the zone of reinforcement on the coefficient of elastic uniform compression Cu by performing cyclic plate load tests in the laboratory. 2) To perform the vertical and horizontal forced vibration tests on the fiber reinforced sand in the laboratory to study the effect of the size of the reinforcement zone as well as the percentage of fiber content on the dynamic response, the coefficient of elastic uniform compression Cu, the coefficient of elastic uniform shear CT and the damping ratio. 3) The influence of the depth of embedment, the number of reinforcement layers and the effect of providing stone columns/ granular piles in the pond ash bed by conducting both vertical and horizontal block resonance tests in the laboratory as well as in the field, supplemented by a few cross borehole tests. The cyclic plate load tests have been performed on un-reinforced and reinforced sand beds at a relative density of 30%. The size of the plate used for tests was 200mm x 200mm of 25mm thickness. The sand beds were prepared in rigid steel tank of sizel.2m x 1.2m x 0.6m for conducting these tests. The percentage of fiber content have been varied from 0 to 2% and the zone of fiber reinforcement below the test plate varied from B x B x B, 2B x 2B x B and 6B x 6B x 2.5B, B being the width of the test plate. The data of cyclic plate load tests has been analyzed to determine the coefficient of elastic uniform compression Cu. The static strength characteristics like the bearing capacity ratio and settlement ratio have also been determined. The slope of the initial portion of pressure vs. elastic settlement plots gives Cu and the confining pressure correction and area correction have been applied to determine the standard values of Cu (corresponding to the confining pressure of 10 kN/sqm and area of lOsqm) for both the un-reinforced and reinforced sand beds. The second part of the experimental investigation on sand consists of performing the vertical and horizontal block resonance tests in un-reinforced and reinforced case. A concrete test block of size 800mm x 400mm x 400mm of M20 grade of concrete was used for performing block vibration tests. A rigid masonry tank of size 1.6m xl.2m x n 1.2m was used to prepare sand beds of varying percentage of fiber content (0 to 2%) and the reinforcement zone ranging from 0 to 2.5 times the width of the test block beyond the edges of the test block. The frequency and amplitude observations were recorded at three different force levels. In the laboratory investigation of the pond ash, both the vertical and horizontal block resonance tests have been performed for the block resting on the surface as well as in embedded condition at two embedment ratios of 1/3 and 2/3. Also block resonance tests were conducted on the pond ash bed provided with the stone columns of 50mm diameter and 800mm depth consisting of the stone chips of 10-12mm size from the surface at a centre to centre spacing of 200mmapart in a grid pattern in the test tank. In the field, the block resonance tests were performed with the large block of size 1500mm x 750mm x 750mmfor surface test only; however the small block of size 800mm x400mm x 400mm was used for surface tests, embedment tests and reinforcement tests etc. at three different eccentricity levels. The embedment ratios adopted in the field were 0.25, 0.50 and 0.75 and the geogrid reinforcement consisted of 1 to 3 layers of Netlon CE-121 of size [3B x (L+2B)] such that there is a projection of B all round beyondthe edges of the block at the vertical spacing of 0.25B, L and B being the length and width of the block respectively. The vertical and the horizontal vibration test data have been analyzed to determine the coefficient of elastic uniform compression Cu and the coefficient of elastic uniform shear CT respectively. The correction for the confining pressure and area of foundation block has been applied to obtain the standard values of Cu and CT. The strain levels associated with the different tests have been determined and the value of Cu and CT are interpreted at various strain levels. In vertical vibration tests, the damping ratio § has been determined by the band width method as applicable for the frequency dependent excitation. In addition to the above field tests, two numbers of cross bore hole tests were also conducted in the field to determine the velocity of propagation of shear waves from which the shear modulus and the Young's modulus were calculated. Further this data was interpreted to obtain the value of Cu at low strain level. Based upon the experimental studies and analysis of data, the following conclusions are drawn. i) In cyclic plate load tests on sand, there is significant increase in the ultimate bearing capacity and appreciable decrease in the total settlements by reinforcing the sand with in polypropylene fibers. With the increase in the percentage of fiber reinforcement, further improvement in the ultimate bearing capacity and more reduction in the total settlement are achieved. However, 1% fiber content and reinforcement zone dimension of (2B x 2B x B) was found to give optimum results. The coefficient of elastic uniform compression Cu decreases with the inclusion of fiber reinforcement in sand, the decrease being dependent upon the percentage and the size of the zone of reinforcement. However, the pressure range to which the Cu value for the fiber reinforced sand corresponds is higher as compared to that of un-reinforced sand. ii) Inblock resonance tests on sand, there is decrease of resonant frequency and increase in the maximum amplitude of vibration with the increase in the percentage of fiber reinforcement as well as the area of the zone of fiber reinforcement both in vertical and horizontal tests. However, the damping ratio £ of the reinforced sand bed decreases with the increase in the percentage and area of the zone of reinforcement. The coefficients of elastic uniform compression Cu as well as the coefficient of elastic uniform shear CT decreases depending on the percentage and area of the zone of reinforcement at low strain levels. iii) For the pond ash, in block resonance tests in the laboratory there is increase in the resonant frequency and decrease in the maximum amplitude values with the increase in the embedment ratio. But the resonant frequency decreases with the increase in strain levels for both horizontal and vertical tests. The damping ratio increases with the increase in strain level as well as the depth of embedment in vertical vibration test. Both the coefficient of elastic uniform compression Cu and the coefficient of elastic uniform shear CT increases with the increase in the depth of embedment of the test block into the pond ash. However, when the pond ash is reinforced with stone columns, there is appreciable increase in the value of Cu and CT respectively and remarkable decrease in the maximum amplitude. iv) In the field investigation of pond ash, the same trend as in laboratory tests was obtained in the surface as well as embedment tests of block vibration. But when the pond ash is reinforced with geogrid layers, there was decrease in the resonant frequency as well as maximum amplitude values, the decrease being more with the increase in number of layers. Also the damping ratio £ of the geogrid reinforced pond ash increases with the increase in number of geogrid layers. But both the coefficient of elastic uniform compression Cu and the coefficient of elastic uniform shear CT decreases with the increase in number of geogrid reinforcement layers at low strain levels. iv Therefore, an important conclusion drawn on the basis of the test results is that though the static strength characteristics of sand is considerably improved upon reinforcing, the coefficient of elastic uniform compression and the coefficient of elastic uniform shear are marginally reduced at the strain levels associated with the block resonance tests. The results of the study will be especially useful while designing the structures or machine foundations on fiber reinforced sand and geogrid or stone column reinforced pond ash as well as embedded foundation in pond ash, where the natural frequency of the soil-foundation system may lie close to the operating frequency of the machine or the structure and the maximum amplitudes exceeds the permissible limits. The maximum amplitudes can be brought under control and the disturbance during the starting and stopping stages of the machine can be reduced. However, in seismically active zones or for the structures likely to be subjected to vibrations, the RDFS is to be cautiously and judiciously used considering the increase in amplitude or decrease of Cu or CT values after weighing against the improvement of performance in static state