Study On The Dilatancy Properties Of Paleo-clay In Hefei Area

Located in the Jianghuai plain,the upper strata of Hefei region is generally covered by the paleo-clay,which is deposited in late Pleistocene or even earlier in the Quaternary period.Under thecondition of that the paleo-clay is subjected to shearing action,the shear stresson the shear surface will lead toshear dislocations,which would trigger decline in soil density and ultimately results in volume expansion of the soil,which infers that paleo-clay possess a characteristic called dilatancy.To examine the mechanism of dilatancy,we make a large amount of investigation on the construction quality of pipe piles,the collapse or extensivedisplacementof the foundation pit.Results indicate that there exists inseparable relationship between these problems and compacting effect,while compacting effect is the external manifestation of the dilatancy.On the basis of the geological structure and hydrogeological data of Hefei area,associated with the characteristics of paleoclimate change,the origin and distribution of paleo-clay in the study area are analyzed,meanwhile,some basic characteristics of paleo-clay,such as overconsolidation,expansibility and fissures,have been examined,the relationship between these properties and the dilatancy is further analyzed.Extensive research on the dilatancy of paleo-clay has been made by methods of geotechnical test data collecting,laboratory test design and field test.The analytical solutions of the compacting effect are obtained throughcavity expansion method,furthermore,by the aid of Abaqus the numerical simulation of the pile sinking process is carried out.Comparingtwo results of the radial,vertical,circumferential displacement and stress fields triggered by the compacting effect,we reap further understanding on the effect of compacting.Based on that,the conclusion is applied to solve engineering problems.It is found that the dilatancy develops mainly while the axial strain is less than 5 percent in the triaxial tests,afterwardsthe dilatancy will not develop and remain stable around 0.6 to 2.8 percent.According to analytic solutions based on cavity expansion method,the average of plastic zone radius of the pile is 2.8 times of the pile diameter,while the result is from 4 to 7 times of the pile diameter according to abaqus numerical numerical simulation.The effect of compaction is generally manifested by the uplift of shallow soil and the radial displacement of deep soil.Some measures are researched for reducing or eliminating the compacting effect,including increasing pile spacing,predrilling,reasonable arrangement of the piling order,reducing piling speed,setting the anti compacting effects ditch and wall.