Modelling soil/rock support as spring support
Modelling soil/rock support as spring support
(OP)
Hi
I often have to model a basement or other ground bearing structure where I have to assign a spring stiffness to model the ground support realistically. If the ground is say a type of sand or clay with say a safe bearing capacity based on likely settlement, I usually calculate the spring stiffness (which in all the software programmes I use is expressed as kN/m, ie deflection for given force) as follows: If SBC is say 150kPa or kN/m2, I can usually find out from the soils report what settlement would occur for this loading. Say 20mm. Therefore my spring stiffness would be 150/0.02 = 7500kN/m. In other words if I applied 7500kN force on a square meter of this material it would settle/deflect by a meter. I then place a spring every square meter. If I wanted to model supports at every 500mm, I would have to reduce the spring stiffness by a factor of 4 as I would have 4 supports per square meter.
What I would like to know is how to calculate the spring stiffness (in kN/m) for rock where I do not know what deflection would occur under load. I was thinking that maybe the elasticity modulus of the material could be used, but cannot figure out how I would do this.
What have you guys done when modelling a rock support? And is there a way that E, the elasticity modulus, can be used to get a spring stiffness? I was hoping to find some information on the net giving spring stiffnesses for various soils and rock but have been unsuccesful.
Looking forward to your replies and opinions
Thanks
Seb
I often have to model a basement or other ground bearing structure where I have to assign a spring stiffness to model the ground support realistically. If the ground is say a type of sand or clay with say a safe bearing capacity based on likely settlement, I usually calculate the spring stiffness (which in all the software programmes I use is expressed as kN/m, ie deflection for given force) as follows: If SBC is say 150kPa or kN/m2, I can usually find out from the soils report what settlement would occur for this loading. Say 20mm. Therefore my spring stiffness would be 150/0.02 = 7500kN/m. In other words if I applied 7500kN force on a square meter of this material it would settle/deflect by a meter. I then place a spring every square meter. If I wanted to model supports at every 500mm, I would have to reduce the spring stiffness by a factor of 4 as I would have 4 supports per square meter.
What I would like to know is how to calculate the spring stiffness (in kN/m) for rock where I do not know what deflection would occur under load. I was thinking that maybe the elasticity modulus of the material could be used, but cannot figure out how I would do this.
What have you guys done when modelling a rock support? And is there a way that E, the elasticity modulus, can be used to get a spring stiffness? I was hoping to find some information on the net giving spring stiffnesses for various soils and rock but have been unsuccesful.
Looking forward to your replies and opinions
Thanks
Seb






RE: Modelling soil/rock support as spring support
What do you hope to achieve by modeling the foundation as an elastic spring? There is a method to determine the spring stiffness by using the subgrade modulus. To get the geotechnical engineer to provide you with this information you will need to request it early in the design stage. Good textbooks such as Bowles with give you estimates for rock.
Personally for rock I would model it as a fixed support.
RE: Modelling soil/rock support as spring support