A method for stability and safety evaluation of ancient city walls with brick-encased soil considering rainfall effect

Abstract

A kind of stability and safety evaluation method of brick-encased earth ancient city wall considering rainfall effect, first obtains the information of surface disease, geometric shape and internal defect of city wall, and establishes a three-dimensional visualization model. Secondly, the point cloud data is optimized to generate a refined surface grid, an integrated numerical model is constructed by fusing internal defects, and material partition, grid division and interface element insertion are completed. Subsequently, mechanical tests of outer brick, rammed earth and connecting interface under different water contents are carried out, and a material parameter library is established. The short-time heavy rain and long-time heavy rain working conditions are set, the spatial and temporal distribution of water content is obtained by simulating rainwater infiltration, and finally the material attribute dynamic assignment is realized by grid numbering and parameter mapping, the structure deformation, crack evolution and stress distribution are calculated, and the stability of city wall is quantitatively evaluated. This method solves the problems of the existing technology lacking dynamic fluid-structure coupling mechanism under rainfall and lacking cohesive force degradation-discrete mechanics analysis model reflecting the continuous to discontinuous deformation process of brick-encased earth ancient city wall.

Description

Technical Field

[0001] This invention belongs to the field of ancient building safety assessment technology, and specifically relates to a method for assessing the stability and safety of brick-encased earthen ancient city walls that takes into account the effects of rainfall. Background Technology

[0002] Ancient city walls are quintessential symbols of human civilization. my country boasts a vast number of ancient city walls, diverse in form and rich in cultural significance, with the earliest dating back six thousand years. Classified by construction materials and techniques, ancient city walls can be categorized into rammed earth structures, masonry structures, and brick-clad earthen structures. As large, immovable cultural relics, damage to ancient city walls results in incalculable losses; protecting them is protecting the history of China's urban development. While my country has begun to prioritize the repair, reinforcement, and scientific protection of ancient city walls, several projects have experienced structural instability and even collapse during construction, and partial collapses have occurred even after repair and reinforcement. Therefore, the need for effective and preventative scientific protection of ancient city walls presents both an urgent need and a significant challenge. The stability and safety assessment of ancient city walls, based on the theoretical analysis method of static equilibrium analysis, simplifies the ancient city walls as retaining walls or gravity structures. Based on the principle of static equilibrium, it calculates the anti-sliding and anti-overturning stability of the wall under loads such as its own weight and water pressure. The formula derivation is intuitive, requiring no complex mechanical modeling, and is suitable for rapid on-site assessment. However, it cannot consider defects such as cracks and internal voids, and only considers the mechanical equilibrium relationship in a two-dimensional plane, ignoring the three-dimensional overall load-bearing characteristics. Furthermore, this method is difficult to simulate and analyze dynamic conditions such as rainfall, and cannot reflect the structural deformation development process. In contrast, numerical simulation methods can compensate for the shortcomings of theoretical analysis methods. They can construct a refined three-dimensional model including cracks, voids, and loose bricks and stones, quantifying the impact of defects on stability, and can also consider the structural deformation development process under rainfall and other influences. To more accurately reflect the degradation characteristics of material properties and interface properties of ancient city walls under long-term environmental influences such as rainfall, and to reflect its deformation process from continuous to discontinuous, a new mechanical analysis model should be considered based on the currently used continuum or discrete mechanical models. A mechanical analysis model of "cohesion degradation-discrete body" conforming to the bearing mechanism characteristics of ancient city walls was established to clarify the coupling mechanism and force transmission mechanism, so as to accurately simulate and analyze the instability and failure mode of ancient city walls. The stability and safety assessment of brick-encased earthen ancient city walls under rainfall has two major challenges: First, existing studies all treat rainfall as an initial condition causing changes in material properties, altering material characteristics at the start of the calculation and analysis, without considering the fluid-structure dynamic coupling effect of rainfall; second, the stability analysis methods for ancient city walls all use continuum mechanics models or discrete mechanics models, which cannot reflect the continuous to discontinuous change process of "small deformation - large deformation - crack generation - instability and failure" during the deformation process of ancient city walls. Summary of the Invention

[0003] The purpose of this invention is to provide a method for assessing the stability and safety of brick-encased earthen ancient city walls that takes into account the effects of rainfall. This method aims to address the technical problems of existing technologies, which currently lack a dynamic fluid-structure interaction mechanism that considers the effects of rainfall, and the lack of a cohesion degradation-discrete mechanics analysis model that can reflect the continuous to discontinuous deformation process of brick-encased earthen ancient city walls.

[0004] To achieve the above objectives, the present invention adopts the following technical solution.

[0005] A method for assessing the stability and safety of brick-clad earthen ancient city walls that takes into account the effects of rainfall includes the following steps: Step 1: Through on-site surveys and non-destructive testing, non-contact point cloud collection of the ancient city wall surface, integration of historical data and ultrasonic ground-penetrating radar detection, information on the surface defects, geometric morphology, internal structure and hidden defects of the ancient city wall is obtained, and integrated to establish a comprehensive three-dimensional visualization model and digital archive. Step 2: Optimize the point cloud data of the ancient city wall surface and construct a refined surface mesh model. Then, integrate the internal detection data to construct an integrated three-dimensional numerical model containing external morphology, surface details and internal defects. Finally, on the basis of the integrated three-dimensional numerical model, carry out material partitioning, masonry layout, non-uniform solid mesh division and interface unit insertion to generate a calculation model for finite element analysis. Step 3: Conduct material mechanical property tests on the outer brick, the internal rammed earth, and the connecting interface to obtain material mechanical property parameters under different moisture contents; Step 4: Based on local rainfall and meteorological information, set up various rainfall conditions, including two types of conditions with equal total rainfall: short-term heavy rain and long-term heavy rain. On the basis of the calculation model, simulate rainwater infiltration to obtain the spatiotemporal distribution of water content at the interface of the outer brick, rammed earth, and mortar joints of the city wall under different rainfall conditions. Step 5: Grid numbering and parameter mapping are performed on the moisture content analysis results to assign material property values; then, structural deformation, crack evolution, and stress distribution under different rainfall conditions are calculated, and the stability of the city wall structure is quantitatively assessed in accordance with relevant standards to clarify the safety level and risk areas.

[0006] Preferably, the specific method of step one includes: Step 1.1: Conduct on-site surveys, investigations, and record the current condition of the ancient city wall, including the masonry type, surface damage, weathering degree, and missing information on mortar joint interfaces. Use a rebound hammer to sample and test the main masonry materials of the city wall, and obtain the basic mechanical performance parameters of the materials by taking the average value of multiple tests. Step 1.2: Using non-contact measurement technology, collect three-dimensional point cloud data of the ancient city wall surface to generate a complete and coherent three-dimensional point cloud model of the city wall surface. Step 1.3: Collect and integrate historical survey cross-sectional maps and existing data of the ancient city wall, sort out the original structural form, masonry process and material composition information of the wall, and build a preliminary internal structural framework model based on the three-dimensional point cloud model of the city wall surface using BIM or three-dimensional modeling software. Step 1.4: Use ultrasonic detection and ground-penetrating radar detection technology to determine whether there are cavities or cracks in the wall; at the same time, locate the size and location of the internal cavities, the extension direction and depth of the cracks, and the distribution of the interfaces of different structural layers. Step 1.5: Overlay and calibrate the detection results from Step 1.4 with the model from Step 1.3 to form a comprehensive three-dimensional visualization model that includes surface morphology, internal structure, and hidden defects.

[0007] Step 1.6: Conduct a comprehensive survey of the historical damage data of the ancient city wall and the design and construction plans during the restoration process, and archive them in a unified manner.

[0008] Preferably, the specific operation process of step two is as follows: Step 2.1: Based on the point cloud data of the ancient city wall surface obtained in Step 1.2, modern digital imaging technology and 3D modeling technology are used to re-optimize the data and construct a refined surface mesh model for numerical simulation. Step 2.2: Based on the internal exploration data of the ancient city wall obtained in Step 1.3, and combined with the constructed refined surface mesh model, digital modeling and integration of internal defects are carried out to form an integrated three-dimensional numerical model that includes external morphology, surface details and internal hidden defects. Step 2.3: Based on the integrated three-dimensional numerical model completed in Step 2.2, and in conjunction with the original masonry style of the ancient city wall, archaeological survey data and historical records, we carry out material zoning, masonry brick layout design, and three-dimensional solid mesh generation and interface unit insertion to generate a calculation model for finite element analysis.

[0009] Preferably, the test procedure for the outer brick in step three is as follows: Step 3.1.1: Extract complete bricks from the outer bricks of different masonry sections and different weathering grades of the ancient city wall, and cut and grind them into standard test pieces that meet the specifications to ensure that the dimensional accuracy and surface flatness of the test pieces meet the test requirements. Step 3.1.2 involves selecting representative bricks and processing them into standard specimens. Uniaxial compressive strength and splitting tests are then conducted under three moisture content conditions: current state, dry state, and saturated state. Freeze-thaw tests are also performed on the saturated specimens to obtain the compressive strength, elastic modulus, splitting tensile strength, and freeze-thaw durability index of the outer bricks under different moisture content conditions.

[0010] Preferably, the test procedure for the internal rammed earth in step three is as follows: Step 3.2.1: Take rammed earth from different depths and compaction degrees inside the ancient city wall, remove impurities and maintain the original particle size distribution and compaction characteristics, and process it into regular specimens according to the geotechnical test specifications to ensure the authenticity and representativeness of the test results. Step 3.2.2: Conduct seepage tests and unsaturated soil property tests on the specimens to obtain the seepage coefficient and unsaturated soil property parameters of the rammed soil; at the same time, conduct direct shear tests on the specimens under current, dry and saturated conditions to obtain the strength parameters of cohesion and friction angle under different moisture contents.

[0011] Preferably, the experimental process for connecting the interface in step three is as follows: Step 3.3.1: The test samples strictly replicate the actual structure of the ancient city wall: the brick-brick joint test specimens use bricks and traditional mortar from the same period as the site; the brick-soil contact test specimens use the original rammed earth of the site and similar wall bricks to simulate the contact state between bricks and rammed earth during actual construction, ensuring that the test specimens fit the real engineering scenario. Step 3.3.2: Conduct brick-brick joint shear tests and brick-soil interface shear tests on the specimens under current, dry and saturated conditions to obtain the cohesion and friction angle strength parameters of the interface under different moisture contents.

[0012] Preferably, the specific operation process of step four is as follows: Step 4.1: Based on the meteorological conditions of the ancient city wall location, the system statistically analyzes the annual rainfall distribution in the area and sets the heavy rainfall conditions according to the rainfall during the rainy season; under the condition that the total rainfall remains unchanged within the set total duration of a single rainfall event, the system sets the conditions for short-term rainstorms and long-term heavy rainfall respectively.

[0013] Step 4.2: Based on the calculation model constructed in Step 2.3, set the boundary conditions for rainfall conditions, including no rainfall, short-term heavy rain and long-term heavy rainfall, and use numerical analysis software to simulate and analyze the rainwater infiltration process and material moisture content changes under different conditions.

[0014] Preferably, the specific operation process of step five is as follows: Step 5.1: Based on the calculation and analysis results obtained in step 4.2, the grids with different water contents and different materials are numbered to form a two-dimensional grid data matrix; Step 5.2: Based on the material mechanical property parameters obtained in Step 3 under different moisture contents, perform parameter mapping by referring to the values ​​in the mesh matrix to assign material parameters. Step 5.3: Using numerical analysis software, calculate the structural deformation, crack evolution, and stress distribution of the ancient city wall under different rainfall conditions, and evaluate its structural stability.

[0015] Preferably, the non-contact measurement technology in step 1.2 specifically involves: using a drone equipped with a high-definition camera and a lidar module for aerial surveying, combined with a portable laser scanner to perform close-range fine scanning of key local areas, generating a complete and coherent three-dimensional point cloud model of the city wall surface; In step 1.3, ultrasonic detection and ground-penetrating radar detection are specifically as follows: ultrasonic detection involves emitting high-frequency ultrasonic waves into the wall and judging the internal voids, cracks, and uneven material density based on changes in sound wave propagation speed and reflected signal intensity; ground-penetrating radar uses broadband electromagnetic pulse signals and analyzes the reflected waveform, amplitude, and travel time of radar waves to accurately locate the size and location of internal voids, as well as the extension direction and depth of cracks.

[0016] Preferably, in step 5.3, the structural stability of the ancient city wall under different rainfall conditions is quantitatively evaluated based on the designed structural displacement limit, material strength design value, allowable crack width, and overall stability safety factor as evaluation indicators.

[0017] Compared with the prior art, the present invention has the following features and beneficial effects.

[0018] 1. The stability and safety assessment method for brick-encased earthen ancient city walls considering rainfall proposed in this invention can accurately reflect the internal and external structural characteristics of the brick-encased earthen ancient city walls, the evolution of material performance parameters under rainfall, and the deformation development process from continuous to discontinuous at the connection interface. This enables refined modeling and quantitative assessment and analysis of the brick-encased earthen ancient city walls. The combination of on-site measurements, model experiments, and numerical analysis can complement and improve the data information, ensuring the accuracy of the simulation analysis, achieving overall structural stability and safety assessment under rainfall, and improving the accuracy of stability risk assessment for ancient city walls.

[0019] 2. This invention achieves full-chain fluid-structure interaction analysis of rainfall infiltration → spatiotemporal evolution of water content → dynamic degradation of material properties → structural response through rainwater infiltration simulation and parameter dynamic mapping. It realizes dynamic coupling of rainfall and fluid structure, overcomes the limitations of traditional methods, and more realistically reflects the gradual deterioration process of the city wall performance under the action of rainfall.

[0020] 3. This invention adopts multi-source data fusion modeling, resulting in a high degree of model fidelity. It integrates UAV / laser scanning, ultrasonic / ground radar detection, historical data and field surveys to construct a refined three-dimensional numerical model that includes external morphology, surface details, internal defects and material partitions, overcoming the shortcomings of traditional simplified models that cannot reflect the complex structure of ancient city walls.

[0021] 4. This invention targets four types of materials / interfaces: outer brick, internal rammed earth, brick-brick joints, and brick-soil contact surfaces. Systematic tests are conducted under three moisture contents: current state, dry, and saturated. Complete parameters such as elastic modulus, tensile strength, c, φ, permeability coefficient, and freeze-thaw durability are obtained, providing reliable data support for numerical simulation.

[0022] 5. Under the premise of equal total rainfall, this invention distinguishes between two typical working conditions: short-term torrential rain and long-term heavy rainfall. It reveals the differentiated impact mechanism of different rainfall patterns on the stability of the city wall. The differentiated rainfall conditions closely resemble real-world environments, thus making the assessment results more valuable for engineering guidance. Simultaneously, this invention, referring to relevant standards, uses quantitative indicators such as displacement limits, strength design values, allowable crack width, and stability safety factors to assess the stability of the city wall, clarifying the safety level and potential risk areas, providing a quantifiable decision-making basis for the scientific preventative protection of ancient city walls. Detailed Implementation

[0023] This method for assessing the stability and safety of brick-clad earthen ancient city walls, taking into account rainfall effects, first involves a comprehensive investigation to obtain historical information and current damage data of the ancient city walls. Then, using this data, a three-dimensional overall numerical analysis model considering the damage is established. Further, model tests are conducted to obtain the material mechanical properties parameters of the outer cladding, rammed earth, and connecting interfaces. Taking into account local rainfall and meteorological data, different rainfall conditions are set to simulate and analyze the infiltration and moisture content changes in the model. Finally, based on the moisture content evolution distribution, corresponding material mechanical parameters are set to simulate the deformation development process of the ancient city walls, thereby achieving a stability performance assessment of the ancient city walls under different rainfall conditions. Specifically, the method includes the following steps: Step one involves obtaining information on the surface defects, geometric morphology, internal structure, and hidden defects of the ancient city wall through on-site surveys and non-destructive testing, non-contact point cloud collection of the ancient city wall surface, integration of historical data, and ultrasonic ground-penetrating radar detection. This information is then integrated to establish a comprehensive three-dimensional visualization model and digital archive. The detailed operation process is as follows.

[0024] Step 1.1: Conduct on-site surveys, inspections, and record the current condition of the ancient city wall, including the masonry type, surface damage, weathering degree, and missing information on mortar joint interfaces. Specifically, missing information on mortar joint interfaces refers to the lack of mortar joint materials, bonding failure, or structural breakage. Use a rebound hammer to sample and test the main masonry materials of the city wall. Select representative test points according to the specifications, avoiding the weathered layer and damaged parts. Take the average value through multiple tests to obtain the basic mechanical performance parameters of the materials, such as compressive strength. Specifically, through a comprehensive and detailed on-site survey of the ancient city wall ruins, we systematically investigated and accurately recorded the various diseases and preservation status of the ruins.

[0025] At the level of masonry construction, the focus is on verifying the overall masonry process, masonry modules, wall orientation and structural integrity of the city wall, and recording in detail the preservation status and damage of key parts such as crenellations, ramparts, and city gates. Regarding surface damage, register each crack on the wall surface, including the length, width, depth and distribution of horizontal, vertical and diagonal cracks, areas of efflorescence peeling, and note the peeling area, thickness and concentrated location. For biological erosion traces, distinguish between plant root invasion, insect infestation or microbial growth traces and traces of human damage, including damage caused by dismantling and scratching. In assessing the degree of weathering, based on the characteristics of the outer wall bricks and exposed rammed earth, such as the looseness of the texture, the degree of surface pulverization, and the roundness of the edges, three levels of weathering are divided into strong weathering, moderate weathering, and weak weathering, and the distribution areas of different weathering levels are clearly defined. During the inspection of mortar joint interfaces, the focus is on recording the material, thickness, and fullness of the mortar joints, as well as whether there are any problems such as cracking, hollowing, detachment, or failure of adhesion to bricks and stones. At the same time, a rebound hammer is used to sample and test the main masonry materials of the city wall. Representative test points are selected according to the specifications, avoiding weathered layers and damaged areas. The average value is obtained through multiple tests to obtain the basic mechanical property parameters of the materials, such as compressive strength.

[0026] Step 1.2 involves using non-contact measurement technology to collect 3D point cloud data of the ancient city wall surface, generating a complete and coherent 3D point cloud model of the city wall surface. The 3D point cloud data includes a massive number of 3D coordinate points on the city wall surface, along with attributes such as color and reflection intensity for each point. It accurately records the macroscopic outline, structural components, brick arrangement, and surface color and material differences of the city wall, serving as the foundational data for subsequent generation of the surface 3D numerical model, locating defects, registering internal detection data, and providing geometric boundaries for the finite element model.

[0027] Specifically, to achieve accurate surveying of the ancient city wall's structure and defects, a non-contact measurement technology combining UAV aerial surveying and portable laser scanners was prioritized to conduct comprehensive, high-precision 3D point cloud data acquisition of the entire surface. The UAVs, equipped with high-definition cameras and LiDAR modules, planned multiple flight routes and angles along the city wall's orientation, focusing on covering the top, facade, and concealed corner areas of the wall. Portable laser scanners are used to perform close-range, high-resolution scans of key areas of the city wall, including gate openings, corner towers, and sections with concentrated defects. High-density sampling intervals are set to ensure that the three-dimensional coordinate information of subtle features such as cracks, peeling, and weathering is captured. Finally, the data collected by the two types of equipment are integrated to generate a complete and coherent three-dimensional point cloud model of the city wall surface, enabling a visual representation of the city wall's appearance, size parameters, and surface defects.

[0028] Step 1.3: Collect and integrate historical survey cross-sectional maps and existing data of the ancient city wall, sort out the original structural form, masonry process and material composition information of the wall, and build a preliminary internal structural framework model based on the three-dimensional point cloud model of the city wall surface using BIM or three-dimensional modeling software. Step 1.4: Use ultrasonic detection and ground-penetrating radar detection technology to determine whether there are cavities or cracks in the wall; at the same time, locate the size and location of the internal cavities, the extension direction and depth of the cracks, and the distribution of the interfaces of different structural layers. Step 1.5: Overlay and calibrate the detection results from Step 1.4 with the model from Step 1.3 to form a comprehensive three-dimensional visualization model that includes surface morphology, internal structure, and hidden defects.

[0029] Specifically, based on the 3D point cloud model of the city wall surface, the collected historical survey cross-sections, archaeological excavation reports, and repair records of the ancient city wall were fully integrated. Information on the original structural form, masonry techniques, and material composition of the wall was compiled, and a preliminary internal structural framework model was built using BIM or 3D modeling software. To further investigate hidden defects within the wall, ultrasonic and ground-penetrating radar technologies were used for penetrating detection. Ultrasonic detection technology emits high-frequency ultrasonic waves into the wall, and based on changes in the propagation speed and reflected signal intensity, it determines whether there are voids, cracks, or uneven material density within the wall, focusing on key stress-bearing areas such as the lower and middle parts of the wall and around the city gate openings. Ground-penetrating radar uses broadband electromagnetic pulse signals, arranging detection profiles along the transverse and longitudinal directions of the wall. By analyzing the reflected waveform, amplitude, and travel time of the radar waves, the size and location of internal voids, the extension direction and depth of cracks, and the interface distribution of different structural layers are accurately located. Finally, the detection results are superimposed and calibrated with the internal structural model to form a comprehensive 3D visualization model that includes surface morphology, internal structure, and hidden defects.

[0030] Step 1.6: Conduct a comprehensive survey of the historical damage data of the ancient city wall and the design and construction plans during the restoration process, and archive them in a unified manner.

[0031] The materials to be archived in a unified manner include, but are not limited to, documentary records, survey reports, design and construction plans, and historical images, in order to ensure the comprehensiveness and authenticity of the materials as much as possible.

[0032] Specifically, to accurately understand the current state of damage and defects of the ancient city wall, a comprehensive and in-depth data survey was conducted, systematically reviewing the historical evolution of its defects and the inherited experience in restoration techniques. The survey focused on collecting relevant data on past defects of the city wall from different historical periods since its construction, as well as complete design and construction plans developed during each stage of restoration projects. All collected data was then standardized, organized, and archived. The types of data covered in the survey should include, but are not limited to: written records of defects, damage, and repairs of the ancient city wall in local chronicles and ancient books; archaeological surveys and defect detection and assessment reports conducted by professional institutions since modern times; complete technical archives of each restoration project, including restoration design drawings, technical plan texts, material selection reports, construction organization designs, process records, and completion acceptance data; and historical imagery from different eras, including old photographs and hand-drawn illustrations. During the data collection process, it is necessary to cross-verify through multiple channels, use documentary records and archaeological discoveries to corroborate each other, compare reports from different periods, eliminate false information, supplement missing details, and ensure the comprehensiveness, authenticity and systematic nature of the archived data to the greatest extent possible.

[0033] Step two involves optimizing the point cloud data of the ancient city wall surface and constructing a refined surface mesh model. Then, internal detection data is integrated to construct an integrated three-dimensional numerical model containing external morphology, surface details, and internal defects. Finally, based on the integrated three-dimensional numerical model, material partitioning, masonry arrangement, non-uniform solid mesh division, and interface unit insertion are carried out to generate a calculation model for finite element analysis. The specific operation process is as follows.

[0034] Step 2.1: Based on the point cloud data of the ancient city wall surface obtained in Step 1.2, modern digital imaging technology and 3D modeling technology are used to re-optimize the data and construct a refined surface mesh model for numerical simulation. First, the raw point cloud data is preprocessed. Point cloud processing software, such as Metashape, can be used to remove redundant and isolated points caused by environmental interference through denoising algorithms. Registration technology is then used to complete the seamless stitching of point cloud data from multiple perspectives and regions, ensuring the continuity and integrity of the data.

[0035] Furthermore, using mesh generation software, such as GeoMagic, the point cloud was surface-meshed to create a refined surface mesh model. Combined with information from the survey of surface defects on the ancient city wall, features reflecting the damage, spalling, and cracks of the outer bricks and exposed rammed earth were added to the numerical model. This model not only fully reproduces the external morphology and dimensional parameters of the ancient city wall but also clearly presents the spatial distribution and detailed features of various surface defects.

[0036] Step 2.2: Based on the internal exploration data of the ancient city wall obtained in Step 1.3, and combined with the constructed refined surface mesh model, digital modeling and integration of internal defects are carried out to form an integrated three-dimensional numerical model that includes external morphology, surface details and internal hidden defects. First, the internal detection data was analyzed and verified layer by layer. Through cross-comparison of multi-source data, using ultrasonic detection results and ground-penetrating radar profiles to confirm the precise spatial coordinates and physical parameters of cavities and fissures, and eliminating invalid data caused by detection errors. Then, in 3D modeling software, based on the analyzed defect information, 3D digital entities of cavities and fissures were precisely created at the corresponding locations on the model according to actual scale. The cavities needed to reproduce their irregular shapes and internal boundary characteristics, while the fissures needed to reflect their penetration and width gradient. Finally, the internal defect model was integrated with the overall numerical model of the ancient city wall. Through coordinate calibration and mesh adaptation, the spatial positions of the cavities and fissures were ensured to be completely matched with the main structure of the wall, ultimately forming an integrated numerical model of the ancient city wall that includes external morphology, surface details, and internal hidden defects. Surface details refer to the millimeter- to centimeter-level geometric features on the outer surface of the city wall, such as brick outlines, mortar joints, and unevenness; surface textures such as color and roughness; and traces of damage such as cracks, peeling, and weathering zones.

[0037] Step 2.3: Based on the integrated three-dimensional numerical model completed in Step 2.2, and in conjunction with the original masonry style of the ancient city wall, archaeological survey data and historical records, we carry out material zoning, masonry brick layout design, and three-dimensional solid mesh generation and interface unit insertion to generate a calculation model for finite element analysis.

[0038] First, based on the differences in the structural layout and material composition of the city wall, the model is precisely divided into material zones, and the material property parameters of each zone are clearly defined to ensure consistency with the actual engineering characteristics. Then, in accordance with the masonry process specifications of the ancient city wall, the size, spacing and overlapping relationship of the bricks are restored in the corresponding material zones according to the actual masonry layout. At the same time, the thickness, distribution and material characteristics of the mortar joints are accurately reproduced to restore the original masonry logic of the ancient city wall.

[0039] After completing the above-mentioned detailed modeling, a solid meshing strategy suitable for the structural analysis of ancient buildings is adopted. According to the structural complexity and analysis accuracy requirements of different regions, the model is divided into non-uniform solid meshes. High-density meshes are used in key stress areas to ensure calculation accuracy, while relatively sparse meshes are used in secondary areas to optimize calculation efficiency. Finally, interface units are precisely inserted at the contact surfaces between the masonry and mortar joints, different material zones, and at the boundaries of identified cracks and voids.

[0040] Step 3: Conduct material mechanical property tests on the outer brick, the internal rammed earth, and the connecting interface to obtain material mechanical property parameters under different moisture contents.

[0041] The testing process for the outer cladding bricks is as follows: Systematic laboratory model tests are conducted to comprehensively test and obtain the key mechanical parameters of the material. Test samples are selected according to the principle of representativeness. Complete bricks are extracted from the outer cladding bricks of different masonry sections and weathering grades of the ancient city wall. These bricks are then cut and polished into standard specimens that meet the specifications, ensuring that the dimensional accuracy and surface flatness of the specimens meet the testing requirements. If sampling from the artifact itself is limited, similar materials are used to prepare the samples.

[0042] The test items cover mechanical property testing under three moisture content conditions: current, dry, and saturated. Uniaxial compressive strength tests were conducted using a microcomputer-controlled electronic universal testing machine. By applying a uniform axial load, the stress-strain curves of the specimen from elastic deformation to failure were recorded, and the compressive strength and elastic modulus under different moisture contents were calculated. Using this equipment in conjunction with a splitting fixture, a splitting test is conducted. By utilizing the principle that the specimen splits along the vertical section under line load, the splitting tensile strength of the specimen under different moisture contents can be indirectly obtained. Freeze-thaw tests were conducted using a rapid freeze-thaw testing machine for concrete. Following the procedure of freeze-thaw cycles and mechanical testing, saturated specimens were placed in an environment of -20℃ to 20℃ and alternately frozen and thawed until the specified number of cycles were reached. The freeze-thaw durability of the bricks was evaluated by comparing the changes in compressive strength and elastic modulus before and after the freeze-thaw cycle.

[0043] Through the above-mentioned multi-condition and multi-index laboratory tests, the core mechanical performance parameters of the outer bricks, such as elastic modulus, splitting tensile strength, and freeze-thaw durability, were obtained under different moisture content conditions.

[0044] The test process for the internal rammed earth is as follows: rammed earth samples of different depths and compaction degrees were taken from inside the ancient city wall. Impurities were removed while maintaining the original particle size distribution and compaction characteristics. The samples were then processed into regular specimens according to geotechnical test specifications to ensure the authenticity and representativeness of the test results. If sampling is restricted by the cultural relic itself, similar materials were used to prepare the specimens. The experiment consists of three parts: First, a seepage test is conducted using a variable head permeameter. By measuring the changes in water head at different time stages and combining Darcy's law, the seepage coefficient of the rammed earth is calculated to determine its permeability. Second, unsaturated soil properties were tested using a triaxial apparatus. By controlling the matrix suction and water content of the specimens, the volumetric deformation, shear strength and other indicators under different suction states were tested to obtain soil-water characteristic curves and related unsaturated soil mechanical parameters, providing a basis for analyzing the impact of rainwater infiltration on wall stability. Third, direct shear tests were conducted using a direct shear apparatus. Different vertical pressures were applied to the rammed soil specimens under three working conditions: current moisture content, completely dry, and completely saturated. By applying a horizontal shear force at a uniform speed, the shear stress-shear displacement relationship curves were recorded during the shearing process. Finally, the key strength parameters such as cohesion (c) and internal friction angle (φ) of the rammed soil under each moisture content condition were calculated, comprehensively revealing the influence of moisture content changes on the shear resistance of rammed soil.

[0045] The experimental process for the connection interfaces is as follows: To obtain the mechanical performance parameters of two key connection interfaces—brick-to-brick mortar joints and brick-to-soil contact surfaces—targeted laboratory model tests were conducted. The test samples strictly replicated the actual structure of the ancient city wall: brick-to-brick mortar joint specimens used bricks and traditional mortars from the same period as the site, such as white lime mortar and glutinous rice mortar, and were made according to the original masonry techniques to ensure that the mortar joint thickness and fullness were consistent with reality; brick-to-soil contact surface specimens used undisturbed rammed earth from the site and similar wall bricks to simulate the contact state between bricks and rammed earth during actual masonry, ensuring that the specimens conformed to the real engineering scenario. Test conditions included three moisture content conditions: current state, dry, and saturated. Shear tests were conducted on brick-to-brick mortar joints and brick-to-soil contact surfaces respectively: a double-shear testing machine was used to control the vertical pressure and horizontal shear rate. During the test, a preset vertical load was first applied to the specimen to simulate the self-weight of the city wall and the pressure of the superstructure, and then a horizontal shear force was applied at a uniform rate. The shear stress-shear displacement curves were recorded throughout the shear process until the specimen failed under shear conditions. By analyzing the test curves under different moisture contents, key strength parameters such as cohesion (c) and internal friction angle (φ) of each interface were calculated, and the influence of moisture content changes on the shear performance of brick-brick and brick-soil interface was obtained.

[0046] Step 4: Based on local rainfall and meteorological information, set up various rainfall conditions, including two types of conditions with equal total rainfall: short-term heavy rain and long-term heavy rain. On the basis of the calculation model, simulate rainwater infiltration to obtain the spatiotemporal distribution of moisture content at the interface of the outer brick, rammed earth, and mortar joints of the city wall under different rainfall conditions; the specific operation is as follows.

[0047] Step 4.1: Combining long-term meteorological observation data of the ancient city wall location, the system statistically analyzes the seasonal distribution characteristics of annual rainfall, monthly rainfall peaks, maximum single rainfall, and extreme rainfall intensity parameters within the region. This clarifies the proportion of rainfall during the rainy season and the periods of concentrated rainfall, providing a realistic meteorological basis for setting operational conditions. Based on this, using the multi-year average rainfall during the rainy season or historical extreme rainfall as a benchmark, a total rainfall threshold for heavy rainfall scenarios is set to ensure that the operational conditions closely match the actual local rainfall intensity.

[0048] While keeping the total rainfall constant, two typical rainfall scenarios are further divided: one is a short-duration rainstorm scenario, which refers to the meteorological rainstorm level standard and sets a shorter rainfall duration, such as within 24 hours, to simulate a high-intensity, concentrated precipitation process, highlighting the characteristics of fast rainfall rate and strong instantaneous impact; the other is a long-duration heavy rainfall scenario, which extends the rainfall duration, such as 72 hours or more, and controls the rainfall intensity to a medium to high intensity level to simulate the long-term infiltration effect of continuous rainfall on the wall.

[0049] Step 4.2: Based on the calculation model constructed in Step 2.3, and combined with the three types of rainfall conditions set in Step 4.1 (no rainfall, short-term heavy rain, and long-term heavy rainfall), set the corresponding boundary conditions and calculation parameters.

[0050] Among them, the no-rainfall condition uses the current measured water content distribution of the ancient city wall as the initial state, without applying any additional rainfall load, as the benchmark for subsequent comparative analysis; the short-term rainstorm and long-term heavy rainfall conditions are based on the set rainfall intensity and duration, setting rainfall boundaries on the top and facade of the model, clarifying the initial flux and spatial distribution of permeability coefficient of rainwater infiltration, and defining the seepage boundary conditions at the bottom and sides of the model to ensure that they conform to the actual hydrological conditions of the ancient city wall.

[0051] Subsequently, using professional numerical analysis software, taking Abaqus as an example, based on the unsaturated soil seepage theory, dynamic simulation analysis was conducted on the rainwater infiltration process and material moisture content changes under three working conditions: by iteratively calculating the infiltration path and infiltration depth of rainwater inside the wall, the moisture content distribution of various parts of the ancient city wall at different time points, including the outer brick, the internal rammed earth, and the mortar joint interface, was tracked in real time, and the range and degree of influence of different rainfall patterns on the wall's humidity field were quantified.

[0052] Step five involves assigning grid numbers and parameter mappings to the moisture content analysis results to determine material property values. Subsequently, the structural deformation, crack evolution, and stress distribution under different rainfall conditions are calculated. The stability of the city wall structure is then quantitatively assessed according to relevant standards to identify the safety level and risk areas. The specific operations are as follows.

[0053] Step 5.1: Based on the calculation and analysis results obtained in Step 4.2, the grids with different moisture contents and different materials are numbered to form a two-dimensional grid data matrix. Specifically, after completing the calculation and analysis of material moisture content changes under different rainfall conditions, including no rainfall, short-term heavy rain, and long-term heavy rainfall, in Step 4.2, the calculation model in Step 2.3 is systematically numbered and a two-dimensional data matrix is ​​constructed based on the results. First, based on the material partitions already divided in the model and the moisture content values ​​corresponding to each grid cell, a unified grid numbering rule is established: the grids are numbered using (Ni, Qi), where Ni represents the grid number value and Qi represents the moisture content value corresponding to that grid, thus constructing a two-dimensional grid data matrix.

[0054] Step 5.2: Based on the mechanical property parameters of various materials such as outer bricks, rammed earth, and connecting interfaces obtained through laboratory model tests in Step 3 under different moisture contents, and combined with the two-dimensional mesh data matrix constructed in Step 5.1, precise material parameter mapping and assignment are carried out. First, an association database of "moisture content range - material type - mechanical parameter" is established. The discrete mechanical parameters obtained from the experiment are classified according to material category and divided into several continuous ranges based on the moisture content values, forming a standardized parameter lookup table. Then, using the two-dimensional mesh data matrix as a carrier, the corresponding material type identifier and moisture content value are extracted by traversing the coding information of each mesh cell in the matrix. Then, based on the value, the corresponding moisture content range in the association database is matched, and the material mechanical property parameters under that range are called. Finally, through the coordinate system calibration of the numerical model and the data matrix, the matched mechanical parameters are mapped to the corresponding mesh cells one by one, completing the material parameter assignment for all meshes in the entire model.

[0055] Step 5.3: Using numerical analysis software, calculate the structural deformation, crack evolution, and stress distribution of the ancient city wall under different rainfall conditions, and evaluate its structural stability. Using professional geotechnical and structural engineering numerical analysis software, taking Abaqus as an example, conduct a systematic calculation and analysis of the structural mechanical response for three types of conditions: no rainfall, short-duration heavy rain, and long-duration heavy rainfall. The calculation process is based on unsaturated soil mechanics theory and the principle of structural deformation coordination, focusing on solving the overall and local structural deformation of the wall under different rainfall conditions, including vertical settlement, horizontal displacement, and angular deformation; the propagation and evolution law of existing cracks, the growth rate of their length and width; and the stress distribution characteristics, including the magnitude and spatial distribution of tensile stress, compressive stress, and shear stress, and identifying stress concentration areas. After the calculations are completed, core evaluation indicators such as structural displacement limits, material strength design values, allowable crack width, and overall stability safety factor are selected in accordance with relevant standards. Combined with the calculated deformation, stress, and crack evolution data, the structural stability of the ancient city wall under different rainfall conditions is quantitatively evaluated. The safety status level, potential risk areas, and disease evolution trends of the wall under each condition are clarified, providing a scientific basis for the subsequent formulation of targeted protection and restoration strategies.

[0056] In this embodiment, the non-contact measurement technology in step 1.2 specifically involves: using a drone equipped with a high-definition camera and a lidar module for aerial surveying, and combining it with a portable laser scanner to perform close-range fine scanning of key local areas, thereby generating a complete and coherent three-dimensional point cloud model of the city wall surface.

[0057] In step 1.3, ultrasonic detection and ground-penetrating radar detection are specifically as follows: ultrasonic detection involves emitting high-frequency ultrasonic waves into the wall and judging the internal voids, cracks, and uneven material density based on changes in sound wave propagation speed and reflected signal intensity; ground-penetrating radar uses broadband electromagnetic pulse signals and analyzes the reflected waveform, amplitude, and travel time of radar waves to accurately locate the size and location of internal voids, as well as the extension direction and depth of cracks.

[0058] In this embodiment, in step 5.3, the structural stability of the ancient city wall under different rainfall conditions is quantitatively evaluated based on the designed structural displacement limit, material strength design value, allowable crack width, and overall stability safety factor as evaluation indicators.

[0059] The above embodiments are not exhaustive examples of specific implementation methods, and other embodiments are also possible. The purpose of the above embodiments is to illustrate the present invention, rather than to limit the scope of protection of the present invention. All applications derived from simple variations of the present invention fall within the scope of protection of the present invention.

Claims

1. A method for assessing the stability and safety of brick-clad earthen ancient city walls considering the effects of rainfall, characterized in that, Includes the following steps: Step 1: Through on-site surveys and non-destructive testing, non-contact point cloud collection of the ancient city wall surface, integration of historical data and ultrasonic ground-penetrating radar detection, information on the surface defects, geometric morphology, internal structure and hidden defects of the ancient city wall is obtained, and integrated to establish a comprehensive three-dimensional visualization model and digital archive. Step 2: Optimize the point cloud data of the ancient city wall surface and construct a refined surface mesh model. Then, integrate the internal detection data to construct an integrated three-dimensional numerical model containing external morphology, surface details and internal defects. Finally, on the basis of the integrated three-dimensional numerical model, carry out material partitioning, masonry layout, non-uniform solid mesh division and interface unit insertion to generate a calculation model for finite element analysis. Step 3: Conduct material mechanical property tests on the outer brick, the internal rammed earth, and the connecting interface to obtain material mechanical property parameters under different moisture contents; Step 4: Based on local rainfall and meteorological information, set up various rainfall conditions, including two types of conditions with equal total rainfall: short-term heavy rain and long-term heavy rain. On the basis of the calculation model, simulate rainwater infiltration to obtain the spatiotemporal distribution of water content at the interface of the outer brick, rammed earth, and mortar joints of the city wall under different rainfall conditions. Step 5: Grid numbering and parameter mapping are performed on the moisture content analysis results to assign material property values; then, structural deformation, crack evolution, and stress distribution under different rainfall conditions are calculated, and the stability of the city wall structure is quantitatively assessed in accordance with relevant standards to clarify the safety level and risk areas.

2. The method for assessing the stability and safety of brick-clad earthen ancient city walls considering rainfall, as described in claim 1, is characterized in that: The specific methods for step one include: Step 1.1: Conduct on-site surveys, investigations, and record the current condition of the ancient city wall, including the masonry type, surface damage, weathering degree, and missing information on mortar joint interfaces. Use a rebound hammer to sample and test the main masonry materials of the city wall, and obtain the basic mechanical performance parameters of the materials by taking the average value of multiple tests. Step 1.2: Using non-contact measurement technology, collect three-dimensional point cloud data of the ancient city wall surface to generate a complete and coherent three-dimensional point cloud model of the city wall surface. Step 1.3: Collect and integrate historical survey cross-sectional maps and existing data of the ancient city wall, sort out the original structural form, masonry process and material composition information of the wall, and build a preliminary internal structural framework model based on the three-dimensional point cloud model of the city wall surface using BIM or three-dimensional modeling software. Step 1.4: Use ultrasonic detection and ground-penetrating radar detection technology to determine whether there are cavities or cracks in the wall; at the same time, locate the size and location of the internal cavities, the extension direction and depth of the cracks, and the distribution of the interfaces of different structural layers. Step 1.5: Overlay and calibrate the detection results from Step 1.4 with the model from Step 1.3 to form a comprehensive three-dimensional visualization model that includes surface morphology, internal structure, and hidden defects. Step 1.6: Conduct a comprehensive survey of the historical damage data of the ancient city wall and the design and construction plans during the restoration process, and archive them in a unified manner.

3. The method for assessing the stability and safety of brick-clad earthen ancient city walls considering rainfall, as described in claim 2, is characterized in that: The specific steps for step two are as follows: Step 2.1: Based on the point cloud data of the ancient city wall surface obtained in Step 1.2, modern digital imaging technology and 3D modeling technology are used to re-optimize the data and construct a refined surface mesh model for numerical simulation. Step 2.2: Based on the internal exploration data of the ancient city wall obtained in Step 1.3, and combined with the constructed refined surface mesh model, digital modeling and integration of internal defects are carried out to form an integrated three-dimensional numerical model that includes external morphology, surface details and internal hidden defects. Step 2.3: Based on the integrated three-dimensional numerical model completed in Step 2.2, and in conjunction with the original masonry style of the ancient city wall, archaeological survey data and historical records, we carry out material zoning, masonry brick layout design, and three-dimensional solid mesh generation and interface unit insertion to generate a calculation model for finite element analysis.

4. The method for assessing the stability and safety of brick-encased earthen ancient city walls considering rainfall, as described in claim 1, is characterized in that: The test procedure for the outer brick in step three is as follows: Step 3.1.1: Extract complete bricks from the outer bricks of different masonry sections and different weathering grades of the ancient city wall, and cut and grind them into standard test pieces that meet the specifications to ensure that the dimensional accuracy and surface flatness of the test pieces meet the test requirements. Step 3.1.2 involves selecting representative bricks and processing them into standard specimens. Uniaxial compressive strength and splitting tests are then conducted under three moisture content conditions: current state, dry state, and saturated state. Freeze-thaw tests are also performed on the saturated specimens to obtain the compressive strength, elastic modulus, splitting tensile strength, and freeze-thaw durability index of the outer bricks under different moisture content conditions.

5. The method for assessing the stability and safety of brick-clad earthen ancient city walls considering rainfall, as described in claim 1, is characterized in that: The test procedure for the internal rammed earth in step three is as follows: Step 3.2.1: Take rammed earth from different depths and compaction degrees inside the ancient city wall, remove impurities and maintain the original particle size distribution and compaction characteristics, and process it into regular specimens according to the geotechnical test specifications to ensure the authenticity and representativeness of the test results. Step 3.2.2: Conduct seepage tests and unsaturated soil property tests on the specimens to obtain the seepage coefficient and unsaturated soil property parameters of the rammed soil; at the same time, conduct direct shear tests on the specimens under current, dry and saturated conditions to obtain the strength parameters of cohesion and friction angle under different moisture contents.

6. The method for assessing the stability and safety of brick-clad earthen ancient city walls considering rainfall, as described in claim 1, is characterized in that: The experimental process for connecting the interface in step three is as follows: Step 3.3.1: The test samples strictly replicate the actual structure of the ancient city wall: the brick-brick joint test specimens use bricks and traditional mortar from the same period as the site; the brick-soil contact test specimens use the original rammed earth of the site and similar wall bricks to simulate the contact state between bricks and rammed earth during actual construction, ensuring that the test specimens fit the real engineering scenario. Step 3.3.2: Conduct brick-brick joint shear tests and brick-soil interface shear tests on the specimens under current, dry and saturated conditions to obtain the cohesion and friction angle strength parameters of the interface under different moisture contents.

7. The method for assessing the stability and safety of brick-clad earthen ancient city walls considering rainfall, as described in claim 3, is characterized in that: The specific steps for step four are as follows: Step 4.1: Based on the meteorological conditions of the ancient city wall location, the system statistically analyzes the annual rainfall distribution in the area and sets the heavy rainfall conditions according to the rainfall during the rainy season; under the condition that the total rainfall remains unchanged within the set total duration of a single rainfall event, the system sets the conditions for short-term rainstorms and long-term heavy rainfall respectively. Step 4.2: Based on the calculation model constructed in Step 2.3, set the boundary conditions for rainfall conditions, including no rainfall, short-term heavy rain and long-term heavy rainfall, and use numerical analysis software to simulate and analyze the rainwater infiltration process and material moisture content changes under different conditions.

8. The method for assessing the stability and safety of brick-clad earthen ancient city walls considering rainfall, as described in claim 7, is characterized in that: The specific steps for step five are as follows: Step 5.1: Based on the calculation and analysis results obtained in step 4.2, the grids with different water contents and different materials are numbered to form a two-dimensional grid data matrix; Step 5.2: Based on the material mechanical property parameters obtained in Step 3 under different moisture contents, perform parameter mapping by referring to the values ​​in the mesh matrix to assign material parameters. Step 5.3: Using numerical analysis software, calculate the structural deformation, crack evolution, and stress distribution of the ancient city wall under different rainfall conditions, and evaluate its structural stability.

9. The method for assessing the stability and safety of brick-clad earthen ancient city walls considering rainfall, as described in claim 2, is characterized in that: The non-contact measurement technology in step 1.2 is as follows: the UAV is equipped with a high-definition camera and a lidar module to conduct aerial surveys, and combined with a portable laser scanner to perform close-range fine scanning of key local areas to generate a complete and coherent three-dimensional point cloud model of the city wall surface. In step 1.3, ultrasonic detection and ground-penetrating radar detection are specifically as follows: ultrasonic detection involves emitting high-frequency ultrasonic waves into the wall and judging the internal voids, cracks, and uneven material density based on changes in sound wave propagation speed and reflected signal intensity; ground-penetrating radar uses broadband electromagnetic pulse signals and analyzes the reflected waveform, amplitude, and travel time of radar waves to accurately locate the size and location of internal voids, as well as the extension direction and depth of cracks.

10. The method for assessing the stability and safety of brick-clad earthen ancient city walls considering rainfall, as described in claim 8, is characterized in that: In step 5.3, the structural stability of the ancient city wall under different rainfall conditions is quantitatively evaluated based on the designed structural displacement limit, material strength design value, allowable crack width, and overall stability safety factor as evaluation indicators.