On The Intrinsic And Additional Property Of Compression Behaviour Of Naturally Sedimentary Soils In Terms Of Depositional Environment

High-speed railway has been constructed on a large scale in most areas of China,especially in eastern areas.The settlement of embankment surface is strictly controlled according to revelant codes in order to ensure the safety and comfortablity of high-speed railway during operation process.The critical point to realize the precise control of the settlement of embankment surface is to evaluate and predict the compression behaviour of ground soil accurately.Based on the profound understanding of the formation process of fabric and bonding between soil particles,the compressibility of naturally sedimentary soils can be divided into intrinsic property and additional one.It is claimed in this paper that the intrinsic property refers to the basic and inherent properties,while the additional compressibility is developed as the fabric and bonding between soil particles is gradually formed in the geological process.By investigating the influencing mechanism of intrinsic and additional properties respectively,the refinement method to evaluate the compressibility of natural soil is established in terms of index properties and important environmental factors.The main work contents and conclusions are listed as follows:I)The technical idea that the compressibility of naturally sedimentary soils should be divided into intrinsic property and additional one is put forward.1)The necessity of dividing the compressibility of naturally sedimentary soils.When soil specimen is completely reconstituted once it settled down,there is no bonding and fabric between soil particles.The soil sample represents innate characteristics,which can be termed as intrinsic property.After the settlement of soil paricles,the microstructure of soil is developed in the endless geological history.Subsequently,the new mechanical property,which can be termed as additional property,is produced.It is remarkable that the origin of intrinsic and additional property is completely different.Therefore,the influencing factors of these two properties are also varying with each other.To quantitatively analyze each factor that could influence the compressibility of soils,the integral properties are necessary to be divided into intrinsic property and additional one.2)The feasibility to conduct study on intrinsic property and additional one respectively.Intrinsic property refers to the mechanical property that is measured on completely reconstituted soil.Thus,natural sedimentary soil can be mixed with water to reach the water content equal to or larger than liquid limit to get completely reconstituted soil.Then the soil sample can be used as the experimental material to get the intrinsic property.Moreover,the compression curve of natural soil lies above the curve of corresponding reconstituted sample,the gap between these two curves represent the evolving law of additional compressibility.Therefore,the formula that investigating the intrinsic and additional properties separately is reasonable.II)The influencing mechanism on intrinsic compressibility and intrinsic compression curve for reconstituted soil1)The oedometer test on Liyao reconstituted clays is performed,and the test data of more than twenty kinds of soils which are reconstituted to initial water content equal to liquid limit is collected.The void ratio under effective vertical stress of 100kPa and the compression index,which are denoted as e₁₀₀^*and C_c^*respectively,are defined as intrinsic compressibility constants.The basic idea that e₁₀₀^*and C_c^*are adopted to represent intrinsic property is put forward.The correlation of e₁₀₀^*and C_c^*with void ratio at liquid limit is discussed,and the influencing mechanism of liquid limit on intrinsic compressibility is revealed.2)Oedometer tests are performed on several soil samples at different water contents for a given kind of soil.With the ratio of intial water content to liquid limit as the indication of the variation for water content,the correltion of e₁₀₀^*and C_c^*with normalized initial water content are discussed.The influencing mechanism of initial water content on intrinsic compressibility is revealed.The evaluation method for intrinsic compressibility constants e₁₀₀^*and C_c^*is eatablished in terms of liquid limit and initial water content.3)Given the limited influence of plasiticiy index,the basic idea,which claimes that plasiticity index should be used to modify the value of e₁₀₀^*and C_c^*evaluated on a basis of liquid limit and initial water content,is proposed.This paper pointed out that compression index should be modified for the soil samples with low plasticity index.Finally,the evaluation method for intrinsic compressibility constants e₁₀₀^*? C_c^*is established with liquid limit and initial water content as critical parameters,as well as with plasticity index as modification factor.4)With e₁₀₀^*and C_c^*evaluated on a basis of index properties,the evaluation method for intrinsic compression curve is proposed in terms of liquid limit,initial water content and plasticity index with the Burland framework as a reference.The reasonability of the proposed method is verified by measured test data.3)Influencing mechanism on additional compressibility and additional compression curve for naturally deposited soil1)Since the compression curve of natural sedimentary soil lies above the compression curve of reconstituted one,the evaluation method of additional compression curve is put forward in accordance with the gap between these two curves.It is also pointed out that additional void index is increased with effective overburden pressure in the pre-yield regime,and decreased in the post-yield regime.Therefore,the maximum additional void index can be obtained at yield stress point.2)One borehole test data is chosen from the study area near Liyao test site located in ancient Yellow River Delta.It is noted by the author that the range of sea level can influence depositional status of natural soil to some extent since late Pleistocene to early Holocene.Furthermore,the rapid deposition since middle Holocene can significantly influence the depositional status and mechanical parameters.Then,depositional rate and range of sea level are chosen as the environmental factors that can have effect on additional properties.3)Oedometer test was performed on Liyao natural soils,and the correlation of depositional rate and years before present is quantitatively evaluated since the middle Holocene.On a basis of depositional age for soils with different buried depth,inverse proportional relationship is established between depositional rate and maximum additional void index for high-sea-level sediments.In addition,the influencing mechanism of depositional rate on additional compressibility is revealed.It is pointed out that the inverse relationship between depositional rate and maximum additional void index should be modified when the sea level remains relatively low.4)It is assumed that additional void index is linearly increased with logarithm of effective vertical stress in the preyield regime,and inversely proportional with effective vertical stress in the post-yield regime.Thus,the evaluation method for additional compression curve is established in terms of depositional rate,sea level and buried depth.Finally,the predicted compression curve can be obtained by adding additional curve to intrinsic compression curve.It is verified that the evaluation method proposed in this study is reasonable when compared with measured compression curve.IV)Simplified evaluation method for integral compressibility on a basis of ordinary geotechnical parameters,1)Standard compression modulus is adopted to represent secant slope of compression curve in low pressure level.Based on the empirical correlation between liquid limit and compression index proposed by Terzaghi,the basic idea is put forward to use liquid limit to reflect tangent slope of compression curve in high pressure level.With initial void ratio as indication of the start point of compression curve,the simplified evaluation method is established for integral compressibility of naturally sedimentary soil.2)The error factors in the evaluation method are discussed,and it is further pointed out that the error caused by the empirical equation between liquid limit and compression index is of subtle influence on the predicted compression curve.Therefore,it is claimed that the proposed method is of ideal availability.