Controlled Modulus Columns-your experience 1

[Robbiee]

Hello,
On one of our new projects, and driven by cutting cost, the ground improvement method of Controlled Modulus Columns is being offered as alternative to using deep foundations.
could you please provide your experience and comments on this method?
The contractor is guaranteeing a uniform soil bearing capacity that suits shallow foundations, strip and spread footings, and limit the differential settlement to 25mm.
The following is unknow to me which I will ask their engineer, but also need your input please:
1- Should the soil engineer of the project approve the suitability of the method?
2- Where boulders will be encountered, would they need to be removed or drill through?
3- How is the uniformity of the soil bearing capacity is achieved? Wouldn't the soil between the “columns” be softer than the column itself?
4- If it happened that one spread footing is sitting on one of the columns, would there not be a concentrated reaction from the column below the footing and hence eccentric reaction?

 

[EireChch]

1- it depends on the project. If they improvement contractor is doing a design and build approach then they provide a warranty. No other approval is needed. In general CMC columns will work in any soils. They may not be the most efficient but would generally be an option for all ground conditions. We are currently working as a third party reviewer for a CMC project, we are approving calculations etc, but ultimate responsibility is still with the contractor. (its a nice contract for us with very limited liability )

2-Likely not practical to remove boulders. What type of CMC are you doing? Bored/grouted or driven? You should have a minimum depth that you need to hit, if the boulder is in that depth then it needs to be drilled through.

3-Yes, it is. The ratio between CMC and soil can be huge i.e. 100++. A transfer layer of gravel is typically installed spread the applied stress.

4-In an ideal world you would design your layout such that a pad say 2x2m is supported with a column in the center, or between 4 columns. However, often the ground improvement is started before the foundation design/layout is complete. Again this is the reason for the transfer layer.