FIELD STUDY ON SOFT SOIL IMPROVEMENT USING CONTINUOUS FLIGHT AUGER (CFA) PILES

Abstract

The continuous flight auger (CFA) piling technique is commonly used where the presence of groundwater or potentially collapsible material reduces the cost-effectiveness of bored piling techniques. Whilst generally considered a soil replacement technique, the process of pumping concrete under pressure during the pile installation can result in a significant improvement of the surrounding soil, particularly in low strength soil conditions. In this study, a case study is analyzed. An oil storage tank was intended to be constructed on soft clay in southern Iraq. The tank has a diameter of 46 m. Cone penetration test, drilling boreholes, and field tests have been conducted in the tank site. A decision was made to improve the mechanical properties of soft clay of the site by CFA piles before construction of the tank. The depth of CFA pile constructed is 16 m, with a diameter of 0.5 m and spacing between piles is 4.2 m. Three types of in-situ tests have been conducted: Conventional plate load test on the soil before and after improvement; large plate load test for soil after improvement; and pile load test on CFA pile. It was concluded that construction of CFA piles shows an increase in the bearing capacity by about 110% for the soil directly around piles and 70% for soil within 2 m from the center of piles. Large plate load test result for the pile with surrounding soil revealed that the ultimate bearing capacity increases to 500 kN/m2. So that the overall loading area showed an increase in the bearing capacity of about 350% when CFA piles are constructed.