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

基础设施系统包括许多公路和铁路设施。 在落基山脉以西的地震敏感区以及美国东部和中部的广阔地区-估计覆盖美国大陆的40%，这些设施的地震工程非常重要。 此类设施的一个关键设计问题是，地震期间是否会发生液化（或饱和砂土的剪切强度损失）。 如果存在这种可能性，则必须重新安置路堤或改善潜在可液化土壤，使改良土壤在预期地震下不会液化。 松散的砂土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？最令人震惊的影响是路堤通过液化土壤的流动破坏。 由于公路和铁路跨越大面积，减轻可液化土壤影响的成本很大，如果在设计中遵循此选项，则与重新调整相关的成本也很大。 为了改善大面积地基，通过控制爆破密实松散砂是一种经济的方法。 爆炸密实化包括在需要处理的松散砂层内放置炸药。 炸药在多次延迟的情况下被引爆，以产生类似于地震的循环载荷。 每次爆炸都会在地下释放大量的气体。 “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，松散砂在爆破后几乎立即压缩，但当进行常规贯入试验以验证密度增加时，这些结果提供了不稳定的结果，有时甚至是违反直觉的结果。 如果在爆炸后不久服用，穿透阻力可能会降低，有时永远不会增加到爆炸前的水平以上。 与此同时，地表在爆破后几乎立即沉降，这意味着地下松散的沙子密度增加。 然而，贯入阻力没有增加表明土壤的强度和刚度显然没有增加。 这就引出了对未来业绩的质疑。 松散的沙子真的已经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行为，例如，气体在地下以何种形式存在，如何消散，以及如何影响砂土对随后的静态和循环应力的反应。 本GOALI研究的目标是开发（i）一种方法，以量化所需的压实量，使土壤抗液化和流动的剪切应力的存在，明确评估的影响，在爆破过程中释放的气体，和（ii）可靠的地面改良的现场验证的手段。 为了实现第一个目标，将进行一个实验室实验计划，以确定本构响应的再造“含气”砂标本。 实验室计划将考虑原地应力和气体浓度，并包括单调和循环测试。 为了实现第二个目标，现场验证计划将在南卡罗来纳州查尔斯顿附近的市政废物处理设施的生产爆破试验段的致密化之前、期间和之后监测土壤和孔隙流体响应。 十多年来，现场一直使用爆破来密实松散的沙子，而Geosyntec顾问公司是该项目的记录工程师。 现场研究将包括孔隙压力测量、地面沉降、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，提供了一个独特的机会来消除设计中的不确定性和验证测试过程中获得的矛盾结果。 除了应用于城市垃圾填埋场，这项研究的结果将直接应用于基础设施系统，包括许多高速公路和铁路。 此外，确定中等密度含气砂的特性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、尾矿坝和存在含气土壤的其他条件产生影响。

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.