PERFORMANCE OF COMPACTED ASH AND SAND FILLS UNDER LOAD

Abstract

Low-lying areas are often required to be developed as sites for construction of residential and industrial buildings. To avoid flooding of the area and place foundations at an appropriate level, formation level of such sites is required to be raised through compacted fills. Further, in situations where top soil is weak, it may be replaced by stronger compacted fill to improve bearing capacity and limit settlement of foundations. Fills may also be required to level undulating ground. Literature reports many cases where, compacted fills have formed an essential part of construction activity. Well-graded sand and gravel containing small percentage of clayey fines are considered the most suitable fill materials. However, it is rare that such materials become available at reasonable costs. Coal ash, produced in enormous quantity (100 million tonnes / annum) in India, can be thought as an alternative fill material. Utilization of ash in geotechnical applications, such as load bearing fills, will help in solving the problem of ash disposal to a great extent and reduce demand on borrow soil, thus saving valuable land. Literature indicates that fly ash and bottom ash are produced in 80:20 proportion. Separate collection of fly ash and bottom ash under 'Dry Mode' of collection is the recommended practice for adoption by thermal power plants. For effective utilization of these ashes, there is a need to establish the relative performance of fly ash, bottom ash and their combinations for structural fill applications. Sand is the other commonly used material as structural fill. It is a common practice in India that river sand available at a nearby location is used as fill material. The river sand, available in wet condition, is not amenable to high degree of 11 compaction due to the bulking of sand grains. Literature (Lade and Yamamuro, 1997 and Pitman et al, 1994) suggests that, on addition of fines, the properties of sand can beimproved. This aspect also needs to beexamined in detail. Adetailed review ofliterature pertaining to the above aspects indicates that: (i) Suitability ofcoal ash has been established till now on the basis ofindex and engineering properties (Gray and Lin, 1972, Martin et al., 1990, Kaniraj and Havanagi, 1996 and Sridharan et al., 1998). Some applications of ash as structural fill have also been reported (Leonards and Bailey, 1982, Martin et al, 1990 and Dayal and Sinha 2000). However, knowledge base on relative performance of fly ash, bottom ash and their combinations under load in various possible fill conditions, including submerged condition of fill of the above materials is lacking, (ii) Addition of fines to sand increases density, imparts some cohesion, reduces $ value and increases compressibility of the mix. However, literature lacks information on the quantitative improvement in the load carrying capacity due to addition of fines and optimum amount of fines which, when added to sand, gives best performance under load, (iii) Compacted ash beds are subjected to capillary stress due to contact moisture and preconsolidation stress due to compaction process. These stresses affect the compressibility of the fill material and reduce settlement of test plates / footings resting on ash beds. However, no method is available for the estimation of settlement of footings resting on such fills accounting for the effect of capillary and preconsolidation stresses. In view of the above, the present study has been directed towards the following objectives: 1• To study the relative performance of compacted fly ash, bottom ash and their combinations under load in various possible fill conditions. in 2. To study the performance of fills of sand with different amount of cohesive and non-cohesive fines under as compacted and submerged conditions. 3. To establish the optimum amount of fines which, when added to sand, gives the best performance under load. 4. To suggest a method for the estimation of settlement of footings resting on compacted ash fills accounting for the effect of capillary and preconsolidation stresses. To achieve the above objectives, a comprehensive experimental investigation programme was planned. The details of studies carried out and the major conclusions from the present study are briefly described below. STUDIES ON COMPACTED COAL ASH Fly ash and bottom ash, procured from thermal power plant at Dadari in Uttar Pradesh, India, were used as test materials. Experimental study consisting of tests for engineering properties and load-settlement behaviour onthe materials wascarried out. Shear strength and compressibility characteristics of the above test materials were studied under the following conditions: (i) As compacted - samples compacted at OMC (Optimum Moisture Content) and tested immediately after compaction (ii) Saturated - samples compacted at OMC and tested after saturating the test specimen, (iii) Aged - samples compacted at OMC and cured for specified period of aging and tested (iv) Aged saturated - samples cured for specified period of aging and tested after saturating the test specimens. Samples were cured in a constant temperature bath for periods of 14, 28, 56 and 90 days. Shear strength parameters (c and <}>) were determined by conducting direct shear tests under consolidated undrained condition of shear. Compressibility characteristics were determined by conducting consolidation tests. To study the performance of compacted ash beds (fly ash, bottom ash and their combinations) under load, load tests were conducted on a circular plate of 150 mm iv diameter. Test beds were prepared in a tank (580 mm diameter and 890 mm high) having provision for raising water table within the test bed. Test beds were prepared by compacting the test materials at their OMC in layers of 80 mm thickness. Each layer was imparted energy equivalent to that in Proctor compaction. The load tests were carried out under following conditions of test beds (i) As compacted - compacted at OMC and load tested immediately after compaction (ii) Submerged - compacted at OMC, submerged and then load tested (iii) Aged - compacted at OMC, allowed for curing for specified period (aging) and load tested, (iv) Aged submerged - compacted at OMC, allowed for specified period ofaging, then submerged and load tested. In order to ensure that the results of load tests have not been affected qualitatively due to test plate being smaller than the standard size, a few load tests were also carried out on a plate of 300mmsize. STUDIES ON FILLS OF SAND AND FINES Alocally available uniform sand was selected as the host sand. Alocal clay soil was chosen as cohesive fines and fly ash as non-cohesive fines. This part of the experimental study also consisted of study of engineering properties and loadsettlement behaviour of compacted sand+fines mix. Engineering properties i.e. compaction, shear strength and compressibility were studied with fines varying from 0to 40 %. Shear strength parameters (c and <j>) were determined by conducting direct shear tests under consolidated undrained condition of shear and compressibility characteristics (e-log prelationship) by consolidation tests, both under as compacted and submerged conditions. Based on the results ofthe above study a few combinations ofsand+cohesive fines (0, 5, 10, 20 and 30%) and non-cohesive fines (5, 10 and 20%) were selected to study the effect of fines on load-settlement behaviour. Load tests were conducted on a plate of 150 mm diameter. Test beds were prepared in asimilar manner as described in the case of ash fills. Load tests were carried out both under (i) as compacted and (ii) submerged conditions. ESTIMATION OF SETTLEMENT OF FOOTINGS ON COMPACTED ASH FILLS The results ofthe present study and the literature suggest that compacted ash fills are subjected to capillary and preconsolidation stresses. In view ofthis, though coal ash is also cohesionless in nature, the existing methods of computing settlement of foundations on cohesionless soils may not be directly applicable to foundations on compacted ash fills. Therefore, an analytical method has been developed to compute the settlement of plate / footing taking into account the effect of capillary and preconsolidation stresses. CONCLUSIONS Based on the above studies, the following major conclusions havebeen drawn: * Bottom ash and bottom ash with 40-50% fly ash are found competent fill materials when examined from their engineering properties as well as load-settlement behaviour both under (i) as compacted and (ii) submerged conditions. * Fly ash beds show very poor performance under submerged condition, a condition likely to occur in. the life time of the structure supported by such fh fills. The load carrying capacity is reduced on submergence to about 1/10 of that exhibited under compaction moisture content. * Load carrying capacity of ash beds improves on aging significantly. An aging period of 28 days is found adequate to reach themaximum strength. * Density, shear strength and load-settlement behaviour of sand improves significantly on addition of both cohesive and non-cohesive fines. Of the two, addition of cohesive fines is found preferable to non-cohesive fines. VI * The optimum amount of fines appears to be that which, when added and compacted at OMC, results in a void ratio equal to that of the host sand at its densest state. For such a combination ofsand+fines, bearing capacity of the test plate is found to be about 1,5 to 2 times of that on a bed of sand alone, both under (i) as compacted and (ii) submerged conditions ofthe test bed. * Ash fills may get submerged during the life time of the structure they support. Therefore, to ensure safe design, an estimate of settlement of footings under submerged condition of the fill is required. This may be estimated from the results of a load test conducted on ash fills at compaction moisture content, using a procedure suggested in the present work which accounts for the capillary and preconsolidation stresses in the load test and prototype footing situations. vn