湿陷性黄土场地湿陷下限深度与桩基中性点位置关系研究

Study of relationship between maximum collapsing depth and neutral point position of pile foundation in collapsible loess ground

湿陷性黄土场地的湿陷下限深度与桩基中性点位置密切相关,为了解决桩基设计方案不当问题,如何准确判定湿陷下限深度显得十分重要。在系统分析并总结宁夏固原桩基系列浸水试验、兰州和平镇黄土浸水试验及其他地区桩基浸水试验研究成果的基础上,揭示了黄土湿陷下限深度与桩基中性点位置确定的相互关系,得出了以下研究成果:(1)指出了均质黄土现场浸水试验水的竖向渗透深度是有限的(一般为20~25 m),现场黄土湿陷发生的下限深度也是有限的,不宜按室内试验的湿陷系数确定的下限深度来直接评判;(2)给出了室内湿陷性评价试验中湿陷系数sδ或自重湿陷系数zsδ随地区和深度变化的取值建议,同时,建议在湿陷量计算时引入深度修正系数和地基土浸水机率系数,初步给出大厚度湿陷性黄土场地不同地区湿陷下限深度评价系数?,可以有效减小湿陷性评价的室内外差异;(3)由室内试验计算确定的湿陷下限深度偏于保守,导致桩基设计过分夸大了负摩阻力的不利作用,造成桩基设计承载力偏低;(4)大厚度湿陷性黄土场地桩的中性点最深位置不宜大于20~25 m,湿陷性评价下限深度小于20 m的场地,宜按评价深度确定中性点位置;给出了桩侧负摩阻力估算方法,并建议桩基负摩阻力平均值取20~35 k Pa为宜。

The maximum collapsing depth of collapsible loess ground is closely related to the neutral point position of pile foundation. In order to solve the problem of improper pile foundation design, the accurate determination of the collapsing depth is of key importance. Based on a systematic analysis of experimental results obtained from Ningxia, Guyuan series of pile foundation and Heping town in Lanzhou ground treatment test and other area immersion test, the relationship between the maximum collapsing depth and neutral point position of pile foundation is revealed. The research results can be divided into the following four points. 1) Because of the limited vertical penetration depth of the homogeneous loess in the field immersion test(generally 20-25 m), the maximum collapsing depth is limited as well. The collapse coefficient in indoor test cannot be directly adopted to evaluate the collapsing depth. 2) The recommended values of the collapse coefficient sδ or dead weight collapse coefficientzsδ varying with the region and the collapsing depth are given. Meanwhile, the depth modification coefficient and earth immersion probability coefficient are suggested to be introduced into calculating collapsing depth and their reference values ? are provided as well; the difference between the indoor and outdoor can thus be effectively decreased. 3) The maximum collapsing depth calculated in indoor test is some kind of over conservative, the adverse effects of negative skin friction are overly exaggerated in most actual projects, rendering the smaller design load-carrying capacity. 4) When the depth of the neutral point of the pile foundation is less than 20-25 m and maximum collapsing depth is smaller than 20 m, the neutral point should be determined by the evaluation depth. The estimation method of negative skin friction is provided and It could be more appropriate if the negative skin friction of foundation pile is set as 20-35 k Pa.