The Long-term Performance Of Typical Improved Soils

Problem soils with poor engineering properties, such as expansive soil, soft soil,laterite soil and silts, are commonly encountered in practical engineering. The universaltreatment technology is improved by adding lime, cement or fly ash. However, moreresearch is focus on improvement mechanism, improvement technology and improvementeffect during site construction. In other words, people take more attention on the earlystrength of improved soils than the long-term strength. In fact, mechanical properties ofimproved soils will produce different extent deterioration under the long-term effect ofatmosphere (freeze-thaw cycle, drying-wetting cycle) and environment (salt solution ionerosion). Unfortunately, current engineering design specification or standard, also did notconsider the negative influence on engineering service brought by the attenuation oflong-term strength. Therefore, based on silts and soft clay, this paper discussed theevolution of the long-term performance of improved soils. And micro-mechanics oflong-term performance degradation was revealed by the microscopic tests, scanningelectron microscopy. Then, some suggestions are given for the engineering protection inthe view of long-term mechanical properties. The main contents and conclusions are asfollows:1) Relying on to silts sampled from a highway, the experimentation on the basiccharacteristics of silts is carried out. The research of compaction characteristics and CBRcharacteristics has been done. The results show that moisture content has very significantinfluence on the strength of silts. Compared with silts, the CBR of cement improved siltshas obvious promotion.2) The freeze-thaw cycle tests and unconfined compressive strength tests are carriedout at different compacted degree and initial moisture contents, which could explore theinfluence law of lime-improved silts and cement-improved silts under freeze-thaw cycleseffect. It is found that the unconfined compressive strength decreases with the number offreeze-thaw cycles increasing. Finally, it reaches a stable stage after6freeze-thaw cycles.The high initial moisture is, the attenuation amplitude of the compressive strength is moreat the same number of the freeze-thaw cycles. In order to find out the influence law of themicroscopic structure under the effect of freeze-thaw cycles, the microscopic tests ofimproved silt are carried out. And then find out the damage mechanism of the microscopicstructure under the effect of freeze-thaw cycles. It shows that different numbers of freeze-thaw cycles and initial moisture contents have less effect on the small pore (less10nm). Freeze-thaw cycles did major damage to the large pores (0.01～100μm) between thesoil aggregate, so it made the strength of improved silt decrease.3) For cement-improved soft clay, in order to find out the influence law of under theeffect of freeze-thaw cycles and initial moisture contents, the freeze-thaw cycle tests andcompression tests are carried out. The results show that the α0.1~0.2of improved soft clayunder low water content (25%,27.5%,30%,32.5%) presents a growth trend of gentle, andthe loss rate of E0.1~0.2is in between25.67%and31.40%. But for high water content(35.0%), the α0.1~0.2increase obviously while the loss rate of E0.1~0.2is as high as54.74%, and the compression performance has significant deterioration.4) The freeze-thaw cycle tests and unconfined compressive strength tests are carriedout. It is found that the unconfined compressive strength mainly decreased after3freeze-thaw cycles with the digital drop of0.66、0.55、0.66、0.73MPa, and the attenuationextent is respectively27.6%,29.7%,59.1%,93.6%. Freezing and thawing cycles showsstrongly influences on the strength of improved soft clay, and the loss rate of strengthbecomes higher with the water content increasing. For finding out the influence law of themicroscopic structure under the effect of freeze-thaw cycles, the microscopic tests ofcement-improved soft clay are carried out. It shows that different numbers of freeze-thawcycles and initial moisture contents have common effect on the pore (0.01～100μm). Thepore develops from d=0.4μm to d≥10μm with the numbers of freeze-thaw cycles and theinitial moisture content increasing.5) In the light of improved silts and improved soft clay, the immersion in salt solutiontests under different days and unconfined compressive strength tests are carried out. Theresults show that the strength of brine soaking is lower than water soaking. For improvedsilts and improved soft clay, the intensity difference of brine soaking and water brinereaches the maximum after6days and3days.At the same time, the calculation of ionictransference is conducted. Consistent with the test results, the adsorption capacity ofimproved silts and improved soft clay samples on the salt ion reaches saturationrespectively soaking5~6days and2~3days.