基于静力触探试验的基坑开挖卸荷单桩水平承载力损失预测研究

选取典型成层地基场地并设计试坑开挖卸荷试验,开展了基于静力触探(CPT)测试的基坑开挖卸荷单桩水平承载力损失程度预测研究。通过在试坑开挖卸荷前后进行CPT原位测试,得到了试坑开挖卸荷前后的CPT贯入锥尖参数变化规律。进而基于CPT测试p–y模型研究了基坑开挖卸荷前后邻近既有单桩的水平承载力损失及桩身弯矩分布特征。分别考察了基坑开挖卸荷后邻近桩基试桩加载模式下的残余水平承载力和桩顶加载联合土体位移共同作用下的卸荷桩基水平承载特性。研究表明,依据真实卸荷土体CPT参数更能准确预测桩基水平承载力损失程度及损失特征,卸荷桩设计阶段须同时考虑卸荷桩较自由场地桩基的水平承载力损失及土体运动位移对桩基水平承载的影响。以上研究为合理确定基坑开挖引起的既有桩基水平承载力损失提供了一种技术思路,同时对卸荷桩水平承载性能评价具有参考意义。     

新型拉压复合型锚杆锚固性能研究Ⅱ：模型试验

针对传统拉力型和压力型锚杆存在受力集中、锚固体与岩土体界面黏结强度发挥不充分、抗拔承载力偏低的问题,研发了一种新型拉压复合型锚杆。通过对传统锚杆及拉压复合型锚杆开展模型试验,对比研究了不同锚杆的极限抗拔承载力及其锚固性能。结果表明：拉压复合型锚杆极限抗拔承载力比传统拉力型锚杆大幅提高,拉压长度比为1∶2和2∶1时,分别提高79%和161%,且具有更好的位移延性和抗变形能力;拉压复合型锚杆峰后残余抗拔承载力显著提高,传统拉力型和压力型锚杆稳定残峰比最大值均不超过0.40,锚头相对拔出变形ξ_s=2.5%时,残峰比平均值分别为0.292和0.259;TC360-12锚杆和TC360-21锚杆稳定残峰比最小值分别不低于0.45和0.60,ξ_s=2.5%时,残峰比平均值分别为0.545和0.790;拉压长度比为2∶1的拉压复合型锚杆即将破坏时,受拉锚固段和承压锚固段协同承载能力更强,界面黏结强度得到充分发挥,锚杆极限抗拔承载力更高。     

圆CFRP-钢复合管约束高强混凝土短柱轴压试验研究

为研究圆CFRP-钢复合管约束高强混凝土短柱轴压受力性能,进行了6个CFRP-钢复合管约束高强混凝土（CFRP-steel composite tubed high-strength concrete,C-STC）柱和2个CFRP约束高强混凝土（CFRP-confined high-strength concrete,CC）柱、1个钢管约束高强混凝土（steel tubed high-strength concrete,STC）柱对比试件的轴压试验研究,得到了试件轴向荷载-位移曲线和CFRP及钢管的应变。结果表明：C-STC柱在轴压荷载作用下发生剪切破坏;约束模式对其前期刚度影响较小,相同CFRP层数的C-STC柱和CC柱的荷载-位移曲线第二线性段斜率近似相等;随着CFRP层数增多,短柱承载力和变形能力均能得到提高;钢管应力分析表明,STC柱钢管在峰值荷载附近屈服,C-STC柱钢管约在荷载-位移曲线第二线性段起点处屈服,钢材强度得到充分发挥。结合试验结果对已有文献中约束混凝土强度计算模型进行验证,并给出了建议的C-STC柱承载力计算式。     

矩形钢管混凝土翼缘-蜂窝钢腹板H形截面组合短柱轴压性能试验研究

为研究矩形钢管混凝土翼缘-蜂窝钢腹板H形截面组合短柱(STHCC)的轴压性能,进行了16根STHCC短柱的轴压静力试验。主要研究参数包括约束效应系数、混凝土立方体抗压强度、翼缘钢管腹板厚度和柱长细比。通过轴压试验得到STHCC短柱试件的试验现象和破坏形态、荷载-位移曲线、钢管翼缘和蜂窝钢腹板的荷载-应变曲线,分析了四参数对STHCC短柱轴压承载力的影响规律及受力机理。结果表明：钢管的外表面会产生吕德尔滑移线,所有试件钢管混凝土翼缘均呈剪切型破坏;试件荷载-位移曲线大致可分为弹性、弹塑性、荷载下降和残余变形等四段。随着约束效应系数和蜂窝钢腹板厚度的增加,试件的轴压承载力逐渐提高;随着长细比的增大,试件的轴压承载力却逐渐下降。最后,通过引入组合效应修正系数和综合影响变量,建立了与试验结果吻合较好的STHCC短柱轴压承载力计算式,并给出了该类轴压短柱的设计建议。     

钢筋混凝土缺角板受弯性能试验研究

为了研究混凝土缺角板的受弯性能,设计制作了角部不同开孔大小的7块钢筋混凝土四边简支板,并进行了静力受弯性能试验。结果表明：缺角板跨中区域钢筋先于板四周区域钢筋屈服,随后屈服范围不断扩展并往四角延伸,最终板顶缺角处沿与板边大致成45°方向的混凝土被压碎;开孔系数(板角所开孔洞边长与跨度的比值)为0.05、0.08、0.10、0.12、0.15和0.20的缺角板极限荷载较矩形板的极限荷载分别降低5.98%、10.35%、21.37%、23.63%、38.50%和49.29%,开裂荷载、屈服荷载基本随开孔系数的增大而降低。根据钢筋混凝土板塑性极限分析的塑性铰线法,对6块不同开孔系数混凝土缺角板提出了4种塑性铰线模式,并利用虚功原理建立缺角板极限荷载计算公式,极限荷载计算值与试验值吻合较好,验证了所提出的塑性铰线模式的合理性,及利用虚功原理建立缺角板极限荷载计算公式的可行性。引入一个与开孔系数相关的系数对塑性铰线模式进行简化,简化公式得出的计算值与试验值吻合较好。     

Bearing capacity of composite ground with soil-cement columns under earth fills: Physical and numerical modeling

Soil-cement columns are widely used to improve soft ground, and the bearing capacity of the formed composite ground is a key design parameter. The currently employed design method was developed for composite grounds under rigid footings, whilst the bearing capacity behavior of composite grounds under earth fills with different degrees of stiffness has rarely been investigated. Hence, the present study attempts to fill this gap. In this investigation, 1-g laboratory model tests are conducted to compare the bearing capacity behavior of composite grounds under a rigid footing and under embankment fill, based on which a numerical model that can capture the strain-softening behavior of soil-cement columns is established. The calibrated numerical model is further employed to perform 144 analyses. The results indicate that the failure mode of composite grounds differs for different types of earth fills: soil failure occurs prior to column failure under soft clay and dredged slurry, whereas column failure is the primary failure mode for composite grounds under embankment fill. This difference in failure mode of composite grounds can be explained using soil arching theories. For different failure modes, different bearing capacity efficiency factors should be used in design.     

Cone-shaped hollow flexible reinforced concrete foundation (CHFRF) – Innovative for mountain wind turbines

This paper presents an innovative type of mountain wind turbine foundation, namely, the cone-shaped hollow flexible reinforced concrete foundation (CHFRF). It consists of a top plate, a base plate and a side wall that are made of reinforced concrete. The cavity of the CHFRF is filled with rubble and soil directly from the excavation for the CHFRF, which means that it can absorb the spoil. A rubber layer is placed beneath the CHFRF to increase the foundation flexibility to resist cyclic and dynamic loadings and to increase the bearing capacity. The great advantages of the CHFRF are the reduction in the usage of concrete and steel and the protection of the vegetation around the wind turbine, compared with conventional mountain wind turbine foundations that are solid structures. It is verified through model tests and a numerical simulation that the CHFRF can provide higher lateral bearing capacity in comparison to the regular circular gravity-based foundation under the same foundation diameter and height, and that the bearing capacity is increased by approximately 33.5% accordingly. It is also found that the rubber layer can effectively reduce the accumulated rotation of the CHFRF under cyclic loading. The accumulated rotation of the CHFRF with a rubber layer having a thickness of 4 mm is decreased by about 50% compared to that of the CHFRF with a rubber layer having a thickness of 2 mm. In addition, the volume of concrete used for the CHFRF is only one-fifth of that used for the circular gravity-based foundation. Therefore, the CHFRF outperforms regular mountain wind turbine foundations.     

Relative density effects on the bearing capacity of unsaturated sand

This study focuses on the impact of relative density on the bearing capacity of unsaturated sand using both theoretical predictions and measurements from physical modeling tests. The theoretical predictions incorporate the effective stress, quantified using the suction stress concept and friction angles obtained from direct shear tests on unsaturated sand specimens at different relative densities and degrees of saturation, into conventional bearing capacity equations. The suction stress values inferred from the failure envelopes were found to match well with values predicted from the soil-water retention curves for sands with different relative densities. Moreover, the bearing capacity values measured in physical modeling experiments involving loading of a circular footing atop unsaturated silty sand layers having different initial degrees of saturation matched well with the predicted bearing capacity values from an effective-stress based model. As expected, the bearing capacity was greater for soils with increasing relative density, but an interesting observation is that a transition from general to local shear failure occurred at a certain combination of relative density and degree of saturation. For the silty sand tested, this transition occurred at a relative density of 0% for degrees of saturation between 4 and 16% and at a relative density of 40% for degrees of saturation between 30 and 90%. General shear failure was always observed at relative densities of 70 and 90%.     

Experimental and theoretical studies on the ultimate bearing capacity of geogrid-reinforced sand

Geosynthetic reinforced soil (GRS) structures have gained popularity in replacing concrete rigid piles as abutments to support medium or small-spanned bridge superstructures in recent years. This study conducted 13 model tests to investigate the ultimate bearing capacity of the GRS mass when sand was used as backfill soil. The GRS mass was constructed and loaded to failure under a plane strain condition. Test results were compared with two analytical solutions available in literature. This study also proposed an analytical model for predicting the ultimate bearing capacity of the GRS mass based on the Mohr-Coulomb failure criterion. The failure surface of the GRS mass was described by the Rankine failure surface. The effects of compaction and reinforcement tension were equivalent to increased confining pressures to account for the reinforcing effects of the geosynthetic reinforcement. The proposed model was verified by the results of the model tests conducted in this study and reported in literature. Results indicated that the proposed model was more capable of predicting the ultimate bearing capacity of the GRS mass than the other two analytical solutions available in literature. The proposed model can be used to predict the ultimate bearing capacity of GRS structures when sand was used as backfill material. In addition, a parametric study was conducted to investigate the effects of friction angle of backfill soil, reinforcement spacing, reinforcement strength, and reinforcement stiffness on the ultimate bearing capacity of the GRS mass calculated with and without compaction effects. Results showed that the ultimate bearing capacity of the GRS mass was significantly affected by the friction angle of backfill soil, reinforcement spacing and strength. Compaction effects resulted in an increase in the ultimate bearing capacity of the GRS mass.     

盾构隧道管片接头抗弯承载力计算模型研究

盾构隧道管片接头承载力的计算是管片设计和服役性能评估的重要部分和关键部分,针对形式相同而细部构造不同的管片接头,提出一套适用于其抗弯承载力计算的理论方法十分重要。鉴于此,文章基于接缝面的不连续特征将接缝面进行适当分区,建立盾构隧道管片接头抗弯承载力计算模型,并给出对应的求解算法。将理论模型的计算结果与接头足尺试验结果进行对比分析,验证理论模型的正确性,然后采用该计算模型对管片接头抗弯承载力进行分析与讨论。结果表明：管片接头抗弯承载力曲线具有明显的非线性特征,随着轴力的增大,接头极限弯矩呈先增大后减小的趋势。混凝土强度对于接头抗弯承载力的影响较为显著,而螺栓强度和螺栓直径对于接头抗弯承载力的影响程度受轴力控制,轴力较小时,上述两参数的增大对于接头抗弯承载力的提升效果明显,轴力较大时效果逐渐减弱,当轴力高于某一阈值后,螺栓对于接头抗弯承载力无影响。文章所提出的抗弯承载力计算模型与研究结果以期为盾构隧道结构设计、试验和性能评估提供重要参考。