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Consolidation behaviour of soft soil subjected to on-land and underwater vacuum preloading.

机译：陆上和水下真空预压下软土的固结特性。

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摘要

Vacuum preloading is a ground improvement technique in which effective stress of soil is increased by reducing pore water pressure. It can be further divided into on-land vacuum preloading and underwater vacuum preloading. To understand the consolidation behaviour of the soil subjected to on-land vacuum preloading and underwater vacuum preloading, theoretical analysis, field investigation, centrifuge modelling and numerical modelling were carried out.;Analytical solutions of average reduction of pore water pressure in soft clay subjected to vacuum preloading were developed. Well resistance, smear effect and vacuum loss are considered in combined radial and vertical consolidation. Variation of soil stiffness and permeability, multi-stage loading procedure are considered in an incremental form. Based on constant coefficient of consolidation, it is found that average degree of consolidation is slightly larger in the presence of vacuum loss than that without vacuum loss (one-way drainage). It is also found that radial consolidation is significant only if the drain spacing ratio is less than threshold n value, which is 7-10 for large diameter drain (e.g., D = 0.5 m) or 20-30 for small diameter drain (e.g., D = 0.1 m).;Observations obtained from field on-land vacuum preloading and centrifuge model tests on underwater vacuum preloading show that the pore water pressure reduces to a suction line with total vertical stress nearly unchanged. The principle of underwater vacuum preloading is similar to that of on-land vacuum preloading with little difference near the edge of treated zone. The increase in vertical effective stress is equal to reduction of pore water pressure for saturated soil, but the increase in horizontal effective stress varies with locations and generally less than that increase in vertical effective stress. The final coefficient of earth pressure in the treated zone ranged from K0 at the bottom of the treated soil layer to nearly unity near the ground surface at the edge of treated zone. The coefficient of earth pressure also varies across the treated area. Negligible lateral displacement was found at lower depths below ground surface or 2.5 H (H thickness of soft soil or depth of vertical drain) away from the edge of treated zone. The water pressure on the membrane plays a positive role on the increase of vertical effective stress. The efficiency (increment of reduction of pore water pressure over increment of water pressure on membrane) is about 0.8 in present centrifuge test with lateral seepage. In field observations, vacuum loss is negligible in a large treated area, whereas vacuum loss is significant in a small treated zone.;Penetration depth of a membrane influences the settlement slightly, whereas its influence is reduced in presence of vertical drains. However, penetration depth of a membrane significantly affects lateral displacement. The inward lateral displacement induced by vacuum preloading when the membrane penetrates through the full depth of soil is 1.25 times of that when penetration depth of a membrane into soil is negligible. The ratio of lateral displacement to vertical displacement is 0.55 when the penetration depth of membrane into soil is negligible, and is 0.70 when the membrane penetrates through the full depth of soil.;The larger the drain penetration depth, the larger the settlement is developed, unless the vertical drain penetrates beyond the optimum penetration depth. For a given penetration depth of sand drains, settlement decreases with increasing drain spacing from 8D to 1H. Given a penetration depth, the largest settlement is developed with drain spacing 8D when sand drains are used. It is found that the bottom upward seepage (vacuum loss) has a negligible influence on the consolidation when the drain spacing is less than 8D. The optimum penetration ratio is about 0.95 when drain spacing is less than 28D and the permeability of sand drain is larger than four orders of that of clay.
机译：真空预压是一种地面改良技术，其中通过降低孔隙水压力来增加土壤的有效应力。它可以进一步分为陆上真空预压和水下真空预压。为了解陆上真空预压和水下真空预压的土壤固结特性，进行了理论分析，现场调查，离心建模和数值模拟。开发了真空预压。在径向和垂直固结中考虑了井阻力，拖影效应和真空损失。土壤刚度和渗透率的变化，多阶段加载过程以增量形式考虑。基于恒定的固结系数，发现存在真空损失的情况下的平均固结度比没有真空损失（单向排水）的平均固结度稍大。还发现仅当排水管间距比小于阈值n值时，径向固结才有意义，对于大直径排水管（例如D = 0.5 m）为7-10或对于小直径排水管（例如，D = 0.5 m）为20-30 D = 0.1 m）.;从现场陆上真空预加载和水下真空预加载的离心模型试验获得的观测结果表明，孔隙水压力降低至吸力线，而总垂直应力几乎不变。水下真空预压的原理类似于陆上真空预压的原理，在被处理区域的边缘附近差异很小。垂直有效应力的增加等于饱和土壤孔隙水压力的减小，但是水平有效应力的增加随位置而变化，通常小于垂直有效应力的增加。处理区的最终土压力系数范围从处理土壤层底部的K0到处理区边缘地表附近的几乎统一。土压力系数在整个处理区域也不同。在距地表以下的较低深度或距处理区边缘2.5 H（软土的H厚度或垂直排水的深度）处发现较小的横向位移。膜上的水压对垂直有效应力的增加起积极作用。在当前的具有侧向渗流的离心试验中，效率（孔隙水压力的降低量相对于膜上水压力的增加量增量）约为0.8。在现场观察中，在较大的处理区域中，真空损失可以忽略不计，而在较小的处理区域中，真空损失是明显的。膜的渗透深度对沉降的影响很小，而在垂直排水沟的存在下其影响减小了。然而，膜的渗透深度显着影响横向位移。当膜穿透土壤的整个深度时，由真空预加载引起的向内横向位移是膜穿透土壤的深度可忽略时的1.25倍。当膜渗入土壤的深度可忽略时，横向位移与垂直位移之比为0.55，而当膜渗入土壤的整个深度时，横向位移与垂直位移之比为0.70。排水深度越大，沉降越大；除非垂直排水管的渗透深度超过最佳渗透深度。对于给定的排沙深度，随着排距从8D增加到1H，沉降会减少。给定穿透深度，当使用排沙孔时，最大排水沟间距为8D。已发现，当排水管间距小于8D时，底部向上的渗漏（真空损失）对固结的影响可以忽略不计。当排水管间距小于28D且排水管的渗透率大于粘土的四倍时，最佳渗透率约为0.95。

著录项

  • 作者

    HU, Liwen.;

  • 作者单位

    Hong Kong University of Science and Technology (Hong Kong).;

  • 授予单位 Hong Kong University of Science and Technology (Hong Kong).;
  • 学科 Engineering Civil.
  • 学位 Ph.D.
  • 年度 2010
  • 页码 519 p.
  • 总页数 519
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

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