Settlement of Large Mats / Rafts

[EireChch]

Hi all,

Settlement related query again...

Interested to hear peoples thoughts on settlement calculations for large rafts/mats/slabs on granular soil. The company where i work now considers a foundation a raft where b x l is greater than 5 x 5m. Elastic theory is then applied using the constrained modulus (which is roughly 1.3 - 1.5 times Youngs Modulus (E')). The idea is that the constrained modulus is more appropriate as the soil is constrained due to the size of the foundation and applied load. I dont really have any issues with this approach however they only use elastic theory and ignore other methods such as Schmertmann, Burland and Burbridge, Christian and Carrier etc.

This is where I do not fully agree, I think these methods are still valid. For example Burland and Burbridges method is base on assessment of 200 case studies. I understand that a lot of these slabs were greater than 5 x 5m. Hence i argued that that is proof that this method is valid.

 

[humanengr]

I'm not a geotech. expert, but you should be able to calculate the soil subgrade modulus based on the size of the mat, then calculate
the settlement accordingly.

 

[Okiryu]

Perhaps you can calculate settlements using different methods and show the results to your company. I think that all methods may be based on the theory of elasticity. Personally, I would use the elastic theory, Schmertmann and Bowles. It will depend also on the size of the mat and the thickness of the sand layer.

 

[Silty]

estimation of settlement can be tricky and most of the times measured settlement can be different from the estimated value. I would suggest calculating with different methods and compare them. I personally used elastic theory using E in many projects with no issues.
I hope that will help.