Small strain stiffness for granite residual soil: effect of stress ratio

Most previous studies have focused on the small strain stiffness of sedimentary soil while little attention has been given to residual soils with different properties. Most studies also neglected the effects of the deviator stress, which is extensively involved in civil engineering. This note considers the effects of the deviator stress on the small-strain stiffness of natural granite residual soil (GRS) as established from resonant column tests performed under various stress ratios. Although increasing the stress ratio results in a greater maximum shear modulus for both natural and remolded residual soils, remolded soil is more sensitive to changes in the stress ratio, which highlights the effects of soil cementation. The data herein offers new insights to understand the stiffness of residual soil and other weathered geomaterials.

Mechanical Properties and Micro Morphology of Granite Residual Soil-Based Fly Ash Geopolymer

In order to explore the polymerization mechanism of granite residual soil, geopolymer containing fly ash, various particle groups, granite residual soil and their composites were prepared with different materials. Sodium hydroxide and sodium silicate are used as alkali activators. In this paper, the compressive strength of geopolymer was studied. The mineral composition and microstructure were tested and analyzed by X-ray diffraction and scanning electron microscope. The results show that geopolymerization can only occur in fine particles due to the presence of amorphous aluminosilicate in granite residual soil. The zeolite phase transition of low polymer in fine-grained reaction is beneficial to enhance the integrity of the sample and improve its compressive strength. The addition of fly ash can accelerate the geopolymerization rate and improve the strength of fine geopolymer, but it can inhibit the occurrence of zeolite phase transformation.

Artificial Ground Freezing Impact on Shear Strength and Microstructure of Granite Residual Soil Under an Extremely Low Temperature

The Artificial Ground Freezing (AGF) method, which is widely used in tunnel excavations, significantly affects the properties of geotechnical materials in frozen walls under extremely low temperatures. In order to simulate the AGF process, the freezing treatment with a temperature of −30°C and thawing treatment temperature of 25°C were performed on natural specimens of granite residual soil (GRS). Subsequently, triaxial (TRX) tests were conducted to evaluate mechanical properties and Nuclear Magnetic Resonance Image (NMRI) tests were applied to detect pore distributions of GRS. To clarify variations of microstructure after freezing-thawing, the relaxation time (T2) distribution curves and T2-weighted images from NMRI results were thoroughly analyzed from the perspective of quantization and visualization. Results show that the shear strength as well as the cohesion of GRS are reduced sharply by the AGF process, while the internal friction angle decreases gently. The pore size distribution (PSD) converted from the T2 curve is constituted of two different peaks, corresponding to micro-pores with diameters from 0.1 to 10 µm and macro-pores with diameters from 10 to 1,000 µm. Under the AGF impact, the expansion in macro-pores and shrinkage in micro-pores simultaneously exist in the specimen, which was verified from a visualized perspective by T2-weighted images. The frost heaving damage on shear strength is attributed to the microstructural disturbance caused by the presence of large-scale pores and uneven deformations in GRS, which is subjected to the AGF impact under an extremely low temperature.

Failure mode of rainfall-induced landslide of granite residual soil, southeastern Guangxi province, China

Granite residual soil landslides are widely distributed in southeastern Guangxi province, China. They are posing a huge threat to local communities and hindering social and economic development. To understand the failure mode of the landslide can provide a scientific basis for early warning and prevention. In this study, it conducted artificial flume model tests to investigate the failure mode of granite residual soil landslide. The macroscopic phenomena of landslides in the flume were summarized. The changes of soil moisture content along with pore water pressure were analyzed. And the differences and commonness in the initiation patterns of landslides were discussed. The results had four aspects. (1) There were significant similarities in the phenomenon of slope failures. In the beginning of the artificial rain, slopes were infiltrated, following by the slope toe soil softened and slipped. Another similar pattern was that continuous rainfall could cause soil crusts and runoff on the slope surface. Short-term low-lying areas and interlocking ditches would appear due to surface runoff and rainwater erosion. (2) The increase of initial dry density enhanced the permeability resistance of rainwater to the residual soil, which led to a delay in the response time of water content and pore water pressure, and a decrease in pore water pressure. Moreover, the fluctuation characteristics of pore water pressure may be related to the type of soil shear deformation. (3) The starting time of a landslide was delayed as the initial dry density and slope angle increased, but it was shortened due to the increase in rainfall intensity. Meanwhile, the initiation pattern changed from a sudden sliding type to a progressive failure type due to the increase of initial dry density. (4) The failure process of the granite residual soil landslide could be classified into five stages: rainwater infiltration, soil sliding at the slope toe, the occurrence of surface runoff and erosion, the formation of a steep free face, and the upper soil sliding. Above research results can provide valuable references for the prevention and warning of granite residual soil landslide in southeast Guangxi.

Influence of Sustainable Biochars Produced from Kitchen Waste, Pig Manure, and Wood on Soil Erosion

The influence of biochars on water retention, mitigating nutrient leaching, and pollutant removal in green infrastructure has been explored in the past. However, there is a lack of understanding on how feedstock (i.e., biomass) would affect biochar physicochemical properties and hence, overall erosion control (including infiltration, surface, and sub-surface runoff) in green infrastructure. The main purpose of this study was to investigate the effect of biochars produced from three different feedstocks (pig manure, wood, and kitchen waste) on the erosion of granite residual soil. Flume experiments were conducted to measure and analyze soil erosion, runoff, and infiltration. The result showed that the runoff and soil erosion of kitchen waste biochar (KWB) samples were reduced by 17.7% and 21.7%, respectively. On the contrary, wood biochar (WB) and pig manure biochar (PMB) were found to enhance runoff and soil erosion. In addition, biochar particles were found in runoff and infiltration in erosion experiment. Thus, it is important to note that measures should be taken to prevent biochar loss when using biochar as a soil amendment. Additionally, the effects of different types of biochar on soil hydraulic and hydrophobicity properties should be taken into account as a selection criterion for choosing amendments in green infrastructure. This study finds that kitchen waste biochar has better performance in improving soil hydraulics and erosion.

Laboratory model test study of the hydrological effect on granite residual soil slopes considering different vegetation types

The problems caused by the interaction between slopes and hydrologic environment in traffic civil engineering are very serious in the granite residual soil area of China, especially in Guangdong Province. Against the background of two heavy rainfall events occurring during a short period due to a typhoon making landfall twice or even two typhoons consecutively making landfall, laboratory model tests were carried out on the hydrological effects of the granite residual soil slope considering three vegetation types under artificial rainfall. The variation in slope surface runoff, soil moisture content and rain seepage over time was recorded during the tests. The results indicate that surface vegetation first effectively reduces the splash erosion impact of rainwater on slopes and then influences the slope hydrological effect through rainwater forms adjustment. (1) The exposed slope has weak resistance to two consecutive heavy rains, the degree of slope scouring and soil erosion damage will increase greatly during the second rainfall. (2) The multiple hindrances of the stem leaf of Zoysia japonica plays a leading role in regulating the hydrological effect of slope, the root system has little effect on the permeability and water storage capacity of slope soil, but improves the erosion resistance of it. (3) Both the stem leaf and root system of Nephrolepis cordifolia have important roles on the hydrological effect. The stem leaf can stabilize the infiltration of rainwater, and successfully inhibit the surface runoff under continuous secondary heavy rainfall. The root system significantly enhances the water storage capacity of the slope, and greatly increases the permeability of the slope soil in the second rainfall, which is totally different from that of the exposed and Zoysia japonica slopes. (4) Zoysia is a suitable vegetation species in terms of slope protection because of its comprehensive slope protection effect. Nephrolepis cordifolia should be cautiously planted as slope protection vegetation. Only on slopes with no stability issues should Nephrolepis cordifolia be considered to preserve soil and water.