Experimental Study On Consolidation And Unloading- Rebound Deformation Of The Plateau Lacustrine Peaty Soil

Peaty soil(general designation of peat and peaty soil) is a kind of special soil composed of organic remainders, humus and minerals. Generally, the peaty soil is of great water content, void ratio, compressibility, rheology and organic matter, and low density, strength properties. Because of its complex composition and special structure, the properties of peaty soil is extremely different compared with other inorganic soils, but there still lack of profound and systematical understanding.There are thick peaty soil widely distributed around the Dianchi Lake and Er Lake basin. With the development of the urban construction carried on, areas around Dianchi Lake and Er Lake are the core districts to be developed. Recently, projects like deep foundation pit, rail transit and belt expressway are under construction or planning, these determine the significant theoretic and practical values of conducting research on its properties, so to guide the engineering design and construction. This thesis conducted a series of laboratory experiments based on peaty soil of Dianchi Lake and Er Lake basin, and systematically analyzed the compression and unloading rebound deformation characteristics, and illustrated the mechanisms. The concrete research contents and conclusions are as follows:(1) Through extensive collection of data of plateau lake facies peaty soil, especially in Kunming, this article studied the statistical characterization, probabilistic model, and correlation of physical property indexes. Statistical analysis showed that the variability is much more obvious than that of sludge soil in eastern shore of China. Analysis of probability model shows that most indexes submit to logarithmic normal distribution to Dianchi peaty soil. Besides, the correlation between indexes and influences of organic matter on different indexes were analyzed.(2) Based on the test results, the secondary consolidation behavior and mechanisms of plateau lacustrine peaty soil were studied, and factors such as the depth of sampling, consolidation pressure(p), loading ratio(R) and pre-consolidation load(pr) that affect secondary consolidation coefficient(Ca) were discussed. Test results showed that: ①the curve of e-1gt were mainly reverse "S", and the secondary consolidation deformation have relatively high percent in the total compressive deformation, ②the Ca of peaty soil increased with the increasing of consolidation pressure to peak value then reduced slowly and stably consistent with some other soils. While the consolidation pressure appropriate peak value of Ca didn’t agree with the preconsolidation pressure (pc), but between 100 to 200kPa.③the mechanism of secondary consolidation were analyzed by means of establishing the micro-structure models of peaty soil, and a conclusion reached to show that the peak value of consolidation pressure was the critical load of changing form porous to tightly state. ④The Ca/Cc ratio almost was a constant value, which was consistent with the values reported in the literature for similar soils.(3) Based on a series of test results, the one-dimensional consolidation coefficient(Cv) and mechanism of plateau lacustrine peaty soil were studied, and the p, R pT, and other factors were discussed. Test results showed that:①the Cv of peaty soil decreased with increasing of consolidation pressure firstly, then reduced slowly and stably. The relationship between p and Cv was a power regression expressed as Cv=mp". ②given the pre-consolidation load conditions, Cv of peaty soil was coordatinted with the nature sample when pr≤100kPa. It was lower than the nature sample once the pr>200kPa, and the Cv changed little with consolidation pressure increased, ③the influences of disturbance, R and loading method to Cv-p was not obvious ④The mechanisms of consolidation were analyzed, which caused the peculiar consolidation properties was rarely structure of natural peaty soil. With consolidation pressure increasing, the structure of peaty soil changed from porous to relatively dense state.(4) A series of experiments were conducted to study the resilience deformation characteristics and mechanism of Kunming platue peaty soil under unloading completely and steply. Test results show that: ①Under unloading steply condition, rebound deformation ratio (λ) is related to the unload ratio(r), and the critical unload ratio (rcr) was 0.5 and the drasticly unload ratio (ru) was 0.9. ②the maximum rebound deformation ratio increased linearly with the increasing of preloading pressure, presenting that the bigger pressure and the larger λ max value will be. ③ While under unloading completely condition, time effects obviously exist and the resilience deformation after unloading was divided into immediate rebound stage, primary rebound stage and secondary rebound stage. The bigger pT, and the less rebound deformation ratio, and the larger residual deformation will be. The secondary rebound coefficients (Cre) were between 0.05 and 0.16; and the Cre value increased as the preloading pressure increasing.