4.1. Changes in collapse intensity and initial resistance of soils against the collapse
Table 1 shows some of the physical, mechanical, and chemical properties of loess soils at sampling points. Generally, it can be claimed that the coefficient of the collapse of the loess soils decreases from the north of Golestan (sandy loess) to the south (clayey loess). Furthermore, the values of liquid limit, percentage of fine clay particles, percentage of moisture, and the degree of initial saturation of soils increase from north of the province southwards. However, the percentage of CaCO3 decreases from north to south of the province. These findings are consistent with the results of previous research (Rezaiy et al. 2011; Salehi et al. 2015).
The collapse intensity of loess soils was determined based on calculated Ic value using Eq. (1) (ASTM D5333 2003):
![](data:image/png;base64,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)
Where ∆h is the vertical deformation of the soil sample after saturation under constant stress, and h0 is the initial height of the soil sample. Variations in the Ic values for samples from these three zones are shown in Fig. 4.
As evident in Fig. 4c, for soil samples from Zone III, the Ic values are generally higher than that of other zones. In the soil samples from Hootan, Alagol and Tange Ali, the highest value of Ic was observed at a pressure of 200 Kpa, and it would drop at higher vertical stresses. The oedometer test results showed that in these samples, under a stress of 200 kPa, the stability of the soil would prevent soil deformation mainly due to the cementation and the suction force between the particles in the natural moisture. However, at the same stress of 200 Kpa, immediately after saturation of the soil sample, the suction force and cementation between the soil particles would vanish, and the soil sample would display the maximum deformation, and thus, the maximum value of Ic would be observed. At vertical pressures greater than 200 kPa, cementation due to calcium carbonate between soil particles would disappear even at natural moisture. As a result, the soil structure and the connections between soil particles would easily break after saturation; subsequently, the soil sample could easily be compressed, and the porosity ratio of the soil sample would reduce significantly. Therefore, at pressures of 400 kPa and above, the collapse due to soil saturation would decrease. As a result, Ic would reduce invertical stress rates up to 1600 kPa.
The Ic deviations corresponding to different vertical pressures are presented in Fig. 4.b for the soil samples of Zone II. As it is evident, in Zone II, in the soils of Cheshmehli and Feraghi, Ic values would increase from 25 to 800 kPa vertical stresses and from 25 to 400 kPa in Gonbad and Kalaleh soils (Fig. 4.b), and then, it would drop in greater pressures. In general, in Zone II soils, the soil structure is more stable than in Zone III. In Cheshmehli and Feraghi soils, the porosity ratio of soil samples would slightly reduce in natural moisture under stresses up to 800 kPa. However, immediately after saturation, the values would change significantly. Therefore, the highest Ic in this region was measured at a stress of 800 kPa. At pressures above 800 kPa, the soil void ratio would decrease significantly before the samples become saturated, and therefore the Ic value decreases at a stress of 1600 kPa.
The clayey loess soil samples collected from Zone I were not collapsible in some areas and had a lower Ic than Zones II and III. These soils usually showed the highest value of Ic at 200 kPa (Fig. 4.c). In Zone I, soil samples had more natural moisture, and the initial degree of soil saturation was high; as a result, the amount of suction force between soil particles was insignificant, and also the percentage of calcium carbonate between soil particles was low. Therefore, the amount of cementation of soil samples was negligible, and only in the Baraftan area, the amount of CaCO3 would reach more than 30%; this was mainly in the form of calcite nodules and, therefore, would not play the role of cement between particles (Salehi et al., 2015). Generally, it can be claimed that the collapse intensity of loess soil would decrease from the north to the south of Golestan Province. This finding is consistent with the results of other researchers.
This research evaluated the soil susceptibility to collapse in Golestan Province by determining the initial collapse stress σi for loess soil samples. Considering the Ic of 1.5% as the boundary between collapsible and non-collapsible soils, the amount of vertical stress corresponding to the coefficient of the collapse of 1.5% was defined as the initial collapse stress in Fig. 4. The values of σi would vary in different zones (Zone I: in the range of 44 to 67 kPa, Zone II: from 56.5 to 100 kPa, and Zone III: from 23 to 45 kPa). Sandy loess soils in Zone III had the lowest value of σi. For example, for soil sample H1, obtained from the Hootan region, the value of σi was 23 Kpa (Fig. 4-c). However, for the CH3 soil sample, this value was about 100 Kpa. Therefore, it can be claimed that more stress would be needed to start the destruction of soil in Zone II, having higher values of σi. Consequently, the maximum Ic in Zone II and III soils were measured at stress levels of 800 Kpa and 200 Kpa respectively, as it is evident in Fig. 4.
4.2. Collapse rate of soil samples
The collapse rate is defined as the time required for the complete collapse of loess soils. This parameter is significant in evaluating the risks of a destructive phenomenon (Cui 2010; Guan 1983; Zhang et al. 2017). In simple words, as the collapse rate increases, the risk of danger would grow.
To determine the collapse rate of soils in the study area, we read the amount of deformation at each loading stage after saturation of the sample at specified intervals. The deformation was continuously read until a deformation value threshold of less than 0.01 mm per hour. The required deformation rates less than 0.01 mm per hour are called stability time. Since the time required for 10% of the final settlement of collapsible soils is considerably longer compared to the time required for 90% settlement of these soils, it has been suggested that for an accurate comparison between the collapse rates of these soils, the time required for 90% of their settlement (T90%) should be used (Zhang et al. 2018) instead of the stability time; therefore, removing the final 10% of the settlement, which requires a considerable amount of time, could help with a more effective and accurate comparison of the collapse speeds of different soils. The experimental results supported (were in favor of) this suggestion. Table (2) shows the stability time and T90% of soil samples of the triple zones.
For instance, for the soil samples of the Hootan (H1) area, the time required for complete settlement (T) was 247 min, which is equivalent to 1.83 times the time required for complete settlement of Cheshmehli sample (CH1), i.e., 135 min. However, the time required for the settlement of 90% (T90%) of the soil samples in the Hootan area (H1) was 14.2 min, equal to slightly more than four times T90% of the soil samples of Cheshmehli (CH1), i.e., 3.5 min.
As can be seen (table2), the lowest and highest T90% values were obtained for the CH1 sample (Cheshmehli) in Zone II (3.5 min) and the SAD1 sample (Saad Abad) in Zone I (122.7 min). The results of the odometer test showed that the T90% for Zone III would vary between 14.2 and 55.4 min. The range was between 3.5 to 76.3 min for Zone II and between 66.33 and 122.7 min for Zone I. With the exception of the central parts of Zone II, namely Gonbad-Kalaleh and Aghabad, which experience more rainfall in these areas, the time required for the settlement of 90% of the soils of Zone III were longer than that of the soil samples of Zone II. Since the soil collapse susceptibility is higher in Zone III compared to Zone II, more time would be needed to reach the complete settlement due to collapse. However, Zone II loess soils have a stronger structure, and soil saturation has less effect on the rate of sudden collapse and degradation of soil structure, and as a result, the samples display a lower Ic value. Since the value of Ic in the soils of this Zone is less than the collapse coefficient of the soils of Zone III, the time required to reach 90% of the settlement due to collapse is less; and more precisely, the collapse rate in Zone II is higher than Zone III. The collapsibility deformation rate of loess is essential when assessing the potential damage caused by collapse since a rapid collapse commonly causes cracking, tilting, and the collapse of construction components (Guan 1983).
This situation in Zone I is not affected by vertical stress changes because of the presence of clay minerals despite having a lower collapse coefficient. Despite having lower settlement due to lower Ic values, Zone I soils require longer 90% settlement time because of the cohesion between soil particles due to the presence of clay matrix.
4.3. Determining the collapse sensitivity of the soil
The criterion of soil collapse sensitivity is presented in Table 3, considering the two parameters of collapse intensity (Ic) and collapse rate (T90%) defined by Zhang et al. (2018).
Using the results of laboratory tests, we divided the soils into four categories based on the value of the Ic:
Ic < 1.5, 1.5 < Ic ≤ 3 3 < Ic ≤ 7, Ic > 7.
Also, soils were divided into four categories based on T90%:
T90% <5 min, 5 < T90% ≤15min, 15min < T90% ≤60 min, T90% >60 min.
Based on two factors, intensity (Ic) and time of collapse (T90%), soils were divided into four categories of: low, medium, medium to severe, and severe collapse sensitivity (Is). To evaluate the accuracy and efficiency of this classification, we calculated Ic and T90% values for 31 points in Golestan Province, and the collapse sensitivity status of loess soils in three Zones I, II, and III was determined (Table 4).
As can be seen, in Zone I, where clayey loess soils are spread, the soil sensitivity was relatively low. In Zone II, where silty loess soils are abundant, the soil sensitivity was severe and, in some places, relatively severe. However, in Zone III, the collapse sensitivity of soil was often relatively severe and rarely severe, considering the higher Ic values of its soil compared to that of Zone II.
4.4. Investigating the relationship between the spatial distribution of sinkholes and Ic and collapse sensitivity of the loess soils
To investigate the relationship between the formation of pseudokarst sinkholes and changes in collapse coefficient (Ic) and collapse sensitivity (Is), we plotted the position of pseudokarst sinkholes on the scattering map of Golestan Province. Nineteen sinkholes in Zone I with an area of 501 km2, 580 sinkholes in Zone II with 3258 km2, and 98 sinkholes in Zone III with 864 km2 were identified recorded (Fig. 5). In this figure, the density of the sinkholes per unit area in each Zone is denoted by colored symbols.
More than 83% of pseudokarst sinkholes were located in Zone II, more than 14% in Zone III, and less than 3% in Zone I. By preparing a map of soil collapse potential risk based on changes in collapse coefficient of loess soils in Golestan Province, the relationship between deviations in collapse coefficient and spatial distribution of sinkholes based on Jennings and Knight (1975) classification and its overlap with the sinkholes' distribution map (Fig. 6) was investigated.
As can be seen, although Zone III is very intense in terms of collapse coefficient, the concentration of sinkholes in this Zone (161 sinkholes equivalent to about 23%) is less than Zone II (517 sinkholes equivalent to more than 71.4%). Zone II is in a severely problematic category. Therefore, it can be claimed that having a higher Ic in Zone III does not necessarily mean a higher risk of pseudokarst sinkholes in this Zone.
By preparing a map of collapse sensitivity of the soil and its overlap with the sinkholes’ distribution map (Fig. 7), the relationship between changes in the degree of collapse sensitivity and sinkholes’ distribution was investigated.
The regional developing regularities of loess sinkholes showed that more than 84.6% of the sinkholes were distributed in the region with a high degree of collapse sensitivity, more than 12.6% in the region with a relatively severe collapse sensitivity, 2% in the region with a moderate collapse sensitivity, and 0.7% in the region with a low collapse sensitivity. More than 91.5% of sinkholes in Zone II have severe collapse sensitivity, and the remaining 8.5% are in the area with relatively high collapse sensitivity. However, in Zone III, more than 60% of the sinkholes are located in the area with severe collapse sensitivity, and less than 40% of the rest are located in the area with relatively severe collapse sensitivity. In Zone I, more than 73% of the sinkholes are located in the area with moderate collapse sensitivity, and the rest are in the area with low collapse sensitivity. Based on these results, it can be suggested that the application of collapse sensitivity index (Is) to investigate the risk of pseudokarst sinkholes in Golestan Province would provide a more appropriate assessment than that of Ic.
The developmental regionalization of loess sinkholes and the loess sensitivity collapse and collapse speed are closely related. The loess sinkholes are often developed in Zone II, where the collapse sensitivity is intense. Some essential characteristics of loess, such as greater silt amounts in the composition, high collapse speed, greater porosity, the disintegrability of silty loess, and overall, higher average soil CaCO3 content, attribute to higher sinkhole formation rate in this Zone, specifically in the northeastern part of Golestan Province. Regarding the regional distribution of sinkholes, it can be seen that in Zone III, the formation of the loess sinkholes is scarcer due to considerably higher proportions of sand (sandy loess). Likewise, the loess sinkholes are less developed in the south of Golestan province with clay loess, having a strong erosion resistance, low collapsibility, and weak disintegrability and permeability.