Generally the Employer is responsible for the accuracy of site information and ground conditions unless the JCT 2011 DB contract has been amended to provide otherwise.
Extensions of time (adjustments to the completion date) are permitted only where a relevant event listed under 2.26 occurs and the works are likely to be delayed beyond completion (2.25). The process is under clause 2.24-2.25. The contractor can give notice of the material circumstances (2.24) or the Contract Administrator can grant an extension based on the information available (2.25.5).
If the default, whether an act or omission, was by an Employer Person (ie a contractor or consultant appointed directly, as you state) then 2.26.6 might apply; the facts might also come under a Change (5.1). If it was by a subcontractor (as normally defined ie employed by the contractor) then it would not. More information would be needed to provide a definitive response.
If the contractor is not granted an extension and the CA has acted reasonably, then the Employer may be able to deduct LADs provided overall completion of the project is delayed and the Employer issues a notice of non-completion under clause 2.28.