Study On Control Standard For Degree Of Compaction Of High Liquid Limit Soil Subgrade Considering The Effect Of Absorbed Bound Water

High liquid limit soils are widely distributed in southern China.High liquid limit soils have high natural water content,high fine particle content,strong hydrophilicity and poor water stability.It is difficult to change high liquid limit soil fillers from natural moisture content to the optimum moisture content by drying in the humid climate and roll in the southern region,so that they can meet the compaction control standard.The existing engineering practice in China shows that,using high liquid limit soil as embankment filling material for expressway can also ensure the long-term stability of roadbed.And reducing the compaction control standard properly can still meet the strength and deformation requirements of subgrade for filling material.However,the compaction control standard has not been unified.Because water content is the key factor affecting the compaction of subgrade and combined water is the main form of water in fine-grained soil.It is of great theoretical and practical significance to study the influence of combined water on the performance of high-liquid limit soil roadbed,and to put forward the compaction control standard of high-liquid limit soil roadbed accordingly for promoting the research of special soil roadbed engineering and green highway construction.In this paper,several soil samples from Hainan,as well as a clayey sand sample from Changsha,were selected.Various tests like scanning electron microscope(SEM),heavy wet compaction,soaked California Bearing Ratio(CBR),and unsaturated consolidation tests were conducted on those soil samples.Absorbed bound water content(ABWC)was measured using a volumetric flask method to evaluate its influence on compaction properties,strength,water stability and compressibility of high liquid limit soil.An equation for estimating lower bound of the degree of compaction of high liquid limit soil was proposed and validated by considering the ABWC as a part of the solid phase.The test results show that the ability to absorb bound water of the high liquid limit soil is much stronger than the ability of the clayey sand due to the presence of a large number of micropores and laminated structures contained in the high liquid limit soil.ABWC is closely related to plastic limit and is about 0.853 times of plastic limit.It can be attributed to the influence of ABWC that the optimum water content(OMC)higher but the maximum dry density is lower for high liquid limit soil compared with the clayey sand.The swelling ratio of CBR specimens of the high liquid limit soils increased significantly after inundation when the initial water content is lower than ABWC.ABWC has an active effect in maintaining the high strength and low compressibility of high liquid limit soil in the state of high water content and can keep stable during the operation period of the embankment.According to the case study of Quan-San Expressway,the embankment constructed under the control of high water content and new standard has small settlement and good long-term stability.The settlement rate of high liquid limit soil roadbed is about 2‰.When using the high liquid limit soil with high water content to fill the embankment,the lower bound of the degree of compaction is not a fixed value but relevant to ABWC and the OMC.The lower bound decreases further when ABWC exceeds the OMC.The rationality and practicability of the new standard are further validated through the analysis of relevant literature and case studies and the application in the filling of high liquid limit soil roadbed of Hainan.For the first time,this paper reveals the internal reasons why the compaction control standard of high liquid limit soil roadbed can be reduced from the perspective of combined water,puts forward and verifies the calculation formula of the lower limit value of compaction control with the optimum moisture content and adsorbed bound water content as parameters.The research results can provide reference for the design and construction of high liquid limit soil roadbed and the revision of relevant technical standards.