Influence of different drill pipe diameters on the blow count of SPT
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摘要: ϕ50钻杆越来越广泛地应用于标准贯入试验中,然而,在进行岩土体工程特性判别时,ϕ50钻杆得到的标贯击数N值是否需要换算以及如何换算,规范中未提供明确的指导。在湛江市某场地开展不同杆径标准贯入试验对比研究,对相关数据进行统计分析,结果表明:标贯击数的大小与杆径之间存在显著的相关关系,且不同杆径间标贯击数的比值与岩土类别、测试深度有关。砂土地层中ϕ42钻杆的标贯击数小于ϕ50钻杆,整体上二者比值为0.89,不同深度下二者比值为0.83~1.01,随着试验深度的增加,击数比值呈现先增加后减小的趋势;黏性土地层中ϕ42钻杆的标贯击数大于ϕ50钻杆,整体上二者比值为1.14,不同深度下二者比值为1.11~1.20,随着试验深度的增加,击数比值基本不变。研究成果可为地区经验积累和工程实践提供参考。Abstract: The ϕ50 drill pipe is increasingly being used in standard penetration tests. However, there is no clear guidance in the specifications on whether and how to convert the blow count N value of the standard penetration tests obtained by the ϕ50 drill pipe when distinguishing the characteristics of rock and soil engineering. A comparative study was conducted on standard penetration tests with different drill pipe diameters at a site in Zhanjiang City. Statistical analysis was conducted on the relevant data, and the results showed that there was a significant correlation between the blow count of standard penetration tests and the drill pipe diameter, and the ratio of blow counts between different drill pipe diameters was related to the soil type and test depth. The blow counts of the ϕ42 drill pipe in sandy soil formations is less than that of the ϕ50 drill pipe, with an overall ratio of 0.89. The ratio of the two at different depths ranges from 0.83 to 1.01. As the test depth increases, the ratio of the blow counts shows a trend of first increasing and then decreasing. The blow counts of the ϕ42 drill pipe in the cohesive soil layer is greater than that of the ϕ50 drill pipe, and the overall ratio of the two is 1.14. The ratio of the two at different depths is 1.11~1.20. As the test depth increases, the ratio of the blow counts remains basically unchanged. The research results can provide reference for regional experience accumulation and engineering practice.
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表 1 测区场地主要地层条件
地层代号 岩性 状态及密实度 ①1 杂填土 松散 ①2 素填土 松散 ⑤1 淤泥 流塑 ⑤2 淤泥质黏土 软塑 ⑤3 淤泥质粉质黏土 软塑 ⑤4 粉质黏土 可塑—硬塑 ⑤5 黏土 软塑—硬塑 ⑤13 泥炭质土 ⑤6 粉细砂 松散—密实、饱和 ⑤7 中砂 松散—中密、饱和 ⑤8 粗砂 松散—密实、饱和 ⑤9 砾砂 松散—密实、饱和 表 2 测区主要地层标贯数量统计
杆径 黏土 粉质黏土 粉细砂 中砂 粗砂 砾砂 ϕ42 262 226 62 156 136 91 ϕ50 224 410 44 252 143 表 3 各类土层标贯次数统计
统计项目 砂土 黏性土 ϕ42 ϕ50 比值 ϕ42 ϕ50 比值 试验个数 445 439 488 634 最小值 3 2 3 4 最大值 38 51 31 50 平均值 17.98 20.22 0.89 13.73 12.02 1.14 变异系数 0.45 0.49 0.42 0.49 标准值 17.32 19.41 0.89 13.29 11.62 1.14 表 4 不同杆径标贯击数统计结果及对比
土层类别 统计项目 试验深度0~10 m 试验深度10~20 m 试验深度20~35 m ϕ42 ϕ50 比值 ϕ42 ϕ50 比值 ϕ42 ϕ50 比值 砂土 试验个数 133 94 113 142 199 203 最小值 3 2 7 7 8 7 最大值 23 36 33 40 38 51 平均值 9.48 11.49 0.83 17.94 17.81 1.01 23.68 25.96 0.91 变异系数 0.40 0.50 0.30 0.33 0.27 0.39 标准值 8.92 10.48 0.85 17.08 16.96 1.01 22.92 24.73 0.93 黏性土 试验个数 124 185 173 211 191 238 最小值 3 4 5 4 7 4 最大值 23 18 26 23 31 50 平均值 9.04 7.51 1.20 12.46 11.27 1.11 17.92 16.20 1.11 变异系数 0.43 0.41 0.35 0.39 0.28 0.37 标准值 8.44 7.13 1.18 11.90 10.74 1.11 17.31 15.55 1.11 -
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