Weather, ground conditions and recovery of cost - JCT SBC/XQ 2016

We have a project which includes forming large areas of playing fields including importing topsoil onto existing clay and then seeding. At the start of the project the levels of existing topsoil where higher than originally anticipated and it was agreed with the client to increase the topsoil depth to 250mm as opposed to the original 150mm in the contract and utilise what was already onsite. The client accepted the saving for this.
A year later we are approaching handover and due to the excessive rainfall incurred over the last 12 months the topsoil originally onsite cannot be used. There is a large amount of standing water on the site and the only way we are able to complete the project is to wait for it to dry out putting the completion date at risk or do a full site strip of 150mm and import clean topsoil which is at a very high cost.
The design and the construction build up has always been the risk of the client and due to the nature of the clay underneath the landscape areas there is nowhere for the surface water or rain to run off, there is also no land drainage on the site. The build up supplied by the client is a standard build up but there is a risk if there’s another wet summer the playing fields will be saturated again.
We’ve issued a notice of a potential relevant event due to the adverse amount of rainfall but that only entitles us to time and not cost. My query is can the cost of the replacement topsoil be recovered through the contract as the design should have included for drainage of the site or is this a weather associated risk which should be borne by the contractor