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

Question

Hello, 
 I want to reduce the height of my model by using a line load (right) instead of modeling the hole height above the tunnel (right) like shown in the following picture. 
 - ground water isn't considered in this model, so no pore pressure 
 - K0-values differ for the layers (green: 0.2, blue: 0.3, yellow: 1.0)

If the line load is activated in the Initial Phase, than it doesn't work with the K0-procedure, because only the body weight will be used for stress calculation and the line load will be ignored, right? 
 I tried two different ways: 
 - adding one more phase for only applying the line load before starting with tunnel excavation 
 - Gravity loading in Initial Phase 
 The result for initial vertical stresses &sigma;' yy are the same as for the original model, but the results for the initial horizontal stresses &sigma;' xx are always different than the original model.

Question: Is there any possibility at all to compute the same initial stresses (vertical AND horizontal), but for a reduced model height? 
 
 Best Regards, Nadja

Vasileios Basas · Accepted Answer

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&rsquo;s consider that the model is at a depth of 130.0m (hreal) and the unit weight is &gamma;real = 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: &gamma;virtual = &gamma;real * 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.

Stefanos Papavasileiou · Answer

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

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

Top Replies