Holdfast Training Services Trial Blog

I’m currently looking at the implications of over excavation on the basement (passive) side of a contiguous piled retaining wall.  Whilst onsite I became aware that the sub-contractor responsible for the basement reduced dig had moved beyond a hold point before another sub contractor had completed the temporary reduced level on the retained (active) side.  The reasons for this are numerous but boil down to poor communication and management of the required design construction sequence.  Figure 1 below is a sketch of the wall cross section during final construction and temporary cantilever states in accordance with the design checks conducted.

Figure 1 – Final construction and temporary construction stages

In order to understand the impact of this error I wanted to analyse the impact of the worst case (figure 2) where the full passive excavation had been completed and no excavation had occurred on the active side.   Note that fortunately work was stopped on site prior to this case occurring!

Figure 2 – Worst case if construction sequence followed incorrectly

The initial WALLUP retaining wall analysis (step 1) gave a minimum pile length for stability of 12m from pile platform level (PPL) to the toe at 39.1mOD.  Re-analysis for the worst case shown above caused an increase in the max BM, shear and displacement of the wall in SLS conditions and a failure in ULS conditions due to passive failure.  This would be a simple conclusion if the wall had been constructed to the 12m length from the original retaining wall design.  However, the contiguous wall is also required to support an axial load for the planned structure above.  The initial analysis of axial capacity (step 2) lead to a minimum pile depth for axial compression of 28.5mOD and a pile length of 20.6m from PPL.  This design was based on resistance provided in skin friction below formation level and a minimum rock socket of 1.5m.   Note however that the initial structural reinforcement design (step 3) only extends to the depth calculated for stability (in step 1) and was based on the maximum moments and shear from this calculation.  The image below summarises the initial pile design results.

Figure 3 – Initial design analysis results

So, drawing conclusions now on the effect of the over excavation shown in figure 2 becomes slightly more difficult.  My initial thoughts are:

1.  The passive failure issue is now removed due to the increase in pile depth.

2.  There is still an increase in BM and shear force that would mean the structural reinforcement would be under designed and could lead to an STR failure.

3.  The structural reinforcement should extend down to the new pile toe depth for stability.

4.  There would be an increase in deflection of the wall which could lead to other issues including eccentric loading in the final permanent state.

I’d welcome any comments or thoughts on the above logic or any conclusions I may have missed.