Soil-structure Interface Research Articles

Thermally induced shear displacement of sand-concrete interface under different stress levels

Energy geostructures are modern technologies developed to exploit shallow geothermal energy. Their installation has recently increased rapidly throughout the world. Their use is devoted to the heating and cooling of buildings, and their operation can induce thermal cycles at the soil-structure interface in the range of 5–40 °C [1]. In addition, their activation would produce thermally-induced stress and strain in structure. Over the last few decades, several full-scale tests have been performed to study the geotechnical aspects of energy piles [2,3]. The main results show that the thermal operations may slightly influence the mechanical performance of the energy pile by increasing the additional thermal compressive stresses and changing the pile axial strain. Numerical studies [4,5] concluded that the temperature increment can lead to thermal expansion of the pile, causing its uplift and the generation of negative shear forces at the pile–soil interface near the pile head. Considering that the thermal operation of energy geostructures leads to temperature change along the piles as well as in the neighbouring soil due to the heat exchange phenomenon, many researchers have investigated the temperature effects on the behaviour of soil and soil–structure interface [6,7,8,9]. Others investigated thermally induced soil deformation using modified triaxial or oedometric apparatuses allowing temperature control [10,11,12]. These results show expansive deformation of coarse-grained soils upon heating and contractive deformation upon cooling for all relative densities. A modified geothermal direct shear box (MGSB) developed at Gustave Eiffel University [13] was used to investigate the effects of temperature cycles on soil–concrete interface. The preliminary results showed that the sand expands upon heating and contract upon cooling. An accumulated contraction and shear displacement were observed after each thermal cycle. However, these preliminary results should be carefully re-analysed since the device was not fully calibrated under various thermo-mechanical loads.
 In this study, the MGSB was utilized after its calibration [13], to investigate the thermally induced vertical and lateral displacements of sand–structure interface. It consists of two half-shear boxes with dimensions of 200 mm x 200 mm x 80 mm. A Peltier module was used to heat and cool water connected to a pipe embedded in the concrete plate, which was used to simulate the pile surface. Normal and shear stresses were applied to represent the geostatic stress of soil and pile load, respectively. Four linear variable differential transformers (LVDTs) were placed in contact with the lower and the upper boxes of the device to measure the shear displacement. One LVDT was placed at the top of the box to measure the vertical displacement of soil. To prevent heat losses, the system was insulated using an insulating foam. Two series of test were performed. The four tests of the first series were conducted on dense (D) sand and the three tests of the second series were on loose (L) sand, with relative densities of about 90% and 30%, respectively. For each density, three normal stress levels were tested: 25, 50 and 100 kPa. For each normal stress, a shear stress equal to 40% of normal stress (45% of the shear strength) was applied. Test D_50 was repeated because of technical problems that happened during the first test. After samples preparation, two phases were carried out. The first phase corresponds to the
 application of normal and shear stresses. Regarding the second phase, 20 thermal cycles were applied, varying the soil temperature between 14 °C and 32 °C.
 The mechanical results showed a good agreement compared to standard direct shear tests. The thermo-mechanical results showed that the succession of soil expansion and contraction upon heating and cooling respectively lead to an accumulated irreversible contraction and continuous shearing displacement after each thermal cycle with different magnitudes, depending on the soil density and stress level. In addition, the rate of these incremental irreversible displacements decreases when the cycle number increases. Moreover, the increase in the stress levels will lead to an increase in the accumulated lateral displacement after 20 cycles, and this increment is significant in loose sand compared to dense sand under the same mechanical conditions. Overall, the thermally induced sand-concrete interface displacements are affected by stress level and strongly depend on the sand density. The thermally induced settlement may be critical in the case of floating energy piles under high mechanical load, and greater than 45% may lead to greater in the case of loose soil would be greater than 0.3 mm. Finally, this study suggests that using a homogeneous stress design method for energy geostructures may adversely affect the design.

Role of Initial State, Material Properties, and Confinement Condition on Local and Global Soil–Structure Interface Behavior during Cyclic Shear

Role of Initial State, Material Properties, and Confinement Condition on Local and Global Soil–Structure Interface Behavior during Cyclic Shear

Shear behavior of the sand-concrete interface under cyclic thermal cy-cles

Energy piles - foundation elements also used as heat exchangers - are subjected to daily and seasonal heating and cooling cycles, which might result in the modification of thermo-mechanical behavior of the soil-pile interface. In order to evaluate the effect of the pile’s geothermal activation on a sand-concrete interface, at laboratory scale, a series of direct shear tests were performed using a device adapted for thermomechanical loading. The constant normal load (CNL) direct shear tests were carried out for three different thermal loading scenarios, i.e., at a constant temperature of 13°C, and for two different cooling-heating (8-18°C) cycles. The volumetric deformation after 10 thermal cycles is evaluated, and the shear results are compared.
 A lot of studies [1, 2, 3, 4, 5, 6, 7, 8] have been performed to investigate the thermal effects on the shear behavior of soil-structure interface through direct shear tests. However, in previous studies all the thermal cycles were performed before the shearing phase. Hence, the effect of temperature cycles on the sand-concrete interface after the shear strength has already been mobilized (i.e., interface shear test) has not been well investigated so far. In fact, in real exploitation conditions, the thermal loading is usually added after a part of the mechanical loading or deformation occurred at the soil-pile interface, given the fact that the geothermal exploitation starts only once the building is in operation. To better understand the effects of thermal cycles on the shear properties of sand-concrete energy piles interface after a prior shear phase, a series of interface direct shear tests were performed in laboratory: (1) Reference Test (RT), shearing under constant temperature of 13°C (undisturbed soil temperature below 5 m in the Parisian region, France); (2) Cyclic Test 1 (CT-1), displacement controlled shearing under constant temperature (13°C), 10 thermal cycles (8 to 18°C), finally displacement-controlled shearing until the constant volume state was generated; (3) Cyclic Test 2 (CT-2), shearing to 1/2 of the residual shear strength, 10 thermal cycles (8 to 18°C), finally displacement-controlled shearing until the constant volume state was generated (CT-2 test). The monotonic displacement-controlled direct shear tests were conducted under constant normal stresses equal to 50 kPa, 100kPa and 150kPa that correspond to the typical normal effective stresses acting on the soil-pile interface at different depths.
 Figure 1 presents the volumetric strain (εv) during the thermal cycles of CT-1 and CT-2. The sand samples are characterized by a contractive behavior during each heating phase and followed by an expansive behavior at the high temperature of 18°C. The samples continue to expand in the next cooling phase, then they start to contract when the low temperature of 8°C is constant. The volumetric strain changes are attributed to the thermal-induced particle rearrangement during the heating-cooling cycles. After the 10 thermal cycles, the sand samples show an overall contractive response. The thermal cycles induce more vertical strain on the samples of CT-2 than CT-1 (see Figure 1). The possible reason is that shearing to 1/2 residual shear strength develops more unstable particle rearrangement in CT-2 samples than shearing to the residual shear strength in CT-1 samples.
 The peak and residual interface friction angles of CT-1 and CT-2 after the 10 cyclic thermal cycles are compared with RT in Table 1. The peak interface friction angle of CT-1 is 28.9°, 0.6° lower than the one of RT (29.5°). Whereas a higher peak interface friction angle is mobilized in CT-2 (31.7°). CT-1 is characterized with a residual interface friction angle of 27.9°, which is 0.5° lower than the 28.4° of RT. However, the difference of residual interface friction angles between CT-2 (28.5°) and RT (28.4°) is only 0.1°. To conclude, the effect of the 10 cyclic thermal cycles on the sand-concrete interface is quite limited, it slightly decreases the peak as well as residual interface friction angles of CT-1 and increases these of CT-2.

Direct shear setup for studying the THM shear response of the soil-structure interface in the context of energy geostructures

Developing pathways for climate-resilient development involves integrating mitigation and adaptation actions, ensuring sustainable development [1]. A climate-resilient development can be achieved through the inclusion of effective mitigation approaches into development planning, reducing vulnerability, conserving ecosystems, and restoring ecosystems [1]. In this regard, energy geostructures are introduced as an effective mitigating approach, providing renewable energy while limiting the emission of greenhouse gases [2-4]. The operation of the system is closely linked to daily and seasonal cycles, which leads to cyclic temperature and water content fluctuations at the soil and the soil-structure interface [4, 5]. Thus, the study of the shear response of the soil–structure interface subjected to different thermo-hydro-mechanical (THM) conditions is of importance. Testing techniques used to study the THM behaviour of unsaturated soils require advanced laboratory equipment, as well as protocols for correcting measured data due to errors in the test conditions and apparatus calibration. This paper presents the development of a new direct shear setup to measure the non-isothermal shear strength of the partially saturated soils and soil-structure interface. The modified setup, a unique one to the authors' knowledge, enables simultaneous control of temperature, matric suction, and mechanical stress state within the soil specimens. The operational temperature range of energy geostructures (i.e., 5°C to 50) is applied through a thermal plate, developed from corrosion-resistant stainless steel with high thermal conductivity, placed at the base of the soil specimen. Matric suction (i.e., in the range of 0 to 100 kPa) is controlled using the axis-translation technique and measured using a pressure transducer connected to the back of the top cap, incorporating the HAE disk, facilitating the measurement (Figure 1) [2, 4-6]. The direct shear setup is modified to accurately measure the shear strength and deformation characteristics of soil samples under controlled laboratory conditions. The design of the device is based on previous direct shear devices but includes several improvements to enhance its accuracy and ease of use [4, 5]. The device has been tested to measure the shear response of both soils and soil-structure interfaces, and the results were compared to those obtained using conventional direct shear devices [2, 4, 7-10]. The results indicate that the new device is accurate and reliable and represents a significant advancement in the field of soil testing.
 In this study, the interface is formed by kaolin clay, a temperature-sensitive clay, and concrete, a structural material widely used for energy geostructures [4, 11]. The soil samples were prepared by static compaction at 30% initial water content () and an initial void ratio () of 1.2. Initially, all samples were inundated with distilled water at room temperature (24°C) [4, 12], followed by sequential hydraulic (i.e., matric suction, = 0 or 70 kPa), mechanical (i.e., net normal load, =100 or 300 kPa), and thermal (i.e., temperature, =24°C or 45°C) loading of the interface. The shearing was initiated after ensuring equilibrium criteria were met. It was necessary to limit the shearing rate to 0.005mm/min and the thermal loading rate to 3°C/hr to maintain the drained condition [4, 13]. Additionally, the vertical displacements associated with hydraulic load were subjected to an equilibrium criterion of 0.025%/day strain rate [14]. As presented in Table 1, the results revealed that the apparent interface friction angle was not significantly affected by matric suction at varying temperatures, but a slight decrease has been observed upon heating at all matric suctions. The apparent adhesion increased in response to temperature increase/decrease, with a decreasing rate as suction increased, while the interface desaturation led to higher apparent adhesion at all temperatures, corresponding to peak and residual values. In interfaces subjected to identical normal stress and temperature but different matric suction values, the shear stress-shear displacement curves showed greater peak stress with increasing matric suction. Furthermore, despite the same net normal stress and matric suction, higher temperatures resulted in slightly lower peak shear stress and less contractive volume change behaviour at the interface. Non-isothermal volumetric behaviour and soil dilatancy play a significant role in governing the THM shear response of the interface [15].

Significance analysis of the factors influencing the strength of the frozen soil-structure interface and their interactions in different phase transition zones

The freezing strength of the contact interface between foundation soil and infrastructure has been a key and difficult problem in the study of frost jacking of piles in cold regions; previous studies have focused on the influence law of experimental factors on freezing strength, and there are few studies on the significance of the effects of factors and their interactions on the freezing strength in different phase transition zones. In this paper, based on the existing test data, the orthogonal experimental design method considering the interactions, the principles of range analysis and variance analysis were used to investigate the significance of the effects of factors influencing the freezing strength in different phase transition zones and their interactions. Finally, a new method for establishing the prediction model of freezing strength based on the multiple linear regression model was proposed. The research shows that there are significant differences in the effects of the experimental factors and their interactions on the freezing strength in different phase transition zones. The freezing strength prediction model established by the new method can better reflect the variation law of freezing strength with factors in different phase transition zones and the influence degree of the interactions between factors.

A critical state three-dimensional multi-shear model for soil-structure interfaces under monotonic and cyclic loading

In this paper, a three-dimensional(3D) multi-shear model of soil-structure interface is established by using the plasticity theory of boundary surface. State parameters are introduced to predict the interface response under different soil densities and normal stress. A constitutive equation is established following the assumption that the interface response is decomposed into macro-response and a series of directional-dependent micro-shear responses. The calculation method of plastic microshear strain is modified, a new calculation method of microscopic plastic shear modulus is introduced, and the calculation parameters are reduced. The proposed model can reasonably describe the variation of stress–strain relationship under 3D shearing conditions and predict the three-dimensional non-linear mechanical behaviour of the contact surface. The proposed model has a unified calculation formula for loose and dense interfaces under different loading conditions. The proposed model can be accurately described the dilatancy characteristics under low normal stress and shear shrinkage under high normal stress. The proposed model can simulate the cyclic response characteristics of the interface, e.g., the normal displacement increases with increasing cycle number, however, the growth rate decreases with increasing cycle number. The validity of the proposed three-dimensional multi-mechanism shear model of soil-structure interface under different boundary and loading conditions is verified by comparing the calculation results with the experimental results.

An analytical method for predicting the soil-structure interaction of axially loaded piles in sands incorporating the two-surface plasticity model

The paper proposed a novel and practical analytical method for predicting the soil-structure interaction in sands. To well capture the stress softening and soil dilatancy behaviors during loading, a developed two-surface plasticity model for interface shearing is adopted to determine the 2-D stress–strain state of the soil-structure interface and the shear band, while the 3-D elastoplastic behavior of soil around the shear band is determined by another developed two-surface plasticity model for triaxial compression. Following a standardized solving procedure, the rigorous derivation for the stress–strain relationship in plastic zone is formulated as a system of first-order differential equations, which are solved as an initial value problem. Combined with the finite difference method (FDM), the load–displacement response of the axially loaded pile and the stress–strain state around the pile are presented. To verify the proposed analytical approach, a FEM simulation is performed using a user-defined subroutine. The soil with different initial void ratios ranging from very dense to loose is also modeled to demonstrate the effect of the initial void ratio on soil-structure response. The proposed analytical approach is also validated against centrifuge tests and field tests, manifesting the validity and capability of the proposed method.

Unified Modeling of Interface between Structured Clay or Crushable Sand and Structure

The purpose of this paper is to develop a unified elastoplastic model for describing both monotonic and cyclic behaviors of natural structured clay–structure and crushable sand–structure interfaces based on the bounding surface plasticity and the critical state concept with three features: (1) a more general yield surface with two shape parameters is adopted to improve the prediction of the shear yield and the critical state of the soil–structure interface, (2) the degradation of bounding surface size and the adhesive normal stress is introduced into the model to consider the shear strength reduction of natural structured clay–structure interface, and (3) the grain breakage effect on the behavior of crushable sand–structure interface is considered by incorporating the location of the critical-state line with the broadening of grain-size distribution induced by grain breakage. Then, explicit simulation schemes with substepping and error control are established for simulating interface shear tests under constant volume, constant normal load, and constant normal stiffness conditions. Finally, comparisons between experimental and numerical results indicate that the unified soil–structure interface model is able to not only predict the state-dependent monotonic stress–strain behavior of structured clay–structure or crushable sand–structure interfaces, but also reproduce the cyclic accumulative contraction and stabilization, as well as the stress degradation observed in both structured clay–structure and crushable sand–structure interfaces.

Effect of degradation on geotechnical behavior of glauconite sands from the U.S. Mid-Atlantic Coastal Plain

Glauconite sand is a challenging sediment that can pose risks to foundation installation and performance due to its tendency to transform from coarse-grained material into fine-grained material due to particle crushing. Glauconite is characteristically green iron potassium mica and smectite often found in sand-sized peloidal form. It forms under reducing conditions within marine depositional environments below wave base and has been found in coastal plain and offshore regions of the U.S. This paper presents an overview of the geological basis for glauconite sand formation, describes its depositional environment and maturation process, and summarizes the state of knowledge of geotechnical challenges in glauconite sands. Laboratory test results are presented for glauconite sands collected from the U.S. Atlantic coastal plain, showing that initial sample conditions, drying methods, and degradation methods affect the initial, temporary, and long-term geochemical and mechanical characteristics of glauconite sands. The degree to which particles crush or degrade from their natural peloidal form to clay-size particles is linked to maturity and depositional method. Intentionally degraded specimens used in laboratory testing can simulate the effect of pile driving through creation of a shear zone of degraded soil along the pile shaft, and quantify its impact on soil-structure interface strength.

Experimental study of the interface evolution behavior and softening mechanism of structure-marine soil

The undrained interface characteristics of marine clay-structures are key to the design of offshore platforms. However, it is difficult to acquire interfacial shear-induced pore pressure measurements employing conventional interface equipment. This paper introduces an interface instrument that is suitable for evaluating marine soil-structure interface characteristics. The developed device is utilized to investigate the undrained interfacial evolution behavior and softening mechanism of specimens with different moisture contents. The results indicate that the interface undrained strength curve exhibits high stiffness initially, and then declines as the strain level increases. The interfacial soil particles experience two stages: obvious contraction and slight dilatancy tendency. The moisture content response to interface shear resistance is mainly through changing interface adhesion. Additionally, the reduction in contact area caused by interfacial particle dilatancy is the internal inducement of interfacial strain softening. The increase in interfacial soil moisture content caused by water migration is another external inducement. Accordingly, the stress development state in the whole shear process and the variation in free water migration toward the interface are revealed, providing an excellent analytical basis for further investigation of undrained interface behavior.

Development of Cyclic Interface Shear Test Device and Testing Procedure to Measure the Response of Cohesive Soil-Structure Interface

Abstract Cyclic loading acting on foundations supporting renewable energy systems affects the soil-structure interface properties and shear resistance. The cyclic shear stress and cyclic displacement applied to the cohesive soil-structure interface may cause volume changes and pore pressure build up in the surrounding soil. This article focuses on the development of a new testing device named the cyclic interface shear test (CIST). The CIST is a fully automated device developed to directly measure the response of cohesive soil-structure interface for foundations subjected to long-term cyclic axial loading. This article describes the newly developed testing device, including the design of the shear head and the three-dimensional (3D) printed interface elements, as well as the control system and data acquisition system. Furthermore, the article presents the sample preparation for clayey soil and the recommended testing procedure for conducting static or cyclic interface shear tests, or both. The developed CIST device is capable of applying one-way, two-way, or combined one- and two-way cycles. Furthermore, the CIST is capable of applying different amplitudes, a range of loading frequencies, and a large number of cycles. To demonstrate the capabilities of the CIST device and to finalize the testing procedure, a series of preliminary CISTs and a static baseline test on cohesive soil-3D printed interface were performed.

A Succinct Review on Soil Structure Interface Thickness

A Succinct Review on Soil Structure Interface Thickness

Modeling of granular soil-structure interface under monotonic and cyclic loading with the nonlinear approach

A constitutive model that can accurately predict the behavior of the soil-structure interface is beneficial to the analysis and design for foundation engineering performance. This paper develops a new model with the nonlinear approach for simulating the monotonic and cyclic behaviors of the granular soil-structure interface. Based on the hyperbolic function, the concept of normalized shear displacement is proposed, and the basic model for the effects of the normal stress, the nonlinear shear modulus, and the friction angle on shear strength during shearing is deduced. On this basis, the Pyke criterion is modified to describe the cyclic loading responses. The relation between the friction angle and the state parameter of the interface in different interface states is established under the framework of critical state soil mechanics (CSSM) theory to further reflect the interface behaviors corresponding to the changes of void. Taking the phase transformation state as a reference state of volume increment, the incremental equation of stress-dilatancy induced by shearing is developed. In addition, the basic model is extended to be able to describe the interface mechanical behaviors under constant normal load (CNL), constant stiffness (CNS), and constant volume (CV) conditions by considering the characteristics of three typical stress loading paths. Only 10 calibrated parameters are required to reproduce the complex properties in the monotonic and cyclic behaviors. The predictive performance of the proposed interface model is examined by the previous experimental results.

ANALYZES OF RUBBER BEDDED, LATERAL LOADED SINGLE PILE BY PLAXIS 2D

In this study, lateral loaded single pile either with rubber bedded were analyzed in cohesionless soil. By rubber bedding how differated the moment values and pile length, head diameter ratio. For this purpose 3 different pile diameter such as 0,40m., 0,60m.,0,80m. 2 different volume rubber bedding and every combination was analyzed with two L/D values. In total 18 plaxis 2D analyzes exist in the study. Lateral loaded single pile interacts with soil either different with rubber bedded one and soil structure interface has been important parameter. Rubber bedding reduces moment values throught the pile length.This reducing factor allows the designer more economical and secured values. On the other hand pile’s different L/D ratios gives varying values on moment either in rubber bedded either in dense sand. By foreseeing this design parameter is important for more economical and efficient designing.

Numerical investigation of the seepage mechanism and characteristics of soil-structure interface by CFD-DEM coupling method

Interfacial seepage on soil-structure interface is common in water-related engineering, such as dam, culvert, water tunnel, foundation pit, etc. As a weak part in the project, seepage failure on soil-structure interface has caused many project accidents. However, there are few studies on the seepage mechanism analysis of soil-structure interface. In this paper, a series of numerical tests of interfacial seepage are carried out using CFD-DEM coupling method. The seepage mechanism and characteristics of different soil layers in the seepage device are analyzed and a formula for calculating the critical hydraulic gradient of the interface is discussed. The main conclusions are as follows: (1) The seepage flow on soil-structure interface will go through three stages: stability stage, transition stage and particle erosion stage; (2) The porosity and flow velocity of soil on the interface are greater than those in the far-field soil at the beginning of seepage flow; (3) The particle erosion on the soil-structure interface occurs earlier and the erosion degree is more serious than that of the far-field soil; (4) The critical hydraulic gradient of the interface rises nonlinearly with the increase of the degree of soil compaction; (5) The formula for calculating the critical hydraulic gradient of the interface could predict the numerical test results well.

Effects of freeze–thaw cycles on the shear stress induced on the cemented sand–structure interface

Freeze–thaw alternation and freeze–thaw cycle erosion are the main reasons for the failure of the cemented soil–structure interface in cold regions. In this study, the direct shear and cyclic direct shear tests of cemented sand–structure interface were conducted in the frozen state (−10 °C) and thawed state (25 °C) at a 300 kPa normal stress under different freeze–thaw cycles (i.e., 0, 1, 5, 10, 15, 20, 25, 30). The experimental results for the shear stresses of the cemented sand–structure interface are presented, and the mechanism of shear stress formation and change was examined through mesoscopic analysis. Variations in shear stress with different freeze–thaw cycles were consistent. Typically, 10 was identified as the critical freeze–thaw cycle number for adfreeze strength, and the hyperbolic model of shear stress–displacement curve in the thawed state was obtained. The peak shear stresses of different freeze–thaw cycles gradually decreased as the shearing cycle number increased until entering a stable stage in frozen state and were close to each other in the thawed state. The internal damage of cemented sand and the formation of ice crystals on the surface increased with the increase in freeze–thaw cycle number. Hence, internal change in cemented sand under freeze–thaw cycles is the fundamental reason for alterations in shear stress. The results of this study shed light on shear stress complexity at the cemented sand–structure interface under freeze–thaw cycles and provide references for practical engineering.

Mechanical test and action mechanism of interface between gravel soil and concrete

The mechanical properties of the gravel soil-concrete structure interface are one of the important issues in academia and engineering. In this research, gravel soil samples were collected from the Three Gorges Reservoir region to carry out simple shear and direct shear tests of the gravel soil-structure interface. The shearing process was simulated with the numerical method. The strength and deformation characteristics of the interface, the applicability of the interface constitutive model, and the failure mechanism of the interface were analyzed. The results show that: 1) The direct shear test curve of the gravel soil-concrete interface shows strain softening with obvious stress peak, while the simple shear test curve shows strain hardening. The stress peak of the direct shear test is about 30% higher than that of the simple shear test. The shear contraction effect of the simple shear test is about 31% larger than that of the direct shear test. 2) The non-linear relationship of the gravel soil-concrete interface can be described by a hyperbola model, which has a good adaptability to the constitutive relationship of the interface in numerical analysis software. 3) The gravel soil-concrete interface gradually develops shear failure from the edge to the inside. The essence of soil-structure interaction is the movement of soil grains. The research results can provide a reference for the analysis of gravel soil-concrete interaction.

Experimental investigation of soil–structure interface behaviour under monotonic and cyclic thermal loading

The effect of temperature on the monotonic and cyclic shearing response of a soil–structure interface is of critical importance for the application of thermal-active geo-structures. To investigate this, soils and soil–concrete interfaces were comprehensively tested with a temperature-controlled direct shear device under both fixed temperatures and thermal/mechanical cycles within the range of 2–38 °C. Monotonic and cyclic shearing with various boundary conditions, including constant normal load (CNL), constant normal stiffness (CNS) and constant volume (CV), were conducted to resemble the conditions that thermal-active-geo-structures may experience. The strength properties of the sand, clay, and sand–concrete and clay–concrete interfaces were partially influenced by heating and cooling under all boundary conditions. However, several effects were observed which could affect the performance of thermo-active structures. Heating cycles caused the clay–concrete interface to be overconsolidated, implying a lower excess pore pressure would be generated during shearing. The cyclic CNS tests suggested that the interface strength could degrade due to (thermally induced) cyclic shear displacements, with this effect strongly related to the state of the soil rather than the temperature directly. In these tests, the medium-dense sand–concrete interface degraded to almost zero shear strength after 5 cycles, whereas the clay–concrete interface asymptotically degraded to around 60% of its strength after 10 cycles.

A review on the thermo-hydro-mechanical response of soil–structure interface for energy geostructures applications

Energy geostructures have been identified as a cost-effective mitigating strategy for the adverse impact of climate change. Operation of energy geostructures results in temperature fluctuation and subsequent water migration, particularly at the soil–structure interface, determining the shear response of soil and soil–structure interface. This state-of-the-art paper brings together experimental data from direct shear tests carried out on the soil–structure interface from several laboratory investigations, presenting a comprehensive review to gain a thorough understanding of the interface response in different thermo-hydro-mechanical states, which is critical in the analysis and design of energy geostructures. First, the evolution of shear strength parameters, i.e., adhesion and friction angle, with matric suction and temperature, are investigated. Then, a more detailed analysis of the impact of matric suction and temperature on the shear strength of the soil–structure interface is provided. Furthermore, a comprehensive discussion is provided in this section on the role of the most recent stress history in determining the non-isothermal shear strength of an interface. Data on the effect of matric suction and temperature on shear parameters of the corresponding fundamental soil is reviewed as a reference to the interface behaviour throughout the study, revealing potential underlying mechanisms. In general, a higher matric suction results in higher shear strength of the interface, whereas non-isothermal variations in adhesion and friction angle may lead to a higher or lower shear strength of a saturated interface.

A state dependent constitutive model for unsaturated soil-steel interfaces considering bonding effect

The frictional behaviours between unsaturated soils and structure are controlled mainly by the complex phenomena that develop within a thin soil layer close to the contact area. The constitutive model for soil-structure interfaces (SSI) must be established to predict the performance of related soil-structuresystems accurately. Multi-stage suction-controlled ring shear (RS) tests with three different upper annular platens were carried out on an equal number of statically compacted samples of Shanghai clay under controlled suction states for a given net normal stress. A state dependent constitutive model for unsaturated soil-structure interfaces based on critical state concept considering bonding effect was proposed. In all stress conditions, the tested and predicted results for smooth interface and rough interface are well matched.