Determination of long-term strength of frozen loess after numerous freeze-thaw cycles

Hong Zhou ,Ze Zhang ,WenJie Feng ,Jiao Ming ,ZhongQiong Zhang

1.State Key Laboratory of Frozen Soil Engineering,Cold and Arid Regions Environmental and Engineering Research Institute,Chinese Academy of Sciences,Lanzhou,Gansu 730000,China

2.Key Laboratory of Mechanics on Disaster and Environment in Western China of Ministry of Education,Lanzhou University,Lanzhou,Gansu 730000,China

3. College of Civil Engineering and Mechanics,Lanzhou University,Lanzhou,Gansu 730000,China

1 Introduction

In China,6.6% of the land area is covered by loess.The loess is widely distributed in the seasonal frozen soil region in the midwest part of the country and is commonly used as a building material for earth dams,embankments,subgrades,and slopes (Lian,2010).However,the freeze-thaw cycle can change the arrangement and connection of the soil particles in loess so that the structure of the soil changes dramatically(Zhanget al.,2012,2013).In cold regions,shear strength is considered one of the essential properties to determine foundation bearing capacity.Because of the fierce rheology in cold regions,practical frozen soil engineering usually uses long-term strength and long-term deformation to design foundations.When soils form new structures,the long-term strength and the long-term deformation changes affect the stability of frozen soil engineering.Therefore,research on the long-term shear strength of soils under the freeze-thaw cycle is getting more and more attention from scholars all over the world.

Since the 1960s,China has successively used various test methods such as spherical die,wedge block shear,direct shear,and triaxial compression to systematically study the long-term shear strength of frozen soil (Zhu,1988).It usually takes much time to determine the long-term strength of frozen soil (including shear strength of frozen soil),but the latter can be tested relatively quickly with a spherical template indenter.When using this device to determine the shear strength of frozen soil,the frozen soil is regarded as an ideal viscous body,which has been proved to be reasonable by many scholars (e.g.,Chamberlainet al.,1972;Ourvy,1985;Zhu,1988).The shear strength of frozen soil increases as temperature decreases,and decreases with longer load durations.The instantaneous strength also increases with water content increases.However,because of the volatile rheology of frozen soil,long-term strength is different from instantaneous strength,which mainly relates to the content of ice and unfrozen water (Ma,1983).Roman and Zhang (2010) tested undisturbed and remodeled moraine loam after 3,6,20,and 40 freeze-thaw cycles,and showed that long-term strength and instantaneous strength decrease as the number of freeze-thaw cycles increases.Direct shear tests on Lanzhou loess after freezing and thawing settlement revealed a decreased degree of soil cohesion after more freeze-thaw cycles,and that the internal friction angle increases slightly with temperature gradient increases.Moreover,for the same freezing temperature gradient,soil cohesion after freeze-thaw first increases and then decreases as dry density increases.Therefore,there is a critical dry density beyond which the soil cohesion after freezing-thawing does not change (Songet al.,2008).Donget al.(2010) measured the cohesion of loess with direct shear tests after repeated freezing-thawing and showed that cohesion first increases and then decreases as the number of freeze-thaw cycles increases.Generally,the cohesion can drop to its lowest values within 10 cycles,and the internal friction angle can be considered to remain essentially unchanged.

So far,there have been many studies on the shear strength of frozen loess cohesion,but less pertinent research on long-term strength and how it changes with the number of freeze-thaw cycles.Although various engineering methods are used to relieve or eliminate the collapsibility of loess and enhance its strength as much as possible (e.g.,compacting and prewetting the loess),the structure and strength of loess subgrade in seasonal frozen soil regions will change and generate large,uneven settlements and possible collapse (Yanget al.,2005).The silt content of Fuping loess is more than 50% and the silt has strong frost susceptibility (Wanget al.,2009).Therefore,in this paper we chose remodeled and saturated Fuping loess as the study object.We measured the change in the long-term strength of soil cohesion after different numbers of freeze-thaw cycles in order to obtain the change law pertaining to long-term strength of soil cohesion under the condition of freeze-thaw cycles.

2 Soil properties and testing procedures

The basic properties of undisturbed Shaanxi Fuping loess are shown in table 1.The test structure diagram is shown in figure 1.The C boxes represent freezing-thawing cycles and their numbers mean cycle index.The remolded and saturated soil samples were prepared with a diameter of 61.8 mm and a height of 20 mm.According to the local meteorological data,the freeze and melt temperatures were respectively designed as-20 °C and +20 °C.Under closed-state conditions,the freeze-thaw cycles were set for 4,6,8,10,50 events in the constant-temperature box.The freezing and thawing times were set at 2 h each.After the freeze-thaw cycles,the frozen samples underwent experimental mechanical property tests at-20 °C in 24 h.To accelerate test stability,15 kg of vertical loading was applied.The strength test was conducted at least six times on each same soil sample.

Figure 1 The test structure diagram

3 Results

The spherical indenter contacts with the surface of the soil sample from the point to the surface under the vertical load,and the inhomogeneity of the soil samples is mainly responsible for the discreteness of the results.Therefore,at least six measurements must be made on each loess sample.As shown in figure 2,it is a reflection of the frozen soil rheological process that the cohesive force became smaller with as the time of tests increased.Under the fixed load,after 24 h (1,440 min) the strength of the frozen loess became mainly stable,and the strength represented in this state was the long-term strength.In all the tests,the strength of the eighth cycle was the largest;at the sixth and tenth freeze-thaw cycles the long-term strength was at the middle level;and it was lowest at the fourth and the fiftieth cycles.Accordingly,the change of the total deformation modulus was similar to the long-term strength.It can thus be concluded that a sample that had a greater total deformation modulus had a stronger ability to resist deformation and had higher strength.

Table 1 Physical properties of the undisturbed soil sample

Figure 2 Changes in equivalent cohesive force and total deformation modulus of frozen loess during freezing-thawing

Figure 3 displays the changes of density,dry density,and void ratio in the tested loess samples.Clearly,the three indices almost correspond with each other,as well as the variation patterns in the first 10 cycles.The density and the dry density decreased while the void ratio increased.Moreover,the density and the dry density were at maximum in the fourth and the eighth cycles,which also had the minimum void ratios.At the fiftieth cycles the density and the dry density were the lowest while the void ratio was the largest.

Figure 3 Changes in some loess physicals properties during freeze-thaw cycles

4 Discussion

Soil physical properties will partly reflect its mechanical properties.However,its long-term strength is the representation of internal structure,mineral composition,the physical properties,and other factors.Because it is not appropriate to determine the long-term strength by only analyzing the physical properties,the time analogy method,which establishes the connection between time and the long-term strength,has been usually used.By way of inducing and strengthening the factors which affect the deformation process,the time analogy method reveals the relationship between the stress,strain,and time.For frozen soil,these factors are temperature,stress,salinity,peat formation,ice content,and so forth.Based on the given elements,the temperature-time analogy method,the stress-time analogy method,and other analogy methods are named.Although the number of freeze-thaw cycles is a direct factor which has great influence on frozen soil,linking the effect of the number of freeze-thaw cycles is not universally done.In this research,the number of freeze-thaw cycles was set as an important factor for predicting long-term soil strength.

Figure 4 shows the fitting curve and the equivalent cohesive force (C) obtained by our experiments.When converting the number of cycles to the actual time elapsed,the whole change curve of strength could not be chosen,so we had to select stable reduction or increase that was very important.As shown in figure 2,the variation of the strength was strong in the first 10 freeze-thaw cycles,whereas the equivalent cohesive force began to decrease steadily after only 8 cycles.Therefore,the test data after those eight cycles has been applied in figure 5;the six groups of data for each sample are averaged.After the freeze-thaw cycles were transformed into time (meaning one cycle =4 h = 240 min),the mechanical experimental results and the time axis could be linked.

Figure 4 The fitting curve between elapsed time and soil cohesive strength

Figure 5 Prediction curves of the long-term strength of frozen loess

Based on the fitting curve in figure 4,figure 5 presents long-term soil strengths predicted according to three different methods.First,Vyalov (1959) produced a logarithmic equation based on long-term damage conditions and creep equations.Then,Roman(1987) assumed the thermal fluctuation characteristics of the failure process and produced a power equation for the long-term strength which includes the dynamic parameters.And finally,Wuet al.(1983) predicted long-term soil strength using the power equation which contains stress parameters as well as the failure time.Figure 5 presents the prediction results from these methods.As shown in figure 5,the calculation results of Vyalov’s equation are larger than results of the other two methods,but the latter two results are relatively close.A lot of tests data processed show that the latter two methods may make the long-term strength higher linear correlation.Moreover,Roman(1987) combined the time of atomic free oscillation and Wuet al.(1983) established a relation between predicted strength with initial strength in calculations that could achieve more accurate results.

5 Conclusions

1) After repeated freeze-thaw cycles,the strength of frozen loess has a general decreasing trend overall.The change of strength in the first 10 cycles is unstable;the cohesive force reaches its peak after only 8 freeze-thaw cycles,and then the strength decreases with the increasing number of freeze-thaw cycles.

2) The relationship between the soil density and the void ratio has a good correlation;in other words,when the density goes down,the void ratio goes up.The variation of these parameters is volatile within the first 10 cycles,and at the eighth cycle the density and dry density are at maximum while the void ratio is at minimum.However,the changes are not completely identical because the long-term strength of frozen loess does not solely depend on its physical properties.

3) By converting the number of freeze-thaw cycles into elapsed time in the tests,three different forecasting methods of long-term soil strength could be assessed.The soil equivalent cohesive force after 10 years,20 years,or 30 years can thus be estimated.

This project is supported in part by the Natural Science Foundation of China (Nos.41301070,41301071),the West Light Program for Talent Cultivation of the Chinese Academy of Sciences(2013-03),and the project sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars of State Education Ministry,granted to Dr.Ze Zhang.

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