The trap door is often used to predict the so-called active (loosened) earth pressure on the tunnel lining and the support system. As is Terzaghi's theory, the main object of the trap door experiment thus far is to evaluate the average magnitude of the active earth pressure which is caused by lowering the trap door. More important and interesting is, however, the information of the passive earth pressure outside of the trap door since the so-called arch action of the surrounding ground makes the vertical earth pressure decreased on the one hand and increased on the other hand. In particular, the latter is more important when the stability of the surrounding ground is discussed. In the present report, a simple formula is proposed to calculate the distribution of the passive earth pressure around the trap door by following Terzaghi's theory on the active earth pressure. It is examined by the data obtained by the trap door experiment as well as the finite element analysis. The result of the comparison is fairly well and the explained formula will give an important idea about the earth pressure around the tunnel support system.
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