标准贯入试验液化判别式细粒修正模型

Fines correction method for SPT-based liquefaction discrimination formula

摘要

《建筑抗震设计规范》液化判别方法与美国国家地震工程研究中心(National Center for Earthquake Engineering Research，NCEER)方法在细粒特征指标上存在显著差异，二者对细粒修正土类的界定存在明显矛盾。本文通过对比和梳理中美液化判别方法的细粒修正模型，结合历史地震液化数据验证判别效果，证实粉砂需做细粒修正。从标贯击数随细粒含量增大而下降的机理、细粒指标的连续一致性和变化敏感性，以及测试难易程度等角度展开分析，得出细粒含量相较于黏粒含量，更适合作液化判别式的特征指标。当细粒含量F_C≤15%时，粗、细颗形成搭配，土体动力稳定性更强，建议对细砂不进行细粒修正；当细粒含量F_C>15%时，细粒产生“滚珠”效应，土体抗液化强度和标贯击数随细粒含量增加而快速下降；当细粒含量F_C≥35%以后，细粒承担土体应力骨架，但细粒骨架弱于砂粒骨架，土体抗液化强度和标贯击数整体较小，细粒修正系数须放缓调整幅度。参照规范方法3/ρ_c，提出的15/F_C可精准体现上述细粒含量的影响规律。结合不同埋深砂土在强地震动作用下的液化判别统一公式，对历史地震液化数据按细粒含量分段判别的效果，以及与NCEER方法判别效果的对比，说明提出的细粒修正模型准确可靠。该模型已被具有示范作用的《建筑工程抗震性态设计通则》修订版采用。

Abstract

There are significant differences between liquefaction discrimination methods of Chinese “Code for seismic design of buildings” and National Center for Earthquake Engineering Research (NCEER) methods in terms of fine-grained characteristic variables, and there are obvious contradictions about soil types for fines correction. By comparing fine correction models of the two methods, and the discriminating results for known liquefaction and non-liquefaction cases of silty sand, it can be concluded that fines correction should be done for silty sands. Based on the mechanism of the decrease of SPT blow count with the increase of fines content, continuity and sensitivity of fine grained characteristic variables, and the difficulty of testing, it is concluded that fines content is more reliable than clay content for the liquefaction discrimination formula. When fines content F _C≤15%, fine grains are located in the voids of coarse skeleton, making the dynamic stability of the soil enhanced. It is recommended not to do fines correction for fine sands. When fines content F _C>15%, fine grains are located in the contact area of coarse grains, dropping liquefaction resistance strength and SPT blow count rapidly with the increase of fines content. When fines content F _C≥35%, fine grains dominate soil skeleton, while fines grain skeleton is weaker than coarse grain skeleton. So the soil liquefaction resistance strength and SPT blow count are smaller, and the fines correction coefficient should change slowly. According to the correction model 3/ρ _c of Chinese code methods, 15/F _C for silty sands and silts is proposed. Combined with the unified formula for predicting sand liquefaction in different buried depths under severe seismic ground motion, identification results of the combined formula for case histories divided to different fines content, and comparisons between discriminating success rate of the combined formula and NCEER methods, showing that the proposed fines correction model is accurate and reliable. The combined formula has been adopted in the revised version of “the general rule for performance-based seismic design of buildings”.

关键词

标贯击数 / 液化判别式 / 细粒含量 / 黏粒含量 / 修正系数

Key words

SPT blow count / liquefaction discrimination formula / fines content / clay content / correction factor

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基金资助

国家自然科学基金项目(52278171)

国家自然科学基金项目(51708525)