1. Introduction
The stability of earth dams under vibration has become one of the most serious problems for civil engineers, particularly after the recent experiences of large earthquakes in the world. To analyze such problems, residual strength parameters have attracted much attention in recent decades due to their great significance in stability analyses when evaluating the influence of large earthquakes on foundation settlements [
1,
2,
3] or the stability of slopes that have experienced landslides [
4]. On the other hand, landslides are clearly one of the most serious natural disasters on earth; they cause great loss of life and severe destruction of the social economy. With the rapid development of urbanization and transportation in China, granite residual soils are used in many areas, especially in coastal areas [
5,
6]. However, under the action of earthquakes, a large number of landslides, vibration subsidence, and ground fissures are caused in such granite residual areas. In recent years, with the continuous improvement of scientific and technological innovations, as well as the attention that is being paid to special problems, research on granite residual soils has been increasing. Granite residual soils are the product of a series of physical and chemical weathering of the parent rock granite. Compared with other kinds of soils, granite residual soils belong to one of the special soils due to the following reasons that are different from other soils. In terms of engineering properties, granite residual soils have been clarified as having higher shear strength and bearing capacity and lower compressibility under natural conditions [
7]. In terms of mineral composition, granite residual soils are mainly composed of quartz and kaolinite and lesser amounts of soluble salt and organic matter. In terms of particle composition, relatively greater amounts of granite residual soils are larger than 0.5 mm or less than 0.075 mm in size, while fewer amounts are between 0.075 and 0.5 mm in size [
8]. However, it should be pointed out that the research on granite residual soils has not formed a complete system, compared with loess or soft soil, due to the delay in starting research on these soils.
The mechanical properties of many kinds of soils have been the object of extensive research for decades. The existing studies on the residual strength of various soils are mainly focused on clay or clayey soil, and a relatively consistent theory has been realized for evaluating the residual strength, like the effect of the shear rate [
9,
10,
11,
12,
13,
14,
15,
16,
17,
18], consolidation ratio [
14,
16], and normal stress [
17,
18] on the residual strength of this clay or clayey soil. Normal stress refers to the weight applied to soil samples during shearing. Moreover, the relationships between residual strength and related indexes, such as the plasticity coefficient, FGPs, CGPs, and so on, have also been shown for clayed soil in a lot of the literature. The plasticity coefficient mainly contains liquid limit, plastic limit, and plasticity index, where the liquid limit refers to the boundary moisture content between the plastic and flowing states of cohesive soil, the plastic limit is the boundary moisture content between the plastic and semi-solid states of cohesive soil, and lastly, the plasticity index can be determined by liquid limit minus plastic limit. However, it should be noted that granite residual soils contain a lot of coarse-grained particles (CGPs), which means the present theory on the residual strength may not be suitable for granite residual soils.
Moreover, there is little research using sensitivity analysis for the strength of granite residual soils. The related indexes for predicting the residual strength of clayed soil, liquid limit, plastic limit, plasticity index, FGPs, and CGPs were selected as the specific factors in this sensitivity analysis under consideration in this study. Therefore, it is worth conducting this study to analyze the effects of different kinds of soil indexes on the residual strength parameters of granite residual soils.
The residual strength parameters of many kinds of soils, namely residual frictional angle
ϕr and cohesion, can be obtained through laboratory tests. Due to the obvious characteristics of their structure and probable fissures, the cohesion of granite residual soils is neither stable nor credible, and it has been suggested that the residual frictional angle be obtained by indoor geotechnical tests that have high credibility [
8]. Thus, this paper is also focused on evaluating the main factors influencing the residual frictional angle by the sensitivity analysis and exploring the possibility of predicting the residual frictional angle by the available indexes for granite residual soils.
The main objective of this study is to expound the action mechanism of fine-grained particles (FGPs) on the residual strength of granite residual soils. At the same time, the possibility of predicting the residual frictional angle of granite residual soils by means of the available indexes based on the sensitivity analysis, including the Atterberg limits, the fractions of different kinds of particles, and the relative grading parameter, , is illustrated, where and are the ratio of FGPs and CGPs, respectively. Moreover, the effect of the fraction of FGPs on the residual strength and peak strength under a normal stress of 201 kPa, as well as a comparison with the published prediction models for the residual frictional angle, is also discussed. In this study, ring shear tests and basic physical property tests, namely grain size analyses, liquid limit tests, and plastic limit tests, were conducted on all samples.
4. Conclusions
Drained ring shear tests, grain size analyses, liquid limit tests, and plastic limit tests were carried out on granite residual soils to clarify the effect of FGPs on the residual strength of granite residual soils. Then, the correlations between the residual frictional angle of granite residual soils and the available indexes, which could be obtained in a short time, were proposed and compared. The results presented have led to the following general conclusions.
The residual state of granite residual soils was able to be determined in a long horizontal displacement when the orientation of the particles occurred and was then kept stable. However, a small amplitude of fluctuation in shear stress still existed and was relatively regular in the residual state. The shape of the shear band was not an ideal flat layer, which was caused by the CGPs, such as the quartz particles. It was seen that the lower the FGP fraction was, the greater the amplitude of the fluctuation would be.
Under the same normal stress, the peak strength and residual strength decreased with an increase in the ratio of FGPs and then remained almost the same when the ratios of FGPs were equal to 85% and 90%, respectively. In addition, the post-peak attenuation tended to increase with the increasing FGPs.
The residual frictional angle is rarely dependent on the ratio of FGPs when the ratio of FGPs is around 90%, which was possibly due to the fact that contact between CGPs and other grained particles hardly ever occurred.
Though predictions of the residual frictional angles seem to be possible for granite residual soils, we have to note that the models proposed in this study are not suitable for different kinds of soils. However, the residual frictional angle of granite residual soils was seen to be closely related to the available indexes used in this study, and it is evident that the influences of kinds of available indexes on the residual frictional angle can be ranked in order of > > > > .