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在材料不可压缩或胀/缩塑性流动情况下，传统低阶单元有体积闭锁问题。以摩尔－库仑模型为例，推导了塑性剪切应变和塑性体积应变的关系，揭示闭锁产生的原因。分析8节点等参元、Wilson非协调元、EAS单元和14节点单元的闭锁性态，表明8节点单元有严重闭锁性，Wilson非协调元也有闭锁性，EAS单元和采用降阶积分的14节点单元能克服闭锁。单元测试和方形基础的承载力计算两个数值算例证实了分析的结果，为土体三维分析中选择有效可靠的单元提供依据。
In the case of materials that are incompressible or bulging / shrinking, the traditional low-order elements have a volumetric locking problem. Taking the Mohr - Coulomb model as an example, the relationship between plastic shear strain and plastic volume strain is deduced, and the reason of blockage is revealed. The closed states of 8-node isoparameters, Wilson non-coordinating elements, EAS elements and 14-node elements are analyzed. It shows that 8-node elements have serious blocking and Wilson non-coordinating elements are also blocking. EAS elements and 14 nodes using reduced- Unit can overcome the lock. The two numerical examples of unit test and square foundation bearing capacity calculation confirm the result of the analysis and provide the basis for selecting effective and reliable unit in soil three-dimensional analysis.