丁选明(1979-), 男, 教授, 博士, 主要从事桩基动力学和软基处理研究, E-mail:
Ding Xuanming (1979-), professor, PhD, main research interests: pile foundation dynamics and soft foundation treatment, E-mail:
为探明斜坡地形条件对桩基竖向承载特性的影响,结合模型试验和数值模拟方法,设计多组斜坡工况和水平对照工况,对单桩竖向承载特性进行研究,在相同桩长条件下对比分析平地、不同坡度的单侧斜坡和连续斜坡地形中桩基的竖向承载力、桩身轴力及桩侧摩阻力的变化规律和荷载传递机理。研究结果表明:在相同坡度条件下,单侧斜坡工况的单桩承载能力小于连续斜坡工况,且桩基竖向承载力随着坡度增大而降低,斜坡影响度呈非线性增长;斜坡地形主要影响桩侧阻力峰值大小,当桩侧阻力出现峰值时,对应的桩端阻力大小接近相等;斜坡地形中,桩身前后存在应力分布差异,坡前位置竖向应力和剪应力大于坡后位置,但剪应力差异仅存在于0~4倍桩径的浅层区域。
In order to study the influence of slope on the vertical bearing characteristics of pile foundation, combined with model test and numerical simulation method, multiple groups of slope conditions and horizontal ground condition were designed to study the vertical bearing characteristics of single pile. The vertical bearing capacity, axial force, side friction and load transfer mechanisms during load-bearing of pile was compared and analyzed in unilateral slope and continuous slope with the same pile length. The results show that:1.Under the same slope condition, the single-pile bearing capacity of unilateral slope is less than that of continuous slope, and the vertical ultimate bearing capacity of pile decreases with the increase of slope gradient, and the slope influence degree increases non-linearly; 2.The slope type mainly affects the peak value of the pile side resistance, and the pile tip resistance is close when the pile side resistance peaks. 3.In the slope, there is a difference in the stress distribution between the front and back of the pile body. The vertical stress and shear stress at the front of the pile body are larger than that at the position back of the pile body, but the difference in shear stress only exists in the area around 0~4 times the pile diameter.
随着山区基础设施建设的不断推进和城市建设用地的日益紧张,越来越多的城市建筑群以及配套的公路、铁路桥梁桩基被设置在山区斜坡等不良地形之上,形成斜坡桩基[
近年来,许多专家和学者从不同角度对斜坡地形中桩基的承载特性展开了研究,得到了一些有益的研究结果。赵明华等[
上述研究表明,斜坡地形桩前土体部分缺失对桩基承载能力产生了不同的影响,但关于斜坡地形中桩基的承载特性和桩身荷载传递机理的研究尚不充分,且实际工程中桩后地形又可分为连续斜坡和平地工况,而考虑桩后地形影响和探讨斜坡桩基与平地桩之间的差异及产生原因的研究还相对较少。为此,笔者结合室内模型试验,利用有限元数值软件对斜坡中的单桩进行三维数值模拟,分析了平地、连续斜坡(桩后地形为连续斜坡)和单侧斜坡(桩后地形为平地)3类地形中单桩的荷载沉降响应、桩身轴力、桩侧摩阻力、桩身应力等的分布规律,并对比分析不同坡度下斜坡对单桩竖向承载能力的影响差异,探讨了斜坡地形对桩基竖向承载特性的影响机理。
试验模型槽的尺寸为2 m×2 m×2.5 m(长×宽×高),配备完善的加载控制系统[
模型试验系统
Model experiment system
试验设计的对比工况分别为水平工况和连续斜坡工况,斜坡坡度设置约为30°,工况设计与应变片布置如
模型试验工况及应变片布置
Model testingconditions and strain gauge layout
模型桩定位俯视图与试验现场照片
Top view of model pile positioning
试验加载采用慢速维持荷载法在桩顶进行竖向加载,每级施加0.5 kN,当持荷状态下桩顶沉降量小于0.1 mm/h时,则认为该级荷载已达到稳定[
提取模型试验所测结果,可绘制出两组对应工况的桩顶荷载沉降曲线见
模型试验桩顶荷载沉降曲线
pile top load-settlement curves by model test
不同荷载作用下桩身轴力及侧摩阻力分布
Axial force and side friction of pile under different loads
结合试验结果可以看出,相比于常见的水平地形,斜坡地形会降低桩基的竖向承载力,影响其轴力和侧摩阻力分布,对桩基的竖向承载产生不利的影响。
采用ABAQUS数值模拟软件建立三维有限元模型,模型土体选用Mohr-Coulomb模型模拟,桩体选用线弹性模型模拟,结合试验所测结果,桩体及土体材料参数选取见
数值模拟材料参数
Material parameters of numerical simulation
材料 | 弹性模量/MPa | 泊松比 | 密度/(g·cm-3) | 黏聚力/kPa | 内摩擦角/(°) | 剪胀角/(°) |
土体 | 00030 | 0.3 | 1.8 | 10 | 25 | 0.1 |
桩体 | 30 000 | 0.2 | 2.5 |
数值模拟计算工况表
Numerical simulation conditions table
工况名 | 地形条件 | 坡度 |
示意图 |
工况H | 水平地形 | 00 | |
工况S15° | 连续斜坡 | 15 | |
工况S30° | 30 | ||
工况S45° | 45 | ||
工况HS15° | 单侧斜坡 | 15 | |
工况HS30° | 30 | ||
工况HS45° | 45 |
选用三维实体模型,C3D8单元,为保证计算精度,对桩及桩周附近区域作网格加密。为降低计算时间成本,根据对称性建立了1/2模型分析。模型底部边界设置固定约束,对称面设置对称约束,侧面边界设置垂直平面方向的约束,顶面为自由边界。
有限元模型尺寸及网格划分图(单位:cm)
Size and meshing diagram of finite element model (unit: cm)
桩侧与桩侧土体的接触模拟选用库伦摩擦模型,结合费康等[
模型建好后依次通过预设地应力场、生死单元控制和导入应力结果的方法进行初始地应力平衡及成桩过程的模拟[
提取对应室内模型试验的工况H和工况S30°的结果,绘制桩顶荷载沉降曲线和桩身侧摩阻力分布,与室内模型试验结果对比见
模型试验与数值的桩顶荷载沉降曲线对比
Comparison of pile top load-settlement curves between model test and numerical simulation
模型试验与数值的桩身侧摩阻力分布对比
Comparison of side friction curvesof pile between model test and numerical simulation
从
各数值模拟工况计算所得桩顶荷载沉降曲线对比如
各工况数值模拟的荷载沉降曲线
Pile top load-settlement curves of different conditions by numerical simulation
为进一步分析坡度对桩基承载的影响效果,将各工况拐点出现时对应的荷载作为该工况的桩基竖向极限承载力
不同工况下竖向承载力及影响度
Vertical bearing capacity and influence degree of different conditions
坡型 | 工况 | 坡度/(°) | 极限承载力/kN | 斜坡影响度/% |
水平 | 工况H | 00 | 4.50 | 0013 |
连续斜坡 | 工况S15° | 15 | 4.40 | 02.13 |
工况S30° | 30 | 4.25 | 05.32 | |
工况S45° | 45 | 3.40 | 23.40 | |
单侧斜坡 | 工况HS15° | 15 | 4.10 | 08.51 |
工况HS30° | 30 | 3.83 | 14.26 | |
工况HS45° | 45 | 3.10 | 29.79 |
为进一步研究地形影响桩基竖向承载力的机理,现取30°连续斜坡和单侧斜坡工况与水平工况对比,即工况H、工况S30°和工况HS30°进行以下对比分析。
1) 初始地应力场对比
不同地形条件下的初始竖向应力场
Initial vertical stress field of different ground conditions
从
不同地形条件生成的初始应力场作用在桩基上会对桩身产生不同的初始应力条件,亦会对后续加载过程中的荷载分布和传递规律产生影响。
2) 桩端桩侧承荷分布 为了对比不同工况下的桩端和桩侧土抗力随外荷载的变化,提取加载过程中的桩端阻力和桩侧阻力变化结果,分别按桩顶竖向位移变化和桩顶竖向荷载变化作桩端、桩侧阻力分布图,如
桩端阻力、桩侧阻力分布
Tip resistance and side resistance of the piles
从
3) 桩身荷载传递规律 提取不同荷载作用下桩身的轴力和侧摩阻力分布如
不同荷载作用下桩身轴力及侧摩阻力分布
Axial force and side friction of pile under different loads
当荷载达到极限承载力后,除了不同地形间存在桩身应力分布差异,在斜坡地形中,桩身在坡前和坡后位置也存在应力分布差异。取桩顶荷载为5 kN时各工况对称界面处坡前和坡后位置的桩身竖向应力及剪应力沿深度分布如
桩身两侧竖向应力与剪应力分布
Normal stress and shear stress on both sides of the pile
由
结合模型试验和数值模拟方法,设计了相同桩长、不同斜坡类型和斜坡角度的单桩承载工况,并与水平工况对比,研究了斜坡地形单桩的竖向承载特性及其影响因素,得出如下结论:
1) 斜坡地形会对桩基承载能力产生削弱影响,在相同桩长和斜坡坡度的条件下,单侧斜坡工况的单桩承载能力小于连续斜坡工况;桩基的竖向承载能力随着坡度的增大呈降低趋势,斜坡影响度呈非线性增长,随着坡度增大,影响效果愈明显,45°单侧斜坡工况可达到约30%。
2) 斜坡桩基桩端和桩侧阻力的发展规律与水平地形相似,桩侧阻力随荷载增大逐渐增大并达到稳定值,桩端阻力随荷载的增长先慢后快,斜坡地形主要影响桩侧阻力峰值大小,当桩侧阻力出现峰值时,对应的桩端阻力大小接近相等。
3) 斜坡地形中桩身前后存在应力分布差异,坡前位置处的桩身竖向应力和剪应力大于坡后位置,但剪应力差异仅存在于0~4倍桩径的浅层区域,而4倍桩径以下区域桩身两侧剪应力的差异很小,趋近相等。
冯忠居.特殊地区基础工程[M].北京:人民交通出版社, 2008.
FENG Z J. Foundation engineering in special areas[M]. Beijing:China Communications Press, 2008. (in Chinese)
邓友生, 赵明华, 邹新军, 等.山区陡坡桩柱的承载特性研究进展[J].公路交通科技, 2012, 29(6):37-45.
DENG Y S, ZHAO M H, ZOU X J, et al. Research progress of bearing characteristics of pile column at steep slope in mountain areas[J]. Journal of Highway and Transportation Research and Development, 2012, 29(6):37-45. (in Chinese)
赵明华, 杨超炜, 陈耀浩, 等.高陡横坡段桩柱式桥梁双桩基础现场试验研究[J].岩土工程学报, 2018, 40(2):329-335.
ZHAO M H, YANG C W, CHEN Y H, et al. Field tests on double-pile foundation of bridges in high-steep cross slopes[J]. Chinese Journal of Geotechnical Engineering, 2018, 40(2):329-335. (in Chinese)
牛富生, 许建聪, 马亢.黄土斜坡桩基竖向荷载传递规律现场试验研究[J].岩土力学, 2014, 35(7):1899-1906.
NIU F S, XU J C, MA K. Field experimental study of transmitted characteristics of pile foundation under vertical load in loess slope[J]. Rock and Soil Mechanics, 2014, 35(7):1899-1906. (in Chinese)
尹平保, 赵明华, 杨超炜, 等.复杂荷载下横坡段桥梁桩基承载特性试验研究[J].土木工程学报, 2014, 47(5):110-117.
YIN P B, ZHAO M H, YANG C W, et al. Experimental study on bearing capacity of bridge piles in cross slopes under complex loads[J]. China Civil Engineering Journal, 2014, 47(5):110-117. (in Chinese)
龚先兵, 杨明辉, 赵明华, 等.山区高陡横坡段桥梁桩基承载机理模型试验[J].中国公路学报, 2013, 26(2):56-62.
GONG X B, YANG M H, ZHAO M H, et al. Load-bearing mechanism model test for bridge pile foundation in high-steep transverse slope[J]. China Journal of Highway and Transport, 2013, 26(2):56-62. (in Chinese)
高博雷, 张陈蓉, 张照旭.砂土中边坡附近单桩水平抗力的模型试验研究[J].岩土力学, 2014, 35(11):3191-3198.
GAO B L, ZHANG C R, ZHANG Z X. Model tests on effect of slopes on lateral resistance of near single piles in sand[J]. Rock and Soil Mechanics, 2014, 35(11):3191-3198. (in Chinese)
程刘勇, 陈善雄, 余飞, 等.竖向荷载下斜坡桩基承载力及影响因素数值研究[J].科学技术与工程, 2013, 13(18):5399-5403, 5422.
CHENG L Y, CHEN S X, YU F, et al. Numerical simulation for vertical ultimate capacity and influencing factors of oblique slope pile under vertical loads[J]. Science Technology and Engineering, 2013, 13(18):5399-5403, 5422. (in Chinese)
陈兆, 陈骅伟, 蒋冲, 等.水平荷载作用下斜坡刚性桩非线性分析[J].土木建筑与环境工程, 2016, 38(3):47-52.
CHEN Z, CHEN H W, JIANG C, et al. Nonlinear analysis of rigid pile in slope under lateral load[J]. Journal of Civil, Architectural & Environmental Engineering, 2016, 38(3):47-52. (in Chinese)
JESMANI M, KASRANIA A, KAMALZARE M, et al. Undrained vertical bearing capacity of pile located near soft clay slope[J]. Journal of Engineering Research, 2015, 3(3):21-38.
JESMANI M, KASRANIA A, KAMALZARE M. Finite element modelling of undrained vertical bearing capacity of piles adjacent to different types of clayey slopes[J]. International Journal of Geotechnical Engineering, 2018, 12(2):147-154.
SAWANT V A, SHUKLA S K. Effect of edge distance from the slope crest on the response of a laterally loaded pile in sloping ground[J]. Geotechnical and Geological Engineering, 2014, 32(1):197-204.
NG C W W, ZHANG L M. Three-dimensional analysis of performance of laterally loaded sleeved piles in sloping ground[J]. Journal of Geotechnical and Geoenvironmental Engineering, 2001, 127(6):499-509.
QU L M, DING X M, WU C R, et al. Effects of topography on dynamic responses of single piles under vertical cyclic loading[J]. Journal of Mountain Science, 2020, 17(1):230-243.
中国建筑科学研究院.混凝土结构设计规范: GB 50010-2010[S].北京: 中国建筑工业出版社, 2015.
China Academy of Building Research. Code for design of concrete structures: GB 50010-2010[S]. Beijing: China Architecture & Building Press, 2015. (in Chinese)
建筑基桩检测技术规范: JGJ 106-2014[S].北京: 中国建筑工业出版社, 2014.
Technical code for testing of building foundation piles: JGJ 106-2014[M]. Beijing: China Architecture & Building Press, 2014. (in Chinese)
费康, 张建伟. ABAQUS在岩土工程中的应用[M].北京:中国水利水电出版社, 2010.
FEI K, ZHANG J W. Application of ABAQUS in geotechnicalengineering[M]. Beijing:China Water Power Press, 2010. (in Chinese)
张建伟, 孔庆梅, 马金栋, 等.复杂荷载共同作用下斜坡上单桩承载特性研究[J].建筑结构, 2014(16):96-98.
ZHANG J W, KONG Q M, MA J D, et al. Study on bearing capacity of the single pile on slope under complex loads[J]. Building Structure, 2014, 44(16):96-98. (in Chinese)
冯忠居, 王航, 魏进, 等.黄土冲沟斜坡桥梁桩基竖向承载特性模型试验研究[J].岩土工程学报, 2015, 37(12):2308-2314.
FENG Z J, WANG H, WEI J, et al. Model tests on vertical bearing performance of bridge pile foundation in loess gulch slope area[J]. Chinese Journal of Geotechnical Engineering, 2015, 37(12):2308-2314. (in Chinese)