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Why doesn't the Leaning Tower of Pisa fall over?

比萨斜塔为什么不倒？

        In 1990, the Italian government enlisted top engineers to stabilize Pisa's famous Leaning Tower. There'd been many attempts to right the tower during its 800 year history, but this team's computer models revealed the urgency of their situation. They projected the tower would topple if it reached an angle of 5.44 degrees, and it was currently leaning at 5.5. No one knew how the tower was still standing, but the crisis was clear: they had to solve a problem that stumped centuries of engineers, and they needed to do it fast.

        1990年，意大利政府招募了一批顶尖工程师来维护著名的比萨斜塔。在过去的八百年中有过许多次修正斜塔的尝试，但计算机模型向这只团队揭示了他们面临的严峻考验。他们预测这座塔将在倾斜角度超过5.44度的时候倒塌，现在它的倾斜角度是5.5。没有人知道为什么斜搭仍旧屹立，但危机就摆在眼前：他们需要解决这个困扰了无数代工程师的问题，并且是迅速解决。

        To understand their situation, it's helpful to understand why the tower tilted in the first place. In the 12th century, the wealthy maritime republic of Pisa set about turning its cathedral square into a magnificent landmark. Workers embellished and enlarged the existing church, and added a massive domed baptistry to the plaza. In 1173, construction began on a free-standing campanile, or bell tower.

        为了更好的理解他们的处境，让我们先来了解比萨塔倾斜的原因。在12世纪，富有的比萨海上共和国着手将主教座堂广场变成它宏伟的地标建筑。工人们扩大并装饰了原有的教堂，并在广场上加盖了带穹顶的洗礼堂。1173年，独立式钟楼的修建也开始了。

        The engineers and architects of the time were masters of their craft. But for all their engineering knowledge, they knew far less about the ground they stood on. Pisa's name comes from a Greek word for "marshy land," which perfectly describes the clay, mud, and wet sand below the city's surface. Ancient Romans counteracted similar conditions, with massive stone pillars called piles which rest on Earth's stable bedrock. However, the tower's architects believed a three-meter foundation would suffice for their relatively short structure. Unfortunately for them, less than five years later, the tower's southern side was already underground.

        参与建造的工程师与建筑师都是那个时代的大师，然而相对工程知识，他们对脚下的这片土地知之甚少。“比萨”一词起源于古希腊词汇“沼泽之地”，恰好完美描述了藏在这座城市地表之下的黏土、淤泥和湿砂岩。相似的条件下，古罗马人将大理石柱立在地表岩床上以增加建筑的稳定性。然而，塔的建筑设计师认为三米的地基足以支撑这个相对较矮的建筑。不幸的是，不到五年，塔的南侧已经陷入地下。

        Such a shifting foundation would normally have been a fatal flaw. If workers added more weight, the pressure from upper stories would sink the structure and fatally increase the lean. But construction halted at the fourth story for nearly a century, as Pisa descended into prolonged warfare. This long pause allowed the soil to settle, and when construction began again in 1272, the foundation was on slightly more stable footing.

        通常情况下地基的倾移将成为致命的缺陷。如果继续增加重量，来自上层的压力将使建筑下沉，从而导致更严重的倾斜。由于比萨陷入长久的战争时期，工程在第四层停滞了将近一个世纪。在这期间，土壤沉降，等到1272年工程再次开始时，地基的基脚变得相对稳定。

        Under the direction of architect Giovanni di Simone, workers compensated for the tower's minor tilt by making the next few floors taller on the southern side. But the weight of the extra masonry made that side sink even deeper. By the time they completed the seventh floor and bell chamber, the angle of the tilt was 1.6 degrees.

        在建筑师乔瓦尼·迪·西蒙尼的指导下，接下来几层的建造中钟楼南侧略高于北侧，以此来修正塔的微小倾斜。然而额外的砖石结构加速了塔楼南侧的下沉。等到第七层及钟室竣工时，塔已经倾斜了1.6度。

        For centuries, engineers tried numerous strategies to address the lean. In 1838, they dug a walkway around the base to examine the sunken foundation. But removing the supporting sand only worsened the tilt. In 1935, the Italian Corps of Engineers injected mortar to strengthen the base. However, the mortar wasn't evenly distributed throughout the foundation, resulting in another sudden drop.

        数个世纪以来，工程师们无数次尝试修正它。1838年，为了探究下沉的地基，他们围绕地基挖掘出一条走道，然而挖走支撑的沙土导致了更严重的倾斜。1935年，为了加固塔基，意大利工程军向基底灌注砂浆，然而砂浆灌注不均匀引发了又一次下沉。

        All these failed attempts, along with the ever-sinking foundation, moved the tower closer to its tipping point. And without definitive knowledge of the soil composition, engineers couldn't pinpoint the tower's fatal angle or devise a way to stop its fall.

        所有失败的尝试，伴随沉降的地基使得倒塌一触即发。由于缺乏对土壤构成的认知，人们无法计算出塔何时会倒也无法阻止它发生。

        In the years following WWII, researchers developed tests to identify those missing variables. And in the 1970s, engineers calculated the curved tower's center of gravity. With this data and new computing technology, engineers could model how stiff the soil was, the tower's trajectory, and the exact amount of excavation needed for the tower to remain standing.

        第二次世界大战结束后，学者们尝试确定这些缺失的变量。20世纪80年代，工程师们计算出塔的重心曲线，再加上新的计算机技术，土质硬度，塔的轨迹以及精确的挖掘量才得以确定。

        In 1992, the team drilled diagonal tunnels to remove 38 cubic meters of soil from under the tower's north end. Then, they temporarily counterbalanced the structure, with 600 tons of lead ingots before anchoring the base with steel cables. More than six centuries after its construction, the tower was finally straightened… to a tilt of about four degrees. No one wanted the tower to fall, but they also didn't want to lose the landmark's most famous feature.

        1992年，施工队挖出倾斜的隧道，从北端的塔基挖走38立方米的土壤。随后，他们用600吨铅锭来过渡性的平衡塔身，最后用钢索来固定地基。在竣工的六个世纪之后，塔的倾斜角终于被修正到4度左右。没有人希望它倒塌，但他们也不想失去这个地标建筑最出名的特征。

        Today the tower stands at 55– or 56– meters tall, and it should remain stable for at least 300 years as a monument to the beauty of imperfection.

        如今，塔高约55到56米，作为缺陷之美的里程碑，它将至少屹立300年。