Mechanical behaviour and shear localisation of gravel soils: experimental study and numerical modelling

Gravelly soils, characterized by a distinctive combination of coarse gravel aggregates and fine soil matrix, are widely distributed and play a crucial role in geotechnical engineering. This study investigates the mechanical behavior of gravelly soil subjected to simulated freeze-thaw (F-T) cycles using triaxial compressive strength tests. The long-term deviatoric stress response of specimens with varying gravel content and initial water content was analyzed under three distinct effective confining pressures (100, 200, and 300 kPa) across different F-T cycles. The results indicate that compressive strength is significantly influenced by gravel content, initial water content, and confining pressure. Notably, the rate of increase in deviatoric stress does not exhibit a proportional rise under confining pressures of 200 kPa and 300 kPa after 40 F-T cycles. However, a direct correlation is observed between deviatoric stress and increasing confining pressure (100, 200, and 300 kPa) over 2-, 4-, and 6-day intervals, this effect is more pronounced at higher confining pressures. The deviatoric stress peaks at different strain thresholds depending on the applied confining pressure; furthermore, no evident strain-softening behavior is observed across the tested conditions. These findings suggests that higher confining pressure inhibits particle displacement and interlocking failure, thereby reducing both the void ratio and axial strain within the soil matrix. Overall, these insights enhance our understanding of the complex interactions among gravel content, water content, confining pressure, and freeze-thaw effects, contributing to the understanding of the compressive strength evolution in gravelly soils under cyclic environmental loading.

Hydraulic conductivity of gravelly soils with various coarse particle contents subjected to freeze–thaw cycles

Study on the Tensile Properties and Application of Gravelly Soil Reinforced by Polypropylene Fiber

The standard Proctor test has been widely used and accepted for characterizing soil compatibility for field compaction control. This paper presents a time- and cost-effective method to predict standard Proctor compaction characteristics of non-gravel and gravelly soils using a proposed small apparatus. This small apparatus is similar to the standard Proctor apparatus and easily introduced into soil mechanics laboratory. A comparison of the compaction characteristics of non-gravel soils measured from the proposed small apparatus and from the standard Proctor apparatus shows that this small apparatus can be used as an alternative to the standard Proctor apparatus and regarded as a practical tool for non-gravel soils. For gravelly soils, gravel content mainly controls compaction energy transmitted to the fine fraction and hence its compaction characteristics. A relationship between the generalized optimum water content of the fine fraction in the gravelly soil and the gravel content is established. This relationship leads to an effective method of predicting standard Proctor compaction characteristics of gravelly soils compacted in standard molds using compaction results from the proposed small apparatus. Comparisons between the predicted and the measured compaction characteristics are in very good agreement.

Scaling law for correcting the gravel content effect due to scalping techniques by DEM investigations

With frequent shocks in Taiwan, located on the Pacific earthquake belt, it is found that injury and structural destruction are often induced by soil liquefaction. From past experience, it is known that soil liquefaction generally occurs in saturated sand or silty fine sand. However, in the areas of Wufeng, Nantou and Armenia where gravelly soils were liquefied, further studies need be conducted on the liquefaction potential of this soil. The Fu Tin Bridge site in Wufeng, where liquefaction occurred during the Chi‐Chi Earthquake, was selected for the liquefaction study in a gravelly deposit. Extensive in situ index tests, including field density, grain size distribution, and water content were conducted in the testing trench. Furthermore, a series of remolded large scale cyclic triaxial tests (15 cm in diameter, 30 cm in height) using the same gravelly soil were performed to study the variation of liquefaction resistance, due to relative density and gravel content. Another series of tests for pure sand in different relative densities were performed to compare the effect of gravel content. The test results show that an increase in the gravel content or the relative density increases its liquefaction resistance, and decreases the axial strain. In addition, the liquefaction resistance of soil with gravel content in the range of 20% to 40% with relative density of 40% shows the same strength as those of pure sand with relative density between 50% and 70%. Finally, a regression equation between the variation of parameters (gravel content and relative density) and the liquefaction resistance can be obtained for gravelly liquefaction assessment.

The measurement of bulk density in gravelly soils (>15% soil particles >2 mm) is more time‐consuming than for other soils. The excavation method, usually employed for measurement of bulk density in gravelly soils, includes excavating a void and calculating volume of the void from the weight and density of the material (e.g. sand and plaster cast) used to fill the void. A 3‐dimensional (3D) scanning system was developed to measure the volume of the void created when using the excavation method. The 3D scanning system combined a time‐of‐flight camera (Kinect ™), the KinectFusion algorithm, MeshLab and a portable computer to produce a 3D model of the void or plaster cast. Experiments were completed at three field sites where soil gravel (>2 mm) content ranged from 35 to 71% to assess the performance of the system. The void volume measured using the 3D scanning system was highly correlated with measurements using the plaster cast method (r = 0.99). The cumulative time taken to measure soil bulk density using 3D scanning was significantly (P < 0.001) less than for the sand replacement at 0–10, 10–20, 20–30 and 30–40 cm depth. The faster measurement of subsurface bulk density is a significant advantage of the 3D scanning system; the time taken to measure bulk density to 40 cm in 10 cm increments using the 3D scanning system was about one‐third of the sand method.

Soil gravel content strongly affects ecological restoration; however, the response and mechanism of plant traits to soil gravel content under the sensitive and fragile natural environment of Qinghai‐Tibet Plateau remains unclear. Herein, soils with three gravel content (10%, 30%, 50%) in the southeastern Tibetan Plateau were selected, and three plant species (one native plants of Elymus dahuricus (Ed), and two introduced ones of Festuca elata (Fe) and Medicago sativa (Ms)) were used in seven planting patterns with different proportions (Fe, Ed, Ms, Fe + Ed (1:1), Fe + Ms (2:1), Ed + Ms (2:1), Fe + Ed + Ms (2:2:1)). Plant traits, phytochemical properties and soil stoichiometric characteristics were measured to explore the interactive effects of soil gravels and plant species on vegetation restoration. Average plant height, coverage, shoot biomass, and total biomass were most affected by plant species (F = 277–611, p < 0.01), followed by gravel content (F = 90–195, p < 0.01) and their interaction (F = 5–51, p < 0.05); root biomass was most affected by gravel content (F = 130, p < 0.01). Among plant species, shoot and root biomass, total biomass overall decreased in the order of Fe + Ed + Ms>Fe>Fe + Ms>Fe + Ed>Ms>Ms + Ed>Ed. Plant total biomass, shoot biomass, root biomass and shoot/root ratio among different soils overall decreased in the order of low> high> medium gravel contents. All plant species were restricted by soil nitrogen except for Ed and Ed + Ms (N:P > 14). In addition, increasing the gravel content in the soil will increase the soil bulk density and reduce the total soil porosity. The total soil porosity is significantly positively correlated with the average plant height, coverage, aboveground biomass, and total biomass of plants (r = 0.78–0.91, p < 0.05). The total nitrogen content, total phosphorus (TP) content, and N: P in rhizosphere soil were significantly positively correlated with the average plant height, coverage, aboveground biomass, root biomass, and total biomass of plants (r = 0.70–0.97, p < 0.05), but soil N was significantly positively correlated with aboveground biomass and total biomass (r = 0.69, 0.71, p < 0.05), and there was no significant difference in soil TP content between them. The gravel content directly affects plant growth by changing bulk density and total porosity, but the combined effect of soil nutrients and plants affects plant growth. Therefore, optimizing the configuration of soil properties (mainly nitrogen and compactness) and plant species (isecologic niche plants) is an effective strategy for ecological restoration in the Qinghai‐Tibet Plateau.

Cyclic and postcyclic simple shear behavior of binary sand-gravel mixtures with various gravel contents

Liquefaction evaluation on sand-like gravelly soil deposits based on field Vs measurements during the 2008 Wenchuan earthquake

Tensile strength is one of the most important properties affecting anti cracking performance of earth core rockfill dam. However, the influence of gravel content on the tensile strength of gravelly soil is still unclear. In the paper, based on the self-developed uniaxial tensile test device, a series of tensile tests were performed on gravelly soils with different gravel content. For gravelly soils with different gravel content, the tensile strength decreases linearly with the increase of gravel content at the optimal water content and maximum dry density. In addition, Empirical formula to calculate tensile strength of gravelly soil based on the gravel content is put forward. The relevant conclusions are helpful to improve the anti-cracking design level of actual earth core rockfill dam.

In this study, large-scale triaxial compression tests are conducted on gravelly soil material. The experimental investigation shows that gravelly soil exhibits complicated volume-dilatation/contraction behaviours that are dependent on stress level. To better describe the mechanical behaviours, an elastoplastic constitutive model is proposed within the frameworks of generalised plasticity and the critical state soil mechanics. An evolution equation of the void ratio is incorporated into the model to describe the effect of a relatively dense or loose state on the volume-change behaviours. The model has 10 material constants that could be determined using a few large-scale conventional triaxial tests. The model is then used to simulate the mechanical behaviours of gravelly soil. Comparisons between the numerical simulations and the test data show that the model is capable of capturing the mechanical behaviours of gravelly soil under various confining pressures. This research can be beneficial for understanding the mechanical behaviours of gravelly soils.

Among other reasons, studies of the liquefaction potential of gravelly soils are limited because of the difficulties involved in preparing uniform specimens, without particle segregation, especially for a well-graded gravelly soil in a dry state for simple shear testing. Errors and difficulties are also involved in compensating for membrane penetration to a gravelly soil specimen in triaxial testing to get reliable data. Thus, innovative approaches for preparing triaxial and simple shear specimens for gravelly soils are introduced and implemented in this study to overcome experimental problems in acquiring accurate test results. In addition to the aim of obtaining reliable testing data on the liquefaction of gravelly soils under initial static shear stress for simulating sloping ground conditions, a study of the effect of various stress paths on the liquefaction resistance of gravelly soils was another goal of this research. In this regard, two sets of cyclic tests using medium-size triaxial and simple shear devices are conducted on a unique soil to compare the liquefaction potential of the tested gravelly soil using these two devices. Results of simple shear tests indicate that as the value of initial static shear stress increases, the cyclic resistance of the tested gravelly soil decreases. However, the results of the triaxial tests show that the variation of cyclic resistance for the tested soil depends on the initial static shear stress level and associated stress reversal conditions. Furthermore, the observed value of the pore water pressure ratio at failure using the strain-based liquefaction criteria for the tested gravelly soil was about 0.85, regardless of the type of testing. In addition, the relationship between the liquefaction resistance of gravelly soils using cyclic triaxial and simple shear devices was obtained.

Evaluating the applicability of conventional CPT-based liquefaction assessment procedures to reclaimed gravelly soils

The influence of soil gravel content on compaction behaviour and pre-compression stress