This paper deals with the lateral-earth-pressure acting against a concrete gravity debris barrier with Shirasu backfill. In-situ tests were carried out in a collapsed land in Shirasu region. Measurements and observations were performed on lateral-earth-pressure, pore-water-pressure, movements of debris barrier, groundwater level, and rainfall. Testing arrangements are shown in Photoes 1 to 3 and in Fig. 1. Table. 1 gives the date of measurements. Physical and mechanical properties of Shirasu backfill are shown in Table 2 and 3, and in Fig. 3. Table 4 indicates the displacement of debris barrier determined by means of triangulation, and the facters in Table 4 are given in Fig. 2. The angle of wall friction was determined by means of the equipment shown in Fig. 4, and test result is shown in Fig. 5. The main results of measurements and observations in-situ are shown in Figs. 6 to 10. Basing on the insitu tests the following conclusions might reasonably be made (1) The gravity debris barrier undergoes movement due to horizontal and rotational displacements in the direction away from Shirasu backfill, consequently the type of distribution of earth-pressure against a wall changes. (2) The lateral-earth-pressure of Shirasu does not show triangular distribution, and does not increase linearly with depth also, as is assumed by the classical earth pressure theories. (3) In the upper part of the wall, the measured lateral-earth-pressures are very larger than the values determined by Rankine's and Coulomb's theories. (4) The influence of apparent cohesion on the development and distribution of earth-pressures of Shirasu was not confirmed, and the large earth-pressures were measured in the part of wall corresponding to cohesion-height. These properties give the important suggestions on the applying of shearing test results to relevant design and safe working.