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孔压静力触探(CPTU)是一种新型的原位测试技术,它可以连续地测量锥尖阻力qc、侧壁摩阻力fs、孔隙水压力u[10],从而直接确定桩基的承载力。然而,在工程实际应用中,静力触探曲线经常会出现非常陡的“峰”或“谷”,这些异常现象主要与砂层的土性、强度的变化有关。目前,常用的桩基承载力计算方法,如建筑桩基规范法[11]、LCPC 法、欧洲法等,都是在CPTU 出现之前提出的,它们忽略了贯入过程中超孔隙水压力对锥尖阻力和侧壁摩阻力的影响,而且桩侧阻力是由锥尖阻力通过经验系数求得的,没有充分利用CPTU 测得的侧壁摩阻力数据。因此,在应用静力触探进行桩基承载力计算时,原始数据的处理方式及超孔压的产生和消散对桩基承载力的预测影响较大。
目前,国内外学者提出了诸多基于 CPTU 测试的桩基承载力计算公式,API RP 2GEO 规范[12]《Geotechnical and Foundation Design Considerations》中推荐的4种方法,并对相应公式进行了适当简化,即 ICP-05 方法[13]、UWA-05 方法[14-15]、NGI-05 方法[16]、Fugro-05 [17]方法,被认为是目前在海洋桩基工程中最为可靠的方法[18]。
其中ICP-05及UWA-05在风电工程的大直径桩基承载力计算中应用较多[19]。UWA-05方法整个框架来源于 ICP-05 方法,本质上是对 ICP -05方法的修改和简化,本文以下计算分析采用UWA-05方法进行承载力分析。
UWA-05方法是由西澳大学(University of Western Australia)提出的一种基于CPTU测试的桩基承载力计算方法,适用于在砂土中施工的开口和闭口钢管桩。该方法综合考虑了以下几个影响桩基承载力的因素:(1)桩安装过程中的土体位移效应;(2)摩阻疲劳;(3)砂-桩界面摩擦角;(4)桩加载过程中径向有效应力的变化;(5)加载方向的影响。其抗压承载力在API规范中采用的简化公式如下:
$$ {Q}_{{\rm{c}}}={Q}_{{\rm{f}},{\rm{c}}}+{Q}_{{\rm{p}}}={\text{π}} D{\displaystyle \int {f}_{{\rm{c}}}({\textit{z}})}d{\textit{z}}+q{A}_{{\rm{p}}} $$ (1) 式中:
Qc ——抗压极限承载力(kN);
Qf, c ——受压状态下桩侧摩阻力提供的承载力(kN);
Qp ——桩端承载力(kN);
fc(z) ——受压状态下桩侧摩阻力,深度z的函数(kPa);
q ——桩端阻力(kPa);
Ap ——桩端截面积(m2),考虑整个端面面积。
对于砂土fc(z)可按下式计算:
$$ {f}_{{\rm{c}}}({\textit{z}})=0.03{q}_{{\rm{c}}}({\textit{z}}){A}_{\text{r}}^{0.3}{\left[\mathrm{max}\left( {\dfrac{L-{\textit{z}}}{D},2} \right)\right]}^{-0.5}\mathrm{tan}{\delta }_{{\rm{cv}}} $$ (2) $$ {A}_{\text{r}}=1-\left( {\dfrac{{D}_{{\rm{i}}}}{D}} \right)^{2} $$ (3) 式中:
qc(z)——锥尖阻力(kPa);
Ar ——桩端环面积与端面积的比值;
Di ——桩身内径(m);
D ——桩身直径(m);
L ——桩入土长度(m);
z ——计算深度(m);
${\delta }_{{\rm{cv}}}$ ——桩土摩擦角(°)。对于黏土可按下式计算:
$$ {f}_{{\rm{c}}}({\textit{z}})={q}_{{\rm{c}}}({\textit{z}})/35 $$ (4) 桩端阻力q可按下式计算:
$$ q = {q_{{\rm{c}},{\rm{av}}1.5D}}(0.15 + 0.45{A_{\rm{r}}}) $$ (5) $$ {q_{{\rm{c}},{\rm{av}}1.5D}} = \int\limits_{L - 1.5D}^{L + 1.5D} {{q_{\rm{c}}}({\textit{z}})} {\rm{d}}{\textit{z}}/(3D) $$ (6) 式中:
${{q}_{{{\rm{c}},{\rm{av}}1}{.5D}}}$ ——桩端上下1.5D范围内的平均锥尖阻力(kPa)。按以上公式计算,研究1#~4#机位抗压承载力计算统计见表1。
表 1 CPTU方法计算桩基侧阻及端阻
Table 1. The CPTU method calculates the side resistance and the end resistance of the pile foundation
序号 桩径/m 总土阻力/kN 桩侧阻力/kN 桩端阻力/kN 1# 3.5 51562 27908 23654 2# 3.5 38399 25290 13109 3# 3.5 50671 32015 18656 4# 5.0 103871 33916 69955 -
高应变动力试桩法是一种通过重锤冲击桩顶,使桩周土发生塑性变形的试验方法。该方法可以在桩顶附近测量力和速度的时程曲线,并利用应力波理论分析桩土体系的相关性质,如单桩极限承载力、土阻力及其分布、桩身完整性等[20]。
工程钢管桩现场高应变测量设置见图3,现场高应变数据采集线缆及设备见图4。
实测曲线拟合法是一种基于实测力和速度信号,或上、下行波曲线,计算土阻力及其分布的数值分析方法。该方法首先设定桩身各层土阻力和其他参数,然后运用波动方程进行拟合计算,得到另一条曲线(如速度或力,下行波、上行波曲线),并与相应的实测曲线进行比较。通过不断调整参数,进行迭代计算,直到达到满意的拟合效果为止。最终确定符合实际桩土体系的各种参数值,并获得桩端阻力和桩周阻力分布。
由于现场1#~3#机位采集为高应变初打数据,4#机位采集数据为10 d后高应变复打数据,考虑到由于复打数据主要反映了沉桩后土体恢复的情况,与设计在位工况承载力更为匹配,因此根据同场址复打(10 d后高应变复打数据)与初打的平均土体恢复系数对比,将1#~3#机位初打数据乘以1.5倍的土体恢复系数,作为本次对比研究的实测依据。
1#~4#机位钢管桩高应变实测总土阻力、桩侧阻力及桩端阻力统计如表2所示。
表 2 现场高应变实测桩基侧阻及端阻分析
Table 2. Analysis of side resistance and end resistance of pile foundation with high strain measured on site
序号 桩径/m 总土阻力/MN 桩侧阻力/MN 桩端阻力/MN 1 3.5 26.758 22.431 4.327 2 3.5 26.782 22.323 4.459 3 3.5 34.510 31.646 2.864 4 5.0 43.398 32.751 10.647
Application Analysis of CPTU Method for Jacket Large Diameter Short Pile Axial Bearing Capacity
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摘要:
目的 目前,我国的海上风电工程中,随着工程开发逐步走向深远海,风电机组最常用的基础结构是桩基础,而最常用的上部结构为导管架结构,两者的结合构成了风电机组的基础支撑体系。在一些深水的浅覆盖层地区,大直径导管架基础的应用需求尤其强烈,而下部桩基础的设计直接决定了方案的可行性及结构的安全性,为验证导管架大直径短桩桩基承载力问题,提出采用孔压静力触探试验(CPTU)的计算方法应用。 方法 通过API规范推荐的CPTU计算方法中的UWA-05方法,进行承载力计算,进一步结合广东某风场高应变实测数据与公式计算结果进行对比分析,并对公式本身端阻差异较大的情况进行了参数拟合。 结果 结果表明:采用CPTU方法计算导管架大直径短桩桩身侧阻时,与实测数据拟合度较好。导管架大直径短桩难以形成有效土塞,在采用CPTU方法计算端阻时,建议仅考虑端部环面积。 结论 由此采用CPTU方法计算导管架大直径短桩承载力是可靠的方法,但在使用时候建议结合试桩数据对比验证,为后续深远海风场利用CPTU进行大直径短桩承载力计算应用提供借鉴。 Abstract:Introduction At present, in China's offshore wind power projects, with the gradual development of the far-reaching sea, the most commonly used foundation structure of wind turbine is the pile foundation, and the most commonly used superstructure is the jacket structure. The combination of the two constitutes the basic support system of wind turbine. In some shallow cover areas of deep water, the application demand of large diameter jacket foundation is particularly strong, and the design of the lower pile foundation directly determines the feasibility of the scheme and the safety of the structure. In order to verify the bearing capacity of the large diameter short pile foundation of the jacket, the calculation method of CPTU is proposed. Method Through the UWA-05 method recommended in the CPTU calculation method of API specification, the bearing capacity was calculated, and the actual measurement of high strain data of a wind field in Guangdong and the formula calculation results were further analyzed, and the parameters of the end resistance of the formula itself were quite different. Result The results show that the CPTU method is used to fit the side resistance with the measured data. It is difficult to form an effective soil plug for the large diameter short pile of the jacket. When calculating the end resistance by using the CPTU method, it is recommended to consider only the end ring area. Conclusion Therefore, it is a reliable method to calculate the bearing capacity of large diameter short pile in jacket, but it is suggested to compare and verify the data of test pile, so as to provide reference for the subsequent application of large diameter short pile in far-reaching sea breeze field. -
Key words:
- large diameter pile /
- bearing capacity /
- static touch probe /
- jacket /
- offshore wind power
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表 1 CPTU方法计算桩基侧阻及端阻
Tab. 1. The CPTU method calculates the side resistance and the end resistance of the pile foundation
序号 桩径/m 总土阻力/kN 桩侧阻力/kN 桩端阻力/kN 1# 3.5 51562 27908 23654 2# 3.5 38399 25290 13109 3# 3.5 50671 32015 18656 4# 5.0 103871 33916 69955 表 2 现场高应变实测桩基侧阻及端阻分析
Tab. 2. Analysis of side resistance and end resistance of pile foundation with high strain measured on site
序号 桩径/m 总土阻力/MN 桩侧阻力/MN 桩端阻力/MN 1 3.5 26.758 22.431 4.327 2 3.5 26.782 22.323 4.459 3 3.5 34.510 31.646 2.864 4 5.0 43.398 32.751 10.647 -
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