王增亮(1994-), 男, 博士生, 主要从事桩土相互作用研究, E-mail:
WANG Zengliang (1994-), main research interest: pile-soil interaction, E-mail:
ZHOU Hang (corresponding author), PhD, associate professor, E-mail:
桥梁桩基周围形成局部冲刷坑时会导致桩基有效埋深减小,增大工程结构的安全隐患。目前研究的冲刷坑模型多为对称形态,而工程实际中的冲刷坑多为非对称形态,使得桩基处于更不利状态。合理计算桩周形成非对称冲刷坑后土体的应力变化是评价桩基承载力的关键之一,目前尚没有严格的理论计算方法。针对该问题,根据试验实测的非对称冲刷坑形态提出了三维非对称冲刷坑的简化模型。将冲刷坑最大深度以上土体重度看做荷载并基于Boussinesq点荷载方程在半无限空间中的应用,推导得到了非对称局部冲刷坑内土体垂直及水平有效应力计算方法。采用有限元中“生死单元法”模拟了半无限空间地基中冲刷坑的形成,并将有限元计算结果与理论计算方法所得结果进行对比,验证了理论计算方法的正确性。基于理论计算方法考虑了冲刷坑内存在桩基的影响,并与有限元计算结果进行了对比,对比结果表明了理论计算方法的可行性。在此基础上,设计了一系列工况,对三维非对称冲刷坑简化模型中的参数进行了敏感性分析,得到了非对称冲刷坑条件下桩周土体的垂直及水平有效应力差的变化规律。
When the local scour hole is formed around the pile foundation, the effective buried depth of the pile foundation will be reduced, which increases the safety hazard of the engineering structure. The scour models in the current research are mostly symmetrical, but the scour holes in engineering practice are asymmetric, which makes the pile foundation in a more unfavorable state. How to calculate the stress change of the soil caused by scour reasonably is pivotal for evaluating the bearing capacity of the pile foundation. However, there is still no strict theoretical calculation method. This paper aims to propose a simplified three-dimensional asymmetric scour hole model based on the asymmetric scour hole shape measured in the experiment. Which is based on the application of Boussinesq's equation in a semi-infinite space and regard the soil above the depth of the scour hole as a load. The calculation method of the vertical and horizontal effective stress of the soil in the asymmetric scour hole is derived. The "active and deactive element" in the finite element method is used to simulate the formation of scour holes in the semi-infinite space foundation, and the FEM results are compared with those obtained from the theoretical calculation method, which verifies the correctness of the theoretical calculation method in this study. Subsequently, based on the theoretical calculation method, the influence of the pile foundation is considered, and compared with the FEM results. The comparison results indicate that the theoretical calculation method in this research is feasible. Based on this, the sensitivity analysis of the parameters in the simplified model of the three-dimensional asymmetric scour hole was carried out, and the changes of the vertical and horizontal effective stress difference of the soil around the pile under the condition of the asymmetric scour hole were obtained.
桥梁作为供公路、渠道、铁路、管线等跨越库区、山谷、河流等其他交通线最常用的工程结构,在经济建设和社会发展中发挥着举足轻重的作用。深水桩基础是现代桥梁建设最主要的基础形式之一,具有体积大、阻水面积大的特点。由于深水桩基础所处的水环境非常复杂,在长期的河流冲刷作用下,桩基础周围土体被掏空,导致桩基承载力下降,从而使桥梁、公路发生破坏。董正芳等[
桥梁桩基冲刷形式按中国分类标准分为长期冲刷(一般冲刷)、收缩冲刷以及局部冲刷。与一般冲刷相比(整个河床的自然冲刷),局部冲刷通常发生在桩基础、桥墩、桥台以及其他过水障碍物处,因此,局部冲刷只发生在桩周附近上覆土层[
桩周形成局部冲刷坑时,关于桩周土体应力的计算,笔者回顾了3种目前被广泛应用于桩基设计规范的方法,即API、FHWA-DP(FHWA driven piles)以及FHWA-DS(FHWA drilled shafts)中建议的方法。其中,FHWA-DP中假设桩周土体应力不受局部冲刷的影响,即土体应力计算时按未发生局部冲刷条件下的河床表面进行计算[
Butch等[
非对称局部冲刷坑纵向和横向剖面图[
Scour longitudinal profiles and scour lateral profiles around pile
三维非对称局部冲刷坑简化模型图
Simplified model of three-dimensional asymmetric local scour hole
非对称局部冲刷坑简化模型剖面图和平面图
Sectional view and plan view of simplified model of asymmetric local scourhole
简化模型中用以表征非对称局部冲刷坑的几何参数包括:桩基上游顶部长度
同时可以发现,用以表征非对称局部冲刷坑的几何参数间存在如式(1)~式(4)所示关系。
如
式中:
非对称局部冲刷坑下,桩周土体垂直有效应力计算分为两部分:第1部分为
式中:
则可以得到非对称冲刷坑内桩周任意深度
式中:Δ
非对称冲刷坑条件下桩周土体水平有效应力计算也分为两部分:第1部分为
式中:
由此,得到非对称冲刷坑条件下桩周任意深度
式中:
为验证提出的非对称冲刷坑内土体应力计算模型的正确性,验证中忽略桩径的影响(即
非对称冲刷坑参数
Parameters of asymmetric scour hole
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1.35 | 0.3 | 2.01 | 0.27 | 0.837 | 0.632 4 |
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( |
1.59 | 39 | 20 | 32 | 0.24 | 0.5 |
有限元计算模型图
Finite element calculation model
提出的简化模型的计算结果与有限元冲刷模拟的计算结果对比如
本文解与有限元计算对比
Comparison of the proposed solution with the FEM
提出的计算模型是基于严格定义在半无限地基中Boussinesq点荷载方程得到的,在应用Boussinesq方程时忽略了桩径的影响。该部分引入桩径的影响,即
有桩时有限元计算模型
Finite element calculation model with pile
有桩条件下非对称冲刷坑参数
Parameters of asymmetric scour hole with pile
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4.05 | 0.9 | 5.03 | 1.23 | 2.4 | 1.2 |
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5.4 | 37 | 18 | 31 | 0.5 | 2.5 |
有桩时本文解与有限元对比
Comparison between the proposed solution and the FEM when pile is considered
由简化模型可以看出,非对称冲刷坑计算模型中的独立变量有9个:
为便于进行垂直有效应力的参数分析,此处引入垂直有效应力率(式(39))来分析非对称局部冲刷坑形成后桩周土体垂直有效应力的变化。
式中:
不同
Variation of effective stress rate around the pile for different
不同
Variation of effective stress rate around the pile for different
不同
Variation of effective stress rate around the pile for different
不同
Variation of effective stress rate around the pile for different
不同
Variation of effective stress rate around the pile for different
不同
Variation of effective stress rate around the pile for different
不同
Variation of effective stress rate around the pile for different
不同
Variation of effective stress rate around the pile for different
不同
Variation of effective stress rate around the pile for different
桩周形成非对称冲刷坑时,桩基下游冲刷剩余土体量大于桩基上游,会造成桩基上、下游土体的水平有效应力存在差异,具体表现为桩基下游土体水平有效应力大于桩基上游。该部分就桩基上、下游的土体水平有效应力差进行参数敏感性分析。由于三维非对称局部冲刷坑模型中冲刷坑的冲刷宽度
不同
Difference of horizontal effective stress varies with depth for different
不同
Difference of horizontal effective stress varies with depth for different
不同
Difference of horizontal effective stress varies with depth for different
不同
Difference of horizontal effective stress varies with depth for different
不同
Difference of horizontal effective stress varies with depth for different
不同
Difference of horizontal effective stress varies with depth for different
不同
Difference of horizontal effective stress varies with depth for different
不同
Difference of horizontal effective stress varies with depth for different
需要注意的是,参数敏感性分析中只是对单一因素进行敏感性分析(即其他参数保持不变),而实际工程中,桩周土体的垂直有效应力及水平有效应力差值为各因素的耦合作用结果。
基于试验实测非对称冲刷坑形态,提出了桩周非对称冲刷坑三维简化模型,且基于Boussinesq点荷载方程得到非对称冲刷坑内桩周土体应力计算方程。基于该计算方法对桩周土体的垂直有效应力及桩基上、下游水平有效应力差进行了计算,并对简化模型中的参数做了土体应力的敏感性分析,得出以下主要结论:
1) 提出的冲刷坑内土体应力计算方法是将局部冲刷坑内最大冲刷深度以上土体(冲刷深度范围内冲刷剩余土体)重度作为外荷载,并基于Boussinesq点荷载方程在半无限空间中的应用得到的。而后采用有限元中“生死单元法”模拟半无限空间内冲刷坑,通过与有限元计算结果的对比,验证了计算方法的正确性。随后在半无限空间中考虑了桩径的影响,采用有限元模拟了桩周非对称冲刷坑,通过有限元计算结果与本文提出的冲刷坑内有桩存在时的理论计算结果进行对比分析,验证了考虑桩径时理论计算方法的可行性。
2) 冲刷坑的形成对坑底以下一定深度土体的垂直有效应力存在较大影响,该深度称之为冲刷坑造成的影响深度,这与API、FHWA-DP中所述一致。提出的计算方法可得到任意形态冲刷坑的影响深度以及桩周土体的垂直有效应力值,弥补了API与FHWA-DP中只可以计算特定形态冲刷坑的不足,且提出的计算模型可以考虑库区等环境下冲刷坑的非对称性的影响,使得理论计算更贴合实际工程。
3) 参数分析表明,桩周形成冲刷坑的尺寸越大,对冲刷坑底以下土体的垂直有效应力影响越大,影响深度也随之增大;参数敏感性分析可以得到,冲刷深度以及冲刷坑宽度的变化对桩周土体的垂直有效应力的影响最大;在影响深度以下,冲刷坑形态参数的改变对土体垂直有效应力没有影响,且影响深度以下土体的垂直有效应力值等于未形成冲刷坑时该处的垂直有效应力。
4) 非对称冲刷坑会造成桩基上、下游侧土体水平有效应力存在差异,具体表现为桩基下游土体水平有效应力大于桩基上游。通过参数分析得到桩基上、下游冲刷深度的差异对桩周土体的水平有效应力差值影响最大,有效重度越大的土体,水平有效应力差值越明显。桩周土体的水平有效应力差也存在影响深度且在冲刷坑底部以下浅层土体差值最大。
董正方, 郭进, 王君杰. 桥梁倒塌事故综述及其预防对策[J]. 上海公路, 2009(2): 30-32.
DONG Z F, GUO J, WANG J J. Summary and prevention countermeasures of bridge collapses[J]. Shanghai Highways, 2009(2): 30-32. (in Chinese)
National Cooperative Highway Research Program. Countermeasures to protect bridge piers from scour: NCHRP Report 593[R]. Washington, D.C. : Transportation Research Board, 2007.
WARDHANA K, HADIPRIONO F C. Analysis of recent bridge failures in the United States[J]. Journal of Performance of Constructed Facilities, 2003, 17(3): 144-150.
SMITH D W. Bridge failures[J]. Proceedings of the Institution of Civil Engineers, 1976, 60(3): 367-382.
BRIAUD J L, TING F C K, CHEN H C, et al. SRICOS: prediction of scour rate in cohesive soils at bridge piers[J]. Journal of Geotechnical and Geoenvironmental Engineering, 1999, 125(4): 237-246.
TSENG W C, KUO Y S, CHEN J W. An investigation into the effect of scour on the loading and deformation responses of monopile foundations[J]. Energies, 2017, 10(8): 1190.
QI W G, GAO F P, RANDOLPH M F, et al. Scour effects on
FISCHENICH C, LANDERS M. Computing scour EMRRP technical notes Collection Vicksburg[M]. U.S. Army Engineer Research and Development Center, 1999.
ARNESON L A, ZEVENBERGEN L W, LAGASSE P F, et al. Evaluating scour at bridges[R]. U.S. National Highway Institute, 2012.
WHITEHOUSE R. Scour at marine structures: A manual for practical applications[M]. London, UK: Thomas Telford Ltd, 1998.
RICHARDSON E V. Evaluating scour at bridges[M]. Washington, DC: Federal Highway Administration, 2001.
ZHANG H, CHEN S L, LIANG F Y. Effects of scour-hole dimensions and soil stress history on the behavior of laterally loaded piles in soft clay under scour conditions[J]. Computers and Geotechnics, 2017, 84: 198-209.
LIN Y J, LIN C. Effects of scour-hole dimensions on lateral behavior of piles in sands[J]. Computers and Geotechnics, 2019, 111: 30-41.
YANG X F, ZHANG C R, HUANG M S, et al. Lateral loading of a pile using strain wedge model and its application under scouring[J]. Marine Georesources & Geotechnology, 2018, 36(3): 340-350.
LIANG F Y, ZHANG H, WANG J L. Variational solution for the effect of vertical load on the lateral response of offshore piles[J]. Ocean Engineering, 2015, 99: 23-33.
DIAB R M A E. Experimental investigation on scouring around piers of different shape and alignment in gravel[D]. Tu Darmstadt, 2011.
BUTCH G K. Scour-hole dimensions at selected bridge piers in New York[C]//U.S. North American Water and Environment Congress & Destructive Water. ASCE. 1996.
HANNIGAN P J, RAUSCHE F, LIKINS G E, et al. Design and construction of driven pile foundations-Volume I[R]. U.S. National Highway Institute, 2016.
O'NEIL M W, REESE L C. Drilled shafts: Construction procedures and design methods[J]. Tunnelling and Underground Space Technology, 1990, 5(1/2): 156-157.
API. API Recommended Practice, Geotechnical and Foundation Design Considerations[S]. Washington, D.C., USA: American Petroleum Institute, 2011.
LIN C, WU R. Evaluation of vertical effective stress and pile lateral capacities considering scour-hole dimensions[J]. Canadian Geotechnical Journal, 2019, 56(1): 135-143.
LIN C. The loss of pile axial capacities due to scour: vertical stress distribution[J]. DEStech Transactions on Materials Science and Engineering, 2017(ictim). DOI:10.12783/dtmse/ictim2017/10056.