Ground treatment by stone column and geopier application

Soil improvement methods are developed for solving the bearing capacity, deformation and stability problems on the earthfills built on soft ground. In this research two of these methods which are stone columns and geopier, were studied. Stone column is frequently used soil improvement method nowadays. It could be used both in cohesive and cohesionless soils for increasing the rate of consolidation, decreasing the settlements, improving the bearing capacity, to lessen the liquefaction potential and for the stability of the earthfills. In US, geopier is a successful and economic practice which is used as a soil improvement method in soft soil region. This is based on a well elaborated method used in weak soil, comprising very hard and dense aggregate compacted with a hammer in the holes formed in the ground. In this research, increasing the bearing capacity of the earthfill on soft clayey soils were studied. Bearing capacity and consolidation behaviour were studied under thick earthfill on soft and medium stiff clay with both stone column and geopier treatment and the improvements were calculated using both the calculation methods in literature and PLAXIS finite elements program. Analysis methods in literature and PLAXIS finite elements analysis have shown the stone column reinforced soil had an increase in the bearing capacity. Also, expected settlement values decreased compared to the untreated soil. Bearing capacity computed by using the classical methods was larger than that found by PLAXIS finite elements program. According to the results of analyses, reinforcement using geopier elements, even with shorter column lengths will provide nearly same improvement with the stone column. The calculated settlement values were larger than the values for stone columns, but smaller than for the untreated soil. Bearing capacity of soil reinforced with geopiers calculated by the classical methods were larger than that found by PLAXIS. In the analyses of the stone column by the literature methods for the stress concentration ratio n=3 value used and the analyses made with PLAXIS revealed that this ratio varies between 2.88 and 3.70 depending on the column diameter. Therefore the value of n=3 seems to be appropriate. In the geopier calculation with the literature methods, stress concentration rate was taken as 6. The analysis made with PLAXIS showed that, the ratio varies between 5.35 and 5.75 depending on the column diameter. Key Words: Earthfill, ground treatment, stone column, geopier, rammed aggregate piers, settlement, bearing capacity.