Objective  With a history of about 870 years, the Jinzhongdu Watergate Site is the largest surviving site of the Jinzhongdu. The wooden structure, as the foundation and main body of the site, plays an important role in the whole watergate structure. The researchers explored the detailed composition of the wooden structure of the watergate site, conducted nondestructive inspection and assessed the defective condition of the wooden components, with the aim of providing a basis for the repair and conservation of the Jinzhongdu Watergate Site, as well as providing a reference for nondestructive inspection and assessment of underground wooden structures of the same type.

  Method  Firstly, the researchers surveyed and calibrated the wooden structure site, and received the typical size, location, amount and interconnection information of the three types of wooden components, including stone fixed timber pile, foundation timber pile and stone lined square. The wooden structure model of the watergate was also restored. Secondly, according to the respective characteristics of the three types of wooden components, the researchers took 236 such woods as examples and conducted on-site testing and laboratory analysis, including knockdown, dimensional measurement, moisture content measurement, micro-drilling resistance testing and tree species identification. Finally, the information from the on-site inspection and laboratory analysis would be summarized, as well as the grades of the condition of the tested wooden components, and the laws of decay defects of wooden components.

  Result  A three-dimensional model of the wooden structure of the Jinzhongdu Watergate Site was reconstructed, and it was concluded that its wooden structure was supposed to be composed of 204 stone fixed timber piles, 771 foundation timber piles and 92 stone lined squares. The main defects of the wooden components of the site were decay and the lack of material caused by decay, and the secondary one was cracked. In terms of single wood members, the decay of overground part was comparatively light, while the decay of the underground part was comparatively serious. The species of wood members at the site were mainly Larix principis-rupprechtii, mixed with Pinus sylvestris var. mongholica.

  Conclusion  The overall condition of the wooden structure of the Jinzhongdu Watergate Site was destructive, and no A-class wooden components in good condition were found. Among the three types of wooden components, the overall condition of the stone fixed timber piles is relatively the best, while the overall condition of the foundation timber piles is relatively the worst. The practice has proved that it is feasible to detect and evaluate the defects of the wooden components of Shuiguan Site by comprehensively using percussion, size measurement, moisture content measurement and micro drilling resistance detection.