Depositional Environment And Geotechnical Properties For The Upper Shanghai Clays

The physical and mechanical properties of a structured natural clay depend on the composite effects of several interacting factors:deposition environment,mineral composition,grain grading and chemical properties of pore water.Soft marine clay with an average thickness of about 30 m was deposited on a surface on top of the Upper Pleistocene sediments in Shanghai.Because of numerous underground projects over last few decades,the engineering properties of Shanghai clay have been well studied.This thesis reconstructs the depositional history of Shanghai clay according to the progradation of the modern Yangtze River delta since the mid-Holocene,and conducts comprehensive research of physical,chemical and mechanical properties of upper Shanghai clays.The conclusions can be obatained as follow:?1?The upper Shanghai clay is a part of modern Yangtze river delta.A worldwide rapid transgression,named as the Zhenjiang Transgression in East China,was observed from 13⁷.5 ka BP.Then,the sea level was stable or decreased slowly until nowadays,and this epoch was just the regression.During the transgressive–regressive cycles,sediments around worldwide coastal region was deposited in marine environment.In the estuary of Yangtze River were huge of sediments deposited as sand bars,and thus the Yangtze Delta?including the Shanghai area?was gradually formed.Layer 6 and sub-layer 5_-4 were formed in a lake and drowned valley environment in different areas.Sub-layer 5_-4 is deposited at the deeply incised valley and Layer 6 is distributed mainly at the southern coastal plain.The sub-layers 5_-3⁵_-1 were deposited in a coastal or swamp environment.layer 4 and layer 3 were deposited in a marine environment?coastal-shallow sea?environment.Layer 2 sediments were deposited in a coastal-estuarine environment and experimenced phyical and chemical weathering as surface crust.There is great difference in salinity of pore water for clays and liquid limit in different sedimentary facies.Where the flow velocity in water is slower,the flocculated sate in particle associations is more.Layer 4 is typical marine clays deposited saline water where the estuary is farthest from coastline,thus the struture is the most strongest among the upper soil layers in Shanghai.?2?Basic physical characteristics of upper Shanghai clays were investigated,combined with some geotechnical engineering investigation reports.Natural water content,initial void ratio,liquid/plastic limit and liquid/plastic index have similar trends of variations with depth change.They are small at top and bottom,while large for Layer 3 and 4 at intermediate depth.Density and OCR have inverse trends.The experimental data showed that the clay content of the Shanghai clay at all depths is less than 50%,especially those at the top and bottom areas,where the clay content is as low as 20%.However,the silt content is more than 50%at all depths.This means the Shanghai clay is a silty clay.?3?The relationship of compression index?C_c?with other physical indexs was studied.Natural water content?w_n?had good linear correlation with initial void ratio?e₀?,but not obvious with liquid limit?w_L?.Since w_n is more easier than e₀ to obtain,it was used to predict C_c.Commonly used linear equation is not suitable for upper Shanghai clays,especially for w_n higher than 45%.Finally,an exponential function[C_c=0.0426(e^{0.0444 w}n-0.794)]was recommended to predict C_c for preliminary engineering design.The function was also verified by compression index of clays at other places in coastal area of eastern China.?4?Oedometer and triaxial tests of intact specimens for Layer 2⁶ Shanghai clays are conducted.Especially for Layer 4,remoulded specimens are else experimented in parallel.In drained triaxial tests,structured clay tends to produce more volumetric strain than the remoulded one.In undrained triaxial tests,structuraled clay shows the same pore water pressure index as remoulded.No obvious differences have been found in strength in both drained and undrained tests.The influence of natural structure on compression index,shear strength,volume change and exceed pore water pressure of Layer 4 are studied by the laboratory tests.In oedometer tests,structured clay with different initial void ratio tends to reach the same void ratio under 1 600 kPa vertical stress,but still not reaches the ICL of remoulded one.?5?The overconsolidaion ratio of upper Shanghai clays were studied through one-dimensional compression and traxial tests.Becaused of gentle curvature of the compression curve over the preconsolidation pressure range due to weak structure of Shanghai clays,work method proposed by Becker is cited to obatin the yield?preconsolidated?pressure in Oedometer test,which showed that OCR of Layer2 and Layer 6 were larger than 2,while those of Layer 3 and Layer 4 were about 1.15,and Layer 5 were closed to 1.5.Drained triaxial tests showed that the axial strains corresponding to peak q/p values and the maximum volumetric strains have different values for various clay layers.Compared to CD test results of reconstituted Shanghai clays,these differences reflect the distribution of overconsolidation ratio obtained in oedometer test.The unite study method composed by Oedometer and triaxial tests will contribute to further understand overconsolidated behaviour of upper Shanghai clays.?6?The structure of natural clay is destroyed in the small strain scale.To research the destruction of Shanghai clay,the procedure of small strain triaxial test is proposed.The principle and recommended methods for installation of Hall Effect device are introduced.The whole test procedure and key points are described in detail.Then some different approaches to attain modulus from stress-strain curves are presented and compared.It is concluded that the modulus is properly calculated based on the stress-strain curve that pass through the origin.Finally,an analysis on the influence of pre-contact forces?q_pre to the small-strain stiffness is presented.The characterization of stiffness in small strain?0.001%⁰.1%?is evaluated for intact Shanghai clay under various drainage conditions.The decay curve of stiffness for specimen under consolidation drained triaxial shear test is more continuous than consolidation undrained shear test.The stiffness in early shear stage measured in this study is significantly higher than that measured from conventional triaxial tests.This research can contribute to establish a small strain testing standard.