Energy, climate change, and waste management are among the crucial global challenges of the 21st century. Following collaboration among researchers and engineers in geotechnical and environmental engineering since the 1980s, a new discipline called geo-environmental engineering has been established with the aim of addressing problems such as waste disposal and the cleaning up of contaminated sites. In more recent years, due to increasing energy demands and depleting natural resources, geotechnics have started to play a major role in the exploration of new forms of energy such as shale gas and methane hydrate, and in energy conservation. This has given birth to a new branch of geotechnical engineering known as geo-energy.

In the USA, there are over 4 million miles (6 million km) of roadways and more than 250 million registered vehicles. Energy lost in the pavement system due to traffic-induced vibration and deformation is enormous. If effectively harvested, such energy can serve as an alternative sustainable energy source that can be easily integrated into the transportation system. It is well known that most piezoelectric materials are also pyroelectric materials, which convert temperature change into electricity. However, the potential of polyvinylidene fluoride (PVDF) as a hybrid piezo-pyroelectric energy harvester has been seldom studied. The uniqueness of this study lies in that the electrical responses of PVDF under coupled mechanical and thermal stimulations are investigated. Through a series of well controlled experiments, it is found that there exists an interesting coupling phenomenon between piezoelectric and pyroelectric effects of PVDF: the voltage generated by simultaneous mechanical and thermal stimulations is the algebraic sum of voltages generated by separate stimulations. This means that there is neither strengthening nor weakening coupling effect when the piezoelectric and pyroelectric phenomena are coupled. This also makes the modeling process of the hybrid piezoelectric and pyroelectric effect straightforward. An estimation of power generation through piezoelectric and pyroelectric effect is conducted, and the overall effects of temperature on hybrid piezo-pyroelectric energy harvesting are discussed.

The soils in seasonal frozen regions freeze and thaw frequently, causing severe frost heave and thaw settlement problems, which bring challenges to piles of photovoltaic stents. In this paper, laboratory tests are conducted with different types of screw piles under freezing conditions, with also using smooth piles for contrast. The aim is to simulate the freezing process of screw piles according to practical working conditions based on the similarity principle. Internal thermal resistance is ignored. The change laws of temperature, displacement, as well as the influence factors of types of screw piles are analyzed. The results indicate that: with a freezing depth of 30 cm, which is half of the pile length, large-double-bladed screw piles perform the best in anti-jacking-up, while all-bladed screw piles perform the worst, independent of the types of soil samples tested. The fitting relationship between jacking-up displacement and freezing depth is also proposed for each type of pile. Results obtained can provide an important reference to site construction in seasonal frozen regions.

Moisture retention capacity (MRC) is a key parameter for the prediction of leachate production of a municipal solid waste (MSW) pile. In this paper, five sets of laboratory tests were conducted in compression cells to characterize the variation of MRC with degradation time and overburden stress. Set A was conducted on the fresh high-food-waste-content (HFWC)-MSW under different degradation conditions and a sustained stress; Set B was on the fresh HFWC-MSW by alternation of degradation time and incremental stresses; Sets C, D, and E were on fresh HFWC-MSW, zero-food-waste-content (NFWC)-MSW, and decomposed MSW, respectively, being subjected to incremental stresses. The following findings were obtained from the test results: (1) The MRC of fresh HFWC-MSW decreased exponentially with degradation time under a sustained stress. The higher waste temperature or oxygen introduction would result in a faster declining of MRC. (2) The MRCs decreased linearly with a logarithmic increase of stress for all the MSW samples with different food waste contents. The MRC of HFWC-MSW was higher than that of NFWC-MSW under a given stress, and the decomposed MSW took the second place. (3) The variation of MRC appeared to be independent of stress path in terms of stress and degradation time. Based on the test results, the dependencies of the MRC of HFWC-MSW on degradation and stress were interpreted. Then, a time- and stress-dependent model was proposed for predicting the MRC of HFWC-MSW. The model was relatively simple and convenient for design purposes, and was verified by the measured data of leachate production at the pretreatment container of Laogang Incineration Plant. Finally, the model was developed to evaluate the dewatering effect of the HFWC-MSW pile. The strategy of combining the degradation-enhancing measures with stress-increasing measures is recommended in a rapid dewatering project.

The use of loess as an earthen final cover material is promising in northwest China which has an arid and semi-arid climate. A full-scale testing facility with an area 30 m long by 20 m wide was constructed at the Xi’an landfill of municipal solid wastes to investigate the performance of an inclined capillary barrier cover. The cover consisted of a compacted loess layer underlain by a gravel layer. The testing facility was well instrumented for a gas permeation test and recording of the soil conditions in terms of volumetric water content, pore gas pressure, and soil temperature. Tests were performed to measure the gas permeability of the compacted loess before and after the planting of vegetation on the cover. The field measurements demonstrate that the capillary break at the fine/coarse soil interface allows the upper compacted loess layer to retain more water, and conversely reduces its gas permeability, which is favorable for reducing landfill gas emissions. When the degree of saturation of the compacted loess was greater than 85%, the gas permeability decreased significantly with a further increment in volumetric water content. The growth of vegetation roots tended to fill the large pores in the upper loosely-compacted loess, resulting in a decrease in gas permeability of one order of magnitude. The influence of soil clods in the compacted loess on gas permeability can be one to two orders of magnitude due to an increase in pore size and a decrease in tortuosity.

The water vapor diffusion can be enhanced by the heating from municipal solid waste, and significantly impact the evaporation process in the earthen final cover. The parameters associated with the water vapor diffusion are usually measured by using the instantaneous profile method. This method is very time-consuming because the drying process lasts a long time. In this study, a bottom heating method is proposed to accelerate the drying process in a loess soil column. A constant temperature of 70 °C is applied at the bottom of the soil column. The thermo-hydraulic response of the loess is monitored along the soil column. A numerical model is developed to simulate the coupled thermo-hydraulic process. The numerical model is used to back analyze the tortuosity τ of the loess for vapor diffusion and the parameter a of an empirical evaporation function. We found that the bottom heating accelerated the drying process of the soil column by almost 22 d compared with the conditions without heating under the same evaporation boundary. Before Day 15, the proportions of the enhanced vapor flux in the total water loss were higher than 50%, dominating the evaporation process. The experimental and numerical study demonstrated that the proposed heating method is able to obtain the parameters of vapor diffusion more efficiently than the conventional method.

In the eastern coastal region of China, many operating highways built over soft ground are vulnerable to severe post-construction settlement. In this study, a technique using jet grouted piles is developed to mitigate post-construction settlement. The piles are installed by drilling boreholes throughout the embankment. The principal stages of the construction process are described, and two field tests in China, on the Lianyan and Linhai highways, are presented. The results revealed that ground heaves of up to 219 mm and 337 mm induced during a short construction period were able to mitigate the settlement of the embankment. The average settling rate was significantly reduced from 60 mm/y to 9 mm/y on the Lianyan highway. Lateral ground displacement on the Linhai highway increased with time during construction, but after construction showed a slight reduction associated with the dissipation of excess pore water pressure. An analytical method was adopted to describe the ground heave due to the jet grouted piles. The ground heave increased with grout pump pressure, but decreased as the embankment load and distance from the pile center increased.

Soil liquefaction can cause disastrous consequences to buildings and human lives. Regular countermeasures against soil liquefaction are often overly expensive for normal buildings and structures. This could be the major reason that liquefaction induced damage is still widely encountered in large- and mid-size earthquakes in recent years. In this paper, a new method for the mitigation of soil liquefaction using the microbially induced soil desaturation is proposed and tested. The desaturation effect in soil is achieved by the generation of nitrogen gas produced from the microbial denitrification process. Some major issues related to this method are experimentally investigated. These include soil desaturation procedures, shapes and distribution of gas bubbles in soil, mechanical responses and liquefaction resistance of desaturated soils, and stability of gas in soils. The desaturation treatment of soils is made simply by introducing denitrifying bacteria and a desaturation solution into soil pores by mixing, flushing, or injection. The degree of saturation can be reduced as the microbial reaction proceeds. Experimental results show that the final degree of saturation is related to the initial nitrate concentration added to the soil: the higher the concentration of nitrate in the desaturation solution, the lower the degree of saturation that can be achieved. The existence of gas bubbles in soil is evidenced by computer tomography (CT) technology. The CT images reveal that gas is in the form of small pockets which has a size a little larger than the mean size of sand grains. It is shown in the shaking table tests that microbially induced desaturation can effectively improve the liquefaction resistance of soil by showing a much lower pore pressure generation, much smaller volumetric strain, and much smaller settlement of the structure in desaturated soil, as compared with those in saturated soil. Triaxial consolidated undrained tests reveal that the desaturation treatment of soil can improve the undrained shear strength of loose sand. The stability of gas is tested under hydrostatic and water flow conditions. The gas phase is stable under the hydrostatic condition, but unstable under water flow conditions. So measures ought to be taken to prevent steady flow in practice.

Considering the great importance of the elastic shear modulus G₀ of unsaturated soils to the serviceability of many geo-structures in geo-energy and geo-environmental engineering, some semi-empirical models have been reported for the G₀ of unsaturated soils. Existing models require at least three parameters and the calibration of the model parameters requires extensive time-consuming unsaturated soil tests. In this study, a simple semi-empirical model is proposed for the hysteretic G₀ of unsaturated soils, requiring only two parameters. The constitutive variables of the mean Bishop’s stress and a bonding variable are adopted for considering the average stress between soil particles and the additional normal forces between soil particles provided by water menisci. The derived equation is applied to simulate the G₀ of unsaturated silts and sands. Comparisons between the measured and calculated results demonstrate that the proposed equation is able to describe the influences of various factors on G₀, including mean net stress, suction, wetting-drying, and void ratio.