6.2.1 Plane strain elements
In terms of shape，plane strain elements are usually categorized into triangular elements and quadrilateral elements.As shown in Figure 6.4，the commonly used triangular elements are the constant strain triangular elements（CST elements），also called the T3 elements（Triangular with 3 nodes）where the strain variation is constant，and the linear strain elements（LST elements），also called the T6 elements（Triangular with 6 nodes）where the strain varies linearly.The commonly used quadrilateral elements are Q4 elements and Q8 elements.The former consist of 4 nodes for each and strains change linearly.They are thereby called low order elements.The latter have 8 nodes for each and the strain changes non⁃linearly.They thus belong to high order elements，as shown in Figure 6.5.
Figure 6.4 Triangular elements
Figure 6.5 Quadrilateral elements
In general excavation analysis，the plane strain elements are used to simulate soils and structural materials.High order plane strain elements，such as a Q8 element，are more recommended，considering the accuracy of analysis.Usually，the accuracy of a Q8 element is better than that of four Q4 elements，as shown in Figure 6.6.
Figure 6.6 Comparison of accuracy between a Q8 element and four Q4 elements