During the landing roll both mainwheels entered soft areas in the strip surface. The aircraft swung through 120 degrees to the left then slid sideways for 17 metres before the right maingear collapsed. The wing struck the ground and was bent upwards. The condition of the strip surface was unsatisfactory because the first 500 metres contained soft spots. The positions of the soft spots were not marked, nor was the strip threshold displaced. The pilot had used the strip previously but on this occasion he did not check its serviceability before the flight.