Durability Mechanisms and Long-Term Stability of Rammed Earth Materials in the Earthen City Wall of Guoyang Ancient City, Shanxi, China

The long-term preservation of earthen wall heritage in Shanxi largely depends on the stability of its rammed earth walls. In this study, the ancient city wall site of Guoyang in Shanxi is taken as a case study. Samples are collected from four directional sections (brick, mortar, and rammed earth), and a nonlinear dynamic finite element method (FEM) with horizontal impact loading is applied, combined with microscopic analyses (SEM, XRF, XRD), to investigate the deterioration mechanisms and structural responses of the brick wall surface. The results show that (1) under identical stress, the north wall exhibits smaller displacement (2.942 × 10−2 mm) than the south wall (4.058 × 10−2 mm), indicating greater lateral stiffness and deformation resistance; (2) the rammed earth particles on the north side show the highest uniformity (CV = 0.4981), while those on the west side display the greatest dispersion (CV = 0.9852), revealing pronounced differences in microscopic stability among the different wall orientations; (3) the north-side rammed earth exhibits a composition typical of high-quality traditional ternary lime–soil mixtures, characterized by high SiO2 (47.06%), high CO2 (19.73%), and low Na2O (2.358%); and (4) the durability of the four wall sections varies with distinct deterioration characteristics. Mineral composition and environmental factors jointly contribute to the differences in wall color and surface conditions. This study provides a reference for the conservation and restoration of similar rammed earth heritage sites.