Bearing capacity and settlement of weak fly ash ground improved using lime – fly ash or stone columns

Abstract

An experimental study was conducted to evaluate the suitability of lime for the improvement of weak fly ash ground. In the study, a series of unconfined and confined compression tests were carried out on cylindrical samples in the laboratory and on cubic samples in the field, of Lime-FA (lime – fly ash) mixtures with various mixing contents of lime and curing times. Some samples were compressed under a soaking condition with water. A series of full-scale physical tests in the field and small-scale physical tests in the laboratory were conducted on a foundation (or rigid plate) on weak fly ash ground improved using Lime-FA or stone columns, which form a composite foundation. Some physical tests were carried out under a soaking condition. From the test and physical model study, it was found that the Lime-FA mixture has a larger shear strength than that of fly ash when the mixing content of lime is larger than 10%. When the weak fly ash ground is improved with Lime-FA columns, the bearing capacity of the fly ash ground is increased, and the settlement is reduced largely. However, when the ground is soaked under water, the corresponding shear strength of the Lime-FA mixture is decreased, the bearing capacity of the Lime-FA composite ground is decreased, and the settlement is increased. A plate loading test with soaking test on a layer of 1.15 m thick fly ash was also done in the laboratory. The test results show that the top fly ash layer is not suitable as a foundation soil layer and should be replaced with other granular soils, rather than simply compacted to a higher density, due to the negative impact of soaking. Results from the test program are presented and discussed.Key words: fly ash, lime, unconfined compressive strength, shear strength, bearing capacity, settlement, soaking.