GOALI: Effects of Gas in Design and Verification of Blast Densification of Liquefiable Sands

基础设施系统包括许多高速公路和铁路的堤防。在落基山脉以西的地震敏感地区以及美国东部和中部的广大地区-估计覆盖了美国大陆的40%，这些设施的地震工程非常重要。这类设施的一个关键设计问题是，在地震发生时，饱和砂土是否会发生液化（或失去抗剪强度）。如果存在这种可能性，则必须重新安置路基或改善潜在的可液化土壤，使其在预期的地震下不会液化。松散的沙子是最容易液化的土壤。在设计过程中会出现两个问题：(i)在给定的地震荷载下会发生液化吗？ （ii）液化的后果是什么？最严重的影响是路基通过液化土壤的流动破坏。由于公路和铁路路堤横跨大片区域，如果在设计中遵循这一选择，减轻可液化土壤影响的成本和与重新调整相关的成本都是巨大的。为了大面积改善地基，控制爆破使松散砂致密化是一种经济的方法。爆破致密化包括在需要处理的松散砂层中放置炸药。炸药会多次延迟引爆以产生循环载荷，类似于地震。每次爆炸都会向地下释放大量气体。“设计”通常依赖于历史或以前的承包商经验，因为没有严格的理论来解释影响致密化过程的参数。案例研究表明，在爆破后，松散的砂粒几乎立即被压缩，但是当进行普通的穿透测试来验证密度的增加时，这些结果提供了不稳定的，有时甚至是与直觉相反的结果。如果在爆炸后不久采取措施，穿透阻力可能会降低，有时不会增加到高于爆炸前水平的水平。同时，地面在爆破后几乎立即沉降，这意味着地下松散砂的密度增加了。然而，穿透阻力缺乏增加表明土壤的强度和刚度显然没有增加。这就引出了关于未来表现的问题。松散的沙子真的被改善到不可能液化的程度了吗？此外，在设计和验证中忽略了释放的气体如何影响土壤的行为，例如，气体在地下以什么形式存在，它如何消散，以及它如何影响砂响应随后的静态和循环应力的行为。这项GOALI研究的目标是开发(i)一种方法，量化在剪切应力存在下使土壤抗液化和流动所需的致密量，并明确评估爆破过程中释放的气体的影响，以及（ii）一种可靠的现场验证地面改善的方法。为了实现第一个目标，将进行一个实验室实验程序来定义重建“气”砂试件的本构响应。实验室计划将考虑原位应力和气体浓度，并包括单调和循环试验。为了实现第二个目标，现场验证程序将监测土壤和孔隙流体在致密化之前、过程中和之后的响应，在南卡罗来纳州查尔斯顿附近的一个城市垃圾处理设施的生产爆破测试部分。爆破已经在现场使用了十多年，以致密化松散的沙子，Geosyntec顾问公司是这个项目的记录工程师。实地研究将包括孔隙压力测量、地表沉降、现场测试、孔隙流体取样、孔隙流体压力测量以及现场气体浓度和成分测量。结合现场和实验室研究来评估爆破过程中释放的气体的影响，为消除设计中的经验主义和验证试验中获得的相互矛盾的结果提供了独特的机会。除了应用于城市垃圾填埋之外，这项研究的结果将直接应用于基础设施系统，包括高速公路和铁路的许多堤防。此外，确定中等密度含气砂的行为将提供目前不存在的有价值的数据，这些数据可能对海上应用、尾矿坝和其他含气土壤存在的条件产生影响。

Infrastructure systems include many embankments for highways and railroads. In seismically sensitive areas west of the Rocky Mountains and over broad areas of the eastern and central US - estimated to cover as much as 40% of the continental US, earthquake engineering for these facilities is very important. A key design issue for such facilities is whether or not liquefaction - or the loss of shear strength of saturated sands - will occur during an earthquake. If such a possibility exists, then one must either relocate the embankment or improve the potentially liquefiable soil to the point where the improved soil will not liquefy under the expected earthquake. Loose sands are the soils most susceptible to liquefaction. Two questions arise during design: (i) will liquefaction occur under a given earthquake loading, and (ii) what are the consequences of liquefaction? The most egregious effect is a flow failure of the embankment through the liquefied soil. Because highway and railroad embankments traverse large areas, the costs of mitigating the effects of the liquefiable soils are large, as are costs related to realignment, if this option is followed in design. To improve the ground over large areas, densification of loose sands by controlled blasting is an economical approach. Blast densification consists of placing charges within the loose sand layer requiring treatment. The charges are detonated with multiple delays to generate cyclic loads, similar to an earthquake. Large amounts of gas also are released in the ground with each explosion. The "design" usually relies on historic or previous contractor experience, as there is no rigorous theory that accounts for the parameters that influence the densification process. Case studies have shown that loose sands compress almost immediately after blasting, but when common penetration tests are conducted to verify the increase in density, these results provide erratic and, at times, rather counterintuitive results. If taken soon after the blast, the penetration resistance may decrease, and at times never increases to levels above the pre-blast level. At the same time, the ground surface settles almost immediately after blasting, implying that loose sands in the subsurface have increased density. However, the lack of increase in penetration resistance suggests that the strength and stiffness of the soil apparently does not. This leads to questions about future performance. Have the loose sands really been improved to the point where liquefaction is not a possibility? Furthermore, how the released gas affects the soil behavior is ignored in design and verification, e.g., what form does the gas take in the ground, how does it dissipate and how does it affect the behavior of the sands in response to subsequent static and cyclic stresses. The objectives of this GOALI research are to develop (i) a methodology to quantify the amount of densification required to make the soil resistant to liquefaction and flow in the presence of shear stresses with explicit evaluation of the effects of gasses released during blasting, and (ii) a means of reliable in situ verification of the ground improvement. To achieve the first objective, a laboratory experimental program will be conducted to define the constitutive response of reconstituted "gassy" sand specimens. The laboratory program will account for in situ stresses and gas concentrations and include both monotonic and cyclic tests. To achieve the second objective, a field verification program will monitor the soil and pore fluid responses before, during, and after densification at a production blast test section at a municipal waste disposal facility near Charleston, SC. Blasting has been used at the site for more than a decade to densify loose sands, and Geosyntec Consultants is the engineer of record for this project. The field studies will include pore pressure measurements, surface settlements, in situ testing, pore fluid sampling, pore fluid pressure measurements, and in situ gas concentration and composition measurements. Combining the field and laboratory studies to evaluate the effects of the released gas during blasting provides a unique opportunity to remove the empiricism in design and the contradictory results obtained during verification testing. In addition to application to municipal waste fills, the results of this research will have direct application to infrastructure systems including many embankments for highways and railroads. Also defining the behavior of moderately dense gassy sands will provide valuable data that currently does not exist which can have an impact in offshore applications, tailing dams and other conditions where gassy soils exist.