Behavior of retaining system of narrow-long deep foundation pit in soft soil in Nansha Port Area

doi:10.3785/j.issn.1008-973X.2022.08.001

Journal of ZheJiang University (Engineering Science)

2022, Vol. 56

Issue (8): 1473-1484 DOI: 10.3785/j.issn.1008-973X.2022.08.001

Behavior of retaining system of narrow-long deep foundation pit in soft soil in Nansha Port Area

Shi-fan QIAO(

),Zi-yong CAI*(

),Zhen ZHANG,Jun-kun TAN

School of Civil Engineering, Central South University, Changsha 410075, China

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Abstract

In order to clarify the characteristics of the retaining system of narrow-long deep foundation pit in soft soil in Nansha Port Area, the behavior of retaining system for foundation pit excavation was studied, according to the measured data of Guangzhou deep and narrow foundation pit with diaphragm wall and internal support as retaining system. Results show that the maximum lateral wall displacement ranges from 0.07%H to 0.38%H, with a mean value of about 0.22%H, where H is the excavation depth. The location of the maximum wall displacements H_δm is between H?6 andH+3, and most are above the excavation surface. The wall deformation mainly occurs in the excavation stage of the second and third layers of soil, accounting for 32.6% and 40.1% of the cumulative deformation respectively. Foundation pit excavation has depth effect, and layered excavation of deep foundation pit is very important for wall deformation control. The main influence depth of wall deformation is about twice the excavation depth of foundation pit, and the spatial effect is significant. The variation characteristics of the vertical reinforcement stress and the lateral displacement of the wall are basically similar. With the excavation depth of the foundation pit, the maximum position moves down gradually, revealing the dynamic adjustment process of the wall deformation and stress. The axial force of the support reaches the maximum value after about two weeks after the support is erected, which shows real-time with the excavation of the foundation pit. The axial force of the multi-layer support structure is adapted dynamically with the excavation and support process of the foundation pit to coordinate the deformation development. When the excavation of the foundation pit is completed, the ultimate stable axial force of reinforced concrete support is about 0.73 times of the design value, the axial force of the first and the second steel support is 0.40 and 0.31 times of the design value, and the steel support design is conservative. On the premise of ensuring the stability of the foundation pit, the optimal design of a support scheme can be considered. Research results have important practical significance for the subsequent safety prediction of similar foundation pits in this area, as well as the guidance of similar engineering design and construction parameter optimization.

Key words： railway tunnel narrow-long foundation pit deep soft soil enclosure system field monitoring

Received: 13 August 2021 Published: 30 August 2022

CLC:

TU 476

Fund: 国家自然科学基金资助项目（51878673）；中国铁路总公司科技研究开发计划重点课题资助项目（2017G007-D，2017G008-J）；中国中铁股份有限公司科技研究开发计划重点课题资助项目（20192001）

Corresponding Authors: Zi-yong CAI E-mail: qiaosf@csu.edu.cn;164801006@csu.edu.cn

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Cite this article:

Shi-fan QIAO,Zi-yong CAI,Zhen ZHANG,Jun-kun TAN. Behavior of retaining system of narrow-long deep foundation pit in soft soil in Nansha Port Area. Journal of ZheJiang University (Engineering Science), 2022, 56(8): 1473-1484.

URL:

https://www.zjujournals.com/eng/10.3785/j.issn.1008-973X.2022.08.001 OR https://www.zjujournals.com/eng/Y2022/V56/I8/1473

南沙港区软土狭长深基坑围护体系性状

为了阐明南沙港区软土狭长深基坑围护体系性状，对广州深厚软土地层采用地连墙加内支撑作为围护体系的狭长深基坑实测分析. 研究结果表明，1) 墙体最大侧移量δ_m的变化范围为0.07%H~0.38%H（H为开挖深度），平均值为0.22%H，最大侧移位置深度H_δm为H－6~H＋3，且大多数位于开挖面以上. 2) 墙体变形主要发生在第2、3层土体开挖阶段，其变形量分别占累积变形的32.6%、40.1%，基坑开挖具有深度效应，深基坑分层开挖对墙体变形控制非常重要，墙体变形主要影响深度约为基坑开挖深度的2倍，空间效应显著. 3) 墙体竖向钢筋应力与侧斜位移变化特性基本相似，随着基坑深度开挖，最大值位置逐渐下移，揭露了墙体变形与应力动态调节过程. 4) 支撑轴力在支撑架设后历时2周左右即达到最大值，随基坑开挖表现出即时性，多层支撑结构的各支撑轴力大小随着基坑开挖支护过程动态调整以协调变形发展，当基坑开挖完成，最终趋于稳定的钢筋混凝土支撑轴力约为设计值的0.73倍，第1、2道钢支撑轴力分别为其设计值的0.40、0.31倍，钢支撑设计偏保守，在保证基坑稳定的前提下，可以考虑支撑方案优化设计. 研究成果对后续该地区同类基坑安全预判以及指导类似工程设计和施工参数优化具有重要的现实意义.

关键词： 铁路隧道, 狭长基坑, 深厚软土, 围护体系, 现场监测

Tab.1 Physico-mechanical parameters of soils

Fig.1 Foundation pit construction and surrounding environment

Fig.2 Profile of supporting structure system

Tab.2 Main excavation account of foundation pit

Tab.3 List of foundation pit monitoring contents

Fig.3 Detailed layout of measuring points

Fig.4 Layout of reinforced concrete support measuring points

Fig.5 Layout of steel support measuring points

Fig.6 Layout of on-site measuring points

Fig.7 Time history curve of vertical displacement of wall top

Fig.8 Time history curve of horizontal displacement of wall top

Fig.9 Time history curve of variation of wall top inclinometer

Fig.10 Relationships between maximum lateral displacement of wall and excavation depth

Fig.11 Relationships between location of maximum lateral displacement of wall and excavation depth

Fig.12 Comparison of maximum lateral wall displacement between long-narrow pit and wide pit

Fig.13 Time history curve of stress variation of vertical reinforcement of wall at different measuring points

Fig.14 Stress curve of vertical reinforcement in wall underdifferent working conditions

Fig.15 Time history curve of axial force variation of reinforced concrete support

Fig.16 Time history curve of axial force change of steel support

Fig.17 Time history curve of earth pressure change outside wall at different measuring points

Fig.18 Variation curve of earth pressure outside wall with depth under different working conditions


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