Before you proceed with your method statement, can i know :

1. How crucial pedestrian bridge to your project. or any option for diversion

2. Who is using the pedestrian route?

3. If involving public then rationally please avoid this idea. The stake is too high. 

But If  for workers then should be OK.

If you want to proceed technically can be done by asking your structure consultant to come out with the detail of the temporary steel structure with span pile that can be use (please refer to your safety regulation document). Approval design must be obtain by relevant consultant and authority. 

Excavation method, please refer to :

1. Piling Work method statement

2. Pile cap work method statement

3. Attachment and binding work between pile cap and  steel structure

If you work as a consultant please do advise your client to avoid such innovation.