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Subgrade is part a road pavement structure designed to accept vehicle loads on it. Therefore must have an optimal carrying capacity and be able to accept the load due to traffic without experiencing significant changes or damages. The increase of heavy vehicle passing the main road cause damage on the pavement surface, such as rutting. For this reason, soil improvement is needed in order to obtain a stable subgrade to increase the performance of the pavement structure. The effort of stabilizing the subgrade soil of expansive clay has been conducted by various stabilizer, such as limestone, cement-fly ash, pozzoland material and others. One of stabilizer materials that can be used as an additive material is phosphoric acid, meanwhile other alternative material for phosphoric acid mixture is coconut shell ash. Soil sample is taken from Gedebage region and the test result on the soil is categorized as expansive soil category. In a soil stabilization with the mixture of 8% coconut shell ash + 10.5% phosphoric acid within 0 day of caring period can increase the carrying capacity of the soil and reduce damage due to fatigue cracking and grooves (rutting) which can improve the performance of flexible pavement structures.