Generating same initial stress for deep tunnel model, but without modelling the height

RE: Generating same initial stress for deep tunnel model, but without modelling the height

Nadja Gibis — Sat, 08 Oct 2022 13:42:27 GMT

Thank you Vasileios! This is the solution I needed.

RE: Generating same initial stress for deep tunnel model, but without modelling the height

Vasileios Basas — Thu, 06 Oct 2022 12:04:40 GMT

Hi Nadja,

There may be another way to successfully generate initial stresses for a deep structure in rock (e.g. a tunnel).

I understand that the main idea is to somehow substitute the huge overburden in your model and simulate it at a smaller scale in your project. So, in the case that the model is too large you can eventually omit the upper part, but you have to compensate for the missing soil weight otherwise non-realistic stresses will be generated. To include the overburden pressure, you can create a thin layer, which will represent the omitted overburden.

As an example, let’s consider that the model is at a depth of 130.0m (hreal) and the unit weight is γreal = 18.0 kN/m3.
We can introduce a virtual layer of 1.0m (hvirtual) on top of the existing model and assign a unit weight estimated from the overburden, which will be: γvirtual = γreal * hreal / hvirtual = 18.0 * 130.0 / 1.0 = 2340.0 kN/m3 . Check the sketch below for a general picture of the whole model, in an example:

In the generation of the initial stresses, in initial phase, you skip having a huge model simulating the overburden and you can substitute it with a layer as "heavy" as you calculate it.

RE: Generating same initial stress for deep tunnel model, but without modelling the height

Stefanos Papavasileiou — Thu, 06 Oct 2022 11:59:12 GMT

Dear Nadja,

Yes, you can model it using the Field Stress option in PLAXIS.

I recommend you check our Reference manual for details (e.g., V22, chapter 7.3.1 Initial stress generation).

We also have this old video but still shows a similar case to yours: https://communities.bentley.com/products/geotech-analysis/w/wiki/45895/response-of-a-rock-mass-to-circular-excavation-non-uniform-in-situ-stress