Study On Time For Unloading Of Preloading Of Embankment Of Expressway On Soft Ground

A lot of soft clays are distributed over the coastal zones and some inland zones of China. The soft grounds under embankment of expressway are usually treated by preloading method. Preloading method can be classed as insufficient-load, equal-load preloading, preloading, surcharge preloading according to the relationship between preloading load and design load of embankment. Unloading for construction of pavement and decision of time for unloading are all encountered by the three preloading methods. Early unloading leads to large postconstruciton settlement which cause not only more maintenance but also vehicle bumping which lowers safety, comfort and velocity of vehicles. Late unloading defers operation of expressway which reduces economic and society benefit. Hence decision of time for unloading of embankment of expressway on soft ground treated by preloading method is very important and is attached much importance to.Postconstruction settlement method and settlement rate method are often employed to decide time for unloading of expressway treated by preloading method. The postconstruction settlement is predicted and settlement rate is calculated by use of monitored settlement. If they are less than allowable value, unloading is permitted. The two methods are so simple and useful that they are widely used.Although decision of time for unloading of expressway treated by preloading method is placed much emphasis on, the result isn't ideal. The practical postconstruction settlement is often larger than that predicted and vehicle bumping occurs extensively. The main reasons are:(1) The components of postconstruction settlement weren't considered completely and properly which lead to postconstruction settlement calculated or predicted less than real. (2) The defference between preloading load and design load of embankment isn't taken into accounted properly .On the other hand, engineers are often confused by such facts:(1)The time gotten by settlement rate method is often later than that by postconstruction settlement method. (2) Settlement rate method lacks theory base. (3) Same settlement rate is adopted by settlement rate method for embankments with different geological conditions and allowable postconstruction settlements.Aiming at the above problems about decision of time for unloading of embankment of expressway on soft ground treated by preloading method, such subjects as follows were investigated through theory analysis, lab tests and in-situ experiments etc, basing on consulting of research background and fully use of current study results. 1. Relationship between settlement of expressway on soft ground treated by preloading and pressure of embankment is studied to take the defference between preloading load and design load of embankment into account properly. Firstly, equation of superimposed stress in ground under embankment is derived and the relationship between superimposed stress in ground under embankment and thickness of embankment is analyzed. The relationship between modulus of compressibility of soft clay and pressure is studied through consolidation test and strain controlled triaxial compression apparatus. It is shown that superimposed stress of ground from surface to depth 20m under embankment is basically in direct proportion to the thickness of embankment and modulus of compressibility of soft clay is basically unchanged when the pressure of embankment is less than 200kPa and it can be induce that the pressure of embankment of expressway is basically in direct proportion to settlement of embankment. Secondly, the relationship between settlement of embankment of expressway and pressure of embankment of expressway is studied analyzed through analysis of relationship between monitored immediate settlement or consolidation settlement of many expressways and pressure of embankment. It is shown that the settlement of embankment of expressway on soft ground is basically in direct proportion to pressure of embankment. Further analysis shows that the settlement of embankment of expressway on soft ground is basically in direct proportion to height of embankment too.2. Relationship between postconstruction settlement and residual settlement, components of postconstruction settlement are analyzed to get postconstruction reasonably and decide time for unloading properly. Calculation method and prediction method of postconstruction settlement of embankment are put forth. It is proposed that postconstruction includes postconstruction compression of embankment, immediate settlement of ground caused by vehicle, recompression settlement of ground caused by pavement, postconstruction initial compression settlement and postconstruction secondary consolidation settlement etc. Except postconstruction primary consolidation settlement other components of postconstruction settlement should take effect of overlay maintenance of pavement into account. It is suggested that secondary consolidation settlement should be calculated with equivalent time. A simper method for calculating equivalent time is proposed and its result is relatively ideal.3. Error of postconstruction settlement predicted by use of monitored surface settlement is analyzed through theory analysis and compression tests and in-situ monitors. The ultimate settlement of each stratum is predicted and total ultimate settlement is gotten. The settlement gotten by the above accumulative prediction method is compared with the ultimate settlement predited by use of surface settlement. It is shown that error of postconstruction settlement predicted by use of monitored surface settlement exists and except ground with thick soft clay stratum under treated zone whose postconstruction settlement should be gotten through settlement of each stratum the error can be neglected.4. Relationship between predicted postconstruction settlement and secondary consolidation settlement is investigated through conventional consolidation tests, long term consolidation tests and in-situ experiments. It is shown that the ratio of time responding to ultimate settlement predicted by hyperbola method and time of primary consolidation is usually between 2 and 4. So the ultimate settlement predicted includes only 30~60% of postconstruction secondary consolidation settlement and postconstruction secondary settlement should be calculated for postconstruction settlement when hyperbola method is used to predict postconstruction settlement.5. Relationship between settlement rate and residual settlement is investigated to integrate postconstruction settlement method and settlement rate method and to decide reasonable settlement rate criterion for unloading. Firstly, error of equation of degree of consolidation proposed by Zeng Guoxi is analyzed. The relationship equations between settlement rate and residual settlement are induced by use of consolidation equation proposed by Zeng Guoxi and Voigt-Kelvin elastic viscoplastic model. It is shown that settlement rate and residual settlement can be demonstrated by a broken line consisting of two straight lines, the intersection point of the two straight corresponds with the end of primary consolidation settlement. So the settlement rate is in proportion to residual primary consolidation settlement and is in direct proportion to residual secondary consolidation settlement. The relationship equation between settlement rate and residual settlement is inferred from hyperbolic time vas settlement curve. It shows that settlement rate is in direct proportion to square of residual settlement. Then, relationship between settlement rate and residual settlement is studied through lab consolidation test, model experiment and in-situ monitoring etc. It is shown that settlement rate and residual settlement can be demonstrated by a broken line consisting of two straight lines, the intersection point of the two straight corresponds with end of primary consolidation settlement. Settlement rate is basically in direct proportion to square of residual settlement.6. Basing on the above studies postconstruction settlement method and settlement rate method which can take into account such factors as postconstruction compression of embankment, recompression settlement of ground caused by pavement, postconstruction secondary consolidation settlement and difference between preloading load and design load of embankment etc are proposed and corresponding equations are induced. It is proven by monitored postconstruction settlement that the results of the methods proposed by the paper are relatively ideal.Finally, further studies needed are simply listed.