The Problem:


1. In our Tunnel Project, I have installed a large diameter circular metal section underground vertically, for downward excavation retaining purposes. I found it too short but I can’t pull it out. As I want to replace it with a longer section to be driven vertically down, sufficient for earth retaining purposes.



Our Answer to you:


This is really unfortunate. Let me explain to you why:


The large diameter circular section (hereinafter called “the Section”) that you installed could have driven through a thick Fill layer not properly compacted, negative skin friction (acting in downwards direction) around the circumference of the Section thus built up i.e. they would increase rapidly as the Fill material in the Fill layer started to settle and the settlement of this Fill layer pulled the Section further down.


As the diameter of the Section is large, the Negative Skin Friction developed is great (as this Negative Skin Friction developed is proportional to the Fill / Section contact area), thus you would find it practically impossible to remove this Section even using your powerful machinery available on site…


The most economical way to resolve this could be as follows:

Instead of removing the Section, have it just remained there. Try to consult a Soil Improvement Specialist to have the relevant portion of the soil below the Section (earth not retained by the Section) treated so as to increase its “Free Standing Capacity”, before you proceed with your downward excavation works any further.


Caution: You would need to proceed with the relevant soil testing to ensure that the earth below are able to “stand erect” after the soil treatment, as it would be dangerous for workers in the shaft to continue any downward excavation, when the soil could just collapse on them and bury them alive.



After note:

In deep foundation design such as piles and caissons, this negative skin friction is added to the other vertical loads in the design of a deep foundation under fill.



Commented by:

Er. Goh Cheow Leng Maria



First drafted on 10 September 2008.

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