【作者】 刘吉福；

【导师】 莫海鸿；

【作者基本信息】 华南理工大学 ， 结构工程， 2010， 博士

【摘要】 我国沿海及部分内陆地区分布着大量软黏土,高速公路软基处理通常采用堆载预压法,根据堆载预压法预压荷载与路基设计荷载的关系分为欠载预压、等载预压和超载预压。三种预压方法均涉及卸载和施工路面的问题,均需确定卸载时机。卸载过早工后沉降大,不但增大养护工作量,而且造成桥头跳车,进而危及行车安全、舒适性和行车速度。卸载过迟延长工期,降低经济效益和社会效益。因此,采用堆载预压法处理的高速公路软基路基确定卸载时机非常重要,并受到重视。工程实践中,采用堆载预压法处理高速公路路基的卸载时机确定通常采用工后沉降法和沉降速率法,利用实测沉降得到沉降速率、预测工后沉降,当沉降速率小于卸载沉降速率标准、工后沉降小于容许工后沉降时,可卸载和施工路面。这两种方法简单实用、应用广泛。虽然非常重视卸载时机确定,但是堆载预压法处理高速公路路基的卸载时机确定效果不理想,大部分高速公路的工后沉降大于预测工后沉降,跳车现象普遍,其原因有:(1)考虑的工后沉降组成不完整;(2)未正确考虑路基预压荷载与设计荷载差值对卸载时机确定的影响。另外,沉降速率法确定的卸载时间往往迟于工后沉降法预测的时间,而沉降速率法缺少理论依据,使人无所适从。对不同的路段、不同容许工后沉降均采用相同的沉降速率标准也是不合适的。针对高速公路软基路基确定卸载时机存在的问题,在查阅文献、借鉴已有成果的基础上,通过理论分析、室内实验、现场测试等手段,开展了以下研究工作:1.为考虑路基预压荷载与设计荷载差值对确定卸载时机的影响,研究了堆载预压法处理的高速公路软基路基荷载与沉降的关系。首先推导了高速公路路基荷载下地基中附加应力公式,利用其分析了地基中附加应力与路基填土厚度的关系;利用压缩实验和应力控制三轴试验研究了软黏土压缩模量与荷载的关系。研究表明,路基荷载小于200kPa时,地基20m深度内的附加应力与路基填土厚度基本成正比、软黏土压缩模量变化不大,从而得到高速公路路基沉降与路基荷载基本成正比的结论。其次,利用高速公路软基路基现场实测资料分析了路基瞬时沉降、固结沉降与路基荷载的关系。研究表明,堆载预压法处理的高速公路软基路基沉降与路基荷载基本成正比,进一步分析表明高速公路软基路基的沉降与路基填土高度也基本成正比。2.为了合理计算工后沉降、确定卸载时机,分析了工后沉降与剩余沉降的关系、工后沉降的组成,提出了工后沉降计算方法及利用实测沉降推算工后沉降的方法。建议工后沉降包含路基工后压缩、汽车荷载产生的瞬时沉降、路面荷载产生的再压主固结沉降、工后原压主固结沉降、工后次固结沉降等,工后主固结沉降不考虑养护加铺的影响,工后沉降其它组成部分应考虑养护加铺的影响。工后次固结沉降建议采用等效时间,提出了等效时间简化计算方法,其计算结果较理想。3.通过理论分析和压缩试验、现场测试等手段分析了利用表面沉降推算工后沉降的误差。对各土层分别推算最终沉降然后累加得到总沉降,将之与利用表面沉降推算的最终沉降进行对比表明,利用实测表面沉降推算成层地基的最终沉降存在一定误差,通常情况下利用表面沉降推算成层地基的工后沉降的误差可以忽略不计,软黏土下卧层较厚的工程误差较大,建议采用各土层的沉降监测资料预测工后沉降。4.利用室内常规压缩试验、长期压缩试验、现场试验研究了推算最终沉降与次固结沉降的关系。研究表明,双曲线法推算的最终沉降对应的时间与主固结时间的之比约为2~4,双曲线法推算的最终沉降只包括了30~60%的工后次固结沉降,采用双曲线法推算工后沉降时还应考虑工后次固结沉降。5.为了统一工后沉降法和沉降速率法、并合理确定卸载沉降速率标准,研究了软基沉降速率与剩余沉降的关系。首先,分析曾国熙固结度公式的误差,利用该公式和Voigt-Kelvin黏弹性模型推导了沉降速率与剩余沉降的关系。研究表明,沉降速率与剩余结沉降成两直线组成的折线关系,两条直线的交点对应主固结沉降的终点,沉降速率与剩余主固结沉降成线性关系,与剩余次固结沉降成正比关系;对符合双曲线的时间~沉降曲线推导分析表明沉降速率与剩余沉降的平方成正比。然后,利用室内压缩试验、模型试验、现场监测资料,分析了沉降速率与剩余沉降的关系,试验表明,软基沉降速率与剩余沉降成两直线组成的折线关系,两条直线的交点对应主次固结的分界点;软基沉降速率与剩余沉降的平方基本成正比关系。6.在上述研究的基础上,提出了考虑路基压缩、路面再压沉降、次固结沉降、荷载大小等因素的工后沉降法和沉降速率法,并推导了相应公式,实测工后沉降表明利用本文方法确定卸载时机效果较理想。最后,对需要进一步开展的工作方向进行了简要的说明。更多还原

【Abstract】 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.更多还原