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首页> 外文期刊>Engineering Geology >Deep enclosures versus pumping to reduce settlements during shaft excavations
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Deep enclosures versus pumping to reduce settlements during shaft excavations

机译：深箱体与泵送相比，可减少竖井开挖时的沉降

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Deep excavations in aquifers may be constructed by combining pumping with the cut and cover method. The enclosures are often deepened more than structurally needed, in order to diminish the risk of heave or fluidisation inside the excavation, and to reduce pumping rates and the associated settlements outside jet-grouting piles are sometimes adopted for lengthening. We analysed the water-proofing efficiency of jet-grouting and the need for water isolation in preconsolidated sediments. We used data obtained from two shaft excavations during the construction of the high speed train tunnel in Barcelona, located adjacent to the Sagrada Familia Basilica. Jet-grouting was characterised using pumping tests before and after the construction of the enclosure. The effectiveness of deepened enclosures was evaluated by comparing settlements caused and discharges required for several dewatering scenarios. Differences between them lie in the depth of the enclosures. Settlements were calculated analytically, using the drawdown obtained from a hydrogeological model, and numerically employing a hydro-mechanical model. Results show that jet-grouting reduced the permeability of the soil (90% reduction, from 5.5 to 0.6 m/d). However, this reduction only affected the pile area, so that the necessary pumping rate and the settlements outside the enclosure were only reduced by 40%. Results also show that settlements due to groundwater pumping are fairly smooth (i.e., differential settlements are small) with low absolute values. Moreover, they recover when pumping ceases. This rather elastic and stiff behaviour reflects the preconsolidated nature of the sediments in Barcelona and the fact that drawdowns concentrate at depth, where the soil is most compact, causing little change in effective stress near the soil surface. Under these conditions, pumping does not pose a serious risk to settlements, and excavation costs can be lowered by constructing the enclosure at the minimum depth required structurally. Also, an intense control of the pumping process may help reducing the conventional safety factors against heave.
机译：含水层中的深基坑可以通过将抽水与切入覆盖法相结合的方式来构造。为了降低基坑内部起伏或流态化的风险，并降低抽水速率，通常会在结构上加深围墙，以降低起伏或流化的风险，有时采用喷浆桩外的相关沉降物来加长。我们分析了喷射灌浆的防水效率以及对预固结沉积物中的水隔离的需求。在巴塞罗那圣家族大教堂附近的巴塞罗那高速火车隧道建设过程中，我们使用了两次竖井开挖获得的数据。喷射灌浆的特征是在建造外壳之前和之后进行泵送测试。通过比较几种脱水方案引起的沉降和所需的排水量，评估了加深围护的有效性。它们之间的差异在于外壳的深度。使用从水文地质模型获得的水位下降数据，并通过水力学模型进行数值分析，从而对沉降进行分析计算。结果表明，喷射灌浆降低了土壤的渗透性（从5.5降低至0.6 m / d，降低了90％）。但是，这种减少仅影响桩的面积，因此必要的抽水率和围护结构外部的沉降仅减少了40％。结果还表明，由于地下水抽取而产生的沉降相当平稳（即差异沉降很小），绝对值较低。而且，当泵停止时，它们会恢复。这种相当有弹性和僵硬的行为反映了巴塞罗那沉积物的预固结性质，并且沉降物集中在土壤最致密的深度处，在土壤表面附近的有效应力几乎没有变化。在这种情况下，抽水不会对定居点构成严重威胁，并且可以通过在结构上要求的最小深度建造围护结构来降低开挖成本。同样，对抽水过程的严格控制可能有助于减少传统的抗起伏安全系数。

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