Dr Benyi Cao

Royal Academy of Engineering Research Fellow; Lecturer in Civil and Environmental Engineering
+44 (0)1483 689280
01A AA 02


My qualifications

BEng in Civil Engineering
Tongji University
MEng in Civil Engineering
Tongji University
PhD in Engineering
University of Cambridge

Previous roles

Research Assistant Professor
Nanjing Institute of Environmental Sciences
Research Associate
University of Cambridge


Research interests

Research projects


Postgraduate research supervision



Huiyang Qiu, Jian Xu, Yizhi Yuan, Eduard Alesi, Xin Liang, Benyi Cao (2024)Low-disturbance land remediation using vertical groundwater circulation well technology: the first commercial deployment in an operational chemical plant, In: Science of The Total Environment173804 Elsevier

Soil and groundwater contamination by organic pollutants from chemical plants presents significant risks to both environmental and human health. We report a significant field trial where a chemical plant in operation showed soil and groundwater pollution, as verified by sampling and laboratory tests. While many remediation methods are effective, they often require the temporary shutdown of plant operations to install necessary equipment. This paper introduces a novel combination of low-disturbance contaminant remediation technologies, including groundwater circulation well (GCW), pump and treat (P&T), and in-situ chemical oxidation (ISCO) technologies, that can be applied on the premises of an active plant without halting production. The groundwater with dissolved contaminants is removed through P&T and GCW, while GCW enhances ISCO that focus on eliminating the remaining hard-to-pump contaminants. Results show: (1) after two years of remediation effort, the contaminant levels in soil and groundwater were significantly reduced; (2) the average concentration reduction rate of four contaminants, including 1,2-dichloroethane, methylbenzene, ethylbenzene, and M&P-xylene, exceeds 98 %; (3) the presented remediation strategy results in the improvement of remediation efficiency. Specifically, the concentration of 1,2-dichloroethane in observation wells dropped from 40,550.7 μg/L to 44.6 μg/L. This study offers a first-of-its-kind commercial deployment of a GCW-based remediation strategy in an active plant setting. Moreover, the combined remediation approach presented here can serve as a model for designing contaminant remediation projects that require minimal operational disruption.

Benyi Cao, Xueying Wang, Abir Al-Tabbaa (2024)Improvement of soil thermal conductivity with graphite-based conductive cement grouts, In: Journal of Geotechnical and Geoenvironmental Engineering American Society of Civil Engineers (ASCE)

Shallow geothermal energy systems (SGESs) are a promising technology for contributing to the decarbonisation of the energy sector. Soil thermal conductivity (λ) governs heat transfer process in ground under steady state, thereby it is a key parameter for SGES performance. Soil mixing technology has been successful in enhancing the shear strength of soils, but is adopted in this paper for the first time to improve soils as a geothermal energy conductive medium for SGES applications. First, the thermal conductivity of six types of soils were systematically investigated and the key parameters analysed. Next, graphite-based conductive cement grout was developed and mixed with the six soils in a controlled laboratory setting to demonstrate the significant increase in soil thermal conductivity. For example, the thermal conductivity of a very silty dry sand increased from 0.19 to 2.62 W/m·K (a remarkable 14-fold increase) when mixed with the conductive grout at a soil-to-grout ratio of 6:1. In addition, the mechanical properties of the cement grouts and cement-mixed soils were examined along with the microstructural analysis revealing the mechanism behind the thermal conductivity improvement.

Guangzhe Zhang, Fenglei Du, Xiaohui Cheng, Xiangyu Li, Anqi Mao, Benyi Cao (2024)Full-scale tests of a long energy pile subjected to separated and coupled thermo-mechanical loads, In: Proceedings of the Institution of Civil Engineers. Geotechnical engineering Emerald Group Publishing

In this paper, four individual full-scale tests were carried out to study the mechanical response of a cast-in-place energy pile beneath a liquefied natural gas (LNG) tank subjected to separated and coupled thermo-mechanical loads. The results show that the temperature profiles displayed a comparable trend in response to pile heating, cooling and recovery. Specifically, the temperature at the mid-depth of the pile fluctuated rapidly, while the changes at both ends were relatively slower. During the thermal stages, when the pile had the flexibility to expand or contract, the observed strain at the pile head significantly deviated from the free thermal strain. In contrast, the strain at the pile toe was relatively aligned with the free thermal strain. The thermally induced stress obtained at the end of the coupled loading-cooling stage was found to exceed the tensile strength of the C40 reinforced concrete. However, under the actual testing conditions, both the settlement and the bearing capacity of the pile remained well within the required values, ensuring the structure of the LNG tank will not be damaged.Highlights

Xueying Wang, Benyi Cao, Christos Vlachakis, Abir Al-Tabbaa, Stuart K. Haigh (2023)Characterization and piezo-resistivity studies on graphite-enabled self-sensing cementitious composites with high stress and strain sensitivity, In: Cement & concrete composites142105187 Elsevier Ltd

Carbon-based conductive fillers have recently been incorporated into a cement matrix to develop an intrinsic self-sensing concrete for data monitoring without the need for embedded, attached, or remote sensors while maintaining or improving its mechanical properties and durability. This paper studies cementitious composites filled with graphite as a novel self-sensing construction material. Experiments were systematically conducted to investigate the dispersion, chemical, mechanical, electroconductivity and piezo-resistivity properties of the composites. Experimental results showed an effective mixing with uniform dispersion of the graphite which acted as an inert filler in the mix and did not alter the microstructure of cement hydration products. Isothermal calorimetry, TGA and rheology tests showed good hydration and adequate workability of the composites with low graphite concentration (≤ 10%), while the effect of adding graphite on the compressive strength is insignificant for graphite concentrations of up to 10%. Monotonic and cyclic compressive test results indicated a repeatable piezo-resistivity performance for low graphite concentration cementitious composites whose stress sensitivity values vary from 0.75 to 7.25%/MPa, and strain sensitivity/gauge factor (GF) 150–1250 with some hysteresis. These combinations showed a stable and reliable piezo-resistivity and the ability to detect damage upon failure.

Zixuan Tang, Benyi Cao, Chrysoula Litina, Vahid Afroughsabet, Christos Vlachakis, Abir Al-Tabbaa (2023)Development of novel self-healing strain-hardening cementitious composites (SH2CC) for dynamic cyclic loading conditions using mineral and polymer admixtures, In: Cement & concrete composites142 Elsevier Ltd

Dynamic cyclic loading commonly encountered in engineering practice can pose great threat to the integrity and durability of concrete infrastructures. Ductile materials with enhanced self-healing performance could provide a promising solution under such conditions. This study proposes and investigates, for the first time, as self-healing SHCC (SH2CC) for dynamic reversed-cyclic (DRC) applications through incorporating a mineral substitute (MS) comprising of reactive MgO and quicklime and triethanolamine (TEA) additive in a high-volume-fly-ash matrix. Different DRC preloads were considered. Results indicated that applying 5% MS combined with 1.5% TEA additive showed optimal crack width control below 12 μm under up to 5 cycles of DRC preloading and the highest potential for mechanical strengthening of modulus of rupture, first cracking strength and flexural stiffness reaching up to 160%, 137% and 210% respectively after healing. Moreover the highest crack sealing efficiency was observed up to 88% under 14d water immersion. Microstructural analysis revealed that MS produced additional hydrates and promoted more carbonates to form in cracks during healing, while TEA effectively paused hydration under air curing but resumed the process upon water contact for massively enhancing crack healing (especially mechanical strengthening). Combining MS and TEA in SH2CC produced complementary healing products both in cracks and fibre-matrix interfaces indicating better filling and bridging of the cracks and demonstrating great potential for enhancing healing performance under aggressive loading conditions. •Novel self-healing SHCC (SH2CC) for dynamic cyclic applications was developed.•SH2CC with combined MgO–CaO-TEA led to optimal healing under dynamic cyclic loading.•SH2CC with combined MgO and CaO substitute improved crack sealing efficiency.•Self-healing SHCC with combined minerals and TEA had full mechanical recovery.•Remarkable healing was achieved with 14d water immersion and 7d air dry.

Meiyang Jiang, Ruixin Chen, Benyi Cao, Fei Wang (2023)The performance of temperature and acid-modified sludge in removing lead and cadmium, In: Environmental science and pollution research international30(30)pp. 76072-76084 Springer Nature

In the present study, the aluminum-containing wastewater treatment residue was modified at 400 degrees C and 2.5 mol/L HCl and used in the removal of Pb and Cd from an aqueous solution for the first time. The modified sludge was characterized by SEM, XRD, FTIR, and BET. Under the optimized conditions, including pH 6, adsorbent dose 3 g/L, Pb/Cd reaction time 120 and 180 min, and Pb/Cd concentration 400 and 100 mg/L, Pb/Cd adsorption capacity was obtained as 90.72 and 21.39 mg/g, respectively. The adsorption process of sludge before and after modification is more consistent with the quasi-second-order kinetics, and the correlation coefficients R-2 are all above 0.99. The fitting of data with the Langmuir isotherm and pseudo-second-order kinetics showed that the adsorption process is monolayer and chemical in nature. The adsorption reaction included ion exchange, electrostatic interaction, surface complexation, cation-pi interaction, co-precipitation, and physical adsorption. This work implies that the modified sludge has greater potential in the removal of Pb and Cd from wastewater relative to raw sludge.

Cong Zhang, P Hu, Qing Liu, Zeyu Lu, Benyi Cao, Yuxin Tang, Tianwei Hao (2023)Biopolymer recovery from waste activated sludge toward self-healing mortar crack, In: The Science of the total environment858(Pt 3)160107 Elsevier

Activated sludge (AS) offers great potential for resource recovery considering its high organic and nutrient content. However, low recovery efficiency and high costs are directing the focus toward the high-valuable resource recovery. This study extracted 71.5 ± 5.9 mg/g VSS of alginate-like exopolysaccharides from AS (ALE/AS) and applied it to mortar as a novel biopolymer agent for crack self-healing. With a mortar crack of 120 μm, addition of 0.5 wt% ALE/AS yielded a high crack closure ratio of 86.5 % within 28 days. In comparison to commercial healing agents, marginal flexural strength reduction with ALE/AS addition (17.9 % vs 30.2-50.5 %) was demonstrated. The abundance of COO group in GG blocks of ALE/AS resulted in a higher cross-link capacity with Ca , while the reduction of hydrophilic residues (e.g., COO and OH) after complexation engendered a lower swelling capacity, which facilitated self-healing and flexural strength maintenance. Molecular dynamics (MD) revealed that lower Ca diffusivity, arising from the stronger electrostatic interactions between the COO groups and Ca , resulted in a high Ca concentration around the cracks, leading to CaCO deposition and healed cracks. The outcomes of this study provided light on ALE-based mortar crack healing and presented a possibility for multi-level AS resource recovery.

A large amount of energy consumed globally is done by buildings, also, buildings are responsible for a great portion of greenhouse gas emissions. With progress in smart sensors and devices, a new generation of smarter and more context-aware building controllers can be developed. Consequently, zone-level surrogate artificial neural networks are used herein, where indoor temperature, occupancy, and weather data are the inputs. A new metaheuristic optimization algorithm, called Chaotic Satin Bowerbird Optimization Algorithm (CSBOA) is employed for the minimization of energy consumption. 24-hour schedules of the heating setpoint of each zone are created for an office building located in Edinburgh, Scotland. Two modes of optimization including day-ahead and model predictive control are applied for each hour. The consumption of energy decreased by 26% during a test week in Feb in comparison to the base case approach of heating. By definition of a time-of-use tariff, the cost of energy consumption is decreased by around 28%.

Yunhui Zhang, Benyi Cao, Hailong Yin, Lite Meng, Wei Jin, Fei Wang, Jian Xu, A Al-Tabbaa (2022)Application of zeolites in permeable reactive barriers (PRBs) for in-situ groundwater remediation: A critical review, In: Chemosphere (Oxford)308136290 Elsevier Ltd

Permeable reactive barrier (PRB) is one of the most promising in-situ groundwater remediation technologies due to its low costs and wide immobilization suitability for multiple contaminants. Reactive medium is a key component of PRBs and their selection needs to consider removal effectiveness as well as permeability. Zeolites have been extensively reported as reactive media owing to their high adsorption capacity, diverse pore structure and high stability. Moreover, the application of zeolites can reduce the PRBs fouling and clogging compared to reductants like zero-valence iron (ZVI) due to no formation of secondary precipitates, such as iron monosulfide, in spite of their reactivity to remove organics. This study gives a detailed review of lab-scale applications of zeolites in PRBs in terms of sorption characteristics, mechanisms, column performance and desorption features, as well as their field-scale applications to point out their application tendency in PRBs for contaminated groundwater remediation. On this basis, future prospects and suggestions for using zeolites in PRBs for groundwater remediation were put forward. This study provides a comprehensive and critical review of the lab-scale and field-scale applications of zeolites in PRBs and is expected to guide the future design and applications of adsorbents-based PRBs for groundwater remediation. [Display omitted] •A comprehensive and critical review on applications of zeolites in PRBs is provided.•Treatability testing for zeolites as reactive media is comprehensively reviewed.•The lab-scale and field-scale applications of zeolites in PRBs is reviewed.•This review can guide the future design and application of adsorbents-based PRBs.

Benyi Cao, Jian Xu, Fei Wang, Abir Al-Tabbaa (2023)Self-healing soil mix cutoff wall materials incorporating reactive MgO pellets, In: Géotechniquepp. 1-12

Recent decades have witnessed the increasing use of soil mixing technology to construct cut-off walls for groundwater control or underground pollutant containment. The strength and permeability of soil mix columns deteriorate considerably when subjected to chemical, mechanical and environmental stresses, leading to the need for repair of crack-originated damage. The development of self-healing grouts could provide more resilient and sustainable soil mix cut-off walls. In this study, for the first time, self-healing soil mix columns incorporating reactive magnesium oxide pellets were developed. The magnesium oxide-based cementitious grout was mixed with a sand soil using a laboratory-scale auger, and the embedded magnesium oxide pellets were ruptured upon cracking, producing expansive healing products due to hydration and carbonation, which can significantly fill the crack volume. The effects of magnesium oxide on the grout and soil mix samples were investigated in terms of both fresh and hardened properties. In addition, the self-healing performance was evaluated by examining permeability recovery and crack closure, and the microstructure and morphology of the magnesium oxide pellets and healing products were analysed. The results demonstrate the great potential of reactive magnesium oxide pellets as a self-healing agent for soil mix cut-off wall materials.

Xiao Chen, Xingyin Chen, Rong Su, Benyi Cao (2023)Optimization Analysis of Natural Ventilation in University Laboratories Based on CFD Simulation, In: Buildings (Basel)13(7)

In recent years, there has been a significant surge in the adoption of natural ventilation for building indoor spaces, garnering widespread attention. However, the research on human comfort optimization strategies closely related to the effect of natural ventilation is still relatively blank. Therefore, we have taken university laboratories as the research object and studied the use of CFD technology to construct numerical models. Based on previous research on the relevant theories of building ventilation and the impact of various air indicators on human comfort, we simulate the indoor airflow organization of buildings, and propose reasonable optimization design strategies based on simulation results and analysis conclusions. Compared to other studies on NV, we propose a completely new indicator, the Average rate of change in air age (ARCA), to assess the rate of improvement in air age. The results show that compared with the wind environment under basic conditions, the optimization strategy proposed by us increases the wind speed area suitable for human beings by about 14.3%, and reduces ARCA by about 53.3% at most.

Feng-Lei Du, Shi-Feng Lu, Jin-Bo Liu, Ben-Yi Cao, Xiao-Guang Zhao, An-Qi Mao, Ning Wang, Benyi Cao (2021)Field test of composite ground improved by different methods in red soil areas of China, In: Proceedings of the Institution of Civil Engineers. Geotechnical engineering174(3)1900269pp. 315-330 Ice Publishing

Large oil tanks are often vulnerable to weak bearing capacity and uneven settlement of foundations, which gives rise to potential losses of both lives and property. Although the pile foundation is commonly used for large oil tanks, its high costs make it less applicable in many cases. Recently, improved ground has been adopted as a tank foundation because it is highly cost-effective. In this study, typical composite grounds improved by dynamic compaction, dynamic replacement and cement fly-ash gravel piles were adopted to work as the foundation for ten large, heavy oil tanks under various geological conditions in red soil areas in China. The results relating to improvement of the composite ground were studied comparatively in detail using in situ tests, including the dynamic penetration test, plate-load test and water injection test. Field studies show that the bearing capacity of the composite ground after treatment is over 260 kPa within the improvement depth, meeting the design requirement for the long-term service of the ten oil tanks. The field test and relevant findings indicate a promising future for composite ground in relation to large, heavy oil tanks.

Shi-Jin Feng, Ben-Yi Cao, Xu Zhang, Hai-Jian Xie (2015)Modeling of leachate recirculation using vertical wells in bioreactor landfills, In: Environmental science and pollution research international22(12)pp. 9067-9079 Springer Nature

Leachate recirculation (LR) in municipal solid waste (MSW) landfills operated as bioreactors offers significant economic and environmental benefits. The subsurface application method of vertical wells is one of the most common LR techniques. The objective of this study was to develop a novel two-dimensional model of leachate recirculation using vertical wells. This novel method can describe leachate flow considering the effects of MSW settlement while also accounting separately for leachate flow in saturated and unsaturated zones. In this paper, a settlement model for MSW when considering the effects of compression and biodegradation on the MSW porosity was adopted. A numerical model was proposed using new governing equations for the saturated and unsaturated zones of a landfill. The following design parameters were evaluated by simulating the recirculated leachate volume and the influence zones of waste under steady-state flow conditions: (1) the effect of MSW settlement, (2) the effect of the initial void ratio, (3) the effect of the injected head, (4) the effect of the unit weight, (5) the effect of the biodegradation rate, and (6) the effect of the compression coefficient. The influence zones of LR when considering the effect of MSW settlement are smaller than those when neglecting the effect. The influence zones and LR volume increased with an increase in the injection pressure head and initial void ratio of MSW. The proposed method and the calculation results can provide important insight into the hydrological behavior of bioreactor landfills.

Xiaohan Wang, Benyi Cao, Guoqing Jiang, Tongxiao Shang, Jian Xu (2023)Non-Invasive Characterization of Subsurface Barriers Constructed via Deep Soil Mixing for Contaminated Land Containment, In: Sustainability (Basel, Switzerland)15(8) Mdpi

Deep soil mixing has been widely used to construct subsurface barriers (cut-off walls) in contaminated sites for contamination containment. Non-invasive geophysical methods are promising for the characterization and assessment of such barriers. The aim of this study was to assess and compare the characterization performance of four geophysical methods (i.e., electrical resistivity tomography, ground-penetrating radar, seismic imaging, and the transient Rayleigh surface wave method) for a subsurface barrier built using soil-mixing technology. The electrical resistivity tomography results show that the overall resistivity of the stratum on the barrier wall increased markedly, and local defects such as pockets of clay appeared as low-resistance anomalies on the resistivity profile. In contrast, the ground radar method failed to make a reasonable evaluation of the quality of the barrier wall because the surrounding environment caused great noise interference. The seismic mapping method had a better performance when the lateral geological conditions were studied. It is also suggested that to improve the signal-to-noise ratio of the surface wave signal, a vibrator with stronger energy should be used, and if conditions permit, the surrounding vibration sources should be shut down during geophysical tests. It is therefore recommended that decision makers and engineers consider using a combination of geophysical methods to evaluate the quality of barrier walls. They should also pay close attention to the specific geological conditions of a survey area, such as the presence of saltwater layers and interference from nearby structures, in order to choose the most appropriate method.

Shi-Jin Feng, Zhen-Bai Bai, Ben-Yi Cao, Shi-Feng Lu, Shu-Gang Ai, Benyi Cao (2017)The use of electrical resistivity tomography and borehole to characterize leachate distribution in Laogang landfill, China, In: Environmental science and pollution research international24(25)pp. 20811-20817

Leachate is a polluting liquid which may cause harmful effects on human health or the environment without a tightly control manner. The leachate management is an important part of the design and operation of bioreactor landfills. To detect the leachate distribution in Laogang Landfill, China, the measurement of electrical resistivity tomography (ERT) was carried out in three areas with different ages. ERT method proved to be an effective non-invasive geophysical method in bioreactor landfills, and the physical properties of waste samples obtained by boreholes were tested in a laboratory. The correlation between the resistivity and the moisture content was described by Archie's law. The result shows that the moisture content of fresh waste is inhomogeneous, while that of aged waste increases with depth. A pseudo 3D model of the moisture content was proposed to improve the understanding of leachate distribution and exhibit the accuracy of the ERT method.

Shi-Jin Feng, Ben-Yi Cao, Hai-Jian Xie, Benyi Cao (2017)Modeling of leachate recirculation using combined drainage blanket-horizontal trench systems in bioreactor landfills, In: Waste management & research35(10)pp. 1072-1083

Leachate recirculation in municipal solid waste (MSW) landfills operated as bioreactors offers significant economic and environmental benefits. Combined drainage blanket (DB)-horizontal trench (HT) systems can be an alternative to single conventional recirculation approaches and can have competitive advantages. The key objectives of this study are to investigate combined drainage blanket -horizontal trench systems, to analyze the effects of applying two recirculation systems on the leachate migration in landfills, and to estimate some key design parameters (e.g., the steady-state flow rate, the influence width, and the cumulative leachate volume). It was determined that an effective recirculation model should consist of a moderate horizontal trench injection pressure head and supplementary leachate recirculated through drainage blanket, with an objective of increasing the horizontal unsaturated hydraulic conductivity and thereby allowing more leachate to flow from the horizontal trench system in a horizontal direction. In addition, design charts for engineering application were established using a dimensionless variable formulation.

S. -J. Feng, B. -Y. Cao, Z. -B. Bai, Z. -Y. Yin (2016)Constitutive model for municipal solid waste considering the effect of biodegradation, In: Géotechnique letters6(4)pp. 244-249 Ice Publishing

With large-scale construction of landfills, the study of the properties of municipal solid waste (MSW) has become an important subject. The objective of this study is to propose a new constitutive model for MSW considering the effect of biodegradation. The indicators representing the degree of biodegradation are first discussed, and the impact of biodegradation on the mechanical properties of MSW is then formulated in volumetric strain by introducing a biodegradation-induced void change parameter, which provides a fundamental understanding of the change in waste properties with long-term biodegradation. On the basis of this, a constitutive model is developed following the framework of critical state soil mechanics, and the proposed model is validated by comparing simulations and experimental data of triaxial tests and of a long-term settlement experiment. Finally, the effects of MSW composition and biodegradation are analysed by parametric studies.

Shi-Jin Feng, Ben-Yi Cao, Xu Zhang, Yang Jiao (2015)Leachate recirculation in bioreactor landfills considering the stratification of MSW permeability, In: Environmental earth sciences73(7)pp. 3349-3359 Springer Nature

Leachate recirculation (LR) was pioneered in the USA in the 1970s as a means to enhance the degradation of landfill wastes, degrade or immobilize harmful compounds within the waste mass, and store excess leachate. LR offers many economic and environmental benefits to municipal solid waste (MSW) landfills. The main objective of this study was to investigate the migration law of LR and develop design guidelines for leachate recirculation systems consisting of spray irrigation when considering the stratification of MSW permeability. A bioreactor landfill will experience more rapid and complete settlement than a traditional landfill, which is mostly attributed to the weight of MSW. The waste at each layer in the landfill experiences a different pressure depending on the depth, which leads to vertical gradients in the void ratio and permeability coefficients of the waste in the landfill. To study LR in bioreactor landfills when considering the stratification of the MSW permeability, a new model was developed based on the law of conservation of mass, the modified Darcy's law, and Stoltz's settlement model. Using this model, the transport law of injected leachate into bioreactor landfills was determined under unsaturated conditions. The effects of various parameters (e.g., permeability stratification, compaction degree, initial void ratio, and injection intensity) on the water content of MSW and unit surface area recirculation were analyzed.

Shi-Jin Feng, Xu Zhang, Ben-Yi Cao, Benyi Cao (2014)Leachate recirculation in bioreactor landfills considering the effect of MSW settlement on hydraulic properties, In: Environmental earth sciences72(7)pp. 2315-2323 Springer Berlin Heidelberg

During leachate recirculation, a bioreactor landfill will experience more rapid and complete settlement, which is mainly attributed to the weight of municipal solid waste (MSW) and its biodegradation. The settlement of MSW may cause the decrease of void ratio of MSW, which will influence the permeability of MSW and the leachate quantity that can be held in bioreactor landfills. In this study, a new one-dimensional model of leachate recirculation using infiltration pond is developed. The new method is not only capable of describing leachate flow considering the effect of MSW settlement, but also accounting separately leachate flow in saturated and unsaturated zones. Moreover, the effects of operating parameters are evaluated with a parametric study. The analyzing results show that the influence depth of leachate recirculation considering the effect of MSW settlement is smaller than the value without considering the effect. The influence depth and leachate recirculation volume increase with the increase of infiltration pond pressure head and MSW void ratio. This indicates that the field compaction of MSW has a great influence on the leachate recirculation.

Shi-Jin Feng, Zheng-Wei Chen, Ben-Yi Cao, Benyi Cao (2016)Three-dimensional modelling of leachate recirculation using vertical wells in bioreactor landfills, In: Waste management & research34(12)pp. 1307-1315

Bioreactor landfills use leachate recirculation to enhance the biodegradation of municipal solid waste and accelerate landfill stabilisation, which can provide significant environmental and economic benefits. Vertical wells are operated as a major method for leachate recirculation systems. The objectives of this article are to analyse the leachate migration in bioreactor landfills using vertical wells and to offer theoretical basis for the design of leachate recirculation systems. A three-dimensional numerical model was built using FLAC-3D, and this model can consider the saturated and unsaturated flow of leachate within anisotropic waste to reflect the actual conditions. First, main influence factors of leachate migration were analysed, including the vertical well height, hydraulic conductivity, and anisotropic coefficient, in a single-well recirculation system. Then, the effects of different configurations of a group-well system were studied and the optimal well spacing was obtained. Some key design parameters (e.g. the recirculation flow rate, volume of impact zone, radius of impact zone and time to reach steady state) were also evaluated. The results show that the hydraulic conductivity has a great impact on the optimal height of vertical wells and uniform configuration is the best option in terms of both volume of impact zone and time to reach steady state.

Shi-Jin Feng, Ben-Yi Cao, Hai-Jian Xie, Benyi Cao (2017)Modeling of Leachate Recirculation Using Spraying–Vertical Well Systems in Bioreactor Landfills, In: International journal of geomechanics17(7)pp. 1-11 American Society of Civil Engineers

AbstractLeachate recirculation in municipal solid waste (MSW) landfills operated as bioreactors offers significant economic and environmental benefits. However, vertical well (VW) recirculation may not be an effective method because gravity will cause the recirculated leachate to flow directly downward to the leachate collection systems at the bottom of the landfill, resulting in a waste of recirculated leachate. One of the solutions to minimize the misapplication of leachate is to enable the leachate to flow in horizontal directions using combined spraying-VW systems, thus enabling it to flow through more MSW. The key objectives of this study were to investigate combined spraying-VW systems, to analyze the effects of applying spraying and VW recirculation systems simultaneously on the leachate migration in landfills, and to estimate some key design parameters (e.g., the steady-state flow rate, the influence radius, and the cumulative leachate volume). In addition, design charts for engineering application were established using a dimensionless variable formulation. The proposed design charts prove both simple and useful.

Benyi Cao, L. P. Souza, Jian Xu, Wenting Mao, Fei Wang, A Al-Tabbaa (2021)Soil Mix Cutoff Wall Materials with Microcapsule-Based Self-Healing Grout, In: Journal of geotechnical and geoenvironmental engineering147(11) Asce-Amer Soc Civil Engineers

Over the last three decades, cutoff walls using soil mix technology have been developed and deployed to deliver in situ containment of contaminated sites. The aggressive contaminated soil environment imposes significant long-term stresses on wall materials, and underground cracking is very difficult to detect and can compromise the integrity of walls. A recent relevant development is the concept of self-healing materials that can be triggered by damage and self-heal without the need for external intervention. This laboratory study developed, for the first time, a microcapsule-based self-healing soil mix cutoff wall material and demonstrated its performance in the healing processes, recovery of compressive strength, and hydraulic conductivity. The developed microcapsule-based cementitious grout was mixed with a sand soil using a laboratory-scale auger, and the embedded microcapsules were triggered upon cracking and released a sodium silicate cargo that healed the cracks. Micro-computed tomography (micro-CT) scan analysis verified the good survivability and uniform distribution of the microcapsules during the auger mixing process. Scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX) and thermogravimetric analysis (TGA) revealed that the released sodium silicate microcapsule cargo reacted with the cementitious matrix to produce healing products in the form of calcium silicate hydrates. The microcapsule-containing posthealing specimens regained 44% of initial compressive strength and showed a recovered hydraulic conductivity only slightly higher than that of the undamaged specimens. The results demonstrated the great potential of microencapsulated sodium silicate as a self-healing agent for cement mixed soil, which could provide more resilient and reliable soil mix cutoff walls.

Benyi Cao, Jian Xu, Fei Wang, Yunhui Zhang, David O'Connor (2021)Vertical Barriers for Land Contamination Containment: A Review, In: International journal of environmental research and public health18(23)12643 Mdpi

Soil pollution is one of the major threats to the environment and jeopardizes the provision of key soil ecosystem services. Vertical barriers, including slurry trench walls and walls constructed with soil mix technology, have been employed for decades to control groundwater flow and subsurface contaminant transport. This paper comprehensively reviewed and assessed the typical materials and mechanical and permeability properties of soil-bentonite, cement-bentonite and soil mix barriers, with the values of mix design and engineering properties summarized and compared. In addition, the damage and durability of barrier materials under mechanical, chemical, and environmental stresses were discussed. A number of landmark remediation projects were documented to demonstrate the effectiveness of the use of barrier systems. Recent research about crack-resistant and self-healing barrier materials incorporating polymers and minerals at Cambridge University and performance monitoring techniques were analyzed. Future work should focus on two main areas: the use of geophysical methods for non-destructive monitoring and the optimization of resilient barrier materials.

Benyi Cao, Livia Ribeiro de Souza, Abir Al-Tabbaa (2020)Organic Contaminant-Triggered Self-Healing Soil Mix Cut-Off Wall Materials Incorporating Oil Sorbents, In: Materials13(24)5802 MDPI

Soil mix cut-off walls have been increasingly used for containment of organic contaminants in polluted land. However, the mixed soil is susceptible to deterioration due to aggressive environmental and mechanical stresses, leading to crack-originated damage and requiring costly maintenance. This paper proposed a novel approach to achieve self-healing properties of soil mix cut-off wall materials triggered by the ingress of organic contaminants. Oil sorbent polymers with high absorption and swelling capacities were incorporated in a cementitious grout and mixed with soil using a laboratory-scale auger setup. The self-healing performance results showed that 500 µm-wide cracks could be bridged and blocked by the swollen oil sorbents, and that the permeability was reduced by almost an order of magnitude after the permeation of liquid paraffin. It was shown by micro-CT scan tests that the network formed by the swollen oil sorbents acted as attachments and binder, preventing the cracked mixed soil sample from crumbling, and that the oil sorbents swelled three times in volume and therefore occupied the air space and blocked the cracks in the matrix. These promising results exhibit the potential for the oil sorbents to provide soil mix cut-off walls in organically-contaminated land with self-healing properties and enhanced durability.

Benyi Cao, Yunhui Zhang, Jian Xu, A Al-Tabbaa (2022)Use of superabsorbent polymer in soil-cement subsurface barriers for enhanced heavy metal sorption and self-healing, In: The Science of the total environment831154708 Elsevier B.V

Conventional subsurface barrier materials for contamination containment deteriorate in aggressive environments and only have a limited exchange/adsorption capacity for heavy metals. This study focused on the potential use of superabsorbent polymer (SAP) in soil-cement subsurface barriers for enhanced heavy metal sorption and self-healing. The SAP adsorption results for lead, copper, zinc and nickel were well fitted by the Langmuir model. The SAP had the highest adsorption capacity for lead at 175 mg/g, and plays a key role in the removal of the heavy metals in an acidic environment. In addition, the incorporation of SAP in soil-cement increased the ductility and had negligible adverse effects on mechanical and permeability properties. When cracks propagate in the matrix, the SAP is exposed to the ingress of water and swells, and this swelling reaction seals the cracks. The SAP-containing soil-cement demonstrated enhanced self-healing performance in terms of the recovery of permeability. The uniform dispersion and the 3D network of the SAP were observed using micro-CT scanning, and good bonding and self-healing mechanism were confirmed by SEM-EDX analysis. The results suggest the significant potential for the SAP-based approach for the development of more resilient subsurface barriers with enhanced heavy metal sorption and self-healing. [Display omitted] •The SAP adsorption were well fitted by Langmuir model with Pb(II) sorption at 175 mg/g.•Brittle stress-strain behaviour of soil-cement changes to a ductile response with SAP addition.•After self-healing, the permeability of SAP-containing soil-cement recovered to the undamaged level.•SAP-based subsurface barriers have enhanced heavy metal sorption and self-healing performance.

Benyi Cao, Yunhui Zhang, A Al-Tabbaa (2022)SEBS-Polymer-Modified Slag-Cement-Bentonite for Resilient Slurry Walls, In: Sustainability (Basel, Switzerland)14(4)2093 Mdpi

In spite of the well-established design and construction approaches of slag-cement-bentonite slurry walls, the materials deteriorate inevitably in contaminated land. The development of effective materials which are sustainable, resilient and self-healing over the lifetime of slurry walls becomes essential. This study, for the first time, adopts a styrene-ethylene/butylene-styrene (SEBS) polymer to modify slag-cement-bentonite materials to enhance mechanical and self-healing performance. The results show that the increase in SEBS dosage results in significantly increased strain at failure, indicating the enhanced ductility thanks to the modification by the deformable polymer. The increased ductility is beneficial as the slurry wall could deform to a greater extent without cracks. After the permeation of liquid paraffin, the SEBS exposed on the crack surface swells and seals the crack, with the post-healing permeability only slightly higher than the undamaged values, which exhibits good self-healing performance. Scanning electron microscopy and micro-computed tomography analyses innovatively reveal the good bonding and homogeneous distribution of SEBS in slag-cement-bentonite. SEBS acts as a binder to protect the slag-cement-bentonite sample from disintegration, and the swollen SEBS particles effectively seal and heal the cracks. These results demonstrate that the SEBS-modified slag-cement-bentonite could provide slurry walls with resilient mechanical properties and enhanced self-healing performance.

Benyi Cao, A Al-Tabbaa (2022)Reactive MgO-based self-healing slag-cement-bentonite slurry walls, In: Cement & concrete composites131104565 Elsevier Ltd

Despite the common use of slag-cement-bentonite slurry trench walls for geotechnical and geoenvironmental applications, the materials deteriorate under mechanical, chemical and environmental stresses. Providing the slurry wall with the capacity of self-healing cracks, could address the concerns. This study proposes and investigates, for the first time, the self-healing slag-cement-bentonite incorporating reactive MgO pellets. The incorporation of the MgO pellets slightly increased the UCS and stiffness, without any significant adverse effects on the engineering properties of the slurry wall material. The crack closure percentages after hydration and carbonation reached over 50% and 80%, when the MgO pellets were added at a dosage of 5% and 10%. The post-healing permeability was as low as 8.4 × 10−9 m/s, satisfying the requirement for slurry walls. TGA and SEM-EDX results show that brucite and hydrated magnesium carbonates produced by the hydration and carbonation processes are the main healing products in the MgO-containing specimens. The uniform dispersion of MgO pellets and healing products was confirmed by micro-CT analysis, and it was observed most of the cracks could be at least partly blocked. The results have collectively demonstrated the great potential of reactive MgO pellets as an effective self-healing agent for the development of more reliable and resilient slag-cement-bentonite slurry walls.

Benyi Cao, Jingtao Chen, Abir Al-Tabbaa (2021)Crack-resistant cement–bentonite cut-off wall materials incorporating superabsorbent polymers, In: Canadian geotechnical journal58(6)pp. 800-810 NRC Research Press

Preserving the integrity of cement–bentonite cut-off walls, particularly in aggressive environments, is critical to their serviceability in polluted sites. The hardened cement–bentonite material in cut-off walls is highly susceptible to desiccation and wet–dry cycles, commonly leading to cracking. The objective of the work presented in this paper was to develop crack-resistant cement–bentonite cut-off wall materials subject to wet–dry cycles. Superabsorbent polymers (SAPs), which are cross-linked polymers that can absorb and retain a large amount of water and swell as a result, were employed for this purpose. It is found that the added SAPs increased the compressive strength by decreasing the water-to-cement ratio and that the strain at failure also increased due to energy dissipative and reinforcement effects. In addition, crack resistance was greatly improved under the imposed wet–dry cycles as the matrix suction was reduced as a result of the reduction of the contact between the free pore water and cement–bentonite particles and the increase of the pore size in the matrix. The morphology and microstructure of the interconnected foam network formed by the SAP films in the matrix were identified with SEM-EDX and micro-CT scan analyses. The results demonstrated the significant potential for SAPs in the development of crack-resistant cement–bentonite cut-off wall materials.

Benyi Cao, L. P. Souza, Fei Wang, Jian Xu, Chrysoula Litina, A Al-Tabbaa (2023)The first microcapsule-based self-healing cement-bentonite cut-off wall materials, In: Géotechnique73(2)pp. 105-114 Ice Publishing

Despite the extensive use of cement-bentonite in contaminated land containment applications, there are still many challenges related to durability. The development of self-healing cement-bentonite materials could provide more resilient, sustainable and reliable cut-off walls with significantly enhanced durability, reduced maintenance costs, enhanced safety and protection against sudden or undetected failure. The objective of this study was to develop self-healing cement-bentonite cut-off wall materials incorporating microcapsules. Microencapsulated sodium silicate, as a healing agent, would be released into cracks when the microcapsules rupture as a result of any damage incurred, and would react with the cement-bentonite matrix to fill and heal the cracks. Novel microcapsules with switchable mechanical properties developed for self-healing cement applications were employed here. The results demonstrated the enhanced average crack mouth healing and recovered permeability performance provided by the microcapsules compared to control cement-bentonite samples. X-ray microcomputed tomography and scanning electron microscopy were applied to investigate the self-healing process of the microcapsule-containing cement-bentonite system. The microstructural analysis confirmed the survivability, uniform dispersion and crack-triggered rupture of the microcapsules, as well as the release of the healing agent and the generation of hydration products within the cracks. These are promising results for the application of the microcapsule-based system for self-healing cement-bentonite cut-off wall materials.

Chrysoula Litina, Benyi Cao, Jingtao Chen, Zijing Li, Ioanna Papanikolaou, Abir Al-Tabbaa (2021)First UK Commercial Deployment of Microcapsule-Based Self-Healing Reinforced Concrete, In: Journal of materials in civil engineering33(6) Asce-Amer Soc Civil Engineers

Consolidating previous research on the development of novel microcapsules for self-healing in cementitious systems, this work forms a base for developing an implementation strategy and guidance for microcapsule-based self-healing technology. The study presents details of the first commercial deployment of this technology, as a ready-mix self-healing additive for commercial application. This involved the on-site construction of two slabs in a new development at the University of Cambridge. This paper describes the optimization of the mix, the structural concept and design, and processing and casting procedures. Prior to application, the compliance and compatibility of the healing additive with the concrete according to specifications and requirements of the design were investigated, validating the use of the developed system. These were complemented by large-scale laboratory testing of the healing efficiency under damage scenarios identified as critical for the on-site application. The performance of the site installation was monitored over 12 months through a combination of nondestructive testing methods. Results are presented with durability indicators confirming the in situ enhanced performance of the proposed self-healing system.

Yunhui Zhang, Fei Wang, Benyi Cao, Hailong Yin, A Al-Tabbaa (2022)Simultaneous removal of Pb and MTBE by mixed zeolites in fixed-bed column tests, In: Journal of environmental sciences (China)122pp. 41-49 Elsevier B.V

•Co-removal feasibility of MTBE and Pb with PRB was assessed in column tests.•Column performance of mixed zeolite reactive media was evaluated.•Clinoptilolite granule and powder were used to examine the effect of particle size.•Removal capacity of clinoptilolite powder almost tripled granule (130.6 vs 45.3 mg/g).•The minimum thickness and longevity became higher with clinoptilolite powder. The co-contamination of metals and organic pollutants, such as Pb and methyl tert-butyl ether (MTBE), in groundwater, has become a common and major phenomenon in many contaminated sites. This study evaluated the feasibility of their simultaneous removal with permeable reactive barrier (PRB) packed with mixed zeolites (clinoptilolite and ZSM-5) using fixed-bed column tests and breakthrough curve modeling. The effect of grain size on the permeability of PRB and removal efficacy was also assessed by granular and power clinoptilolite. The replacement of granular clinoptilolite by powder clinoptilolite largely reduced the breakthrough time but increased the saturation time nearly fourfold. The column adsorption capacity of clinoptilolite powders almost tripled that of clinoptilolite granules (130.6 mg/g versus 45.3 mg/g) due to higher specific surface areas. The minimum thickness and corresponding longevity of PRB were calculated as 7.12 cm and 321.5 min when 5% of granular clinoptilolite was mixed with 5% ZSM-5 and 90% sand as mixed PRB reactive media compared with 10.86 cm and 1230.2 min for the application of powder clinoptilolite. This study is expected to provide theoretical support and guidance for the practical application of mixed adsorbents in PRBs. Graphical Abstract [Display omitted] .