| Expansive soil is a special clay soil with engineering properties such as water swelling,disintegration,softening,water shrinkage,dry cracking and hardening.The damage caused by such engineering characteristics to the engineering structures such as roadbed,foundation and slope constructed in expansive soil areas often has the characteristics of long-term potential,suddenness and recurrence.Such damage often causes huge economic losses.In the production activities of various cold zone projects,the nature of the expansive soil and its changes with the freeze-thaw state must be fully considered.Three improved expansive soil schemes were used to compare with ES to reduce or eliminate the expansion and contraction of expansive soil.The test soil was taken from Linyi City,Shandong Province,and the expansive soil was moderately weak expansive soil.Two kinds of specimens were prepared with the measured optimum moisture content and maximum dry density as the control indexes,and the dimensions were 3.91 cm × 8 cm and 6.18 cm × 2 cm for the strength test and the expansion ratio test,respectively.The test selected a total of 7d,28d,60d,90d four ages,pure expansive soil(ES),cement improved soil(ES-C),steel slag powder+cement improved soil(ES-SSP-C),steel slag powder+cement+sodium hydroxide improved soil(ES-SSP-C-SH)four sets of samples,set-5℃,-10℃,-15℃ three temperatures for expansive soil freeze-thaw cycle test.The expansion potential of the expansive soil is obtained by the volume change during the freeze-thaw cycle,and combined with the difference of the mass change rate,it is found that the volume change rate difference is proportional to the difference of the mass change rate.The volume change of ES shows continuous "frozen and swell"characteristics,and the volume is gradually increasing,which reflects the accumulation of damage to the ES during the freeze-thaw cycle.In the gradual increase of age,the volume expansion ratio of the improved soil decreases.The trend indicates that the older samples have stronger volume control ability in the freeze-thaw cycle.The volume change rate of the ES-SSP-C-SH sample in the multiple freeze-thaw cycles is not much difference,indicating it’s the ability to maintain sample stability is stronger.At-15℃,the volume change rates of ES and improved soil reached the maximum.Compared with ES-C and ES-SSP-C-SH samples,the difference in mass change rate of ES-SSP-C sample at-10℃ is the smallest,the volume change rate is also the smallest,and the expansion state is also the weakest.With the increase of the number of freeze-thaw cycles,the expansive soil sample changed from unstable state to dynamic steady state,and its freezing shrinkage and swelling amount became more and more stable.The ES-SSP-C<ES-C<ES-SSP-C-SH<ES sample is known from the sample of free expansion rate and free expansion ratio,and the ES-SSP-C sample is resistant to low temperature due to the addition of steel slag powder.The material with good volume stability makes the expansion rate the lowest;the added NaOH provides Na+for the interlayer structure of montmorillonite,which makes the free expansion rate and free expansion ratio of ES-SSP-C-SH sample larger.In the one-dimensional no-load expansion rate test,the three improved soil samples showed a phenomenon of a large amount of sinking after a short expansion at different ages and temperatures,and the freeze-thaw cycles caused the samples to show up over-consolidated soil properties.However,the above phenomenon does not occur in ES.The expansion rate of ES sample increases with the decrease of temperature and the increase of the number of freeze-thaw cycles.The maximum expansion rate of-15℃ can reach 14.32%.The ES-SSP-C-SH sample has the best performance in the one-dimensional expansion ratio sample compared to the other two sets of samples.The value of the no-load expansion ratio in each temperature is small,and the lower the temperature,the tolerance the ability is stronger than the other two improved soils.The unconfined compressive strength test shows that the strength of ES has nothing to do with the age of curing,only related to the water content,and the water content is inversely proportional to the strength.The relationship between the unconfined compressive strength of improved soil samples was ES-SSP-C-SH>ES-C>ES-SSP-C at 7day,28day,60day and 90day ages,ES-SSP-C-SH sample improvement program is more superior.Under the action of freeze-thaw cycles,the strength of various soils has different degrees of damage,and the maximum intensity attenuation is generated at the 1st freeze-thaw(F-T)cycle.The strength loss rate of the ES-C sample is greater than that of the ES-SSP-C sample due to the anti-freeze and durability effect of the steel slag powder.In freeze-thaw cycles at different temperatures,the strength loss increases with decreasing temperature and the lowest at-15℃.The undrained and unconsolidated(UU)triaxial shear test shows that when the 100 kPa confining pressure is used,the ES-C sample is more likely to break down after the shear equilibrium and the deviatoric stress decreases rapidly compared with the other two sets of improved soil samples.The shear strength index c and cp values are greatly improved in the improved soil compared with the ES.The most improved one is the ES-SSP-C-SH sample.The internal friction angle is much larger than that of ES,which improves the slope stability.Electron microscopy of ES and improved soil samples before and after freezing and thawing were carried out from low magnification and high power microscope.Compared with ES,the distribution of three improved soil particles was very dense,and the gap between the particles was also smaller.The compactness of the ES-SSP-C samples in the three improved soils was slightly worse than the other two.The hydration product ettringite(AFt)was found in the ES-C sample under high magnification,and the material has a certain swelling property.More C-S-H was found in the ES-SSP-C-SH sample at high magnification,which played a direct role in the connection of soil particles and the improvement of strength.The study also found that the lower the temperature,the greater the influence on the ES and the improved soil sample.The fracture and pore development at-15℃ are more obvious,which further explains the reason why the volume deformation rate and the strength decay rate reach the maximum value. |
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