Technical Note No.6 Trench Lining Systems - Design ... - Groundforce

Note that the allowable pressures at the maximum stated depths comfortably exceed the TBG
pressure requirements.
As a further guide for the selection of the best system, the tractive forces required to extract
boxes can be considerable. The following table can be used as a guide for excavator sizing.
Trench Box Extraction Forces
Where the end wall of a trench requires support it is essential that no
lateral load is applied to the box struts. Purpose made end closure panels
are available for closing off the ends of boxes if required. Any contractor
designed end support members must thrust off the ends of the box panels.
The structural adequacy and integrity of non-standard end support
members is the responsibility of the contractor.
Note: Groundforce does not recommend the practice of “flying” trench boxes above the base
of the excavation as this can potentially overload the lower struts.
Drag Boxes
Drag boxes as their name suggests are intended to be pulled or
dragged through the excavation as the work proceeds. They are
therefore designed to be used in pre-dominantly stable ground
conditions in battered excavations.
They therefore act as shields to protect the workforce rather that
provide positive ground support as in the case of a trench box.
Drag box plates do have a working load capacity commensurate
with their height and would evidently provide support in the case of ground movement.
However it is not industry policy to provide site-specific designs with this equipment
type. Any assessment for the use of drag boxes should be carried out at site level where first
hand knowledge of ground conditions and hence short-term ground stability is available.
Rev E – 06/04/10