Experimental and theoretical studies on modeling of uplift bearing characteristics of screw anchors in sandy soil foundation
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摘要: 目前多锚片螺旋锚上拔承载的理论研究需人为引入临界埋深比和临界间距比的概念，这将一个连续演化问题割裂成多种特殊工况，但两个临界值的取值标准又存在不统一的问题，导致理论的工程应用存在风险。基于模型试验并结合数字照相测量技术开展砂土地基中螺旋锚上拔承载特性研究，主要得到以下结论：①单、双锚片螺旋锚上拔荷载位移曲线的变化均可分为增长区间和震荡下降区间两个阶段，埋深比越大，荷载峰值越高，但随之增强的土拱效应也导致曲线震荡幅度越显著。②抗拔承载力突破因子Nγ随埋深比的变化曲线存在峰值特征，先快速增大后缓慢减小；下层锚承载力发挥系数η随间距比的增大呈先快后缓的增加趋势。③多锚片螺旋锚的各层锚片对应的破坏滑动面均可用随埋深比增大长短轴比逐渐减小的椭圆来刻画，首层锚滑动面与同埋深下单锚片滑动面形态一致，下层锚滑动面的椭圆长短轴之比受到间距比变化的影响可分为3个阶段。④多锚片螺旋锚的首层锚和下层锚各自有3种滑动面工况，对应3种力学模型，这取决于埋深比和间距比，螺旋锚总的上拔承载力学模型由它们组合而成。⑤推导建立了多锚片螺旋锚上拔承载力计算方法，该方法可以考虑任意锚片数量、埋深比和间距比，并通过对4个松砂和中密砂地基中试验案例的计算验证了该方法的有效性。Abstract: At present, it is necessary to introduce the concepts of the critical burial depth ratio and critical spacing ratio artificially into the theoretical researches on the uplift bearing of screw anchors with multiple blades, which divides a continuous evolution problem into two special working conditions. However, the criteria of the two critical values are inconsistent, leading to risks in the engineering application of the theory. Based on model tests and combined with the digital photographic measurement technology, the studies on the uplift bearing characteristics of screw anchors in sandy soil foundation are made, and the main conclusions are drawn as follows: (1) The variation of the uplift load displacement curves of the screw anchors with single and double blades can be divided into two stages, growth interval and oscillation decline interval. The larger the burial depth ratio, the higher the peak load, but then the enhanced soil arching effects also lead to the more significant oscillation amplitude of the curve. (2) The curve of the breakout factor Nγ with the burial depth ratio has a peak characteristic, which increases rapidly first and then decreases slowly. The bearing capacity playing coefficient η of lower blade increases rapidly and then increases slowly with the increase of spacing ratio. (3) The failure sliding surface corresponding to each layer of the screw anchor with multi-blades can be characterized by an ellipse with the decreasing axis ratio as the depth ratio increases. and the sliding surface shape of the first blade is consistent with that of the single blade with the same burial depth. The axis ratio of the ellipse of the sliding surface of the lower anchor can be divided into three stages under the influences of spacing ratio. (4) There are three sliding surface conditions of the first-layer anchor and the lower-layer anchors, respectively, corresponding to three mechanical models, which depend on the buried depth ratio and the spacing ratio. The overall mechanical model for the screw anchors is composed of them. (5) A method for calculating the uplifting capacity of the screw anchors with multiple anchor plates is developed to consider any number of anchor blades, buried depth ratio and spacing ratio. The effectiveness of the proposed method is verified by the calculation of four test cases in loose sand and medium-dense sand foundation.
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图 1 典型双锚片螺旋锚竖向拉拔破坏模式
Figure 1. Typical uplift failure modes of screw anchors with double blades
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图 4 空腔变化与荷载位移曲线对应关系
Figure 4. Relationship between cavity change and load-displacement curve (H/D=8)
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图 8 空腔变化与滑动面形成的相关力学机制
Figure 8. Mechanical mechanism of cavity variation and sliding surface formation
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图 9 单锚片滑动面随埋深比变化
Figure 9. Variation of sliding surface with burial depth ratio of single blade
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图 11 双锚片滑动面随埋深比变化（S/D=1）
Figure 11. Variation of sliding surface with burial depth ratio of double blades (S/D=1)
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图 12 滑动面随间距比变化（H/D=2）
Figure 12. Variation of sliding surface with spacing ratio(H/D=2)
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图 13 滑动面随间距比变化（H/D=6）
Figure 13. Variation of sliding surface with spacing ratio(H/D=6)
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图 15 下层锚和相同埋深比单锚的a/b对比
Figure 15. Comparison of a/b between lower anchor and single anchor with same burial depth ratio
表 2 试验用砂物理力学指标
Table 2 Physical and mechanical indexes of test sand
Dr Cu Cc φ/(°) ρdmin/
(g·cm-3)ρdmax/
(g·cm-3)γ/(kN·m-3) 0.57 2.5 1.1 33.1 1.616 1.766 16.56 
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表 3 试验工况
Table 3 Test conditions
锚片数 首层锚埋深比H/D 锚片间距比S/D 1 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 0 2 1, 2, 4, 6, 8 1.0, 1.5, 2.0, 3.0, 4.5, 6.0
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表 4 双锚片螺旋锚滑动面组合形态
Table 4 Combined forms of sliding surface of screw anchors with double blades
工况 1 2 3 4 5 6 7 8 9 首层滑动面 ① ② ③ ① ② ③ ① ② ③ 下层滑动面 Ⅰ Ⅰ Ⅰ Ⅱ Ⅱ Ⅱ Ⅲ Ⅲ Ⅲ
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表 5 试验案例基本情况
Table 5 Basic information of experimental cases
案例 内摩擦角φ/(°) 重度γ/(kN·m-3) 相对密实度Dr/% 直径D/mm 埋深比(H/D) 间距比(S/D) 一 33.1 15.56 57 50 1/2/4/6/8 1.0/1.5/2.0/3.0/4.5/6.0 二 30.0 14.44 23~32 50 4/5 1.5/2.0 三 31.0 13.82 33~42 50 5 0/1.0/1.5/2.0 四 32.0 19.00 44 20 6 0/1.5/3.0/4.5/6.0
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