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在软弱地基土层上设计建筑物时，除了考虑基础下土层的承载力必须达到安全要求外，地基土的长期沉降变形也同样重要。本文在追踪一工程实例中，发现依据传统土力学观念在深厚沉积土层中，标准贯入击数Ｎ值已达１５～２５之地层，设计４５ｍ长摩擦桩基础之谷仓结构物，在使用１９年后仍产生因差异沉降过量导致之严重结构破坏。传统土力学较偏于可靠的承载力，对于各土层的先期固结应力，仅能由单向度固结试验求得，但实际工作时，取得完全不扰动土样几乎不可能，因而亦很难取得正确的先期固结应力，便无法计算正确沉降量。故设计者常因本身主观的观念，造成研判的疏忽。本文提出“由Ｎ值判断各土层超固结比（ＯＣＲ）法及应力还原法”观念，由不同深度土层的标准贯入击数Ｎ值及地质历史，可协助设计者于设计阶段便能轻易而明确地判断不同深度土层的固结比（ＯＣＲ），精确计算各土层沉降量，以便能事先设计出防止类似本实例于１９年后才发现的疏忽。
In the design of buildings on soft foundation, it is also important to consider the long-term settlement of the foundation soils, in addition to considering the safety of the bearing capacity of the ground under the foundation. This paper traces a project example and found that according to the traditional concept of soil mechanics in the deep sedimentary soil, the standard penetration number N value has reached 15 ~ 25 stratigraphy, the design of 45m long friction pile foundation structure of the barn, the use of After 19 years, there was still serious structural damage caused by excessive differential settlement. The traditional soil mechanics is more reliable than the bearing capacity. For the pre-consolidation stress of each soil layer, it can only be obtained by one-way consolidation test. However, it is almost impossible to obtain completely undisturbed soil samples in practical work. It is difficult to obtain the correct pre-consolidation stress and the correct settlement can not be calculated. Therefore, designers often because of their own subjective ideas, resulting in judgmental negligence. In this paper, we put forward the concept of “OCR method and stress reduction method for determining soil layers based on N value”. Based on the standard penetration number N and geological history of different depths of soil layers, we can assist designers in designing phase The consolidation ratio (OCR) at different depths can be easily and clearly determined, and the settlement of each soil layer can be precisely calculated so as to prevent the negligence discovered in 19 years from this example.