Abstract：In the construction of roadbed，it is still an important work to find a fast，nondestructive and precise measurement method for soil compactness. The method for measuring the compactness of soft soil by instant hammering，and the signal analysis method based the wavelet and AR spectrum analysis are put forward. By the analysis of signal measured in lab，the characteristic parameters are picked-up，and some useful conclusion is obtained. The main peak frequency is increased with the dry density under condition of same water content，but decreased with the water content under condition of same dry density. The analysis result shows that the method of instant hammering is feasible for determining compactness of soft soil.
