Things that you should consider are the stage at which you install the shotcrete layer, it's properties and the configuration of the support (do you cover the complete cross-section or part of it). First you should make sure that everything is as expected in an analysis with out the seismic force and as soon as you are sure that your reference case (i.e. analysis without seismic forces) is right then you should apply the seismic force. Without more details about your modelling it is hard tell what causes that response.

I agree with Ioannis on this. Please review all input parameters and boundary conditions... 

Application of shotcrete 'per se' shouldn't have any major influence on the safety factor value. However, in the case of zero cohesion material (with shallow sliding surface) the FoS value of shotcreted cut can differ from that of the non-protected cut. 

One could then rise a question whether is it possible to apply shotcrete on zero cohesion slope?

Thanks Ioannis and Zoran...

The rock slope is of 13m high with almost 45 deg gradient at the slope face with roller support on the lateral sides and hinge support at the bottom boundary.

The shotcreting is applied to the bottom part of the slope for a length of approx. 10 m. The application of shotcrete is in the form of a reinforced concrete beam of compressive strength 30 MPa (E =27 GPa)  and with a wire mesh of 6 mm dia. The shotcreting is applied in addition to end anchored bolts of tensile capacity 125 kN.

I had been facing the same issue for a while for different slopes. Please do explain if there are any other important parameters to be taken care of while deciding on the input parameters for the supports.

Mithresh,

I believe that you have encountered this problem because your horizontal seismic force is not oriented well, as to produce negative effects on slope stability.

What is orientation of your slope left to right or right to left?

By definition applied quazi-static load should reduce safety factor value.

Please, change the orientation of your horizontal force by 180 deg and perform analyisis again.

Let as know about the results...

yea...the horizontal seismic force is applied against the orientation of the slope i.e; slope facing towards right and seismic force towards left. Seismic force when  applied in the direction of the slope has been observed to reduce the FoS value. But I wanted to know what causes this response and most importantly how can the effect of shotcrete be shown to improve the stability of the slope without the use of the constant seismic force.

Now try to remove the liner (shotcrete) and leave seismic force oriented as it is.

You should obtain the same results as in the case with shotcrete.

As I have mentioned in my previous posts shotcrete in general should not produce higher FoS value. In other words, global stability of the slope should be the same in the case with and with out shotcrete. One benefit from the shotcrete could be in the case when you encounter material that is prone to slaking. Shotcrete protects face of the slope from slaking, which in turn produces shallow failures. This effect can be simulated as follows:

1. In the case of non-shotcreted slope introduce artificial layer several meters thick (to simulate slaking) at face of the excavation. Reduce strength and deformation parameters accordingly. Your critical sliding surface will now be limited to this weak/slaked zone.

2. In the case shotcrete is applied immediately after excavation you can avoid above-mentioned procedure.

Hope this helps.

Thank you very much...The concept is more clear now...So what I have understood is that... shotcrete only protects the face of the excavation which otherwise would induce slope face failure due to the weak zone at the face. Also shotcrete does not influence the internal stability of the slope which is thus not refelcted on the FoS.

Mithresh,

As well stated by Zoran, the shotcrete layer is not going to contribute significantly in the global stability of the slope but you mentioned the presence of bolts which are changing the stability conditions. My question is how far away do they extend in the slope? Are they extending further than the potential failure plane or are they contained in the ground that is going to slide? Again, this is something that you should investigate without seismic forces under quasi-static conditions first. I also agree with Zoran on the fact that you need to be really careful when you apply your boundary conditions and your loading conditions (magnitude and orientation). Finally, is your model large enough so that you eliminate the effect of your boundaries?

Iaonnis,

The bolts are extended beyond the failure plane and yes, the model is large enough to ignore the effect of boundaries as it does not cause any influence on the failure plane.

Hello Mithresh,

Thanks a lot for the clarification. So based on that, these are the steps that I would follow for such a project:

1. Slope analysis without support and without seismic forces.

2. Slope analysis with bolts without seismic forces

3. Slope analysis with bolts and seismic forces

4. Slope analysis with bolts, shotcrete but without forces 

5. Slope analysis with everything

Your reference in such a case is going to be the first analysis and figuring out the impact for all different scenarios is getting a lot easier as you have a solid case which you can compare to.

I would personally add one more scenario:

- Analysis in the case of extreme hydrological conditions (shotcrete + anchors + suitable valeue of ru coefficient or other value, without seismic forces)

Thank you so much for the suggestions...Hope it solves my query...:)