Waldos notes...

There is a whole set of trench wall modules. 
The trenches include both various L, T and X patterns and also include some sections that descend for changing terrain.

)))))))))))))))))))))

INsanes Note: there was a problem with three models in this set, they had missing materials...

models/props_fortifications/trench_high_zigstraight_512.mdl
models/props_fortifications/trench_high_zigy_512.mdl
models/props_fortifications/trench_high_zigzag_512.mdl

I have found them and they are different in the .MDL file, now added the missing textures in this download...
dod\materials\models\trench

All other trench walls use...
dod\materials\models\props_fortifications

Please credit Waldo if you use.

Cheers

INsane