Pore features and seepage characteristics of mixed granular materials are of great significance in many subjects, including engineering, for instance geotechnical engineering, petroleum extraction, hydrogeology, environmental science and hydraulic engineering. In this paper, two types of mathematical models are proposed that consider different packing methods for porosity and permeability – namely, the filling model (FM) and the replacement model (RM). On this basis, incomplete coverage (overlap between large and small grains during the replacement process) is considered as the ‘replacement model considering incomplete coverage’ (RMCIC) and the ‘replacement model considering incomplete coverage and roundness’ (RMCICR). Roundness is considered in the ‘filling model considering roundness’ (FMCR), the ‘replacement model considering roundness’ (RMCR) and the RMCICR. Four kinds of natural sand are chosen as the source material, with median grain sizes of 1733, 1050, 449 and 190 μm. All models can be divided into two components – one in which the porosity decreases with large grain ratios and the other in which the porosity increases. When roundness is considered, the RMCICR model has the largest porosity, and the FMCR model has the lowest porosity. In addition, the porosity increases when roundness is considered for all models in this study. The porosity in the RMCICR model better coincides with the test results, which can be explained by the consideration of both incomplete coverage and roundness. The order of porosity values is as follows: RMCICR, RMCR, RMCIC, RM, FMCR and FM. Permeability presents a tendency similar to porosity – that is, permeability decreases first and then increases. Moreover, the slope in the declining stage is relatively small, and the slope in the ascending stage is relatively large. The ratio of large grains to minimum porosity and permeability has significant differences. The results suggest the effectiveness of previous mathematical models.

The existence of a large number of abandoned salt caverns in China has posed a great threat to geological safety and environmental protection, and it also wasted enormous underground space resources. To address such problems, comprehensive utilization of these salt caverns has been proposed both currently and in the future, mainly consisting of energy storage and waste disposal. Regarding energy storage in abandoned salt caverns, the storage media, such as gas, oil, compressed air and hydrogen, have been introduced respectively in terms of the current development and future implementation, with site-selection criteria demonstrated in detail. The recommended burial depth of abandoned salt caverns for gas storage is 1000–1500 m, while it should be less than 1000 m for oil storage. Salt cavern compressed air storage has more advantages in construction and energy storage economics. Salt cavern hydrogen storage imposes stricter requirements on surrounding rock tightness, and its location should be near the hydrogen production facilities. The technical idea of storing ammonia in abandoned salt caverns (indirect hydrogen storage) has been proposed to enhance the energy storage density. For the disposal of wastes, including low-level nuclear waste and industrial waste, the applicable conditions, technical difficulties, and research prospects in this field have been reviewed. The disposal of nuclear waste in salt caverns is not currently recommended due to the complex damage mechanism of layered salt rock and the specific locations of salt mines in China. Industrial waste disposal is relatively mature internationally, but in China, policy and technical research require strengthening to promote its application. Furthermore, considering the recovery of salt mines and the development of salt industries, the cooperation between energy storage regions and salt mining regions has been discussed. The economic and environmental benefits of utilizing abandoned salt caverns have been demonstrated. This study provides a solution to handle the abandoned salt caverns in China and globally.

Strength and stiffness of stabilized/solidified (S/S) sediments are supposed to be degraded by freeze–thaw cycles (FTs) when applied in constructions in cold regions. In order to reveal the degradation mechanisms, FTs were first simulated on S/S sediments in the laboratory. Then, macrotests including unconfined compression strength (UCS) tests and direct shear tests were conducted on S/S sediments at different FTs to investigate the strength and stiffness changes and microanalyses including scanning electron microscope analysis (SEM), mercury intrusion porosimetry (MIP), and nitrogen adsorption porosimetry (NAP) to investigate the microstructure changes. Macrotests show that strength indexes decrease gradually, stiffness parameters and brittleness increase first and then decrease, the compaction stage of stress–strain curves diminishes first and then increases, the decrease rate of friction angle decelerates first and then accelerates, and the decrease rate of cohesion decelerates first and then keeps stable, for S/S sediments in FTs. Microanalyses have precisely captured the damage chain of S/S sediments: meso and macropores inside sediment aggregates expand gradually during FTs; as a result, sediment aggregates are fractured and stripped off from the solidification skeleton gradually; losing support of sediment aggregates, solidification skeletons are adjusted first and then collapsed when sediment aggregate fracture reaches a limit value. Based on macro and microfindings, a damage model to explain strength and stiffness degradations by microstructure damage is successfully established. Based on the established damage model, a reasonable idea is proposed for promoting the resistance of S/S sediments to FTs.

Stabilized/solidified (S/S) sediments are eroded by dry–wet cycles (DWs) when applied in an atmospheric environment. The microporous structures of S/S sediments, including their size, shape, and distributions, are sensitive to DWs and closely related to their macro properties. Therefore, multiscale micropore measurements, including scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP), and nitrogen adsorption porosimetry (NAP), were conducted on S/S sediment samples subjected to different DWs to elucidate the micro-damage mechanisms of S/S sediments under DWs, in conjunction with unconfined compression strength (UCS) tests. The results indicated that, as DWs increased, the strength of the S/S sediments decreased, and pores/cracks developed due to the expansion of the calcium silicate hydrate (CSH) skeleton pore structures and the shrinkage of the sediment aggregate pore structures. Pores that accounted for most of the volume were those in the sediment aggregates in the range of 10² nm < d < 10⁴ nm (d represents the pore diameter), while smaller pores in the range of d < 10² nm contributed 90% of the pore area. Pores in the CSH skeletons (d > 10⁴ nm) increased with DWs, while those in the sediment aggregates decreased with DWs due to the expansion and shrinkage forces originating from the sediment particles with pore sizes of d < 10 nm. The plastic deformation of the pores in the CSH skeletons and sediment aggregates jointly controlled the strength of S/S sediments, and the adjustment of those pores gradually reduced the decrease rate of UCS. The revealed damage mechanisms of S/S during DWs provide theoretical foundations for optimizing S/S additives and expanding the engineering applications of the S/S sediments.

In applications of stabilized/solidified (S/S) sediments, the organic matter inside the original sediments will decompose and release fulvic acid (FA) and humic acid (HA) gradually, which may reduce the durability of the S/S sediments. It is critical to understand the long-term internal erosion mechanism to improve the durability of S/S sediments. Thus, S/S sediments solidified by commonly used cement, lime, and metakaolin were first prepared in the laboratory. Reasonable proportioning of the additives was determined through orthogonal tests. Unconfined compression strength tests, measurements of organic environment indexes and chemical environment indexes, and scanning electron microscopy analysis were then conducted after different curing periods of the S/S sediments to explore the erosion mechanism. The following results and conclusions were obtained. 1) The microstructure of S/S sediments is composed of a sediment aggregate core, a hydrated calcium silicate (CSH) shell covering the core, and calcium hydroxide (CH) submerged in the CSH shell. 2) HA and FA are released gradually by organic matter decomposition inside the core and are consumed by neutralization reactions with solidification products. 3) The dissipation process of organic matter and the induced damage process of S/S sediments can be partitioned into three stages: in the first stage, HA and FA jointly neutralize the solidification products inside the CSH shell, the CSH shell is eroded gradually, and the S/S sediments lose strength gradually; in the second stage, the neutralization reactions are still inside the CSH shell, but only occur between FA (not HA) and the solidification products, the CSH shell breaks down at the end of this stage, but the S/S sediments do not lose much strength until the end point; and in the third stage, neutralization reactions take place outside the CSH shell between HA (but not FA) and the solidification products, and the S/S sediments lose strength dramatically. It was also concluded that HA (rather than FA) is more likely to neutralize solidification products in a more alkaline environment; HA has much stronger chemical affinity with Ca²⁺ ions than FA; HA controls the strength loss of S/S sediments, whereas FA controls the working status of HA; and the developments of HA content and Ca²⁺ ion concentration reflect the strength loss of S/S sediments.