Dear reader in last part we have discussed about total settlement and maximum differential settlement. In our next post we will learn about secondary influence that can lead to foundation settlement. Here we will learn about settlement rate and maximum angular distortion.
c. Settlement rate
During construction work gradually loads are applied to foundation soil say 2-3 years or may be several years where skyscrapers are founded. Some structures need to go in operation within shortest expected duration say highway, dam etc. when socio-economical or political aspects are considered.
So we can leave some settlement during its service and essentially safe fraction of consolidation before structure is left to service. In this regard we have to know rate of settlement as most construction work have time constraint.
Again settlement consideration for building is not same for highway construction. Different methods are introduced by geotechnical engineers to change rate of settlement, some have discussed here discussed in this blog, and some are coming soon.
d. Maximum angular distortion:
This term is expressed as δ/L. For the expression we can easily realize that it is nothing but differential settlement (not maximum differential settlement) between two locations in foundation soil divided by distance between two locations but should be less than tilt.
Now what is tilt? MacDonald and Skempton (1986) defined this term as rotation of entire structure. The maximum angular distortion is the highest value of δ/L as shown in fig above, again the location of maximum angular distortion not necessarily to locate at maximum settlement or total settlement.
Now how these parameters are used in foundation design?
At first total/maximum or maximum differential settlement of structure proposed for in-situ soil conditions are determined. Thus allowable settlement rates are determined and finally they are compared to maximum settlement parameters.
If these parameters exceed allowable settlement values, the foundation type required or required mitigation measures to limit settlement are designed.