Organic clay has, usually, more than 10% organic content. Its improvement technology is different
from that of an-organic clay, therefore the research about organic clay improvement is not as
intensive as the research about improvement of an-organic clay. This research investigated the
change of unconfined compression strength (qu) of organic clay stabilized with lime and class F of fly
ash. Soil samples were taken from four different site : Wates, Kalasan, Piyungan and Playen. Those
three soil samples have different organic content (OC), and engineering characteristics. Three
different Fly ashes (FA-X, FA-Y and FA-Z) were purchased from local supplier. The three fly ashes
have different engineering characteristics, and lime was also bought from local material store. The
dry oven soils were mixed with 5, 15, and 25% of fly ashes (FA-X, FA-Y and FA-Z) then optimum
water content of soils were added to the blend. The blend were formed in unconfined compression test
samples the cured on 7, 14, 21 and 28 days curing period. Finally, the unconfined compression tests
were performed on all soil samples. The results indicate that the change of unconfined compression
strength depend on organic content (OC) of soils and fly ashes, the degree of acidity (pH) of soils,
the CaO conten and ratio of CaO/SiO2 in the fly ashes, fly ashes proportion and the curing period.
The qu of stabilized soil decrease proportional to the increase of OC in the soil and fly ashes. With
respect to pH, the optimum improvement of qu is on the value of pH around 4, and it decreases when
pH is more than or less then 4. When CaO content on FA increase, the qu of the stabilized soil is also
increase, and the best improvement of qu, was found on CaO/SiO2 around 0.30. The qu of the mixed
improve significantly due to improvement of FA in the blend. The significant improvement can be
noted on the content of FA 18 to 25%. Due to the increase curing time the qu of lime and flay ash
stabilized soil increases. The sharp improvement occur on the curing period 21 to 81 days, because
pozzolanic reaction start happening on that period. It can be compared to 7 to 15 days of curing, in
this period only the cation exchanges reactions occur not cementation of the stabilized soil, therefore
the improvement of qu is not as sharp as on the blend cured 21 to 28 days.