自重湿陷性黄土场地桩基浸水荷载试验研究
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摘要
随着我国近年来在湿陷性黄土地区基础建设的迅猛发展,特别是大直径桩基在该地区的普遍应用,现有的许多理论研究,尤其是对桩基负摩阻力的研究已不能满足日益发展的工程需要,因此桩基的现场原位试验对其设计理论的完善和实际工程应用都具有十分重要的意义。
本文结合实际工程需要,通过自重湿陷性黄土中桩基的现场大面积试坑浸水试验,运用滑动测微等先进测试技术,探讨了自重湿陷区天然状态及浸水状态下单桩竖向承载力的变化情况,对自重湿陷性黄土浸水过程中桩侧负摩阻力的变化规律及浸水对桩竖向承载力的影响等进行了研究。
通过浸水前后桩的极限承载力对比分析,前者的单桩极限承载力远大于后者。对桩基先加载到工作荷载后再浸水,之后再次加载至破坏与先浸水后加载至破坏,其单桩极限承载力是完全一致的,但是浸水造成的桩顶附加沉降可达工作荷载下桩顶沉降的6.3倍,湿陷性黄土区桩基设计首先要考虑的是变形问题,之后才是承载力问题。
考虑桩基在实际工程中与土层相互作用的特殊性,基于最小二乘法原理并考虑可控制点约束的因素,结合excel软件运用VB编制了滑动测微数据分析程序,对实测数据进行拟合,取得了较为精确的拟合曲线,为试验数据的进一步分析,提供了有利的条件。本文根据实测结果,分析了负摩阻力大小的发生发展规律,以及负摩阻力与桩顶沉降、桩土相对位移之间的关系。通过分析可知:(1)浸水后桩侧负摩阻力很快产生,但在浸水过程中达到稳定的时间较长,停水后负摩阻力又有所增长,在“浸水期”末达到最大,“浸水后”加载过程中负摩阻力的减小也有一个过程;(2)桩顶沉降大的桩,其最终负摩阻力小,但浸水过程中的负摩阻力大小排序未必严格等同于最终负摩阻力;(3)加载时桩与土间微小的相对位移即可激发侧阻力的产生,浸水湿陷造成土的沉降稍大于桩的沉降时正摩阻力即消失并产生负摩阻力,且负摩阻力随着桩土相对位移的增大而增大,并随着浸水湿陷的稳定而趋向稳定。中性点的产生是一个较快增长——稳定——加载上升的过程,其深度要超过实测自重湿陷下限深度。
With the rapid development of engineering construction in collapsible loess region recently, especially widespread application with the big diameter piles in this region, many theories now, especially the one about negative skin friction (NSF) on piles, are not perfect enough to fit the requirements of engineering practice. So the field tests for piles are quite significant to perfect the design theories and satisfy the application for engineering practices.
According to the actual needs of engineering, the article discussed the natural situation of self-subsidence area and change situation of vertical bearing capacity of single pile in soaking state, which used the vanguard technology such as sliding measured micro technology, through the local extensive soaking trail pit tests of pile foundation of self-weight collapse loess. At the same time, the article do some research in changes of negative skin friction of pile shaft of self-weight collapse loess and influences of the soaking to pile vertical bearing capacity.
The ultimate bearing capacity of the piles has distinct difference because of the immersion. The ultimate bearing capacity of single pile which is not submerse is more lager than the one immerged. The pile which load the work load first then water it ,after that load it to failure compare with the pile which load to failure directly after watering, they have the very same ultimate bearing capacity. But the extra settlement of the pile begotten by immersion is 6.3 times than the one which is caused by working load. The pile design in collapsible loess region must consider the deformation more important than the bearing capacity.
Considering the particularity of pile foundation interaction with soil horizon in actual engineering, the article draw up a procedure of analysis sliding measured micro data using VB assembly language combined to EXCELsoftware ,which is based on the principle of least square method and the controllable restriction's factors. Then we fit the measured data and obtained comparatively accurate fitting curve, which provide the advances to further analysis of test data. According to the measured results, the article analysis the development discipline of negative friction, and relation of negative friction、pile top settlement and pile-soil relative displacement. We know that :(1) the negative friction will generate quickly after soaking, but its steady time is gong to be long in soaking process, and it will increase to some extent when we cut off the water supple. The negative friction can reach to maximum in terminal phase of soaking. It will take a little time to decrease the negative friction in loading procedure of after soaking.(2)the pile with high pile top settlement has the lower negative friction finally. But the ordering of negative friction in soaking process not equal to final negative friction strictly.(3)the micro relative displacement between pile and soil will arouse side resistance. The positive friction will disappeared and the negative friction will generate when settlement of soil is bigger than pile's which is caused by soaking collapse. The negative friction enlarged with the higher of pile-soil relative displacement, and reach to steady with stabilize of soaking collapse. Neutral point which has a process: increase fast early days first, then keep stabilization and ascend along with the continuous load finally.
本文结合实际工程需要,通过自重湿陷性黄土中桩基的现场大面积试坑浸水试验,运用滑动测微等先进测试技术,探讨了自重湿陷区天然状态及浸水状态下单桩竖向承载力的变化情况,对自重湿陷性黄土浸水过程中桩侧负摩阻力的变化规律及浸水对桩竖向承载力的影响等进行了研究。
通过浸水前后桩的极限承载力对比分析,前者的单桩极限承载力远大于后者。对桩基先加载到工作荷载后再浸水,之后再次加载至破坏与先浸水后加载至破坏,其单桩极限承载力是完全一致的,但是浸水造成的桩顶附加沉降可达工作荷载下桩顶沉降的6.3倍,湿陷性黄土区桩基设计首先要考虑的是变形问题,之后才是承载力问题。
考虑桩基在实际工程中与土层相互作用的特殊性,基于最小二乘法原理并考虑可控制点约束的因素,结合excel软件运用VB编制了滑动测微数据分析程序,对实测数据进行拟合,取得了较为精确的拟合曲线,为试验数据的进一步分析,提供了有利的条件。本文根据实测结果,分析了负摩阻力大小的发生发展规律,以及负摩阻力与桩顶沉降、桩土相对位移之间的关系。通过分析可知:(1)浸水后桩侧负摩阻力很快产生,但在浸水过程中达到稳定的时间较长,停水后负摩阻力又有所增长,在“浸水期”末达到最大,“浸水后”加载过程中负摩阻力的减小也有一个过程;(2)桩顶沉降大的桩,其最终负摩阻力小,但浸水过程中的负摩阻力大小排序未必严格等同于最终负摩阻力;(3)加载时桩与土间微小的相对位移即可激发侧阻力的产生,浸水湿陷造成土的沉降稍大于桩的沉降时正摩阻力即消失并产生负摩阻力,且负摩阻力随着桩土相对位移的增大而增大,并随着浸水湿陷的稳定而趋向稳定。中性点的产生是一个较快增长——稳定——加载上升的过程,其深度要超过实测自重湿陷下限深度。
With the rapid development of engineering construction in collapsible loess region recently, especially widespread application with the big diameter piles in this region, many theories now, especially the one about negative skin friction (NSF) on piles, are not perfect enough to fit the requirements of engineering practice. So the field tests for piles are quite significant to perfect the design theories and satisfy the application for engineering practices.
According to the actual needs of engineering, the article discussed the natural situation of self-subsidence area and change situation of vertical bearing capacity of single pile in soaking state, which used the vanguard technology such as sliding measured micro technology, through the local extensive soaking trail pit tests of pile foundation of self-weight collapse loess. At the same time, the article do some research in changes of negative skin friction of pile shaft of self-weight collapse loess and influences of the soaking to pile vertical bearing capacity.
The ultimate bearing capacity of the piles has distinct difference because of the immersion. The ultimate bearing capacity of single pile which is not submerse is more lager than the one immerged. The pile which load the work load first then water it ,after that load it to failure compare with the pile which load to failure directly after watering, they have the very same ultimate bearing capacity. But the extra settlement of the pile begotten by immersion is 6.3 times than the one which is caused by working load. The pile design in collapsible loess region must consider the deformation more important than the bearing capacity.
Considering the particularity of pile foundation interaction with soil horizon in actual engineering, the article draw up a procedure of analysis sliding measured micro data using VB assembly language combined to EXCELsoftware ,which is based on the principle of least square method and the controllable restriction's factors. Then we fit the measured data and obtained comparatively accurate fitting curve, which provide the advances to further analysis of test data. According to the measured results, the article analysis the development discipline of negative friction, and relation of negative friction、pile top settlement and pile-soil relative displacement. We know that :(1) the negative friction will generate quickly after soaking, but its steady time is gong to be long in soaking process, and it will increase to some extent when we cut off the water supple. The negative friction can reach to maximum in terminal phase of soaking. It will take a little time to decrease the negative friction in loading procedure of after soaking.(2)the pile with high pile top settlement has the lower negative friction finally. But the ordering of negative friction in soaking process not equal to final negative friction strictly.(3)the micro relative displacement between pile and soil will arouse side resistance. The positive friction will disappeared and the negative friction will generate when settlement of soil is bigger than pile's which is caused by soaking collapse. The negative friction enlarged with the higher of pile-soil relative displacement, and reach to steady with stabilize of soaking collapse. Neutral point which has a process: increase fast early days first, then keep stabilization and ascend along with the continuous load finally.
引文
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[7]Bozozuk M.Beating capacity of a pile preloaded by downdrag[A].Proceedings 10th Intemational Conference on Soil Mechanics and Foundation Engineering,Stockholm,June,15-19,1981,2:631～636
[8]Fellenius B.H.,Downdrag on piles in clay due to negative skin friction[J].Canadian Geotechnical Journal.9(4):323～337
[9]陈福南,龚晓南,马时冬.桩的负摩阻力现场试验及三维有限元分析[J],建筑结构学报,2000,21(3):77-80
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[12]李大展,滕延京,何颐华等.湿陷性黄中大直径扩底桩垂直承载性状的试验研究[J].岩土工程学报,1994,16(2):11-21
[13]张炜,隋国秀,黄土中钻孔灌注桩荷载传递性状的试验研究[A],岩土工程青年专家学术论坛文集[C].北京:中国建筑工业出版社,1998,173-181
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