崔强,童瑞铭,刘生奎,鲁先龙
(1 中国电力科学研究院,北京 102401;2 甘肃省电力设计院,兰州 730050)
基础尺寸对碎石土地基扩底基础上拔承载力影响的现场试验研究
崔强1,童瑞铭1,刘生奎2,鲁先龙1
(1 中国电力科学研究院,北京 102401;2 甘肃省电力设计院,兰州 730050)
碎石土;原状土;输电线路;扩底基础;上拔承载力
原状土扩底基础(输电线路工程中又称“掏挖基础”)是指利用人工或机械的方法在原状土地基中钻(挖)成基础设计外形的基坑,然后将钢筋骨架和混凝土直接浇注于基坑而成的原状土基础,已广泛应用于输电线路、高耸建筑物等结构的基础工程中。此类基础的上拔承载力主要受基础结构强度、基础几何特征及其周围土体的物理力学性质影响。在高电压等级输电线路中,杆塔基础往往承受几百吨乃至上千吨的上拔荷载。在工程设计中,为了提高基础承载力,往往通过增加桩径、埋深及扩大头的尺寸等一系列技术手段来提高基础的上拔承载力。然而,如何确定最优的基础尺寸、使基础上拔承载力最大是基础优化设计的核心问题。
针对基础抗拔承载特性影响因素也有一些研究。Matsuo[7-8]通过室内模型试验分析了砂土和黏土中扩大头形状对基础上拔承载力影响,表明在底板混凝土方量一定条件下,圆形扩大头较方形扩大头承载能力高;Dickin等[9-10]通过离心机试验分析了埋深、扩底直径、填土密度和基底扩展角对砂土中扩底桩上拔特性的影响;Hesham等[11]通过模型试验分析了砂土中锥形桩的承载力随围压增长的变化规律,得出高围压下锥形桩的上拔承载力与等截面桩上拔承载力相差不大的结论;刘文白等[12]应用颗粒流理论研究了黄土中扩底桩抗拔承载特性,发现增加扩底桩扩大段的高度对提高承载力有效;陈仁朋等[13]通过大尺寸模型试验得出在非饱和及饱和粉土中,扩底桩深宽比在1~3范围内变化时,上拔极限承载力对应的上拔位移基本不变,当深宽比达到5时,上拔位移有显著增加;喻皓[14]通过数值计算得出黏土地基条件下,短桩的扩大头对桩体的变形影响较大,而长桩影响较小;李保中等[15]研究了主柱和扩底尺寸对中国500 kV线路工程中粉土地基扩底基础上拔、下压余度的影响规律。上述研究的对象均为细粒土中的抗拔基础,而针对碎石土这类粗粒土中的基础抗拔承载特性均未涉及。
综上所述,目前针对基础尺寸参数对碎石土地基扩底基础抗拔承载特性的研究较少。笔者以戈壁滩碎石土地基中的扩底基础为研究对象,基于正交试验设计理论,选取立柱直径、基础深宽比、基底扩展角3个参数为影响因素,以基础上拔承载力为分析指标,通过现场上拔静载试验,分析试验基础的抗拔承载特性以及上述各参数对承载力的影响程度,提出适用于戈壁滩碎石土地基原状土扩底基础的工程优化措施。
1.1 试验场地条件
试验场地位于甘肃省张掖市高台县境内原330 kV张—嘉一回线173#塔位附近,如图1所示。根据地勘资料,试验场地的地质参数推荐值见表1。
图1 试验场地Fig 1 The test site
Table 1 Physical mechanic index of ground soil
土层层深/m天然重度γ/(kN·m-3)内摩擦角φ/(°)粘聚力c/kPa黄土状粉土0.115248碎石6.52142卵石>102140
1.2 加载系统与加载方案
上拔加载装置由千斤顶、加荷梁、连接框架和反力基座组成,其中,反力基座采用承压桩,承压桩具有足够的刚度,以确保满足地基强度和变形的要求,试验加载装置见图2。
图2 加载装置
试验采用分级加载,加载初期可根据经验采用快速荷载法,当基础变形较大时,采用慢速维持法[16],以确保地基土的承载性能得以充分发挥。加载过程中通过布置在基顶表面的位移传感器测定上拔位移。
原状土扩底基础上拔承载力受基础几何尺寸参数的影响,如立柱直径d、扩展角θ、深宽比ht/D、扩底直径D等,见图3。不同因素对基础承载力影响程度不同,为了综合分析上述各参数对基础承载力的影响,同时考虑各因素之间的相关性,分别选取立柱直径d、深宽比ht/D、基底扩展角θ为因素,每个因素取3个水平进行,如表2所示。同时将其他不控制基础承载力的参数(见表3)设为定值。其中立柱直径d以A表示,深宽比ht/D以B表示,基底扩展角θ以C表示。
图3 试验基础示意图
采用3因素4水平的L9(34)标准的正交表进行设计,以基础上拔承载力为评价指标,基础设计方案见表4。
表2 各因素及水平
Table 2 All factors and levels
水平立柱直径d/mm深宽比ht/D基底扩展角θ/(°)18001.510212002.520316003.530
表3 其他基础尺寸参数
Table 3 Other size parameters of foundation mm
表4 L9 (34) 正交试验表
Table 4 Orthogonal experiment for L9(34)
基础编号1(A/mm)2(B)3(C/(°))1#1(800)1(1.5)1(10)2#1(800)2(2.5)2(20)3#1(800)3(3.5)3(30)4#2(1200)1(1.5)3(30)5#2(1200)2(2.5)1(10)6#2(1200)3(3.5)2(20)7#3(1600)1(1.5)2(20)8#3(1600)2(2.5)3(30)9#3(1600)3(3.5)1(10)
图4 1#~9#基础QS曲线Fig 4 Q-S curves of 1~9#
Table 5 The calculated slowly varying rates of load-displacement curves of test foundations
基础编号A点B点荷载/kN位移/mm荷载/kN位移/mm((B位移-A位移)/(B荷载-A荷载))/%1#3901.945032.7234.242#10803.1132039.7510.183#13001.37286038.372.034#10801.83234037.612.485#20000.92580027.100.646#12003.34220044.403.427#30001.52750032.100.588#13001.43364048.011.799#24000.8720028.240.49
3.2 承载力影响因素分析
表6 基础上拔承载力试验结果
Table 6 Uplift bearing capacity of test foundations
基础编号d/mmht/Dθ/(°)Qcu/kNScu/mm1#8001.5104507.052#12001.52013209.243#16001.530286011.454#12002.510234014.505#16002.520580022.446#8002.530220023.607#16003.510750020.648#8003.520364024.239#12003.530720021.10
表7 基础上拔承载力直观分析结果
Table 7 Analysis on directly perceive method on uplift bearing capacity of test foundations
基础编号因素ABC基础上拔承载力Qcu/kN11114502122132031332860421323405221580062322200731275008323364093317200K16290463010290K2108601034010760K3161601834012260K1209715433430K2362034473587K3538761134087R32904570657
同一列中,最大值与最小值之差为极差,即表7中的R。R越大,该列对应的因素对指标Qcu影响最明显。由表7可知,各列的极差R差异较大,表明各因素的水平改变时对指标Qcu的影响程度不同。从表中各列极差大小可知,在立柱直径、深宽比、基底扩展角3个因素中,对基础上拔承载力影响最为敏感的因素是深宽比,其次是立柱直径,最不敏感的是基底扩展角。
为直观比较,通过Qcu与各水平之间的变化趋势来分析各因素对基础上拔承载力的影响程度,分析结果见图5。从图中可以看出,因素B对Qcu影响显著,当深宽比从1.5增加到3.5时,Qcu指标增加4倍左右;因素A次之,Qcu随立柱直径的增加而增加,当立柱直径从800 mm增加到1 600 mm时,Qcu指标增加2倍左右,增加幅度较因素B要小很多;因素C对Qcu影响最不显著,当扩展角从10°增加到30°时,Qcu指标变化很小,接近1.2倍,趋势线近乎一条平行于X轴的直线,表明工程中增加基础深宽比对于提高基础上拔承载力最为有效,可作为一种基础优化设计的推荐做法;而增加扩展角,一方面对于提高基础承载能力效果不明显,同时,施工中宜引起塌孔和掉石现象,增加了施工的风险性,设计中建议取值不大于15°。
图5 各因素和水平下上拔承载力的变化趋势Fig. 5 the trend of uplift bearing capacity under different factors and leve
基于正交试验分析方法,对戈壁滩碎石土地基中不同几何尺寸参数的9个扩底基础进行了承载力特性试验,并分析得出影响基础上拔承载力最敏感的几何尺寸参数,获得的主要结论和建议如下:
2)正交试验结果表明,戈壁滩碎石土地基中扩底基础尺寸参数深宽比、立柱直径、基底扩展角的取值对基础上拔承载力影响程度由大到小依次为:深宽比、立柱直径、基底扩展角。
3)在戈壁滩碎石土地基中进行原状土扩底基础的设计过程中,建议优先考虑通过增加深宽比来提高基础上拔承载力。由于扩展角的增加对基础承载力影响不明显,且易导致施工过程中的塌孔和掉石现象,建议扩展角不大于15°。埋深和扩展角一定条件下,立柱直径的增加势必导致深宽比减小,同时基础本体混凝土方量也大幅度提高,建议立柱直径以满足施工安全和踏脚板大小即可,而不建议采用提高立柱直径来增加基础上拔承载力的做法。
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(编辑 胡英奎)
Experimental analysis of the influence of foundation size parameters on the uplift bearing capacity of the spread foundation in gravel soil
CuiQiang1,TongRuimin1,LiuShengkui2,LuXianlong1
(1.China Electric Power Research Institute, Beijing 100192, P. R. China;
2.Gansu Electric Power Design Institute, Lanzhou 730050, P. R. China)
The uplift bearing capacity of the spread foundation is determined by not only soil physical and mechanical characteristics but also its geometry shapes and scales. The foundation size parameters to determine the minimum quantity of concrete, and the maximum uplift bearing capacity, are key to optimize the foundation design. In order to study their effect on the uplift bearing capacity of the foundation, the undisturbed soil spread foundation in Gobi gravel soil is took as the case. Nine full size test foundations are designed, which use orthogonal test method selected column diameter, the extension angle of the foundation foot and the depth-to-width ratio as influencing factors and the uplift bearing capacity of the foundation as examining goal. The in-site static load tests are carried out in the Gobi gravel soil located northwest China. Load-displacement curves of nine test foundations are obtained and the uplift bearing capacity corresponding are concluded. An index named gradual change ratio is proposed to expressing plastic deformation characteristic of soil foundation under pull load. Meanwhile, the results of the tests show that the sensitivity sort of these geometry factors on the uplift bearing capacity is the depth-to-width ratio, column diameter, and the extension angle of the foundation foot. The conclusion achieve in this work is that adding the depth-to-width ratio in foundation design is an effective measure for improving the uplift capacity of the spread foundation in Gobi gravel soil.
gravel soil; undisturbed soil; transmission line; spread foundation; uplift bearing capacity
2016-04-01
国家电网公司科技项目(GCB17201400162)
崔强 (1980- ),男,博士,高级工程师,主要从事输电线路岩土工程试验检测研究,(E-mail)everjsl@126.com。
Foundation item:State Grid Corporation Science and Technology Project(No.GCB17201400162)
10.11835/j.issn.1674-4764.2016.06.003
TU475
A
1674-4764(2016)06-0017-07
Received:2016-04-01
Author brief: Cui Qiang (1984- ), PhD, senior engineer, main research interest: test detection on geotechnical engineering in transmission line project,(E-mail) everjsl@126.com.