Trench & Manhole Box System Design Methodology
It is common practice to specify the structural capacity of a trench box or manhole box system in terms of the Uniform Panel Pressure (measured in kN/m²) that a box panel can safely sustain.
A simple and conservative verification can be performed by comparing this value to the worst case calculated soil pressure. Although, this method often gives very conservative results.
A more detailed analysis of boxes used in various typical arrangements can be found in Groundforce’s Shoring Manual for Shallow Excavations which can be accessed online via the Groundforce Technical Library.
Calculating Uniform Panel Pressure
The most basic check is to assume a block of uniform pressure behind the whole box panel. But this can actually lead to unsafe results. We generally check a box against three load patterns.
In accordance with the latest European standards and up to date research, Groundforce ensure safety whilst maintaining economy with a rigorous assessment of each critical component. There are many variations of boxes in the Groundforce fleet, but the main structural elements remain the same. These are: Box Plates, Struts and Lifting Lugs.
Box Plates
These resist the earth pressure and are verified against the following:
Lifting Lugs
These resist the weight of the box during lifting and the extraction (frictional/suction) forces during removal. These are verified against elongation and tearing of the holes and against shear of the welded connection.
Struts
These resist the compressive and tensile forces generated by the box plates and are verified against buckling of main section and bending, shear and bearing of the pinned connection.
Closure Panels
Lateral loads on box struts should be avoided wherever possible because the struts are designed to take axial loading. Any lateral load applied (e.g. by sheeting the end of a box and using the struts to support the sheets) will reduce the overall strength of the box significantly and should be referred to the Groundforce technical department.
End closure panels can be provided that rest upon the ends of the box panels and do not adversely affect the struts.
