Research On CBR Evaluation Method And Classification Scheme For Expansive Soils Fill

According to the real condition that envelope and water barrier method is used to build trial embankments with expansive soils core of different shrink-swell potential, the research group of Changsha University of Science and Technology built nine sections of trial embankments in four strips highway of Yunnan, Hunan and Guangxi provinces. These trial embankments have gained good engineering treatment results and economy environmental protection benefit.In order to promote the application of the new technique and ensure the strength and the stability of the subgrade, it is necessary to evaluate the strength of expansive soil fill correctly and establish a corresponding classification scheme.Considering the characteristics of the expansive soils and the situation that the CBR (California Bearing Ratio) of the soils seldom satisfied the requirement of current specification for subgrade design to embankment filler (CBR≥3%), some unreasonable aspects, such as the method of soaking sample, the surcharge during soaking, and the moisture content of preparing sample, are particularly analyzed, as the routine CBR test method is used to estimate the bearing capability of expansive soils. Then, according to the real condition that envelope and water barrier method is used to build trial embankments with expansive soils core, a modified CBR test method evaluating the real bearing capability of special soils such as expansive soils is studied and recommended. The method includes preparing sample on the condition of heavy compacting and the optimum moisture content by wet compaction test, changing the water infiltration method from top to lateral and increasing the weight of surcharge on the sample during soaking sample. The reasonability and feasibility of the modified method are testified by comparing the testing results of routine and improved test on six types of soils including expansive soils and inexpensive soils.Likewise, from the project reality, following some old classification indexes and developing new indexes, the paper study the engineering properties of expansive soil and the main factors deeply such as strength, deformation and construction performance when built trial embankments using expansive soils directly. According to numerous laboratory results on engineering properties of expansive soils and the experience in build trial embankments with expansive soils, using the results of modified CBR test, it puts forward a new classification scheme of expansive soils used as embankment fill based on three indices, CBR strength, swelling rate of CBR sample in the modified CBR test and consistency. The corresponding indexes were verified through tests on three typical expensive soils. The results show that this system can reflect the basic characteristics of the expensive soil and the engineering properties as embankment fill, furthermore it is very easy and convenient to obtain the three indexes. The scheme can enlarge the range of expensive soil used as embankment fill.The proposed modified CBR test and the establishment classification scheme of expansive soils used as embankment fill provide engineers with references to the design and construction of expensive soil embankments.