Variation Of In-situ Parameters And Mechanism Analysis Of Marine Clay In Pore Water Salinity Fluctuation Region In Lianyungang

The soft marine clay,widely distributed in Lianyungang,North of Jiangsu,comprises high pore water salinity and montmorillonite minerals.The engineering properties of clay are affected by the interaction between them.The parameters used in design are based on the current pore water salinity environment.However,with the permeation of fresh water and the leaching of salinity,the physical and chemical properties of soft clay change simultaneously,leading to engineering problems like landslide,failing of ground bearing capacity,differential settlement,and increasing deformation,etc.Thus,it is necessary to have a proper cognition on engineering properties of marine soft clay and their evolution.In-situ test is capable of acquiring engineering parameters under in-situ stress state by a more effective and economic way compared with conventional investigation approach.However,the current analytical framework has not taken the pore water salinity effect under consideration.In this study,the influences of pore water salinity effect on in-situ parameters were investigated,and the major conclusions are listed as follows:(1)Cone penetration test(CPTu),Vane shear test(VST),and Dilatometer test(DMT)were conducted at 12 sites in Lianyuangang marine clay.The results showed that a thick soft marine clay layer lied between the surface hardshell and underlaying hard clay.The undisturbed shear strength,remoulded shear strength and sensivity generally increases with the increase of depth.The correlation among the in-situ parameters proves using adaptive cone factors chosen according to the stability of cone resistance curve to estimate shear strength was more accurate.(2)The fine content and clay content of marine clay in this region varies from 87.2%~99.2%and 41.2%~79.4%respectively.The mineral consists of quartz,anorthose,feldspar,calcite and clay minerals,where the illite/smectite sensitive to salinity effect is generally over 10%.The salinity in pore water varies from 0.51%~6.25%with an average of 2.7%.As a result,it is of high possibility that the clay in this region is under the influence of pore water salinity effect.(3)The influence of pore water salinity on the classification parameters of the Soil Behavior Type chart(SBTn),the Eslami-Fellenius chart and the DMT chart was discussed respectively.Theoretical solutions of cavity expansion in modified Cam clay were proposed to interpret CPTu.It was inferred that pore water salinity effect could influence the classification parameters and lead to the deviations of projections in classification chart.The normalized cone resistance(Q_t)and normalized friction ratio(F_r)in SBTn chart magnify almost simultaneously with critical stress ratio M,leading the projection to move towards up-left on the chart.Despite the parameters used in Eslami-Fellenius chart were also affected,soft clay was generally identified for the low sensitivity of their parameters to the change of pore water salinity.(4)The isotropic consolidation test and undrained consolidation test were conducted to investigate the mechanical properties of clay with different salinity concentration.The results showed that the internal friction angle,critical stress ratio and shear strength increased with the increase of salinity concentration.(5)Model tests of cone penetration and vane shear test in clay with salinity concentration of0.4%,3%,6%consolidated under 53.6kPa were conducted.The results showed that compared with the sample with 0.4%salt concentration,the tip resistance and undisturbed undrained shear strength were higher in the sample with 3%concentration,while the sleeve friction and remoulded shear strength were lower.The properties of sample with 3%and 6%salinity concentration showed few differences.(6)A numerical simulation combined with theoretical calculation based on the parameters measured by laboratory test was conducted to obtain the SBTn classification parameters of clay with different salinity concentration.The results showed that the sleeve friction of clay with lower salinity concentration was lower than that with higher concentration,while the tip resistance and the excess pore water during penetration were higher,leading the projection to move towards up-left on SBTn chart.The osmotic suction was introduced to build a constructive model based on SBTn chart to modify the influence of pore water salinity effect on soil classification,on which the projection was moved towards right-down to the actual position where the osmotic suction equals to zero.