Some Studies on Feasibility of Using Flyash in Canal Embankment

Abstract

Though flyash is successfully in use for highways and road embankments, till date its use in canal embankment is hardly addressed by the researchers. To open up new direction for bulk utilization of this thermal power plant waste this study is taken up. This study addresses the feasibility of using flyash for canal embankments. Flyash from two sources, (i) GEB, Gandhinagar (GF) and (ii) Torrent power plant (TF) and locally available soil (AS) is used in the study. In this study the issues considered are (i) compaction characteristics of flyash (ii) seepage characteristics of flyash (iii) erodibility / dispersibility of flyash (iv) assessment of collapse potential of flyash (v) stability of flyash embankment and (vi) assessment of liquefaction potential of flyash. Physical characterizations and chemical composition tests showed both flyash as class-F type. Shear tests are performed on unsaturated and saturated flyash samples prepared at its MDD-OMC. Study reveled that on saturation flyash looses its cohesion and marginal increase in angle of internal friction is observed. Compaction plays very important role in the behavior of an embankment. Compaction characteristics of flyash used in this study with other class-F flyash from the literature review are studied to develop the relation between MDD and OMC. The collapse potential shows there is less risk of collapse when flyash is compacted at its MDD and OMC. Pin hole test results suggest both flyash as erodible material. Coefficient of permeability of flyash and layered flyash is studied to decide the percentage of soil and number of layers to be put in layered flyash embankments. From the test results it was decided to put one layer of soil replacing 30% of flyash in between the two layers of flyash because for that arrangement gives minimum coefficient of permeability for both fiyashes. Primary embankment section was assumed with 9.0 m height with a slope of 2:1 without berms with other parameters as per the IS 7112 (2002). The results reveled that embankment remained stable for all conditions (Just after construction, steady seepage and for sudden drawdown) of stability of slopes. After analytically checking the stability of assumed slope, the model embankments are tested in small laboratory geocentrifuge. Modeling technique is given in detail. Keeping embankment material, its placement density, thickness of middle soil layer as constant, series of centrifuge tests were executed. Parameters varied were (i) slope inclination and (ii) thickness of the cover. Scaled down models was subjected to constant g-value of 300. From the pilot testing it was observed that the section which remained stable just after start of the test remained stable even after 30 minutes of the centrifugation time. The slopes which failed were failed within very few minutes after start of the test. So it was decided to run the test for 15 minutes to observe the static behavior of the models under self weight. Vertical displacement was observed during test with the help of two LVDTs, one placed on the top of the embankment and the other on the slope of the embankment. No models show development of cracks during testing except models with thin cover tested for sudden drawdown condition of stability. So it can be said that thickness of the cover plays important role in stability of embankment. During testing in small geocentrifuge it was felt that use of sophisticated instrumentations becomes almost impossible. Small geocentrifuge can be used to study quality of the model behavior rather than the quantity. Four dynamic triaxial tests were performed to assess the liquefaction potential of flyash and layered flyash under dynamic loading. It was observed that flyash sustains 5-10 cycles of dynamic loading. From this study it is concluded that flyash is weak in sustaining dynamic loading so before using it in replacement of geomaterial, proper stabilizing treatment should be given to make it more resistant against dynamic loading. Relationship between stress ratio and No. of stress cycles was also developed for class-F flyash.