Seismic Ovalization Analysis of Tunnel Permanent Linings

RE: Seismic Ovalization Analysis of Tunnel Permanent Linings

Osman Murat SARSILMAZ — Tue, 11 Jul 2023 11:45:26 GMT

Thanks agian for all suggestions and comments

RE: Seismic Ovalization Analysis of Tunnel Permanent Linings

Berk Demir — Tue, 11 Jul 2023 10:52:52 GMT

I assume with Hoek Brown model, even with Rinter = 1.0, you should not have high tensile forces, because tensile strength of rock is very small as you stated. So, for both no-slip and full-slip case, you are calculating very low axial forces.

Don't confuse no-slip / full-slip with low tension capacity of the rock. If you use Rinter=1.0, that means no-slip in FEM. So, that will also ease your job.

I suggest you go with Hoek-Brown and Rinter=1.0 & Rinter=0.1, check to see which governs, and note that in your report. I think, if you are talking about standards of Road Directorate of Turkey, this should be under 6.4 - numerical methods. Because, if you do otherwise, you will end-up thinking and thinking. But if you crunch the numbers, you will see that there is around 60 times difference between axial forces. Compression is OK, but since you have to use this force also as tension due to sin(2*theta) thing, that means also tension.

The hand calculations are extremly overconservative - even though I used them to do some checks of my calculations before. It's not the Turkish Directorate that puts that requirement actually. It's Wang's recommendations. Whole standard is the translation of Wang's and Hashash's works. (See the second screenwhot below.)

Anyhow, we definitely exceeded the purpose of the forum. So, I suggest you keep going with the methodology I recommended and never use linear elastic soil material in these types of analyses. Only use when the strength does not matter - like purely settlement analysis.

RE: Seismic Ovalization Analysis of Tunnel Permanent Linings

Osman Murat SARSILMAZ — Mon, 10 Jul 2023 12:34:45 GMT

Thanks very much Berk for all suggestions.

About the tunnel - liner interaction, in order to get conservative results which suggested (actually forced) by spesification, I modeled 2 phases

To simulate no - slip case I assumed Rinter=1 (I took axial loads from this case)
To simulate full - slip case I assumed Rinter=0.1 (I took moment and shear force from this case).

About the MC model you are rigth I didn't give any tensile resistance, so I turned my model to HB model and the tensile resistance of rock mass calculated by plaxis is around 13 kPa

The results are similar like MC model, there is no axial tension on the lining (with only seismic loads).

Actually the main question here is: Can we trust this no tension behaviour. Because there is an analytical method which calculate +- same amount of axial load. And there is a spesification which leads us to use analytical methods.

This is a really interesting topic for me I think we should talk to the professors who wrote the spesification.

Thanks again for all comments

RE: Seismic Ovalization Analysis of Tunnel Permanent Linings

Berk Demir — Mon, 10 Jul 2023 06:34:03 GMT

Hi Osman,

I have been on this position before. I know what you are experiencing. The problem is caused by following:

You have two loads - static and seismic. When you are calculating the seismic load for your reinforcement calculation, you actually take both static and seismic into account. We know that seismic load creates tension due to ovalization. You can double check that by Wang or comparing the axial forces after the seismic forces in Plaxis. Seismcity creates tension.

The problem is, when you are on a soft soil, even though the ovalizations are high (and tension is also high), the resulting axial force does not go negative, because the axial force due to static forces are high. So, in the end you end up with compression, which is very favourable.

But in very hard rock, the static axial force is very low because rock is very good. Therefore, when you apply high seismicity, the ovalization creates tensile forces that exceed compression forces.

But, there is another problem with your model. Modelling rock as linear elastic material gives the material infinite tensile strength, which creates more ovalization forces. This is in-line with no-slip assumption, but in reality rock has a limited (though very high) tensile strength. However, as far as I see this is a NATM tunnel, so maybe you can consider lower Rinter values to relief the axial forces if you use Hoek Brown model.

With MC, you get no forces, because when it try to ovalize, the rock-tunnel interface just slips, because you probably didn't enter any tensile capacity to the model. So, it's completely different from what you have in linear elastic model.

This is a fun and niche part of tunnel engineering, enjoy!