"The 9th High-level Forum on Geotechnical Principal Structure Theory was held on 26 October 2023 at Shenzhen University. The conference was organised by the Geotechnical and Geotechnical Engineering Division of the Chinese Society of Civil Engineers, and hosted by the College of Civil and Transportation Engineering of Shenzhen University, the Institute of Future Underground Cities of Shenzhen University, and the Department of Civil and Environmental Engineering of The Hong Kong Polytechnic University. Academician Chen Xiangsheng, President of SZU, was the advisor of the Academic Committee, Professor Yao Yangping of Beijing University of Aeronautics and Astronautics (BUAA) was the director of the Academic Committee, Professor Jin Yinfu of SZU was the director of the Organising Committee, Professor Yin Zhenyu of The Hong Kong Polytechnic University (HKPU) and Professor Su Dong of HKPU were the vice-directors of the Organising Committee, and a number of first-class experts and scholars served as members of the Academic Committee and Organising Committee.

Academician Chen Xiangsheng, President of the School, and Professor Yao Yangping of Beijing University of Aeronautics and Astronautics delivered opening speeches. Academician Chen Xiangsheng introduced at the opening ceremony of the conference that the predecessor of the School of Civil and Transportation Engineering of SZU was the Department of Structural and Municipal Engineering, and after forty years of development, the School has gradually developed from a single discipline into a comprehensive school covering several first-level disciplines, and pointed out that with the development of science and cognitive progress, the concept of "intrinsic structure" has been extended and promoted in the fields of engineering, physics and materials science. He also pointed out that with the development of science and cognitive progress, the concept of "homeostasis" has been extended and promoted in the fields of engineering, physics and material science, etc., and the homeostatic model has been endowed with wider performance, and the functionality and accuracy of the model have been greatly developed. This forum will discuss the latest progress and application of the constitutive model, so that we can better understand and apply the constitutive model, and contribute more ideas to the scientific and technological development of the development of resilient underground space. Finally, Academician Chen Xiangsheng expressed his deep gratitude to all the guests attending this conference, and wished that all of them would gain a lot from this forum, and join hands to make efforts for the continuous development of geotechnical constitutive theory and the construction of resilient cities.

In his speech, Prof Yao Yangping shared the development and achievements of the High-level Forum on Geotechnical Principal Structure Theory and pointed out that the theme of this conference is to discuss the principal structure theory in geotechnical and subsurface engineering, and hoped to strengthen the cross-fertilisation of disciplines through the exchange of the forum and to jointly assist the development of the discipline. Finally, Prof Yao expressed his deep gratitude to the organisers and contractors of the conference, and wished that the Ninth High-level Forum on Geotechnical Principal Structure Theory would be a complete success.

After the opening ceremony, the conference entered the academic report session. Professor Yao Yangping of Beijing University of Aeronautics and Astronautics made a report entitled "UH Model Based on Three Basic Mechanical Properties of Soil", which focused on the theoretical innovations and engineering applications of the UH model and pointed out that the establishment of the Unified Hardened Hensile Structural Model (UH model) for sandy clays has realised the expectation of the founder of Cambridge School, Wroth-Houlsby, that "a single model can be applied to all types of soils by adjusting parameters". A single model that can be applied to all types of soils by adjusting the parameters", as expected by Wroth-Houlsby, the founder of Cambridge School.

Professor Zhao Chenggang of Beijing Jiaotong University made an academic report entitled "Uncertainty in the Prediction of Soil Mechanics Theory", in which the report discussed such aspects as the principle of effective stress could not be used as a rigorous scientific method, and it was an incomplete approximation to consider only the effect of effective stress in soil mechanics, pointing out that the prediction results obtained from the current theory of soil mechanics had a large degree of uncertainty.

Professor Huang Maosong of Tongji University gave a lecture entitled "Small Strain Intrinsic Simulation and Engineering Application of Saturated Soft Clay", in which he pointed out that soft clay can have significant initial elastic anisotropy and full-strain range stress path related nonlinearity, proposed a saturated clay intrinsic model system based on Intergranular Strain (IGS), which can realise the simulation of the nonlinear characteristics of the full strain range stress path related to the soft clay, and developed a soft clay engineering numerical computation platform based on the IGS intrinsic model, which realises the application of multi-engineering scenarios.

Professor Yang Guanghua, Honorary President of Guangdong Institute of Water Resources and Hydropower Research, gave an academic report entitled "Generalisation and Practicality of the Eigenstructural Model of Soil". In the report, Professor Yang Guanghua talked about the generalised potential theory, which can provide an easy-to-understand, easy-to-apply and versatile method for the establishment of soil constitutive model, so that simple and practical models can be established flexibly and conveniently according to the engineering needs and improve the engineering applications.

Professor Jidong Zhao of the Hong Kong University of Science and Technology gave an academic report entitled "Multi-physics and Multi-scale Simulation of Geotechnical Large Deformation Problems", which discussed the multi-scale properties of geotechnical materials, the common problems of multi-physics processes, and multi-scale simulation methods for large deformation problems.

Professor Yin Jianhua of the Hong Kong Polytechnic University gave an academic report entitled "From the elastic-viscoplastic ontological theory of soils to a simple and practical method for calculating the consolidation and settlement of soft soils", which introduced the one-dimensional and three-dimensional time-dependent stress-strain ontological models of soils, the nonlinear creep and compression functions, the direct measurement of the effective stresses in saturated soils, and coupled numerical methods and a simple method of calculating the consolidation and settlement of cohesive soils.

Professor Su Dong of Shenzhen University made an academic report titled "Methods of Controlled Generation of Irregular Particle Morphology and Discrete Element Simulation", which systematically introduced the methods of obtaining, reconstructing and evaluating the geometrical morphology of irregular particles, put forward the method of particle cloning based on the compensating radius, as well as the method of controlled generation of particles combined with the equiaxialised reconstruction and stretching algorithms, etc., and summed up the laws of the influence of the distribution of the multilayered-multidispersed geometrical morphology on the physical and mechanical characteristics of the soil body.

Professor Sun De'an of Shanghai University made an academic report entitled "Triaxial Test Methods of Compacted Bentonite for Cushioning Materials", which introduced the definition and range of suction in soil, water content and suction of cushioning materials, the mechanical theory of unsaturated soil, triaxial test of soil, triaxial test methods under high suction, and so on.

Professor Lu Dechun of Beijing Institute of Technology gave an academic report entitled "Numerical Computational Methods for Geotechnical Elastoplastic Models", which discussed numerical methods for non-smooth problems, numerical methods for non-linear problems, stress updating algorithms for numerical differential foundations, and integral non-local methods for softened models.

Researcher Wei Changfu from Wuhan Institute of Geotechnics, Chinese Academy of Sciences, gave an academic report titled "Effective Stress Model for Unsaturated Soil", which systematically introduced the key issues in establishing effective stress model for unsaturated soil, including the generalised effective stress formula, chemical-mechanical coupling of saturated soil, the intrinsic behaviour of unsaturated soil, and the dewatering and shrinkage process of soil body.

Professor Yang Zhongxuan of Zhejiang University made a academic report entitled "A New Method of Constructing State-Related Soil Structural Models Based on Stress Trial Tests", which introduced the method of discrete element stress trial tests, revealed the development law of elastic and plastic deformation of soil under different stress paths, constructed isotropic and anisotropic soil structural models that can reasonably consider state relevance, and verified the validity of the models.

Professor Liu Enlong of Sichuan University made an academic report entitled "Non-local Multiscale Ontological Model of Granular Materials", which pointed out that the two-scale micropolar theory ontological model of saturated granular materials can completely eliminate the mesh-dependence problem, and the micromechanics-based ontological model within the framework of the micropolar theory can reflect the strain-hardening (softening) phenomenon and the body-shrinking (body swelling) phenomenon of granular materials.

Professor Zhenyu Wan of the Hong Kong Polytechnic University gave an academic report entitled "Simulation and Application of Fine Particle Transport in Granular Soils", which introduced the simple constitutive model considering the role of fine particles in soils, the development of a tool for numerical analysis of hydraulic coupling, and the development of the PI-MR-NN model.

Academician Chen Xiangsheng of Shenzhen University made an academic report entitled "Thinking from the Mechanical Properties Test of Artificially Frozen Clay to Digital Intelligent Civil Engineering", which discussed the experimental study of the mechanical properties of artificially frozen clay, the transformation of geotechnical properties - in-situ sensing - intelligent prediction, and the change of the development of the big civil engineering, and shared the history, current situation, and future planning of the College of Civil and Transportation Engineering and the Institute for the Future of Underground Cities of Shenzhen University.

During the meeting, the participating experts, scholars and students had a lively discussion on the contents of the academic report.

At the end of the conference, Prof. Yangping Yao delivered a closing speech and thanked again the conference organisers and contractors for their hard work, as well as all the participating experts and scholars for their wonderful sharing and support.

The success of this conference further promotes academic exchanges in this field, actively promotes the development of the disciplinary field by discussing the cutting-edge issues of geotechnical theories and their applications in the field of ductile subsurface space development, stimulates the power and vitality of innovation, and links and cooperates in order to burst into new advantages.