Abstract：As the transmission layer between segments and surrounding rock, the backfill layer has important influence on the mechanical characteristics of segments. However, the current design method for shield TBM neglects the influence of the backfill layer, and has differences from the engineering practice. According to the actual distribution of backfill in shield TBM tunnel, the formula for calculating backfill layer and ground reaction coefficient is deduced, and the design model for segment structures is established. It is concluded that due to the existence of the backfill layer, the equivalent resistance coefficient of the surrounding rock and backfill layer is different from that of the surrounding rock. The change degree of the resistance coefficient is related to the mechanical parameters of the surrounding rock and the backfill. Because the backfill behind the segments is thick at upper part and thin at lower part, the distribution of resistance coefficient is different. The distribution of resistance coefficient in rock grade II is small at upper part and large at lower part like a duck egg. The resistance coefficient in rock is grade III and IV, close to be uniform. The resistance coefficient in rock grade V is large at upper part and small at lower part like an inverted duck egg. The calculated results of the segmental force based on the structural design model for backfill are closer to the field test values, and the proposed method for the equivalent resistance coefficient and the design model are verified.
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