Evaluation of compressibility
Benatti and Miguel (2013) proposed the structural models for colluvial and lateritic soil based on their three stages of collapsible behavior, taking into account the effects of matrix suction and cementation. Analogously, intact loess also has typical characteristics of unsaturated and intergranular cementation. As shown in Fig.4, the compression of loess can be divided into three stages in e-logσv coordinates: (Ⅰ) The skeleton particles become tighter, with mild linearly decrease in void ratio at small vertical stress; (Ⅱ) Gradually breakage of interparticle cementations and aggregates, the collapse of the overhead structure, with decreasing void ratio at an increasing rate of change in void ratio at intermediate vertical stress; (Ⅲ) Completely breakage of interparticle cementations and aggregates, particles rearrangement, with decrease sharply in void ratio at great vertical stress. However, it is worth noting that these stages are not completely separated, for example, there are also cementation breakage and particle moving in stage Ⅰ, only that elastic compaction is predominant. A curved connection (Stage Ⅱ) between two straight lines (Stage Ⅰ and Stage Ⅲ) indicates that the interparticle cementation gradually fractures with increasing vertical pressure. In ln(1+e)-logσv plane, the pore ratio varies nearly linearly with increasing vertical pressure in the two phases of elastic compaction and particle rearrangement, and the vertical pressure corresponding to the intersection of the two linear segments is determined as yield stress (Sridharan et al.,1991; Cheng et al.,2020; Leng et al.2021).
Fig.5(a) shows the compression curves of intact loess with different initial water contents in the e-logσv plane. The void ratio of all specimens decreases with vertical stress; with the same initial pore ratio, the higher the initial water content, the faster the void ratio decreases. Following the method proposed by Sridharan et al. (1991), the yield stress of intact loess with different initial water content determined and showed in Fig.5(b), (c), (d), when the water content of specimens is low, the two straight sections are evident (Fig.5(b)); when the water content of specimens are very high, the slop of the two straights is almost the same (Fig.5(d)). The intersection of two lines gradually moves to the left as the water content increases, meaning that the higher the water content, the smaller the yield stress of the intact loess. The yield stress decreases sharply and then stabilizes as water content increases, as shown in Fig.6, which relationship can be well fitted by the power function based on water content w0. This result is consistent with the conclusion of other papers (Chen et al.2006; Leng et al.2021), presented in Fig.6. It is also worth noting that the yield stress of the intact loess is approximately 530kPa, which is significantly higher than the cover earth pressure, and they (Munoz-Castelblanco et al.,2012; Sadeghi et al.,2019) presumed that the apparent over-consolidation should be related to the soil-forming process, where the cementation between the skeletal particles provides great cohesion.
The compression index (Cc) is the slope of the compression curve that can be defined as Cc= ∆e/∆logσv, which evolves with vertical stress to evaluate the compressibility of the intact loess. Fig.7 shows the variation curve of the compression index with vertical pressure, and it can be seen that the compression index of each specimen increases with increasing vertical pressure. However, the evolution of the compression index of specimens with different water contents follows different paths. The compression index of loess with low water content loess increases slowly and then sharply with vertical stress, whereas the compression index of loess with high water content is high and increases rapidly before becoming slow. Further observation of the evolution of compression index shows that the difference of evolution path of compression index seems to be related to yield stress, when the vertical stress exceeds the yield stress, the compression index will increase greatly. The evolution of the compression index with vertical stress is related to the yield stress, which is similar in other soils (Perisic et al.,2019; Rezania et al.,2020). The further increase of compressibility of low water content loess after the vertical stress exceeds the yield stress may arise from two aspects: one the one hand, from the fracture of the inter-particle cementation, the sudden reduction in strength and a great deal of compression space remains after the previous loading; on the other hand, the increase in saturation due to volume compression also contributes to the compressibility of unsaturated soil(Rezania et al.,2020), as can be seen from Table 2, although the water content of the specimen decreases after compression, the saturation increases.
Evaluation of secondary compressibility
The compression of soils consists of two stages: the primary compression stage, in which the effective stress is increased by the extrusion of pore water and pore gas resulting in the soil volume being compressed; and the secondary compression stage, in which the effective stress is constant and packing density of soil particles increases slowly with time (Mitchell et al.,2005). The study of the secondary compression characteristics of soils is of great significance in evaluating the long-term stability, especially in the loess areas of China, when evaluating the settlement deformation caused by long-term irrigation in loess tableland, the secondary compression deformation can not be ignored.
It is not easy to determine accurately and quickly the demarcation point of primary and secondary compression in the void ratio versus the logarithm of time(e-logt). Mataic et al. (2016) suggested that the secondary compression of soft clay soil occur from the time period of 6-24h for each load increment; summary of loess secondary compression shows that the time corresponding to the demarcation point of primary and secondary compression of loess gradually elapses with the increase in vertical stress and is distributed within 46-200min (Ge et al.2015; Zhi et al. 2018). To reduce the complexity and subjective error of determining the demarcation point, it is considered in this paper that the secondary compression is the period from 200min to deformation stability (the vertical deformation is less than 0.002mm per day). During this time period, the void ratio decreases linearly with the logarithm of time, which slope is defined as the secondary compression index and is usually expressed as Cα=∆e/log(t2/t1), where t1 is the time of demarcation point of primary and secondary compression, t2 is the time of deformation stability, and ∆e is the change in void ratio during the secondary compression stage.
Fig.8 shows the variation of the void ratio with the logarithm of time for loess with different water content, 7.14% (natural water content), 18.03%, 25.10%, and 35.02%(saturated), separately. For all specimens, the void ratio decreases with time at each loading, the rate of change in the void ratio was great high at the beginning of loading, after which it started to decline. Higher volume changes occur in the primary compression stage or secondary compression stage with an increase in vertical stress, particularly for vertical stress more elevated than the yield stress, which is more obvious in the low water content specimens.
Fig.9 compares the variation of void ratio with the logarithm of time for specimens with different water content at similar vertical stresses. It is clear that the rate and magnitude of secondary compression of loess increase with the increase of water content at low vertical stress (Fig.9(a)); however, with the increase of vertical stress, the secondary compression characteristics of loess with different water content tend to be similar. At σv=3200kPa, the e-logt curves of each specimen in the secondary compression stage are parallel to each other (Fig.9(d)). It can be concluded that the effect of water content on the secondary compression of loess is gradually weakened with increasing vertical stress. The water softens the cementitious materials in loess (Wen and Yan,2014), and for unsaturated soils, water menisci at the interparticle contacts apply tensile pressure to the skeleton particles for preventing particle rearrangement (Rezania et al.,2020), which lead to the increase of loess secondary compressibility with the increase of water content in low vertical stress. Interparticle cementation and suction hinder the development of secondary compression. For low water content loess, with the increase of vertical stress, the interparticle cementation is gradually destroyed, and the decrease of volume result in an increase in saturation and a decrease in suction, which leads to the secondary compressibility enhanced; for the loess with high water content, it has greater volume change than the loess with low water content at the similar vertical stress, higher density makes the skeleton particles form inter-locking structure, thus slowing down the development of secondary compression.
Fig.10(a) shows the variation curves of the secondary compression index (Cα) with vertical stress for loess with different water contents. It can be seen that the secondary compression index of the low water content loess increases with the increase of vertical stress, and the secondary compression index of higher water content loess increases to the peak and then slightly decreases to constant with the vertical stress, which is similar to the results of other literature (Perisic et al.,2019; Rezania et al.,2020). Comparing with the compression index curves (Fig.7), it can be found that they show a similar variation pattern in logarithmic of vertical stress. A large number of studies (Mesri,1987; Zhang et al.,2007; Santagata et al.,2008; Carlos,2018) have concluded that Cα/Cc is a constant value independent of stress level and specimen type (Undisturbed or remodeled). However, some studies have found a poor linear correlation between Cα and Cc, Zhang and Wang (2012) speculated that the structure of soft clay prevents its Cα/Cc from a constant value; Mataic et al. (2016) found that the Cα/Cc value of Perniö clay increases with effective stress and then decreases and converges to a constant value (0.036). The Cα/Cc values of the low water content loess firstly increased with the vertical stress to peak and then gradually decreased and stabilized (Fig.10(b)), showing a similar variation pattern as the result of Perniö clay (Mataic et al.2016). However, a little different is that the Cα/Cc values of the higher water content loess gradually decreased and tended to stabilize, and did not show a peak, the Cα/Cc values of all specimens converged to around 0.025 eventually.
Microstructure analyses
Microstructure of intact and saturated loess
This section describes the microstructure of intact and saturated loess in terms of four elements: particle pattern,contact relation,bonding material,pore size distribution, to illustrate the effect of water content on the microstructure of loess and to provide a basis for further analyzing the effect on compressibility.
Fig.11(a), (b) is the SEM images of intact loess, where the overall microstructure such as skeletal particles and pore distribution are observed in the low magnification image, and the surface morphology of the particles, interparticle contact, and bonding material are observed in the high magnification image. In addition to silt and sand, aggregates are also an essential part of skeleton particles in intact loess, which are formed when a large number of clay particles clumped together on their own or when clay particles and carbonates bind the silt and sand (Gao,1980; Li et al.,2016; Liu et al.,2016; Pihlap et al.,2021). In the deposition, the skeleton particles are randomly and loosely arranged, making the loess form an opening metastable structure. Lanzhou intact loess has the typical microstructure characteristics of loess: silt, sand, and aggregates are contacted point-to-point, and there are only a few bonds at the contact, forming an overhead pore structure (Fig.11(a)). The cementation of clay and calcium carbonate in loess plays a vital role in the metastable structure (Cliek,2001; Smalley and Marković,2014; Li et al.,2016; Liu et al.,2016; Yates et al.,2017; Meng and Li.,2019), Fig.11(b) clearly shows that clay particles are distributed on the surface and contact point of skeleton particles and do not exist alone. Secondary calcium carbonate can occur due to the precipitation of carbonates (Pihlap et al.,2021), which reinforce the bonding structure and help to trap clay particles at skeleton particles contacts (Smalley and Marković, 2014). Although it is difficult to observe the secondary calcium carbonate directly, some experiments show that the strength of the loess decrease obviously after removing the calcium carbonate, which can also prove the cementation of the secondary calcium carbonate (Meng and Li, 2019). The high compressibility of intact loess and its sensitivity to water mainly comes from the overhead pore structure and the softening effect of water on cementation materials, where the high porosity provides space for compression deformation, and the hydrophilicity of clay minerals makes the compressibility of loess extremely sensitive to the change of water content. The microstructure of saturated loess shows in Fig.11(c), (d), which is still dominated by overhead pore structure, but compared with intact loess, the distribution of skeleton particles and pores is more uniform, and the macropores are closed. The most significant phenomenon is the disappearance of the aggregates prevalent in the intact loess after saturation.
Mercury intrusion porosimetry (MIP) has been proved to be one of the reliable methods to measure pore distribution of soil, which has been widely used in loess (Ng et al.,2016; Wang et al.,2018). Lei (1987) divided pores of loess into four types according to the pore radius: large pores (>16μm), mediate pores(4-16μm), small pores(1-4μm) and micropores(<1μm), which is referred to in this paper. Fig12 shows the cumulative pore volume curves and pore distribution curves of intact loess and saturated loess. There are two peaks in the intact loess and saturated loess pore distribution curves (PSDs), indicating two major pore groups in the loess structure. The first pore group with a dominant mediate pore diameter is about 9.7μm for intact loess and about 8.3μm for saturated loess, and the second pore group with a small pore has a nearly identical diameter of about 3.5μm for intact and saturated loess, which indicate that pore size decreases slightly after saturation. The effect of saturation on the pore structure of loess is significant in the pore volume. The cumulative intrusion pore volume of intact loess with 0.28 decreases to 0.23 after saturating, indicating self-weight collapse due to inundating. On the other hand, in intact loess mediate pore accounts for 69.38% of the total pore volume and decreases to 41.31% after saturation, and small pores increase from 15.67% to 35.33%; therefore, it can be seen that mediate pore collapsed when the loess was inundated, leading to the transformation from mediate pores to small pores. In terms of total pore volume and pore size, inundation reconstructs the pore structure of loess, makes it more homogeneous, and causing some damage to the original microstructure of the intact loess.
Microstructure of loess with different water content after compression
The compression tests show that the water content has a significant influence on the compressibility of the loess, and the damage of hydraulic effect on loess microstructure is also analyzed. However, to reveal the evolution of loess compressibility under wetting and loading through the change of loess microstructure under hydro-mechanical effect, it is necessary to understand the microstructure changes of loess with different moisture content after compression. Therefore, three post-compression samples of loess with different water content are selected for SEM and MIP, which are 7.14% (natural state), 18.03% (saturation 50.03%), and 35.03% (saturated state).
Fig.13 shows the microscopic images of the loess with different water content after compression, which can be seen that the microstructure of saturated loess is subject to drastically change compared to others. For intact loess (Fig.13a, b), aggregates and intergranular cementation are destroyed under load, overhead structures collapse, the intergranular pores become small, and part of which is filled with clay particles, and the skeleton particles contact more closely. However, it is still dominated by the point-to-point and point-to-face contact of skeleton particles and retains some microstructure characteristics of undisturbed loess. Comparison between the microscopic images of the sample with natural water content (Fig.13a) and the microscopic images of samples with higher water content (Fig.13c, e) suggests that the microstructure is closer and homogeneous as the water content is increased. From details (comparison Fig.13b and Fig.13d, f), the microstructure changes from overhead structure to interlocking structure with the water content are increased. It can be summarized by the fact that the microstructure of loess has undergone a radical transformation under the action of saturating and loading; in terms of microstructure, even after compression with high stress, the intact loess still retains some original structure and has the potential to compress further under higher loads or inundation.
Fig.14 shows the pore size distribution curves of the loess with different water content after compression. Compared to the cumulative intruded pore volume of the intact loess (Fig.12), compression leads to a significant reduction in the total pore volume of the loess, and the reduction in pore volume decreases more with increasing water content. The pore size distribution curves of the compressed specimens show a more significant change from a bimodal peak to a single peak compared to the undisturbed loess specimen, regardless of the water content. This is because the mediate pores collapse to the small pores during the compression process, which is reflected in the pore size distribution curve as the bimodal peak of the PSD is eliminated and transformed into a single peak. Moreover, in response to an increase in water content, the height of single peak decreases, and the peak that defines the dominant pore diameter on the PSD shifts to the left. In other words, compression with high pressure can not eliminate the collapsible pores of intact loess, and wetting can promote the further collapse of mediate pores to form a more compact structure.
The evolution of microstructure and pore distribution due to wetting and loading is consistent with the compression responses of intact loess. Under low vertical stress, microstructure damage due to wetting is one of the key factors for the great difference of compressibility of loess; Under high vertical stress, the intact loess still retains some original microstructure and pore space, but microstructure damage is aggravated to form a compact interlocking structure due to wetting.