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Arbuscular mycorrhizal fungi (AMF) typically provide a wide range of nutritional benefits to their host plants, and their role in plant water uptake, although still controversial, is often cited as one of the hallmarks of this symbiosis. Less attention has been dedicated to other effects relating to water dynamics that the presence of AMF in soils may have. Evidence that AMF can affect soil hydraulic properties is only beginning to emerge. In one of our recent experiments with dwarf tomato plants, we serendipitously found that the arbuscular mycorrhizal fungus (Rhizophagus irregularis ‘PH5’) can slightly but significantly reduce water holding capacity (WHC) of the substrate (a sand–zeolite–soil mixture). This was further investigated in a subsequent experiment, but there we found exactly the opposite effect as mycorrhizal substrate retained more water than did the non-mycorrhizal substrate. Because the same substrate was used and other conditions were mostly comparable in the two experiments, we explain the contrasting results by different substrate compaction, most likely caused by different pot shapes. It seems that in compacted substrates, AMF may have no effect upon or even decrease the substrates’ WHC. On the other hand, the AMF hyphae interweaving the pores of less compacted substrates may increase the capillary movement of water throughout such substrates and cause slightly more water to remain in the pores after the free water has drained. We believe that this phenomenon is worthy of mycorrhizologists’ attention and merits further investigation as to the role of AMF in soil hydraulic properties.

Water uptake dynamics of superabsorbent polymers (SAP) in soil is of key importance for the optimum application of these materials in environmental engineering and agriculture, so goal of this paper is to determine time dependent values of coefficient of permeability for various SAP-soil mixtures. Retaining water in soil is a key requirement in critical zones to support plant growth. There is an urgent need for technologies that can increase soil water retention, given the increasing prevalence of droughts and scarcity of clean water as the climate changes, combined with the rising demand for food by a growing world population. SAPs are materials that can absorb significant amounts of water, and thus have tremendous potential to help increase water retention in soil. However, while some studies have characterized the equilibrium swelling behavior of SAPs in soil, how their addition influences the time-dependent flow of water through soil remains poorly understood. Here, we address this gap in knowledge by directly measuring the coefficient of permeability of SAP-soil mixtures, testing different soil grain sizes, SAP grain sizes, and different SAP-soil ratios. We find that SAP addition can dramatically hinder the flow rate of water through soil—reducing the permeability by several orders of magnitude, and in some cases causing complete blockage of water infiltration, at mass fractions as small as 1%. In this scenario coefficient of permeability of 1.23 × 10−4 m/s dropped by a factor of ~10 after 14 min, a factor of ~100 after 36 min, and by nearly a factor of ~1000 after 63 min, eventually causing complete blockage of infiltration after 67 min. Authors concluded that in this particular situation the size and quantity of SAP particles was enough to nearly completely fill the available pore space resulting in rendering the soil column almost completely impermeable. Moreover, we demonstrate that these effects are well-described by a simple hydraulic model of the mutual interactions between SAP and soil grains, providing more generally-applicable and quantitative principles to model SAP-soil permeability in applications. Ultimately, this work could help evaluate the optimal proportions and grain sizes of SAPs to use for a given soil to simultaneously achieve a desirable permeability along with increased water holding capacity in the plant root zone.

Rubber-soil mixtures are known to have mechanical properties that enable their use in backfills, road construction or geotechnical seismic isolation systems. The complexity of these mixtures comes from adding soft (i.e. rubber) particles that increases the number of particle properties to consider when studying the macroscopic behaviour. The distinction between sand-like and rubber-like behaviour is normally presented in relation to the rubber content and size ratio between particles. It is however unknown how the change on the mixture gradation affects the mechanical behaviour of RSm. Entropy coordinates condense the entire particle size distribution (PSD) to a single point on a Cartesian plane, accounting for all the information in the gradation. Grading entropy coordinates have been used to study typical geotechnical behaviours of mostly incompressible (i.e. sand) soils. In this study, entropy coordinates are used to analyse the correlation between the small-strain stiffness and liquefaction susceptibility of RSm and their PSDs. The results suggest that entropy coordinates can be used effectively on RSm as an alternative means of assessment of typical soil behaviours, being also able to distinguish between sand-like and rubber-like behaviours. Based on the 30 PSDs analysed, it is also evidenced that internal stability criterion proposed by Lőrincz (1986) can be used to predict the liquefaction susceptibility of RSm. The normalised base entropy (A) has also been shown to increase with the rubber content, which is linked to a lower liquefaction susceptibility, due to the supporting effect of rubber particles on strong-force chains formed of sand particles.

Phosphorus (P) is an essential element for all life forms, and P-availability thus an important driver of a functioning agriculture. However, phosphate rock resources for P-fertilizer production are only available in a few countries. Therefore, P-recovery from waste materials has become of increasing interest during the last decade and has been investigated worldwide. In order to characterize potential novel P-fertilizers made from recycled materials, a large array of P-compound characterizations, chemical extractions and growth experiments were performed. This review bundles the work carried out in that field over the last years. Overall, P-fertilizers from recycled materials show a broad range of P-compounds with very different chemical structure and solubility. Growth experiments performed to assess their fertilizing effects display high variations for most of the products. While these experiments have demonstrated that some fertilizers made of recycled materials may reach P effects in the same order of magnitude as water-soluble phosphate rock-based fertilizers, an important limitation in their interpretation is the fact that they often vary considerably in their experimental design. The existing data show clearly that standardization of growth experiments is urgently needed to achieve comparable results. Standard chemical extractants used to assess the chemical solubility of P-fertilizers were found to be of limited reliability for predicting plant P uptake. Therefore, alternative methods such as sequential fractionation, or the extraction of incubated soil/fertilizer mixtures with standard soil extractants or with P sink methods should be tested more intensively in the future to provide alternative options to predict the P-availability of fertilizers from recycled materials.

Soil-rock mixture is a commonly used geotechnical material used in many construction projects, such as slopes, tunnels, and dams. The shear strength of soil-rock mixture is its key property and is affected by many factors. This study aimed to investigate the shear strength characteristics of soil-rock mixture and the influences of moisture and stone content on shear strength parameters. Soil-rock mixture samples with four different stone and moisture contents were fabricated and tested using a large-scale direct shear test apparatus under four vertical pressures. The results demonstrated that the shear properties of the soil-rock mixture showed significant Mohr Coulomb failure criteria for all stone contents. As the moisture content increased, the shear strength of the soil-rock mixture first increased by 10~18% and then decreased after w = 12% to the residue value. The change in cohesion and internal friction angle of soil-rock mixture with different moisture contents shared a similar trend. For w < 12%, the cohesion and internal friction angle increased with moisture content, and for w > 12%, the two indexes obviously decreased. As the stone content increased from 30% to 60%, the shear strength of the soil-rock mixture increased by 82~174%. The internal friction angle increased linearly with stone content, while the cohesion of the mixture first increased and then decreased after the stone content reached 50%. The results can help in the designation and application of soil-rock mixture.

In the analysis of stability for soil–rock mixture (S-RM) slopes, the limit equilibrium method (LEM) has limitations in capturing the properties of discrete materials. Therefore, this study proposes the use of the discrete element method (DEM) to analyze the stability of S-RM slopes. A novel approach for building a discrete element model of S-RM slopes is presented, which enables the development of S-RM slope models with varying rock contents, block size distributions, and compactness. The factor of safety (FoS) for S-RM slopes is proposed based on the gravity overload coefficient, and a slope stability criterion in the discrete element calculation cycle is established. The results indicate that for rock contents below 60%, the FoS for S-RM slopes is lower than that for pure soil slopes, suggesting that a limited number of rock blocks may potentially have a negative impact on slope stability. Only when the rock content is greater than 60%, do the rock blocks contact each other and form force transmission between the main rock skeleton, which can enhance the stability of S-RM slopes. In addition, there is a significant localization of deformation in the shear zone, as evidenced by the larger particle rotation compared to other areas.

An interlayer existed between the ballast layer and subgrade in the conventional railway substructure. Considering that the shear strength τ of the interlayer soil was influenced by the changes in the ballast grain content and water content, this aspect was explored in the present study. Monotonic triaxial tests were fulfilled, which considered five coarse grain contents fv and three water contents of fine soil wf. The results showed that the growth in fv contributed to an increment in τ of the soil mixture under both saturation and unsaturation. Conversely, in previous studies, the growth of fv induced an increment in τ under saturation, but a decline in that under unsaturation. This was explained by the competing influences of fv and suction ψ: in previous studies, increasing fv induced a decline in the dry density of the fine soil fraction ρd–f, which contributed to a decline in ψ. When the negative influence of declining ψ outweighed the positive influence of the incrementing fv, the τ of the soil mixture decreased. Meanwhile, modelling of the τ–ψ relationship in the soil mixture with varying fv was performed. This proposed model was examined using the test results from both the present and previous studies, which shows its reasonably good performance.

Inertial instability-induced slope failure is one of the most destructive seismic hazards encountered in China. Depositional slopes are widely distributed across the south-western part of China. To study the seismic performance of rock–soil mixture deposit (RSMD) slopes in detail, physical and mechanical tests of air-dried RSMD, and a 1:50 geometric scale, generalised RSMD slope centrifugal shaking table modelling tests were conducted. The acceleration response, slope deformation, and instability mode of the air-dried RSMD slope under the continuous action of four earthquakes with different intensities are analysed. Besides, the dynamic finite-difference numerical simulations were performed with FLAC to simulate the centrifuge model test. The numerical simulation results were compared with the test results in terms of the horizontal acceleration response, time history of crest settlements, and the slope failure characteristics. The results show that the horizontal peak ground acceleration (PGA) amplification factors of the RSMD slope reflect typical elevation, and surface, amplification effects. In addition, the spectral distribution of the seismic wave has changed significantly with propagation along the slope elevation. The energy of input seismic wave in a frequency range close to the natural frequency of an RSMD slope is significantly amplified. During the test, it was observed that the failure of an air-dried RSMD slope occurred when the PGA reached 0.216 g, and the settlement at the crest of the slope was obvious. The failure mode was dominated by shallow collapse.

Based on the 3D laser scanning technology, the surface morphologies of the rock blocks collected from the soil–rock mixture (S–RM) were scanned. The numerical model of the S–RM with different rock block proportions were established. To study the deformation and failure characteristics of the S–RM during the formation and evolution of the shear band, the numerical large-scale direct shear tests under different normal stresses were performed on these S–RM samples, the failure points were recorded by the detection of contact failure. Base on the step accumulation of particle’s spin, the rotation angles of rock blocks were monitored in the simulation. A parameter Pθ is introduced to describe the evolution characteristics of the shear band. Results show that the, Pθ of the S–RM with the rock block proportion of 60% is obviously greater than the others. The formation and evolution process of the shear band in the S–RM was consistent with the evolution process of the local particle’s rotation. The rotation of the soils always preceded the rotation of the rock blocks, which reflects the “bully” deformation characteristic of S–RM in the shear band formation process. In the form of the failure distribution, the interior failure of the S–RM with low rock block proportions was mainly the thin band-shaped destruction between the soils. However, higher rock block proportion caused more interactions between soils and rock blocks in the S–RM, and the destruction presented a thick cloud-like distribution. The shear fractures in the soils and tensile fractures along the contact planes between soils and rock blocks are the main failure pattern of S–RM under shear tests.

Soil rock mixture (SRM) is a common geo-material, which usually needs to be treated by grouting reinforcement. The grouting effect is tied to the grouting method, grout property, and geological condition, which are difficult to be quantitatively analyzed. The main obstacle in developing a rigorous theory for evaluating the grouting effect is the heterogeneity of SRM. This paper aims at developing a preliminary theoretical model and an empirical formula for predicting the overall strength of grouted SRM based on the actual morphological structure of grout-rock skeleton and its heterogeneity. A discrete element model was established for analysis of grout vein structures and validation of the theoretical approach. Two morphological parameters called “grout vein uniformity” and “block-skeleton conversion ratio” were proposed to quantify the influence of spatial distribution of grout vein and rock blocks in grouted SRM. Finally, an empirical formula was established for estimating the uniaxial compressive strength of grouted SRM, with a complete description of rock conversion ratio as a function of grout proportion and rock proportion. The ability of the approach to capture the influence of the grouting effect was verified by comparing the predicted values with the numerical values and the laboratory test results of this study. The improvement of the mechanical property of the stratum can be quickly assessed according to the obtained correlations as a general rule. Specifically, the normalized UCS increases in a linear fashion over grout proportion at a rock-soil ratio between 0.6 and 0.8. However, the theoretical model may overestimate the strength when the rock-soil ratio is higher than 0.9. It is open to improvement by further studies for the systemization of a more rigorous and robust approach in estimating and accommodating the uncertainties when applied in grouting operation guidance in the real world.