The first thing you need to ascertain is who is responsible for the design. From your question it appears that the Employer has provided an indicative design suggesting that you are responsible for the design. If that’s the case then you need to check the standing of the indicative design but I suspect it will be caveated and the design is your responsibility.
As for the ground conditions encountered, you need to check if the standard compensation event clause 60.1(12) is intact and not modified or deleted by a Z clause. If it remains intact then you will need to demonstrate that the conditions encountered had “such a small chance of occurring that it would have been unreasonable to have allowed for them”. if that's the case you need to notify the PM that you consider a CE has occurred (see clause 61).
Whilst checking the above you should submit early warning and start discussing possible mitigation wit the PM.
As for the ground conditions encountered, you need to check if the standard compensation event clause 60.1(12) is intact and not modified or deleted by a Z clause. If it remains intact then you will need to demonstrate that the conditions encountered had “such a small chance of occurring that it would have been unreasonable to have allowed for them”. if that's the case you need to notify the PM that you consider a CE has occurred (see clause 61).
Whilst checking the above you should submit early warning and start discussing possible mitigation wit the PM.