Preliminary Study of the Seismic Performance of Improved Ground Sites during the 2010-11 New Zealand Earthquakes

新西兰南岛最近发生了一系列强烈地震，对整个坎特伯雷地区造成了重大破坏。第一次地震是2010年9月的迪尔菲尔德地震，震级为7.1级，位于克赖斯特彻奇以西40公里处。震中附近发生了强烈的地面震动，测得峰值地面加速度(PGA)高达1.3g。克赖斯特彻奇市中心的摇晃程度中等(PGA约。0.2g)。未加固的老旧砖石建筑遭受了严重破坏，凯阿佩和克赖斯特彻奇的住宅区也遭受了严重破坏，那里发生了土壤液化等地面故障。2011年2月，基督城附近发生了第二次规模较小的6.3级地震。虽然这次地震规模较小，但由于震源较浅，距离市区仅10公里，造成的破坏较大。震中附近的震动非常严重，PGAS高达2.2克。在克赖斯特彻奇市中心，震动非常强烈(PGA约。0.6-0.8克)，大约是9月7.1级地震的三倍。破坏包括严重的土壤液化，以及几座多层建筑和未加固的砖石结构的倒塌。近200人遇难。第三次6.3级地震发生在2011年6月，震中距离克赖斯特彻奇13公里。这次地震进一步削弱了在之前的地震中受损的结构，并造成了适度的液化。坎特伯雷中部的大部分地区被饱和的软粉土和松散的沙子覆盖。克赖斯特彻奇尤其覆盖着位于海滩沙丘沙子后面的沼泽沉积物，以及已经被排干的河口和泻湖。这些软弱沉积物在整个地区的普遍存在意味着该地区极易受到液化和其他形式的地震引起的地面破坏的影响。为了减少预期的破坏，近年来已经使用了各种工程方法来加固克赖斯特彻奇多层建筑、大型市政设施和住宅开发项目下的土壤。最常见的土壤改良方法是石桩振动压实，这是一种施工方法，它利用插入地面的大型振动探头，随着石块的加入而缓慢取出，形成由致密、压实的石料与原生土壤混合而成的圆柱形柱体(即“石柱”)。碎石桩的设计目的是通过防止液化和其他形式的地面破坏，如地基土壤中巨大的、无法容忍的沉降，来增强地震性能。与当地工程师、研究人员和政府官员合作，我们收集了克赖斯特彻奇遭受强烈震动的10个改良土壤场地的数据。虽然一些经过处理的场地表现良好(即，与附近受损的未改善场地相比，地面损坏很少或根本没有发生)，但令人惊讶的是，有些场地表现不佳，特别是在2月份的地震期间。在最近修建市政设施和多层建筑的许多处理场地上，发生了意想不到的地面坍塌和大范围沉降，导致设施遭到灾难性破坏和拆除。一些改进后的网站表现良好，而另一些则表现不佳，原因尚不清楚。然而，已经提出了几个假设。首先，一些网站受到的震动超过了它们的设计水平。我们还怀疑，目前用于设计石柱的工程方法可能会导致对其有效性的高估。我们最近对其他地震加固场地的现场和数值研究表明，这种地基处理在减少地震破坏方面的效果往往不如目前的设计方法所预测的那样有效。这项奖励将资助前往新西兰收集每个网站的数据并进行分析，以帮助解决这些设计问题。我们将与坎特伯雷大学的研究人员和当地工程师合作。主要的智力优势是，我们的研究可能会表明目前的设计方法是不保守的。我们也有独一无二的机会研究遭受远高于设计水平的震动的场地。更广泛的影响是，我们的发现可能会影响国际建筑实践。我们的结果还将告知克赖斯特彻奇社区的利益相关者和决策者，他们为了可持续重建，正试图评估哪些改进技术有效，哪些无效，以及应该为未来的项目指定哪些方法。最后，这项研究将使我们能够更好地了解地震减灾实践的成本效益权衡，从而提高未来地震期间已建设施和生命线的安全性和可靠性。该奖项由国际科学与工程办公室共同资助。

The south Island of New Zealand has been recently subjected to a series of strong earthquakes that caused significant damage throughout the Canterbury region. The first event, the September 2010 Deerfield Earthquake, was magnitude 7.1 and located 40 km west of Christchurch. Strong ground shaking occurred near the epicenter where peak ground accelerations (PGA) up to 1.3g were measured. Shaking in central Christchurch was moderate (PGA approx. 0.2g). Older unreinforced masonry buildings suffered heavy damages, as did residential developments where ground failures, such as soil liquefaction, occurred in Kaiapoi and Christchurch. A second smaller earthquake of magnitude 6.3 struck near Christchurch in February 2011. Although this earthquake was smaller, it produced more damage because it was shallower and located only 10 km from the city. Shaking was extreme near the epicenter, with PGAs up to 2.2g. In central Christchurch, the shaking was very strong (PGA approx. 0.6 - 0.8g), about three times higher than that from the magnitude 7.1 September earthquake. Damage included significant soil liquefaction along with the collapse of several multi-story buildings and unreinforced masonry structures. Nearly 200 people were killed. A third earthquake of magnitude 6.3 struck in June 2011 and was centered 13 km from Christchurch. This shock further weakened structures damaged in previous events and caused moderate liquefaction. Much of central Canterbury is underlain by saturated soft silts and loose sands. Christchurch in particular overlies swamp deposits located behind beach dune sands, and estuaries and lagoons that have been drained. The prevalence of these soft and weak deposits throughout the region means the area is highly susceptible to liquefaction and other forms of earthquake-induced ground failure. To reduce anticipated damages, various engineering methods have been used in recent years to strengthen the soils beneath multi-story buildings, large-scale municipal facilities, and residential housing developments in Christchurch. The most common soil improvement method has been vibrodensification with stone columns, a construction method that employs large vibrating probes that are inserted into the ground and slowly withdrawn as stone is added to form a cylindrical column of dense, compacted stone mixed with the native soil (i.e., a "stone column"). Stone columns are designed to bolster earthquake performance by preventing liquefaction and other forms of ground damage such as large, intolerable settlements in the foundation soils. Working with local engineers, researchers, and public officials, we collected data for 10 improved soil sites that were subjected to strong shaking in Christchurch. While some treated sites performed well (i.e., little to no ground damage occurred relative to unimproved nearby sites with damage), it was surprising that some did not perform well, especially during the February earthquake. Unexpected ground failure and large settlements occurred at numerous treated sites where municipal facilities and multi-story buildings were recently built, leading to catastrophic damages and demolition of the facilities. The reason some improved sites performed well and others did not is unclear. However, several hypotheses have been proposed. First, some sites were shaken harder than their design levels. We also suspect that current engineering approaches used for the design of stone columns may lead to an overestimation of their effectiveness. Our recent field and numerical studies of improved sites from other earthquakes suggest such ground treatment is often much less effective in reducing earthquake damages than current design methods predict. This award will fund travel to New Zealand to collect data for each site and performing analyses that can help to resolve these design issues. We will collaborate with Canterbury University researchers and local engineers. The main intellectual merit is that our study may show current design methods to be unconservative. We also have the unique opportunity to study sites subjected to shaking far above their design levels. The broader impact is that our findings could impact international building practices. Our results would also inform stakeholders and decision makers in the Christchurch community who, in an effort to rebuild sustainably, are trying to assess what improvement technologies worked and what did not, and what methods should specified for future projects. Finally, this research will allow us to develop a better understanding of the cost-benefit tradeoff for earthquake mitigation practices, thereby increasing the safety and reliability of constructed facilities and lifelines during future earthquakes.This award is co-funded by the Office of International Science and Engineering.