Temporary lateral earth pressures during construction

[kauri]

I am designing a basement that extends about 5m beneath an existing house, 4.5m deep. I am planning the design around constructing soldier piles along the perimeter of the basement inside the house prior to excavation, with a top-down shotcrete methodology.

When designing a propped or tied-back retaining wall, there will be a period of time during which the supports or anchors will not have been installed. In this case I would follow the usual procedure to check the retaining wall as a pure cantilever. With anchors, I understand that they are installed during excavation so the wall does not retain the full height. In my case, I have a rib & infill type ceiling tied in to the top of the retaining wall, which will be one of the last elements constructed.

I am struggling to find guidance on how to address this temporary case. One of my references mentions this failure mode (see page 61, case (f) here). It seems rather conservative to design the temporary and permanent cases with essentially identical parameters. Else I might as well ignore the ceiling diaphragm altogether. The only factor I can think to change is my earthquake return period, though this has no impact on the gravity case. I might consider reducing the gravity Factor of Safety, but I have seen no such guidance indicating I should do so.

Would appreciate your thoughts.
Oh, and of course I will I have another meeting with my geotech engineer next week.

 

[SlideRuleEra]

I might consider reducing the gravity Factor of Safety, but I have seen no such guidance indicating I should do so.

See Table 4-1, "Retaining Wall Stability Criteria" on page 4-5 of US Army Corps of Engineers Retaining and Flood Wall Engineering Manual" for guidance on reduced factor of safety for temporary conditions (Case R2).

 

[kauri]

I am not convinced Case R2 is applicable though. In line with my understanding, Case R1 "includes the usual loads possible during construction which are not considered short-duration loads." I would not consider the cantilevered case to be "short-term".

I am considering installing temporary diagonal braces between perpendicular walls.