Numerical analysis of desiccation, shrinkage and cracking in low plasticity clayey soils

This paper presents a numerical study of the desiccation processes of low-plasticity clayey soils that usually result in shrinkage and often in cracking. For the theoretical development of the numerical model, concepts of Unsaturated Soils Mechanics and of classical Strength of Materials are used as a framework for formulating phenomena such as water flow in deformable porous media and cracking. The mathematical formulation of the problem and its implementation in a hydro-mechanical coupled model is presented, in order to simulate fluid flow and cracking in soils, for which the FEM and the node release technique is combined. The code developed has been used to perform several numerical analyses on radial sections of cylindrical soil specimens subjected to a drying process for which experimental laboratory data was available. The objective of these simulations is to determine the mechanisms by which the soil shrinks and cracks during desiccation. The results show the capabilities of the approach to reproduce the main features of the problem, with desiccation, shrinkage, and cracking being reproduced consistently during a desiccation cycle. The model also highlights the key role of the displacement and suction boundary conditions in the development of cracks as a consequence of tensile stress fields. Finally, the model has revealed the necessity of further research in the study of the soil-container and soil-atmosphere interaction in order to reproduce with more accuracy the changes in the main variable

Development of a new advanced elastoplastic constitutive model that considers soil behavior at small strains: the EPHYSS model

The Elastoplastic Hysteretic Small Strain (EPHYSS) model is an advanced elastoplastic model as a result from the combination of the Hysteretic Quasi-Hypoelastic (HQH) model that considers strain-induced anisotropy and can reproduce the nonlinear reversible, hysteretic and dependent on recent history soil behavior, and the Cap-Cone Hardening Soil Modified (HSMOD) model that can reproduce soil plastic behavior. EPHYSS model uses state variables that define different short and long-term memory levels which provide it with robustness for the reproduction of soil hysteretic behavior and confers it a great versatility and adaptability to experimental results. It also corrects some inconsistencies of the Hardening Soil with Small Strain Stiffness (HS-SS) model of Plaxis whose effects can have a considerable influence on the numerical simulations of boundary problems. The performance of EPHYSS and a comparison with the HS-SS model is presented in some experimental and numerical tests, and in a boundary value problem of a large excavation in Barcelona.Peer ReviewedPostprint (published version

Protecting sensitive constructions from tunnelling: the case of World Heritage buildings in Barcelona

Permission is granted by ICE Publishing to print one copy for personal use. Any other use of these PDF files is subject to reprint fees.Construction of the tunnel for the high-speed Madrid–Barcelona–France railway link across central Barcelona became a major technical and social challenge due to the impact of the tunnel on nearby historic buildings (two of them, the Sagrada Familia basilica and Casa Milà, being United Nations Educational, Scientific and Cultural Organization (Unesco) World Heritage structures). Protection of sensitive buildings from tunnelling-induced movements relied on the construction of a stiff pile wall, separating the tunnel from historic sites. This paper first presents a simplified procedure to analyse the wall–tunnel interaction in a straightforward manner. The main features of the tunnel, excavated by means of an earth pressure balance machine in tertiary clays and sands below the water table, are then described. Details of the design of the wall that was finally built are presented. Issues that were particularly important include the groundwater flow constraints and the use of small-strain soil stiffness properties to obtain realistic settlements. General criteria to design the protection wall are also presented. The good performance of the wall resulted in negligible tunnelling impact on the sensitive structures. The measured and predicted displacements are compared, suggesting that this type of solution is adequate to protect historic structures from tunnelling.Peer ReviewedPostprint (published version

Low lateral stiffness underground structures for improved seismic performance: application to the Kobe Daikai station

Underground cut-and-cover structures are commonly designed as rigid box sections; however, in practical applications, connections between walls and slabs are frequently rather hinged (because of ease of construction). The abovementioned rigid configurations are highly sensitive to seismic ground motions, due to their important lateral stiffness and internal hyperstaticity; conversely, structures with articulated (or sliding) members have a smaller lateral stiffness, and would be significantly less affected by seismic waves, as would simply accommodate the imposed strains. This flexible solution has been widely considered in practice, but has received little attention from the academic community; this paper tries to close this gap by investigating preliminarily the seismic performance of box-section underground structures with hinged or sliding members. The well-known Daikai Station, damaged by the 1995 Kobe earthquake is analyzed in this paper as a highly relevant case study. An alternative solution is proposed for that station; both simplified and precise calculations have been performed. The simplified calculations are linear static analyses of the station-soil system; the soil-structure interaction is represented by a simple classical model. The precise calculations are nonlinear time-history analyses of an integrated finite element model of the station and the surrounding soil. Both types of analyses refer to the traditional and the proposed solutions of the station. The results of the static and dynamic analyses are satisfactorily compared; they prove that the proposed flexible solution is fully feasible and provides better seismic performance. Finally, another paper by the same authors presented a supplementary case study on a 2-story 3-bay subway station; the outcomes of these two studies could contribute to ground this constructive solution for shallow underground rectangular cut-and-cover structures in seismic areas (both for new construction and retrofit). Noticeably, this approach can be utilized for both cast-in-place and precast structures.This research has been partially funded by the Spanish Research Agency (AEI) of the Ministry of Science and Innovation (MICIN) through project with reference: PID2020-117374RB-I00 / AEI / 10.13039/501100011033. The study of Mr. Xiangbo Bu in the Technical University of Catalonia (UPC-BarcelonaTech) is funded by Chinese Government Scholarship (CSC No. 201906560013). These supports are gratefully acknowledged.Peer ReviewedPostprint (published version

Novel seismic design solution for underground structures. Case study of a 2-story 3-bay subway station

This paper proposes an innovative seismic design approach for shallow rectangular cut-and-cover underground subway or railway stations. The traditional approach is to design rigid frame-like structures by connecting rigidly the main horizontal and vertical structural elements (side walls, top, bottom and intermediate slabs, and central columns); on the contrary, the proposed strategy consists of joining them by means of hinged and sliding connections, in order to obtain structures whose lateral stiffness is almost zero. The objective of this approach is to be able to adapt to the transverse racking motion imposed by the seismic ground motion without significantly increasing the internal forces in the structural members. The aforementioned flexibility of the joints is achieved by interposing rubber bearings between the connected structural elements. As a case study, an existing 2-story 3-bay subway station located in Southwest China is redesigned with the proposed technology; its seismic performance is numerically investigated by performing nonlinear dynamic analyses for a number of horizontal transverse input ground motions (accelerograms) representing the site seismicity. Such inputs are scaled to fit PGAs ranging from 0.1 to 0.6 g. As expected, the results of the time-history analyses reveal that the seismic damage to the structural members is significantly alleviated in the sliding-hinged alternative solution. This conclusion can be understood as a preliminary confirmation of the satisfactory seismic performance of the proposed technology.This work has received support from the Chinese Government Scholarship (CSC No. 201906560013) with UPC - Barcelona Tech - CSC joint project, which is gratefully acknowledged.Peer ReviewedPostprint (published version

Boundary effects in the desiccation of soil layers with controlled environmental conditions

This article presents the results of an experimental research carried to investigate the mechanics of cracking of soil layers under drying conditions. The tests were conducted under controlled laboratory conditions and in an environmental chamber with circular and rectangular specimens to investigate the effect of the boundary conditions (size, shape, and aspect ratio of the specimens and containers) on the process of initiation and propagation of cracks and on the final crack pattern at the end of desiccation. The tests in the environmental chamber were conducted with imposed temperature and relative humidity and provided new insight into the mechanics of the formation of cracks in a drying soil, and they showed that cracks can initiate either at the top, bottom, or at both surfaces of the drying specimen. The results also reveal how the crack patterns are controlled by the existing mechanical and hydraulic boundary conditions. The cracks seem to form sequentially in patterns that can be explained by three key factors: stresses higher than the tensile strength, the direction of the generated stresses, and the stress redistribution in the vicinity or inside the newly formed domain. In order to substantiate the sequential nature of the crack pattern formation, experimental evidences showing the existence of a cracking sequence during the laboratory desiccation experiments are presented and analyzed.Peer ReviewedPostprint (published version

Numerical analysis of desiccation, shrinkage and cracking in low plasticity clayey soils

This paper presents a numerical study of the desiccation processes of low-plasticity clayey soils that usually result in shrinkage and often in cracking. For the theoretical development of the numerical model, concepts of Unsaturated Soils Mechanics and of classical Strength of Materials are used as a framework for formulating phenomena such as water flow in deformable porous media and cracking. The mathematical formulation of the problem and its implementation in a hydro-mechanical coupled model is presented, in order to simulate fluid flow and cracking in soils, for which the FEM and the node release technique is combined. The code developed has been used to perform several numerical analyses on radial sections of cylindrical soil specimens subjected to a drying process for which experimental laboratory data was available. The objective of these simulations is to determine the mechanisms by which the soil shrinks and cracks during desiccation. The results show the capabilities of the approach to reproduce the main features of the problem, with desiccation, shrinkage, and cracking being reproduced consistently during a desiccation cycle. The model also highlights the key role of the displacement and suction boundary conditions in the development of cracks as a consequence of tensile stress fields. Finally, the model has revealed the necessity of further research in the study of the soil-container and soil-atmosphere interaction in order to reproduce with more accuracy the changes in the main variables.Postprint (published version

Numerical and experimental study of initiation and propagation of desiccation cracks in clayey soils

This paper presents the fundamentals and the mathematical formulation to study desiccation cracking in soils based on Unsaturated Soil Mechanics as well as a numerical analysis of a previous desiccation test program. The numerical approach implemented in MATLAB is used in 2D simulations on radial sections of the cylindrical specimens and in a theoretical study of the stress field in plane strain conditions. The numerical analysis, based on two stress stare variables (total net stress and suction) is consistent and in good agreement with the experimental results, including the location of cracks and time of crack initiation.Peer ReviewedPostprint (author's final draft