简介概要

碎石桩复合地基桩土应力比的时效分析

来源期刊:中南大学学报(自然科学版)2008年第4期

论文作者:赵明华 马缤辉 张玲

文章页码:831 - 837

关键词:碎石桩;复合地基;桩土应力比;时效;

Key words:gravel-pile; composite foundation; pile-soil stress ratio; time effect

摘    要:

对碎石桩复合地基的时效特性进行研究,根据桩体及桩间土体应力随时间的变化规律,考虑碎石桩复合地基中土体径向、竖向双向排水固结的特点,基于线弹性状态下桩体及桩间土体的应力-应变关系,分别给出考虑时效的桩体与桩间土体竖向应力计算方法,进而建立桩土应力比时效计算的双曲线模型,得到考虑时效的桩土应力比计算方法。研究结果表明:碎石桩复合地基桩土应力比随时间呈双曲线变化,其值与不考虑时效时相比可增大1倍,时效非常明显,在具体计算时不容忽视;双曲线时效模型能够有效地模拟碎石桩复合地基桩土应力比的时效规律;利用桩土应力比时效模型对某工程实例进行计算、分析,理论桩土应力比随时间的变化曲线与实测曲线较吻合。

Abstract: Through studying the time effect characteristics of gravel-pile composite foundation, the stresses variation rules of both gravel-pile body and the surrounding soil along with the time were analyzed. Considering the bidirectional drained-consolidated characteristics of the surrounding soil where the water in the soil can be drained vertically and drained laterally, a calculation method considering time effect for stress of pile and that of surrounding soil was put forward according to the linear elastic theory analysis of the stress-strain relationship between pile and surrounding soil. A hyperbolic model of pile-soil stress ratio considering the time effect was established and a new calculation method for pile-soil stress ratio which took the time effect into account was obtained. The results show that the pile-soil stress ratio of gravel-pile composite foundation obeys the hyperbolic curve along with the time and the value of that is bigger than that of the time effect unconsidered, and the time effect of pile-soil stress ratio is so obvious that it can’t be neglected. Meanwhile, the hyperbolic model of pile-soil stress ratio can effectively adjust itself to the rules of the time effect of pile-soil stress ratio and the calculation result shows that the theoretical calculating curve that describes the relationship between the time and pile-soil stress ratio agrees well with the measured one obtained by the proposed method.

基金信息:国家“863”计划资助项目



详情信息展示

碎石桩复合地基桩土应力比的时效分析

赵明华,马缤辉,张  玲

(湖南大学 岩土工程研究所,湖南 长沙,410082)

摘  要:对碎石桩复合地基的时效特性进行研究,根据桩体及桩间土体应力随时间的变化规律,考虑碎石桩复合地基中土体径向、竖向双向排水固结的特点,基于线弹性状态下桩体及桩间土体的应力-应变关系,分别给出考虑时效的桩体与桩间土体竖向应力计算方法,进而建立桩土应力比时效计算的双曲线模型,得到考虑时效的桩土应力比计算方法。研究结果表明:碎石桩复合地基桩土应力比随时间呈双曲线变化,其值与不考虑时效时相比可增大1倍,时效非常明显,在具体计算时不容忽视;双曲线时效模型能够有效地模拟碎石桩复合地基桩土应力比的时效规律;利用桩土应力比时效模型对某工程实例进行计算、分析,理论桩土应力比随时间的变化曲线与实测曲线较吻合。

关键词:碎石桩;复合地基;桩土应力比;时效

中图分类号:TU 473.1        文献标识码:A         文章编号:1672-7207(2008)04-0831-07

Analysis of time effect of pile-soil stress ratio in

 gravel-pile composite foundation

ZHAO Ming-hua, MA Bin-hui, ZHANG Ling

(Geotechnical Engineering Institute, Hunan University, Changsha 410082, China)

Abstract: Through studying the time effect characteristics of gravel-pile composite foundation, the stresses variation rules of both gravel-pile body and the surrounding soil along with the time were analyzed. Considering the bidirectional drained-consolidated characteristics of the surrounding soil where the water in the soil can be drained vertically and drained laterally, a calculation method considering time effect for stress of pile and that of surrounding soil was put forward according to the linear elastic theory analysis of the stress-strain relationship between pile and surrounding soil. A hyperbolic model of pile-soil stress ratio considering the time effect was established and a new calculation method for pile-soil stress ratio which took the time effect into account was obtained. The results show that the pile-soil stress ratio of gravel-pile composite foundation obeys the hyperbolic curve along with the time and the value of that is bigger than that of the time effect unconsidered, and the time effect of pile-soil stress ratio is so obvious that it can’t be neglected. Meanwhile, the hyperbolic model of pile-soil stress ratio can effectively adjust itself to the rules of the time effect of pile-soil stress ratio and the calculation result shows that the theoretical calculating curve that describes the relationship between the time and pile-soil stress ratio agrees well with the measured one obtained by the proposed method.

Key words: gravel-pile; composite foundation; pile-soil stress ratio; time effect

随着碎石桩复合地基处理技术在软土地基处理工程中的广泛应用,其理论研究也取得了较大的进展[1-2],但仍存在许多问题有待探讨,尤其是桩土应力比的计算尚有待进一步研究。桩土应力比是碎石桩复合地基计算中的重要设计参数,但因其影响因素很多,理论计算较困难。目前,不少学者对此进行了深入研究,从不同的角度考虑桩土应力比的影响因素[3-5],如:赵明等[3]从考虑桩体侧向膨胀时桩土共同工作机理出发,提出基于p-y曲线法的桩土应力比计算方法,但假定碎石桩为刚性线性强化材料;刘杰等[4]在考虑桩土共同作用的基础上讨论桩土应力比与置换率及桩周土变形模量等影响因素的关系,利用弹性理论推导出桩处于自身极限平衡状态时桩土应力比的计算公式,但假设桩体材料的屈服服从摩尔-库仑准则;Han等[5]则从数值分析角度对桩土应力比的影响因素进行分析。这些方法均考虑桩土共同作用以及各种因素的影响,提高了桩土应力比计算的精度,但在桩土应力比的时效方面,尚未能给出精确合理的计算方法,难以反映桩土应力比随时间的变化情况。而另一方面,大量试验研究表明[6-9],桩土应力比具有时间效应,且随着时间的延长而增大并趋于稳定,因此,在计算分析桩土应力比时考虑时间效应具有重要的理论与工程实际意义。为此,本文作者对碎石桩复合地基的时效特性进行研究,根据桩体和桩间土体应力随时间变化的规律,提出一种能考虑时效的桩土应力比计算方法。

1  碎石桩复合地基时效特性分析

碎石桩复合地基在成桩过程中对桩间土体产生扰动,土体强度下降;但随着时间推移,土体强度将逐渐恢复。此外,碎石桩成桩后为桩间土体提供了良好的排水通道,有利于土体加速固结,土体强度进一步提高。而碎石桩的承载力主要取决于桩间土体的侧向压力。桩间土体强度提高,对桩体产生的侧向压力增大,桩体承载力提高,进而使整个碎石桩复合地基承载力随时间的延长而增长,直至桩周土体固结完成时,复合地基承载力才趋于稳定。在受荷初期,土体强度较低,桩土应力明显向桩体集中,桩体竖向应力较大,土体竖向应力则相对较小;随着荷载的提高和作用时间的延长,桩周土体由于碎石桩体的存在而加速排水固结,土体强度提高,压缩性降低,桩土相对刚度比开始减小,荷载逐渐向土体转移,土体竖向应力增长较快,桩体竖向应力增长幅度较小。最终土体应力和桩体应力均趋于稳定值。这就是碎石桩复合地基的时间效应。下面分别分析桩体及桩间土体的应力时效,以便对碎石桩复合地基的时效进行进一步探讨。

1.1  桩体应力时效分析

桩体应力时效主要表现为受荷初期桩体竖向应力增长较快;随着荷载的提高和时间的延长,桩体竖向应力增长幅度逐渐减小,并最终趋于稳定。

根据桩体竖向应力随时间的变化特征,可设桩体竖向应力随时间的变化规律为[10]

1.2  土体应力时效分析

土体应力时效主要表现为受荷初期土体竖向应力较小,增长较慢;随着荷载的提高和作用时间的延长,土体竖向应力增长幅度逐渐变大,并最终在一定范围内趋于稳定。一般认为,其原因是桩间土体在荷载作用下产生固结和蠕变,使得荷载向桩体集中,从而导致桩土应力比n随时间的延长而增大[11]。其中桩间土体固结沉降是导致荷载转移和土体应力发生变化的主要原因。因此,土体固结沉降计算是土体应力时效分析的关键。下面从土体固结理论方面入手对土体应力时效进行计算。

1.2.1  土体固结沉降计算

碎石桩的设置为桩间土体提供了竖向和径向2种排水固结途径,桩间土体的固结属于三维轴对称固结问题,类似于砂井地基固结模式,但又有所不同,主要表现在:

a. 在径向固结方面,碎石桩复合地基的井阻和涂抹作用以及桩土刚度比明显比砂井地基大;

b. 在竖向固结方面,砂井地基主要应用太沙基一维固结理论,假定外荷载为一次瞬间加在土体之上并在固结过程中保持不变,并假定土体渗透系数和固结系数在固结过程中保持不变,其与碎石桩复合地基固结的实际情况不符。

因此,需对常规的砂井地基固结模式进行改进,在计算中考虑桩间土体的实际固结特点。

由于碎石桩复合地基的涂抹作用与井阻作用相比较小,故不考虑碎石桩复合地基的涂抹作用,可得碎石桩复合地基的径向平均固结度和竖向平均固结度分别为[12]

可由下式计算[3]

根据Carrillo定理可得复合地基总固结度为:

将式(2)和(3)代入式(4)可得:

为简化计算,对式(5)取第1项m=0计算,可满足工程精度要求[12],故有:

由固结度的定义有:

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