The Xi'an Bell Tower in Shaanxi Province, China, has been preserved for more than 600 years. However, due to the deterioration of brick and rammed earth in the bell tower foundation (BTF), the ancient building has suffered serious damages, such as seepage, efflorescence, peeling, and cracking. The seepage causes and mechanisms of the BTF are investigated in this work. The distribution of damages in the BTF is analyzed by on-site investigation, and the moisture filed of the BTF is monitored by in-situ tests and model tests under different types of rainfall and different compaction degree of rammed earth. The results show that: (1) The brick-soil structure within 3.0 m below the top of the BTF and 2.0 m above the bottom of the BTF is seriously affected by rainfall infiltration. (2) Heavy rainfall infiltration, aging of the top anti-seepage layer and the cracking of the side wall in the BTF are the main reasons for the water seepage. The irrigation water for the plantings has no influence on the BTF when the ground around the BTF is hardened. (3) The decreasing of the dry density (or increasing of the pore ratio) of the rammed earth is the second reason for the water seepage. When the dry density of the rammed earth is decreased by 10, the depth of infiltration can be increased by 40, and the wetting degree can be increased by 50. Appropriately increasing the dry density of the rammed earth can effectively reduce the depth and impact of rainfall infiltration.
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