Non linear behaviour of few sands and its engineering implication

Abstract

There are vast deposits of sands along the banks of river Yamuna flowing through various parts of northern India. These deposits of sand contain varied amount of fines classified as silt and clay. Engineering behaviour of clean sand has been well investigated in the past few decades whereas a little effort was made to characterise sand with varied proportion of fines. Hence proper characterization of these sands with varied proportion of fines becomes a matter of great importance, since it provides support to various structures such as structural rafts and deep foundations to multi-storeyed buildings and supporting ancillary services. In the present work, a set of standard tests are performed on the sample of sands with varied proportions of fine collected from various construction sites to study their engineering behaviour. The soil samples are analysed for grain size analysis, moisture content, density tests including maximum and minimum void ratio, liquid limit, plastic limit, relative density, relative compaction and shear strength etc. The results thus obtained have been co-related with the field observation and site specific record of SPT test. A series of triaxial tests have been performed on different samples with fine contents in the range of 5 to 25% by weight. The shear stress and shear strain data are plotted to interpret the shear strength and dilatancy characteristics of Yamuna sand with varied proportion of fines. The stress strain behaviour of silty sand varies significantly due to the presence of fines, relative density, and confining pressure. The presence of fines modifies the grain size distribution compared to clean sand which indirectly affects the peak and critical friction angle. On the basis of stress strain plots of silty sands at varied confining pressure, fine content and relative compaction, the non-linear behaviour of Yamuna sand was evaluated based on non-dimensional and non-linear shear strength parameters. The strength behaviour of silty sand can be interpreted from its relative compaction, mean confining pressure and relative dilatancy. Use of relative compaction has an advantage of using itself with respect to the maximum density instead of its relation with minimum density. It is relatively easy to interpret relative compaction since in the field test natural density can be directly obtained in relation to the maximum density. The non-linear and non-dimensional strength parameters Qaf and Raf were evaluated and the results thus obtained are compared with some other sands and also with the back calculated values from the previously published literature on sand containing fines. It has been concluded that the non-linearity of Yamuna sand containing significant proportion of fines can be fairly represented by correct interpretation of non-dimensional strength parameters Qaf and Raf. It has been observed that the Yamuna sand containing silts has comparable values (Qaf =25 to 40) for evaluation of shear strength at varied relative compaction. The presence of silt affects the strength properties of Yamuna sand significantly as reflected by the changes in the values of Qaf. The value of Qaf ranges from 50 to 30 when silt content increases from 0 to 15%. With further increase in silt (beyond 15%) Qaf drops to 5 at a silt content of 25% which shows that behavior of sand containing nearly 25% fines is no longer simulates to clean sand but the soil behavior is nearer to that of silt. Based on the values of new strength parameter of Yamuna sand containing varied proportions of fines, engineering implication namely bearing capacity and liquefaction potential of Yamuna sand are evaluated. The bearing capacity of Yamuna sand with fines can be easily evaluated on the basis of non- linear, non-dimensional shear strength parameters using the field value of relative compaction without performing the triaxial test. Bearing capacity of Yamuna sand with varied proportion of fines first increases when fine contents are up to 10% then decreases with increase in fine content due to reduction in the value of Qaf. The data of SPT test from Yamuna basin was collected in NCR of Delhi. The liquefaction potential of silty sand has been evaluated by Seed & Idriss method. Simultaneously the liquefaction potential was evaluated using relative compaction based on the newly established shear strength parameters Qaf and Raf. It is shown that the model technique is convergent with that of Seed & Idriss method for sandy soils while it diverges for silty sand. Also the liquefaction potential has been validated using the empirical relation developed in the present study and relative compaction obtained using field SPT data. It is proposed to use the present technique of liquefaction potential evolution (CRR) based upon Qaf, Raf and Rc specially for silty sand.