I am aiming to simulate a deep excavation near an existing tower with 4 levels of below grade parking (shaded in grey). The approximate loading of the building is 10 ksf. Therefore I have simulated the material (shaded in grey) as a non-porous Mohr Coloumb material with stiffness and strength parameters equivalent to concrete. The density is0.187 kips/ft3 in order to achieve the 10 ksf vertical pressure at the base of the building. I have also added a negative interface on the outside of the building with Rinter of 0.6. The soil layers are simulated with the Hardening Soil model. I was wondering if this approach makes sense and if not, what is the best way to simulate a nearby burried structure.
That doesn't look bad for me. I usually model the basement walls with plate element (and add horizontal beams on possible slab locations) and add the total building load as a distrubuted load at the bottom. Of course plate elements have zero unit weight. With this, I can take the small deformation of the building into account too while in your case it is very rigid. But, of course, all of these are rough approximations to buildings that we have not designed. Rinter is all other subject and nobody can give you the correct answer. Just one note, I would make sure that building does not have any piles etc. to support that excavation since it is not very shallow. If they have piles, you might have a little trouble with the anchors. Lastly, click on embedded beams of anchors' bond length and select the behaviour as grout body, as far as I can see it is not correct right now - you can make sure that by checking if there is any small green rectangles at the beginning or end of the embedded beam.
P.S.: The contractors usually do not like the first anchor that you have there crossing through other anchors. I can imagine that in 3D, they do not intersect, but considering tolerances and other possible construction problems, if they catch the strands during drilling, you might have problems since anchor will lose its force.
Thank you Berk! I will give it a try with the plate elements and compare the two results. Regarding the grouted body, the color is misleading because I chose a green color for my embedded beam rows. I changed it to light blue now and there are no green rectangles. I simulated the grouted body as massive circular beam with a diameter of 6".
Answer Verified By: Boris Kolev
Hi Berk,
I have a follow up question regarding using plate elements with 0 weight to model the building. I am seeing some unexpected deformations that don't correspond to what we've observed in the past with similar deep excavations - we usually see higher deformations at the top of the wall and lower towards the bottom. I have a suspicion that the interaction between the soils in the wedge and the building foundation walls is not modeled correctly. When you use this approach (stiff plate elements) to model parkade levels, how do you model the interaction between the plates and the soils? Thank you in advance for your help.
Hi Boris, your model seems correct. It is your expectations that you should reconsider. "Higher deformations at top" is never a case for laterally supported excavations - if you see higher deformations at top for laterally supported excavations, you should reconsider your design. You can only see that on cantilever excavations.
Also, now you have much less load at the back of your retaining wall. You do not have any lateral load from building (was that the case when you model with linear elastic soil), so, it is very easy to carry all the load at upper levels which even shows that you don't need all those soil nails.
Thanks Berk. How do you typically initialize your stresses when it comes to the building loading? I Tried both the K0 procedure and the Gravity loading. Gravity loading seems to be working better, however I still see quite a lot of lateral movement in the model after I start the excavation. I understand that I am unloading quite a bit of load on the right side so that is likely the cause - see plots from the third excavation stage. I have set up the model with the following steps currently:
1) Phase 1 - Gravity loading step - activating the building surcharge and the lane surcharge.
2) Phase 2 - Set displacements to 0. Install Soldier pile plate element for the excavation
3) Phase 3 and onwards - Excavation stages.
Boris, everything seems OK. This is becoming consulting rather than software help, but as I said, it looks OK.