The paper proposes to geotechnical engineers a new concept to identify active clays based on the meaning of a graphical representation – the soil chart – originated from a Romanian Standard, that unifies the Casagrande's chart (LL-PI), the Skempton's chart (I_A) with the granulometric curve of the investigated soil. The normalized area of the soil chart Aⁿ = f(LL; PI; I_A;P₉₀ ;A_2μ) is considered a composite indicator of the soil swelling-contraction potential. Thus, a new index is proposed, based on a similar structure with the soil liquidity index (I_L=1-I_C) to estimate the soil swelling-contraction potential, as the activity coefficient. This one correlates the normalized areas of the soil chart for two clays developing extreme behaviour, kaolinitic clay (Aⁿ_C) and montmorillonitic clay respectively (Aⁿ_M), with the normalized area of the investigated soil (Aⁿ_O), resulting in the activity coefficient (C_A). This new and original concept has been calibrated by laboratory tests using kaolinitic and montmorillonitic clays from Romania and as the investigated soil, and active clay present in the flat area of the Bahlui River, frequently found as the foundation soil in Iasi city and county. The obtained results compared with the other indices ( U_L; I_A; A_2μ; C_p; w_s; C_v, w₁₅; q_max; p_u) presented in the corresponding literature are considered relevant and useful to acknowledge the activity coefficient (C_A) as a new physical parameter integrating all the other classical ones, that characterizes the intensity of the swelling-contraction phenomena anticipated during the geotechnical design on construction sites on active clays.