Sunday, 27 July 2014

ERSS - What on earth is that?

Many countries in Asia have romantic sounding nicknames, "the land of the rising sun", " the pearl of the orient" and I was attracted a while ago to work in the romantically sounding "Venice of the East" which after a few years dawned on me as perhaps being based on the simple recognition of the fact that cities are subject to widespread flooding and occasional subsidence.... Now for those living in Singapore, (and travelling . like me, down the PIE, KPE, MCE and AYE to get to work) eventually realise that we inhabit the "Island of the Acronym"  hopefully a tourist draw in its own right one day...

So to add one more to that evergrowing list, we have the "ERSS" or Earth Retaining and Stabilizing System. This is usually defined as "Any structure, structural system or other means used to maintain the shape of excavation during construction, earth filling or cutting" We are using such a structure to help us make a big wall in the car park adjacent to the tent plaza.

I suppose it could also be called PTPDS - That is to say the" Prevention of the Tent Plaza disappearing system" as it the civil engineering structure that is supporting the tent plaza whilst we dig out the basement car park and build a new retaining wall to support it. This is one of the more complex areas of construction in this project and this is a simple explanation of what we are doing..

This chap on youtube explains this "top down" construction method much better than me in this video but we have a few particular variations on our site that are not shown in  this video.

Step one for us is to drive in "Soldier Piles" these are simple steel I beams driven vertically into the ground in a line, (I suppose like soldiers standing to attention?) we then infill these piles with timber planks, called timber lagging...

Here you can see the soldier piles driven into the ground and being filled in with the timber lagging.
We then add another steel Ibeam horizontally between the soldier piles. This is called a waler and further reinforces this system and this prevents the solider piles from bending under the weight of the soil and other pressures behind them and coming out of line.

Soldier piles and waler complete - we can now move on to add more support  to the waler by bracing it back against the finished foundations. Whilst this is costly in terms of time as we have to wait for the foundations to catch up, it is by far the safest and most prudent way of construction in this school environment. 
Here you can clearly see the series of struts to add a supporting force to the waler by bracing against the foundations and ensure absolutely no chance of movement of the ERSS. Safety is paramount in this project.  Note the think layer of concrete on the slope of soil to be removed. This prevents erosion of the soil in the heavy rain of Singapore, as this soil erosion could cause pollution to the surrounding waterways.
A close up of the plate that we attach to the foundations which we then attach the structs to hold them firmly against the ERSS
Once these struts are in place, we can safely remove move soil and continue to add the timber planks down the soldier piles 

A nice top down view of a completed section of ERSS. It was completed in sections with progressive construction as each section was complete. In this photo you can see the area on the left being prepared for the foundations and the area on the right undergoing excavation.
The final strip of foundations being cast alongside the completed ERSS.  Once these foundations are complete, we can start to remove the struts. Overall a complex construction sequence completed safely and efficiently.  Phew...

Behind the finished retaining wall. We will now remove the steel (as much as we can!) and then backfill the gap with soil