Abstract: A deep excavation projects is located on the Taipei soft deposit. The penetration depth of the retaining system that was evaluated by the building codes showed that the safety factors against heaving are satisfied with the requirement of the codes. But a serious damage still occurred. In the re evaluating works of the project, the authors find that the traditional soil mechanics is inconsistent with the real stressstrain behavior of the soft deposit. Soft ground is always a fuzzy term in geotechnical engineering field. There is no clear qualitative definition on it till now and is short of complete theoretical basis. The traditional concept on the soft ground is based on the elasto plastic model of the overconsolidated clay. From the monitored results in practical deep excavation cases, it is found that there exists many unreasonable contradictions. A tri linear stress-strain model is necessary to simulate the real behavior of the soft deposit.