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

新西兰南岛最近遭受了一系列强烈地震，在整个坎特伯雷地区造成了重大破坏。 第一次是2010年9月迪尔菲尔德地震，震级为7.1级，位于基督城以西40公里处。 震中附近发生了强烈的地面震动，测得的峰值地面加速度（PGA）高达1.3g。 基督城中部的震动是中等的（PGA大约是1.5米）。0.2g）。 旧的无钢筋砌体建筑遭受了严重的破坏，凯阿波伊和基督城的住宅开发也发生了土壤液化等地面破坏。 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，但它造成的破坏更大，因为它较浅，距离城市只有10公里。 震中附近的震动非常剧烈，PGA高达2.2克。 在基督城中心，震动非常强烈（PGA约。0.6- 0.8g），比9月7.1级地震高出约3倍。 破坏包括严重的土壤液化沿着几座多层建筑物和未加固的砖石结构倒塌。近200人丧生。 第三次6.3级地震发生在2011年6月，震中距离基督城13公里。这次地震进一步削弱了在以前的地震中受损的结构，并造成中度液化。坎特伯雷中心的大部分地区下面都是饱和的软淤泥和松散的沙子。 特别是基督城，其上覆盖着位于海滩沙丘砂后面的沼泽沉积物，以及已经排干的河口和泻湖。 这些松软的沉积物在整个地区的普遍存在意味着该地区非常容易发生液化和其他形式的地震引起的地面破坏。 为了减少预期的损害，近年来已经使用了各种工程方法来加强基督城多层建筑、大型市政设施和住宅开发下的土壤。 最常见的土壤改良方法是用石柱进行振动密实，这是一种采用大型振动探头的施工方法，该振动探头插入地面，并随着石头的加入而缓慢撤回，以形成与原生土壤混合的致密压实石头的圆柱形柱（即，“石柱”）。 石柱的设计是为了通过防止液化和其他形式的地面破坏，如基础土壤中的大的，无法忍受的沉降，来增强地震性能。 我们与当地工程师、研究人员和政府官员合作，收集了基督城10个遭受强烈震动的改良土壤场地的数据。 虽然一些治疗部位表现良好（即，相对于未改善的附近受损场地，几乎没有地面损坏），令人惊讶的是，有些没有表现良好，特别是在2月的地震中。 在许多新近建造市政设施和多层建筑物的处理地点发生了意外的地面塌陷和大面积沉降，导致设施的灾难性损坏和拆除。一些改进的网站表现良好而另一些网站表现不佳的原因尚不清楚。 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，目前的工程方法用于石柱的设计可能会导致高估其有效性。 我们最近对其他地震中改善的场地进行的实地和数值研究表明，这种地基处理在减少地震破坏方面的效果往往比目前的设计方法预测的要差得多。该奖项将资助前往新西兰收集每个站点的数据，并进行有助于解决这些设计问题的分析。 我们将与坎特伯雷大学的研究人员和当地的工程师合作。 主要的智力价值是，我们的研究可能会显示目前的设计方法是不保守的。 我们也有独特的机会来研究受到远远超过其设计水平的震动的网站。 更广泛的影响是，我们的研究结果可能会影响国际建筑实践。 我们的研究结果也将告知利益相关者和决策者在基督城社区，在努力重建可持续发展，正在试图评估哪些改善技术的工作和没有，以及什么方法应该指定为未来的项目。 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，从而提高未来地震中建筑设施和生命线的安全性和可靠性。

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.